Terminal and bridging fluorine ligands in TiF₄ as studied by ¹⁹F NMR in solids

To examine bonding nature of fluorine ligands in a metal coordinated system, ¹⁹F high-resolution solid-state NMR has been applied to TiF₄, which bears both bridging and terminal fluorines. Observed 12 isotropic signals are assigned to 12 crystallographically different fluorines (6 terminal and 6 bridging fluorines) in TiF₄ by referring to the calculated isotropic shifts using density functional theory (DFT). The isotropic chemical shift (δ_iso) for terminal F (F_T) appears at high frequency (420-480 ppm from δ(CCl₃F) = 0 ppm) with large shielding anisotropy Δσ ∼ 850 ppm. Whereas the δ_iso and Δσ values for bridging F (F_B) are moderate; δ_iso ∼ 0-25 ppm and Δσ ∼ 250 ppm. The origin of the observed high-frequency shift for F_T is ascribed to the second-order paramagnetic shift with increased covalency, shorter Ti-F bonds, and smaller energy difference between the occupied and vacant orbitals. Examination of the orientation of the shielding tensor relative to the molecular structure shows that the most deshielded component of the shielding tensor is oriented along the Ti-F bond. The characteristic orientation is consistent with a Ti-F σ bond formed by d_YZ of Ti and p_z of F. Further, we show that the selectively observed spinning sideband patterns and the theoretical patterns with the calculated Δσ and η (shielding asymmetry) values are not consistent with each other for F_B, indicating deficiency of the present DFT calculation in evaluating Δσ.