Structure and stability of the [TCNE](2)(2-) dimers in dichloromethane solution: a computational study

The solution behavior of TCNE](.-), which forms long-living pi-TCNE]22- dimers, is computationally studied by B3LYP and MCQDPT/CASSCF(2,2) calculations (a multiconfigurational quasi-degenerate perturbative calculation using a CASSCF(2,2) wavefunction, which properly accounts for the dispersion interaction). B3LYP calculations indicate minimum-energy TCNE](2)(2-)(dichloromethane)(4) aggregates, a solvent where pi-TCNE](2)(2-) dimers are spectroscopically observed. Their existence is attributed to TCNE](.-)...solvent interactions that exceed the TCNE](.-)...TCNE](.-) repulsion. The lowest energy minimum at the B3LYP level corresponds to an open-shell singlet electronic structure, a metastable minimum where the shortest interanion C...C distance is 5.23 A. A slightly less stable minimum is also found for the closed-shell singlet when double-occupancy of the orbitals is imposed, but it converts into the open-shell singlet minimum when the double occupancy is relaxed. At the MCQDPT/CASSCF(2,2) level, the only minimum is for the closed-shell singlet (24.0 kcal/mol (101 kJ/mol) more stable than the dissociation products), consistent with experimental enthalpy of dimerization of TCNE](.-) in dichloromethane solutions. It has an interanion C...C distance of 2.75 A and is in accord with the UV-vis experimental properties of the TCNE](.-) solutions.