Relaxation Processes in Aromatic Methyl-Groups. I. 2-Chloro-Toluene and 2,6-Dichloro-Toluene

Abstract

Three series of selectively deuterated toluenes, 2-chlorotoluenes and 2,6-dichlorotoluenes have been synthesized, and their methyl group ¹H n.m.r. relaxation pathways have been determined by ¹H, ²H and ¹³C n.m.r. spin-lattice relaxation time measurements. ¹H spin-lattice relaxation in the methyl groups of these series occurs predominantly through an intramethyl H-H dipolar mechanism as well as through the spin-rotation mechanism. Dipolar spin-lattice relaxation rates for intramethyl H-H pairs are 0.012, 0.020 and 0.025 s^-1 for toluene, 2,6-dichlorotoluene and 2-chlorotoluene respectively, suggesting a decrease in the rate of methyl group rotation in this order. Ab initio molecular orbital calculations on the same compounds show that the theoretically predicted barrier to methyl group rotation increases in the order toluene < 2,6-dichlorotoluene < 2-chlorotoluene, supporting the experimentally derived results.