Anomalous rotational resonance spectra in magic-angle spinning NMR

Magic-angle spinning NMR spectra of samples containing dilute spin-1/2 pairs display broadenings or splittings when a rotational resonance condition is satisfied, meaning that a small integer multiple of the spinning frequency matches the difference in the two isotropic shift frequencies. We show experimental rotational resonance NMR spectra of a C-13(2)-labeled retinal which are in qualitative disagreement with existing theory. We propose an explanation of these anomalous rotational spectra involving residual heteronuclear couplings between the C-13 nuclei and the neighboring H-1 nuclei. These couplings strongly influence the rotational resonance C-13 spectrum, despite the presence of a strong radiofrequency decoupling field at the H-1 Larmor frequency. We model the residual heteronuclear couplings by differential transverse relaxation of the C-13 single-quantum coherences. We present a superoperator theory of the phenomenon and describe a numerical algorithm for rapid Liouville space simulations in periodic systems. Good agreement with experimental results is obtained by using a biexponential transverse relaxation model for each spin site. (C) 1999 Academic Press.