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 + C 2 H 4 f ZrC 2 H 2 + H 2 . 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 HZrC 2 H 3 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 HZrC 2 H 3 to the strongly bound dihydrido species H 2 ZrC 2 H 2 , 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 from the triplet to singlet metallacyclopropane complex competes effectively, perhaps dominating at low collision energy. As in earlier work, B3LYP places key transition state energies too high by 6-9 kcal/mol. The mPW1PW91 density functional gives much more realistic energies.

Bimolecular rate constants, primary products, and kinetic isotope effects for the reactions of Y (4d 1 5s 2 , 2 D) with C 2 H 4 and C 2 D 4 and with C 3 H 6 and C 3 D 6 are measured in a fast flow reactor at 300 K with He/N 2 buffer gas at 0.8 Torr. The H 2 and D 2 elimination products and Y(alkene)-stabilized complexes are detected using single photon ionization at 157 nm and time-of-flight mass spectrometry. We find a small normal isotope effect (k H /k D ) 1.75 ( 0.12) for the reaction with ethylene but no significant isotope effect (k H /k D ) 1.06 ( 0.07) for the reaction with propylene. We use density functional theory in its B3LYP and mPW1PW91 forms with a large basis set to characterize stationary points on the doublet potential energy surface for the reaction Y + C 2 H 4 f YC 2 H 2 + H 2 . Theory finds no energy barrier to the formation of a long-range Y-ethylene complex. Subsequent steps involving CH bond insertion by metallacyclopropane complexes are consistent with earlier work. However, a new, low-energy path involves concerted rearrangement of the HYC 2 H 3 insertion intermediate directly to a weakly bound, product-like complex with no exit channel barrier to elimination products. Theory also provides a set of geometries and vibrational frequencies for use in statistical rate models of the hot metallacyclopropane complex decay. The preferred model, consistent with the collection of Y + ethylene experimental data, requires no adjustments to the mPW1PW91 energies. As in earlier work, B3LYP places key transition state energies too high by 6-9 kcal/mol. The available evidence suggests that nonadiabatic and/or steric effects contribute to the reaction inefficiency at room temperature.