Alkaline Earth Chloride Hydrates: Chlorine Quadrupolar and Chemical Shift Tensors by Solid‐State NMR Spectroscopy and Plane Wave Pseudopotential Calculations

Abstract

A series of alkaline earth chloride hydrates has been studied by solid-state (35/37)Cl NMR spectroscopy in order to characterize the chlorine electric field gradient (EFG) and chemical shift (CS) tensors and to relate these observables to the structure around the chloride ions. Chlorine-35/37 NMR spectra of solid powdered samples of pseudopolymorphs (hydrates) of magnesium chloride (MgCl(2).6H(2)O), calcium chloride (CaCl(2).2H(2)O), strontium chloride (SrCl(2), SrCl(2).2H(2)O, and SrCl(2).6H(2)O), and barium chloride (BaCl(2).2H(2)O) have been acquired under stationary and magic-angle spinning conditions in magnetic fields of 11.75 and 21.1 T. Powder X-ray diffraction was used as an additional tool to confirm the purity and identity of the samples. Chlorine-35 quadrupolar coupling constants (C(Q)) range from essentially zero in cubic anhydrous SrCl(2) to 4.26+/-0.03 MHz in calcium chloride dihydrate. CS tensor spans, Omega, are between 40 and 72 ppm, for example, Omega= 45+/-20 ppm for SrCl(2).6H(2)O. Plane wave-pseudopotential density functional theory, as implemented in the CASTEP program, was employed to model the extended solid lattices of these materials for the calculation of their chlorine EFG and nuclear magnetic shielding tensors, and allowed for the assignment of the two-site chlorine NMR spectra of barium chloride dihydrate. This work builds upon our current understanding of the relationship between chlorine NMR interaction tensors and the local molecular and electronic structure, and highlights the particular sensitivity of quadrupolar nucleus solid-state NMR spectroscopy to the differences between various pseudopolymorphic structures in the case of strontium chloride.

24 T High-Resolution and -Sensitivity Solid-State NMR Measurements of Low-Gamma Half-Integer Quadrupolar Nuclei 35Cl and 37Cl

“…As evident from this figure, 1D experiments at 24 T result in excellent experimental powder lineshapes that fit extremely well with the simulated lineshape, which highlights the importance of the use of such ultra-high field. 19,22-24 a substantial disagreement is observed for the L-histidine·HCl·H2O. This should be attributed to the different crystalline form of L-histidine·HCl·H2O used here (purchased from Kanto Kagaku, Japan, and used without further purification) with respect to that reported earlier.…”

“…Therefore, the quadrupolar and isotropic chemical shift (δiso) parameters were easily extracted from the spectrum observed at 24 T with a SIMPSON simulation: (χQ, η, δiso) = (4.26 MHz, 0.75, 110 ppm) despite these parameters hardly to obtain from that at 14.1 T. These parameters agree well with the previously reported values. 19,20 The distortions in the lineshape observed at 14.1 T are mainly due to the combined effect of the broad lineshape and the dead time in NMR measurements. The first few sampling points of the FID were found to be absent in the NMR experiments during the dead time due to some technical difficulty in observing the signal just after the rf pulses.…”

“…This should be attributed to the different crystalline form of L-histidine·HCl·H2O used here (purchased from Kanto Kagaku, Japan, and used without further purification) with respect to that reported earlier. 19,32 This observation is a good example for showing how quadrupolar/ chemical shift interactions are sensitive to the local environment. It should be interesting to point out the nuclear spin precession can also be observed in the presence of quadrupolar interactions even without the magnetic field.…”

“…[1][2][3][4][5][6] While 1 H, 13 C, 15 N, 19 F and 31 P are routinely observed nuclei in organic-solids, 29 Si represents the majority of NMR observations in inorganic solids. Indeed, being associated with the spin quantum number I = 1/2, and a reasonably high natural abundance and gyromagnetic ratio (γ) which determine the Larmor frequency, these nuclei are relatively easy to observe.…”

“…Since chloride is a quite important nucleus in pharmaceutical, organic, and inorganic chemistry, both 35 Cl and 37 Cl have been extensively studied. [19][20][21][22][23][24][25][26][27] To demonstrate the advantage of the high magnetic field, we propose a simple theoretical model a) to predict the sensitivity enhancement at higher magnetic fields, and b) to estimate the isotope effect. In the present study, this model is experimentally verified utilizing 35 Cl and 37 Cl solid-state NMR at two different magnetic fields.…”