The Rotational Spectrum and Conformational Structures of Methyl Valerate LAM NGUYEN Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA)

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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

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

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

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.

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. Different conformations

Conformational analysis MP2/6-311++G(d,p), 11 stable conformers Conformers VIII-XI : more than 4.5 kJ·mol−1 higher in energy than the most stable conformer I  not observable  discard Except conformer V, all other conformers possess C1 symmetry.

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

Conformer I Two methyl internal rotors Barrier heights : 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

Molecular beam FT microwave spectroscopy, 2 – 26.5 GHz Measurements Molecular beam FT microwave spectroscopy, 2 – 26.5 GHz High resolution Broadband scan Experimental accuracy: 2 kHz Doppler effect A – E splittings Broader lines (unresolved splittings from the alkyl methyl rotor)  4 kHz Series of automatically recorded spectra in the high resolution mode 250 kHz step width, 50 decays per step Frequency range : 9  13.5 GHz

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

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

Rigid-Rotor (A Species) Assignments Expt. Calc. Q-branch b-type transitions shifted by up to 2.0 GHz

Internal Rotation (E Species) Assignments Expt. Methoxy methyl group Calc. Barrier to internal rotation : estimated to be 420 cm1 Polar-coordinates of the internal rotor axis : taken from the geometry optimized at the MP2/6-311++G(d,p) level

Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 MP2 XIAMMP2 4407.0 656.2 (13 %) 932.3 34.6 (4 %) 897.4 51.0 (6 %)

Within the experimental accuracy Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 MP2 XIAMMP2 4407.0 656.2 (13 %) 932.3 34.6 (4 %) 897.4 51.0 (6 %) Within the experimental accuracy

Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 MP2 XIAMMP2 4407.0 656.2 (13 %) 932.3 34.6 (4 %) 897.4 51.0 (6 %)

Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 MP2 XIAMMP2 4407.0 656.2 (13 %) 932.3 34.6 (4 %) 897.4 51.0 (6 %)

Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 MP2 XIAMMP2 4407.0 656.2 (13 %) 932.3 34.6 (4 %) 897.4 51.0 (6 %)

Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 MP2 XIAMMP2 4407.0 656.2 (13 %) 932.3 34.6 (4 %) 897.4 51.0 (6 %)

Basis set variation Conformer I was definitively assigned ! a-type 6 ← 5 MP2/6-311++G(d,p) level fails ! Which level works ?

Basis set variation Conformer I was definitively assigned ! a-type 6 ← 5 MP2/6-311++G(d,p) level fails ! Which level works ? MP2/cc-pVDZ

Barrier heights in methyl alkanoates PAR. Unit XIAM A MHz 5063.17500(71) B 897.72973(19) C 846.44127(19) J kHz 0.27385(64) JK −6.7550(49) K 68.77(14) J −0.01929(32) K 2.611(82) V3,1 cm–1 417.66(68) F0 GHz 157.98(22) (i1,a) ° 37.793(93) (i1,b) 52.293(93) (i1,c) 87.8136(46) NA/NE 85/83  3.3 coupling 422.0 cm1 Methyl acetate 429.3 cm1 Methyl propionate 425.1 cm1 Similar structures Methyl butyrate 417.7 cm1 Methyl valerate