Lowest singlet excited state geometries, rotational constants and molecular electrostatic potentials of some substituted benzenes: an ab initio study

Ground and lowest singlet excited state geometries of certain substituted benzenes (fluorobenzene, chlorobenzene, p-difluorobenzene, p-dichlorobenzene, p-fluorochlorobenzene and aniline) were optimized using the ab initio RHF procedure employing the 6-31+G* basis set. The calculations were also carried out using the 6-311++G** basis set for two molecules. Excited states were generated using configuration interaction involving singly excited configuration (CIS). The calculated lowest singlet transition energies of the molecules agree with experimental ones satisfactorily. It is found that our calculations reproduce the ground and excited state rotational constants A, B and C of the molecules obtained from high-resolution spectroscopy quite well. Excited state geometries of the molecules have not been determined experimentally precisely but certain approximate estimates of the changes in bond lengths and bond angles consequent to excitation are available in the literature. There is a satisfactory agreement between these and our calculated changes in ring bond lengths and bond angles of the molecules consequent to excitation. Studies of excited state molecular electrostatic potential (MEP) maps are scarce in the literature. Our study of ground and excited state MEP maps of the molecules has revealed several interesting features some of which, e.g. the ortho, para directing property of the substituents in the ground state, are in agreement with experiment. It is indicated that reactions at the ortho, para positions in the ground state would take place in planes located much above the ring planes and there is an appreciable modification in this respect following excitation of the molecules. (C) 2000 Elsevier Science B.V. All rights reserved.