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