The electronic structure of conjugated systems with large effective charges

Pariser-Parr-Pople (PPP) calculations have been carried out for the sydnone, sydnoneimine, and sydnoneimine cation molecules. Different possibilities of selecting Coulomb two-electron integrals _v were examined. Characteristics of the electronic structure of the five-membered rings and of the exocyclic bonds of the computed molecules were examined. The energies of the lower excited states were calculated by the method of interaction of a finite number of configurations. It was shown that the energy of the lowest triplet level of these molecules must lie close to the energy of the ground state. To some extent this fact explains the considerable reactivity of the given compounds.