How the hydrogen bond in NH${}_4$F is revealed with Compton scattering

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.