Molecular dynamics and structures of amine boranes of the type R3N.BH3. III. NMR investigation of (CH3)3N.BH3

Abstract

For pt.II see ibid., vol.16, p.6137 (1983). Proton second moments, spin-lattice relaxation times in the laboratory and rotating frames and 11B spin-lattice relaxation times in the laboratory frame have been measured as functions of temperature. The results can only be accounted for in terms of non-equivalent molecules in the unit cell in the ratio 2:1. Phase I (T<344K) is dominated by threefold reorientations of BH3 and CH3 groups and the (CH3)3N moiety. Methyl and borane groups have the same activation energies associated with their threefold reorientations. Evidence has been found for the softening of a lattice mode in phase II (359>T>344K). The shallow T1 rho (H) minimum in this phase is attributed to this mechanism. In phase III (366>T>359K) T1(B)<<T1(H) due to the strong quadrupolar relaxation of the 11B spin system.