Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors

Abstract

A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupolar nuclei in static powders is analyzed. By acquisition of the QE spectrum during a Carr−Purcell−Meiboom−Gill (CPMG) train of selective π pulses, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum. The applicability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experimental 87Rb (RbClO4 and RbVO3) and 59Co spectra (Co(NH3)5 Cl3), it is demonstrated that the quadrupolar CPMG experiment represents a useful method for studying half-integer quadrupolar nuclei exhibiting large quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 are observed for the samples studied.

High-Field Chlorine NMR Spectroscopy of Solid Organic Hydrochloride Salts: A Sensitive Probe of Hydrogen Bonding Environment

“…The relatively low values of γ and moderately large quadrupole moments for these nuclei make them excellent candidates for high-field NMR studies. Smith has recently reviewed the progress made in solid-state NMR of low-γ nuclei and has summarized the available solid-state NMR parameters for 35 Cl. 2 To date, single-crystal 35 Cl NMR studies have been carried out on NaClO 3 , 6 Mn(CO) 5 Cl, 7 tris sarcosine calcium chloride, 8 NH 4 ClO 4 , 9 and betaine calcium chloride dihydrate.…”

“…Smith has recently reviewed the progress made in solid-state NMR of low-γ nuclei and has summarized the available solid-state NMR parameters for 35 Cl. 2 To date, single-crystal 35 Cl NMR studies have been carried out on NaClO 3 , 6 Mn(CO) 5 Cl, 7 tris sarcosine calcium chloride, 8 NH 4 ClO 4 , 9 and betaine calcium chloride dihydrate. 10 Perchlorate anions have been a popular focus of chlorine solidstate NMR studies due to the high local symmetry at chlorine.…”

“…16 This study focuses on the chlorine NMR interaction tensors of the perchlorate anion in several inorganic perchlorates and establishes chlorine solid-state NMR as a viable and informative technique for the investigation of these types of materials. Their experiments show that the chlorine chemical shifts for a wide variety of perchlorate salts fall in the narrow range of ∼980 to 1005 ppm relative to NaCl(aq) and that the 35 Cl nuclear quadrupolar coupling constants range from 0.3 to 3.0 MHz.…”

Testing the sensitivity limits of 33S NMR: An ultra-wideline study of elemental sulfur

“…MAS has been demonstrated to be useful for measuring small C Q values via the satellite transitions (STs) [6], as well as for studying chemical shielding anisotropy (CSA) [8]. Signal enhancement techniques such as QCPMG [15] or population transfer experiments (which transfer polarization from the STs to the CT [16]) have been used to boost the signal and reduce experimental times significantly [9,10,[12][13][14]. Finally, much larger C Q values have recently been observed from transition metal disulfides [11] and potassium sulfate phases [14] using a combination of high magnetic field strengths, QCPMG enhancement and the ''frequency stepping" method, in which the broad spectra are acquired in a number of separate sections whose widths are determined by the excitation bandwidth of the pulse sequence used.…”

Slow turning reveals enormous quadrupolar interactions (STREAQI)

“…In general, such systems are still studied using static samples, and some variant of frequency sweeping. While some improvement in sensitivity has been gained by spikelet spin-echo methods [9], in general these spectra require cumbersome acquisition of multiple free induction decays across the width of the central transition, which may be several MHz, exceeding the effective spectrometer bandwidth by a factor of ten or more. In many respects, these systems represent the last dogged holdouts, resisting the revolution in NMR that began with the implementation of Fourier transform and coherent averaging methods.…”