Some Calculations on the Hydrogen Bond

In this paper we present a perturbation approach to the calculation of the energies of weak to moderately strong H bonds. The method is based on an extension of London's theory of intermolecular forces to the region of nonnegligible intermolecular overlap. The H-bond energy is obtained as a sum of terms which are of first order (Coulomb, exchange) and of second order (induction, dispersion, charge transfer) in the intermolecular potential. The calculations have been performed for a linear model system A—H···B as a function of the orbital exponents of Atoms A and B, the ionicity parameter k of the A—H bond, and the H···B distance rHB. The results show that stable H bonds may be formed even when A—H is nonpolar and that the dipolar attraction due to A—H polarity is a good measure for comparing the stabilities of H bonds having a common acceptor B and equal rHB. The effect of H-bond formation on the A—H bond length is discussed briefly.