HO + OClO Reaction System: Featuring a Barrierless Entrance Channel with Two Transition States

Abstract

Chlorine-containing compounds play a significant role in the troposphere and are key players in the stratosphere. The free radical compound OClO reacts with HO free radicals, but the existing experimental kinetics data are limited and uncertain. In the present theoretical investigation, the reaction mechanism, rate constants, and product branching ratios for the HO + OClO reaction system were computed over wide temperature and pressure ranges and compared with the existing experimental data. Stationary points on the singlet potential energy surface (PES) were calculated at high levels of theory, and the kinetics parameters were computed using several methods, including variational transition state theory (VTST) and RRKM/master equation techniques. The computed PES is in reasonable agreement with previous calculations, and the computed rate constants and branching ratio are in good agreement with the recent experiments. The results are used as the basis for recommendations for atmospheric chemistry modeling. The PES along the reaction path forming the peroxy bond has a steplike structure and only a very weakly bound prereactive complex, and yet it still supports two transition states along the reaction path. This feature may also be present in other reactions in which electrostatic forces align the approaching reactants in an unfavorable orientation at long distances, thus requiring a dramatic geometry change before reaction can take place.

Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

“…The optimized geometries at each point were used to obtain the rotational constants, the vibrational zero‐point energy (ZPE) corrections, and frequencies orthogonal to the reaction path. The CVTST rate constant for a given temperature is identified as the minimum “trial” CVST rate constant as a function of distance along the reaction coordinate 31–33 . Using the simulated kinetics constants, a sensitivity analysis was performed using the KINTECUS 34 program to determine the weight of the proposed dissociation pathways at different pyrolysis temperatures.…”

“…30 For dissociation channels, canonical variational transition state theory (CVTST) was used to locate the transition state assuming that the path corresponded to a bond being broken, that is, a barrierless The CVTST rate constant for a given temperature is identified as the minimum "trial" CVST rate constant as a function of distance along the reaction coordinate. [31][32][33] Using the simulated kinetics constants, a sensitivity analysis was performed using the KINTECUS 34 program to determine the weight of the proposed dissociation pathways at different pyrolysis temperatures. When the pyrolysis temperature is increased to 688 C and beyond, new peaks appear in the mass spectrum at m/z 44 (CO 2…”

Ab-initio studies of thermal unimolecular decomposition of furan: A complementary deterministic and stochastic master equation model

“…The HIR parameters were determined with the use of MSMC-GUI 67,68 , the details of which can be found in our previous work. 69 The reaction path degeneracy 70 For reactions having no intrinsic energy barriers, Inverse Laplace Transform (ILT) approach 71 was employed to obtain the microcanonical rate constants, k(E). For the calculations of k(E), the density of states (DOS) of the reacting species and the empirical Arrhenius parameters (A∞ and E∞ ≈ E0) of the high pressure limiting rate coefficients are required.…”

BrHgO• + C2H4 and BrHgO• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Prereactive Complexes and Bifurcation

“…For both dissociation of PRCs, these rate constants were treated using the quasimicrocanonical approximation. In other words, the position of the canonical variational transition state was used to compute k ( E ). , At each temperature and pressure simulated, we determined the fraction, f ([M], T ), of PRCs going on to react. Accordingly, k ([M], T ) is given by k VTST ( T ) × f ([M], T ).…”