A Microwave Study of Hydrogen-Transfer-Triggered Methyl-Group Rotation in 5-Methyltropolone
Vadim V. Ilyushin, Emily A. Cloessner, Yung-Ching Chou, Laura B. Picraux, Jon T. Hougen, Richard Lavrich
We present here the first experimental and theoretical study of the microwave spectrum of 5-methyltropolone, which can be visualized as a 7-membered aromatic carbon ring with a five-membered hydrogen-bonded cyclic structure at the top and a methyl group at the bottom. The molecule exhibits two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is particularly interesting because transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the molecule, which then triggers a 60 internal rotation of the methyl group. Measurements were carried out by Fourier-transform microwave spectroscopy in the 8 GHz to 24 GHz frequency range. Theoretical analysis was carried out using a tunneling-rotational Hamiltonian based on a G12m extended-group theoretical formalism. Our global fit of 1015 transitions to 20 molecular parameters gave a root-mean-square deviation of 1.5 kHz. Tunneling splittings associated with a hypothetical pure hydrogen transfer motion and with a pure methyl-top internal rotation motion were determined to be 1310 MHz and 885 MHz, respectively.
, Cloessner, E.
, Chou, Y.
, Picraux, L.
, Hougen, J.
and Lavrich, R.
A Microwave Study of Hydrogen-Transfer-Triggered Methyl-Group Rotation in 5-Methyltropolone, Journal of Molecular Spectroscopy, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=903562
(Accessed December 8, 2023)