Structure and Stability of the [TCNE]₂^2-Dimers in Dichloromethane Solution:  A Computational Study

Abstract

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.

On the interactions between poly(ethylene oxide) and graphite oxide: A comparative study by different computational methods

“…52-54), but, sometimes, it presents certain limitations, specially for the simulation of π interactions, where it can overestimate the interaction energy, specially in cases where a small basis set has been used (smaller than triple zeta). [55][56][57] The use of a most precise method such as Coupled-Cluster Singles Doubles and Non-iterative Triples Correction (CCSD(T)) 58 [28][29][30][31]40 depending on the quality of the exchangecorrelation functional, such as the description of transition metals, thermochemical properties, medium-range correlation energy, barrier heights or non-covalent bond interactions. Among all those, the former is the one in which we would like to focus our interest in this work.…”

Reversible Spin Pairing in Crystalline Organic Radicals

“…Calculations indicate that in the case of TCNE •− , for example, solvation has an important role to play in stabilising the dimer pair. 10 Similarly solvent dependent studies on TTF •+ have revealed H dim in the region -16 to -38 kJmol −1 reflecting the important role of solvation effects on stabilising/destabilising the dimer. In other cases it may be likely that ion pairing can play an important role in stabilising such dimers.…”

Evidence for Multicenter Bonding in Dianionic Tetracyanoethylene Dimers by Raman Spectroscopy

“…Long, multicenter CÀC bonds within dimers of neutral or charged radicals, for example, p-[TCNE] 2 2À (TCNE = tetracyanoethylene, Figure 1) have been established from spectroscopic (UV/Vis/NIR, ESR, NMR and IR) studies, structural (crystallographic) evidence, and theoretical support. [1][2][3][4][5][6][7][8] [TCNE] 2 2À is stable in the solid state as a consequence of the attractive cation + ···anion À interactions exceeding the repulsive cation + ···cation + and [TCNE]C À… [TCNE]C À interactions, and is assisted by the dispersion and bonding energetic components acting between the two [TCNE]C À fragments. [6,8] Thus, p-[TCNE] 2 2À anion is diamagnetic, and has an observed closed-shell singlet ground state.…”

“…[1][2][3][4][5][6][7][8] [TCNE] 2 2À is stable in the solid state as a consequence of the attractive cation + ···anion À interactions exceeding the repulsive cation + ···cation + and [TCNE]C À… [TCNE]C À interactions, and is assisted by the dispersion and bonding energetic components acting between the two [TCNE]C À fragments. [6,8] Thus, p-[TCNE] 2 2À anion is diamagnetic, and has an observed closed-shell singlet ground state. Its long multicenter bond is best described as a two-electron/four-center (2e À /4c) bond, [1,8] with an approximately 2.9 intradimer separation.…”

“…[6,8] Thus, p-[TCNE] 2 2À anion is diamagnetic, and has an observed closed-shell singlet ground state. Its long multicenter bond is best described as a two-electron/four-center (2e À /4c) bond, [1,8] with an approximately 2.9 intradimer separation. To date there are approximately 25 [TCNE] 2 2À structural determinations, and although the cations range from point charges with significant electrostatic interactions, for example, Tl + , [9] to distant bulky cations, for example, [Cr(C 6 H 6 ) 2 ] + , [10] the [TCNE] 2 2À dimer is essentially identical.…”