MP2, DFT-D, and PCM study of the HMB-TCNE complex:: Thermodynamics, electric properties, and solvent effects

Geometry, thermodynamic, and electric properties of the pi-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 x 10(-10) m and the corresponding BSSE corrected interaction energy is -51.3 kJ mol(-1). 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 x 10(-10) m, respectively) than our MP2 gas phase value. Our estimate of the distance in CCl4 on the basis of PCM solvent effect study is also larger (3.06-3.16 x 10(-10) m). The calculated enthalpy, entropy, Gibbs energy, and equilibrium constant of HMB-TCNE complex formation in gas phase are: Delta H-0 = -61.59 kJ mol(-1), Delta S-c(0) = -143 J mol(-1) K-1, Delta G(0) = -18.97 kJ mol(-1), and K 2,100 dm(3) mol(-1). Experimental data, however, measured in CCl4 are significantly lower: Delta H-0 = -34 kJ mol(-1), Delta S-c(0) = -70.4 J mol(-1) K-1, Delta G(0) = -13.01 kJ mol(-1), and K = 190 dm(3) mol(-1). 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(-1) which is very close to our PCM value 6.5 kJ mol(-1). 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 pi-EDA complexes. (C) 2008 Wiley Periodicals, Inc.