Mobility of the monomers in the Van der Waals molecule (N2)2. Comparison with N2 crystals

Abstract

The N₂–N₂ intermolecular potential has been obtained from ab initio calculations and represented in two analytic forms: a spherical expansion and a site–site potential (with different sites for each contribution to the potential). It is shown that, in the range of the Van der Waals minimum, the short-range exchange repulsion is the dominant anisotopic contribution, not the multipole–multipole interactions; this repulsion is mainly responsible for the (crossed) equilibrium configuration of the (N₂)₂ dimer. Using this potential in lattice-dynamics calculations for solid N₂(in the ordered α and γ phases) with the harmonic and self-consistent phonon methods, yields generally very good agreement with experimental data: lattice structure, cohesion energy, translational phonon frequencies and their pressure dependence, and pressure dependence of the librational frequencies. The values of the librational frequencies and their temperature dependence are less well reproduced, however, especially going towards the orientational order–disorder, α–β phase transition; this is probably due to the larger amplitudes of the librations in the crystal and the failure of the self-consistent phonon model to deal with these. The (N₂)₂ dimer, for which we have made preliminary (rigid-rotor–harmonicoscillator) calculations is floppier than the crystal. The barriers to internal rotation are rather low (20 and 40 cm^–1, dimer binding energy 125 cm^–1) and only one or two states in each internal-rotation direction correspond with “locked-in” N₂ rotations (librations); the higher states will be (hindered) internal rotations.