INTRAMOLECULAR MOTIONS IN A SERIES OF CRYSTALLINE BENZYLAMMONIUM BROMIDES AND DIBENZYLAMINES STUDIED BY CP MAS NMR/

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 NM R. The results of dynamic NMR studies on the solids suggest that there is a dramatically wide range of molecular motions occurring in this s imple series of compounds. A combination of 2D CPEXSY, dynamic line sh ape analyses and T-1 rho measurements reveals the considerable extent of intramolecular group motions including rotations of methyl, tert-bu tyl, tert-amyl and phenyl groups, Rates of rotation and activation par ameters for these molecular motions are derived where appropriate. In the case of benzyl-tert-butylammonium bromide, where two independent m olecules 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 mo l(-1). The extent of the motions observed suggests that commonly held prejudices about the rigidity of molecules in crystalline solids need revising.