Molecular dynamics and structures of amine boranes of the type R3N.BH3. II. NMR investigation of H3N.BH3

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Abstract
For pt.I see ibid., vol.16, p.6129 (1983). Proton second moment, T1H.T1 rhoHand T111Bmeasurements over the temperature range 330K>T>46K are reported. At low temperatures tunnelling of the protons of the NH3group influences the second moment and T1 rhoHresults seriously. An analysis of the results has yielded a tunnelling frequency ft=1.4 MHz at 85K. In the low-temperature orthorhombic phase (T225K) the BH3groups and possibly the NH3groups rotate freely. The activation energy associated with the BH3motion is 1.4+or-0.1 kcal mol-1. The high degree of plasticity of the crystals in this phase is attributed to the increased disorder. Strong evidence for piezoelectricity has been found in the tetragonal phase. The orientation of the B-N bonds is the same in both phases and, except for a very slow motion that starts at about 300K, no evidence has been found for a reorientation of the entire H3N.BH3unit. The temperature dependence of the unit-cell dimensions is discussed in terms of the reorientational rates of BH3and NH3groups.