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

The large effects on the singlet-tripler energy difference, Delta E-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 Delta E-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/B-31G* calculations give Delta E-ST = 11.0 kcal mol(-1) for m-benzoquinodimethane, which is very close to the experimental value of Delta E-ST = 9.6 +/- 0.2 kcal mol(-1). At the same level of theory Delta E-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 Delta E-ST in these three diradicals is discussed. As expected, for 1,3-naphthoquinomethane CASPT2N predicts a value of Delta E-ST that is only slightly larger than that in m-benzoquinomethane. However, the calculated value of Delta E-ST = 11.6 kcal mol(-1) is 7 kcal mol(-1) lower than the value measured by photoacoustic calorimetry.