15N cross-polarization using the inversion-recovery cross-polarization technique and11B magic angle spinning NMR studies of reference compounds containing B—N bonds

Borane–ammonia complexes, trialcynylborazine and N-trimethylcycloborazane were characterized by 15N and 11B solid-state magic angle spinning (MAS) NMR. They are representative of compounds containing B—N bonds and presenting various N and B environments, either tri- or tetracoordinated. Inversion–recovery cross-polarization (IRCP), a spectral editing technique first introduced for 13C, was extended to 15N. For all compounds, characterized by either weak or strong 15N–1H dipolar couplings, the polarization inversion behavior follows the theoretical equations describing the spin dynamics associated with the IRCP sequence. This technique thus proved to be an excellent tool for identifying 15N sites according to their proton environments. The isotropic chemical shift, quadrupolar coupling constant and asymmetry factor were extracted from the 11B MAS NMR spectra. However, the unusual shape of the center band was tentatively related to the presence of residual 11B–14N dipolar coupling, which could be possibly reduced at higher fields.