Electrostatic binding in the first-row AH and A2diatomic molecules

Abstract

The charge migration that converts two overlapping spherical atoms into a bound molecule produces attractive Hellmann-Feynman fields at the two nuclear positions, offsetting the repulsive field that each nucleus encounters on penetrating inside the charge cloud of its neighbour. The magnitudes of these fields correlate positively with the molecular dissociation energies. The contribution of the σ valence orbitals to the field at a first-row nucleus varies regularly from binding in lithium to anti-binding in fluorine. The electrostatic effect of these orbitals is weakened by reversed polarization inside the 2s nodal sphere and is further opposed by a tiny but effective exchange polarization of the core. The major binding role in the first row thus falls to the π orbitals, which, like the σ orbital in hydrogen, have no radial node to inhibit a uniformly binding forward polarization. The net electrostatic interaction of two spherical atoms, derived with neglect of antisymmetry, is always binding. Especially in the heavier first-row A₂ molecules, this classical Coulomb attraction provides more of the binding energy than either charge accrual on the bond axis, which is opposed by the exclusion principle, or contraction towards the nuclei, which is characteristic of hydrogen but hardly occurs in first-row atoms.