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 Moller–Plesset (MP2) level of theory. The structures are stabilized by two basic types of intermolecular interactions: the CH-O and the CH-pi interaction. The CH-pi interaction appears in two variants, depending on which molecule provides the hydrogen atom and which molecule the pi system. The MP2 results indicate that the CH-pi interaction between one of the hydrogen atoms of acetylene and the pi 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/mo 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-pi interactions are furthermore analyzed with the help of the atoms in molecules (AIM) theory.
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