Hydrogen Bonds in NH₄F and NH₄HF₂ Crystals. Comparison of Electron Density Distribution Obtained by X-ray Diffraction and by Quantum Chemistry

Abstract

The quality of the Hartree−Fock and the density functional methods for the description of hydrogen bonds is judged by comparing quantitatively the outcome of calculations on the hydrogen bonds in ammonium (bi)fluorides with the results of accurate electron density studies by X-ray diffraction. X-ray data and analysis for NH4F were taken from the literature; those for NH4HF2 were collected in a single-crystal diffraction experiment. A total of 8522 reflections was measured, yielding the structure factors of 2333 independent reflections. Rint (F2) = 0.03. A multipole refinement converged to R w (F2) = 0.034. Quantum chemical calculations were carried out with the CRYSTAL package, using the Hartree−Fock and the density functional methods and 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 experiment valid, the theoretical densities were converted to structure factors that 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 critical points, in reciprocal space the structure factors as functions of the scattering angle. The GGA results showed the best agreement with experiment, with excellent agreement both in the very short and the longer hydrogen bonds. The curvatures of the electron density distributions along the hydrogen bonds and perpendicular to it reveal the character of these bonds.

Anionic Species (FH)xF- in Room-Temperature Molten Fluorides (CH3)4NF·mHF

“…Moreover, the position of this peak suggests that the (FH) 2 F - anion may be the major anionic species in the NH 4 F·2.0HF melt, considering that the distance of F−F correlations for (FH) x F - anions ( x = 1−5) are 2.253, 2.349, 2.425, 2.491, and 2.542 Å, respectively. The peak at 3.0 Å (peak 4) indicates the distance of the N−F interaction between NH 4 + and (FH) x F - ions in the liquid state because its distance in the NH 4 F crystal is 2.8 Å . In the g ( r ) for the NH 4 F·3.0HF melt, the peak assigned to the F−F correlation in the (FH) x F - anion shifted to 2.40 Å from 2.36 Å in that for a NH 4 F·2.0HF melt.…”

How the hydrogen bond inNH4Fis revealed with Compton scattering

“…4,5 While ice has been the subject of many studies, NH 4 F has not been as widely studied. The crystal structure has been determined by Adrian and Feil, 6 the electrostatic properties have been studied by van Beek et al, 7 and Ab initio calculations have been carried out by Alavi et al 8 and by van Reeuwijk et al 9 Given the ionic nature of ammonium salts, it is tempting to consider the force between the NH 4 + and F − ions to be essentially an electrostatic interaction. Nevertheless, charge-density distribution does not provide a good criterion for establishing the existence of hydrogen bonds, as shown, for example, by Alavi et al 8 for this compound.…”

Hydrogen bonding probes electron density variations at the basic center in substituted alkyl benzoates: Theory and experiment

“…Electron densities and their distribution over the crystal space may be obtained from high-resolution X-ray diffraction studies. [10][11][12] For separate molecules, however, theoretical computations remain the only accessible option.…”