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 T1ρ 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.