1-(ARYLCHLOROMETHYL)PYRIDINIUM CHLORIDES - INVESTIGATION BY X-RAY SINGLE-CRYSTAL DIFFRACTION AND SEMIEMPIRICAL (PM3, AM1, MNDO) CALCULATIONS

The molecular structures of 1-[chloro(2-methoxyphenyl)methyl]pyridiniu m chloride and 1-[chloro(3-nitrophenyl)methyl]pyridinium chloride were studied by X-ray single crystal diffraction and semiempirical calcula tions (PM3, AMI, and MNDO) in comparison with model compounds to give a deeper insight into reactivity of these pyridinium salts. Feasibilit y of the nucleophilic substitution of the chlorine atom in such cation s finds its explanation in a relatively strong exocyclic N-C bond, whi ch suppresses the substitution of the pyridine ring system. PM3 was fo und to be sufficiently accurate to predict the structure of such catio ns correctly. For both cations, nonsymmetric conformations were observ ed in crystalline phase being mainly determined by the chlorine substi tuents at the exocylic C-alpha atoms. The acidity of the methylene hyd rogens promotes the formation of intermolecular C-H ... Cl- hydrogen b onds in both compounds. (C) 1998 Elsevier Science B.V.