A surface for which the electrostatic potential is negative delineates regions in a molecule that are subject to electrophilic attack. It can help you to rationalize the widely different chemistry of molecules that are structurally similar.

Optimize the geometries of benzene and pyridine using the HF/3-21G model and examine electrostatic potential surfaces corresponding to 100 kJ/mol. Describe the potential surface for each molecule. Use it to rationalize the following experimental observations: (1) Benzene and its derivatives undergo electrophilic aromatic substitution far more readily than do pyridine and its derivatives; (2) protonation of perdeuterobenzene (C₆D₆) leads to loss of deuterium, whereas protonation of perdeuteropyridine (C₅D₅N) does not lead to loss of deuterium; and (3) benzene typically forms π -type complexes with transition metals whereas pyridine typically forms σ -type complexes.