A deuterium NMR spectroscopic study of solid BH3NH3

Deuterium nuclear magnetic resonance (NMR) powder spectra and spin-lattice relaxation times (T-1) are used to measure the deuterium quadrupolar coupling constants (QCCs) chi(BD) and chi(ND) and to investigate Chemolecular reorientation of the BD3 and ND3 groups in solid deuterated borane monoammoniate, BD3NH3 and BH3ND3, respectively. In the high-temperature, tetragonal, phase (above 225 K) the following Arrhenius parameters are obtained from the temperature;dependent T-1: E-a= 5.9 +/- 0.5 kJ/mol and tau(infinity),= 1.1 x 10(-13) s for BD3NH3;E-a 7.3 +/- 0.8 kJ/mol and tau(infinity), = 4.4 x 10(-14) s for BH3ND3. In the low-temperature, orthorhombic, phase the following parameters are obtained: E-a = 26.4 +/- 1.4 kJ/mol and = 1.2 x 10(-17) a for BD3NN3; E-a,= 13.7 +/- 0.9 kJ/mol and tau(infinity) = 5.7 x 10(-15) s for BH3ND3. Here tau(infinity) is proportional to the inverse of the usual Arrhenius preexponential factor, A. Deuterium Line shape measurements for the low-temperature phase of BD3NH3 yield E-a = 25 =/- 2 kJ/mol and tau(infinity) = 4.7 x 10(-19) s. These dynamic factors indicate that the molecule is probably undergoing whole molecule rotation above the phase transition but the BH3 and NH3 groups are undergoing uncorrelated motion in the low-temperature phase. Deuterium quadrupolar coupling constants of 105 +/- 10 and 200 +/- 10 kHz were determined for BD3NH3 and BH3ND3, respectively. Molecular orbital (MO) calculations (CI(SD)/6-31G(d,p)// MP2/6-31G(d,p)) for the isolated molecule yield values of 143 and 255 kHz. MO calculations also show that the deuterium quadrupolar coupling constants chi(BD) and chi(ND) are relatively insensitive to all molecular structural parameters except the B-H and N-H bond lengths, respectively. It is suggested that the large decrease in the QCC on going from the gas phase to the solid state may be due to a slight lengthening of the B-H and N-H bonds, possibly a result of attractive B-H ... H-N interactions.