Two-dimensional Nutation Echo Nuclear Quadrupole Resonance Spectroscopy

Abstract

We discuss two new two-dimensional nuclear quadrupole resonance experiments, both based on the principle of nutation spectroscopy, which can be used to determine the asymmetry parameter, and thus the full quadrupolar tensor, of spin-3/2 nuclei at zero applied magnetic field. The first experiment is a simple nutation pulse sequence in which the first time period (t1) is the duration of the radiofrequency exciting pulse; and the second (t2) is the normal free-precession of a quadrupolar nucleus at zero-field. After double Fourier-transformation, the result is a 2 D spectrum in which the first frequency dimension is the nutation spectrum for the quadrupolar nucleus at zero-field. For polycrystalline samples this sequence generates powder lineshapes; the position of the singularities, in these lineshapes can be used to determine the asymmetry parameters η in a very straightforward manner, η has previously only been obtainable using Zeeman perturbed NQR methods. The second sequence is the same nutation experiment with a spin-echo pulse added. The virtue of this refocussing pulse is that it allows acquisition of nutation spectra from samples with arbitrary inhomogeneous linewidth; thus, asymmetry parameters can be determined even where the quadrupolar resonance is wider than the bandwidth of the spectrometer. Experimental examples of 35Cl, 81Br and 63Cu nutation and nutation-echo spectra are presented.

Solid-State 137Ba NMR Spectroscopy: An Experimental and Theoretical Investigation of 137Ba Electric Field Gradient Tensors and Their Relation to Structure and Symmetry

“…66,67 The η Q value obtained from our NMR experiments is likely more accurate, since the previously reported η Q value was estimated from calculations and quadrupolar parameters of analogous systems, and not directly measured in the NQR experiments (NQR experiments on spin-3/2 nuclei do not readily permit determination of η Q , 68 unless special techniques are applied). 69,70 The barium atom is coordinated to four oxygen and five chlorine atoms (Scheme 1d), with Ba-O bond distances varying from 2.844 to 2.887 Å and Ba-Cl distances varying from 3.131 to 3.340 Å. 56 The chlorine and oxygen atoms are arranged such that no simple geometrical assignments of shape can be made; in fact, the arrangement of atoms has been described "approximately as a square antiprism with one square face enlarged to accommodate the fifth Cl -ion".…”

Off-Resonance Effects in Two-Dimensional NQR Spectroscopy Using a Single Crystal

“…Recently two-dimensional nutation nuclear quadrupolar resonance (NQR) experiments have been described in which the two-dimensional time-domain signal S(t 1 , t 2 ) is detected during the free induction decay (FID) for time t 2 for varying radiofrequency (rf) pulse widths t 1 (1,2). These experiments have the advantage that the nutation frequencies of chemically equivalent but physically inequivalent (identical electric field gradient (efg) tensors being oriented in different directions) sites are different, leading to a resolution and identification of such sites.…”

Two-dimensional nuclear magnetic resonance correlation spectroscopy at zero field

“…The first such applications [1][2][3] were directed at extracting the asymmetry parameter, ordinarily unavailable for spin 3/2 nuclei at zero field. For spins 5/2 and higher, can be obtained from the ratios of two or more spectral frequencies observable for each species.…”