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DOI: 10.1039/A706409H (Paper) Reference Section for: J. Chem. Soc., Perkin Trans. 2, 1998, 1037-1044

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

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David A. Hrovat, Mark A. Murcko, Paul M. Lahti and Weston Thatcher Borden

Abstract

The large effects on the singlet–triplet energy difference, ΔEST, 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 ΔEST 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 ΔEST = 11.0 kcal mol–1 for m-benzoquinodimethane, which is very close to the experimental value of ΔEST = 9.6 ± 0.2 kcal mol–1. At the same level of theory ΔEST = 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 ΔEST in these three diradicals is discussed. As expected, for 1,3-naphthoquinomethane CASPT2N predicts a value of ΔEST that is only slightly larger than that in m-benzoquinomethane. However, the calculated value of ΔEST = 11.6 kcal mol–1 is 7 kcal mol–1 lower than the value measured by photoacoustic calorimetry.

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