Intramolecular motions in a series of crystalline benzylammonium bromides and dibenzylamines studied by CP/MAS NMR

Abstract

A series of 15 compounds including ammonium bromides containing one or two benzyl groups with H, methyl, isopropyl, tert-butyl and tert-amyl substituents and dibenzylamine with N-isopropyl-, N-tert-butyl- and N-tert-amyl substituents have been synthesised and studied by CP/MAS NMR. The results of dynamic NMR studies on the solids suggest that there is a dramatically wide range of molecular motions occurring in this simple series of compounds. A combination of 2D CPEXSY, dynamic line shape analyses and T_1ρ measurements reveals the considerable extent of intramolecular group motions including rotations of methyl, tert-butyl, tert-amyl and phenyl groups. Rates of rotation and activation parameters for these molecular motions are derived where appropriate. In the case of benzyl-tert-butylammonium bromide, where two independent molecules of the compound exist in the asymmetric unit it is shown that the independent processes of tert-butyl rotation in the two molecules have vastly different activation energies that differ by ca. 16 kJ mol^–1. The extent of the motions observed suggests that commonly held prejudices about the rigidity of molecules in crystalline solids need revising.