1. 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

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

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

4. 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.

5. 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

6. Conformational analysis
MP2/ 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.

7. Conformer I Most stable conformation of methyl valerate
No linear alkyl chain
A = MHz, B = MHz, C = MHz  near prolate top
a = 1.43 D, b = 0.39 D, c = 0.92 D  a-, b-, and c-type transitions

8. 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

9. 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

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

11. 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

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

13. 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/ G(d,p) level

14. Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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 %)

15. Within the experimental accuracy
Molecular Parameters
PAR.
Unit
XIAM A
MHz
(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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

16. Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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 %)

17. Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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 %)

18. Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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 %)

19. Molecular Parameters PAR. Unit XIAM A MHz 5063.17500(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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 %)

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

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

22. Barrier heights in methyl alkanoates
PAR.
Unit
XIAM A
MHz
(71) B
(19) C
(19) J
kHz
(64)
JK
−6.7550(49) K
68.77(14) J
− (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)
(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