Assessing distortion of the AF 6 − (A=As, Sb) octahedra in solid hexafluorometallates(V) via NMR spectroscopy

Abstract

Arsenic and antimony hexafluoride salts have played an important role in the history of both solution and solid-state NMR spectroscopy. Here, solid polycrystalline KAsF6 and KSbF6 have been studied via high-resolution variable temperature 19F, 75As, 121Sb, and 123Sb solid-state NMR spectroscopy at high magnetic field (B0 = 21.14 T). Both KAsF6 and KSbF6 undergo solid-solid phase transitions at approximately 375 and 301 K, respectively. We use variable temperature NMR experiments to explore the effects of crystal structure changes on NMR parameters. CQ(75As) values for KAsF6 at 293, 323, and 348 K are −2.87 ± 0.05 MHz, −2.58 ± 0.05 MHz, and −2.30 ± 0.05 MHz, respectively, the sign determined via DFT calculations. In the higher temperature cubic phase, CQ(75As) = 0 Hz, consistent with the crystal symmetry at the arsenic nucleus in this phase. In contrast, CQ values for 121Sb and 123Sb in the cubic phase of KSbF6 are nonzero. E.g., at 293 K, CQ(121Sb) = 6.42 ± 0.10 MHz, and CQ(123Sb) = 8.22 ± 0.10 MHz. In the higher temperature tetragonal phase (343 K) of KSbF6, these values are 3.11 ± 0.20 MHz and 4.06 ± 0.20 MHz, respectively. CASTEP calculations performed on the cubic and tetragonal structures support this trend. Isotropic indirect spin-spin coupling constants are 1J(75As,19F) = −926 ± 10 Hz (293 K) and −926 ± 3 Hz (348 K), and 1J(121Sb,19F) = −1884 ± 3 Hz (293 K), and −1889 ± 3 Hz (343 K). Arsenic-75 and antimony-121,123 chemical shift values show little variation over the studied temperature ranges.