A QUANTUM-THEORETICAL STUDY OF SOME AROMATIC NITRILES BY THE SEMIEMPIRICAL LCAO–SCF–MO METHOD

Semiempirical LCAO–SCF–MO calculations have been carried out on some aromatic nitriles in an attempt to predict spectral transitions. Mataga's approximation has been applied to the Coulomb repulsion integrals and a set of parameter values which gives a reasonable prediction of the observed singlet–singlet transition energies of pyridine and benzonitrile was used. The calculations were applied to pyridine, benzonitrile, 2-, 3-, and 4-cyanopyridine, 1,2-, 1,3-, and 1,4-dicyanobenzene, 2,6-dicyanopyridine, and 1,2,4,5-tetra cyanobenzene. Computed singlet–singlet transition energies are in fair agreement with experiment. However, calculated singlet–triplet transition energies are, in general, lower than the experimental values by about 1 eV. The SCF ground state MO's and orbital energies are reported together with the wave functions for the four lowest-energy excited singlet states for each molecule.