Breakdown of the vibrationally adiabatic approximation in the early-barrier CH3 + HBr → CH4 + Br reaction

Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: integral cross sections, rate constants and microscopic mechanism

“…48. The interesting observation that the excitation function switches from activated (initially zero and rising starting from a threshold energy) to capture-type (diverging with decreasing collision energy) that can be drawn from the figures obtained by Wang et al, 29 which show that the extra attraction can even override the presence of a positive potential barrier. This same phenomenon was observed with reactions with early or late, low or high positive barriers (H + H 2 O, H + HF, H + HCl), [49][50][51][52][53] all displaying capture-type excitation functions but, in contrast to the present reaction only at high vibrational excitation of the breaking bond.…”

“…The vibrational excitation of H 0 Br was found to enhance the reactivity by roughly a factor of two, while the excitation of the umbrella mode of CH 3 has the opposite effect. Wang et al 29 performed eight-dimensional quantum dynamics calculations using the analytical PES of Espinosa-Garcia and found similar vibrational effects, but from their results positive activation energy can be inferred.…”

The Role of Zero-Point Vibration and Reactant Attraction in Exothermic Bimolecular Reactions with Submerged Potential Barriers: Theoretical Studies of the R + HBr → RH + Br (R = CH3, HO) Systems

“…These rules apply to reactions with positive potential barriers, and not much is known about how the presence of the prereaction potential well interferes with them. For reaction R1 , the currently available information on how the vibrational excitation of the HBr molecule, i.e., the breaking bond, influences the reaction cross sections comes from the theoretical study by Y. Wang et al, 30 who performed reduced-dimensional quantum scattering calculations on reaction R1 on an analytical potential energy surface. 31 On the PES they used, the barrier to reaction is positive, and accordingly, the excitation functions they obtained for the reaction of HBr in the vibrational ground state correspond to what is referred to as activated behavior: the cross sections are zero below a threshold and rise slowly above it.…”

Eight-Dimensional Wave Packet Dynamics Within the Quantum Transition-State Framework: State-to-State Reactive Scattering for H2 + CH3 ⇆ H + CH4

“…Extending the understanding of chemical reactivity at the most detailed level of quantum-state resolution toward increasingly large systems is a subject of fundamental importance. Recent research in this area focuses on reactions of methane with different atoms as prototypical examples. , Impressive progress has been achieved in the experimental studies of X + CH 4 → HX + CH 3 (X = H, F, Cl, and O) reactions and their isotopologues. − The accurate theoretical description of these reactions is still a challenging task. , Reduced-dimensional wave packet dynamics calculations explicitly including seven or eight dimensions were reported for the H + CH 4 ⇆ H 2 + CH 3 reaction and their isotopologues, − the F­( 2 P) + CHD 3 → HF + CD 3 reaction, the Cl­( 2 P) + CH 4 → HCl + CH 3 reaction and its isotopologues, ,− the O­( 3 P) + CH 4 → HO + CH 3 reaction, , and the HBr + CH 3 → Br + CH 4 reaction . Full-dimensional quantum dynamics calculations for these six-atom reactions are even more challenging, and reports are limited to the H + CH 4 → H 2 + CH 3 reaction − and the H + CHD 3 → H 2 + CD 3 reaction. − Almost all full-dimensional calculations employed the quantum transition-state (QTS) framework − and the multiconfigurational time-dependent Hartree (MCTDH) approach , to obtain reaction rates, , initial state-selected reaction probabilities, − ,− and state-to-state reaction probabilities .…”

“…16,17 Reduceddimensional wave packet dynamics calculations explicitly including seven or eight dimensions were reported for the H + CH 4 ⇆ H 2 + CH 3 reaction and their isotopologues, 18−25 the F( 2 P) + CHD 3 → HF + CD 3 reaction, 26 the Cl( 2 P) + CH 4 → HCl + CH 3 reaction and its isotopologues, 14,27−29 the O( 3 P) + CH 4 → HO + CH 3 reaction, 30,31 and the HBr + CH 3 → Br + CH 4 reaction. 32 Full-dimensional quantum dynamics calculations for these six-atom reactions are even more challenging, and reports are limited to the H + CH 4 → H 2 + CH 3 reaction 33−41 and the H + CHD 3 → H 2 + CD 3 reaction. 42−45 Almost all full-dimensional calculations employed the quantum transition-state (QTS) framework 46−50 and the multiconfigurational time-dependent Hartree (MCTDH) approach 51,52 to obtain reaction rates, 33,34 initial state-selected reaction probabilities, 35−39,42−45 and state-to-state reaction probabilities.…”