Assignment of electronic transitions by geometry optimization

Abstract

Adiabatic excitation energies, excited state geometries, excited state charges, bond orders and dipole moments have been obtained for HCN, CO2,H2CO, HFCO, FzCO, ethylene, trans-butadiene, furan, pyrrole and uracil using the SINDO1 semi-empirical method with configuration interaction. Our results generally agree with those of ab initio calculations and experiment satisfactorily. Geometry optimization is found to mix configurations differing in their allowedness in vertical excitation from the ground state, which in turn helps in the assignment of spectral transitions. The V excited singlet state of trans-butadiene and various excited states of furan, pyrrole and uracil have been found to be appreciably non-planar. The single and double CC bonds are found to exchange positions due to the lowest triplet and singlet transitions of furan and pyrrole. The first triplet and first singlet transitions of uracil have been found to be of zr-T r* and ~r -or* types respectively in agreement with recent experimental findings.

Applied quantum chemistry

“…The combination of CNDO/2 geometry optimization with CNDO/S transition energy calculations proved to be a successful compromise in some cases [1224]. Other approaches are the M NDO-CI method [1249] and a special INDO-type parametrization scheme, the SINDO with Cl [919].…”