An ab initio and matrix isolation infrared study of the 1 : 1 C2H2-CHCl3 adduct

The details of weak C-H ...pi interactions that control several inter and i ntramolecular structures have been studied experimentally and theoretically for the 1:1 C2H2-CHCl3 adduct. The adduct was generated by depositing acet ylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evi denced by shifts in the vibrational frequencies compared to C2H2 and CHCl3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6 -311+G(d,p) levels. Both the computational and experimental data indicate t hat the C2H2-CHCl3 complex has a weak hydrogen bond involving a C-H ...pi i nteraction, where the C2H2 acts as a proton acceptor and the CHCl3 as the p roton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl3 and a hydrogen in C2H2 The combi nation of the C-H ...pi interaction and the secondary Cl ... H interaction determines the structure and the energetics of the C2H2-CHCl3 complex. In a ddition to the vibrational assignments for the C2H2-CHCl3 complex we have a lso observed and assigned features owing to the proton accepting C2H2 submo lecule in the acetylene dimer. (C) 1999 Elsevier Science B.V. All rights re served.