Molecular quadrupole moments for the series of fluoro- and chlorobenzenes

Ab-initio molecular quadrupole moments for benzene and the complete series of fluoro- and chlorobenzenes are reported. In addition, quadrupole moments for two methylbenzenes and for naphthalene are also presented. Molecular geometries are fully optimized using a local spin density approximation in a density functional theory code (DGAUSS) that solves the Kohn-Sham equations within the linear combination of Gaussian type orbitals approach. These geometries are used to calculate the RHF/6-31G** permanent electric moments. It is found that the employment of a reasonable geometry is sufficient to obtain reliable quadrupole moments, Calculated values for the quadrupole moment tensor component perpendicular to the aromatic ring (Q(zz)) are in excellent agreement with available experimental data. Nuclear and electronic contributions to Q(zz) are also calculated and compared against experiment. The main trends of the results, i.e., the variations of Q(zz) with the nature and number of substituents in the aromatic ring, are discussed in terms of charge redistributions in the molecules. Standard basis set effects within the RHF scheme are analyzed for the series of fluorobenzenes. It is concluded that for the type of molecules studied here it is very important to use polarization functions, while diffuse functions do not improve the calculated quadrupole moment values.