NMR chemical shifts in ionic fluorides

19F MAS NMR Investigation of Strontium Substitution Sites in Ca2+/Sr2+ Fluorapatite Solid Solutions

“…For a given cation M, the σ-bonding contribution to δ(θ) is a σ sin 2 (θ), and the π-bonding contribution is a π cos 2 (θ). 17 The predicted values of the shift tensor components δ 11 , δ 22 , and δ 33 for the peak at 79.6 ppm (configuration B in Figure 5) obtained using a π and a σ values for Ca-F and Sr-F bonds from ref 16 are equal to 137, 51.4, and 36.9 ppm, respectively. The isotropic shift calculated from these three predicted component values and used in the following calculations is 75.1 ppm, which is not too far from the observed shift.…”

“…The isotropic chemical shifts of structures B and C in Figure can be predicted by assuming that the shielding components from individual cation to fluoride anion bonds are additive, as discussed in ref . In that approach, which is based upon the work of Gagarinskii and Gabuda, the 19 F chemical shift differences for different alkaline earth fluorapatites are viewed as arising from different contributions to the paramagnetic term in the Ramsey equation for chemical shielding, the diamagnetic term remaining nearly constant for a negatively charged fluoride anion. The paramagnetic term arises from the overlap of cation and anion orbitals, and by considering the measured chemical shift anisotropies of the fluorapatites it can be broken down into angular-dependent contributions from both σ- and π-bonding, with coefficients a σ and a π , respectively.…”

“…The smallest chemical shift component δ 33 is obtained when the external magnetic field is parallel to a Sr−F bond. From the equations in ref , which are based on those in ref , the chemical shift tensor components δ 11 , δ 22 , and δ 33 of configuration B in Figure can be represented by Here δ(θ)(M) refers to the chemical shift due to the paramagnetic shielding term when the magnetic field makes an angle θ with respect to the M−F bond axis. For a given cation M, the σ-bonding contribution to δ(θ) is a σ sin 2 (θ), and the π-bonding contribution is a π cos 2 (θ) .…”

“…From the equations in ref , which are based on those in ref , the chemical shift tensor components δ 11 , δ 22 , and δ 33 of configuration B in Figure can be represented by Here δ(θ)(M) refers to the chemical shift due to the paramagnetic shielding term when the magnetic field makes an angle θ with respect to the M−F bond axis. For a given cation M, the σ-bonding contribution to δ(θ) is a σ sin 2 (θ), and the π-bonding contribution is a π cos 2 (θ) . The predicted values of the shift tensor components δ 11 , δ 22 , and δ 33 for the peak at 79.6 ppm (configuration B in Figure ) obtained using a π and a σ values for Ca−F and Sr−F bonds from ref are equal to 137, 51.4, and 36.9 ppm, respectively.…”