INTERPRETATION OF THE HYDROGEN-BOND ENERGY AT THE HARTREE-FOCK LEVEL FOR PAIRS OF THE HF, H2O, AND NH3 MOLECULES

The Pauli blockade method (PB) for studying intermolecular interactions is equivalent to the Hartree-Fock procedure, allowing however an insight into various physically different energy components. The method has been applied to nine hydrogen-bonded complexes to analyze the factors leading to formation of the hydrogen bond. It turned out that at the minimal basis set level of approximation the hydrogen bond is controlled by the electrostatic interaction between the unperturbed proton donor and proton acceptor (about 75-79% of the attractive interaction at the equilibrium distance). The next contribution (about 14-16%) comes from the exchange interaction of monomers, which in the PB method may be interpreted as a correction due to the Pauli exclusion principle to the electrostatic interaction of the electrons with parallel spins. Finally, a still significant contribution (5-6%) is associated with the proton-acceptor polarization due to the field of the proton donor. The component resulting from the polarization of the proton donor by the proton acceptor is usually much smaller. The above-mentioned proton-acceptor deformation is mainly due to the electrostatic interaction with its partner. In contrast to that, the less important proton-donor deformation results from the valence repulsion exerted by the proton acceptor. All the significant components result from the first iteration of the PB method. The PB method turned out to be only moderately more expensive than the conventional SCF procedure and easy to implement into existing SCF programs. © 1990 American Chemical Society.