Interaction of the atoms in molecules of pyridine and its derivatives according to the results of ab initio calculations

Nonempirical quantum-chemical calculations of pyridine and its 2-, 3-, and 4-X-substituted derivatives (X = F, Cl, Br, Me, and Et) by RHF/6-311G(d) and MP2/6-311G(d) methods indicate an alternation of charges on the atoms of the pyridine ring and of the occupancy of their valence p_y-orbitals. This is caused by the polarization of bonds under the action of the charges of the atoms geminal to C(n). Bonding molecular orbitals in these molecules, formed as a result of the p_y-orbitals, occurring in the plane of the pyridine ring, are not an indication (characteristic) of p, π-conjugation between the unshared electron pair of the heteroatom of a substituent X and the π-electron system of the ring. The results of the calculations by these methods did not differ in principle. Dedicated to Academician of the Russian Academy of Sciences M. G. Voronkov in honor of his 85^th birthday. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1671–1681, November, 2006.