Theoretical studies of proton transfer in (CH₃CHO-H-OCHCH₃)⁺

The geometric structures as well as the transfer of a proton in the complex of proton-bridged acetaldehyde dimer (CH₃CHO)₂H⁺ are studied theoretically by ab initio calculations. Four isomeric structures of the complex are found, and among them the cis-cis complex is calculated to be the most stable conformation. The energy difference between the least and the most stable conformation is less than 1 kcal/mol. A symmetric double well is found for proton transfer on the potential-energy hypersurface. A modified procedure was introduced while the energy barrier of proton transfer was calculated. With the transition structure fully optimized (allowing mutual movement of the two aldehyde moieties during proton transfer) the barrier is calculated to be 1.73 kcal/mol, significantly smaller than that (4.15 kcal/mol) calculated with a fixed distance between the two aldehyde subunits in the process of transfer. Each equilibrium structure in cis-cis and trans-trans conformations of (CH₃CHO)₂H⁺ has C_s symmetry with angular O-H-O, whereas the transition structure belongs to point group C_2h. Comparison of these results with those of the formaldehyde cluster ion (HCHO)₂H⁺ is also presented.