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.
Citation: Journal of Molecular Spectroscopy
Pub Type: Journals
Hydrogen Transfer, Internal Rotation, Large-Amplitude Motion, Microwave Spectrum, Tunneling Hamiltonian, Tunneling Splittings