For reactions of gas phase, ground state, neutral transition metal atoms from the 4d series with alkanes and alkenes, we combine 300 K kinetics measurements with ab initio electronic structure calculations to infer mechanisms in some detail. The theoretical method PCI-80 with zero-point energy corrections to the bare potential surface apparently produces bond energies, reaction exothermicities, and even saddle point energies accurate to within 2-3 kcal/mol, provided that the correct ground state has been located, which is sometimes difficult. The reactions fall into two general categories: termolecular stabilization of long-lived M(hydrocarbon) complexes and bimolecular elimination of Hz. By using the ab initio energies and vibrational frequencies in a statistical unimolecular rate theory (RRKM theory), we can model the lifetimes of M(hydrocarbon) complexes to assess the plausibility of a saturated termolecular mechanism at 1 Torr He. Termolecular examples include the reactions of Pd with alkanes to form long-range v2 complexes; the reactions of Rh and Pd with alkenes to form n complexes; and probably the reactions of Y, Zr, Nb, Rh, and Pd with cyclopropane to form CH or CC insertion complexes. In other reactions, all of the evidence indicates a bimolecular H2 elimination mechanism. Rhodium is unique among the 4d metal atoms in effecting HZ elimination from ethane and larger alkanes. Yttrium, zirconium, and niobium almost surely insert in CH bonds of ethylene and larger alkenes, ultimately eliminating H2. We discuss the general requirements on the pattern of atomic electronic states that pennit efficient CH bond activation and H2 elimination. The good agreement between the observed reaction rates and the PCI-80 calculations lends confidence to future efforts to apply ab initio techniques to more complicated catalytic systems, including condensed phase reactions involving ligated metal centers.
We survey the reactivity of ground-state, neutral transition metal atoms from the left-hand side of the 4d series (Y through Mo) with eight alkanes and alkenes. Effective bimolecular rate constants are measured in 0.5-0.8 Torr of He in a fast flow reactor at 300 K with laser-induced fluorescence detection. None of the four ground-state metal atoms reacts with linear alkanes. Mo reacts slowly and Y, Zr, and Nb react rapidly with alkenes. From the absence df a measureable pressure dependence of the rate constant over the limited He range of 0.5-0.8 Torr, we infer that bimolecular elimination chemistry occurs. We relate the pattern of reactivity to recent ab initio calculations of the geometry and binding energy of M-C2H4 and H-M-C2H3 for 4d-spries transition metals.
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