The anisotropic interaction between nitrogen molecules from solid state data

Abstract

The development of an angle-dependent interaction potential for slightly nonspherical molecules is outlined, concentrating on molecular nitrogen. The suggested N2–N2 pair potential combines the radial dependence of the Barker type potential used for the rare gases with a modification of the treatment of anisotropy by the Berne–Pechukas Gaussian overlap model. This potential includes quadrupolar and long range dispersion contributions as determined from experiment or semiempirical studies. The valence potential parameters are determined by a fit of beam scattering and solid state measurements. This potential provides satisfactory agreement with Raman and neutron scattering measurements only when the anharmonic contributions to the lattice potential energy are included. A comparison of the suggested potential with a number of other presently available potentials, determined from a variety of measurements, as well as recommendations as to the limits of possible use are made. Possible steps which would lead to future improvements of the N2–N2 pair potential are outlined.

Hindered internal rotations in Van der Waals molecules and molecular crystals

“…Also these modes do not soften sufficiently with increasing temperature. This is not surprising, however, as the librational displacements are actually not small even at low temperature (estimated [3] root-mean-square dis placement 17 deg at T = 20 K). When the temperature rises to the a -¡3 phase tran sition point, the N 2 rotations in the crystal become more and more free.…”

“…This is the more satisfactory since no adjustments of the ab initio potential to the crystal data have been made (in contrast with the usual semiempirical treatments [3]). For the translational phonon modes the agreement is almost perfect: the frequencies are only 4% higher than the neutron scattering results ( 11% higher if one neglects anharmonicity).…”

“…Although it has also been advocated [3] that molecular crystals and, in particular, their phonon dispersion relations, are very useful in this respect, we note that the route from the intermolecular potential to the phonon frequencies that can be compared with experiment, involves approximations, such as the assumption of pairwise additivity for the potential and the (usually harmonic) model for the lattice dynamics. For simple Van der Waals molecules, on the other hand, accurate dynamical calculations can be performed which lead directly from a given potential to the rovibrational spectrum.…”

“…Apart from extensive work [3,4] on solid N 2 , which has two ordered (a and 7 ) phases and one orientationally disordered (/?) phase, there is also much interest in gas and liquid properties [5][6][7].…”

“…phase, there is also much interest in gas and liquid properties [5][6][7]. An important part of this work is concerned with extracting empirical N 2 -N 2 interaction potentials [3] from the experimental data. Recently, a detailed N 2 -N 2 potential has been obtained from ab initio calculations [8,9].…”