PowerPoint-Präsentation

PowerPoint-Präsentation

The Rotational Spectrum and Conformational Structures of Methyl Valerate LAM NGUYEN Laboratoire Interuniversitaire des Systmes Atmosphriques (LISA) Universit Paris Est Crteil 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 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 Different only

trans esters are considered. conformations 3 Conformational analysis MP2/6-311++G(d,p), 11 stable conformers Conformers VIII-XI : more than 4.5 kJmol1 higher in energy than the most stable conformer I not observable discard Except conformer V, all other conformers possess C 1 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 cm1 (Methyl acetate : 422 cm 1, Methyl propionate : 429 cm1) A-E splittings up to a few tens of MHz

Butyl methyl group : 1000 cm1 no observable splittings 6 Measurements Molecular beam FT microwave spectroscopy, 2 26.5 GHz High resolution Experimental accuracy: 2 kHz Doppler effect A E splittings Broader lines (unresolved splittings from the alkyl methyl Broadband scan Series of automatically recorded spectra in the high resolution mode 250 kHz 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 65

76 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 MP2/6-311++G(d,p) level 9 Molecular Parameters PAR. Unit XIAM A

MHz 5063.17500(71) 4407.0 656.2 (i13 %) B MHz 897.72973(19) 932.3 34.6 (4 %) C MHz 846.44127(19) 897.4 51.0 (6 %) J kHz 0.27385(64) JK kHz 6.7550(49) K kHz 68.77(14) J

kHz 0.01929(32) K kHz 2.611(82) V3,1 cm1 417.66(68) F0 GHz 157.98(22) (ii1,a) 37.793(93) (ii1,b) 52.293(93) (ii1,c) 87.8136(46) NA/NE 85/83 kHz 3.3

MP2 XIAMMP2 10 Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) 4407.0 656.2 (i13 %) B MHz 897.72973(19) 932.3 34.6 (4 %) C MHz 846.44127(19) 897.4 51.0 (6 %) J kHz 0.27385(64) JK

kHz 6.7550(49) K kHz 68.77(14) J kHz 0.01929(32) K kHz 2.611(82) V3,1 cm1 417.66(68) F0 GHz 157.98(22) (ii1,a) 37.793(93) (ii1,b) 52.293(93)

(ii1,c) 87.8136(46) NA/NE 85/83 kHz 3.3 MP2 XIAMMP2 Within the experimental 10 Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) 4407.0 656.2 (i13 %) B MHz 897.72973(19) 932.3 34.6 (4 %)

C MHz 846.44127(19) 897.4 51.0 (6 %) J kHz 0.27385(i64) JK kHz 6.7550(i49) K kHz 68.77(i14) J kHz 0.01929(i32) K kHz 2.611(i82) V3,1 cm1

417.66(68) F0 GHz 157.98(22) (ii1,a) 37.793(93) (ii1,b) 52.293(93) (ii1,c) 87.8136(46) NA/NE 85/83 kHz 3.3 MP2 XIAMMP2 10 Molecular Parameters PAR. Unit XIAM A

MHz 5063.17500(i71) 4407.0 656.2 (i13 %) B MHz 897.72973(i19) 932.3 34.6 (i4 %) C MHz 846.44127(i19) 897.4 51.0 (i6 %) J kHz 0.27385(64) JK kHz 6.7550(49) K kHz 68.77(14) J kHz

0.01929(32) K kHz 2.611(82) V3,1 cm1 417.66(68) F0 GHz 157.98(22) (ii1,a) 37.793(93) (ii1,b) 52.293(93) (ii1,c) 87.8136(46) NA/NE 85/83 kHz 3.3 MP2

XIAMMP2 10 Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(i71) 4407.0 656.2 (i13 %) B MHz 897.72973(i19) 932.3 34.6 (i4 %) C MHz 846.44127(i19) 897.4 51.0 (i6 %) J kHz 0.27385(64) JK kHz 6.7550(49)

K kHz 68.77(14) J kHz 0.01929(32) K kHz 2.611(82) V3,1 cm1 417.66(68) F0 GHz 157.98(22) (ii1,a) 37.793(93) (ii1,b) 52.293(93) (ii1,c) 87.8136(46)

NA/NE 85/83 kHz 3.3 MP2 XIAMMP2 10 Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(i71) 4407.0 656.2 (i13 %) B MHz 897.72973(i19) 932.3 34.6 (i4 %) C MHz 846.44127(i19) 897.4 51.0 (i6 %)

J kHz 0.27385(64) JK kHz 6.7550(49) K kHz 68.77(14) J kHz 0.01929(32) K kHz 2.611(82) V3,1 cm1 417.66(68) F0 GHz 157.98(22) (ii1,a)

37.793(i93) (ii1,b) 52.293(i93) (ii1,c) 87.8136(i46) NA/NE 85/83 kHz 3.3 MP2 XIAMMP2 10 Basis set variation a-type 6 5 Conformer I was definitively assigned ! MP2/6-311++G(d,p) level fails ! Which level works ? 11 Basis set variation

a-type 6 5 Conformer I was definitively assigned ! MP2/6-311++G(d,p) level fails ! Which level works ? MP2/cc-pVDZ 11 Barrier heights in methyl alkanoates Unit XIAM A MHz 5063.17500(71) B MHz 897.72973(19) C MHz 846.44127(19) J kHz 0.27385(64) JK kHz 6.7550(49) K kHz

68.77(14) J kHz 0.01929(32) K kHz 2.611(82) V3,1 cm1 417.66(i68) F0 GHz 157.98(22) (ii1,a) 37.793(93) (ii1,b) 52.293(93) (ii1,c) 87.8136(46) NA/NE 85/83

kHz 3.3 Methyl acetate Methyl propionate Methyl butyrate Methyl 422.0 cm1 couplin g 429.3 cm1 425.1 cm1 417.7 cm1 Similar structur es PAR. 12

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