ELECTRONIC-STRUCTURE AND CONFORMATIONAL FLEXIBILITY OF 1,4-DIHYDROAZINES AND THEIR 4-YLIDE DERIVATIVES

The conformational flexibility of 1,4-dihydropyridine, 1,4-dihydropyri midine, 1,4-dihydropyridazine, 1,4-dihydro-1,3,5-triazine, and their 4 -oxo, imino, and methylene derivatives was studied by the semiempirica l quantum-chemical AMI method. It was demonstrated that the replacemen t of the methylene group in the dihydroazine ring by the exocyclic dou ble bond results only in an increase in the rigidity of the heterocycl e rather than leading to a loss of its conformational flexibility. It was suggested that nonplanar conformations of rings in ylide derivativ es are stabilized by the nonaromatic cyclic pi-system. introduction of the exocyclic double bond does not cause a substantial change in the pi-electronic structure of the heterocycle. The aromaticity indices l( 6) and l(6)(pi) were calculated. The correlation between these indices and a change in the energy upon bending of the heterocycle was establ ished.