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 …

Large Amplitude Motion LAM Internal rotation Nitrogen inversion tunneling W. Stahl operation – 13 molecules studied Diethyl amine Nguyen and Stahl J. Chem. Phys. 135, (2011).

Internal rotation C s : methyl acetate C 1 : isoamyl acetate C s : methyl propionate Jelisavac et al JMS.(2009). Tudorie et al JMS 2011 Nguen, Stahl JMS 2010 Nguen and Stahl ChemPhysChem 2011 Nguyen et al Mol. Phys.2010

Internal rotation Frequency AEAAAE EAEE* 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 V 3, 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 & 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 about 40 kJ/mol around θ ii 1 Cs – Symmetry + 26 C1-Symmetry

quantum chemical calculation 10 on MP2/ G** level of theory lilian w. sutikdja, 23 rd February 2012 Experimental Value VS. Quantum Chemical Calculation Rotational Constants A, B, and C Angles between the internal rotation axis and the inertial axes a, b, and c axes

isoamyl acetate 12 spectral analysis & results ConstantUnitXIAMBELGI-C 1 (PAM)MP2/ G** AGHz (12) (33)3.307 BGHz (18) (30)0.725 CGHz (22) (81)0.702 JJ kHz (67)  JK kHz0.2849(42) KK kHz3.986(21) JJ kHz (53)  JK kHz-3.91(12) D pi2j kHz61.55(16) D pi2- kHz32.68(47) F0F0 GHz (21) (32) II uÅ (47) (72)3.197 V3V3 GHz (39) (28) cm (14) (93) kJ/mol (16) (12) ∡ (i,a) degrees (8) (17) ∡ (i,b) degrees (54) (71) ∡ (i,c) degrees96.912(18) (28) RmskHz Reduced barrier % 1.8% 1.7%

Methyl proprionate: a two-top inequivalent methyl rotor

Two-step diagonalization for the two-top problem H RAM = H tor + H rot + H c.d + H int 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: H rot = AJ a 2 + B R J b 2 +C R J c 2 + q 1 J a p 1 + q 2 J a p 2 + r 1 J b p 1 + r 2 J b p 2 H c.d usual centrifugal distorsion terms H int higher order torsional-rotational interactions terms : cos3    cos3  2, p 1, p  and global rotational operators like J a, J b, J c

Operator a Parameter b Value c / cm -1 Jz2Jz2 A (48) Jx2Jx2 B (17) Jy2Jy2 C (15) −J 4 ΔJΔJ 6.180(21)∙10 -9 −J 2 J z 2 Δ JK 30.17(16)∙10 -9 −J z 4 ΔKΔK 167.2(13)∙10 -9 −2J 2 (J x 2 −J y 2 ) JJ (50)∙10 -9 p12p12 F1F d p22p22 F2F d p 1 p 2 F 12 −0.50 d (1/2)(1−cos3α 1 )V 3, (25) (1/2)(1−cos3α 2 )V 3, (15) (1/2)(1−cos3α 2 )J 2 V 3,2J − (90) J z p 1 q1q d J z p 2 q2q (92) J x p 1 r1r1 − d J x p 2 r2r2 − (56) Methyl propionate, Parameters determined by the BELGI-2tops code rotation Cent. Dist. Potential barriers Internal rotation constants Related to   and   Interaction between the 2 tops

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 C s FRAME N-methylacetamide: N. Ohashi, J. T. Hougen, R. D. Suenram, F. J. Lovas, Y. Kawashima, M. Fujitake, and J. Pyka, JMS 2004 V 3 (1)=73 cm -1 V 3 (2)=79 cm -1 ; Methyl Acetate : Williams et al, J. Trans. Faraday Soc 1970; Sheridan et al JMS 1980, Kelley And Blake, Ohio state 2006 : Astrophysical importance! V 3 (1)=100 cm-1 V 3 (2)=425 cm-1

Methyl Acetate: energy levels JK a K c 3 sets of internal rotation splittings : (AA,EA). V 3 = 100 cm -1  1 = a few GHz (AA,AE). V 3 = 425 cm -1  2 = a few MHz (AA,EE). Interaction between the 2 tops  a = 1.64 D,  b = 0.06 D 0 0 ±1 ± 1 0 ± 1 1 ±1  1  2 Permutation-inversion group G 18 Without torsion Top 1 Top 2 Interaction

The new code: BELGI-2tops a new two-C 3v -top program was written in 2009: 1. For low, medium or high barriers 2. 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 H RAM = H rot + H tor + H int + H c.d. RAM = Rho Axis Method (axis system) for a C s (plane) frame : get rid of J x p  Constants1 1-cos3  p2p2 JapJap 1-cos6  p4p4 Jap3Jap3 1V 3 /2F  V 6 /2k4k4 k3k3 J2J2 (B+C)/2*FvFv GvGv LvLv NvNv MvMv k 3J Ja2Ja2 A-(B+C)/2*k5k5 k2k2 k1k1 K2K2 K1K1 k 3K J b 2 - J c 2 (B-C)/2*c2c2 c1c1 c4c4 c 11 c3c3 c 12 JaJb+JbJaJaJb+JbJa D ab or E ab d ab  ab  ab d ab6  ab  ab Torsional operators and potential function V(  ) Rotational Operators 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 1962 Lees and Baker, 1968 Herbst et al 1986

PsPAM = Pseudo Principal Axis Method: Get rid of all J x J y, J y J z, and J z J x terms Constants 1 1-cos3   p  2 J a p  1-cos6   p  4 J a p  3 1.V 3 /2F  V 6 /2k4k4 k3k3 J 2.B bar FvFv GvGv LvLv NvNv MvMv k 3J J z 2. A-B bar k5k5 k2k2 k1k1 K2K2 K1K1 k 3K J b 2 - J c 2. (B-C)/2c2c2 c1c1 c4c4 c 11 c3c3 c 12 JaJb+JbJaJaJb+JbJa D ab d ab  ab  ab d ab6  ab  ab Torsional Operators = f(      p   p   Rotational Operators Kirtman et al. 1962; Lees and Baker 1968; Herbst et al Operator = (rotation)x(torsion)

Global approach for two tops : Ohashi’s model. H tor = F 1 p F 2 p F 12 p 1 p 2 + (1/2) V 31 (1-cos3  1 ) + (1/2) V 32 (1-cos3  2 ) +V 12c (1-cos3  1 ) ( 1-cos3  2 ) +V 12s sin3  1 sin3  2 H rot = AJ z 2 + BJ x 2 + CJ y 2 + cent.distorsion H int = r 1 J x p 1 + r 2 J x p 2 + q 1 J z p 1 + q 2 J z p 2 +B 1 p 1 2 J x 2 + B 2 p 2 2 J x 2 +B 12 p 1 p 2 J x 2 + C 1 p 1 2 J y 2 + C 2 p 2 2 J y 2 + C 12 p 1 p 2 J y 2 +q 12p p 1 p 2 (p 1 +p 2 ) J z +q 12m p 1 p 2 (p 1 -p 2 ) J z +...

Overview of Existing Two-Top Programs Name Authors What it does? Method programs for rotational spectroscopy (Z. Kisiel) _____________________________________________________________________ XIAM Hartwig up to 3 sym tops « IAM » Potential Function fit Maederup to one quad Often 1MHz Obs-Calcs nucleusAr-acetone, (CH 3 ) 2 SiF 2 _____________________________________________________________________ ERHAM Gronerone or two Effective v t states fit internal rotors Fourier series for Torsional of sym. C 3v or C 2v Tunneling Splittings J up to 120. High Barrier acetone, diMEether _____________________________________________________________________ SPFIT/ Pickettone or two internalPotential Function fit SPCATrotors, sym or asym.propane _____________________________________________________________________ OHASHI Ohashitwo C 3v internal rotorsPotential Function fit Hougen C s or C 2h Frame A and E species fit together 1 kHz accuracy, but very slow N-methylacetamide, biacetyl

Overview of Existing Two-Top Programs(suite) Nameauthors what it does? Method ______________________________________________________________________ JB95Plusquellic one internal rotorPAM but can be used for 2 tops in top-top interaction is smallalanine dipeptide, peptide mimetics... graphical interface

the conformers & nomenclature 29 the geometries of isoamyl acetate lilian w. sutikdja, 23 rd February x 3 x 3 = 27 cis-conformers  iii ≈ 180°  iv ≈ 180°  v ≈ 60°, -60° 11 a a 3 -( 16 P 8, 20 M 8 ) aa(P, M)

the conformers & nomenclature conformers of isoamyl acetate lilian w. sutikdja, 23 rd February cis-conformers 1 C s – Symmetry + 26 C 1 -Symmetry 1 C s – Symmetry + 26 C 1 -Symmetry calculation on MP2/ G** level