Trends in Stability for N18 Cages

Abstract

Nitrogen molecules Nx have been extensively studied for their potential as high energy density materials (HEDM). Cages of three-coordinate nitrogen have been studied to determine the structural features that result in the most stable isomers. N12 cages, for example, have been shown to follow a general trend that molecules with more pentagons in the network are more stable than other molecules. Larger Nx cages (24 or more atoms) do not follow this trend, favoring cylindrical structures over spherical ones with larger numbers of pentagons. To determine which trends intermediate-sized Nx cages follow, a series of N18 cages are studied by theoretical calculations of their stability. Geometries are optimized by using Hartree−Fock theory and perturbation theory (MP3), with single energies calculated with the fourth-order perturbation theory (MP4). The correlation-consistent CC-PVDZ basis of Dunning is employed. The major result is a loose trend favoring pentagons, with the most stable molecules being the ones with an optimal combination of pentagons and triangles.