A Solid-State Multinuclear Magnetic Resonance Investigation of Hexamethylborazine

Abstract

Analyses of 11B, 13C, 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 11B nuclear quadrupolar coupling constant (CQ), 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 13C CP/MAS NMR spectra provide the carbon−boron isotropic indirect spin−spin coupling constant, Jiso, the sign of CQ(11B), the relative orientations of the boron electric field gradient (EFG) and the 13C−11B dipolar coupling tensors, and the motionally averaged 13C−11B 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 CQ(2H)-1 (i.e., τjump ≥ 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 11B 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.