Singlet–triplet excitation energies of naphthyl cations: High level composite method calculations suggest a singlet ground state

Singlet–triplet excitation energies (ES–T) were calculated for the phenyl, 1-naphthyl, and 2-naphthyl cations using a broad range of model chemistries, including semiempirical, Hartree–Fock, density functional, Moller–Plesset perturbation, composite, coupled cluster, and quadratic configuration interaction methods and various basis sets. Substantial model chemistry dependent ES–T results were obtained for all three cations with correspondingly minimal basis set size effects. G4/G4MP2 composite method well-to-well (and adiabatic) ES–T for the phenyl, 1-naphthyl, and 2-naphthyl cations are 101.9/102.3 (101.7/102.0), 20.4/18.8 (19.3/17.8), and 21.6/21.4 (20.8/20.7) kJ/mol, respectively. All composite methods predict a substantially positive ES–T for the 1- and 2-naphthyl cations, and are in both quantitative and qualitative disagreement with many other model chemistries (particularly density functionals such as B3LYP) in estimating both the magnitude and sign of the singlet–triplet excitation energy for the 1- and 2-naphthyl cations. Composite method approaches suggest both the 1- and 2-naphthyl cations are ground state singlets with sufficiently large ES–T such that the population of the corresponding triplet state should be negligible, and thereby non-observable, where experimental conditions operate under thermodynamic control.