Selective dephasing of OH and NH proton magnetization based on 1H chemical-shift anisotropy recoupling

A method for selectively suppressing the signals of OH and NH protons in H-1 combined rotation and multiple-pulse spectroscopy (CRAMPS) and in H-1-C-13 heteronuclear correlation (HETCOR) solid-state NMR spectra is presented. It permits distinction of overlapping CH and OH/NH proton signals, based on the selective dephasing of the magnetization of OH and NH protons by their relatively large H-1 chemical-shift anisotropies. For NH protons, the N-14-H-1 dipolar coupling also contributes significantly to this dephasing. The dephasing is achieved by a new combination of heteronuclear recoupling of these anisotropies with H-1 homonuclear dipolar decoupling. Since the 180degrees pulses traditionally used for heteronuclear dipolar and chemical-shift anisotropy recoupling would result in undesirable homonuclear dephasing of proton magnetization, instead the necessary inversion of the chemical-shift Hamiltonian every half rotation period is achieved by inverting the phases of all the pulses in the HW8 multiple-pulse sequence. In the HETCOR experiments, carefully timed C-13 180degrees pulses remove the strong dipolar coupling to the nearby C-13 spin. The suppression of NH and OH peaks is demonstrated on crystalline model compounds. The technique in combination with HETCOR NMR is applied to identify the CONH and NH-CH groups in chitin and to distinguish NH and aromatic proton peaks in a peat humin. (C) 2002 Elsevier Science (USA).