Discrimination of rotational isomers of 2-phenylethanol by dispersed fluorescence spectroscopy

Abstract

Laser-induced dispersed fluorescence (DF) spectra for excitations of several low-energy bands near the S₁ origin (0⁰₀) of 2-phenylethanol have been measured in a supersonic free jet expansion. The spectrum measured for excitation of a weak band at 0⁰₀ +48 cm^-1 has been found to resemble the -excited emission spectrum. This weak band has been assigned to the S₁ ← S₀ origin of a higher energy conformer. Spectral analyses with the aid of ab initio (DFT/B3LYP/6-311++G**) calculations reveal that the frequencies of the benzenoid 6a and a few other ring vibrational modes are sensitive to molecular shape. Comparisons of these frequencies with those obtained by theoretical calculations suggest that the higher energy species is the anti conformer of the molecule. This conformational assignment based on vibronic analysis agrees with the quantum chemistry predictions at the MP2/cc-pVTZ level of theory that the anti is the second preferred conformational isomer having 7.03 kJ/mol more energy than the most stable gauche form. No spectral feature corresponding to emission from an excited 1:1 water complex of the molecule was apparent in the dispersed fluorescence when the exciting laser was tuned across the 0⁰₀ +48 cm^-1 band. This behavior is in contrast to conclusions of two previous studies that the 48 cm^-1 band in the LIF excitation spectrum is due to a 1:1 water complex.