| Abstract
| - By using density functional theory (DFT) method at the B3LYP/6-311+G* level, it has been shownthat the preference for the singlet-state phenyl cation can be dramatically increased relative tothe triplet state by introducing onium cationic substituents in the para position. At this level oftheory, the singlet ground state for the parent phenyl cation was found to be lower than the tripletby 19.6 kcal/mol. Introduction of electron-donating substituent groups, namely −NMe2, −NH2, and−SMe in the para position, strongly favors the triplet state. The −OMe and −SH groups cause asimilar 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 reversalof the relative stabilities in favor of the singlet ground state are indicated with −SH2+, −SMeH+,and −NH3+ showing the largest singlet/triplet energy difference. The −N2+ group in the para positionhas 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-anthracenylcation the triplet minimum lies 13.4 kcal/mol below the singlet minimum. Introduction of activatinggroups, i.e., OH and NMe2 at the 4-position of the 1-naphthyl cation, greatly increases the relativestability of the triplet state. Upon heteroatom protonation, the singlet/triplet energy gap issubstantially narrowed but the triplet state still remains lower in energy. Structural features inthe resulting dications are discussed and compared with the corresponding monocations. The NBOcharges at the cationic centers were also compared. We have also calculated the dediazoniationand decarbonylation energetics for mono- and bis-o-trimethylsilyl-substituted benzenediazoniumand benzoyl cation to aryl cations in order to probe the effect of β-silyl stabilization of the positivecharge.
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