Singlet-triplet excitation energies of substituted benzenes: A G4 theoretical study

Singlet-triplet adiabatic (AES-T) and well-to-well (WWES-T) excitation energies of variously substituted phenyl cations were calculated at the G4 level of theory. The G4 ES-T estimates range from 19 to 40 kJ/mol higher than prior density functional theory based predictions for these cations, and suggest that ES-T and ground state multiplicity structure-property trends for phenyl cations previously proposed in the literature may be incorrect. Among the substituents considered, meta-substituted phenyl cations with electron donating groups have singlet ground states, and are not isoenergetic with the corresponding triplet states as previously claimed. Depending on the functional group, orthoor parasubstituted phenyl cations may also be ground state singlets despite the presence of an electron