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

Abstract

Singlet–triplet adiabatic excitation energies (AES−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 AES−T estimates range up to 40 kJ/mol higher than prior density functional theory (DFT)-based predictions for these cations and suggest that AES−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 AES−T of the phenyl cation. Electron-donating substituents on the phenyl cation substantially lower the AES−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.