Intermolecular Interactions in Weak Donor-Acceptor Complexes from Symmetry-Adapted Perturbation and Coupled-Cluster Theory: Tetracyanoethylene-Benzene and Tetracyanoethylene-p-Xylene

The interactions in the complexes of tetracyanothylene (TCNE) with benzene and p‐xylene, often classified as weak electron donor–acceptor (EDA) complexes, are investigated by a range of quantum chemical methods including intermolecular perturbation theory at the DFT‐SAPT (symmetry‐adapted perturbation theory combined with density functional theory) level and explicitly correlated coupled‐cluster theory at the CCSD(T)‐F12 level. The DFT‐SAPT interaction energies for TCNE–benzene and TCNE–p‐xylene are estimated to be −35.7 and −44.9 kJ mol−1, respectively, at the complete basis set limit. The best estimates for the CCSD(T) interaction energy are −37.5 and −46.0 kJ mol−1, respectively. It is shown that the second‐order dispersion term provides the most important attractive contribution to the interaction energy, followed by the first‐order electrostatic term. The sum of second‐ and higher‐order induction and exchange–induction energies is found to provide nearly 40 % of the total interaction energy. After addition of vibrational, rigid‐rotor, and translational contributions, the computed internal energy changes on complex formation approach results from gas‐phase spectrophotometry at elevated temperatures within experimental uncertainties, while the corresponding entropy changes differ substantially. Complexes of tetracyanoethylene with benzene and p‐xylene (see picture), often classified as weak electron donor–acceptor complexes, are investigated by a range of quantum chemical methods including intermolecular perturbation theory at the DFT‐SAPT level and explicitly correlated coupled‐cluster theory at the CCSD(T)‐F12 level.