An Extended Multireference Study of the Singlet and Triplet States of the 9,10-didehydroanthracene Diradical.

The 9,10-didehydroanthracene is an aromatic diradical produced by the Bergman cyclization of a benzannulated 10-membered enediyne. It is a 1,4 diradical, similar to p-benzyne. Here we study the spin state occupancy of the ground state of 9,10-didehydroanthracene by employing multireference methods (MR-CISD and MR-AQCC) with different basis sets (cc-pVDZ and cc-pVTZ) and active space sizes (CAS (2,2) through CAS (8,8)). At the CAS (8,8) MR-AQCC/cc-pVDZ level of theory, we find a two-configurational singlet ground state with an adiabatic Δ EST of 6.13 kcal/mol. Unpaired electron density populations and dominant electronic configuration interactions were used to analyze the features of the 9,10-didehydroanthracene diradical.