Ab initio and matrix isolation study of the acetylene-furan dimer

Five acetylene-furan dimer structures are identified using ab initio calculations at the second-order Møller-Plesset (MP2) level of theory. The structures are stabilized by two basic types of intermolecular interactions: the CH⋯O and the CH⋯π interaction. The CH⋯π interaction appears in two variants, depending on which molecule provides the hydrogen atom and which molecule the π system. The MP2 results indicate that the CH⋯π interaction between one of the hydrogen atoms of acetylene and the π system of furan as found in structure A is the strongest interaction, followed by the in-plane CH⋯O interaction in the second most stable acetylene-furan dimer structure B. A matrix isolation study shows the acetylene-furan dimer to exist in an argon matrix, but likely rather as structure B than as A. High level coupled cluster calculations with up to triple excitations (CCSD(T)) yield the interaction energy of structure A as about -2.4 kcal/mol in the complete basis set limit and find structure B to be nearly isoenergetic with -2.3 kcal/mol. This is confirmed in calculations employing the density functional theory combined with symmetry adapted intermolecular perturbation theory (DFT-SAPT) approach yielding interaction energies of -2.3 and -2.0 kcal/mol for A and B, respectively. DFT-SAPT also helps to understand the importance of the electrostatic, induction and dispersion interaction energies and their respective exchange counterparts for the stability of the various acetylene-furan dimer structures. The CH⋯O and CH⋯π interactions are furthermore analyzed with the help of the atoms in molecules (AIM) theory.