The relative reactivity of CH3D molecules with excited symmetric and antisymmetric stretching vibrations

Abstract

Experimental and theoretical studies explore the reactivity of the symmetric and the antisymmetric stretching vibrations of monodeuterated methane (CH3D). Direct infrared absorption near 3000 cm−1 prepares CH3D molecules in three different vibrationally excited eigenstates that contain different amounts of symmetric C–H stretch (ν1), antisymmetric C–H stretch (ν4), and bending overtone (2ν5) excitation. The reaction of vibrationally excited CH3D with photolytic chlorine atoms (Cl, 2P3/2) yields CH2D products mostly in their vibrational ground state. Comparison of the vibrational action spectra with the simulated absorption spectra and further analysis using the calculated composition of the eigenstates show that the symmetric C–H stretching vibration (ν1) promotes the reaction seven times more efficiently than the antisymmetric C–H stretching vibration (ν4). Ab initio calculations of the vibrational energies and eigenvectors along the reaction coordinate demonstrate that this difference arises from changes in the initially excited stretching vibrations as the reactive Cl atom approaches. The ν1 vibration of CH3D becomes localized vibrational excitation of the C–H bond pointing toward the Cl atom, promoting the abstraction reaction, but the energy initially in the ν4 vibration flows into the C–H bonds pointing away from the approaching Cl atom and remains unperturbed during the reaction. A simple model using vibrational symmetries and vibrational adiabaticity predicts a general propensity for the greater efficiency of the symmetric stretch for accelerating the reaction in the vibrationally adiabatic limit.

Tracking the energy flow along the reaction path

“…Theory predicted that as the Cl atom approaches the H end of CHD 3 , the chemical interaction induces a static curvature coupling (i.e., coupling of a vibrational mode to the reaction coordinate induced by the curvature of the reaction path) between the C-H stretching (v 1 ) motion and the reaction coordinate, resulting in a significant decrease of its frequency in the transition-state region (19)(20)(21)(22) and allowing energy flow between this mode and the reaction coordinate. Similar behavior was found for the CD 3 umbrella mode (v 3 ), yet other modes show little variation in frequencies.…”

“…By using the reaction path Hamiltonian approach (26), previous ab initio calculations of isotopically analogous reactions mapped out the minimum energy path and the evolution of the vibrational frequencies of various modes along the reaction path (19)(20)(21)(22). By adding the theoretically calculated vibration frequencies (with isotope corrections) to the minimum energy path, we connected the vibrational energy levels according to their symmetries, employing the rule that energy levels for vibration of the same symmetry do not cross (26).…”

“…The notation of 2 i refers to the vibrational mode 2 (the umbrella-bend) with i-quantum excitation.) Pair-correlated state and angular distributions were then exploited, in light of available ab initio theory (19)(20)(21)(22), to unveil the microscopic reaction pathways. We further sharpened the comparison with the result of a ground-state reaction at either the same initial translation energy (E c ) or the higher E c with an equivalent amount of total energy (vibration ϩ translation).…”

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,…”

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

“…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.…”