Ab initio Molecular orbital (MO) calculations with the 4–31G, 6–31G (split valence) and 6–31G^* (split valence + polarized) basis sets have been carried out on monofluorobbenzene ando-, m-, p-difluorobenzenes to study ground state properties such as orbital energies, gross orbital charges, net atomic charges and dipole moments. The redistribution of charges consequent to excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) has also been calculated. The results so obtained have been compared and discussed in the light of semi-empirical calculations and available experimental observations. The present study suggests the intermingling of the σ and π orbitals; however, the highest occupied and lowest unoccupied molecular orbitals are of π-symmetry indicating that most of the chemical and spectroscopic properties are controlled by π-electrons. It is also noted that the substituent fluorine is a weak π-donor and a strong σ-attractor. On exciting an electron from HOMO to LUMO it is observed that certain reactions may proceed through excited states. Though, in general, the semi-empirical andab initio methods predict the same trend, the latter should be applied in order to explore the finer details of the chemical properties for the molecules considered in the present investigation.

Ab initio computations of harmonic frequencies for the three isomeric difluorobenzenes have been carried out using the MO technique with split valence 3–21 G basis set. The computed frequencies have been compared with the experimental frequencies. The computed frequencies have been scaled with empirical scale factors in order to correct for the systematic errors originating in the limitations of the theoretical model. It was found necessary to use different scaling factors for the planar and non-planar modes.