Singlet and Triplet Reaction Paths for Gas-Phase Zr + C2H4 by Density Functional Theory

Abstract

We use the B3LYP density functional theory with a large basis set to characterize stationary points on both triplet and singlet potential energy surfaces for the gas-phase reaction Zr + C2H4 → ZrC2H2 + H2. The previously described stepwise rearrangement path occurs on the triplet surface, requiring passage over a substantial exit channel barrier. A new, lower energy triplet path involves concerted rearrangement of the HZrC2H3 insertion intermediate directly to a weakly bound, product-like complex with no exit channel barrier to triplet products. A new low-energy singlet path involves stepwise rearrangement from HZrC2H3 to the strongly bound dihydrido species H2ZrC2H2, which then dissociates to singlet products over a small exit channel barrier of 4 kcal/mol. We argue that the singlet path is more consistent than either triplet path with the experimental product kinetic energy distribution, which peaks at 3−5 kcal/mol. This in turn suggests that access of the singlet surface via fast intersystem crossing f...