The mass-resolved excitation spectra of jet-cooled guaiacol (2-methoxyphenol) and 4-methylguaiacol (2-methoxy-4-methylphenol) have been measured. For guaiacol, two rotational isomers were identified. The potential barriers for internal rotation of the methoxy group were determined by molecular mechanics calculations for the ground state S0 and by fitting to the experimental data for the first excited state S1. The potential energy barrier for hindered rotation of the methoxy group is found to be dependent on the hydroxy group orientation with respect to the methoxy group and increases upon excitation. The potential energy barrier is described by a term V1 due to the interaction with the hydroxy group and a term V2 due to the interaction with the ring. When the hydroxy group is pointing away from the methoxy group, the values are V1 = 146 cm-l, V2 = 580 cm-l for S0 and V1 = 238 cm-1, V2 = 560 cm-l for S1. When the hydroxy group is pointing toward the methoxy group, these values are V1 = 1074 cm-l, V2 = 550 cm-l for S0 and V1 = 1100 cm-l, V2 = 500 cm-l for S1.

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Journal J. Phys. Chem.
Citation
Konijn, S. W, Steenvoorden, R. J. J. M, Kistemaker, P. G, & Weeding, T. L. (1994). Mass-resolved excitation spectra of guaiacol and 4-methylguaiacol and geometry assignment of the minimum energy conformations of guaiacol. J. Phys. Chem., 98, 5399–5403.