How the hydrogen bond inNH4Fis revealed with Compton scattering

Abstract

In order to probe electron wave functions involved in the bonding of NH 4 F, we have performed Compton scattering experiments in an oriented single crystal and in a powder. Ab initio calculations of the Compton profiles for NH 4 F and NH 4 Cl are used to enlighten the nature of the bonds in the NH 4 F crystal. As a consequence, we are able to show significant charge transfer in the ammonium ion which is not observable using other methods. Our study provides a compelling proof for hydrogen bond formation in NH 4 F.The distinction between ionic and hydrogen bondings is not always clear. 1 Ammonium salts in general are similar to the potassium and rubidium salts in crystal form and in other physical properties, 2 and thus are clearly ionic in nature. This similarity is due to the size of the ammonium ion, NH 4 + , which is almost equal to the size of K + and Rb + ions. Remarkably, NH 4 F, instead of having a CsCl or NaCl structure like other ammonium salts which are ionic in nature, is isostructural to ice I h with NH 4 + and F − ions alternating in the oxygen positions. Besides, ammonium fluoride and ice can form mixed crystals. 3 The fact that ammonium fluoride deviates from the standard isomorphism and prefers to adopt the ice structure is ascribed to the presence of significant hydrogen bonding in this compound. 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. In this work we investigate the nature of chemical bonding in NH 4 F and show that, using the study of electron momentum distributions, we detect the presence of hydrogen bonding in ammonium fluoride.Recently, Compton scattering, which is inelastic x-ray scattering at large energy and momentum transfers, 10,11 has provided fundamental information on the quantum nature of the hydrogen bond in ice and water. 12-18 The measured Compton profile ͑CP͒ J͑p z ͒ represents the double integral of the ground-state total electron momentum distribution ͑EMD͒ ͑p͒:where p z lies along the scattering vector of the x rays. Our Compton scattering experiments on NH 4 F will show how electronic states are modified by the hydrogen bond in contrast to the ionic scenario of NH 4 Cl.The CPs of the crystalline a and c directions, and the spherical average of a powder were measured at the European Synchrotron Radiation Facility in Grenoble on high energy beamline ͑ID15B͒, using the scanning Compton spectrometer. 19 The energy of incident photons was set to 29.74 keV by use of ...

Oxygen disorder in ice probed by x-ray Compton scattering

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Pressure-induced transitions in solid nitrogen: Role of dispersive interactions

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