The Rotational Spectrum and Conformational Structures of Methyl

Methyl valerate Methyl pentanoate Chemistry : Linear aliphatic ester (methyl alkanoate) Natur : Fruit

Conformational analysis Methoxy methyl group Butyl methyl group Methyl internal rotations no new conformations

Conformational analysis MP 2/6 -311++G(d, p), 11 stable conformers Conformers VIII-XI : more than

Conformer I Most stable conformation of methyl valerate No linear alkyl chain A =

Conformer I Methoxy methyl group Butyl methyl group Two methyl internal rotors Barrier heights

Measurements Molecular beam FT microwave spectroscopy, 2 – 26. 5 GHz High resolution Experimental

Rigid-Rotor (A Species) Assignments Expt. Calc. 8

Rigid-Rotor (A Species) Assignments 6← 5 7← 6 Expt. Calc. R-branch a-type J =

Rigid-Rotor (A Species) Assignments Expt. Calc. Q-branch b-type transitions shifted by up to 2.

Internal Rotation (E Species) Assignments Expt. Calc. Methoxy methyl group Barrier to internal rotation

Molecular Parameters XIAM MP 2 PAR. Unit XIAM A MHz 5063. 17500(71) 4407. 0

Basis set variation a-type 6 ← 5 Conformer I was definitively assigned ! MP

Barrier heights in methyl alkanoates Unit XIAM A MHz 5063. 17500(71) B MHz 897.

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The Rotational Spectrum and Conformational Structures of Methyl Valerate LAM NGUYEN Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA) Université Paris Est Créteil WOLFGANG STAHL Institute of Physical Chemistry, RWTH Aachen University 1

Methyl valerate Methyl pentanoate Chemistry : Linear aliphatic ester (methyl alkanoate) Natur : Fruit ester (odorants of fruits, flowers, wines. . . ) Spectroscopy : Conformations, internal dynamics 2

Conformational analysis Methoxy methyl group Butyl methyl group Methyl internal rotations no new conformations 3

Conformational analysis Methoxy methyl group Butyl methyl group Methyl internal rotations no new conformations trans or cis esters ➔ cis esters much higher in energy not observable under our experimental conditions discard ➔ only trans esters are considered. 3

Conformational analysis MP 2/6 -311++G(d, p), 11 stable conformers Conformers VIII-XI : more than 4. 5 k. J·mol− 1 higher in energy than the most stable conformer I not observable discard Except conformer V, all other conformers possess C 1 symmetry. 4

Conformer I Most stable conformation of methyl valerate No linear alkyl chain A = 4407. 0 MHz, B = 932. 3 MHz, C = 897. 4 MHz near prolate top a = 1. 43 D, b = 0. 39 D, c = 0. 92 D a-, b-, and c-type transitions 5

Conformer I Methoxy methyl group Butyl methyl group Two methyl internal rotors Barrier heights : ➔ Methoxy methyl group : about 420 cm 1 (Methyl acetate : 422 cm 1, Methyl propionate : 429 cm 1) A-E splittings up to a few tens of MHz ➔ Butyl methyl group : 1000 cm 1 no observable splittings 6

Measurements Molecular beam FT microwave spectroscopy, 2 – 26. 5 GHz High resolution Experimental accuracy: 2 k. Hz Doppler effect A – E splittings Broader lines (unresolved splittings from the alkyl methyl rotor) 4 k. Hz Broadband scan Series of automatically recorded spectra in the high resolution mode 250 k. Hz step width, 50 decays per step Frequency range : 9 13. 5 GHz 7

Rigid-Rotor (A Species) Assignments Expt. Calc. 8

Rigid-Rotor (A Species) Assignments 6← 5 7← 6 Expt. Calc. R-branch a-type J = 6 ← 5 and J = 7 ← 6 transitions shifted by 0. 5 GHz 8

Rigid-Rotor (A Species) Assignments Expt. Calc. Q-branch b-type transitions shifted by up to 2. 0 GHz 8

Internal Rotation (E Species) Assignments Expt. Calc. Methoxy methyl group Barrier to internal rotation : estimated to be 420 cm 1 Polar-coordinates of the internal rotor axis : taken from the geometry optimized at the MP 2/6 -311++G(d, p) level 9

Molecular Parameters XIAM MP 2 PAR. Unit XIAM A MHz 5063. 17500(71) 4407. 0 656. 2 (13 %) B MHz 897. 72973(19) 932. 3 34. 6 ( 4 %) C MHz 846. 44127(19) 897. 4 51. 0 ( 6 %) J k. Hz 0. 27385(64) JK k. Hz − 6. 7550(49) K k. Hz 68. 77(14) J k. Hz − 0. 01929(32) K k. Hz 2. 611(82) V 3, 1 cm– 1 417. 66(68) F 0 GHz 157. 98(22) (i 1, a) ° 37. 793(93) (i 1, b) ° 52. 293(93) (i 1, c) ° 87. 8136(46) NA/NE 85/83 k. Hz 3. 3 MP 2 10

Basis set variation a-type 6 ← 5 Conformer I was definitively assigned ! MP 2/6 -311++G(d, p) level fails ! Which level works ? 11

Basis set variation a-type 6 ← 5 Conformer I was definitively assigned ! MP 2/6 -311++G(d, p) level fails ! Which level works ? MP 2/cc-p. VDZ 11

Barrier heights in methyl alkanoates Unit XIAM A MHz 5063. 17500(71) B MHz 897. 72973(19) C MHz 846. 44127(19) J k. Hz 0. 27385(64) JK k. Hz − 6. 7550(49) K k. Hz 68. 77(14) J k. Hz − 0. 01929(32) K k. Hz 2. 611(82) V 3, 1 cm– 1 417. 66(68) F 0 GHz 157. 98(22) (i 1, a) ° 37. 793(93) (i 1, b) ° 52. 293(93) (i 1, c) ° 87. 8136(46) NA/NE 85/83 k. Hz 3. 3 422. 0 cm 1 coupling Methyl acetate 429. 3 cm 1 Methyl propionate 425. 1 cm 1 Similar structures PAR. Methyl butyrate 417. 7 cm 1 Methyl valerate 12