Buckybowls and Heterobuckybowls -...

Chapter V: Buckybowls and Heterobuckybowls

Chapter V: Buckybowls and Heterobuckybowls

5.0: Abstract

A benchmark study at vanous ah lnrrlo HF, pure and hybnd dens~ty funct~onal

theory levels is done on the synthet~cally elusive buckybowl, sumanene (ClIHl2), for the

first t ~ m e to examlne ~ t s vanous phys~cochem~cal propertles Based on the bowl-to-bowl

Inversion calculated at the B3LYPlcc-pVTZ level, an lnterestlng pred~ct~on IS made that

~t would undergo slow lnverslon near room temperature ~f synthes~zed Clues are

prov~ded as to why some of the synthet~c attempts were fut~le and potentla1 mutes for the

successful synthesis of this compound are suggested Calculat~ons at the hybnd DFT

level on C ~ ~ H P ~ , where the charge (Z) of the system IS var~ed from -3 to +3 lnd~cates that

the curvature and lnvers~on bamer decreases as more electrons are added to the system

The electron afinities and lonlzatlon energles are calculated and the redox behav~or 1s

compared to that of corannulene, wh~ch revealed that the C,-fragment 1s a better electron

acceptor and a donor S ~ t e specific heteroatom subst~tut~on was shown to be a s~mpler

way of modulat~ng the curvature, bowl r ~ g ~ d ~ t y and other phys~cochem~cal propertles of

buckybowls, whlch results In a new class of compounds, namely, heterobuckybowls

Dens~ty funct~onal theory calculations were done on a serles of heterosumanenes.

C18HbX3. X = 0, NH, CH2, BH, S, PH, PH,. Si, S I H ~ and AIH The bowl-to-bowl

lnverslon barr~er and curvature decrease with the Increase In the srze of the heteroatom

The planar form 1s computed to be the mlnlmum energy structure when X = SI, S1H2 and

AIH Structure-energy relat~onsh~p study on t h ~ s senes of heterobowls ~nd~ca tes that the

bowl-to-bowl inversion dynam~cs can be explained by a m~xed quart~c-quadrat~c

funct~on The thermodynam~c and klnetlc stablht~es of these heterobowls were assessed

based on homodesm~c equations and chem~cal hardness values respect~vely The stra~n

energy build up along two poss~ble synthet~c routes are calculated and the synthetic

feaslb~lit~es of thls class of compounds are d~scussed

5.1: Introduction

The discovery of fullerenes stimulated the contempomy chemistry researchers to

investigate and unravel the unique structural, elmtronic, optical, biological, magnetic and

other properties of this potentially promising class of compounds.14 The attempts

towards the synthesis of C u (1) by rational means were futile till recently (Scheme 5.1),

however these attempts opened an exciting area of buckybowls, the fragments of

fu~lerenes.'.~ The interest in the novel C U N ~ polycyclic compounds, buckybowls, may

be attributed to their ability to mimic some of the novel physico-chemical properties

exhibited by fullerenes.'" Recently, Scott and co-workers have achieved the landmark

synthesis of Cw starting h m a C,-symmetric fragn~ent.~"~ Corannulene (2) and

sumanene (3) are the pristine buckybowl structures, which are readily recognized as

fundamental structural motifs of C60, retaining Cs and C3 axes respectively (Scheme

5.1).".'~ The trivial name sumanene is bestowed on this novel bowl shaped curved

aromatic, tricyclopenta(defjkllpqr)triphenylene, based on the Sanskrit (also in Hindi)

word 'suman' meaning flower.".t2 Though the synthesis of corannulene (1) was reported

by Barth and Lawton more than 30 years ago, it drew renewed attention only in the post

fullerene era.') Recently more than half a dozen efficient alternative syntheses are

reported on corannulene and it is now available in preparative quantities.14 On the other

hand, it is disheartening to note that a limited number of attempts which are reported to

accomplish sumanene (3) are futile so far.'."." The one attempt towards a synthes~s of

sumanene was that of Mehta el al, wherein they start from a suitable tnphenylene denvattve and

adopt a sequential ring closure strategy."," Only the first two bridges could be assembled tn this

strategy and closure of the crucial third bridge could not be achieved, which was attributed to the

enormous build-up of strain energy.'* The attempts towards the synthesis through transition

metal complexes as precursors also were not successfu~.~' Still, we feel alternative routes

should be actively explored to achieve the synthesis of this key C,-symmetric buckybowl

sumanene (3) considering the fact that the syntheses of much more complicated and

highly strained analogs met with ~ u c c e s s . ' ~ ~ ~ ~ However, Otsubo and co-workers have

successhlly synthesized a hetero analogue of sumanene, where the three CHI groups in

sumanene are replaced by S atoms, and characterized by X-crystallographic studie~. '~ A

large number of interesting bowl like moieties, which fonn a part of Cm have been

Chapter V Buckybowls and heterobuckybowls 168

synthes~zed and were shown to e x h ~ b ~ t novel structural, chem~cal and physlcal

propertlcs 5'8.1622

Scheme 5.1

Corannulenc (2), Cs, Sumanene (3). Cj,

Corannulene (3) was unamb~guously character~red as a flex~ble molecule

exh~b~t lng rapld bowl-to-bowl ~nvers~on." '~ whereas h~gher buckybowls, whlch form a

part of C60. are found to be n g ~ d bouls 2 5 2 8 Computations played a plvotal role In

understand~ng and modellng the novel propertles of buckybowls "" W l e a large

number of theoret~cal stud~es of hlgh soph~st~catlon have appeared on corannulene?' to

our knowledge only sem~emp~ncal calc~tlat~ons have b e ~ n done on sumanene l 2 The

bowl-to-bowl lnverslon bamer of sumancnc pred~cted by uslng MNDO IS 24 2 kcal mol ' w ~ t h a bowl depth of 1 15 A l 2 Therefore, 11 IS expected to he a n g ~ d buckybowl In

contrast to corannulene (2) whose lnverslon bamer IS 10 2 kcal mol w ~ t h a bowl depth

of 0 89 1$ 24 Prev~ous theoret~cal stud~es lnd~cate that adequate bas~s set funct~onal~ty

and ~nc lus~on of dynamic electron corrclat~on are necessary to obtaln rel~able estimates " 27 However, In some cases, smaller bas~s sets and Hdrtrce-Fock methods place one In the

ballpark for predlctlng bowl-to-bowl lnverslon bamer, geometry and other phys~co-

chemlcal propertles probably due to fortu~tous cdncellat~on of errors 2 5 2 1 Therefore, ~t 1s

qulte dangerous to amve at any conclus~on that sumanene (3) 1s locked Into a slngle bowl

confomatlon based on sem~emp~ncal data For a molecule of t h ~ s slze the conventional

ab rnrrlo post-Hartree Fock w ~ t h good qual~ty bas~s sets are proh~b~tlvely expensive

Consequently, Dens~ty Funct~onal Theory (DFT) becomes the only vlable altemattve to

Include the effects of dynamic electron correlat~on at present 1'3SThus, the appllcatlon of

density h c t l o n a l methods for electron~c structure calculat~ons has reached a peak In the

Chapter V: &ckybowls and hcterobuckybowls 169

last few years.'5'3K A detailed analysis of the structure, inversion barrier, suitability of

various computational procedures, vibrational spectra, population and charge analyses of

sumanene are presented in the first part of this chapter.

The stability of hllerenes in various oxidation states has bmn a fascinating

observation; C60 was found to be stable as a hexaanion as well as a hi~ation.' '~ These

ionized forms showed interesting properties, such as superconduction. Corannulme was

found to exhibit equally interesting redox properties and was reported even before the

discovery of fu~lerenes.~'" Mono-, di-, tri- and tetraanion of coranulme have been

generated using controlled reduction by alkali metals, Li and K, and were characterized

based on optical absorption, electron paramagnetic resonance (EPR) and nuclear

magnetic resonance (NMR) spectroscopies.42 Corannulene tetraanion was found as a

dimer, where four of the Li atoms are sandwiched between the two corannulene moieties

and two Li atoms lie in the outer surface of each corannulene nuclei." Corannulene

monocation was generated and gas-phase ionlmolecule reaction with C60 has been

st~died.~' Theoretical studies concerning the structure and inversion baniers of dianion

and tetraanion of corannulene was reported by Rabideau and coworkers." Recently, the

vibronic coupling, the Jahn-Teller effects and vibrational structures in negatively charged

corannulenes were reported45,46 Synthesis and characterization of dianion and octaanion

of 1,s-corannulenyloctane (two corannulene units tethered by a hydrocarbon chain), a

supramolecular octaanion, were done.47 The authors also have determined the inversion

baniers of the dianion using variable temperature 'H NMR and found that the inversion

barrier of the dianion is lowered compared to the neutral moiety.47 h u l a t e d

corannulenes have been studied extensively both experimentally and theoretical^^!^"^ The anions of benzannulated corannulenes have been studied by NMR spectroscopy and

magnetic properties were calcu~ated.~~ Hybrid density functional theory calculations were

done to examine the electronic structure, curvature, bowl-to-bowl inversion barria, the

electron accepting and donating ability and other physicochemical properties of the bowl

shaped ~ 2 1 ~ : unit where the charge is varied from -3 to +3 (Scheme 5.2).

Chapter V. Buckybwlr and hcterobuckybowls 170

Scheme 5.2

Z

In fullerene chemistry, heterohedral fullerenes, where one or more of skeletal C

atoms are replaced by heteroatoms, such as B, SI and N, have attracted cons~derable

attention and these dopyballs are cons~dered to modulate the superconduct~ng, electrical

and redox propert~es of fullerenes j2 The first heterobuckybowl to be synthes~zed 1s

tnthlasumanene, 11s (Scheme 5 3), and was charactenzed uslng X-ray crystallography,

whlch revealed that 11 is a shallower bowl compared to corannulcne '(I We have proved

that slte spec~fic heteroatom substltut~on 1s the s~mplest means of tailoring the curvature,

bowl n g ~ d ~ t y and stabll~ty of buckybowls 53 55 Modulating the curvature and the bowl-to-

bowl lnverslon barner of buckybowls has been an lnterestlng suhject " '' Annulat~on of

five membered nng to the nm of corannulene seems to arrest the howl-to-howl Inversion

at room temperature6 Const~ctlng a cyclophane bndge In the cornnulene nucleus was

found to be another strategy for locklng the bowl to one of 11s confonnat~ons '- Decamethyl substituted corannulene and benzannulated cormnulenes dre found to be

shallower and possess lower lnverslon bamer compared to the pnstlne corannulene

rnolety 49.58 S~egel and co-workers reported that a double well potential, given by mrxed

quart~ciquadrat~c funct~on, seem to model the lnverslon dynam~cs In a senes of

corannulene denvatlves 59 In this context. ~t 1s ~nterestlng to test the appl~cab~l~ty of the

structure-energy correlations ~n the inversion dynamics of heterobuckybowls

CIS-tnphenylene and CIS-tnndene are the two planar CJ-symmetnc hydrocarbons,

wh~ch map on the C60 surface m addltlon to the planar benzene and 6-rad~alene l 2 Both of

Chapter V: Buckybowls a d hcterobuckybowb 171

them are accessible in quantities and are amenable to structural functiona~ization.~~~~ The

syntheses of sumanene7'"." and trithia~umanene~~ were unsuccessful when started from

triphenylene derivatives, in contrast, a trindene analogue yielded hithias~manene.'~

Requirements for introducing bond alternation in benzene have been a subject of interest

for a long The significant bond alternation that exists in triphenylene ( 1 2 ) , ~ ~ ' " ~

tnndene (13CHz) and cw6' prompted us to critically analyze the bond length alternation

in this class of compounds. Therefore, an examination of bond alternation in trindenes

and heterosumanenes, in the flat as well as in the bowl forms as a function of substituent

would be interesting in its own right (Scheme 5.3). The heterosumanenes have 18-n

electrons and 18-sp2 centers for X = CH2, pH3 and SiH2; 18-n electrons and 21-sp2

centers for X = BH, Al and Si; 24-n electrons and 21-sp2 centers for X = NH, PH, 0 and

S. "Annulene - within an annulene", where both the rim and hub attain the aromatic

Huckel count has been evoked to account for the stability in fused polycyclic aromatic

systems.42 Thus according to this model, the heterobuckybowls are stable only for X =

NH, PH, 0 and S with a 6 n hub and an 18n rim electron count. The effect of replacing

the three unique CHz groups in sumanene (3) by X = 0, NH, BH, S, PH, PHI, Si, SiH2

and AIH on the geometry, curvature, inversion barrier and the feasibility of their

syntheses are studied here using hybrid density functional theory calculations. The

synthetic feasibility of these heterobuckybowls is explored through two possible routes:

namely the triphenylene and trindene routes.

Scheme 5.3

X = 0, NH, CH2. BH, S, PH, PH3, Si, SiH* and AIH


5.2: Benchmark Calculations on Sumanene

5.2.1: Computational Details

The planar and bowl forms of sumanene (3) were fully optlm~zed wlthln Dlh and

CI, symmetry constraints respectively, ustng the default gradlent techn~ques ~mplemented

In the Gauss~an 98 program package 6R Frequency calculat~ons were performed at several

representatwe levels, whlch unequ~vocally characterize the C,, bowl structurc as a

mlnlmum whlle the corresponding planar DII, structure as the translt~on state, for bowl-to-

bowl inversion To scrutlnlze the sensltl\,lty of the geometries of mlnlnia and transltlon

state and the bowl-to-bowl lnverslon barriers, the calculat~ons were done at oh rn~t ln

(HF), pure and hybrld denslty functional theo~y levels w ~ t h an array ol bas~s sets, us~ng

the Gauss~an 98 package Beckc's gradlent-corrected exchange funct~onal (B)"" and

hybr~d three-parameter funct~onal (81).'" werc used In conjunction with non-local

correlat~on prov~ded by Lee, Yang, Parr (LYP)." Perdew 86 ( ~ 8 6 ) ~ ' ~ and Perdew 91

( ~ ~ 9 1 ) ' ~ whlch contam both local and non-local terms The qual~ty of basis sets arc

systematically Improved startlng from the mlnllnal STO-3G hasls set to Pople's douhle

and tr~ple-j quallty basls sets w ~ t h addcd polarlrdt~on funct~ons, vlz, 3-21G. 6-JIG* 6-

31G**, 6-31 lG*, and 6-3llG** " Add~tlonally, Dunning's correlat~on consistent bas~c

set, cc-pVDZ, was used for both the RHF and DFT calculat~ons '' Hybr~d DFT methods

m conjunction w ~ t h the cc-pVDZ bass set hdvc heen shown to glve adequate descnptlon

of hamonlc frequenc~es for thls class of con~pounds Opt~m~zatlons lnclud~ng dlfl'use

funct~ons on carbons met w ~ t h numerous SCF convergence problems and could not hc

carled out However slngle pant calculat~ons ale carr~cd out uslng 0-31 1+G* 6-

31 l++G* and C C - ~ V T Z ' ~ basis sets, cc-pVTZ a tr~ple j qual~ty bass set wh~ch ~ncludes

one set o f f funcllons and two sets ol'd-polar~zat~on funct~ons on carbon, glves r ~ s c to 798

bass functions for CIIH12 (2) Thc three popular senl~emp~r~cal schemes. MNDO -' AM^," and P M ~ , " were also performed to ascertain thclr sultabll~ty Naturd! populdtlon

analysls was done, In add~tlon to the routlne Mulhken analys~s, to obtaln natural atomlc

and group charges, I e , the charges on hydrogens are summed up wlth heavy atoms to

whlch they are llnked Natural Bond Orb~tal (NBO)" analys~s IS performed uslng the

subrout~ne Implemented In the Gausslan 98 program package Unscaied vlbrat~onal

frequenc~es are used to obtain zero-polnt energy and enthalpy correct~ons In denslty

functional methods, and the scaling factor of 0.89 is adopted for H w F o c k b d

methods." All the calculations were done using the Gaussian 98 suite of programs. The

best values for optimized geometric parameters are those obtained at the B3LYPI6-

31 1G** level as it is the largest basis set employed and energetics obtained at the

B3LYPlcc-pVTZ level.

5.2.2: Results and Discussion

5.2.2.1: Equilibrium Geometries

Table 5.1 gives the principal geometric parameters for the minimum energy bowl

structure obtained at various levels of theory, based on the labeling given in Scheme 5.4.

Similarly, the principal geometrical parameters for the bowl-to-bowl inversion transition

state are given in Table 5.2. Expectedly, HF consistently underestimates all bond lengths.

The geometries are virtually identical at all DFT levels. The bond alternation is

overestimated at semiempirical and HF levels, especially with inadequate basis sets. The

bond alternation in sumanene (3) is higher in the hub six-membered ring when compared

to its flank six-membered ring. Thus, surprisingly r2 and r3a have very similar bond

lengths to that of rla, which is in turn closer to the aromatic C-C bond length. Improving

the basis set quality further to 6-31G' does not bring in any significant changes in the

geometries and bond alternations. The central six membered ring witnesses significant

bond alternation in 3, which is only slightly lower than that present in Cm; 1.398 and

1.455 A,"' In contrast, five of the flank six membered ring bond lengths are essentially

identical, with a slightly elongated rim bond (r4). It may be noted that the bond

alternat~on in central six-membered ring of triphenylene (12) is found to be very

significant both by experimental66 and theoretica~"~ studies; with the endo and exo bonds

measuring 1.41 and 1.47 A respectively.

Scheme 5.4

spoke, 12 rim, r4 - vertex

hub(6), rla

Chapter V Buckybowls and hctcrobuckybawk 174

Table 5.1 Selected geometric parameters of sumanene (3) at vanous levels of theory All values are gven m A

Level rla rlb r2 r3a r3b r4 Al' 1\2b

MNDO 1 391 1454 1429 1401 1 546 1444 0063 0053

B3PW9lIcc-pVDZ 1 387 1432 1 399 1400 1 545 1429 0045 0042

a) n l = rlo-ria, 16 m c UU~IY ~IICIII~LIVII 1x8 LISL LEU" -lrryr.. ,,. - b) A2 = r4-rla, IS the bond alternat~on In the flank benzene nng In 3

Chapter V: Buckybowk cad hctvobuckybowls 175

Table 5.2: Selected geometric parameters of the bowl-to-bowl inversion transition state of swnanene at various levels of theory. All the values are given in A.

Level rla r lb r2 r3a r3b 14 A1' ub MNDO 1.373 1.426 1.405 1.414 1.580 1.468 0.053 0.095

B3PW9lIcc-pVDZ 1.366 1.398 1.376 1.418 1.587 1.453 0.032 0.087 a) A1 = rlb-rla. 1s the bond alternat~on In the hub benzene nng In bowl-to-bowl lnverslon

translt~on state shucNre of sumanene. b) A2 = r4-rla, 1s the bond alternat~on In the flank benzene nng In bowl-to-bowl ~nvers~on

trans~tlon state structure of sumanene.

While going from the minimum energy bowl to the flat transition state structure,

the hub and spoke bond lengths (rla, r lb and 12) are substantially shrunk, while the bond

Chapter V. Buckybawls and heterobuckybowls 176

lengths flank and nm (r3a. r3b and r4) are elongated at all levels of theory, consistent

w ~ t h the observat~ons In the bowl structures T h ~ s 1s to be expected as the planar form

enforces shnnklng of hub SIX-membered nng and elongat~on of nm and flank bonds to

accommodate angular straln Consequently, the bond altematlon 1s much hlgher In the

peripheral SIX membered nng compared to the hub one, wh~ch IS In complete contrast

w ~ t h the sltuatlon In bowl

5.2.2.2: Curvature, Bowl Depth and Polarity

Haddon's rr-Orb~tal AXIS Vector (POAV) anglex2" an angle between the normal

vector to the pyramldal base and the ~deal~zed CJ synimetnc C-C bonds, 1s the most

popular measure of pyram~dal~zat~on at a glven sp2 carbon center An angle of 90"

correspond to a flat carbon center, corannulene and Cu, have POAV angles of 98 2 and

101 6" respectrvely *'' The POAV angles at the three un~que (hub, nm-quat and nm) sp2

centers, along w ~ t h the bowl depth and the dlpole moment obtalned at vanous levels of

theory are glven In Table 5 3 Companson of POAV angles between corannulene (2) and

sumanene (3) at the hub posit~on clearly lndlcate that the latter IS more pyramldal and the

curvature IS closer to that of C60 POAV angle IS found to be nearly lnsensltlve to the

level of calculat~on employed

Scheme 5.5

Bowl depth 1s defined as the dlstance between the planes f o n e d by the central

hub atoms and that formed by the nm carbon atoms (Scheme 5 5) Howcver, bowl depth

e x h ~ b ~ t s greater fluctuat~ons as a funct~on of method and the best computed value 1s about

1 14 obtalned at B3LYPicc-pVDZ level Flgure 5 1 glves the plot of vanat~on of bowl

depths as a functlon of levels of theory employed Here, bass set of the qual~ty equal to

or h~gher than 6-31 lG* or cc-pVDZ IS requlred to get the correct plcture Buckybowls

are polar in contrast to the planzr aromatlc hydrocarbons due to the anlsotroplc

d ~ m b u h o n of x-electrons and the C-H bonds, wh~ch tnggcr separatlon of charges

Consequently the base of the hub n charged and the depth of the bowl n proport~onal to

charge separatlon Natural charges obtiuned by NBO analys~s at 6-31 IG* bas~s set at HF

and B3LYP levels of theory are given in Figure 5 2 The computed dlpole moment (p) of

around 2 5 Debyes IS substantial for pure hydrocarbons The d~pole moment was found to

be very sensitwe to the level of theory employed and semlemp~ncal or ab lnirro and DFT

methods without the polanzat~on funct~onality In the basis sets are found to be

madequate The value of p 1s zero for the flat structure due to symmetry. also as the

curvature or bowl depth Increases the value of p 1s also expected to increase

Unexpectedly, sem~em~~rlca l levels, whlch show higher bowl-depths, gavc lower dipole

moments

Figure 5.1 The vanatlons In the bowl depth of sumanene (5.3) at vanous levels of theory

Chaptv V Buckybowls and hetvobuckybowls 178

Table 5.3 POAV angles obta~ned for the hub, nm-quat and nm posltlons obta~ned uslng POAV3 program for the op t~m~zed geometnes of sumanene at vanous levels of theory Bowl depth, BD (A) and d~pole moment, p (Debyes) are also glven

Level Hub Rim-auat R ~ r n BD' u

B3PW9llcc-pVDZ 98 9 94 7 92 3 1 144 2 336 a) Bowl depth is mter-planar distances between the two best planes formed by the hub and nm

Chapter V: Buckybawls ond hetcrobuckybavls 179

Figure 5.2: Natural charges in sumanene obtained at HF and B3LYP (underlined) levels with 6-3 11 G* basis set.

5.2.2.3: Bowl-to-bowl Inversion Barrier

Bowl-to-bowl inversion vividly exhibited by corannulene at room temperature is

arguably the most striking feature of buckybowls. The total energies along with the bowl-

to-bowl inversion barriers at various levels of theory are given in Table 5.4. HF level

shows near insensitivity to the inversion barrier from 3-21G basis set onwards and was

found to give satisfactory description. Figure 5.3 gives the magnitude of bowl-to-bowl

inversion barrier at various levels of theory. Although the calculated barrier is essentially

similar at HF and DFT levels and less sensitive to the basis set above double-6 quality,

density functional methods seem to require a basis set of cc-pVDZ or triple-6 quality.

The energies reported up to entries 27 are the energetics obtained on the geometries at the

same level of theory. Further, single point energy calculations were done at B3LYP level

using cc-pVTZ, 6-311+G* and 6-31 I H G * basis sets. All the semiempirical levels are

found to be inadequate to quantitatively predict the barrier height, and among them

MNDO was found to be better than the rest. Similar observations were made in the bowl-

to-bowl inversion barrier of corannulene, where MNDO gives reasonable estimate and

AM1 and PM3 proved to be quite unsatisfactory.'' Mer the enthalpy correction, done at

Chapter V. Buckybowls and heterobuckybowls 180

B3LYPIcc-pVDZ level, the best estlmate for the bowl-to-bowl lnverslon bamer 1s 16 9

kcaVmol whlch IS s~gn~ficantly smaller compared to the earlier MNDO estlmate of 24.2

kcaYmol Cons~denng the same frequency factor for both corannulene (2) and

sumanene (3) lnverslon dynamics, the rate constant for the later process 1s about 2 5 s I

lnd~cat~ng that one observes slow real tlme lnverslon near room temperature The

actlvatlon bamer for thls Inversion seems to be the llm~tlng case for locklng Into a slngle

conformat~on and w ~ l l prove to be extremely sens~t~ve to the temperature

Figure 5.3 Bowl-to-bowl lnvcrslon barr~er of sumanene (3) versus method of calculat~on

However, the accuracy of the bamer he~ght 1s hard to prove In the absence of

expenmental ev~dence Yet, based on the track record of DFT methods and also looklng

at the near Insensltlvl(y of the harner he~ght to the bass set and the type of exchange

correlat~on funct~onal, ~t is very unl~kely that the error In the pred~cted barr~er he~ght will

be more than 2 kcalimol, thus maklng an upper and lower l~mlts of 19 and 15 kcallmol

Chapter V Buckybowls and hetcrobuckybowls 181

Table 5.4 The semi-empmcal enthalp~es of fonnahon and ab rnlho and DFT total energles (m hartrees) for the bowl structure and the planar translhon state of sumanene and the bowl-to-bowl lnverslon bamers, AE* (m kcal mol I) obbuned at vanous levels of theory

Level

1) MNDO

2) AM1

3) PM3

4) HFISTO-3G

5) HFi3-21G

6) HFl6-31G*

7) HFl6-3 1G"

8) HFI6-31 IG*

9) HFi6-3 11G"

10) HFicc-pVDZ

I I) B3LYPiSTO-3G

12) B3LYPi3-21G

13) B3LYPl6-31G'

14) B3LYPl6-31G0*

15) B3LYPl6-31 IG*

16) B3LYPl6-31 IGg*

17) B3LYPIcc-pVDZ

18) BLYPi6-31G'

19) BLYPIcc-pVDZ

LO) BP8616-31'3%

21) BP86lcc-pVDZ

22) B3P8616-3 1 G*

23) B3P861cc-pVDZ

24) BPW9116-31G'

25) BPW9llcc-pVDZ

26) B3PW9116-31G'

27) B3PW9llcc-pVDZ

28) B3LYPl6-311+G*'

29) B3LYPl6-3 1 I++G*'

30) B3LYPlcc-pVTZa

Bowl (C,,) Transrt~on state (Dlh)

0 22760 0 26645 24 2

0 27262 0 32843 35 0

0 23093 0 28097 31 4

-792 48746 -792 45228 22 1

-...-.. rl.... -~~ -

B3LYPIcc-pVDZ level 1s also rncluded 1; the A? values

Chapter V: Buckybowls ond hctcrobuckybowls 182

5.2.2.4: Effect of Temperature on the Inversion Dynamics

Figure 5.4 gives the effect of temperature on the bowl-to-bowl inversion

frequency. Assuming that the activation free energy is 16.9 kcaUmol, the bowl structure

remain locked in a single conformation around 250 K. Therefore, the inversion dynamics

is expected to exhibit interesting features near room temperature. The inversion dynamics

of sumanene (3) reminiscent of the situation in cyclohexane and a simple variable

temperature 'H nuclear magnetic resonance (NMR) experiment will be suffice to follow

the dynamics. Whereas a proper substituent, which distinguishes the protons in its two

different bowl forms, is essential for following the inversion dynamics of

corannu~ene."'~

Figure 5.4: The plot of bowl-to-bowl inversion frequency, the first order rate constant, versus temperature. The inset includes a broader range of temperatures, and the inversion frequency is in the order of lo5 s-' units.

5.2.2.5: Frontier orbitals

The ordering of the frontier orbital energies is essentially the same at all the levels

of theory, both HOMO and LUMO are doubly degenerate. Figure 5.5 depicts the one

electron energy ordering in the frontier range and shapes of the doubly degenerate

C h a p t v V: Buckybowls and heterobuckybowls 183

HOMO and LUMO at HF level. The nature of the frontier orbitals at various levels of

theory was examined using MOPLOT program.s5 The ordering of one-electron energy

levels in the frontier region was found to be similar at all levels of theory. The vertical

ionization potential value of 7.45 eV for sumanene at HFIcc-pVDZ level, and the orbital

energies are virtually insensitive to the basis set. The relevance and utility of one-electron

wave functions in DFT methods are highlighted recently, which provides confidence in

believing the vertical ionization potential values.86

LUMO (E)

HOMO (E)

Figure 5.5: The frontier orbitals of sumanene and the shapes of the doubly degenerate HOMO and LUMO.

5.2.2.6: Computed Vibrational Spectra

The predicted infra-red (IR) spectrum of sumanene is given in Figure 5.6. A1 and

E irreducible representations are IR active while Az is inactive due to symmetry. Hartree-

Fock based methods tend to overestimate the vibrational frequencies by about

However, B3LYP together with double-6 and polarization quality basis set was found to

be quite satisfactory for reproducing experimental vibrational frequencies of aromatic

Chapter V: Buckybowb d hhctvobuckybowls 184

hydrocarbons." The normal modes of vibration of 3 are given as the following symmehy

adapted linear combinations:

r v l b = 17AI + 14A2 + 31E

The computed IR spectrum indicates that there are around three very intense vibrational

fundamentals at 3025, 3086 and 3178 cm-', which belong to E, A1 and E irreducible

representations respectively. Among which the doubly degenerate band (E) at 3178 cm.'

was computed to be the most intense band, a trend which is reproduced at all levels of

theory. The other two intense vibrational fundamentals appear in the regions of 800 and

1400 cm-I, which belong to AI and E irreducible representations respectively. The broad

features obtained at this level are essentially similar to the other levels of theory

employed here.

Figure 5.6: The B3LYPIcc-PVDZ computed vihratlonal spectrum of sumanene

5.2.2.7: Prediction of a Feasible Synthetic Strategy

We have evaluated the relative ease using computational methods of

accomplishing the synthesis of sumanene (3) and trithiasumanene (11s) through two

representative routes namely, the triphenylene and trindene routes (Figure 5.7). The strain

energy build-up in each of the steps in sequential ring closure pathways, obtained from

Chapter V: Buckybowls and hetcrobwkybowlr 185

the reaction energies, for both the routes are given in Figure. 5.7. The cornlation of the

strain energy build-ups along the triphenylene and trindme mutes obtained at the

B3LYPlcc-pVDUIMND0 level is given in Figure 5.8. In the triphenylene route, all steps

lead to increase in strain with the third step computed to be endothermic by more than 50

kcallmol, which makes the task of the synthesis almost impossible. In the thia-analogue,

while the first two steps have much less strain energy build-up. the final step requires

almost 40 kcallmol, which accounts for the failure of the attempt by Klemm et 01.~' The

lower strain energy for the thia-analogue can be traced to the fact that larger atoms

subst~tuted at the rim of the bowl facilitate ring

Triphenylene route

Trlndene route

&x - -57.0 x ----r -43.1 2 9

-53.4 -45.3 -37.3 1 X

Figure 5.7: Strain energy (kcal mol-') build-up in a sequential ring closure strategy along the triphenylene and trindene routes towards the synthesis of 5.3 and 5.11s at the B3LYPIcc-pVDZllMNDO level. Bold and ordinary correspond to X = CHI and S respectively.

Astonishingly, the trindene route shows substantial negative reaction energies

indicating the ease with which its synthesis can be accomplished. The reported synthesis

can easily be understood looking at the huge exothermicities of -53.4, -45.3 and -37.3

kcallmol for the first, second, and third bridge closures en-route to 11s. What is more

important is that the steps in the route towards the hydrocarbon compound are more

exothermic when compared to the thia-analogue. Therefore, according to these

Chapter V: hckybowls and hcterobuckybowls 186

calculations, once the synthesis of the precursor 17CH2 is achieved, the sequential ring

closures will follow immediately to accomplish the elusive sumanene, CIIHII.

Calculations indicate that in the trindene route, the hydrocarbon analogues have lower

strain energies compared to the thia-analogues. The strain energy build-ups along the

triphenylene and trindene routes obtained at various levels of theory are given in Tables

5.5 and 5.6 respectively. The qualitative trend obtained by any of the levels of theory

employed is the same.

Table 5.5: The heats of reactions obtained at various levels by following triphenylene route to obtain sumanene (3) and trithia-sumanene (11s). All values are given in kcal mol-'.

--A

X = CHI X = S Method 145 -+ 15X-t 16X-t 'Ex'' l X 3 15X 16X 11s

AM1 11.8 36.2 50.4 0.5 18.1 37.0 MNDO -2.3 19.6 46.4 -13.5 2.1 30.2

PM3 6.4 30.2 45.0 -0.6 14.5 26.9 B3LYPlcc-pVDZ' 7.3 33.4 53.4 -1.3 12.5 37.5 B ~ L Y P ~ C C - ~ V D Z ~ 6.0 33.0 55.8 -1.8 13.9 40.2

a) Single ooint calculation on AM1 optirnlzed aeometrtes. . u .

b) Single point calculation on MNDO opt~rn~zed geornetrles

Table 5.6: The heats of reactions obtained at various levels by following the trindene route to obtain sumanene (3) and trithia-sumanene (11s). All values are given in kcal mol".

X = CH2 X = S Method 17X-t 18X-r 19X+3 I:;x+ l a x - + 19X-r

18X 19X 19X 11s AM1 -42.6 -28.1 -30.2 -42.0 -33.8 -26.7

MNDO -46.2 -29.7 -24.1 -47 0 -35.8 -23.5

B3LYPlcc-pVDZa -56.8 -43.4 -43.0 -54.5 -47.1 -38.5

B ~ L Y P ~ C C - ~ V D Z ~ -57.0 -43.1 -42.9 -53.4 -45.3 -37.3

a) Single point calculation on AM1 optimized geornehles. b) Single po~nt calculation on MNDO opt~rnlzed geometries.

Chapter V: Buckybowk and hctvobuckybowk 187

1 , I

0 1 2 3

# of bridges

Figure 5.8. The correlation of the strain energy build-ups in the sequential ring closure strategy along the triphenylene and trindene routes towards the synthesis of 5.3 and 5.1 1 s obtained at the B3LYPIcc-pVDZ//MNDO level.

To examine the inherent strengths/weaknesses of the two strategies adopted, the

isodesrnic equations are set up as given in Scheme 5.6, The results of the isodesmic

equations are given in the same scheme. Beyond any doubt, these calculations indicate

that the trindene route is definitely a better choice to achieve the synthesis of surnanene.

The lower strain energy for the hydrocarbon analogue may be traced to the presence of

aromaticity in the thia-analogue, which is absent in the former. We feel that this

computational study gives boost to the synthetic efforts towards sumanene using the

trindene route.

Scheme 5.6

5.3: ~ 2 1 ~ 9 ~ (2 = -3 to +3)

5.3.1: Computational Details

Inlt~ally, the bowl and the planar forms of slnglet and mplet states of 4, 6, 8 and

10 and doublet states of 5.7 and 9 were optlmlzed w~th C,, and Dlh symmetry constralnts

respectlvely However, due to SCF convergence problems, lower symmetry constralnts

were used for the bowl and planar forms for some cases (wde mnfia) Hybnd dens~ty

funct~onal B3LYP method with 6-31G' basls set was employed for the optlmlzat~on and

frequency calculat~ons UB3LYP procedure was adopted for the open shell systems and

triplet states of 4, 6, 8, and 10 The statlonary polnts obtalned were characterized by

Frequency calculat~ons, wh~ch designated some bowl structures as trans~t~on states and

some planar forms as second order saddle polnts Thc C3, howl form of 7 (tnplet) was

found to be a second order saddle pomt The true mlnlma and the correspond~ng bowl-to-

bowl lnverslon trans~t~on states were obtnned In all the cases by following the normal

modes of the lmaglnary frequencies All calculat~ons were performed uslng Gaussian 98

sulte of programs Graphical Interface program, Moplot was used to exarnlne the nature

of the normal modes of the v~brat~onal frequenc~es and the molecular orb~tals '' 5.3.2: Results and Discussion

The nature of the statlonary polnts obtalned are d~scussed first followed by the

dlscuss~on on the sal~ent geometnc parameters of the bo\\l structures and the

correspond~ng saddle polnts Then the d~scusston on the energetics, bowl-to-bowl

lnverslon bamers and the redox behav~or are presented The molecular orb~tal ordenng In

the C,,-bowl stnlcture obtained usmg the extended Huckel method 1s glven In F~gure 5 9

The degeneracy of the HOMO and LUMO, and closeness of the A1 and A1 orb~tals

suggest that the symmetric forms would undergo d~stort~on due to v~bronlc ~nteractlons

We have also performed calculat~ons on the tnplet states of 4, 6 . 8 and 10 Statlonary

polnts correspond~ng to the C,, and Djh forms of 4 (tnplet), 5. 6 (s~nglet), 7, and 8

(tnplet) could not be obtnned due to SCF convergence problems C3, form of 7 (tnplet)

could he obtalned, however, the Djh form encountered symmetry break~ng problem

Therefore, we resorted to optimlze the bowl and planar forms of these specles w~th lower

symmemes, namely C, and CZV respectlvely Bowl structures of 9 and 10 w~th C3, po~nt

p u p s are found to be trans~tlon states and the C,, form of 10 (tnplet) n computed to be

Chapter V Buckybowls and heterobuckybawls 189

a second order saddle polnt Stm~larly, the C,-bowl forms of 6, 4 (tnplet) and 8 (tnplet)

were charactenzed as hansltlon states The geometries of these molecules wen sl~ghtly

dtstorted along the normal modes correspond~ng Imagtnary Frequenc~es and opt~m~zed to

obtain the true mlnlma All the bowl-to-bowl lnvenlon trans~t~on states correspond to the

planar structures The D ~ I , form of 10 and Clv forms of 5 and 10 (tnplet) were found to be

second order saddle polnts The true transition states correspond~ng to the bowl-to-bowl

lnverslon were obtalned In all the cases and ngorously charactenzed - A, -6 50

+ - E -1046

Energy (eV) + A, -1201

4 - 1 2 0 3

Figure 5.9 The molecular orb~tal ordenng In CzlHp In the C,, form obtalned uslng the extcnded Huckel method The ordenng 1s not to the scale The orb~tal energles are given In eV


In t h ~ s sectlon, the Important geometric features of c ~ ~ H / are analyzed with

speclal reference to the vanatlon of curvature and bond length altemat~on as the charge

on the system 1s vaned The hub bond lengths (Scheme 5 4) ln bowl form of c ~ ~ H ~ ) - are

almost equal to the bond lengths In benzene, but the nm and the flank bond lengths are

elongated compared to the aromatlc bond lengths The concept of 'annulene w ~ t h n an

annulene' has been used to explan stabll~ty of the neutral and tetraamon of

cormulene 41 42 In this case, the tnanlon can be cons~dered as two concentric (4n+2)n-

con~ugated systems (6n16C and 1BnIlSC) connected to each other Thls 1s supported by

Choptv V: Buckybowls and hetvobuckybowls 190

the charge present in the inner and outer rings. The central benzene ring possesses a

charge of -0.443 and the outer 1871-annulene has a total group charge of -2.556 in the

singlet state of 4. Similarly the monocation (8) can be viewed as benzene moiety within a

14x-conjugated monocation. The mulliken charge analysis of 8 indicates that the benzene

ring has a charge of 0.051 and the outer annulene has a total group charge of 0.946,

indicating that most of the charge is located on the periphery of the bowl. The hub and

the spoke bond lengths are substantially shortened when going from the bowl to the

planar form, whereas the flank and the rim bond lengths are elongated as expected. Bond

the difference between the longest and the shortest bond lengths, in the

central benzene h n g of heteroatom substituted sumanenes, showed interesting variations

depending on the n-electron count (vide infra). Therefore, it would be interest~ng to note

the variation of bond alternation as the charge of the system is varied. The bond

alternation in the hub six membered ring is the minimum when Z = -3 (0.008 A). This

gradually increases from 0.008 A to 0.067 A from Z = -3 to 0. and slides down slightly

&om 0.067 A to 0.059 A when we go from Z = 0 to +3.

Bowl depth, the interplanar distances between the two planes formed by the hub

atoms and the atoms in the rim, is a useful measure 10 evaluate the curvature in

buckybowls. In the species considered in this study, the definillon of bowl depth is not

unambiguous due to lack of symmetry in some of the howl structures. Hence the

Haddon's n-Orbital Axis Vector (POAV) angle which was widcly and successfuly

employed to gauge the curvature of a given center in bowls was employed in the present

study.82.83 The correlation of the POAV angles in the hub, rim-quat, vertex and nm

positions as the charge on the system varies is given in F~gure 5.10. The average of the

POAV angles was taken to gauge the curvature of the bowls at d~fferent centers. Such an

approach was used successfully to measure the curvature of b ~ c k ~ b o w l s . ~ ~ ~ The POAV

angle at the hub position gradually increases from 4 to 8 and is virtually the same for the

cations. Similar to the hub position, the POAV angle at the rim-quat position gradually

increases as the charge on the system decreases up to +2 and decreases slightly for 10.

POAVvenCx is the maximum for 4 and minimum for 10 whereas POAV,, does not vary

systematically against the charge. In general, the bowl is more shallow for the trimion (4)

and deeper for the cations.

Chapter V: Buckybowls and hctvobuckybowb 191

-+- Rim-quat

98 'J U Rim

Figure 5.10: The correlation of the average POAV angles at various positions with the charge present on the system.

5.3.2.2: Energetics and Bowl-to-bowl Inversion Barrier

The total energies, the point group, the number of imaginary frequencies, and the

relative energies of all the stationary points and the bowl-to-bowl inversion bmiers of all

the minimum energy bowl structures obtained at the B3LYPi6-31G1 level are given in

Table 5.7. Figure 5.1 1 gives the correlation of the bowl-to-bowl inversion barriers of

C~IH: for all Z. The triplet state of 6 is computed to bc more stable than the

corresponding singlet state by 1.5 kcal/mol and the triplet state of 10 lies only 1.3

kcallmol higher compared to singlet 10. Whereas in the other two cases, namely 4 and 8,

the energy difference between the singlet and the triplet states are more than 30 kcal/mol.

The energy difference between the minimum energy bowl structure and the saddle points

corresponding to the bowl stmctures are less than 1 kcallmol in all cases except in 9 (4.3

Chapter V: Buclqbowls ond heterabuckybowls 192

kcallmol). Similarly, the difference between the energies of the bowl-to-bowl inversion

transition states and the higher order saddle points are very less (0.1 to 0.4 kcallmol). The

bowl-to-bowl inversion barriers of these species vary from 12.2 and 40.2 kcallmol with

singlet states of 4 and 9 having the least and the maximum values (Table 5.7). This is

similar to the situation in corannulene where the inversion hanier varies from 3.2 to 14.2

kcal/mol from the neutral to tetraanion. It is to be noted that inversion harrier of

sumanene (C21H12) is 16.8 kcal/mol obtained at the same level of theory (Section 5.2.2).

The inversion barrier gradually increases from -3 to +I and for the cations, the barrier

remains almost the same. The inversion barrier of the triplet state of 4 is more than that of

the corresponding singlet state. In contrast, the bowl-to-bowl ~nversion barriers of the

triplet states are lower than those of the singlet states for 6,8 and 10.

Singlet Doublet 1 I Trylet 1

Figure 5.11: The correlation of the bowl-to-bowl inversion barriers with the charge present on the system.


Table 5.7 The total energles (E), point group, number o f lmaglnary frequencies (NIMAG) and the relat~ve energles (AE) and bowl-to-bowl lnverslon bamers (AE') of ~ 2 ~ ~ 9 ~ for Z = -3 to +3 obtaned at the B3LYPI6-31Ga level

Structure Point ~ r o u p ~ u ~ t l p ~ l c ~ t y AE AE' (hamees) 'IMAG (kcallmol) (kcallmol)

1 Bowl C,, 1 -805 24246 0 0 0 12 2 cs 3 -805 18340 1 37 1 C, 3 -805 18384 0 36 8 18 7

Planar Dlh 1 -805 22309 1 12 2 Cz, 3 -805 15412 1 55 4

2 Bowl C, 2 -805 49042 0 0 0 20 1 Planar C,, 2 -805 45825 2 20 2

(-5 2 -805 45835 1 20 1

3 Bowl C, 1 -805 58870 1 0 8 ct 1 -805 58994 0 0 0 29 6 C,, 3 -805 59228 0 -1 5 25 7

Planar CZ, 1 -805 54271 1 29 6 D,h 3 -805 55128 1 24 3

4 Bowl C , 2 -805 54051 0 0 0 33 2 Planar C?, 2 -805 48762 1 33 2

5 Bowl Cj, 1 -805 33197 0 0 0 39 9 cs 3 -805 27943 1 33 0 Ct 3 -805 27949 0 32 9 34 9

Planar 1 -805 26837 1 39 9

C2, 3 -805 22386 1 67 8

6 Bowl C,, -804 90941 1 4 3 C, 2 -804 91634 0 0 0 40 2

Planar Dl,, 2 -804 85231 1 40 2

'i Bowl C,, 1 -804 34060 1 0 5 C, I -804 34134 0 0 0 39 1

c,, 3 -804 33879 2 1 6

cs 3 -804 33933 0 1 3 32 4

planar D , ~ I -80427841 2 39 5 c,,, 1 -804 27901 1 39 1

cz, 3 -80428725 2 33 9

C, 3 -804 28762 1 33 7


5.3.2.3: Redox Behavior of CllHg

The Koopman's (-ELUMO), vert~cal and adtabatlc electron affin~t~es of CIIHP (7)

up to addlt~on of three electrons obta~ned at the B3LYP16-31G* level are glven In Table

5 8 S~mllarly, the ~onlzabon energles are dep~cted In Table 5 9 We also have calculated

the vert~cal electron~c affin~t~es and ~onlzat~on potent~als for corannulene at the same

level of theory and ~ncluded In the correspond~ng tables The ad~abat~c electron affinlt~es

obta~ned at HF and MP2 levels from the prevlous study are incorporated In Table 5 8 454"

The electron affin~ty of 7 col~espondmg to the add~t~on of the first electron 1s posltlve

~nd~catlng that the monoanlon (6) 1s more stable than the neutral form, wh~ch 1s a rad~cal

The second and thlrd electron affin~t~es are computed to be negative, however.

cornpanson of the correspond~ng values of corannulene ~nd~cates that C ~ I H P (7) IS eas~ly

reduc~ble The second and th~rd lomzatlon energy values are also slrnllar to, In fact, lower

than that of corannulene Thus, the C3 syrnrnetnc fragment 1s even more faale compared

to corannulene The energy gamed due to skeletal reorganlzatlon after ~on~zatlon 1s lower

for the add~t~odrernoval of first two electrons and maxlmum for the third electron Thus,

the C, symmetnc fragment seems to be deal to extst In vanous ox~dat~on states

Therefore, ~t may be lnterest~ng to attempt the synthes~s of the ~on~zed C ~ I H ~ ' specles

startmg from an appropnate precursor

Table 5.8 The Koopman's, vertlcal and ad~abat~c electron affin~t~es and the reorganlzatlon energles (Av A) of CllH9 (7) and corannulene obtamed at B3LYPl6-31G* level up to add~t~on of third electron All values are glven In eV

Corannulene I 1 57 0 04 -2 75 (-0 37)

- I1 - -4 35 -4 35 (-6 20) - 111 - -12 09 -13 51 (-1440)

. -.-.A,"

,115 II",II"II,I* C'CCl,",, ,,,,,,.l.l ". --.- -.-."..- -- v.-.. -. ...- - - - - . . .- -..

level taken from reference 46 The values m parentheses correspond to HFl6-31G" level

Chapter V Buckybowk and heterobuckybwls 195

Table 5.9 The Koopman's, vertical and adlabatlc lonlzabon energles and the reorgamzatlon energies (Av A) of C I I H ~ (7) and corannulene obtalned at B3LYPl6-31G* level up to removal of thlrd electron All values are glven In eV

lonlzat~on Energy Koopman's Vertical (V) Adlabat~c (A) Av A

C?,H9(7) I 4 31 5 82 5 67 0 14 11 9 69 1721 16 96 0 22 111 14 15 33 09 32 63 0 46

Corannulene I 5 94 7 49 - I1 - 19 71 -

111 34 94 -

5.4: Heterobuckybowls

5.4.1 : Computational Detalls

The planar D3h Structures (C3h for X = PH3) of all the heterosumanenes (11X)

considered In t h ~ s study were optim~zed withln the symmetry constraints uslng the hybnd

dens~ty funct~onal B3LYP method wlth the cc-pVDZ bass set Frequency calculat~ons

were done at the same level to ascertain the nature of the statlonary pomts The Dlh

structures of 1 1 0 , l l N H , 11CHz (3). l l B H and 11s were charactenzed as translt~on

states corresponding to the bowl-to-howl lnverslon process However, the C3h structure

of IIPH.? and the Djh structures of IlSi, llSiH2 and l lAlH were charactenzed as

mlnlma on thelr respectlve potentla1 energy surfaces 87 For X = 0, MI, CH2, BH, S and

PH the mlnlmum energy bowl structures were obtalned by follow~ng the normal modes

correspond~ng to the respectlve lmaglnary frequencies The resultant bowl structures w ~ t h

CI, symmetry have been charactenzed as the mrnlma by frequency calculat~ons In all

cases All the Djh forms of heterotnndenes (13X), C I Z X ~ H ~ were also optim~zed and the

statlonary points were charactenzed by frequency calculat~ons at the B3LYPIcc-pVDZ

level The planar structures of 13X were charactenzed as m~nlma except for 13PH, 13Si

and 13AIH, whlch were found to be th~rd order saddle polnts B3LYPIcc-pVDZ level

has been proven to be a reliable method for predictmg the equll~bnum geometnes,

lnverslon barners and h a r m o ~ c fiequencles of buckybowls especlaliy when post-SCF

methods are not practical for molecules of thrs slze l5 26" In th e synthetic strateg~es

Chapter V Buckybavls and heterobuckybarls 196

toward hettrosumanenes, all the specles were opt~rnlzed w~thout any symmemc

wnstmnts and charactenzed as mlnlma at the MNDO level " T ~ I S is followed by slngle

polnt calculat~ons at the B3LYPIcc-pVDZ level on all the MNDO opt~m~zed pants In

both the synthetic routes All the calculat~ons were done ustng Gausslan 98 sulte of

programs bs The nature of the normal modes were examlned uslng the MOPLOT

graphical interface program '' 5.4.2: Results and Discussion


The equlhbnwn geometries and the bond length alternat~ons (A) In the trlndenes

(2X) will be discussed first The sal~ent structural parameters, w~th a spec~al attention to

the bond length alternat~on in the central benzenoid nng, of the heterosumanenes (1 1X)

as well as the bowl-to-bowl inversion transit~on states (11X') are given next

5.4.2.1.1: Trindenes, 13X

Tnndcnes may be mewed as three substituted cyclopentadienes annulated to a

benzene nng mainta~mng the three-fold axls These cyclopentadlenes can be class~fied

Into three types based on the electron count, (I) 6n-systems 130, 13NH, 13S, 13PA (11)

conjugated 4n-systems 13BH. 13Si, 13AIH and (111) uncon~ugated 4n-systems 13CH2,

13PH3, 13SiH~

The important skeletal parameters and the bond length alternatlon (A) In the

central SIX-membered nng of the subst~tuted tnndenes 13X (X = 0 , NH, CHI, BH S PH

PHI, SI, SIHZ and AIH) obtalned at the B3LYPIcc-pVDZ level are given in Table 5 10

Scheme 5 7 illustrates the notations used Annulat~on to benzene rlng reduces ~ t s

symmetry, the extent of localizat~on of the double bonds is represented by bond length

alternation (A), whlch 1s the d~fference between the bond lengths (A = 12 - rl) un~formly

for all the structures cons~dered here (Scheme 5 7) Very low bond alternat~on is

exhlblted for 130, 13NH, 13s and 13PH where the augmented five membered nngs are

aromatlc T h ~ s 1s In sharp contrast to the sltuatlon m tnphenylene (I), whlch exh~h~ts

slgnlficant bond altemat~on When the augmented five membered nngs are non-aromatic,

I e , when X = CHI, pH3 and SIH~, greater bond alternat~on resulted The h~ghest bond

altemat~on 1s seen In cases where the five membered nngs are con~ugated 471 systems,

when X = BH, SI and AM Thus, the aromatlclty of the augmented five membered nng

Chapter V Buckybmls and heterobwkybowls 197

cntlcally controls the bond alternatlon m the central SIX membered nng of the

heterotnndenes (Rgure 5 12) A maxunum A value of 0 092 A 1s observed for 13B,

whch IS slmllar to the ever observed hrgh alternatlon of 0 091 .& In tnmulated benzene

type systems In none of the cases, rl or r2 IS close to the aromatrc bond length except In

13B where rl IS 1 393 These results lndlcate that the electronic factors are chrefly

responsrble for bond length altematron

Scheme 5.7

Table 5.10 The pnnclpal bond lengths and the bond length altematrons (A) rn the central SIX membered nng of the heterotnndenes (13X) and tnphenylene (12) obtalned at the B3LYPJcc-pVDZ level The expenmental bond lengths for CM, tnphenylene (12) and corannulene are also grven All values are glven rn .&

Sficture Po~nt group r l r2 A' r3 Cm(1) Ih 1 401h 1458' - 0 057

12 1 423

D>h 1 469 0 046

( 1 41 1)' (1 470)' -

(0 059) 2 ' 2 5 , 1 410d 1408' - -0 002 130 Dlh 1 454 1 454 1 369 0 000 13NH D3h 1 452 1 446 1388 -0006 13CH2 (3) Dl h 1 462 1 489 1 358 0 027 13BH D>h 1 393 485 1 420 0 092 13s DI 1463 449 377 -0014 13PH CI, 1 470 48 1 366 0011 13PH' Dlh 1 462 446 402 -0016

13PH, Clh 1472 501 0029

13Si D3h 1 467 528 362 0 061

13SiHt Dlh 1480 512 361 0 032

13AIH D I ~ 1 489 538 365 0 049

a) A = r2 - rl IS the bond alternatlon rn the central six membered nng b) The expenmental bond lengths for CW are taken from reference 67 c) The experimental bond lengths for hlphenylcne (12) are taken from reference 66, here the

planar structure 1s considered d) The expenmental bond lengths for corannulene are taken from reference 89

Chapter V: Buckybowls and hetvobuckybowlr 198 - --

0 NH S PH CH, PHI S I H ~ BH SI AIH

Figure 5.12: The plot of bond length altematlon (A = r2 - r l , Scheme 5.7) in the central six-membered rings of the heterotrindenes (13X), bowl and flat forms of heterosumanenes (11X and 11X') as a functlon of the substituent. The substituents are grouped according to their contribution to n-conjugation and electron count. (Note that size was taken as a criterion to arrange X's in other places. The A in Cocl (1) and triphenylene (12) are given for comparison.

5.4.2.1.2: Heterosumanenes, 3X

The pnncipal geometric parameters and the bond length altemat~ons In the hub and nrn of

the minimum energy structures (11X) and the corresponding transltlon state structures

(11X') of all the heterosumanenes obta~ned at the B3LYPicc-pVDZ level are glven In

Table 5.11. The notations used are illustrated ~n Scheme 5.7. Whlle golng From 3X to

l lX ' , (when X = 0, NH, CHI, BH, S), the hub (r l and 1'2) and the flank (r3) bond lengths

are contracted, whereas the rim bond lengths (r4, r5, r6) are elongated. The extent of

contractionielongat~on while going from bowl to the planar form for all the geometnc

parameters gradually decreases with the increase In the size of X, with an exception In

case of l l P H , which may be traced to the fact that 11PHt is not the true transition state

corresponding to the bowl-to-bowl inversion process. The contraction of hub bond

lengths and the elongation of rim bond lengths are due to the strain energy build up in

golng from the bowl structure to the flat translt~on state In IlNH, there 1s a substant~al

pyram~dal~zat~on around all N The mmlmurn energy bowl geometry has C,, sbucture,

the 'H's are away fmm the hub leavlng the lone pars on the concave face of the bowl

Table 5.11 Pnnc~pal geometric parameters and the bond length alternat~ons ID the hub (Ahuh) and the nm (4,) of the heterosumanenes (11X and 11X') obtalned at the B3LYPJcc-pVDZ level All values are glven In )i

110' l INH I INH' I ICH, IICH; llRH 11BH' 11s 11s' l lPH IIPH'

IIPH,

llSl l ISIHZ llAlH Dlh 1407 1470 1429 1971 1396 1415 0063 0033

7) = r2 - 11, IS the bond length alternation in the hub six membered nng b) An,, IS the maxnnum bond altematlon in the nm six membered nng

However, the Djh form was charactenzed as the true transltlon state for the bowl-to-bowl

lnverslon In contrast to the tnndenes, the bond lengths In the central SIX-membered nng

of trlsuhstltuted sumanenes are close to the aromatlc C-C length In most of the cases The

bond alternat~on In the hub SIX-membered nng (AM) IS always found to be lower In 11X'

than that In the C3, (bowl) form, 11X for all X In contrast, substant~al Increase In the

alternat~on In the n m SIX-membered (A,,,) 1s observed wh~le golng from 11X to 11X'

T h ~ s d~fference IS found to gradually decrease w ~ t h the slze of the heteroatom and reverts

for 11PH (I e , A,,, (IIPH) > A,,(lIPH')) hub ln both 11X and I lX ' vanes wlth the n-

Chapter V Buckybarls and heterobuckybowls 200

electmn count as observed In the case of bond alternat~on m tnndenes (F~gure 5 12) 4 . a

when X = 0, NH, S and PH are lower than that when X = CHI, pH,, SIHZ wh~ch are In

turn lower when X = B, S1, AIH Eventhough. A,, of 11X shows slmllar vanattons, A,,,

of 11X' 1s found to be straln dependent, larger the size of the hetematom more IS the

alternat~on So In general In thls class of compounds. In both tnndenes and sumanenes,

the bond length alternat~on In the hub SIX membered nng IS mainly controlled by

electronic factors and IS ~ndependent of the slze of the heteroatom

5.4.2.2: Curvature, Bowl Depth and Polarity

One of the straightforward measures of evaluating the curvature In symmetric

buckybowls IS the bowl depth (BD), wh~ch IS the lntetplanar d~stance between the two

planes formed by the hub and the rrm atoms (Scheme 5 5) The curvature of the

hetembuckybowls seems to be solely controlled by the sue of the substltuent, whlch 1s In

agreement w ~ t h our prevlous studles (Flgure 5 13) Accordingly, a b~gger heteroatom

placed on the penphery of the howl flattens ~t whlle a smaller atom Increases the depth of

the bowl Flgure 5 13 clearly establishes the relatlonshlp between the slze of the

heteroatom and depth of the bowl Haddon's n-Orbital AXIS Vector (POAV) angles have

been measured for all the huckybowls uslng the POAV3 program 82 The bowl depth,

POAV angles at the hub, nm-quat and the nm carbon atoms and the dlpole moment (p)

ohtamed at the B3LYPIcc-pVDZ level are glven In Table 5 12 The flat structures (X =

PH3, SI, S I H ~ and AIH) have zero bowl depth, no d~pole moment and POAV angles as

90" W ~ t h the Increase In the slze 01 the heteroatom, decrease In the bowl depth and the

POAV angles IS seen as ev~dent from Tablc 5 12 and F~gures 5 13 and 5 14 In all the

cases as expected, POAV angle at the hub 1s greater than that at the nm-quat, whlch IS In

turn greater than the POAV angle obta~ned at the rrm poslt~on except In 3PH, whlch IS

due to the puckered hydrogen atoms Dlpole moment In buckybowls anses due to the11

curvature and the presence of heteroatoms From 3NH to 3S, the dlpole moment IS found

to decrease w ~ t h the bowl depth 3 0 havlng the maxtmum bowl depth w ~ t b Interspersed

oxygen atoms IS expected to show a hlgher d~pole moment, but a lower value of 1 9 7

Debye IS observed Thls 1s because the d~pole due to the oxygen atoms and the d~pole

anslng from the curvature act opposlte to each other In thls case alone and the dlrectlon

Chapter V Buckybowls and hctvobuckybowls 20 1

of the resultant dlpole 1s found to be opposlte compared to others The reason for the

h~gher value of dlpole moment of 3PH 1s traced to the three heavily puckered hydrogens

attached to the P atoms

Table 5.12 The bowl depth, BD (In A) POAV angles (In degree) at the hub, nm-quat and the nm carbons (POAVh,b, POAV.,,,., and POAV,, respectively) and the d~pole moment (p In Debye) for the heterosumanenes (11X) obtaned at the B3LYPIcc-pVDZ level

Structure' BD (A) P0AVh.h POAV,,,, POAV., p (Debye) 110 1 492 101 9 96 5 93 0 197

llNH 1 336 100 8 95 6 92 6 4 88

llCH, 1 143 98 8 94 9 92 3 2 10

IlBH 0 914 96 9 95 2 91 8 127

11s 0 656 95 2 92 9 91 0 1 03

l lPH 0 118 91 0 91 8 90 0 1 33

a) For. 1 IPH,, 11Si. 1 I SIH, and I IAIH, the planar structure 1s the mlnlmum, so the bowl depth w ~ l l be zero and the POAV angles wlll be 90'

Figure 5.13 The vanatlon of bowl depth (BD) of the heterosumanenes (1 1X) obtaned at the B3LYPIcc-pVDZ level

'I

1 5 -

1 0 - ,-. 5 . 2 0 5 -

0 0 -

J 1 I

0 NH CH, BH S PH [PH,, SI, SiH,, AIH]

Chapter V: Buckybowb and heimbuckybowk 202

-0- rim-quat

1 r , 0 NH CH2 BH S PH [PH,, Si,

SiH,, AltI]

Figure 5.14: The plot of the POAV angles at the hub, rim-quat and rim carbons of the heterosumanenes (1 1X) computed at the B3LYPicc-pVDZ level of theory.

5.4.2.3: Bowl-to-bowl Inversion Barrier

A gradual decrease in the bowl-to-bowl inversion barriers (AE') with the increase

in the size of X is found for the heterosumanenes, 11X (Table 5.13). AE' of 1 1 0 and

l l N H are so high that, if synthesized they are not expected to undergo bowl-to-bowl

inversion at room temperature, i.e. they will he locked to a single bowl conformation.

However, sumanene, IlCHt is expected to undergo slow inversion near room

temperature (Section 5.2.2):' Rapid bowl-to-bowl inversion near room temperature for

l lBH and 1 IS is evident from their AEt values. The heterosumanenes, 11 Pttl, 1 ISi,

l lSiHt and l lAlH are no more bowls, as the planar structures correspond to min~ma on

the respective potential energy surfaces. The flat D3h s t ~ c t u r e of l lPH has been

characterized as a third order saddle point, the normal modes of the three imaginary

frequencies correspond to the pyramidal inversion at the three phosphorous atoms. Since

none of the normal modes corresponding to the imaginary kequencies represent the

m i t i o n state for the bowl-to-bowl inversion, l l P H also does not undergo inver~ion.~'

However, the skeleton (devoid of hydrogens) of the C,, structure of IIPH, which is

Chapter V Buckybowb and heterobuckybwls 203

chatactenzed as a m~n~mum, 1s v~rtually flat w~th the hydrogens connected to the three

phosphorous atoms heavlly puckered Therefore, the bowl-to-bowl n-,version barner

seems to be exclusively controlled by the sue of the substttuent and Independent of any

electron~c factors

Table 5.13 The bowl-to-bowl mverston bamer, AE' (in kcal/mol), the enthalpy of actlvatlon, AH* (In kcallmol), the reactlon energles, AE(5 1) and AE(S 2) of the reactions

5 1 and 5 2 (In kcallmol) and the chemtcal hardness, q (In eV), of the heterosumanenes (11X) obtalned at the B3LYPlcc-pVDZ level

X A E ~ AH' AE(5 1) AE(5 2) (kcallmol) (kcalimol) (kcallmol) (kcallmol) 'I (eV'

110 69 6 68 3 130 0 80 7 2 14

a) The energy d~fference between the C,, and Dlh structures IS 85 b kcallmol, wh~ch in thls case does not correspond to the Inversion bamer

b) The equil~brium structure is flat

5.4.2.4: Homodesmic Equations and Chemical Hardness

The thermodynam~c stablllty of the beterosurnanenes, 3X are assessed uslng the

homodesni~c equatlons 5 1 and 5 2 glven In Scheme 5 8, the corresponding reactlon

energles AE(5 1) and AE(5 2) obtatned at the B3LYPlcc-pVDZ level are glven In Table

5 13 Although the ordenng of the thermodynam~c s tab~l~t~es are very slmllar In both

cases, eq 5 2, where tnndene (13CHz) IS taken as reference unifomly glves higher

stabtlity compared to eq 5 1 where tnphenylene (12) IS taken as reference The stabiltty

as such seems to be controlled by two factors namely the slze of the heteroatom and the

total n-electron count of the huckybowl A general feature 1s that the thennodynamic

stab~l~ty Increases as the slze of the heteroatom increases, slmply due to the favorable

stratn energy However, 24x-systems (X = 0, S NH and PH) exhib~t lower stability

partly because of the way the homodesmtc equatlons are set up Slnce these two factors

Chapter V Buckyboluls and heterobuckybowls 204

compete ln dec~dlng the stabil~ty, d~rect correlat~on between the thermodynam~c stablllty

IS not possible when taklng the 18n and 2471 electron systems together However, w~thln a

glvcn rr-electron count the s ~ z e of the heteroatom exclus~vely controls the thermodynamic

stab111ty of the heterobowls, a result wh~ch IS consistent wlth preblous observations '' " Compared to sumanene, only 110 , l l N H and 11s are found to be unstable 11s. which

IS computed to be less stable than sumanene and most other heterosumanenes, has been

synthesized recently

Scheme 5.8

Chemical Hardness (q), wh~ch may be defined as 11 = (El I~M~-EIIOM0)/2r w~thln

the Koopman's approxlmatlon, was found to be a good lneasure of the stab~l~ty of the

system The measured q values for all the heterosumanenes (11X) cons~dered here are

glven In Table 5 13 Surpnsmgly, sumanene (IICHI) was computed to he the hardest of

all cons~dered compounds and so ~t IS the most stable In that sense Thls analysls also

lndlcates that q values are too low for X = BH, SI, and AlH lnd~catlng that these

compounds will be very react~ve and might provide severe challenges to synthet~c

attempts Thus, q lnd~cates that 18n-systems wlth fully extended n-framework are less

stable and the stablllty of the rest are comparable Both the homodesm~c equations and

chem~cal hardness analysis po~nt that t h ~ s class of compounds IS fatrly stable and

s~nthetlc attempts t~wards these compounds should be rewading

5.4.2.5: Structure - Energy Relationship

The importance of shucture-energy correlat~ons In eluc~dat~ng the reactton

mechantsms was recognized long tlme back Burg and Dubler-Steudle, In a nng

lnverston study, showed that a quartlc funct~on fits the actlvat~on energy of a nng

lnverslon and the d~stort~on from the planarity in a senes of metallocyclopentenes ""

Slegel et al have tested that the quartic funct~on excellently fits the expenmentally (or

computat~onally) detenntned tnverslon bamer, for a senes of substituted corannulenes,

and thelr bowl depth '' However, Burg and Slegel et al have shown that a closer analysts reveals that

there IS an attractwe and repulsive part whtch results In a double well potentla1 59 60 The

double-well potentlal 1s a funct~on of x, whlch has quartic mlnus quadratic term as glven

Ineq 5 3

E = ax4 - bx2 eq 5 3

At the equtttbnum geometry of the howl structure, I e , at the mlnlma, the first denvatlve

of energy wlth respect to the reactlon coordinate should van~sb, (AEIAx) = 0

Therefore, 4ax3 - 2bx = 0 eq 5 4

Solving eq 2, we get, x = 0, x2 = bl2a b = 2ax,,' eq 5 5

- A E = E (x,,) - E (xo), xCq IS the howl depth at the mtnlma and xo 1s that m the transition

state, whlch 1s zero

-AE: = (ax,: - 2a x,: xCq2) - 0 = -axeq4 eq 5 6

So, a quart~c funct~on n~cely correlates the bowl-to-howl inversion bamer and the

bowl depth m this class of compounds (eq 5 6), as shown In Ftgure 5 15 F~gure 5 15

reveals that at htgher bowl depths the potenttal 1s much steeper and In contrast the

potentla1 1s much sofier at smaller bowl depths, s~milar to the S~egel's structure-energy

correlat~ons of substituted corannulenes Therefore, small perturbat~ons due solvent or

stenc effect by adding bulky groups, for the flat mtnlma and bowls w ~ t h smaller

curvature, should lead to substant~al changes m the geometry However lugher levels of

perturbation are required to change the shape and bowl depth for the deeper bowls The

single well potentla1 observed for I l X , X = PH, PH,, SI, S I H ~ and AlH, indicates that the

Chaptv V Buckybowls and hetcrobuckybowls 206

energy Increases along the bowl lnverslon mode when x dev~ates from zero Of cowse,

the shape of the slngle well depends on the nature of subst~tuent, and only an ~deal~zed

p~cture 1s gven m Figure 5 15 These slngle well and double well potent~als are

remlnlscent of the sltuatlon m the pyram~dal lnverslon of BH, and NH3 respectively

-100 I . , . , . , . , . , . I

-3 -2 -1 0 I 2 3

Bowl Depth (A)

Figure 5.15 The plot show~ng the correlat~on of bowl depth and the bowl-to-howl lnverslon barner (shown In th~ck dotted Ilnes) Thln dotted l~nes correspond to the emp~ncal double well potent~als for the bowl structures and slngle well potentla1 ior the flat mlnlma The thlck dotted l~ne is fit w~ th a quartlc funct~on and the dotted llnes by a mlxed quart~c/quadrat~c funct~on The continuous llne corresponds to the computed results

Trans~t~on metal complexat~on to fullerenes IS restncted mostly to q2 type, q5 and

q6 coordlnatlons are rare 22~u l l e r enes , a ~ t h thelr splayed orb~tals rendenng reduced

overlap w~ l l be unfavorable due to them n g ~ d geometry In terms of orb~tal compatth~l~ty 9'

Also due to the Inherent ng~dity of fullerenes, they do not leave much scope for chem~cal

man~pulat~ons In contrast, the heterohuckybowls, wlth the~r flex~ble geometry prov~dcs

much scope for q6cornple~at~on~ e~ther w~ th the hub or the nm benzenold nngs

Chapter V: Buckybowls od hetmbuckybauls 207

5.4.2.6: Synthetic Feasibilities

Ring closure strategies starting from a suitably functionalized hydrocarbon

skeleton were found to be the most successful synthetic strategies for b u ~ k ~ b o w l s . ~ ' ~ We

conceived two idealized CJ-symmetric pathways, namely a triphenylene route and a

trindene route, which could target the buckybowl in a sequential ring closure process (see

Figure 5.7).

Triphenylene Route

0 1 2 3

# of bridges

Figure 5.16: The reaction energies (AE) corresponding to the strain energy build up, obtained for the sequential ring closing to form heterosumanenes (11X) via the triphenylene route.

The reaction energies of the dehydrogenations (triphenylene route) 14X -r 15X

-i 16X + 11X and the reaction energies of the isomerization reactions (tnndene route)

17X + 18X + 19X + 11X were calculated (Figure 5.7) using the B3LYPIcc-pVDZ

single point calculations done on the corresponding MNDO equilibrium geometries. The

reaction energles obtained via the triphenylene and the trindene routes are plotted against

the number of bridges and given in Figures 5.16 and 5.17 respectively. Along the

triphenylene route, the strain build up increases and the third bridging becomes crucial

and hence the synthesis of both l lCHl and 11s could not be achieved through this route.

Olaptv V: Buckybowls ond hctvobuckybowls 208

The energy difference decreases with the increasing size of the atom. The strain build up

in each step when X = PH, Si, SiH2 and AIH indicates that the synthesis of these

compounds might be achieved by following the triphenylene route itself. Along the

trindene route, all the reaction energies are negative indicating the feasibility of the

formation of 11X. Figures 5.16 and 5.17 indicate that as the size of the heteroatom

increases, the strain energy build up becomes uniform in both triphenylene and trindene

routes. A closer look at the figures also indicate that 18n systems are much more facile

than the 24n system. But, it is to be noted that the synthesis of suitably funct~onalized

precursors is the key in all these cases and our computational results ensure that once a

tindene precursor is prepared, effecting the sequential ring closures is easier. So, the

syntheses of 11X may be achieved once the precursors for the tnndcne route are

achieved. Our theoretical studies clearly account for the success of Otsubo et al In the

synthesis of trithias~manene.2~ 11s in the trindene route. Similarly, the failure to achieve

the final ring closure step for 11S, by Klemm et a16' and 3 by Mehta et al." are obv~ous

from the above analysis (see also Section 5.2.2.5).

Trindene Route

# of bridges

Figure 5.17: The reaction energies, which correspond to the strain energy build up, obtained for the sequential ring closing to form heterosumanenes (11X) via the trindene route.

Chapter V Buckybowls and hctvobuckybowls 209

5.5: Conclusions

Thls chapter presents the study of structure, curvature, bowl ng~dlty, and other

phys~cochemlcal properties of the eluslve key structural mottf of buckm~nlsterfullerene,

sumanene (3), c ~ I H ~ ' , where Z IS vaned from -3 to +3 (4 to 10) and some

heterobuckybowls (11X) uslng ab lnltro and denslty funct~onal theory methods

The bowl-to-bowl lnverslon bamer for sumanene (3) IS pred~cted to be 16 9

kcallmol at B3LYPlcc-pVTZllB3LYPIcc-pVDZ level lncludlng the enthalpy correction,

whlch IS found to be about 7 kcaVmol lower compared to the earller semlemp~ncal-

MNDO result Importantly, the rev~sed estlmate predlcts slow bowl-to-bowl lnverslon

near room temperature This IS because the bamer IS clearly htgher than that of

corannulene (2) as well as many other famlllar dynamlc processes, where all of them

exhlblt rapld lnverslon at room temperature, such as a) between hvo tub conformers of

~ ~ c l o o t a t e t r a e n e ~ ~ b) the lnverslon between the two char forms of cyclohexaneg3 c)

pyramidal lnverslon of a m r n o n ~ a ~ ~ and d) automenzatlon of cyc lobutad~ene~~ Thus the

bowl-to-bowl Inversion bamer of sumanene 1s "In the ballpark" to provlde lnterestlng

perturbatlon In the dynamlcs near mom temperature, as a funct~on of many factors, vlz

temperature, solvent, countcnon etc The bowl depth, cunature (POAV angle) and

polanty (p) of sumanene are found to be hlgher compared to corannulene The

vlbratlonal spectrum of sumanene has been calculated by the hybnd denslty functional

B3LYP method wlth a bas15 set of spl~t valence plus polanzatlon qual~ty The feas~btl~ty

of two representatwe synthettc strategies was analyzed and the computed straln energy

bulld-ups lndlcate that the hlgh bullt-ln straln IS not a bottleneck and calls for the

attention of synthet~c organrc chemlsts towards the synthes~s o f sumanene (3)

The curvature present In the mlnlmum energy bowl structures of ~ ~ ~ 1 - l q ' (where Z

= -3 to +3) gradually Increases from -3 to +1 and however remalns constant lndlcatlng

l~tt le vanatlon In curvature on further removal of electrons The tnplet states of 6 and 10

are comparable tn energy to the corresponding singlet states, whereas those of 4 and 8 Ile

above 30 kcaVmol compared to the slnglet states Neutral C ~ I H ~ (7) has a posltlve

electron afintty value mndlcatmg that t h ~ s 1s less stable than the monoanlonlc counterpart

Companson of the electron affin~ty and lonlzat~on energy values of C21H9 w ~ t h those of

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