Computational study of the electronic structure of substituted phenylcarbene in the gas phase

Gas-phase calculations for ortho-, meta-, and para-substituted phenylcarbenes demonstrate that aromatic ring substituents can have a large effect on the singlet-triplet splitting in these molecules. For example, p-aminophenylcarbene is predicted to have a singlet-triplet splitting (DeltaE(ST)) of 0.7 kcal/mol, which is 4.7 kcal/mol smaller than that of the parent phenylcarbene. However, when the mesomeric effect of pi-donation is removed, as in the meta-substituted molecules, this affect is greatly attenuated. m-Amino substitution lowers the DeltaE(ST) to 5.2 kcal/mol, a change of only 0.2 kcal/mol from the parent phenylcarbene. The sensitivity of phenylcarbene to substitution is judged by the slope of the line correlating the singlet-triplet gaps to linear free energy (LFE) parameters. In this study, the best linear correlation for DeltaE(ST) in the para-substituted phenycarbenes is observed when plotted against sigma(p)(+). The resultant slope (rho) is 5.0. The origin of this effect is preferential interaction of aryl substituents with the singlet rather than the triplet species as demonstrated by electron density analyses.