High-Resolution ¹⁹F MAS NMR Spectroscopy: Structural Disorder and Unusual J Couplings in a Fluorinated Hydroxy-Silicate

Abstract

High-resolution (19)F magic angle spinning (MAS) NMR spectroscopy is used to study disorder and bonding in a crystalline solid. (19)F MAS NMR reveals four distinct F sites in a 50% fluorine-substituted deuterated hydrous magnesium silicate (clinohumite, 4Mg(2)SiO(4)·Mg(OD(1-x)F(x))(2) with x = 0.5), indicating extensive structural disorder. The four (19)F peaks can be assigned using density functional theory (DFT) calculations of NMR parameters for a number of structural models with a range of possible local F environments generated by F(-)/OH(-) substitution. These assignments are supported by two-dimensional (19)F double-quantum MAS NMR experiments that correlate F sites based on either spatial proximity (via dipolar couplings) or through-bond connectivity (via scalar, or J, couplings). The observation of (19)F-(19)F J couplings is unexpected as the fluorines coordinate Mg atoms and the Mg-F interaction is normally considered to be ionic in character (i.e., there is no formal F-Mg-F covalent bonding arrangement). However, DFT calculations predict significant (19)F-(19)F J couplings, and these are in good agreement with the splittings observed in a (19)F J-resolved MAS NMR experiment. The existence of these J couplings is discussed in relation to both the nature of bonding in the solid state and the occurrence of so-called "through-space" (19)F-(19)F J couplings in solution. Finally, we note that we have found similar structural disorder and spin-spin interactions in both synthetic and naturally occurring clinohumite samples.

Computational 19F NMR. 1. General features

“…Some early computational work (aimed at benchmarking and validation) included F 2 as test molecule, which proved to be challenging [26][27][28]. Interest in modeling 19 F shifts has steadily continued, and several papers have appeared in the last decade aimed at general structural issues, solution chemistry and physical organic chemistry, [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] solid-state issues (especially the prediction of chemical shift tensors), [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] J-couplings [65][66][67][68][69][70][71][72]…”

Modeling Ti/Ge Distribution in LiTi2–xGex(PO4)3 NASICON Series by 31P MAS NMR and First-Principles DFT Calculations

“…[46][47][48] In order to maintain geometrical restrictions deduced by XRD structural refinements, different subgroups of R͞ 3c have been considered to describe octahedral cation arrangements. In Figure 4, subgroups relating R͞ 3c and R3 S.G. are analyzed.…”

Exploiting Periodic First-Principles Calculations in NMR Spectroscopy of Disordered Solids

“…4 One advantage of such calculations is that it is possible to vary systematically the values of both parameters and observe the effect upon the calculation. An example is shown in Figure 1a, where the calculated 29 Figure 1b). For nuclei with I = 1/2, such as 29 Si, the most commonly calculated parameter is the isotropic shielding, σ iso .…”

“…An example is shown in Figure 1a, where the calculated 29 Figure 1b). For nuclei with I = 1/2, such as 29 Si, the most commonly calculated parameter is the isotropic shielding, σ iso . Although the anisotropy of the shielding tensor (i.e., its magnitude or "span", Ω (= δ 11 -δ 33 ), and shape or "skew", κ (= 3(δ 22 -δ iso )/Ω) is also calculated, this is often more difficult to measure experimentally when spectra contain multiple distinct resonances (as it is removed by the rapid magic-angle spinning (MAS) used to improve resolution and sensitivity), and only more recently have experiment and calculation been compared.…”