Shapes of Molecules by Millimeter-Wave Spectroscopy: 2-Phenylethanol

Abstract

We have detected one conformer of 2-phenylethanol and determined its shape using Stark-modulated free jet microwave spectroscopy. By comparing observed and ab initio predicted spectroscopic rotational constants and hydroxyl-deuteration isotopic substitution coordinates, the species has been identified with a gauche conformer, designated as 1, in which the hydroxyl hydrogen atom is involved in an intramolecular hydrogen bond with the π-electron cloud of the aromatic ring. Stark effect measurements yielded electric dipole moment values of μa = 1.251(3) D and (μb2 + μc2)1/2 = 1.102(10) D, and hence μtotal = 1.667(10) D. Previously published molecular mechanics and RHF level ab initio calculations predict that other conformers are of comparable stability to 1. However, when electron correlation is included at the MP2/6-31G(d,p) level, the predicted energy of 1 is substantially (ca 8 kJ mol-1) lower than that of all the other conformers. This is in full accord with our identification of only one conformer in our spectroscopic searches at a pre-expansion temperature of 363 K.