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 pip i direction (parallel to the HCO3 layers and parallel to the H bonds), the pisigma direction (parallel to the layers and orthogonal to the H bonds), and the sigmasigma direction (orthogonal to the layers and or thogonal 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 assum ption is supported by the fact that the free-atom model (anions HCO3- without H bonds) is not able to describe our experimental results. A m olecular orbital-LCAO-self-consistent model has been used to calculate the anisotropies with different size clusters, and taking into accoun t or not the environment of the dimeric anion (HCO3)(2)2- containing c onta two H bonds. The calculated anisotropies based on free K2 (HCO3)2 yield the main features of the experimental anisotropies with respect to the pipi direction. More sophisticated calculations involving K4(H CO3)4 and K6(HCO3)6 with the dimeric anion (HCO3)2(2-) surrounded by p oint charges have not improved the agreement with the experiment. More over, none of these calculations is able to describe the small measure d anisotropy between pisigma and sigmasigma directions.