Optimized geometries and total energies for the S0 and S1 states of five monochlorophenols and phenol were calculated using a 10-electron, 8-orbital CAS SCF/6-31G(d). We investigated the effects of a substituent chlorine atom and the S1 ← S0 excitation on the geometries and charge distributions. It is common to all monochlorophenols that the substituent chlorine atom makes the C-O bond shorten and that an empirical rule with respect to the internal ring angle concerned with the substituent chlorine atom holds true. The S1 ← S0 excitation enlarges the carbon ring and reduces the C-H, C-O, and C-Cl bond lengths. The O-H bond length and the C-O-H bond angle are almost invariant upon excitation. A structural approach based on van der Waals radii has clarified that cis-o-chlorophenol has a hydrogen bond in the S1 state and that the interaction between the chlorine and oxygen atoms in a trans-o-chlorophenol affects its geometries for the S0 and S1 states. As to the origin energies of the S1 ← S0 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 electronic spectra of monochlorophenols. The relative stabilities of rotational isomers, and dipole moments as well, have been also studied in relation to experimental results.
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