Geometry, thermodynamic, and electric properties of the -EDA complex between hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) are investigated at the MP2/6-31G* and, partly, DFT-D/6-31G* levels. Solvent effects on the properties are evaluated using the PCM model. Fully optimized HMB-TCNE geometry in gas phase is a stacking complex with an interplanar distance 2.87 ϫ 10 Ϫ10 m and the corresponding BSSE corrected interaction energy is Ϫ51.3 kJ mol Ϫ1. As expected, the interplanar distance is much shorter in comparison with HF and DFT results. However the crystal structures of both (HMB) 2 -TCNE and HMB-TCNE complexes have interplanar distances somewhat larger (3.18 and 3.28 ϫ 10 Ϫ10 m, respectively) than our MP2 gas phase value. Our estimate of the distance in CCl 4 on the basis of PCM solvent effect study is also larger (3.06 -3.16 ϫ 10 Ϫ10 m). The calculated enthalpy, entropy, Gibbs energy, and equilibrium constant of HMB-TCNE complex formation in gas phase are:, and K ϭ 2,100 dm 3 mol Ϫ1 . Experimental data, however, measured in CCl 4 are significantly lower:, and K ϭ 190 dm 3 mol Ϫ1 . The differences are caused by solvation effects which stabilize more the isolated components than the complex. The total solvent destabilization of Gibbs energy of the complex relatively to that of components is equal to 5.9 kJ mol Ϫ1 which is very close to our PCM value 6.5 kJ mol Ϫ1 . MP2/6-31G* dipole moment and polarizabilities are in reasonable agreement with experiment (3.56 D versus 2.8 D for dipole moment). The difference here is due to solvent effect which enlarges interplanar distance and thus decreases dipole moment value. The MP2/6-31G* study supplemented by DFT-D parameterization for enthalpy calculation, and by the PCM approach to include solvent effect seems to be proper tools to elucidate the properties of -EDA complexes.

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Formation of tetracyanoethylene—benzene 1: 1 and 1: 2 complexes was modeled using the Møller—Plesset second-order theory (MP2) and polarized continuum model (PCM). The MP2 calculated geometry of 1: 1 complex presents a plane-parallel C 2υ sandwich structure with interplanar distance 3.05 × 10−10 m, while the 1: 2 complex has D 2h geometry where the planparallel distance is equal to 3.09 × 10−10 m. The MP2 calculations indicate that the main part of formation enthalpy in both complexes is dispersion energy due to intermolecular electron correlation. The calculations also show that the formation entropy destabilizes both complexes since from the two constituent molecules one complex molecule arises. The MP2/6-31G* procedure showed to be a suitable tool for the estimation of the relative importance of 1: 2 complexation compared to the 1: 1 complexation. In the gas phase the ratio of the equilibrium constants of both complexes K 1:2/K 1:1 = 0.09 was calculated. The presence of solvent, treated by the PCM, further destabilized the 1: 2 complex with respect to the 1: 1 complex. The ratio K 1:2/K 1:1 in CH2Cl2 calculated by the PCM method was 0.022, i.e. the 1: 2 complex was almost 50 times less stable than the 1: 1 complex, which is in agreement with available experimental data. According to the calculations, solvent always destabilizes complex with respect to the isolated (solvated) components.It was also found that charge polarization in the 1: 2 complex with respect to that in the 1: 1 complex was not strictly additive due to the presence of the second benzene molecule in the 1: 2 complex. Non-additive were also formation enthalpy, entropy, polarizability, charge transfer from donors to acceptor molecule and other properties. This fact is caused by a slightly changed interaction between constituent molecules in the 1: 2 complex in comparison with the 1: 1 complex as well as by the interaction between benzene molecules in the 1: 2 complex which is missing in the 1: 1 complex.Preliminary CIS/6-31G* theoretical study regarding a few first-electron (electron charge transfer) transitions in both complexes indicates the presence of Frenkel excitn and Davydov transition energy splitting in the 1: 2 “supercomplex” with the first allowed π → π* absorption transition at λ = 355 nm, while the first allowed transition in the case of 1: 1 complex was characterized by λ = 392 nm with the oscillator strength only half of that of the 1: 2 complex, which is in agreement with experiment. These unexpected large hypso-and hypochromic effects predicted by the theory could allow to overcome difficulties of the experimental determination of the 1: 2 complexation.

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SCF, MP2, DFT(B3LYP) and the polarizable continuum model (PCM) were used to study geometry, charge distribution and energetics of the π-EDA complex formation between tetracyanoethene (TCNE) and benzene both in gas phase and in various polar solvents (cyclohexane, dichloromethane and water). MP2/6-31G*, MP2/6-31+G*, MP2/6-31G*(0.25) calculations have shown that geometry of the complex is planparallel with interplane distance of 3.05 × 10-10 m on the MP2/6-31G* level and the complexation energy is equal to -6.8 to -8.95 kcal/mol, while dominant contributions to the complexation energy come from intermolecular correlation and energy. The PCM continuum model of polar solvents describes well both the Gibbs energy of solvation of individual solutes and the difference between the complex and its constituents and also agrees with the experimental finding that the polar solvent effect decreases the complexation constant of the π-EDA complex formation by a factor of 2-4 when chloroform is replaced by more polar dichloromethane, and by a factor of 9, when tetrachlormethane is replaced by dichloromethane. It seems that the solvation Gibbs energy of the π-EDA complex formation always prefers stability of solvated constituents to that of the solvated complex. The electrostatic polarization Gibbs energy of solvation is responsible for the tendency of complexation constants to decrease with increasing solvent polarity; however, non-electrostatic terms contribute as well. While the enthalpy of complexation between benzene and TCNE in gas phase is about -10.0 kcal/mol due to the negative complexation entropy ∆(∆S) = -22.56 cal/mol K, the ∆G of complexation is -3.8 kcal/mol. The solvation part of the complexation Gibbs energy in dichloromethane is +5.14 kcal/mol (PCM-SCF/6-31G* calculation) so that complexation constant K = 0.1 dm3/mol in this solvent was found.