CROATICA CHEMICA ACTA CCACAA 60 (4) 605-612 (1987)

CCA-1757YU ISSN 0011-1643

UDC 547.567:(539.194+543.878)Original Scientific Paper

Skele ton Deformations and Electronic Structure ofp-QuinonemethideSergey G. Semenov

Department of Quantum Chemistry, Leningrad University, 198904 Leningrad, USSR

and

Ser'gey M. Shevchenko*,;Rudjer Bošković" Institute, 41000 Zagreb, Croatia, Yugoslavia

Received February 18, 1987

The electronic structure of the Iignin subunit p-quinone-methide (4-alkylidene-2,5-cyclohexadiene-l-one) was calculated bythe CNDO/S3method. The results indicate a quinoid structure forthe ground state, the transformation into diradical being possibleonly by photoexcitation. There is no reason to ascribe zwitterionicstructure to any p-quinonemethides. Chemical reactivity of p--quinonemethides and the effect of skeleton deformations onreactivity are analyzed in detail.

INTRODUCTION

The electronic structure of p-quinonemethides (4-alkylidene-2,5-cyclohexa-diene-1-ones) is of interest because of their place in the chemistry of lignin t,2

and sterically hindered phenols." In spite of theoretical-." and experirnental"arguments in favour of the »classic« quinoid structure of these compoundsin the ground state many assumptions may be found in literature that somethermal reactions proceed through p-quinonemethide intermediates in theirdiradical or zwitterionic forms, these assumptions being based on obsoleteand probably erroneous data." Therefore, we have examined theoretically indetail the electronic structure of p-quinonemethides and its dependence onmolecular geometry. It should also be mentioned that the high chemicalreactivity of the simplest compounds belonging to this series imposes limi-tations on experimental approaches.

RESULTS

CNDO/S3 calculations? have been performed for p-benzoquinone (1, testcompound) and the simplest p-quinonemethide using geometry parameterscorresponding to quinoid (2), planar zwitterionic (3), and zwitterionic withperpendicular orientation of benzenoid and methylene fragment (4) forms.

* Reprint requests to Dr. S. M. Shevchenko, Department of Organic Chemistry,Leningrad Forest Technical Academy, 194018Leningrad, USSR (permanent address).

G06 S. G. SEMENOV AN"D S. M. SCHEVCHENKO

In addition, calculations have been performed for amore complicated corn-pound resembling p-quinonemethide subunits of the lignin macromoleculemore closely, in its quinoid (5) and plan ar zwitterionic (6) forms.

aa -Oa O o O O

C~d cQd ~OC~ ~OCH'e f e .f-

~ ~H qCH, H ~ CH{",oh. H r.; H H \

\H H

1 Z J. ~ ~.§.

The geometry parameters were chosen on the basis of experimental datafor p-quinodimethane," p-benzoquinone and related compounds.? The acceptedbond lengths are listed in Table I, the C-H length being 110 pm. Valenceangles of the quinonemethide framework in the zwitterionic forms weremade equal to 120°, in the quinoid forms (cbd) to 118J, and (egf) to 122°. Fordetails of the geometry accepted for .'i and 6 see ref.5 Wave functions andexcited states energies were calculated taking into account 80 single excitedconfigurations, corresponding to singlet-singlet electron excitations from oneof ten highest occupied levels to one of eight lowest unoccupied levels.

TABLE I

Bond Lengths Accepted jor the Systems 1-6, pm

Bond 1 2,5 3,4,6

ab 123 123 140bc 148 148 140ce 136 136 140eg 148 146 140gh 123 138 151

The results are presented in Tables II-VI*. The calculated (Table II)and experimental electron absorption spectra of 1 are in good agreement,corroborating the adequacy of parametrization used.!? The CNDO/S3 methodbeing parametrized for electron level calculations may give unsatisfactoryresults for other molecular properties. So, in order to check the reliabilityof qualitative conclusions, CNDO/2 calculations of bond indices, atomic charges,valence indices, and relative energies of different forms were performed,as well. It is to be mentioned that the CNDO/2 method was parametrizedto reproduce the ab ini tio density matrix.

Bond indices (lAB and lAB', Table III) were calculated according to for-mulae:11-13

* Bond indices lAB and atomic charges qA of 5 and 6 are listed in paper".

STRUCTURE OF p-QUINONEMETHIDE 607TABLE II

Calculateti Electron Transitions*

Compound Excited Excitation Wave Oscillator Dominatingor form state energy, length, strength electron

eV nm transition

Sl 3.03 409 O (j_~Jt*S2 3.67 333 O O'_~jt*

1 S3 4.05 306 O Jt_~Jt+ *S4 5.05 245 0.654 Jt+~Jt+ *S5 5.77 215 O (j_~Jt_*

TI 1.46 850 O Jt+~Jt+*T2 2.77 448 O 1t_~Jt+ *T3 3.55 350 O Jt_~.rr:_*

2 T4 3.66 339 O (j_~Jt+ *Sl 3.66 339 O (j_~Jt+*S2 4.27 290 0.837 Jt+~1t+ *S3 4.48 277 0.007 iL~Jt+*

TI 0.71 1750 O Jt+~Jt+ *T2 2.22 558 O 1t.-~j[+ *T3 3.16 392 O Jt_~Jt_*

3 Sl 3.26 380 0.009 1t_----?1t+*

S2 3.34 372 0.767 Jt+~+Jt*S3 3.53 352 O n_~Jt+*T4 3.53 352 O n_~j['I-*

TI 1.11 1118 O Jt+~Y*Sl 1.11 1118 O Jt+~Y*

4 T2 2.55 487 O Jt_~Y*S2 2.55 487 O iL->Y*

T1 1.42 871 O Jt~Jt*T2 2.77 447 O Jt~Jt*Ta 3.59 446 O Jt~Jt*

5 Sl 3.82 325 0.677 Jt~Jt*S2 3.84 323 O n~Jt*T4 3.84 323 O n~Jt*S3 4.31 288 0.177 Jt~Jt*

TJ 0.87 1426 O Jt~Jt*T2 2.30 538 O Jt~Jt*Sl 2.94 422 0.405 Jt~Jt*

6 T3 3.28 379 O Jt~Jt*S2 3.28 378 0.386 Jt~Jt*S3 3.68 337 O n~Jt*T4 3.68 337 O n~Jt*

* MO classification for 1 corresponds to the symmetry point group of p-quinone-methide (C2v). Oscillator strengths were calculated according to formula i»« = 2/3 En,Uon2 (En-energy, and ,uon-dipole moment of electron transition).

lAB = ~ ~ I (st;' PS'('lab 12;aEA bEB

I'AB = ~ ~ (PS)ab (PS)ba'aEA bEB

S. G. SEMENOV AND S. M. SCHEVCHENKO

TABLE III

Bond Indices

rAB (CNDOj2) rAn 1\"1 2 3 4 1 2 3 4 1 2 3 4

1.81 1.79 1.54 1.28 1.73 1.68 1.29 1.18 1.71 1.65 1.27 1.171.06 1.06 1.18 1.26 0.96 0.99 1.16 1.25 0.92 0.95 1.12 1.211.86 1.84 1.70 1.54 1.84 1.80 1.61 1.46 1.80 1.75 1.56 1.421.06 . 1.07 1.16 1.24 0.96 1.03 1.15 1.28 0.92 0.99 1.11 1.24

1.80 1.62 1.18 1.76 1.47 0.91 1.72 1.43 0.85

608

Bond

abbcceeggh

TABLE IV

Atomic Charges,. a. u.

Atom qA (CNDOj2)

1 2 3 4 1 2 3 4 2 3 4

abceghHeHeHh

-0.22 -0.26 -0.33 -0.52+0.23 +0.24 +0.20 +0.19-0.02 -0.06 -0.04 -0.08-0.02 +0.03 +0.03 +0.07+0.22 +0.03 +0.03 -0.10

-0.02 + 0.01 + 0.33+0.02 +0.01 +0.02 +0.01+0.02 +0.01 -0.00 -0.02

+0.02 +0.03 +0.07

-0.42 -0.46 -0.60 -0.66+0.28 +0.27 +0.21 +0.16-0.04 -0.08 -0.07 -0.09-0.04 -0.04 -0.03 -0.06+0.28 -0.04 -0.05 -0.13

-0.08 +0.04 +0.26+0.11 +0.10 +0.10 +0.10+0.11 +0.09 +0.08 +0.07

+0.09 +0.12 +0.16

-0.45 -0.49 -0.60 -0.66+0.29 +0.28 +0.20 +0.16-0.07 -0.10 -0.10 -0.11-0.07 -0.08 -0.07 -0.09+0.29 -0.04 -0.05 -0.11

-0.15 -0.06 +0.12+0.15 +0.13 +0.14 +0.14+0.15 +0.13 +0.12 +0.10

+0.12 +0.16 +0.21

TABLE V

Atomic Valence Indices

Atom VA (CNDOj2) VA V'A

2 3 4 2 } 4 1 2 3 4

a 2.04 2.03 1.91 1.51 1.93 1.88 Ul 1.37 1.91 1.87 1.50 1.37b 3.97 3.97 3.97 3.92 3.77 3.78 3.78 3.84 3.71 3.72 3.73 3.78c 3.99 3.99 3.99 3.98 3.91 3.91 3.91 3.90 3.82 3.82 3.83 3.8e 3.99 3.99 3.99 3.98 3.91 3.92 3.92 3.91 3.82 3.84 3.84 3.83g 3.97 4.00 4.00 3.86 3.77 3.90 3.88 3.71 3.71 3.84 3.81 3.6h 3.99 3.97 3.35 3.93 3.74 2.94 3.84 3.65 2.83

where P is the density matrix, and S is the overlap matrix of all the atomicorbitals.

Atomic charges (qA and qA', Table IV) were calculated according to for-mulae:

qA = ZA - ~ na' na = (S'/' PS'/')au;a€A

q'A = ZA - ~ nu', na' = (PS)aa'a€A

where na and na' are the Lowdin and Mulliken a-th orbital electron popu-lations of atom A, respectively; ZA - core charge of the latter.14,15

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610 S. G. SEMENOV AND S. M. SCHEVCHENKO

Atomic valence indices (VAand VA', Table V) were calculated accordingto formulae:14,16

VA = ~ (2na-na2)- ~ ,I (S'/'PS'/')aa' 12;aEA "r"'" EA

VA' = ~ (2 na' - na'2) - ~ (PS)aa' (PS)a'a'aEA ar" ,,' EA

All the data obtained using different approaches are in good agreement,leading to the same general conclusions. It is essential to note the similarityof properties of the simplest p-quinonemethide and the substituted one,resembling the lignin subunits more closely. Therefore, we think that theconclusions based on the investigation of simple molecules are transferableto the natural polymer.

DISCUSSION

The data presented here enable us to analyze the problem of electronicstructure of p-quinonemethides in different terms. Let us consider the possibi-lity for p-quinonemethide to exist in zwitterionic form. The calculated electrontransitions of quinoid and zwitterionic forms are quite different (Table II),being in agreement with experiment6,17.18only in the case of quinoid structures(2, 5). Moreover, CNDO/2 calculation has predicted a considerable increaseof energy as a result of the conversion of 2 to 3. However, such energy esti-mations cannot help to decide whether the high energy form corresponds toa minimum on the potential energy surface. The data presented in Table IIIindicate that this is not the case. In fact, bond indices of form 3 calculatedunder the assumption of its benzenoid structure (i. e. equal bond lengths inthe ring) alternate appreciably. Consequently, in-plane geometry deformationof 2, making it more similar to the hypothetic zwitterion 3, is not accompaniedby the expected change in electronic structure. It is characteristic that atomiccharges (Table IV) are also insensitive to this deformation, so formula 3 inprinciple does not reflect even the structure of related vibrational exicitedstates.

The analysis of atomic valence indices (Table V), being now a popularmethod to differentiate between »norrnal« molecules, diradicals and zwit-terions,19,20leads us to the same conclusion. All the atoms of 2 have »norrnal«indices, undoubtedly indicating the quinoid structure; a transformation into3 brings about only an insignicifant change in the valence index of atom O".

Thus, it may be supposed that no in-plane skeleton vibration of p-quino-nemethides changes their »classic« quinoid structure. In spite of this, thetwisting of the methylene »tail«, just as expected, results in a significantchange of electronic structure. The calculation of the saddle point of twisttransition 4 has predicted a decrease of ring bonds alternation (Table III),considerable polarization (Table IV), and obvious deviation of valence indicesof the atoms O:, and Ch (Table V) from »normal« values (»subvalent«2o atoms).

Consequently, p-quinonemethide has zwitterionic structure only near thesaddle point of the twist transition. However, it is well known that torsionbarriers are extremely high in the alkene series. The CNDO/2 calculation hasresulted in 2.8 eV energy difference between 3 and 4; experimental estimationof the alkene torsional barrier is about 2.5 eV.21 In general, the electronic

STRUCTURE OF p-QUINONEMETHIDE 611structure of such a reactive intermediate as p-quinonemethide is of interestprimarily with respect to its reactivity. It is evident that the structure 4corresponds to the high energy saddle point on the potential energy surface,and must not be considered as an intermediate (the latter corresponding tolocal minimum). This opinion is supported by high stereoselectivity of thereactions of nucleophilic addition to substituted p-quinonemethides." thesrereoselectivity being improbable if the reaction proceeds through a twistedform like 4.

Consequently, there is no reason to connect the chemical reactivity ofp-quinonemethides and the possibility of their existance in zwitterionic form.However, it must be pointed out that thermal excitation of in-plane and twistvibrations must raise their reactivity as substrates in nucleophilic additionreactions. The data listed in Table VI in all the cases showa significant energydifference between the LUMO 1t+ * and other MO levels, and consequently highprobability of p-quinonemethides reacting with mild nucleophiles to beorbitally controlled. In this case the reaction rate increases with decreasingLUMO energy. LUMO energy decreases during the transition of quinoidstructure 2 to structures 3 and 4. The regioselectivity of orbitally controlledreactions is determined by AO contributions in the frontier MO. Even inthe classic quinoid structure 2 a contribution of AO (O) prevails, being thereason for the well known high regioselectivity of p-quinonemethide re-actions.P It is of interest that skeleton deformations accompanied by increasedreactivity (transformation of 2 into 3 or 4) also result in amore pronouncedLUMO localization on atom C'', and consequently in higher regioselectivity,which is in contradiction with the widespread view on the »activity -selectivity« relationship. This feature is helpful in explaining the high regio-selectivity of the reactions of nucleophilic addition to p-quinonemethides atelevated temperatures. A comparison of data obtained for simple p-quinone-methide and its derivative resembling typical lignin subunits more closelydemonstrates no qualitative differences.

The probability for p-quinonemethides to exist in diradical form mayhe estimated by analysis of the data presented in Table II. According toSalem and Rowland'", diradicals are characterized by singlet-triplet degene-racy. In principle, experiments (ESR, electron absorption spectra), as well ascalculations, leave no doubt that in the ground state p-quinonemethides areclosed-shell compounds. The calculated energies of singlet-triplet excitationin any case are too high to consider the corresponding structure as a diradical.Only some excited states may have diradical nature. Minimum energies ofsuch states, characterized by singlet-triplet degeneracy in casesof 2, 3, 5, and6, are unattainable by thermal excitation (Table II). The energy of such astate is low only in the case of 4 (1.11 eV) however, this structure correspondsto the high energy saddle point on the potential energy surface of p-quino-nemethide. So, diradical forms of p-quinonemethides may be created onlyby photoexcitation, and may play an important role only in photochemicalreaction mechanisms. The absence of diradical participation in thermalreactions of p-quinonemethides is also supported by the high stereoselectivityof such reactions.P-š!

r

612 S. G. SEMENOV AND S. M. SCHEVCHENKO

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SAZETAK

Deformacija skeleta i elektronska struktura p-kinonmetida

Sergej G. Semenov i Sergej M. Ševčenko

Elektronska struktura podjedinice Iignina, p-kinonmetida (4-alkiliden-2,5-ciklo-heksadien-1-on), izračuna na je s pomoću CNDO/S3 metode. Rezultati upućuju nakinoidnu strukturu temeljnog stanja, pa je transformacija u diradikal mogućajedino fotoekscitacijom. Pokazuje se da ne postoji razlog za pripisivanje zwitterion-ske strukture bilo kojem p-kinonmetidu. U radu se potanko analizira kemijskareaktivnost p-kinonmetida kao i utjecaj deformacija skeleta,

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