Cross Polarization with High-Speed Magic-Angle Spinning

Abstract

Cross-polarization magic-angle-spinning 13C NMR has become an important analytical method for the study of organic solids. At high field strengths, the high magic-angle-sample-spinning (MASS) rates required to suppress spinning sidebands can substantially suppress the dipolar couplings among the abundant protons. This results in dramatic changes in the cross-polarization (CP) dynamics. For samples with both weak homonuclear and heteronuclear dipolar couplings, the matching condition is only 1 ∼ 2 kHz wide at 10 kHz MASS rates. The maximum CP signal for a sample which fills the receiver coil is also rather weak, with an amplitude which is nearly the same as the Bloch decay signal. In addition, intensity distortions are observed in CP spectra due to resonance-offset effects. In this paper it is shown that if the phases of the two RF channels are simultaneously inverted many times during the CP contact, the matching width can be broadened to about 8 kHz, the S magnetization doubled, and intensity distortions significantly reduced. These useful features of rapid simultaneous phase-inversion CP with high-speed MASS are shown by theory to be the result of spin dynamics different than those encountered in static samples where slower phase inversion is employed.