Selective dephasing of OH and NH proton magnetization based on (1)H chemical-shift anisotropy recoupling.
A method for selectively suppressing the signals of OH and NH protons in (1)H combined rotation and multiple-pulse spectroscopy (CRAMPS) and in (1)H-(13)C 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 (1)H chemical-shift anisotropies. For NH protons, the (14)N-(1)H dipolar coupling also contributes significantly to this dephasing. The dephasing is achieved by a new combination of heteronuclear recoupling of these anisotropies with (1)H homonuclear dipolar decoupling. Since the 180 degrees 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 (13)C 180 degrees pulses remove the strong dipolar coupling to the nearby (13)C 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.