PICTURING THE TRANSITION-STATE REGION AND UNDERSTANDING VIBRATIONAL ENHANCEMENT FOR THE CL-]HCL+CH3 REACTION(CH4)

Comparison of state-to-state differential cross sections for methane i n the ground vibrational state to methane with one quantum of asymmetr ic stretch excitation probes the effect of C-H stretch excitation on t he reaction of atomic chlorine with methane. We previously reported st ate-to-state differential cross sections and HCl product state populat ion distributions for the vibrationally excited reaction. Here we repo rt analogous measurements of the reaction for methane in the vibration al ground state. Photolysis of molecular chlorine produces chlorine at oms that react with methane molecules at 0.16 eV collision energy. Cal ibrated resonance-enhanced multiphoton ionization (REMPI) determines t he product state distributions, and the core-extraction technique meas ures the angular scattering distribution. The product HCl(v=0) is form ed with a cold rotational-state distribution and is strongly back scat tered. The product state and angular scattering distributions for the ground-state reaction are consistent with a line-of-centers model in w hich the cone of acceptance is only narrowly open. The rotational-stat e distributions and comparisons to thermal rate data indicate that the C-H-Cl angle must be constrained in the transition-state region. One quantum of C-H asymmetric stretch vibrational excitation enhances the rate of reaction at a collision energy of 0.16 eV by a factor of 30 +/ - 15 (+/-2 sigma). The behavior of the ground-state reaction is in mar ked contrast to our earlier results for the reaction of chlorine atoms with C-H stretch-excited methane, for which the state-to-state angula r scattering distributions were consistent with a widely open cone of acceptance. By using the approximation that hard-sphere scattering des cribes the relation between impact parameter and scattering angle, we can transform the measured state-to-state differential cross section i nto the distribution of impact parameters that lead to reaction, which forms what we call a b map. This b map pictorially shows that the gro und-state reaction occurs only for head-on collisions (with small impa ct parameters), whereas C-H stretch vibrational excitation allows reac tivity to spread to the periphery of the methane molecule. The data in dicate that the mechanism of vibrational enhancement is opening of the cone of acceptance and lessening the necessity for collinearity of th e C-H-Cl angle in the transition-state region.