Uniform chemical shift scaling: Application to 2D resolved NMR spectra of rotating powdered samples

Abstract

Chemical shift scaling experiments which reduce the number and intensity of sidebands in high field magic angle sample spinning (MASS) spectra are described. They rely on the fact that the scaling pulse sequence reduces the effective size of the shift anisotropy, so that moderate speed spinning results in spectra devoid of sidebands of appreciable amplitude. In principle, the technique allows a realization of the full signal strength available in high field MASS spectra. In addition a number of other interesting features have emerged from the experiments. First, the problem of decoupling in the presence of a scaling sequence has been solved by arranging the rf field strengths to mismatch the Hartmann–Hahn condition. Second, it is shown that the scaling cycles used in the experiments are uniform over a wide frequency range with respect to both the scaling factor and the linewidth. Third, under certain conditions, the frequencies for scaling and spinning are about the same size and consequently new features appear in the spectra which we refer to as combination sidebands, combination artifacts, and zero and ν_r artifacts. We believe these features will be shared by many multiple pulse/MASS spectra. Finally, we demonstrate that a complete separation of centerbands and sidebands in high field MASS spectra is possible by incorporating chemical shift scaling into a two‐dimensional experiment.