Signal-to-noise enhancement of NMR spectra of solids using multiple-pulse spin-echo experiments

Abstract

Three multiple-pulse spin-echo experiments that have been used to enhance the sensitivity of broad powder patterns obtained from NMR spectra of spin-1/2 and half-integer quadrupolar nuclei in polycrystalline solids are examined. Specifically, the Carr-Purcell Meiboom-Gill (CPMG), the Mansfield-Waugh (MW-4), and a modified version of the CPMG experiment (modified-CPMG) are described and compared. Use of these pulse sequences results in transforming the broad powder lineshapes into a series of narrow spikelets whose overall envelope mimics that obtained from the normal Hahn-echo experiment. The CPMG and modified-CPMG methods can result in a considerable saving in experiment time and are particularly powerful in studying the central transition of halfinteger spin quadrupolar nuclei. Artifacts generated by the MW-4 sequence or caused by imperfections in the pulses in the CPMG sequences are explained using a product operator analysis.

Recent technique developments and applications of solid state NMR in characterising inorganic materials

“…4(b)) consists of a train of selective π-pulses which refocus the magnetisation in the transverse plane allowing multiple spin-echoes to be recorded during each acquisition, thereby enhancing the amount of signal obtained in each scan which has been reviewed in detail [27,28]. The last period of the pulse sequence is extended to ensure full acquisition of the free induction decay.…”

Direct detection of nitrogen-14 in solid-state NMR spectroscopy

“…This is now a widely-used signal enhancement technique in solid-state NMR, and has been reviewed elsewhere [80]. The defining feature of QCPMG is an excitation pulse followed by a repeating loop containing both a refocusing pulse and an acquisition period.…”

Solid-state NMR spectroscopy of the quadrupolar halogens: chlorine-35/37, bromine-79/81, and iodine-127

“…157 We refer the reader to the recent reviews by Wasylishen and coworkers on these topics. 158,159 These methods have thus far not been widely applied to the quadrupolar halogens.…”

“…It is difficult to speculate on the reason that such large spans were reported previously, as no spectra are shown in the paper of Segel and Vyas. 209 To deal with the very broad 127 A recent report by Wasylishen and coworkers 159 has demonstrated the utility of enhanced-QCPMG methods for the acquisition of 127 I NMR specta of periodates, e.g. KIO 4 .…”