Singlet-triplet excitation energies of substituted phenyl cations: a G4(MP2) and G4 theoretical study

Singlet-triplet adiabatic excitation energies (AE(S-T)) of the parent and variously substituted phenyl cations, as well as the parent benzannelated derivatives up to anthracenyl, were calculated at the G4(MP2) and G4 levels of theory. The G4(MP2)/G4 AE(S-T) estimates range up to 40 kJ/mol higher than prior density functional theory (DFT)-based predictions for these cations and suggest that AE(S-T) and ground state multiplicity structure-property trends for phenyl cations previously proposed in the literature need to be re-assessed at higher levels of theory. In general, Hartree-Fock, DFT, and semiempirical methods do a poor job describing the singlet-triplet excitation energetics of these systems. Only modest effects of different solvation models (SMD, IEF-PCM, and C-PCM) and different polar protic through apolar aprotic solvents are evident on the calculated AE(S-T) of the phenyl cation. Electron-donating substituents on the phenyl cation substantially lower the AE(S-T) to an extent where some functional groups (-NH2, N(CH3)(2), OCH3, and SCH3) can result in triplet ground states depending on their position relative to the cation. In contrast to the phenyl and 1- and 2-naphthyl cations, which are predicted to be ground state singlets, the three parent anthracenyl cations will be ground state triplets.