DFT study of substituted and benzannelated aryl cations: substituent dependency of singlet/triplet ratio

By using density functional theory (DFT) method at the B3LYP/6-311+G level, it has been shown that the preference for the singlet-state phenyl cation can be dramatically increased relative to the triplet state by introducing onium cationic substituents in the para position. At this level of theory, the singlet ground state for the parent phenyl cation was found to be lower than the triplet by 19.6 kcal/mol. Introduction of electron-donating substituent groups, namely -NMe(2), -NH(2), and -SMe in the para position, strongly favors the triplet state. The -OMe and -SH groups cause a similar but smaller effect, whereas -OH substitution results in an energetically identical system. Protonation of these substituent groups form onium-phenyl dications for which a complete reversal of the relative stabilities in favor of the singlet ground state are indicated with -SH(2)(+), -SMeH(+), and -NH(3)(+) showing the largest singlet/triplet energy difference. The -N(2)(+) group in the para position has a similar effect. Benzannelation also increases the relative stability of triplet aryl cation. Whereas the 1-naphththyl and 2-naphthyl cations are energetically identical, in the 9-anthracenyl cation the triplet minimum lies 13.4 kcal/mol below the singlet minimum. Introduction of activating groups, i.e., OH and NMe(2) at the 4-position of the 1-naphthyl cation, greatly increases the relative stability of the triplet state. Upon heteroatom protonation, the singlet/triplet energy gap is substantially narrowed but the triplet state still remains lower in energy. Structural features in the resulting dications are discussed and compared with the corresponding monocations. The NBO charges at the cationic centers were also compared. We have also calculated the dediazoniation and decarbonylation energetics for mono- and bis-o-trimethylsilyl-substituted benzenediazonium and benzoyl cation to aryl cations in order to probe the effect of beta-silyl stabilization of the positive charge.