RELATIVE ENERGETICS OF C-H AND C-C BOND ACTIVATION OF ALKANES - REACTIONS OF NI-ENERGY SURFACES( AND FE+ WITH PROPANE ON THE LOWEST ENERGY (ADIABATIC) POTENTIAL)

Reactions of Fe+ and Ni+ with propane, propane-2-d(1), propane-2,2-d(2 ), propane-1,1,1-d(3), propane-1,1,1,3,3,3-d(6) and propane-d(8) are e xamined to gain insight into the mechanism and energetics for the H-2 and CH4 elimination channels. The questions of C-H and/or C-C bond act ivation and the relative contributions from primary and secondary C-H bond activation are addressed. Total cross section measurements indica te that ground-state Ni+(D-2) and Fe+(D-6) react with propane ineffici ently, 13% and 7.5% of the Langevin collision cross section, respectiv ely, with CH4 loss favored over H-2 loss by a factor of 4.0 for Ni+ an d 2.8 for Fe+. For reactions with C3D8, the total cross sections decre ase by factors of 3.8 for Ni+ and 4.4 for Fe+ relative to C3H8, with t he dehydrogenation channel enhanced over demethanation for both Ni+ an d Fe+. Kinetic energy release distributions (KERDs) from nascent metas table Ni(propane)(+) and Fe(propane)(+) complexes were measured for H- 2 loss and CH4 loss. For H-2 loss, the distribution is bimodal. Studie s using propane-2,2,d(2) and propane-1,1,1,3,3,3-d(6) indicate that bo th primary and secondary C-H insertions are involved as initial steps. Initial secondary C-H insertion is responsible for the high-energy co mponent in the bimodal KERD, which is much broader than predicted from statistical theory, indicating that a tight transition state leads to the final products. The low-energy component for H-2 loss involves in itial primary C-H insertion and appears to be statistical, suggesting little or no reverse activation barrier as the system separates to pro ducts. The kinetic energy distribution for demethanation is statistica l and is very sensitive to the energy of the rate-limiting C-H inserti on transition state. A lower limit for the energy of this transition s tate is obtained by modeling the experimental kinetic energy release d istribution for demethanation using statistical phase space theory. Th e barrier reduces the contribution of high angular momentum states to the final products, thus reducing the high-energy portion of the produ ct kinetic energy distribution. Modeling the cross section, the isitop e effect, and the KERD for CH4 loss using statistical phase space theo ry indicates that the barrier for C-H bond insertion is located 0.10 /- 0.03 eV below the Ni+/C3H8 asymptotic energy and 0.075 +/- 0.03 eV below the Fe+/C3H8 ground-state asymptotic energy. All data can be exp lained by initial C-H insertion, without the need to invoke initial C- C bond activation for ground-state Fe+ and Ni+ reacting with propane a t low kinetic energy.