Sensing the Molecular Structures of Hexan‐2‐one by Internal Rotation and Microwave Spectroscopy

Abstract

Using two molecular jet Fourier transform spectrometers, the microwave spectrum of hexan‐2‐one, also called methyl n‐butyl ketone, was recorded in the frequency range from 2 to 40 GHz. Three conformers were assigned and fine splittings caused by the internal rotations of the two terminal methyl groups were analyzed. For the acetyl methyl group CH3COC3H6CH3, the torsional barrier is 186.9198(50) cm−1, 233.5913(97) cm−1, and 182.2481(25) cm−1 for the three observed conformers, respectively. The value of this parameter could be linked to the structure of the individual conformer, which enabled us to create a rule for predicting the barrier height of the acetyl methyl torsion in ketones. The very small splittings arising from the internal rotation of the butyl methyl group CH3COC3H6CH3 could be resolved as well, yielding the respective torsional barriers of 979.99(88) cm−1, 1016.30(77) cm−1, and 961.9(32) cm−1.

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6‐Dimethylfluorobenzene

“…A "pseudo-C s " conformation leads to a barrier height of approximately 180 cm À 1 and a C 1 conformation is connected to a value of about 240 cm À 1 . [45][46][47] In another study, Herbers et al pointed out that π-conjugations from the phenyl group significantly affect the acetyl methyl torsion of acetophenone derivatives and the barrier height raises to 600 cm À 1 . [44] Obviously, the methyl torsional barrier can be applied as a direct measure for steric and electronic contributions in the molecules.…”

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan‐2‐one

“…[14] Apart from electrostatic and steric effects, the length of the alkyl chain at the other side of the carbonyl group might also affect methyl torsional barriers. The barriers to internal rotation of the acetyl methyl group observed for methyl n-alkyl ketones previously studied [2][3][4] suggest a trend: The longer the alkyl chain, the lower the barrier. Moreover, there is a tendency for a plateau curve, i. e. once the alkyl chain reaches a sufficient length, the barrier remains at a constant value.…”

“…In many cases, its value is caused by structural aspects and functional groups of the molecule, making the internal rotor a spectroscopic "detector" of the structure. To explore this link between structural configurations and barriers in ketones, investigations on a series of saturated methyl n-alkyl ketones, including butan-2-one, [1] pentan-2-one, [2] hexan-2-one, [3] and heptan-2-one, [4] have been performed.…”

“…Conformers of the C 1 class always show a barrier to internal rotation of approximately 240 cm À 1 , while conformers of the C s class exhibit barrier heights around 180 cm À 1 . [2][3][4] In this work, the situation in ketones containing an acetyl methyl group will be further examined.…”

“…We were interested to find the limit of the plateau curve and decided to extend the series by studying octan-2-one, also called methyl hexyl ketone. As changes of the barrier heights induced by the alkyl chain length are rather small, the methyl alkyl ketone series is ideally suitable to study this effect because in the three previous investigations on pentan-2-one, [2] hexan-2-one, [3] and heptan-2-one [4] similar sets of parameters have been consistently applied. Since the values of the torsional barriers are quite sensitive to the set of parameters in use, this enables a reliable comparison.…”