Intramolecular motions in crystalline phosphonium salts studied by 31P and 13C CP–MAS NMR spectroscopy

Abstract

Three homologous trimethylphosphonium bromides with benzyl (1), 2-phenylethyl (2) and 3-phenyl- propyl (3) groups were synthesised and their internal conformational motions were examined by solid-state NMR techniques. For compounds 2 and 3 a combination of dynamic line shape changes and T_1ρ measurements of ¹³C and ³¹P nuclei allow the activation parameters for trimethylphosphonium group rotation in the crystalline solid to be determined. There is excellent agreement between the three independent but complementary NMR methods on the rates of rotation and the activation parameters for the rotation processes. In 1 the rate of rotation of the trimethylphosphonium group is too rapid to be measured.