1. MICROWAVE SPECTROSCOPY OF 2-PENTANONE
M. ANDRESEN, H. V. L. NGUYEN, W. STAHL
Institut fur Physikalische Chemie, Raum Aachen, Germany
I. KLEINER
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA),
CNRS, Universités Paris Est et Paris Diderot, Créteil, France

2. Objectives
Study relatively large organic molecules (esters and ketones, amines) to obtain very precise molecular structures and compare with ab initio calculations
odorant molecules, phytohormones, alarm pheromone : what are their molecular structure ?
A complex issue …

3. Internal rotation and structure of alkyl acetates
Collaboration with Institute of Physical Chemistry, RWTH Aachen (Germany):
W. Stahl, L. Nguyen, H. Mouhib, T. Attig, Y. Zhao, D. Jelisavac, L. Sutikdja, R. Kannengisser
N-butyl acetate
pear
N-pentyl acetate
rose
N-hexyl acetate
Fruity
Isoamyl acetate Banana oil
JMS 2013
JMS 2013
JMS 2014
JMS 2012
The barrier to internal rotation of the acetyl methyl group in acetates is always around 100 cm−1
due to an almost C2v symmetric charge distribution over the COO group

4. Internal rotation and structure of ketones
Methyl ethyl
ketone
Allyl acetone
Methyl vinyl
ketone
Diethyl ketone
V3 = 772 cm-1
V3 = 183 cm-1
V3 = 233 cm−1
V3 = 434 cm-1 ap
377 cm-1 sp
Nguen and Stahl,
ChemPhysChem 2011
Nguyen et al,JCP 2014 3
Tulimat et al JMS 2015
Wilcox et al
JMS 2011
barrier heights to internal rotation vary in a wide range.
accurate predictions of the barrier height of an acetyl methyl group are still difficult to make and no conclusive trends could be determined.
ketones containing an acetyl methyl group as internal rotor exhibit barriers in the range from about 170 cm−1 to 440 cm−1.
No systematic study of ketones by MW spectroscopy
- Too many atoms to determine their structure by isotopic substitution
- Large internal rotation splittings, serve as test of our models

5. Methyl propyl ketone (MPK) or 2-Pentanone
additive in the food industry
pheromone in bee secretions

6. Methyl Butyl ketone and Methyl propyl ketone (MPK)

7. Ab initio calculations
MP2, B3LYP and B3PW91 method in combination with 26 different basis sets each
The rotation about the C1 – C5 axis corresponds to the internal rotation of the acetyl methyl group and does not create new conformations .The same situation occurs for the rotation about the C10 – C13 axis, which refers to the internal rotation of the respective alkyl methyl group
Plan C1-C5-C7 and plan C5-C7-C10:
-60, +60, 180°  32 = 9 conformers but 4 are stable

8. Energy of the lowest conformers of 2-pentanone

9. Rotational constants, dipole moments and relative energies of the most stable conformers of 2-pentanone (MPK) C1 Cs
MP G(d,p),

10. Internal rotation in 2-pentanone: 2 terminal methyl groups
alkyl methyl group V cm−1
acetyl methyl group
V3 = 170 cm−1 – 250 cm−1,

11. Potential energy curves of the acetyl methyl group of conformer 1 of methyl propyl ketone (acetyl methyl group)
RED experimental value
the shape of the curves is not entirely C3 symmetric, which is probably due to the bent structure of the conformer
BLUE: MP2/ G(d,p)
GREEN: MP2/cc-pVDZ
ORANGE: B3LYP/6-311G(3df,3pd)
PURPEL: B3PW91/6-311G(3df,3pd)

12. Potential energy curves of the acetyl methyl group of conformer 2 of methyl propyl ketone
Surprisingly, the barrier height of the acetyl methyl group of conformer 2 of MPK is only cm−1, instead of cm−1 as for conformer 1
double minimum for calculations at the MP2/ G(d,p) and the B3LYP/ G(df,pd)
 Near Cs structure
BLUE: MP2/ G(d,p)
GREEN: B3LYP/ G(df,pd)
ORANGE: B3PW91/6-31G(3df,3pd)

13. Experimental technique at Aachen
Molecular beam Fourier transform microwave (MB-FTMW) spectrometers
a gas mixture of approximately 1%
of 2-pentanone in helium
expanded through a pulsed
nozzle into the cavity at a total
pressure of about
200 kPa
vapor pressure of 41 hPa
Frequency ranges: 3 to 26.5 GHz

14. Analysis of Conformer 2 : Cs
No c-type transitions found
Arrows: First A transitions assigned
Obs
Calc
Comparison of the experimental microwave spectrum and the spectrum reproduced with XIAM of conformer 2 of methyl propyl ketone in the frequency range from 9.90 to GHz
A species ____
E species ____
____ assigned to Conformer 1
calculations at the MP2/ G(d,p) allow a good Initial prediction

15. FT-MB-MW spectrometer Aachen University: High resolution mode
Conformer 2: Cs.
AA-EA: splittings due to V3 = 186 cm-1
AA-AE: splittings due to V3=1000 cm-1
EE and EE*: coupling of the two tops
200 kPa
Res: 2 kHz isolated lines
GHz

16. Molecular parameters of conformer 2 of methyl propyl ketone obtained from the XIAM Fit AA/AE in comparison to the BELGI-Cs Fit

17. Two-tops fit using XIAM for Conformer 2
=218 transitions
V31 = )(74) cm-1
V32= 1036(16) cm-1
use BELGi-Cs-2Tops to improve rms

18. Analysis of Conformer 1 : C1
Obs
Calc
A species ___
E species ___
305 lines assigned to Conformer 1, 218 lines assigned to Conformer 2
27 unassigned lines (with squares) : conformer 3 or 13C ?

19. Molecular parameters for the conformer 1 (C1 symetry)
with XIAM and BELGI-C1 transformed in the PAM system
BELGI-C1 1-top
_________
(22)
(71)
(64)
163.9fixed
(80)
26.87
71.66
71.11
5.3
105/91
XIAM 1top
XIAM 2-tops
Fit with another 2-top
code (C1 symetry)
N(AA, AE)

20. MP2/6-311++G(d,p) and B3LYP/6-311++G(df,pd) level of theory : 
Conformer 2 :“near-Cs” structure, alkyl chain is tilted out of the Cs plane by ca. 10°
B3PW91/6-31G(3df,3pd) level and the experimental results -
“true-Cs” structure
same phenomenon has been observed for
conformer 4 of Methyl butyl ketone (Andresen et al unpublished)
methyl ethyl ketone (2-butanone) Kroll et al 2014, Nguyen et al 2014
methyl neopentyl ketone (Zhao et al JMS 2012)

21. methyl neopentyl ketone (Zhao et al JMS 2012)
methyl ethyl ketone
(2-butanone)
Nguyen et al 2014

22. Conclusions
the barrier heights of two different conformers of MPK differ by ca. 50 cm−1 ( cm−1 vs cm−1).
Contrarly to acetate : the influence of the conformational structure on the barrier height is negligible. For ketone: more sensitive to their environnement. The CH3 “smells” the structure more!

23. 2 categories of conformers in ketones
“out-of-plane” structure, C1 symmetry and acetyl methyl barrier heights of about 240 ± 10 cm−1
conformer 1 of MPK ( cm−1)
allyl acetone ( cm−1)
methyl isobutyl ketone (250 cm−1)
B) “in-plane” conformers with Cs symmetry or near-Cs and barrier heights of about 180 cm−1
- conformer 2 of MPK
( cm−1)
- ethyl ketone ( cm−1)
- methyl neopentyl ketone
(174.1 cm−1)

24. Thank you !

27. Potential energy curves of the propyl methyl group of conformer 1 of methyl propyl ketone (alkyl methyl group)
RED experimental value
BLUE: MP2/ G(d,p)
GREEN: MP2/cc-pVDZ
ORANGE: B3LYP/6-311G(3df,3pd
PURPEL: B3PW91/6-311G(3df,3pd)

28. Rotational constants for the 4 lowest conformers of 2-pentanone
Experimental A B C