Anomalous rotational resonance spectra in magic-angle spinning NMR

Magic-angle spinning NMR spectra of samples containing dilute spin-1/2 pair s display broadenings or splittings when a rotational resonance condition i s satisfied, meaning that a small integer multiple of the spinning frequenc y matches the difference in the two isotropic shift frequencies. We show ex perimental rotational resonance NMR spectra of a C-13(2)-labeled retinal wh ich are in qualitative disagreement with existing theory. We propose an exp lanation of these anomalous rotational spectra involving residual heteronuc lear couplings between the C-13 nuclei and the neighboring H-1 nuclei. Thes e couplings strongly influence the rotational resonance C-13 spectrum, desp ite the presence of a strong radiofrequency decoupling field at the H-1 Lar mor frequency. We model the residual heteronuclear couplings by differentia l 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 w ith experimental results is obtained by using a biexponential transverse re laxation model for each spin site. (C) 1999 Academic Press.