Geometric and electronic structure of dicyanofuroxan by experiment and theory

Dicyanofuroxan (3,4-dicyano-1,2,5-oxadiazole 2-oxide), the precursor to the novel NCCNO species, has been studied in the solid and gas phases to obtain both structural and electronic information. The solid-state structure determined by X-ray diffraction gives an orthorhombic space 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 geometries in the asymmetric unit. The gas-phase molecule is characterized by HeI photoelectron, HeI and HL(alpha,beta,gamma) photoionization and IR spectroscopies. The vibrational data is also supported by a Raman study of the solid, The equilibrium geometry of dicyanofuroxan obtained from ab initio calculations at the HF and MP2/6-31G* levels lends support to the crystallographic result of an asymmetric planar five-membered ring with three quite different N-O bonds, including a very short (and strongly polarized) exocyclic N-oxide group. Nevertheless, both HF and MP2 calculations are in poor quantitative agreement with the solid-state structure. Density functional theory (B3-LYP) is, however, much more in accord with the crystallographic result, as indeed, it is with the vibrational data.