The relative reactivity of the stretch–bend combination vibrations of CH4 in the Cl (2P3/2)+CH4 reaction

Abstract

Direct infrared absorption prepares CH4 in two nearly isoenergetic vibrationally excited states, the symmetric stretch–bend combination (ν1+ν4) and the antisymmetric stretch–bend combination (ν3+ν4), for a study of the effect of stretching vibrations of CH4 on the reaction, CH4+Cl(2P3/2)→CH3+HCl. Comparison of intensities in the action spectra with those in the simulated absorption spectra shows that vibrational excitation of methane to the ν1+ν4 state promotes the reaction more efficiently than excitation to the ν3+ν4 state by a factor of 1.9±0.5. The reaction of methane in both vibrational states produces a substantial fraction (35%) of the CH3 products with the umbrella mode (ν2) excited, which we attribute to the presence of the bending vibration (ν4) in the combination states. The similarity of the vibrational population distributions of the products for the two excitations implies that the differences in the action spectra arise from the relative reactivity of the states rather than from population of different product states. This result is consistent with theoretical calculations that predict a stronger coupling of the symmetric stretching vibration to the reaction coordinate than the antisymmetric stretch. Analyzing the infrared laser power dependence of the signal with a simple two state model shows that the reaction cross section of the ν1+ν4 state of methane is 19±5 times larger than that of methane molecules in their ground or thermally populated vibrational states.

Empirical Valence Bond Theory Studies of the CH4 + Cl → CH3 + HCl Reaction

“…The outcome of the reaction can be controlled by vibrational excitation of specific bonds or vibrational modes; [27][28][29] for example, excitation of the symmetric stretch-bend combination promotes the reaction twenty times more efficiently than exciting the antisymmetric stretch-bend combination. 30 However, Liu and coworkers showed that excitation of the CH stretch in the CD3H molecule facilitated the reaction to a lesser extent than increasing the collision energy by a corresponding amount. 31 This observation is contrary to the predictions of the Polanyi rules.…”

Bond-Selective and Mode-Specific Dissociation of CH3D and CH2D2 on Pt(111)

“…Two iso-energetic reactant states can lead therefore to rather different reaction probabilities. One example of a mode-specific gas-phase reaction is the H-abstraction reaction of CH 3 D by a chlorine atom, reported by Yoon et al 8,9 The authors used their state-resolved reactivity data to calculate state-specific reaction cross sections for the ν 1 , ν 4 , and 2ν 5 normal modes of CH 3 D. They find the symmetric stretch vibration (ν 1 ) to be 7 times more reactive than the antisymmetric stretch (ν 4 ) whereas the bending overtone (2ν 5 ) had no detectable influence on the reactivity. Furthermore, the reaction of vibrationally excited CH 3 D with chlorine was reported to be bond-selective.…”

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,…”