Hydrogen bonds in NH4F and NH4HF2 crystals. Comparison of electron densitydistribution obtained by X-ray diffraction and by quantum chemistry

The quality of the Hartree-Fock and the density functional methods for the description of hydrogen bonds is judged by comparing quantitatively the out come of calculations on the hydrogen bonds in ammonium (bi)fluorides with t he results of accurate electron density studies by X-ray diffraction. X-ray data and analysis for NH4F were taken from the literature; those for NH4HF 2 were collected in a single-crystal diffraction experiment. A total of 852 2 reflections was measured, yielding the structure factors of 2333 independ ent reflections. R-int (F-2) = 0.03. A multipole refinement converged to R- w (F2) = 0.034. Quantum chemical calculations were carried out with the CRY STAL, package, using the Hartree-Fock and the density functional methods an d a 6-311G** basis set. Both the local density and the generalized gradient (GGA) version of the latter were applied. To make a comparison with experim ent valid, the theoretical densities were converted to structure factors th at then were subjected to the same multipole refinement as the experimental ones. The differences between the theoretical and the experimental results are shown-by comparing in direct space the electron density in the bonding regions in the crystals and the topological characteristics at the bond cr itical points, in reciprocal space the structure factors as functions of th e scattering angle; The GGA results showed the best agreement with experime nt, with excellent agreement both in the very short and the longer hydrogen bends. The curvatures of the electron density distributions along the hydr ogen bonds and perpendicular to it reveal the character of these bonds.