The effect of substituents on electronic states' ordering in meta-xylylene diradicals: qualitative insights from quantitative studies.

Equation-of-motion spin-flip coupled-cluster method with single and double substitutions (EOM-SF-CCSD) is employed to study how substituents affect the electronic states' ordering in meta-xylylene diradicals. The electronegativity of substituents and the incorporation of a heteroatom are found to have a negligible effect. The effect of charges on energy gaps is much more pronounced, in agreement with the proposal of Dougherty and co-workers [J. Am. Chem. Soc. 118, 1452 (1996)]. Resonance structure theory and molecular orbital analysis are employed to explain this phenomenon. The changes in the exocyclic C-C bond length in substituted meta-xylylenes, derived from equilibrium structures calculated by using analytic gradients for the EOM-SF-CCSD method, support the original resonance theory explanation by West et al. However, a similar resonance-theory-based reasoning fails to explain the quantitative difference between positively and negatively charged systems as well as the observed strong stabilization of an open-shell singlet state in the N-oxidized pyridinium analog of meta-xylylene.