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

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 …

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

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

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

Methyl Butyl ketone and Methyl propyl ketone (MPK)

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

Energy of the lowest conformers of 2-pentanone

Rotational constants, dipole moments and relative energies of the most stable conformers of 2-pentanone (MPK) C1 Cs MP2-6-311++G(d,p),

Internal rotation in 2-pentanone: 2 terminal methyl groups alkyl methyl group V3 1000 cm−1 acetyl methyl group V3 = 170 cm−1 – 250 cm−1,

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: MP2/6-311++G(d,p) GREEN: MP2/cc-pVDZ ORANGE: B3LYP/6-311G(3df,3pd) PURPEL: B3PW91/6-311G(3df,3pd)

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 188.76 cm−1, instead of 238.59 cm−1 as for conformer 1 double minimum for calculations at the MP2/6-311++G(d,p) and the B3LYP/6-311++G(df,pd)  Near Cs structure BLUE: MP2/6-311++G(d,p) GREEN: B3LYP/6-311++G(df,pd) ORANGE: B3PW91/6-31G(3df,3pd)

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

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 12.15 GHz A species ____ E species ____ ____ assigned to Conformer 1 calculations at the MP2/6-311++G(d,p) allow a good Initial prediction

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 9.0 - 25.0 GHz

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

Two-tops fit using XIAM for Conformer 2 =218 transitions V31 = 188.755)(74) cm-1 V32= 1036(16) cm-1 use BELGi-Cs-2Tops to improve rms

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 ?

Molecular parameters for the conformer 1 (C1 symetry) with XIAM and BELGI-C1 transformed in the PAM system BELGI-C1 1-top _________ 6.49826 (22) 2.153824(71) 1.929372(64) 163.9fixed 238.360(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)

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)

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

Conclusions the barrier heights of two different conformers of MPK differ by ca. 50 cm−1 (238.59 cm−1 vs. 188.76 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!

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 (238.59 cm−1) allyl acetone (233.145 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 (188.76 cm−1) - ethyl ketone (183.170 cm−1) - methyl neopentyl ketone (174.1 cm−1)

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

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