Quadrupolar Isotope-Correlation Spectroscopy in Solid-State NMR

Abstract

Quadrupolar solid-state NMR carries a wealth of structural information, including insights about chemical environments arising through the determination of local coupling parameters. Current methods can successfully resolve these parameters for individual sites using sample-spinning methods techniques applicable to quadrupolar I ≥ 1 nuclei, provided second-order central transition broadenings do not exceed by much the spinning rate. For large quadrupolar coupling ( C Q ) values, however, static acquisitions are often preferable, leading to challenges in extracting local structural information. This study explores the use of two-dimensional QUadrupolar Isotope Correlation SpectroscopY (QUICSY) experiments as a means to increase the NMR spectral resolution and enrich the characterization of quadrupolar NMR patterns under static conditions. QUICSY seeks to correlate the solid-state NMR powder line shapes for two quadrupolar isotopes belonging to the same element via a 2D experiment. In general, two isotopes of the same element will have different nuclear quadrupole moments, gyromagnetic ratios, and spin numbers but essentially identical chemical environments. The possibility then arises of obtaining sharp “ridges” in these 2D correlations, even in static samples showing large quadrupolar effects, which lead to second-order line shapes that are several kilohertz wide. Moreover, pairs of quadrupolar isotopes are recurrent in the periodic table and include important elements such as 35,37 Cl, 69,71 Ga, 79,81 Br, and 85,87 Rb. The potential of this approach is explored theoretically and experimentally on two rubidium-containing salts: RbClO 4 and Rb 2 SO 4 . We find that each compound gives rise to distinctive 2D QUICSY line shapes, depending on the quadrupolar and chemical shift anisotropy (CSA) parameters of its sites. These experimental line shapes show good agreement with analytically derived 2D spectra relying on literature values of the quadrupolar and CSA tensors of these compounds. The approach underlined here paves the way toward better characterization of wideline NMR spectra of quadrupolar nuclei possessing different nuclear isotopes.

Deuteron-proton isotope correlation spectroscopy of molecular solids

“…For L-histidine.HCl.H2O deuterated from water-d at room temperature (295 ±3 K), the H/D exchange yields are in the range of 20-36% for different labile sites such as -NH in the heterocycle, NH3 and water protons, however, a high degree of H/D exchange of up to 90% can be obtained by recrystallizing the same compound at elevated temperatures. [19] Relatively high yields of deuteration and shorter relaxation delays associated 2 H sites allow us to rapidly detect a 2 H MAS spectrum (Figure 1b), in which the water-D, ND3 and the D7/D5 peaks (10)(11)(12)(13)(14)(15)(16)(17)(18) ppm are observed.…”

“…However, only a handful of studies have reported the solid-state 2D correlation NMR spectra of isotopes of the same nuclei. [18,19] The limitations arise from the Larmor frequencies of many of these isotopes being too close together to be separated and simultaneously tuned in NMR probes, or due to the large differences in their natural abundance or nuclear spin properties (I = ½ and I >½ ) or a combination of these. For example, spins with large quadrupolar interactions require specific pulse sequences to excite and correlate them in a 2D NMR experiment.…”

“…Examples of acquiring 2D isotope correlation spectra between two quadrupolar nuclei as well as between spin ½ and quadrupolar nuclei have been proposed in recent studies. [18,19] H isotope correlation spectroscopy (iCOSY) approach that benefits from fast MAS and high magnetic fields. [19] Since 2 H is a quadrupolar isotope and exhibits a low natural abundance (I = 1, natural abundance = 0.015%), it was required to partially isotopically enrich the proton sites in small molecules.…”

Visual Display Interface (VDI): A MATLAB Software Library For Simulating and Processing In-Vivo Magnetic Resonance Spectroscopy and Spectroscopic Imaging Data

“…VDI is already in use by several groups and underlies the processing of data in multiple publications over the past several years [14][15][16][17][18][19][20] . To facilitate its widespread dissemination, the current manuscript describes the general philosophy behind the framework, and its main classes and the way they interact among themselves.…”