Theoretical study of the electronic structure of [tetrathiafulvalene](2)(2+) dimers and their long, intradimer multicenter bonding in solution and the solid state.

The long, multicenter bonding in tetrathiafulvalenium cation radical dimers, TTF(2+), observed in both the solid state and in solution, were computationally investigated via B3LYP, CASSCF(2,2), and MCQDPT/CASSCF(2,2) calculations. The stability of TTF(2+) dimers originates from TTF...anion(-) or TTF...solvent electrostatic interactions, whose sum exceeds the TTF...TTF repulsion, in solution, or the sum of the TTF...TTF and anion(-)...anion(-) repulsions in solid state. MCQDPT/CASSCF(2,2) calculations indicate that the electronic ground state of the TTF(2+) dimers is a closed-shell single state with non-negligible contribution of the open-shell singlet, as is experimentally observed. This ground state is not well reproduced in B3LYP or CASSCF(2,2) calculations.