STRUCTURE AND ROTATIONAL DYNAMICS OF ISOAMYL ACETATE AND METHYL PROPIONATE STUDIED BY MICROWAVE...
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Transcript of STRUCTURE AND ROTATIONAL DYNAMICS OF ISOAMYL ACETATE AND METHYL PROPIONATE STUDIED BY MICROWAVE...
STRUCTURE AND ROTATIONAL DYNAMICS OF ISOAMYL ACETATE AND METHYL PROPIONATE STUDIED BY MICROWAVE SPECTROSCOPY
W.STAHL, H. V. L. NGUYEN, L. SUTIKDJA,
D. JELISAVAC, H. MOUHIB
Institut fur Physikalische Chemie, Raum Aachen, Germany
I. KLEINER
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA),
CNRS, Universités Paris Est et Paris Diderot, Créteil, France
Objectives
Study relatively large organic molecules
(esters and ketones) to obtain very precise
molecular structures and compare with ab
initio calculations
Fruit esters and odorant molecules
Relationship odor-molecular structure ?
A complex issue …
LLarge arge AAmplitude mplitude MMotionotion
LAMLAM
Internal rotationInternal rotation Nitrogen inversion Nitrogen inversion tunnelingtunneling
W. Stahl operation – 13 molecules studied
Diethyl amine Nguyen and StahlJ. Chem. Phys. 135, 024310 (2011).
H3C CH3O
O
H3C
CH3
O
O
H3CO
O
O
O
H3CO
O
H3C
H3C CH3
OCH3H3C
O
Internal rotation
Cs: methyl acetate
C1 : isoamyl acetate
Cs : methyl propionate
Jelisavac et alJMS.(2009).
Tudorie et alJMS 2011
Nguen, StahlJMS 2010
Nguen and StahlChemPhysChem 2011
Nguyen et alMol. Phys.2010
Internal rotation
Frequency
A EAA AE EA EE*EE
L. NGUYEN
Experimental technique at Aachen
Molecular beam Fourier transform microwave (MB-FTMW) spectrometers
Frequency ranges: 4 to 26.5 GHz (big cavity)
26.5 to 40 GHz (small cavity)
High resolution mode
Experimental technique
Two different modes of the spectrometers
Scan mode
Methyl propionate
Strategy: ab initio / two internal rotation codes / experimental data
1) ab initio calculation (MP2) or quantum chemical calculation (DFT) A, B, C for all conformers
2) use the scan spectrum to assign transitions for the A species
3) guess values for V3, I and use XIAM to predict the E species
4) use the high resolution spectrum to assign E species
5) fit the A-E data with XIAM and BELGI 6) compare with ab initio values the structural
parameters (A,B,C, angles of the methyl group)
XIAM BELGI
the conformers & nomenclaturethe conformers & nomenclature
9
the geometries of isoamyl acetate
lilian w. sutikdja,
3 x 3 x 3 = 27 cis-conformers
Cis
trans conformers:high torsional barrier of about40 kJ/mol around θii
1 Cs – Symmetry+26 C1-Symmetry
calculation on
MP2/6-311++G** level
quantum chemical calculationquantum chemical calculation
10
on MP2/6-311++G** level of theory
lilian w. sutikdja, 23rd February 2012
Experimental ValueVS.
Quantum Chemical Calculation
Rotational Constants A, B, and CAngles between the internal rotation axis and
the inertial axes a, b, and c axes
Table 2. Molecular constants of isoamyl acetate, rotamer aM(M,a), obtained by a global fit
using program BELGI-C1.
Operatora Parameterb BELGI-C1 notation
Unit Valuec
Pa2 A OA GHz 2.9582257(28)
Pb2 B B GHz 1.0353123(24)
Pc2 C C GHz 0.69050142(76)
{Pa,Pb} Dab DAB GHz -0.8512215(32) {Pa,Pc} Dac DACI GHz 0.0077626(33) –P4 J DJ kHz 0.41400 (26) –P2Pa
2 JK DJK kHz -4.1946(38) –Pa
4 K DK kHz 10.4281(89) –2P2(Pb
2–Pc2) J ODELN kHz 0.11786(17)
–[Pa2,(Pb
2–Pc2)] K ODELK kHz -0.5507(10)
P2 F F GHz 150.55277d
(1/2)(1–cos 3) V3 V3 cm-1 93.98242(93) PaP RHORHO unitless 0.01168289(25) Pa
3P k1 AK1 MHz 0.02940(23) (1–cos 3)P2 Fv FV MHz -1.78951(80)
Ne RMS deviation/kHz
... A species 140 3.1
... E species 118 2.6
... total 258 2.9
isoamyl acetateisoamyl acetate
12
spectral analysis & results
Constant Unit XIAM BELGI-C1 (PAM) MP2/6-311++G**
A GHz 3.2799235(12) 3.2809144(33) 3.307B GHz 0.7115582(18) 0.7129805(30) 0.725C GHz 0.6897250(22) 0.69014460(81) 0.702J kHz 0.15753(67) JK kHz 0.2849(42) K kHz 3.986(21) J kHz -0.01094(53) JK kHz -3.91(12) Dpi2j kHz 61.55(16) Dpi2- kHz 32.68(47) F0 GHz 149.195(21) 149.3048(32) I uÅ2 3.38737(47) 3.384861(72) 3.197V3 GHz 2816.64(39) 2817.522(28) cm-1 93.953(14) 93.98242(93) kJ/mol 1.12393(16) 1.124280(12) ∡(i,a) degrees 60.2293(8) 60.18940(17) 62.496∡(i,b) degrees 30.7241(54) 30.68609(71) 28.021∡(i,c) degrees 96.912(18) 96.6220(28) 94.946Rms kHz 17.0 2.9
Reduced barrier 8.31500 8.31756
0.8%
1.8%
1.7%
Methyl proprionate: a two-top inequivalent methyl rotor
Two-step diagonalization for the two-top problem
HRAM = Htor + Hrot + Hc.d + Hint
1) Diagonalization of the torsional part of the Hamiltonian :
Eigenvalues = torsional energies
2) A low set of torsional Eigenvectors x rotational wavefunctions are then used to set up the matrix of the rest of the Hamiltonian:
Hrot = AJa2 + BRJb
2 +CRJc2 + q1Jap1 + q2Jap2 + r1Jbp1 + r2Jbp2
Hc.d usual centrifugal distorsion termsHint higher order torsional-rotational interactions terms : cos3 cos32 , p1, p and global rotational operators like Ja, Jb , Jc
Htor = F1 p12 + F2 p2
2 + F12 p1p2 + (1/2) V31 (1-cos31) + (1/2) V32 (1-cos32)
+V12c (1-cos31) ( 1-cos32) +V12s sin31sin32
Operatora Parameterb Valuec / cm-1
Jz2 A 0.3471830 (48)
Jx2 B 0.0721304 (17)
Jy2 C 0.0605222 (15)
−J4 ΔJ 6.180(21)∙10-9
−J2 Jz2 ΔJK 30.17(16)∙10-9
−Jz4 ΔK 167.2(13)∙10-9
−2J2(Jx2−Jy
2) J 0.9294(50)∙10-9
p12 F1 5.60674d
p22 F2 5.6417d
p1 p2 F12 −0.50d
(1/2)(1−cos3α1) V3,1 819.97 (25)
(1/2)(1−cos3α2) V3,2 428.537 (15)
(1/2)(1−cos3α2)J2 V3,2J −0.0002903 (90)
Jz p1 q1 0.54d
Jz p2 q2 0.590664(92)
Jx p1 r1 −0.0734 d
Jx p2 r2 −0.053176(56)
Methyl propionate, Parameters determined by the BELGI-2tops code
rotation
Cent. Dist.
Potential barriers
Internal rotationconstants
Related to and
Interaction betweenthe 2 tops
Constant Unit XIAM BELGI-Cs-2topsa Calc.b Exp.c – Calc. (%) A GHz 9.515022(35) 9.51687(21) 9.4824 0.0326 (0.34%) B GHz 2.147746(40) 2.148733(52) 2.1522 –0.0045 (–0.21%) C GHz 1.811769(35) 1.814411(45) 1.8133 –0.0015 (–0.08%) J kHz 0.18577(87) 0.18527(61) JK kHz 0.9968(72) K kHz 4.936(40) J kHz 0.02803(16) K kHz –0.271(22) V3,1 cm-1 820.46(99) 819.97 (25) 956.2 –135.7 (–16.5%) I,1 uÅ2 3.15862 (fixed)d 3.25155(11) 3.161 (i1,a) ° 32.53(46) 34.4583(22) 33.83 –1.30 (i1,b) ° 57.46(46) 55.5417(22) 56.17 1.29 (i1,c) ° 90.0 (fixed) e 90.0 (fixed) 89.95 s1 65.0381 (derived) 64.9988 V3,2 cm-1 429.324(23) 428.537 (15) 509.2 –79.9 (–18.6%) I,2 uÅ2 3.15862 (fixed) d 3.19871(70) 3.217 (i2,a) ° 156.356(19) 154.827(21) 149.86 6.50 (i2,b) ° 66.356(19) 64.827(21) 59.86 6.50 (i2,c) ° 90.0 (fixed)e 90.0 (fixed) 90.00 Dpi2J,2 kHz 27.7(29) Dpi2-,2 kHz –70.6(13) s2 33.8215 (derived) 33.7595 /Nf
kHz 3.4 / 282 3.3 / 282 A/NA
g kHz 3.4 / 55 E1/N E1 kHz 2.6 / 55 E2/NE2 kHz 3.6 / 58 E3/NE3 kHz 3.4 / 57 E4/NE4 kHz 3.4 / 55
Conclusions
Combining ab initio, microwave spectroscopy in a MB and effective hamiltonian methods to study rather large esters led to rather consistent results, apart sometimes for the moment of inertia of the top (non-rigidity effects from the rest of the molecule).
- How can we study the excited torsional states now ?
- Are any of those studies of any relevance for the understanding of the odor-molecular structure relationship?
From microwave spectroscopy to perfum analysis ?
(2S,5S)-Cassyrane (2S,5R)-Cassyrane
important blackcurrant odorants for perfumery, the two cassyrane stereoisomers were studied by high resolution microwave spectroscopy
(2S,5S)-Most fruity (2S,5R)-
H. Mouhib, W. Stahl, M. Lüthy, M. Büchel, P. Kraft, Angew. Chem. Int. Ed. 2011, 50, 5576
For structural correlations:
gas-phase structure of the most fruity (2S,5S)-Cassyrane superposed with the (+)‑(2S,4R)-Oxane (“Cassis base 345B”)
For the superposition:
Cassyrane fixed (black)
(+)‑(2S,4R)-Oxane superimposed on the structure by rotation (in silver)
No deformation of bond lengths and angles
Methyl groups which have a direct effect on the olfactory properties of Cassyrane Oxane only overlay well if C-5 of Cassyrane is (S)-configured and the C-4 of Oxane is (R)-configured.
19
Cassyrane – Superposition Analysis –
H. Mouhib, W. Stahl, M. Lüthy, M. Büchel, P. Kraft, Angew. Chem. Int. Ed. 2011, 50, 5576
Columbus 2009: A NEW PROGRAM FOR NON-EQUIVALENT TWO-TOP INTERNAL ROTORS WITH A Cs FRAME
N-methylacetamide: N. Ohashi, J. T. Hougen, R. D. Suenram, F. J. Lovas, Y. Kawashima, M. Fujitake, and J. Pyka, JMS 2004
V3(1)=73 cm-1 V3(2)=79 cm-1 ;
Methyl Acetate : Williams et al, J. Trans. Faraday Soc 1970;Sheridan et al JMS 1980, KelleyAnd Blake, Ohio state 2006 : Astrophysical importance!
V3(1)=100 cm-1 V3(2)=425 cm-1
Methyl Acetate: energy levels
JKaKc
3 sets of internal rotation splittings :
(AA,EA). V3 = 100 cm-1
1 = a few GHz
(AA,AE). V3 = 425 cm-1
2 = a few MHz (AA,EE). Interaction
between the 2 tops a = 1.64 D, b = 0.06 D
0 0
0 ±1
± 1 0
± 1 1±1 ±1
1 2
Permutation-inversion group G18
Without torsion
Top 1 Top 2 Interaction
The new code: BELGI-2tops
a new two-C3v-top program was written in 2009:
1. For low, medium or high barriers2. With high accuracy (obs-calcs < 1 kHz)3. With high computational speed
Begin with Ohashi’s two-top program, but use:
1. Two-step diagonalization (Herbst, BELGI)2. Banded matrix computational methods suggested in 2009 ?
Theoretical Model: the global approach for one top
HRAM = Hrot + Htor + Hint + Hc.d.
RAM = Rho Axis Method (axis system) for a Cs (plane) frame : get rid of Jxp
Constants 1 1-cos3 p2 Jap 1-cos6 p4
Jap3
1 V3/2 F V6/2 k4 k3
J2 (B+C)/2* Fv Gv Lv Nv Mv k3J
Ja2 A-(B+C)/2* k5 k2 k1 K2 K1 k3K
Jb2 - Jc
2 (B-C)/2* c2 c1 c4 c11 c3 c12
JaJb+JbJa Dab or Eab dab ab ab dab6 ab ab
Torsional operators and potential function V()
Ro
tati
on
al O
per
ato
rs
Hougen, Kleiner, Godefroid JMS 1994
= angle of torsion, = couples internal rotation and global rotation, ratio of the moment of inertia of the top and the moment of inertia of the whole molecule
Kirtman et al 1962Lees and Baker, 1968 Herbst et al 1986
PsPAM = Pseudo Principal Axis Method:Get rid of all JxJy , JyJz , and JzJx terms
Constants 1 1-cos3
p
2 Jap 1-cos6
p
4
Jap3
1 . V3/2 F V6/2 k4 k3
J2 . Bbar Fv Gv Lv Nv Mv k3J
Jz2 . A-Bbar k5 k2 k1 K2 K1 k3K
Jb2 - Jc
2 . (B-C)/2 c2 c1 c4 c11 c3 c12
JaJb+JbJa Dab dab ab ab dab6 ab ab
Torsional Operators = f(p p
Rot
atio
nal O
pera
tors
Kirtman et al. 1962; Lees and Baker 1968; Herbst et al. 1986
Operator = (rotation)x(torsion)
Global approach for two tops : Ohashi’s model.
Htor = F1 p12 + F2 p2
2 + F12 p1p2 + (1/2) V31 (1-cos31) + (1/2) V32 (1-
cos32) +V12c (1-cos31) ( 1-cos32) +V12s sin31sin32
Hrot = AJz2 + BJx
2 + CJy2 + cent.distorsion
Hint = r1 Jxp1 + r2 Jx p2 + q1 Jzp1 + q2 Jzp2
+B1 p12Jx
2 + B2p22Jx
2 +B12 p1p2Jx2 + C1 p1
2Jy2 + C2 p2
2Jy2 + C12 p1p2Jy
2
+q12p p1p2 (p1+p2) Jz +q12m p1p2 (p1-p2) Jz + ...
Overview of Existing Two-Top Programs
Name Authors What it does? Method http://info.ifpan.edu.pl/~kisiel/prospe.htm: programs for rotational spectroscopy (Z. Kisiel)_____________________________________________________________________XIAM Hartwig up to 3 sym tops « IAM » Potential Function fit
Maeder up to one quad Often 1MHz Obs-Calcsnucleus Ar-acetone, (CH3)2SiF2
_____________________________________________________________________ERHAM Groner one or two Effective vt states fit
internal rotors Fourier series for Torsionalof sym. C3v or C2v Tunneling SplittingsJ up to 120. High Barrier
acetone, diMEether_____________________________________________________________________SPFIT/ Pickett one or two internal Potential Function fitSPCAT rotors, sym or asym. propane
_____________________________________________________________________OHASHI Ohashi two C3v internal rotors Potential Function fit Hougen Cs or C2h Frame A and E species fit together
1 kHz accuracy, but very slow N-methylacetamide, biacetyl
Overview of Existing Two-Top Programs(suite)
Name authors what it does? Method ______________________________________________________________________JB95 Plusquellic one internal rotor PAM
but can be usedfor 2 tops in top-topinteraction is small alanine dipeptide, peptide
mimetics ...
graphical interface
http://physics.nist.gov/Divisions/Div844/facilities/uvs/jb95userguide.htm
the conformers & nomenclaturethe conformers & nomenclature
29
the geometries of isoamyl acetate
lilian w. sutikdja, 23rd February 2012
H
iBu H
O
COCH31
3
11M1
11
COCH3
H iBu
H
1
11
8
1M11
iBu
H H
O
COCH31
3
11
11a1
COCH3
H H
iBu
1
11
8
1a11
H
H iBu
O
COCH31
3
11
11P1
COCH3
iBu H
H
1
11
8
1P11
3 x 3 x 3 = 27 cis-conformers
iii≈ 180° iv≈ 180°v≈ 60°, -60°
11a1-14a3-(16P8, 20M8)11a1-14a3-(16P8, 20M8)aa(P, M)
the conformers & nomenclaturethe conformers & nomenclature
30
27 conformers of isoamyl acetate
lilian w. sutikdja, 23rd February 2012
27 cis-conformers
1 Cs – Symmetry+
26 C1-Symmetry
calculation on
MP2/6-311++G** level