Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl+CH4 reaction

Abstract

The effects of two nearly isoenergetic C-H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl(2), CH(4), and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (nu(3)=3019 cm(-1)) of CH(4) is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (nu(1)=2917 cm(-1)) of CH(4) is prepared by stimulated Raman pumping. Photolysis of Cl(2) at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290+/-175 cm(-1) (0.16+/-0.02 eV). Finally, the nascent HCl or CH(3) products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH(3) products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. [J. Chem. Phys. 119, 9568 (2003)] report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(v=1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(v=0,J) and have more rotational excitation. The HCl(v=0,J) products are predominantly back and side scattered. Measurements of the CH(3) products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(v=0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.

On the origin of mode- and bond-selectivity in vibrationally mediated reactions on surfaces

“…When vibrationally non-adiabatic couplings are introduced, n 1 and n 3 differ in their ability to couple to lower energy vibrations, including the totally symmetric vibrational ground state, and thereby free vibrational energy for reaction coordinate motion. While similar patterns of mode-and bond selectivity were observed for methane activation on surfaces and for the gas phase bimolecular reaction of Cl with methane or its isotopologues, [69][70][71] the extent of bond-selectivity reported to date is greatest in gas-surface reactions. This most likely results from the extremely short surface residence times for methane, which severely limit the extent of IVR that can occur prior to the gasphase reagent's collision with the repulsive wall of the PES.…”

Quasi-Classical Trajectory Calculations Analyzing the Dynamics of the C−H Stretch Mode Excitation in the H + CHD3 Reaction

“…In a series of pioneering studies, Zare and co-workers [1][2][3] and Crim and co-workers [4][5][6] analyzed the effects of both reactant stretching and bending excitations on the dynamics of polyatomic systems.…”

Velocity map imaging the dynamics of the reactions of Cl atoms with neopentane and tetramethylsilane

“…[21][22][23][24][25][26][27][28][29][30][31][32] The experimental measurements and dynamical calculations for H-atom abstraction reactions involving simple alkanes illustrate a wealth of dynamical behaviour. For example, the shape of the transition state, with near linear Cl-H-C moiety, is reflected in the low rotational excitation of HCl products; scattering angles are largely determined by impact parameter and their distributions can vary with product rotational and vibrational quantum states; 1 and the reactions of Cl atoms with methane and partially deuterated isotopologues exhibit reagent vibrational mode specificity, [4][5][6][7][9][10][11][12]14,15,[33][34][35][36][37][38][39][40] electronically non-adiabatic pathways, 29,31,41,42 and evidence for scattering resonances. 43 Reactions of functionalized organic molecules (RH = alcohols, 24,44,45 amines, 46 alkyl halides 23,47 and linear and cyclic ethers 24,44,…”

“…In an early dynamical study, Flynn and co-workers employed infra-red absorption spectroscopy of the DCl products of reaction of Cl atoms with d 12 -cyclohexane, 3 but the subsequent combination of resonance enhanced multi-photon ionization (REMPI) or vacuum ultraviolet (VUV) ionization with velocity resolution of reaction products has proved most informative. The velocity information was initially derived from analysis of time-of-flight (TOF) profiles obtained in a TOF mass spectrometer, [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] but velocity map imaging (VMI) 19,20 has since become the method of choice. [21][22][23][24][25][26][27][28][29][30][31][32] The experimental measurements and dynamical calculations for H-atom abstraction reactions involving simple alkanes illustrate a wealth of dynamical behaviour.…”