Methylene spectral editing in solid-state 13C NMR by three-spin coherence selection

Abstract

A robust new solid-state nuclear magnetic resonance (NMR) method for selecting CH 2 signals in magic-angle spinning (MAS) 13C NMR spectra is presented. Heteronuclear dipolar evolution for a duration of 0.043 ms, under MREV-8 homonuclear proton decoupling, converts 13C magnetization of CH 2 groups into two- and three-spin coherences. The CH 2 selection in the SIJ (C H H) spin system is based on the three-spin coherence S x I z J z , which is distinguished from 13C magnetization ( S x ) by a 1H 0°/90° pulse consisting of two 45° pulses. The two-spin coherences of the type S y I z are removed by a 13C 90° x-pulse. The three-spin coherence is reconverted into magnetization during the remainder of the rotation period, still under MREV-8 decoupling. The required elimination of 13C chemical-shift precession is achieved by a prefocusing 180° pulse bracketed by two rotation periods. The selection of the desired three-spin coherence has an efficiency of 13% theoretically and of 8% experimentally relative to the standard CP/MAS spectrum. However, long-range couplings also produce some three-spin coherences of methine (CH) carbons. Therefore, the length of the 13C pulse flipping the two-spin coherences is increased by 12% to slightly invert the CH signals arising from two-spin coherences and thus cancel the signal from long-range three-spin coherences. The signal intensity in this cleaner spectrum is 6% relative to the regular CP/TOSS spectrum. The only residual signal is from methyl groups, which are suppressed at least sixfold relative to the CH 2 peaks. The experiment is demonstrated on cholesteryl acetate and applied to two humic acids.