Theoretical studies of electronically excited states

The electronic spectrum of CH{dollar}sb3{dollar}Cl, {dollar}rm CHsb2Clsb2{dollar}, and a nonplanar non-conjugated diene (closely related to norbornadiene) are studied theoretically, using multi-configurational second-order perturbation theory (CASPT2) and configuration interaction singles (CIS) methods with extended basis sets. In all systems studied, both the valence and Rydberg states are considered. In the study of CH{dollar}sb3{dollar}Cl the validity of weak coupling limit of the Rydberg electron and the core is discussed. This limit is found to be too simplistic.; The (n {dollar}to sigmasp{lcub}*{rcub}{dollar}) transitions in the electronic spectrum of {dollar}rm CHsb2Clsb2{dollar} are studied. Contrary to earlier assignments, it is suggested that not all the valence (n {dollar}to sigmasp{lcub}*{rcub}{dollar}) transitions contribute to the lowest energy feature in the electronic spectra. The calculations predict that only four of these states contribute to the lowest peak in the spectra and that the other four states are appreciably higher in energy.; The electronic spectra of a non-conjugated diene is calculated. The calculations comprise all valence ({dollar}pitopisp{lcub}*{rcub}{dollar}) and all Rydberg ({dollar}pito3{dollar}s, 3p, and 3d) transitions. The net electronic coupling between the two ethylenic groups is discussed in terms of the through-bond, through-space, and dipole-dipole coupling mechanisms.; Most of the excitation energies calculated with the CASPT2 method are in good agreement with experiment. The CIS excitation energies tend to be too high due to the neglect of correlation energy, but by use of a constant reduction, the CIS results can be brought into fairly good agreement with experiment.; The CIS method is used to study site-selective photochemistry of R-CO copolymer. The electronic couplings of the CO groups in their excited states are studied by means of calculations on model systems. Based on these calculations, the mechanism of photo-degradation of copolymer is explained.