METHOXY GROUP NONPLANARITY IN OMICRON-DIMETHOXYBENZENES - SIMPLE PREDICTIVE MODELS FOR CONFORMATIONS AND ROTATIONAL BARRIERS IN ALKOXYAROMATICS

Photoelectron spectroscopy and ab initio STO-3G calculations provide evidence that o-dimethoxybenzene and derivatives exist in predominantly nonplanar conformations in the gas phase. By contrast, the preferred conformations of methoxy bcnzene and m• and p-dimcthoxybenzene arc planar, and the nonplanar structures are significantly less stable. For both hy-droxy-and methoxybenzenes, the STO-3G calculated barriers to rotation about the Ar-OR bond decrease in the order wi-di-< mono-< p-di-< o-di-. Partition coefficient, dipole moment, and dielectric relaxation measurements on o-dimethoxybenzene are consistent with the presence of nonplanar conformations in solution. On the other hand, the crystal structures of compounds bearing o-dimethoxy substituents indicate a preference for planar conformations: of 32 examples found in the literature-, only two structures contain a nonplanar methoxy group. The relevant differences between the gas-phase, solution, and crystal conformations of o-dimethoxy substituted aromatics are discussed and rationalized. The origin of the rotational barriers in methoxy-and hydroxybenzenes is analyzed in terms of both resonance and frontier molecular orbital arguments which indicate that the preferred conformations are those maximizing two-electron stabilizing interactions and minimizing four-electron destabilizing interactions. A simple method of predicting rotational conformational preferences, based on ir charge densities, is proposed and used to explain the influence of substitution on the hallucinogenic properties of several polyalkoxyamphe-tamines. © 1979, American Chemical Society. All rights reserved.