N2–N2 interaction potential from a b i n i t i o calculations, with application to the structure of (N2)2

Abstract

The short range electrostatic and (first order) exchange contributions to the N₂–N₂ interaction energy have been calculated ab initio as a function of the N₂ orientations and the distance (139 geometries). Using a numerical integration procedure, the results have been represented analytically in the form of a spherical expansion. At R=0.3 nm this expansion is accurate to better than 0.5% if we include the first 18 independent terms, to 2% if we truncate after L_A=L_B=4, and to 16% if we truncate after L_A=L_B=2. In combination with the long range multipole expansion results (electrostatic R⁻⁵, R⁻⁷, R⁻⁹ terms, dispersion R⁻⁶, R⁻⁸, R⁻¹⁰ terms) calculated by Mulder et al., this yields an anisotropic N₂–N₂ interaction potential in the region of the van der Waals minimum, which can be fairly well represented also by a site–site model. The potential is in good agreement with the available experimental data for the gas phase and for the ordered (α and γ) crystal phases of solid N₂. The structure of the van der Waals molecule (N₂)₂ is discussed; its energy is lowest for the crossed structure: ΔE_m=1.5 kJ/mol, R_m=0.35 nm (for the isotropic potential the well characteristics are ΔE_m=0.75 kJ/mol and R_m=0.417 nm). The (staggered) parallel and the T‐shaped structures are slightly higher in energy. The internal N₂ rotation barriers vary from 0.2 kJ/mol (17 cm⁻¹) to values comparable with the dissociation energy.