A COMPARATIVE X-RAY-DIFFRACTION STUDY AND AB-INITIO MO CALCULATIONS ON AMIDOCYANOPYRIDINIUM METHYLIDE

The crystal structure of amidocyanopyridinium methylide, C8H7N3O, has been determined using X-ray diffraction. The compound crystallizes in the monoclinic system, space group P2(1)/c, with cell dimensions a = 4 .888 (1) Angstrom, b = 11.931 (2) Angstrom, c = 13.406 (2) Angstrom, b eta = 105.84 degrees (1) and Z = 4. The structure has been refined to a final R = 0.040 for 2500 reflections. The ylide carbon-pyridinium ni trogen bond distance is 1.422 (4) Angstrom. The refined structure was found to be significantly non-planar. The crystal structure of amidocy anopyridinium methylide is thus the second one, after that of dicyanop yridinium, in which the pyridine ring and the carbanion are almost cop lanar. A comparative study using the ab initio procedure shows good ag reement with X-ray diffraction data. Indeed, standard deviations betwe en the calculated and the experimental values are 0.017 Angstrom and 0 .685 degrees for the bond lengths and bond angles, respectively. The e nergy difference between the calculated and experimental geometries is only 6.902 kcal mol(-1). (C) 1997 Elsevier Science B.V.