Intermolecular Potentials for N2 Molecules and the Lattice Vibrations of Solid α-N2

Abstract

The lattice energy, crystal structure, and optical properties in the far infrared and Raman spectrum have been used as a basis to test various forms of an intermolecular potential for N₂. The librational mode frequencies observed in the Raman spectrum are exclusively characteristic of the anisotropic part of the intermolecular potential. It was found that the quadrupole–quadrupole interaction term does not give an adequate description of the angle‐dependent part of the intermolecular potential for nitrogen molecules at distances corresponding to the solid. On the other hand, an atom–atom or dumbbell intermolecular potential has been found to give very good agreement between calculated and experimental frequencies of all optical lattice modes. Potentials of the 6–12 type and 6‐exponential types have been used. In order to obtain good quantitative fit to the librational frequencies an effective interatomic distance in the molecule had to be used which is 20% smaller than the actual bond length. Also intensity ratios in the infrared and Raman spectra were used as criteria for the quality of the potential models. In addition it was shown that the molecular orientations as observed in the solid are accounted for by the potential models.