NMR study of a single crystal of KHF2. II. proton magnetic shielding tensor

The proton magnetic shielding tensor of the hydrogen bond in the bifluoride ion [FHF]− of KHF2 has been determined at 93 MHz, applying fluorine decoupled proton multiple pulse techniques on the central line of the dipolar spectrum. Principal values (with respect to TMS) of the nearly axially symmetric tensor are σxx = −39.7 ± 1.0, σyy = −32.4 ± 1.0, σzz = +8.8 ± 1.0 ppm. The most shielded direction is the bond axis, the least shielded one is perpendicular to the plane of the ions. The anisotropy of 44.8 ppm is the largest proton anisotropy ever reported so far. The results are analyzed using Gierke and Flygare's atom-dipole approximation of the magnetic shielding tensor. Theory and experiment are brought into close agreement by introducing empirically a reduced mean square electron cloud radius which accounts for the overlap of the shielded nucleus by the electrons of the neighbouring nuclei. The analysis of the proton shielding tensor suggests that the hydrogen-fluorine bond in the [FHF]− is strongly ionic