Rotational Spectrum, Tunneling Motions, and Potential Barriers of Benzyl Alcohol

The rotational spectra of benzyl alcohol and of its OD isotopologue have been assigned and measured in a supersonic expansion, either with pulsed-jet Fourier transform microwave or free jet absorption millimeter wave spectroscopy. The spectrum is consistent with a gauche conformation of the oxygen atom, characterized by a θ (OC7-C1C2) dihedral angle of approximately 55°. Such a configuration is 4-fold degenerate, corresponding to minima with θ ≈ (60°, (120°. The four equivalent minima are separated by two kinds of barrier, corresponding to θ ) (90°, and 0 or 180°. Only the θ ) (90° barriers are low enough to generate a tunneling splitting, which has been measured in a spectrum strongly perturbed by tunneling interactions. The observed splittings diminish considerably upon deuterium substitution. The tunneling splittings are consistent with a barrier about 280 cm-1 and high level ab initio calculations predicting a 320 cm-1 barrier.