THE ELECTRONIC-STRUCTURE OF HYDROGEN-BONDING IN ALPHA-OXALIC ACID DIHYDRATE

Compton spectra of alpha-oxalic acid dihydrate single-crystal slices and powder samples were measured in a helium atmosphere using 59.537 keV Am-241 radiation. In the anisotropic case, the scattering vector was oriented parallel to the shortest hydrogen bond, in the direction of the C = O bond, parallel to the C - O bond and perpendicular to the (a, c)-plane. The comparison of theoretical reciprocal form factors, calculated with a local density-approximation (LDA) method for an alpha-(COOH)2.2H2O cluster in triple-zeta basis-set quality, with the experimental data has demonstrated that there is a considerable influence of intermolecular interactions on the electronic structure. Further calculations including the intermolecular interactions of the first coordination sphere by symmetrical orthogonalisation of the LDA wave functions clearly improved the agreement of experiment and theory. A theoretical analysis of the reciprocal form factor B(a) (s) in the direction of the three different hydrogen bonds of alpha-oxalic acid dihydrate on the basis of the cluster calculations has shown that in the region above s = 1.9 angstrom all these bonds are dominated by electrostatic attraction and exchange repulsion of the donor and the acceptor system.