Chemical-shift γ-encoding nuclear magnetic resonance for uniaxially oriented matter under sample spinning

Abstract

Two approaches are proposed by which chemical shift anisotropies (CSAs) in uniaxially oriented matter can be obtained from the positions of sharp peaks under sample spinning. Both rely on the removal of broadening due to the γ dependence of the resonance frequency, where γ is the third angle of the Euler angles describing the orientation of the chemical shift tensor in the sample symmetry-axis frame. In one approach, an inversion pulse train recovers the γ-dependent CSAs, and the free-induction decay is transformed into sharp lines by γ-encoding transformation. In the other, the free-induction decay signal due to γ-dependent anisotropic chemical shifts is observed by rf-field inhomogeneity-compensated rotary resonance, and the CSAs are represented as the positions of sharp lines by Fourier transformation. The effects of finite pulse width, rf-amplitude missetting, rf-field inhomogeneity, and resonance offset are theoretically and experimentally investigated for both approaches. When the orientation of the sample symmetry axis in the CSA principal axis system distributes around the central orientation to some extent, peaks in γ-encoded two-dimensional (2D) spectra under off-magic-angle spinning show two-dimensionally broadened line shapes. It is theoretically shown that the orientational distribution can be directly obtained by transforming the 2D spectrum on the basis of the one-to-one correspondence between the peak position and the symmetry axis orientation. The C13 CSA in uniaxially drawn polyethylene and the C13 CSA with the orientational distribution in uniaxially drawn polypropylene are obtained from the γ-encoded 2D spectra.