A Two Transition State Model for Radical−Molecule Reactions:  A Case Study of the Addition of OH to C₂H₄

Abstract

A two transition state model is applied to the study of the addition of hydroxyl radical to ethylene. This reaction serves as a prototypical example of a radical-molecule reaction with a negative activation energy in the high-pressure limit. The model incorporates variational treatments of both inner and outer transition states. The outer transition state is treated with a recently derived long-range transition state theory approach focusing on the longest-ranged term in the potential. High-level quantum chemical estimates are incorporated in a variational transition state theory treatment of the inner transition state. Anharmonic effects in the inner transition state region are explored with direct phase space integration. A two-dimensional master equation is employed in treating the pressure dependence of the addition process. An accurate treatment of the two separate transition state regions at the energy and angular momentum resolved level is essential to the prediction of the temperature dependence of the addition rate. The transition from a dominant outer transition state to a dominant inner transition state is predicted to occur at about 130 K, with significant effects from both transition states over the 10 to 400 K temperature range. Modest adjustment in the ab initio predicted inner saddle point energy yields theoretical predictions which are in quantitative agreement with the available experimental observations. The theoretically predicted capture rate is reproduced to within 10% by the expression [4.93 x 10(-12) (T/298)(-2.488) exp(-107.9/RT) + 3.33 x 10(-12) (T/298)(0.451) exp(117.6/RT); with R = 1.987 and T in K] cm3 molecules(-1) s(-1) over the 10-600 K range.

Epoxide as a precursor to secondary organic aerosol formation from isoprene photooxidation in the presence of nitrogen oxides

“…Recent VARIFLEX calculations of rate constants for OH addition to hydrocarbons containing carbon double bonds have revealed similar differences. Greenwald et al (34) observed for the OH + ethene addition reaction that decreasing the energy of the transition state analogous to TS 2 by 1.0 kcal·kmol −1 , which is within the expected accuracy of their ab initio calculations, results in good agreement with experimental data between 300 and 400°K. A similar approach was adopted by Zador et al (35) in their computational investigations of the OH + propene addition reaction.…”

The OH-Initiated Oxidation of 1,3-Butadiene in the Presence of O2 and NO: A Photolytic Route To Study Isomeric Selective Reactivity

“…Recently, this treatment has been applied to the case of the hydroxyl radical addition to ethylene. 49 At high temperatures, the outer transition state becomes unimportant and the kinetic model can be accurately approximated by including only the flux through the inner transition state. At 300 K, the inner transition state serves as the dominant bottleneck such that an inclusion of both transition states only reduces the rate by 30%.…”

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

“…Initially at large intermolecular distances the OH and unsaturated molecule will form a weakly bound van der Waals complex, and only if rearrangement of this to form a chemically bound adduct occurs via a submerged barrier (barrier is below the 65 reagent asymptote) will the reaction be fast at very low temperatures. 2,3 In this paper we report rate coefficients at low temperature for reactions of OH with ethene, ethyne and propyne, all of which have been observed in ISC 4 and planetary 70 atmospheres 5 . The reactions proceed via addition of OH to the double or triple bond forming a stable adduct, which is either stabilised by collisions or dissociates back to the reagents, leading to a pressure dependent rate coefficient.…”