Simplified Protocol for the Calculation of Multiconformer Transition State Theory Rate Constants Applied to Tropospheric OH-Initiated Oxidation Reactions

Abstract

Chemical kinetics plays a fundamental role in the understanding and modeling of tropospheric chemical processes, one of the most important being the atmospheric degradation of volatile organic compounds. These potentially harmful molecules are emitted into the troposphere by natural and anthropogenic sources and are chemically removed by undergoing oxidation processes, most frequently initiated by reaction with OH radicals, the atmosphere’s “detergent”. Obtaining the respective rate constants is therefore of critical importance, with calculations based on transition state theory (TST) often being the preferred choice. However, for molecules with rich conformational variety, a single-conformer method such as lowest-conformer TST is unsuitable while state-of-the-art TST-based methodologies easily become unmanageable. In this Feature Article, the author reviews his own cost-effective protocol for the calculation of bimolecular rate constants of OH-initiated reactions in the high-pressure limit based on multiconformer transition state theory. The protocol, which is easily extendable to other oxidation reactions involving saturated organic molecules, is based on a variety of freeware and open-source software and tested against a series of oxidation reactions of hydrofluoropolyethers, computationally very challenging molecules with potential environmental relevance. The main features, advantages and disadvantages of the protocol are presented, along with an assessment of its predictive utility based on a comparison with experimental rate constants.

Conformational Sampling for Transition State Searches on a Computational Budget

“…Conversely, these benchmarks illustrate the hazards, including overlooked reaction pathways and inaccurate barriers, of neglecting conformational sampling in TS searches. This also comports with the findings of several recent studies across multiple application domains regarding the qualitative importance of conformational sampling for TS searches. ,,− …”

Computational Study on the Reaction of β-Hydroxyethylperoxy Radical with HO2 and Effects of Water Vapor

“…Because of the multiconformer nature in reactants, products, and transition states, the multiconformer transition state theory (MC-TST) was utilized for rate coefficient prediction. − The equation of MC-TST for a unimolecular process is given by eq where κ is the tunneling correction factor calculated by the asymmetric Eckart method, k B is the Boltzmann constant, h is the Planck constant, E i and E j are the zero-point corrected energy of transition state conformer i and reactant conformer j relative to the lowest-energy conformer, and Q TS, i and Q R,j are the partition functions of transition state conformer i and reactant conformer j .…”

Teoria do Estado de Transição ao Serviço da Química Verde

“…De facto, para se conseguir usar esta equação de uma forma sistemática e o mais automatizada possível, torna-se necessário ter um protocolo computacional previamente preparado que consiga lidar com as consequências da análise conformacional ao HFPE em estudo. O desenvolvimento de um protocolo eficiente foi nosso objeto de estudo nos últimos anos [23,42], tendo sido testado na reação (6) [23,[42][43][44][45]. No conjunto das 16 reações estudadas, o referido protocolo não só prevê com um grau de precisão considerável as cinco constantes de velocidade que já tinham sido obtidas experimentalmente (diferença entre valores teóricos e experimentais bem abaixo de uma ordem de magnitude [42]), como também prevê as restantes constantes de velocidade dentro da tendência expectável de reatividade, de acordo com a literatura existente [35,36,42,46].…”

“…O desenvolvimento de um protocolo eficiente foi nosso objeto de estudo nos últimos anos [23,42], tendo sido testado na reação (6) [23,[42][43][44][45]. No conjunto das 16 reações estudadas, o referido protocolo não só prevê com um grau de precisão considerável as cinco constantes de velocidade que já tinham sido obtidas experimentalmente (diferença entre valores teóricos e experimentais bem abaixo de uma ordem de magnitude [42]), como também prevê as restantes constantes de velocidade dentro da tendência expectável de reatividade, de acordo com a literatura existente [35,36,42,46]. Considerem-se dois exemplos específicos.…”