An ab initio study of the 19F nuclear magnetic resonance chemical shielding tensor in the hydrogen difluoride ion FHF − and its variation with bond length and angle deformation

Abstract

The hydrogen difluoride ion FHF has long been of interest as a prime example of strong hydrogen bonding [ 11. Structural studies of simple salts of the anion, using in the main neutron diffraction techniques [2,3] but including some solid state NMR investigations [4-61, have consistently revealed a linear symmetric structure for the anion (Dh). One exception is the p-toluidinum salt, in which the hydrogen difluoride anion is reported to be asymmetric owing to the formation of a secondary F. . H . . . N hydrogen bond between the anion and the organic cation [7]. The hydrogen-bond energy has been reported from thermochemical estimates and ab initio theoretical calculations to lie in the range 155-255 kJmol_’ [l]. A value in this range is typical of that expected for a single covalent bond energy. This suggests that it is appropriate to regard the hydrogen difluoride anion as a covalently bonded species and to use methods of calculation appropriate to this type of bonding. In this short communication we report the results of an ab initio study of the NMR chemical shielding tensor for an isolated symmetric hydrogen difluoride anion. It is well known that secondorder properties such as the chemical shielding