STRUCTURE OF DICYANO-PYRIDINIUM METHYLIDE AND AMIDOCYANO-PYRIDINIUM METHYLIDE USING AB-INITIO AND SEMIEMPIRICAL METHODS

Taking into account the X-ray diffraction patterns of dicyano-pyridini um methylide and amidocyano-pyridinium methylide, comparative ab initi o and semi-empirical studies have been performed. The aim of this stud y is to elucidate some electronic and structural properties of the cyc loimmonium ylides. In fact, we found that the positive charge of the y lide nitrogen atom is delocalized on the pyridine ring. The ylide carb on atom is found to be insignificantly charged. This proves that the f ormal negative charge often accepted for the ylidic carbanion is deloc alized. Thus, we demonstrate the existence of the resonance interactio n between the ylidic carbanion and the positive aromatic ring as a fac tor of stability for planar cycloimmonium ylides. The negative charges found for the cyclic carbon atoms adjacent to the ylide nitrogen atom precludes any nucleophilic addition on the or carbon atoms in the het erocyclic ring. This is in good agreement with the experiments, where no nucleophilic addition on such atoms has ever been observed. Among t he six semi-empirical methods used in this paper, the AM1 method is th e most appropriate in the study of planar cycloimmonium ylides. This m ethod is found to reproduce the experimental values and ab initio resu lts with good accuracy. (C) 1997 Elsevier Science B.V.