Charged stacks of dithiin, diselenin, thianthrene and selenanthrene radical cations: long range multicenter bonds.

The bonding mechanism of charged stacks of thianthrene and selenanthrene radical cations is studied using quantum-chemical methods. The investigation of the nature of the electronic ground state and the electronic structure via gas-phase multireference calculations brings insight into the interactions of such dimers and trimers as found in molecular crystals. Since thianthrene and selenanthrene are the dibenzo-homologues of dithiin and diselenin, the latter are taken as model systems to study the influence of the ring systems. For all investigated systems, the singlet state is the ground state. Multicenter bonds are formed between the singular occupied orbitals of the radicals. All dimers are found to be metastable. The thianthrene trimers, which are experimentally found in molecular crystals are also stable in the gas phase, while the analogue selenanthrene trimers are not.