Charge-density distribution of heptasulfur imide (S7NH)

Abstract

Heptasulfur imide, S₇NH, was studied by X-ray diffraction at both 100 and 300 K: space group Pnma, Z= 4, M_r= 239, a= 7.842(2), b= 13.115(2), c= 7.6219(7)Å, at 100 K; R= 0.028 and 0.029 for 2517 and 994 reflections at 100 and 300 K respectively. The crown shape of the compound is similar to that of cyclooctasulfur, with one sulfur atom replaced by an NH group. The experimental deformation density distribution reveals a large positive accumulation between the sulfur and nitrogen atoms; small accumulations were found between the sulfur atoms. A multipole refinement shows much clearer features in the bonding and lone-pair electron regions. The results of theoretical calculations by the extended-Hückel molecular orbital, AM1 and ab initio methods are compared with experimental results. There is evidence of bent S–N bonds with endocyclic maxima in the deformation density distribution. The electrondensity distribution may be affected by the packing of molecules in the solid so that it differs from that of the isolated molecule. The net atomic charges from the multipole refinement indicate that N is negative, H is positive and S are neutral. However, those obtained from the ab initio method show a positive charge on the sulfur atom attached to the NH group.