Analyses of ~(11)B, ~(13)C, and ~2H NMR spectra of solid hexamethylborazine, I, provide conclusive evidence for rapid in-plane jumps of the borazine ring at room temperature. Boron-11 NMR spectra of magic-angle spinning (MAS) samples, acquired at low (4.7 T), moderate (9.4 T), and high (18.8 T) external applied magnetic field strengths, have been simulated to yield the ~(11)B nuclear quadrupolar coupling constant (C_1), asymmetry parameter, and isotropic chemical shift; their values at 298 K are 2.98 ± 0.03 MHz, 0.01 ± 0.01, and 36.0 ±0.4 ppm, respectively. Simulations of ~(13)C CP/MAS NMR spectra provide the carbon-boron isotropic indirect spin-spin coupling constant, J_(iso), the sign of C_Q(~(11)B), the relative orientations of the boron electric field gradient (EFG) and the ~(13)C-~(11)B dipolar coupling tensors, and the motionally averaged ~(13)C-~(11)B dipolar coupling constant. Variable-temperature ~2H NMR spectra of a partially deuterated sample of I indicate that the in-plane jumps of the borazine ring are slow with respect to C_Q(~2H)~(-1) (i.e., τ_(jump) ≥ 10~(-4) s) at temperatures less than 130 K. Over the temperature range 180 to 128 K, ~2H NMR line shape analysis yields an activation energy of 30.1 ± 1.5 kJ mol~(-1) for the in-plane jumps of the borazine ring. Although a precise experimental determination of boron chemical shift anisotropy was impeded by intramolecular and intermolecular boron-boron dipolar interactions and heteronuclear nitrogen-boron dipolar interactions, simulations of high-field ~(11)B NMR spectra of a stationary sample of I suggest a value of 55 ± 15 ppm for the motionally averaged span of the chemical shift tensor. Lastly, high-level ab initio and density functional theory calculations provide values of the boron EFG tensor and the boron and nitrogen magnetic shielding tensors for a rigid molecule of I.
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