A novel assessment of the role of the methyl radical and water formation channel in the CH₃OH + H reaction

Abstract

A number of experimental and theoretical papers accounted almost exclusively for two channels in the reaction of atomic hydrogen with methanol: H-abstraction from the methyl (R1) and hydroxyl (R2) functional groups. Recently, several astrochemical studies claimed the importance of another channel for this reaction, which is crucial for kinetic simulations related to the abundance of molecular constituents in planetary atmospheres: methyl radical and water formation (R3 channel). Here, motivated by the lack of and uncertainties about the experimental and theoretical kinetic rate constants for the third channel, we developed first-principles Car-Parrinello molecular dynamics thermalized at two significant temperatures - 300 and 2500 K. Furthermore, the kinetic rate constant of all three channels was calculated using a high-level deformed-transition state theory (d-TST) at a benchmark electronic structure level. d-TST is shown to be suitable for describing the overall rate constant for the CHOH + H reaction (an archetype of the moderate tunnelling regime) with the precision required for practical applications. Considering the experimental ratios at 1000 K, k/k ≈ 0.84 and k/k ≈ 15-40, we provided a better estimate when compared with previous theoretical work: 7.47 and 637, respectively. The combination of these procedures explicitly demonstrates the role of the third channel in a significant range of temperatures and indicates its importance considering the thermodynamic control to estimate methyl radical and water formation. We expect that these results can help to shed new light on the fundamental kinetic rate equations for the CHOH + H reaction.

Temperature Dependence of Rate Processes Beyond Arrhenius and Eyring: Activation and Transitivity

“…However, as traditionally implemented, TST is unable to cope with systems with strong deviation from Arrhenius behavior (Masgrau et al, 2003). The chemical reactions for which quantum tunneling effects play an important role are those where Arrhenius plots show a concave curvature (Limbach et al, 2006; Silva et al, 2013; Sanches-Neto et al, 2017): this is the most important case of sub -Arrhenius kinetics for elementary reactions, but in complex processes it may show up, e.g., when concurrent reactions contribute to the mechanism (Hulett, 1964; Perlmutter-Hayman, 1976; Vyazovkin, 2016).…”

Kinetics of the OH+HCl→H2O+Cl reaction: Rate determining roles of stereodynamics and roaming and of quantum tunneling

“…No major differences were found using either the Bell‐1935 or Bell‐1958 formulas: also, it can be seen that both formulations, involving minimal effort with respect to much more elaborated treatments, can estimate with satisfactory agreement the rate constants of the reaction for the whole wide range of investigated temperatures. Conversely, as expected, the d ‐TST does not describe the range of experimental data for low temperature, where tunneling effects become more dominant, because its validity is limited to weak tunneling . In the lower panel in Figure , we also show rate constants for the isotope substituted reaction, OH + DCl, obtained with the Bell and d ‐TST formulas at MP2/aug‐cc‐pVTZ level of calculation.…”

“…Conversely, as expected, the d-TST does not describe the range of experimental data for low temperature, where tunneling effects become more dominant, because its validity is limited to weak tunneling. [14,42] In the lower panel in Figure 4, we also show rate constants for the isotope substituted reaction, OH + DCl, obtained with the Bell and d -TST formulas at MP2/aug-cc-pVTZ level of calculation. Our results are in reasonable agreement with previous experimental and theoretical values.…”

DFT exploration of [3 + 2] cycloaddition reaction of 1H-phosphorinium-3-olate and 1-methylphosphorinium-3-olate with methyl methacrylate

“…† It is also observed that calculations at low level functionals provide better explanation in some of the studies of organic reactions. [38][39][40] Thus, only the results obtained at the B3LYP/6-31G(d) computational level are herein reported.…”