Effects of Bending Excitation on the Reaction of Chlorine Atoms with Methane

Abstract

Many chemical reactions are accelerated by heating the reagents. This effect is caused by more energy being partitioned into the reagents electronic, translational, vibrational, and rotational degrees of freedom that ultimately becomes available for overcoming the reaction barrier. Which degrees of freedom are most effective at driving chemical reactions? For endoergic reactions involving an atom and a diatom, Polanyi [1] showed that vibrational excitation of the reagent diatom is the most effective means of overcoming the reaction barrier because the stretching motion along the diatomic internuclear axis efficiently couples to the reaction coordinate, that is, the set of motions that transforms the reagents into the products. For reactions involving polyatomic reagents, the simple concepts associated with atom + diatom reactions become complicated by the 3NÀ6 extra degrees of freedom available for internal motions of a nonlinear N-atom polyatomic molecule. Intuitively, stretching vibrations of polyatomic molecules are expected to have the largest effects on abstraction reactions because energy is placed directly into the breaking bond and they most resemble the stretching vibration of diatomic molecules. This notion is confirmed for the Cl + CH 4 reaction, where one quantum of antisymmetric stretch excitation enhances the reaction by a factor of approximately 30.[2]The effects of reagent bending vibrations on chemical reactions, on the other hand, are less intuitive because they require the concerted motion of three or more atoms in the reagent. Is the energy in these modes available for overcoming the reaction barrier? Unlike the stretching vibrations, the bending vibrations are low-frequency and consequently have less energy. They do not obviously map onto the reaction coordinate. Moreover, it is known from previous studies of the Cl + CH 4 reaction [3] that internal energy placed in more than one CÀH stretch of methane remains localized in the methyl product, that is, all the internal energy is not available to appear in internal motion of HCl or in translational motion of the escaping product pair. The effects of bending vibrations have been largely unexplored experimentally despite the fact that these low-frequency vibrations are more easily populated at thermal temperatures. To date, the influence of bending vibrations on chemical reactions remains ambiguous, and many theoretical studies of direct polyatomic reactions treat the system in reduced dimensionality, simply assuming that these low-frequency modes play little or no role.[4]Herein we present direct measurements of the effects of bend-excitation on an atom + polyatom reaction system under single collision conditions. We have chosen the Cl + CH 4 reaction [Eq. (1)] both as a prototype and for its practical importance to combustion and atmospheric chemistry. This reaction is thought to be responsible for the removal of chlorine in the stratosphere, and kinetic studies have shown a nonlinear variation of the logarithm of the rate constant versus the r...

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

“…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.…”

“…[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,…”

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

“…If vibrational excitation of the methyl radicals is to be useful in promoting nonadiabatic reactivity and thus production of Cl * , the vibrational motion ͑which is mainly of the CH 3 umbrella bending mode͒ must be retained along the reaction coordinate, at least as far as the TS region. Prior work has clearly shown, however, that a vibrationally adiabatic picture of the reverse Cl+ CH 4 reaction does not hold for bending vibrations, 19 and we expect that the initial bending mode excitation of CH 3 is quenched in the entrance channel of the PES for its reaction with HCl. The symmetric stretch excitation of CH 3 might, in contrast, be preserved along the reaction pathway but it occurs for only 12% of the fast CH 3 radicals 40 and cannot explain the observed Cl * branching of 15%, as fast and slow CH 3 radicals are produced in a 1: 2.1 ratio.…”

“…The reverse reaction of Cl atoms with CH 4 ͑and isotopologs resulting from partial or complete deuteration͒ is well established as a benchmark polyatomic reactive system for studying scattering and vibrational mode and bond specific dynamics. [15][16][17][18][19][20][21][22][23][24] Reaction of Cl * with CH 4 adiabatically correlates with an electronically excited state of the CH 3 radical, and nonadiabatic formation of the energetically accessible CH 3 ͑X 2 A 2 Љ͒ + HCl products is not observed for collisions with energies just above the energetic barrier 25,26 but does occur at much higher collision energies. 27 In studies of the kinetics of collisional processes between Cl * atoms and small hydrocarbons, Matsumi et al 28 demonstrated that the rate of SO quenching of Cl * to grounda͒ Present address: Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan.…”

Unraveling the Nature of Chemical Reactivity of Complex Systems 3

“…Yet, the measured branching ratios (the red bars) showed significant variance with that from the ground-state reaction (either the green or blue bars). In particular, the adiabatically correlated (0, 1 1 ) s pair accounts for only 30% of total reactivity of this isotope channel (a value identical to the other isotope channel may be fortuitous), in significant deviation from the adiabatic expectation or the spectator paradigm (34,35) that the initial excitation of the unreactive C-H bond survives as the (0, 1 1 ) s product pair. Therefore, the initial C-H excitation is counterintuitively not a mere spectator when a D atom is abstracted.…”