Basis set dependence of the molecular electrostatic potential topography. A case study of substituted benzenes

The basis set dependence of the topographical structure of the molecular electrostatic potential (MESP), as well as the effect of substituents on the MESP distribution, has been investigated with substituted benzenes as test cases. The molecules are studied at HF-SCF 3–21G and 6–31G∗∗ levels, with a further MESP topographical investigation at the 3–21G, double-zeta, 6–31G∗, 6–31G∗∗, double-zeta polarized and triple-zeta polarized levels. The MESP critical points for a 3–21G optimized/6–31G∗∗ basis are similar to the corresponding 6–31G∗∗ optimized/6–31G∗∗ ones. More generally, the qualitative features of the MESP topography computed at the polarized level are independent of the level at which optimization is carried out. For a proper representation of oxygen lone pairs, however, optimization using a polarized basis set is required. The nature of the substituent drastically changes the MESP distribution over the phenyl ring. The values and positions of MESP minima indicate the most active site for electrophilic attack. This point is strengthened by a study of disubstituted benzenes.