Exploring the features on the OH + SO₂potential energy surface using theory and testing its accuracy by comparison to experimental data

Abstract

Ab initio theory has been used to identify the pre-reaction complex in the atmospherically important reaction between OH + SO2, (R1), where the binding energy of the pre-reaction complex was determined to be 7.2 kJ mol-1. Using reaction rate theory, implemented with the master equation package MESMER, the effects of this complex on the kinetics of R1 at temperatures above 250 K have been investigated. From simulations and fitting to the experimental kinetic data, it is clear that the influence of this pre-reaction complex is negligible and that the kinetics are controlled by the inner transition-state that leads to the product, HOSO2. While the effect of this complex on the thermal kinetics is small it potentially provides an efficient route to remove energy from vibrationally excited OH. The fitting to the past experimental data reveals that this inner transition-state is submerged with a barrier -0.25 kJ mol-1 below the entrance channel, which is outside the range predicted from the best theoretical calculations. The data fitting also yielded ΔR1H0K equal to -(109 ± 5.6) kJ mol-11 and a more precise expression for k∞1(T), (5.95 ± 0.83) × 10-13 × (T/298)-0.11±0.27.

Kinetic study of the OH + ethylene reaction using frequency‐modulated laser absorption spectroscopy

“…Similar results were reported and discussed by Golden for the OH + C 2 H 4 reaction, and more recently by Medeiros et al. for the OH + SO 2 reaction which, like reaction 1a, involves the formation of a weakly bound (1.7 kcal mol –1 ) prereaction complex that proceeds over a submerged barrier to HOSO 2 product …”

“…18 Extracting kinetics from a PES involving a prereaction complex is of significant interest as the local energy minima can cause strongly non-Arrhenius behavior and influence the collision-induced energy transfer process. [34][35][36][37] In the case of reaction 1a, the magnitude of the vdW complex well preceding reaction is comparable to both the computed submerged barrier height (0.1-2.24 kcal mol -1 ), 14,23 and the expected accuracy of high-level calculations (± 1-2 kcal mol -1 ). Consequently, theoretical studies of the OH + C 2 H 4 reaction have reported barrier heights both above 18,19 and below the entrance channel, [13][14][15][16]20,23 depending on the level of theory employed.…”

“…Collision‐induced energy transfer was treated using an exponential down model with the parameter 〈Δ E 〉 down defined as the average amount of energy transferred through a deactivating collision with the bath gas. Experimental data can be used to significantly constrain the input parameters used in chemical master equation calculations . Given the aforementioned variation in ab initio estimates of the zero‐point‐corrected energy of the inner TS relative to the reactants (herein referred to as ZPCE–Inner TS), the ZPCE–Inner TS value was allowed to vary within the uncertainty limits (–2.1 to 1.9 kcal mol –1 ) of the current high‐level theoretical estimates .…”

“…Experimental data can be used to significantly constrain the input parameters used in chemical master equation calculations. 36,[64][65][66] Given the aforementioned variation in ab initio estimates of the zero-point-corrected energy of the inner TS relative to the reactants (herein referred to as ZPCE-Inner TS), the ZPCE-Inner TS value was allowed to vary within the uncertainty limits (-2.1 to 1.9 kcal mol -1 ) of the current high-level theoretical estimates. 13 The present measurements were combined with all the aforementioned experimental k 1a values reported in the literature, [29][30][31]33 with the exception of the measurements by Zellner and Lorenz, 32 and treated as a single data set in the one-dimensional master equation analysis, with only the ZPCE-Inner TS and ⟨ΔE⟩ down treated as adjustable fitting parameters.…”

“…High‐level calculations have shown that long‐range attractive interactions between the reactants result in the formation of a weakly bound (1.9 kcal mol –1 ) van der Waals (vdW) complex, which proceeds through a loose transition state into the C 2 H 4 OH adduct radical well . Extracting kinetics from a PES involving a prereaction complex is of significant interest as the local energy minima can cause strongly non‐Arrhenius behavior and influence the collision‐induced energy transfer process . In the case of reaction 1a, the magnitude of the vdW complex well preceding reaction is comparable to both the computed submerged barrier height (0.1–2.24 kcal mol –1 ), and the expected accuracy of high‐level calculations (± 1–2 kcal mol –1 ).…”