The diazo group as an example of an isolated 15N-14N nuclear spin pair: a static and variable spinning frequency...

The 15N-14N heteronuclear spin pair present in the polycrystalline compound 5-methyl-2-diazobenzenesulphonic acid hydrochloride singly labelled with 15N at either the alpha- or the beta-position is investigated by solid-state nuclear magnetic resonance. The 15N spectrum is obtained both under static and spinning conditions. Unusually well resolved fine structure is observed in both cases and, in particular, the static spectra allow a direct and straightforward measurement of the 15N-14N internuclear dipolar coupling in addition to values of the principal components of the shielding tensors for 15N labelling (at either the alpha- or the beta-positions of the diazo moiety) and their orientation relative to a molecular frame. We also present a preliminary theoretical discussion concerning the origin of the observed spinning frequency dependence of the sideband fine structure.