Direct and indirect proton-proton coupling in quantum-chemical theory of H-bonded materials

The direct (electrostatic) and indirect mechanisms of proton-proton couplin g in II-bonded solids of different nature and dimensionalities are treated. The 3D crystals of the KH2PO4 family, squaric acid (H2C4O4. 2D) as well as the OD K3H(SO4)(2)-like compounds are examined as examples. We found out t he role of direct and indirect mechanisms in the formation of coupling para meters (J(ij)) of the Ising model, commonly applied to describe the thermod ynamic and dielectric properties of ii-bonded materials. It was shown that the evaluated contribution to any J(ij) due to the direct mechanism does no t exceed 30 K for all the H-bonded materials of interest. In the cases of t he KH2PO4 family and squaric acid these contributions are minor in comparis on with the indirect one, whereas the relative direct contribution in the I sing parameter can be more significant in the case of the K3H(SO4)(2)-like materials. (C) 2000 Elsevier Science B.V. All rights reserved.