For pt.I see ibid., vol.16, p.6129 (1983). Proton second moment, T1
H.T1 rho
H and T1
11B measurements over the temperature range 330K>T>46K are reported. At low temperatures tunnelling of the protons of the NH3 group influences the second moment and T1 rho
H results seriously. An analysis of the results has yielded a tunnelling frequency ft=1.4 MHz at 85K. In the low-temperature orthorhombic phase (T<225K) threefold classical reorientations of the NH3 and BH3 groups have been identified. The activation energy associated with the BH3 reorientation is 6.0+or-0.1 kcal mol-1 while that of the NH3 group is 2.3+or-0.1 kcal mol-1. In the tetragonal phase (T>225K) the BH3 groups and possibly the NH3 groups rotate freely. The activation energy associated with the BH3 motion 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.BH3 unit. The temperature dependence of the unit-cell dimensions is discussed in terms of the reorientational rates of BH3 and NH3 groups.
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