Investigation of Magnetic Shielding in Xenon Difluoride Using Solid-State NMR Spectroscopy and Relativistic Density Functional Theory

Abstract

The xenon and fluorine magnetic shielding tensors, σ, of XeF2 are characterized using solid-state 129Xe and 19F NMR spectroscopy and nonrelativistic and spin−orbit relativistic zeroth-order regular approximation density functional theory (ZORA DFT). Analysis of 129Xe and 19F NMR spectra acquired with magic-angle spinning at several spinning rates indicates that the Xe and F magnetic shielding tensors are axially symmetric, as dictated by the crystal symmetry. The isotropic 129Xe chemical shift is −1603 ± 5 ppm with respect to OXeF4 (neat liquid, 24 °C) and the Xe magnetic shielding anisotropy, Ω, is 4245 ± 20 ppm, the first anisotropy measured directly for a xenon compound. The parallel component of the experimentally determined xenon chemical shift tensor, δ∥ = −4433 ppm differs from δ(Xe(free atom)) by ∼1000 ppm, providing the first experimental demonstration that relativistic effects play an important role in the nuclear magnetic shielding for xenon. Both the sign and magnitude of the isotropic indirect 129Xe,19F nuclear spin−spin coupling constant are determined, −5560 ± 50 Hz. Analysis of the 19F NMR spectra yield Ω(F) = 150 ± 20 ppm. The ZORA DFT method has been employed to calculate σ(Xe) and σ(F) for isolated XeF2 and XeF4 molecules, as well as σ(Kr) and σ(F) for an isolated KrF2 molecule, at the relativistic and nonrelativistic levels of theory. Spin−orbit relativistic DFT results for Ω(Xe) are in very good agreement with those determined experimentally and highlight the importance of relativistic effects.