Obtaining high-fidelity [formula omitted] powder spectra in anisotropic media: Phase-cycled Hahn echo spectroscopy

Abstract

In the spectroscopy of isolated spin-1 nuclei in anisotropic media, the spectral distortion resulting from the inability to record the initial part of the free-induction decay was eliminated by using the quadrupolar echo technique. Wideline spin- 1 2 spectroscopy is subject to the same problems associated with finite receiver recovery time as is spin-1 spectroscopy; however, these problems have been generally ignored in the past. Through the use of a very simple and well-known pulse sequence (90°-τ-180°) the chemical shift interaction, among others, can be refocused to generate a so-called Hahn echo. If magnetic dipolar interactions with like nuclei and fluctuating interactions with other nuclei may be neglected, then the refocusing of the magnetization will be complete and an undistorted spin- 1 2 powder spectrum can be obtained by Fourier transforming the signal starting at the peak of the echo. The practical aspects of implementing the Hahn echo technique are discussed. A phase-cycling scheme is proposed to prevent spectral distortion due to misset pulse lengths. The effects of finite pulse lengths are investigated, and the theoretical single pulse and echo distortion factors are demonstrated to differ from those which were determined for quadrupolar echo spectroscopy. Experimental verification of some of the theoretical predictions is provided.