Multiple transition states in chemical reactions: Variational transition state theory studies of the HO2 and HeH+2 systems

Abstract

The results of a microcanonical variational transition state theory study of the HO₂ and HeH⁺₂ systems are reported. The calculations were carried out using a modification of the Wardlaw–Marcus flexible transition state theory method in which the sum of states N(E,R) is calculated directly. For the HO₂ system, the results obtained using the Melius–Blint potential surface are in excellent agreement with previously reported variational transition state theory calculations of Rai and Truhlar on this system. In particular, two transition states were found for the HO₂→OH+O half‐reaction. However, calculations using the Lemon–Hase surface produced only one transition state. From calculations carried out on surfaces constructed by combining the two original surfaces and calculations carried out using an orbiting transition state variational model (i.e., ignoring the angular part of the potential), it is shown conclusively that the two transition states found for the Melius–Blint surface arise from the peculiar shape of the minimum reaction path for this surface. Also, the orbiting transition state calculations suggest that phase space theory can describe this half‐reaction on the Lemon–Hase surface over a large energy range provided that the potential along the minimum reaction path is used in the phase space theory calculations. Other calculations, in which the parameters of the Lemon–Hase surface were adjusted, were also carried out in order to determine the conditions necessary to support multiple transition states. The results of these calculations suggest that multiple transition states are more likely to occur when a strong bottleneck is present in the angular part of the potential, especially when the intermediate complex exists in a shallow well. These predictions were confirmed by the results obtained for the HeH⁺₂ system using the McLaughlin–Thompson–Joseph–Sathyamurthy surface. Two transition states were found for the HeH⁺₂→HeH⁺+H half‐reaction and calculations using the orbiting model showed that the origin of the multiple transition states is not the minimum reaction path as in the case of the Melius–Blint surface. Furthermore, the energy at which ‘‘transition state switching’’ occurs is in reasonable accord with the energy at which phase space theory calculations begin to diverge from the experimental data on the cross section for this system. Lifetime calculations for the pseudocomplex lying between the two transition states suggest that the observation of multiple transition states may be possible using femtosecond transition state spectroscopy. Finally, calculations on the HeH⁺₂→He+H⁺₂ half‐reaction showed that one transition state exists for this half‐reaction at low energy, while at higher energy there are no transition states.