Vibrational vs. Translational Energy in Promoting a Prototype Metal-Hydrocarbon Insertion Reaction David L. Proctor and H. Floyd Davis Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY Abstract The reaction Y + CH 4  HYCH 3  YCH 2 + H 2 is initiated by C-H insertion involving an 20 ± 3 kcal/mol potential energy barrier. The reaction is studied in crossed molecular beams under two different conditions with nearly the same total energy. One experiment is carried out at a collision energy of 15.1 kcal/mol with one quantum of CH 4 antisymmetric ( 3 ) stretching vibrational excitation (8.63 kcal/mol), the other at a collision energy of 23.8 kcal/mol. The reaction cross section for C-H stretch excited methane (  s ) is found to be at least a factor of 2.2 times larger than for ground state methane (  g ) at the same total energy. Observed Mode-specificity Cornell Rotatable Source Crossed Molecular Beams MachineResults Results and Conclusions References Cited Several groups have studied the mode- and bond- specific reactivity of excited vibrational states of methane isotopologues in gas-phase hydrogen atom abstraction by chlorine atoms. (1,2) Recently a direct comparison of vibrational with translational energy for this reaction has been completed. (3) Dissociative adsorption of methane on metal surfaces, the rate limiting step in the steam reforming of methane, has also shown sensitivity to the methane vibrational state. (4-8) Many important chemical reactions, particularly those involving transition metal catalysts, involve insertion, rather than abstraction. Here, we performed the first systematic study of the relative reactivity of the methane antisymmetric stretch (ν 3 ) in a prototype insertion reaction, Y + CH 4 → YCH 2 + H 2, which has a large (20 ± 3 kcal/mol) insertion barrier. 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Rev. Lett. 94: Maroni P, et a.l (2005) State resolved gas-surface reactivity of methane in the symmetric C-H stretch vibration on Ni(100). Phys. Rev. Lett. 94: Wittborn AMC, Costas M, Blomberg MRA, Siegbahn PEM (1997) The C-H activation reaction of methane for all transition metal atoms from the first three rows. J. Chem. Phys. 107: ReactantIsotopologueRelative reactivity findingRef. ClCHD 3 bending > translation > antisymmetric stretch(3) Ni(111)CH 4 ν 3 > translation(4) Ni(100)CH 4 translation > ν 3 (5) Pt(111)CH 4 translation > 2ν 3 (6) Ni(111)CH 4 ν 3 >> 3ν 4 (7) Ni(100)CH 4 ν 3 >> 3ν 4 (7) Ni(100)CH 4 ν 1 >> ν 3 (8) YCH 4 ν 3 > translation this work Isoenergetic Collision Conditions A pulsed beam of neutral metal atoms is produced by laser ablation of a rotating metal rod in a stream of carrier gas supplied by a pulsed valve. This pulse is temporally refined to ~7μs by a mechanical chopper wheel. Velocity is tuned by changing the carrier gas. A pulsed beam of CH 4 is supplied by a second valve. The velocity is tuned by changing the proportion of CH 4 in the gas mixture. Metal-containing products are photoionized at 157nm by a pulsed F 2 laser in the extraction region of a quadrupole mass spectrometer. Time- of-flight distributions are measured by recording the ion signal as a function of the 157nm laser firing time. Chopper Wheel Two experiments were performed, the first with relatively fast beams giving a center of mass collision energy of 23.8 kcal/mol (above the insertion barrier), and the second with nearly identical total energy split as 15.1 kcal/mol (below the insertion barrier) in translation and 8.63 kcal/mol in vibration (one quantum of the ν 3 stretch). The two total energy distributions are shown at left. Signal at M/e=103 (YCH 2 + ) is observed at the higher collision energy and at the lower collision energy with the OPO on, but not at the lower collision energy with the OPO off. Light Source for Vibrational Excitation: Infrared Pulsed Optical Parametric Oscillator/Amplifier 1064nm pump Oscillator signal (~806nm) 532nm pump R70% 800nm HR 800nm & 532nm Beam dump λ/2 plate λ/2 plate High energy plate polarizer Fiber launch for seed DFB laser 40dB Faraday isolator Expanding telescope KTP 60˚, AR 532nm & 800nm KTP, 45˚ Oscillator idler (~1567nm) & Amplifier idler (~3018cm -1 ) Beam dump KTA, 45˚ Reducing telescope The beam of a homebuilt pulsed OPO/OPA tuned to the Q(1) line of the CH 4 ν 3 band at cm -1 intersects the CH 4 beam 5mm upstream of the interaction region. The saturation of the transition is determined to be near complete by monitoring the YCH 2 signal at the center of mass scattering angle as a function of OPO energy. The TOF spectra and laboratory angular distributions could be simulated using an isotropic CM angular distribution kinetic energy release distribution (P(E)) peaked at 3 kcal/mol, extending to 11 kcal/mol. Energy (kcal/mol) The relative reactivity was calculated as the ratio of the total CM product flux (calculated during fitting) normalized by the relative beam fluxes. The flux for vibrationally excited CH 4 included a calculation of the pumping fraction. Taking all factors into account we find that the total reactive cross section of CH 4 ( ν 3 =1) is ≥ 2.2 times that for ground state CH 4. The observed behavior is somewhat analogous to 3- atom systems with “late” potential energy barriers. Assuming vibrationally adiabatic behavior, reactant vibrational excitation provides access to lower energy transition state geometries for insertion. Analogous studies of reactions involving bending and symmetric stretching levels are in progress. Together, these studies provide the first fundamental insight into how insertion chemistry may be promoted by different forms of reactant energy. This research was supported by the National Science Foundation under grant CHE