ABINITIO MOLECULAR-ORBITAL STUDY OF 2-MONOSUBSTITUTED AND 4-MONOSUBSTITUTED PYRIDINES

Ab initio molecular orbital calculations have been performed to investigate the structural, energetic, and electronic properties of the 2- and 4-R-pyridines, with R including the isoelectronic saturated groups CH3, NH2, OH, and F and the unsaturated groups C2H3 (vinyl), CHO, and CN. The equilibrium geometry of the pyridine ring is essentially unchanged in the equilibrium structures of the 2- and 4-R-pyridines, independent of the nature of the substituent and its position. In all cases, the 2-R-pyridines are more stable than the corresponding 4-R-pyridines. Except for substitution of F in the 4 position, substitution in the 2 or 4 position of a ύ donating group has a stabilizing effect in pyridine relative to benzene, while substitution of the ύ withdrawing groups CN in the 2 or 4 position or CHO in the 4 position has a destabilizing effect relative to benzene. Analyses have also been made of substituent effects on electron distributions, n and ύ orbital energies, and dipole moments of the substituted pyridines. © 1979, American Chemical Society. All rights reserved.