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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.
Journal of Molecular Spectroscopy


Hydrogen Transfer, Internal Rotation, Large-Amplitude Motion, Microwave Spectrum, Tunneling Hamiltonian, Tunneling Splittings


Ilyushin, V. , Cloessner, E. , Chou, Y. , Picraux, L. , Hougen, J. and Lavrich, R. (2010), A Microwave Study of Hydrogen-Transfer-Triggered Methyl-Group Rotation in 5-Methyltropolone, Journal of Molecular Spectroscopy, [online], (Accessed May 25, 2024)


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Created November 9, 2010, Updated October 12, 2021