Proton and boron-11 nuclear spin relaxation and the molecular tumbling of nido-decaborane in perdeuterotoluene solution. An interesting transition in solute–solvent interaction behaviour

Abstract

The variations with temperature of the 1H and 11B longitudinal relaxation times, T1, for B10H14 in CD3C6D5 solution show that there is a transition from anisotropic motion at lower temperatures to essentially isotropic tumbling at higher temperatures. The dominant relaxation mechanisms are (11B–1H) and (1H–1H) dipolar for the 1H nuclei and quadrupolar for the 11B nuclei. The activation energy EA for the high-temperature isotropic tumbling is 9.9(5) kJ mol–1, and analysis of the 1H relaxation at lower temperatures shows activation energies ExA≅ 20, EyA≅ 18 and EzA≅ 15 kJ mol–1 about molecular cartesian axes x, y and z, z being the C2 axis of the molecule. Proton nuclear-shielding behaviour permits description of the modes of solute–solvent interaction and shows that ΔH and ΔS for the motional transition are +25 ± 3 kJ mol–1 and +110 ± 20 J mol–1 K–1, respectively. Some of the observations have important wider implications.