S-0 and S-1 states of monochlorophenols: Ab initio CASSCF MO study

Optimized geometries and total energies for the So and S, states of five mo nochlorophenols and phenol were calculated using a 10-electron, 8-orbital C ASSCF/6-31G(d). We investigated the effects of a substituent chlorine atom and the S-1 <-- S-0 excitation on the geometries and charge distributions. It is common to all monochlorophenols that the substituent chlorine atom ma kes the C-O bond shorten and that an empirical rule with respect to the int ernal ring angle concerned with the substituent chlorine atom holds true. T he S-1 <-- S-0 excitation enlarges the carbon ring and reduces the C-H, C-O , and C-CI bond lengths. The O-H bond length and the C-O-H bond angle are a lmost invariant upon excitation. A structural approach based on van der Waa ls radii has clarified that cis-o-chlorophenol has a hydrogen bond in the S -1 state and that the interaction between the chlorine and oxygen atoms in a trans-o-chlorophenol affects its geometries for the S-0 and S-1 states. A s to the origin energies of the S-1 <-- S-0 transitions, the present method can compete in terms of accuracy with a 8-electron, 7-orbital CASSCF/6-31G (d,p). The increasing order of the calculated origin transition energies of monochlorophenols and phenol is in qualitative agreement with experimental results. Zero-point corrections are important in identifying the electroni c spectra of monochlorophenols. The relative stabilities of rotational isom ers, and dipole moments as well, have been also studied in relation to expe rimental results.