13C NMR relaxation and computational study of anisole and derivatives in the solution state

Abstract

13C NMR spin–lattice relaxation times and nuclear Overhauser effects were measured at several temperatures for the methoxyl methyl carbon and the phenyl ring carbons in neat samples and in dilute cyclohexane solution for anisole, 4‐methylanisole, and 4‐chloroanisole. Similar measurements were made for 2‐methylanisole, 2‐methyl‐4‐bromoanisole, and 2,4,6‐trimethylanisole in dilute cyclohexane solution. Density functional theory (DFT) computations were performed on anisole, 4‐chloroanisole and 2,4,6‐trimethylanisole to obtain the minimum energy structures and the potential energy barriers to the internal rotations of the methoxyl group. The shortest distance between a methoxyl methyl hydrogen and the ortho hydrogen in anisole is 1.920 Å. The DFT results point to steric interactions that arise thereof as the principal source of the energy barriers to the internal rotation of the methyl or of the methoxyl group. The carbon relaxation data are consistent with the existence of noncovalent intermolecular interaction, especially π − π stacking interaction. The nuclear magnetic resonance and DFT results are discussed with reference to the rotational characteristics of the methoxyl methyl and the anisotropy in the reorientational motion of anisole and its derivatives in dilute cyclohexane solution. Copyright © 2012 John Wiley & Sons, Ltd.