Crystal structure of hexamethylborazine and the possible impact of directionally preferred rotational dynamics on 14 N overtone solid-state NMR Spectra

Inspired by a report describing the sensitivity of the 14N overtone NMR spectrum to the macroscopic sense of sample rotation with respect to the applied magnetic field (Gan et al., J. Chem. Phys. 149, 064201 (2018)), we consider the possible impact of directionally preferred molecular rotational dynamics on 14N overtone solid-state NMR spectra. Solid hexamethylborazine (HMB) was previously shown to exhibit rapid ring hopping about its principal axis via 2H and 11B solid-state NMR, and is used here as a model without directionally preferred dynamics. We report the single-crystal X-ray structure of HMB and the 14N overtone solid-state NMR spectra of powdered HMB samples. These are the first such spectra for the borazinyl-type functional group. The centres of gravity of the spectra follow the known dependence on magic-angle spinning rate. Spin dynamical simulations of the 14N overtone NMR spectra when the nitrogen site undergoes six-fold site hopping are described. These simulations reveal sensitivity to any directional bias in the six-fold hopping. While the experimental spectra do not allow us to fully explore this effect experimentally, the results suggest that 14N overtone solid-state NMR could provide a unique probe of the efficiency of molecular machinery in favourable cases.