Singlet-triplet gap in α-n-dehydrotoluene and related biradicals:: An ab initio configuration interaction study

By means of ab initio difference-dedicated configuration interaction (DDC12) calculations, singlet-triplet gaps of alpha-n-dehydrotoluene (n = 2, 3, or 4), 2-o, 2-m, 2-p, and alpha-3-didehydro-5-methyl-6-hydroxybenzene, 4, biradicals have been determined. Calculations have been performed with a 6-31G** basis set. A triplet ground state has been found for both the ortho and para isomers of 2, with quite similar gaps: 6.5 and 6.8 kcal mol(-1), respectively. For the meta-substituted compounds, 2-m and 4, the ground state is found to be a singlet with a gap of -2.1 kcal mol(-1) for both compounds. The intermediate in the synthesis of 4 and 2,6-dimethylcyclohexa-2,5-dien-1-on-4-ylidene 3, has also been considered. Its triplet ground state does not present a pure carbenic character, but the single electrons are strongly localized in different centers.