GEOMETRIC AND ELECTRONIC-STRUCTURE OF DICYANOFUROXAN BY EXPERIMENT AND THEORY

Dicyanofuroxan (3,4-dicyano-1,2,5-oxadiazole 2-oxide), the precursor t o the novel NCCNO species, has been studied in the solid and gas phase s to obtain both structural and electronic information. The solid-stat e structure determined by X-ray diffraction gives an orthorhombic spac e group Pna2(1), with a = 10.2578(14), b = 10.8818(12) and c = 10.2259 (15) Angstrom. There are two independent molecules with similar geomet ries in the asymmetric unit. The gas-phase molecule is characterized b y HeI photoelectron, HeI and HL(alpha,beta,gamma) photoionization and IR spectroscopies. The vibrational data is also supported by a Raman s tudy of the solid, The equilibrium geometry of dicyanofuroxan obtained from ab initio calculations at the HF and MP2/6-31G levels lends sup port to the crystallographic result of an asymmetric planar five-membe red ring with three quite different N-O bonds, including a very short (and strongly polarized) exocyclic N-oxide group. Nevertheless, both H F and MP2 calculations are in poor quantitative agreement with the sol id-state structure. Density functional theory (B3-LYP) is, however, mu ch more in accord with the crystallographic result, as indeed, it is w ith the vibrational data.