Electronic structure of zwitterionic diamino-meta-quinonoid molecules: identity of UV absorption bands

Quantum chemical calculations using the multi-state multiconfigurational MS-CASPT2 method, in conjunction with the 6-31G(d,p) basis set, have been applied to investigate the electronic structure of the parent diamino-meta-quinonoid molecule containing a six-membered carbon ring coupled with two exocyclic CO bonds situated in a meta position, along with two NH2 substituents. A particular attention has been paid to its electronic spectrum. Our results obtained using the MS-CASPT2 method do not agree with the recent CIS results by Sawicka et al. [Chem. Phys. Lett. 362 (2002) 527] on the identity of two observed UV absorption bands centered at 350 and 343 nm. A new interpretation on the origin of these bands is proposed.