Variable-angle correlation spectroscopy in solid-state nuclear magnetic resonancea)

TLDR

Previous solid‑state NMR correlation methods relied on sudden mechanical flippings or multiple‑pulse sequences. The study introduces a new solid‑state NMR experiment to correlate anisotropic and isotropic chemical shifts of inequivalent nuclei in powdered samples. The method acquires spectra by spinning the sample and recording signals at various angles between the rotation axis and magnetic field, then processes these signals to extract correlation information, as demonstrated on 13C NMR anisotropy of glycine, cysteine, and p‑anisic acid. The variable‑angle‑spinning signals evolve according to a distribution linking isotropic frequencies to chemical‑shift anisotropies, and Fourier transformation yields a two‑dimensional NMR spectrum that correlates isotropic and anisotropic line shapes, as demonstrated on 13C NMR of glycine, cysteine, and p‑anisic acid.

Abstract

We describe here a new solid-state nuclear-magnetic-resonance (NMR) experiment for correlating anisotropic and isotropic chemical shifts of inequivalent nuclei in powdered samples. Spectra are obtained by processing signals arising from a spinning sample, acquired in independent experiments as a function of the angle between the axis of macroscopic rotation and the external magnetic field. This is in contrast to previously proposed techniques, which were based on sudden mechanical flippings or multiple-pulse sequences. We show that the time evolution of variable-angle-spinning signals is determined by a distribution relating the isotropic frequencies of the spins with their corresponding chemical shift anisotropies. Fourier transformation of these data therefore affords a two-dimensional NMR spectrum, in which line shapes of isotropic and anisotropic interactions are correlated. Theoretical and experimental considerations involved in the extraction of this spectral information are discussed, and the technique is illustrated by an analysis of 13C NMR anisotropy in glycine, cysteine, and p-anisic acid.