Theoretical investigations at the AM1, RHF/3-21G, RHF/6-31G*, MP2/6-31G*//RHF/3-21G, and MP2/6-31G*// RHF/6-31G* levels of theory have been used to study the gas-phase rotational barrier in anisole. Geometries at all levels are similar. Vibrational frequency analyses indicate that the coplanar structure is a local minimum at both the AM1 and RHF/3-21G levels. The conformer with the methoxy substituent perpendicular to the phenyl ring is a transition structure at the AM1 level but is a minimum at the RHF/3-21G level. A transition structure between these minima is located at both the RHF/3-21G and the RHF/6-31G* levels. The potential at either ab initio level is very flat between Φ = 45° and 90° but is rather more steep with AM1. Calculations at the MP2/6-31G*//RHF/3-21G and MP2/6-31G*//RHF/6-31G* levels indicate that the perpendicular conformer is slightly higher in energy than the transition structure. The 4-fold term for rotation is small, but very significant, based on the ab initio energy profile. Molecular mechanics parameters derived for the AMBER force field have been used in an isothermal-isobaric simulation of liquid anisole to assess the effect of the liquid structure on the dihedral angle population. The population of the intermediate values of Φ is slightly higher than in the liquid than is predicted from the gas-phase energy profile, suggesting a slightly decreased barrier to rotation in the liquid, in direct contrast to experiment. © 1990 American Chemical Society.