The long, multicenter bonding in tetrathiafulvalenium cation radical dimers, [TTF](2)(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)(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)(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.
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.
Abstract:The structure and stability of the -TCNE 2 2À dimers in K 2 TCNE 2 aggregates is revisited trying to find if the origin of their two-electron/four-centers CÀ ÀC bond are the electrostatic K þ -TCNE À interactions or the dispersion interactions between the anions. The study is done at the HF, B3LYP, CASSCF (2,2), and MCQDPT/CASSCF (2,2) levels using the 6-31þG(d) basis set. Our results show that the only minima of this aggregate that preserves the -TCNE 2 2À structure has the two K þ atoms placed in equatorial positions in between the two TCNE À planes. When the K þ atoms are placed along the D 2h axis of the anions the structure is not a minimum. The main energetic component responsible for the stability of these aggregates comes from the cation-anion interactions. However, a proper accounting of the dispersion component (as done in the MCQDPT/CASSCF (2,2) calculations) is needed to make the closedshell singlet more stable than the open-shell singlet. Thus, the bond results from the combination of the electrostatic and dispersion components, being the first the dominant one. The optimum geometry of the closed-shell singlet is very similar to the experimental one found in crystals.
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