Structure and Dynamics in Metal Tetrahydroborates. I. Nuclear Magnetic Resonance Studies of Zirconium and Hafnium Tetrakis(tetrahydroborates)

The identical temperature dependence of the pmr spectra of Zr(BH₄)₄ and Hf(BH₄)₄ is due to the variable rate of 10B and 11B quadrupolar spin-lattice relaxation. Intramolecular rearrangement processes are still rapid at –80°. Quantitative analysis of the spectra as a function of solution viscosity yields results at least partially in accord with the hydrodynamic model for molecular reorientation, though inertial effects are also probably operative. Within experimental error, both molecules have the same nuclear quadrupole coupling constants, which are estimated to be: 10B, 3.5 ± 0.6 MHz; 11B, 1.7 ± 0.3 MHz, and the same activation energies for molecular reorientation, 3.1 ± 0.1 kcal/mol.