A study of anisotropy in the momentum density of KHCO₃ using high-resolution Compton scattering

We report directional Compton profiles of KHCO₃ measured along the pi pi direction (parallel to the HCO₃ layers and parallel to the H bonds), the pi sigma direction (parallel to the layers and orthogonal to the H bonds), and the sigma sigma direction (orthogonal to the layers and orthogonal to the H bonds) using a high-resolution Compton spectrometer at the LURE-DCI synchrotron facility in France operating at 19.9 keV. These measurements confirm the significant anisotropy of the electron distribution in KHCO₃, which we associate with the H bonds. This assumption is supported by the fact that the free-atom model (anions HCO₃^- without H bonds) is not able to describe our experimental results. A molecular orbital-LCAO-self-consistent model has been used to calculate the anisotropies with different size clusters, and taking into account or not the environment of the dimeric anion (HCO₃)₂^2$containing two H bonds. The calculated anisotropies based on free K₂(HCO₃)₂ yield the main features of the experimental anisotropies with respect to the pi pi direction. More sophisticated calculations involving K₄(HCO₃)₄ and K₆(HCO₃)₆ with the dimeric anion (HCO₃)₂^2$surrounded by point charges have not improved the agreement with the experiment. Moreover, none of these calculations is able to describe the small measured anisotropy between pi sigma and sigma sigma directions.