Electronic structure study of the reaction C2H4+ → C2H2++H2

The photoionization of ethylene (C2H4) and the behavior of the molecular ion (C2H4+) in its diverse electronic states have been subject of numerous experimental and theoretical studies. The first ethylene cation fragments from the dissociative process; i.e., acetylene cation (C2H2+) and vinyl cation (C2H3+), appear on the second ionization band. The papers in literature, though enlightening, leave behind some unclarified points and doubts about the C2H4+ --> C2H2+ + H-2 dissociative process. As for example: whether some energetic barriers exist or not along the steps followed. Hence, the main goal of this paper is to carry out a systematic research on the C2H4+ -->C2H2++H-2 dissociative process using quality electronic structure calculations to determine the least energy pertinent pathway and to characterize the stationary points along this pathway. Geometry optimization and vibrational frequencies calculations were carried out at the B3LYP, MP2 and CCSD (T) using the 6-311G(d,p) basis set. To establish the stationary points connectivity in the transitional states, characterized as the first order saddle point, intrinsic reaction coordinate (IRC) calculations were performed. In order to obtain more accurate electronic energies, single point calculations were performed at the CCSD(T)/6-311 + +G(3d,2p) theoretical level in the CCSD(T)/6311G(d,p) optimized geometries. All energies were corrected for zero point energy (ZPE) contributions, calculated at the CCSD)(T)/6-311G(d,p) level. Calculated AE at high theoretical level, CCSD(T)/6-311++G(3df,2p)//CCSD(T)/6-311 G(d,p), presents an excellent concurrence with the experimental values. More accurate barrier energies are established using CCSD(T)/6-311++G(3df,2p)//CCSD(T)/6-311G(d,p) level. (C) 2006 Wiley Periodicals, Inc.