The influence of the heteronuclear dipolar interaction on nuclear magnetic resonance spectra of quadrupolar nuclei

A theoretical analysis is presented of the influence of spin-spin interactions on the nuclear magnetic resonance (NMR) spectra of quadrupolar nuclei. Specifically, both the dipolar and J coupling between a spin-1/2 nucleus and a spin-3/2 nucleus are considered. Results of the analysis indicate that in practical situations the dipolar interaction between the two nuclei does not cause ‘residual dipolar couplings’ in multiple-quantum magic-angle-spinning NMR spectra where only symmetric transitions, m ↔ −m, are involved. However, the dipolar interaction is predicted to perturb any given pair of satellite transitions, m ↔ m −1 and 1 − m ↔ −m (m ≠ 1/2), differently. Although this effect may be difficult to observe for powder samples, it should be observable in single-crystal NMR spectra of quadrupolar nuclear which are strongly dipolar coupled to a neighbouring spin-1/2 nucleus. A vector model is introduced to supplement the perturbation approach and provide a simple rationalization of the dipolar effects in NMR spectra of spin pairs involving quadrupolar nuclei.