Microwave spectrum and ab initio calculations of ethylbenzene: potential energy surface of the ethyl group torsion

Abstract

The weak rotational spectrum of the ground and three vibrational states belonging to the ethyl group torsional progression of ethylbenzene has been measured using conventional microwave spectroscopy. The small dipole moment (μ_a = 0·49(4), μ_c = 0·2(1) D) has been determined from a second order Stark effect, and is compared with a variety of ab initio calculations at the RHF/6-31G, MP2/6-31G, RHF/6-31G*, MP2/6-31G* and RHF/6-31G** levels. The potential energy surface for the rotation of the ethyl group with respect to the ring has also been estimated on the basis of vibrational satellite intensities, shifts of rotational constants upon torsional excitation, and of ab initio calculations. A conformation in which the ethyl group is perpendicular to the nominal ring plane is found to be the most stable.