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

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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 LA=LB=4, and to 16% if we truncate after LA=LB=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: ΔEm=1.5 kJ/mol, Rm=0.35 nm (for the isotropic potential the well characteristics are ΔEm=0.75 kJ/mol and Rm=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.