Calcium-43 chemical shift and electric field gradient tensor interplay: a sensitive probe of structure, polymorphism, and hydration

Abstract

Calcium is the 5th most abundant element on earth, and is found in numerous biological tissues, proteins, materials, and increasingly in catalysts. However, due to a number of unfavourable nuclear properties, such as a low magnetogyric ratio, very low natural abundance, and its nuclear electric quadrupole moment, development of solid-state (43)Ca NMR has been constrained relative to similar nuclides. In this study, 12 commonly-available calcium compounds are analyzed via(43)Ca solid-state NMR and the information which may be obtained by the measurement of both the (43)Ca electric field gradient (EFG) and chemical shift tensors (the latter of which are extremely rare with only a handful of literature examples) is discussed. Combined with density functional theory (DFT) computations, this 'tensor interplay' is, for the first time for (43)Ca, illustrated to be diagnostic in distinguishing polymorphs (e.g., calcium formate), and the degree of hydration (e.g., CaCl2·2H2O and calcium tartrate tetrahydrate). For Ca(OH)2, we outline the first example of (1)H to (43)Ca cross-polarization on a sample at natural abundance in (43)Ca. Using prior knowledge of the relationship between the isotropic calcium chemical shift and the calcium quadrupolar coupling constant (CQ) with coordination number, we postulate the coordination number in a sample of calcium levulinate dihydrate, which does not have a known crystal structure. Natural samples of CaCO3 (aragonite polymorph) are used to show that the synthetic structure is present in nature. Gauge-including projector augmented-wave (GIPAW) DFT computations using accepted crystal structures for many of these systems generally result in calculated NMR tensor parameters which are in very good agreement with the experimental observations. This combination of (43)Ca NMR measurements with GIPAW DFT ultimately allows us to establish clear correlations between various solid-state (43)Ca NMR observables and selected structural parameters, such as unit cell dimensions and average Ca-O bond distances.

Calculations of solid‐state 43Ca NMR parameters: A comparison of periodic and cluster approaches and an evaluation of DFT functionals

“…The largest residuals in both computational methods arise from Ca(NO 3 ) 2 , an error which has been discussed elsewhere. [11] The slopes (|m| 5 1.35 6 0.06 and |m| 5 1.37 6 0.07 for GIPAW and cluster calculations, respectively) relative to the ideal case (unity) suggests a substantial…”

“…[23] In principle, better agreement with experimental values can be obtained when the optimal model chemistry is used. [11] suggest a systematic overestimation of 43 Ca chemical shifts associated with the PBE functional and Profeta et al report inability of the PBE functional to predict 17 O NMR parameters in CaO. For example, the calculations presented in Ref.…”

“…[23] In principle, better agreement with experimental values can be obtained when the optimal model chemistry is used. Survey of a wide array of model chemistries for 43 Ca is important because several studies have noted difficulties associated with the prediction of NMR parameters of 43 Ca sites, or of nearby sites in the same material [11,24] when limited and selected model chemistries were used. For example, the calculations presented in Ref.…”

“…For example, the calculations presented in Ref. [11] suggest a systematic overestimation of 43 Ca chemical shifts associated with the PBE functional and Profeta et al report inability of the PBE functional to predict 17 O NMR parameters in CaO. [24] To the authors' knowledge, no systematic study has been undertaken to investigate model-chemistry effects on computed 43 Ca NMR parameters.…”

A DFT-based comparative equilibrium study of thermal dehydration and hydrolysis of CaCl2 hydrates and MgCl2 hydrates for seasonal heat storage

“…Computational models can be successfully used to investigate the in-depth behavior of salt hydrates in different temperature regimes. 4,[15][16][17][18] The hydration of Ca 2+ ions has been successfully investigated using Density Functional Theory (DFT), 19,20 molecular dynamics simulations 18,21 and validated by experiments. 22,23 Iype et al 4 characterized the H-bonds present in MgSO 4 hydrates using DFT calculations.…”