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 b enzenes (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 carr ied out using the 6-311++G** basis set for two molecules. Excited states we re generated using configuration interaction involving singly excited confi guration (CIS). The calculated lowest singlet transition energies of the mo lecules agree with experimental ones satisfactorily. It is found that our c alculations reproduce the ground and excited state rotational constants A, B and C of the molecules obtained from high-resolution spectroscopy quite w ell. Excited state geometries of the molecules have not been determined exp erimentally precisely but certain approximate estimates of the changes in b ond lengths and bond angles consequent to excitation are available in the l iterature. There is a satisfactory agreement between these and our calculat ed 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 st ate MEP maps of the molecules has revealed several interesting features som e 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 re actions at the ortho, para positions in the ground state would take place i n planes located much above the ring planes and there is an appreciable mod ification in this respect following excitation of the molecules. (C) 2000 E lsevier Science B.V. All rights reserved.