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 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 coworkers. 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 Dougherty and coworkers. However, similar resonance theory based reasoning fails to explain quantitative difference between positively and negatively charged systems, as well as observed strong stabilization of an open-shell singlet state in the N-oxidized pyridinium analogue of meta-xylylene.