Structure and dynamics of ND3BF3 in the solid and gas phases:: A combined NMR, neutron diffraction, and ab initio study

The decrease in D-A bond lengths, previously reported for some Lewis acid/base complexes, in going from the gas to the solid phases is investigated by obtaining an accurate crystal structure of solid ND3BF3 by powder neutron diffraction. The B-N internuclear distance is 1.554(3) Angstrom, 0.118 Angstrom shorter than the most recent gas-phase microwave value and 0.121 Angstrom shorter than the single molecule geometry optimized (1.672 Angstrom, CISD/6-311++G-(d,p)) bond length. The crystal structure also shows N-(DF)-F-...-B hydrogen bonds. The effects of this change in structure and of intermolecular hydrogen-bonding on nuclear magnetic shielding (i.e., chemical shifts) and the nuclear quadrupolar coupling constants (QCC) are investigated by ab initio molecular orbital and density functional theory calculations. These calculations show that the nitrogen (N-15 and N-14) and boron (B-11 and B-10) chemical shifts should be rather insensitive to changes in r(BN) and that the concomitant changes in molecular structure. Calculations on hydrogen-bonded clusters, based on the crystal structure, indicate that H-bonding should also have very little effect on the chemical shifts. On the other hand, the B-11 and N-14 QCCs show large changes because of both effects. An analysis of the B-10{F-19} line shape in solid (ND3BF3)-B-10 yields a B-11 QCC of +/-0.130 MHz. This is reasonably close an earlier value of +/-0.080 MHz and the value of +/-0.050 MHz calculated for a [NH3BF3](4) cluster. The gas-phase value is 1.20 MHz. Temperature-dependent deuterium T, measurements yield an activation energy for rotation of the ND3 group in solid ND3BF3 of 9.5 +/- 0.1 kJ/mol. Simulations of the temperature-dependent T-1 anisotropy gave an E-a of 9.5 +/- 0.2 kJ/mol and a preexponential factor, A, of 3.0 +/- 0.1 X 10(12) s(-1). Our calculated value for a [NH3BF3](4) cluster is 16.4 kJ/mol. Both are much higher than the previous value of 3.9 kJ/mol, from solid-state proton T-1 measurements.