Theoretical study of solvent effect on π-EDA complexation II. Complex between TCNE and two benzene molecules

Abstract

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.

MP2 and DFT study of IR spectra of TCNE‐methylsubstituted benzene complexes: Is charge transfer important?

“…On the other hand, we are aware of some deficiencies with respect to the size of the basis set applied in this work. It was shown in our study of Be-TCNE [13] and HMB-TCNE [12] (Kysel' et al, unpublished results) that more diffuse and polarization functions are needed to more accurately describe long-distance intermolecular interactions (e.g., the binding energy of the complex obtained with the 6-31ϩG(d) basis set at the MP2 level was larger by about 20% compared with the MP2/6-31G(d) result). Nevertheless, the study suggests that the 6-31G(d) basis set is a reasonable choice leading to the accuracy of Ϯ0.1 ϫ 10 Ϫ10 m for the interplanar distances and 10% error in the dipole moment.…”

“…between theoretical and experimental data for HMB-TCNE [5,6,9] can be partly explained by the overestimated interplanar distance in the complex obtained at the HF and DFT levels. As it follows from previous studies on stacking complexes [12][13][14], for proper description of the dispersion interactions in these complexes, the proper inclusion of the electron correlation (EC) effects seems to be crucial. In our recent article [9], we made a detail study on thermodynamics, IR spectra, and some electric properties of the HMB-TCNE complex.…”

“…Geometry optimizations, calculation of both harmonic and anharmonic vibrational modes, the interaction energy (including its decomposition), and electric properties for methylbenzenes-tetracyanoethylene (NMB-TCNE) complexes in gas phase have been performed at the MP2 and partly also at the DFT levels using the 6-31G(d) and 6-311G** basis sets. A comparison of MP2 and CCSD(T) results on the geometries and binding energies of Be-TCNE complexes [12] (Kysel' et al, unpublished results) suggests that the MP2 method seems to be appropriate to account for the main part of the EC effects, though it still slightly underestimates the interaction energies. On the other hand, we are aware of some deficiencies with respect to the size of the basis set applied in this work.…”

“…The computational strategy was based on our previous experience with theoretical studies of -EDA complexes [9,12,13]. Geometry optimizations, calculation of both harmonic and anharmonic vibrational modes, the interaction energy (including its decomposition), and electric properties for methylbenzenes-tetracyanoethylene (NMB-TCNE) complexes in gas phase have been performed at the MP2 and partly also at the DFT levels using the 6-31G(d) and 6-311G** basis sets.…”

“…1) we located this peak at ϳ2,252 cm Ϫ1 . We also compared the first two peaks measured by Stires Be-TCNE (⌬Q ϭ 0.064) [12] Be-TCNE-Be (⌬Q ϭ 0.103) [12] HMB-TCNE (⌬Q ϭ 0.168) [9] In connection with the intensities of both CN and CAC vibrations we mention the following: the CAC stretching vibration which is due to symmetry forbidden in TCNE itself but rapidly (exponentially) increases its intensity with increasing number of methyl groups (see Table III). The intensity is here borrowed both from CH 3 torsion vibrations which strongly interact with CAC stretch and also from the deformation of the TCNE plane in the benzene complexes.…”