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 theta (OC(7)-C(1)C(2)) dihedral angle of approximately 55 degrees. Such a configuration is 4-fold degenerate, corresponding to minima with theta approximately +/-60 degrees, +/-120 degrees. The four equivalent minima are separated by two kinds of barrier, corresponding to theta = +/-90 degrees, and 0 or 180 degrees. Only the theta = +/-90 degrees 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.