Mass-resolved excitation spectra of guaiacol and 4-methylguaiacol and geometry assignment of the minimum energy conformations of guaiacol

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.