Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy

Abstract

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V 3 potential barriers of about 22 cm −1 and 584-588 cm −1 for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations.

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

“…The electrostatic characteristic of the "pseudo-C s " geometry is the decisive factor. (ii) Ketones of the "phenyl class" show barrier heights of around 600 cm À 1 (see Table 3), [49] which is much higher than the values of 180 cm À 1 and 230 cm À 1 found for the C s and C 1 class, respectively. The reason is probably the πelectron conjugation between the carbonyl group and the phenyl ring, indicating the important role of electrostatic interactions on the acetyl methyl torsion in ketones.…”

“…Recently, Herbers et al extended the "two-class" concept of the acetyl methyl barriers in ketones to include a "phenyl class". [49] There, a phenyl ring instead of an alkyl chain is attached on one side of the carbonyl group. The barrier heights are between 550 cm À 1 and 630 cm À 1 .…”

“…The barrier heights are between 550 cm À 1 and 630 cm À 1 . Members of this class are acetophenone, [50] 4-methylacetophenone, [49] acetovanillone, [51] and 6-hydroxy-3-methoxyacetophenone. [51] A "mesomeric class" enclosing methyl vinyl ketone [52] and ionone [53] might also exist, since both molecules contain an unsaturated α,β-bond in their alkyl chains.…”

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

“…[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.…”

“…Since checking the assignment with combination difference loops was not possible because only b-type transitions are allowed, we verified the assigned frequencies by Separately Fitting the Large Amplitude Motion Species using the program SFLAMS. [44] The lines of the (00) symmetry species follow a rigid rotor pattern and could be fitted well to a standard deviation of 2.3 kHz using an effective Hamiltonian H ¼ H r þ H cd comprising rotational and quartic centrifugal distortion terms of Watson's S reduction in its I r representation.…”