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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 V3 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.
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https://hal.u-pec.fr//hal-03182961
Contributor : Ha Vinh Lam Nguyen <>
Submitted on : Friday, March 26, 2021 - 6:14:36 PM
Last modification on : Thursday, April 15, 2021 - 3:08:17 PM

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Sven Herbers, Sean Fritz, Piyush Mishra, Ha Vinh Lam Nguyen, Timothy Zwier. Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy. Journal of Chemical Physics, American Institute of Physics, 2020, 152 (7), pp.074301. ⟨10.1063/1.5142401⟩. ⟨hal-03182961⟩

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