The effects of heteroatom substitution on the singlet–triplet energy differences in diradicals—ab initio calculations of ΔEST in meta-benzoquinomethane and in 1,3-naphthoquinomethane

Abstract

The large effects on the singlet–triplet energy difference, ΔE_ST, of substituting oxygen atoms for one or two methylene groups in trimethylenemethane and in 2,4-dimethylenecyclobutane-1,3-diyl are discussed. In contrast, CASSCF and CASPT2N/6-31G* calculations predict only small changes in ΔE_ST on substituting oxygen atoms for one or two methylene groups in m-benzoquinodimethane. After corrections for differences in zero-point energies and heat capacities, CASPT2N/6-31G* calculations give ΔE_ST = 11.0 kcal mol^–1 for m-benzoquinodimethane, which is very close to the experimental value of ΔE_ST = 9.6 ± 0.2 kcal mol^–1. At the same level of theory ΔE_ST = 9.3 and 11.8 kcal mol^–1 are computed for, respectively, m-benzoquinomethane and m-benzoquinone. The reasons why substitution of oxygen atoms for one or two methylene groups is predicted to have such a small effect on ΔE_ST in these three diradicals is discussed. As expected, for 1,3-naphthoquinomethane CASPT2N predicts a value of ΔE_ST that is only slightly larger than that in m-benzoquinomethane. However, the calculated value of ΔE_ST = 11.6 kcal mol^–1 is 7 kcal mol^–1 lower than the value measured by photoacoustic calorimetry.