Molecular Structure of 2-(4-Aminophenyl)ethanol Studied by Supersonic Jet Spectroscopy

A weak hydrogen bonding such as the OH/π interaction between the OH group and the π electron system plays an important role in the conformational preference of a flexible molecule. However, the experimental data showing the effect of the substituent on the OH/π interaction have not been reported so much. In this article, laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectra of 2-(4-aminophenyl)ethanol (APE) were measured for the first time. Almost all bands observed in the spectral region of 33150-34150 cm^-1 in the LIF excitation spectrum were successfully assigned based on the DF spectra and the quantum chemical calculations at M06-2x/6-311+G(d,p) and MP2/6-311+G(d,p) levels. Ten conformers were predicted by the quantum chemical calculations, resulting from the orientations of the hydroxyethyl and amino groups. However, the molecular structure of the amino group was assumed to adopt a planar conformation due to a low inversion barrier between syn and anti conformations. The bands in the LIF excitation spectrum for APE were found to be due to three conformers. The most stable among the three conformers was Ggπ, in which the OH group is directed toward the π electron system of the benzene ring, others were At and G conformers. For APE, the energy gap between Ggπ and At, which has little OH/π interaction, was larger than that of 2-phenylethanol. The amino group would increase the negative charge on the benzene ring, causing the enhancement of the OH/π interaction. The CN bond length in the excited state was shorter than that in the ground state, and the OH bond length was longer. These results suggest that the OH/π interaction should be stronger in the excited state than in the ground state.