Gas-phase Hydrogen Abstraction Reactions Research Articles

Quantitative relationship between rate constants and molecular structure descriptors for the gas phase hydrogen abstraction reactions

The abstraction of hydrogen by general radicals has a wide role in environmental and also in technological processes because it results in reactive free radicals that play a vital role in atmospheric chemistry and also in biochemical processes. In addition to experimental studies, the theoretical modelling of this elementary reaction has been important for understanding and predicting respective rate constants. In this paper, molecular descriptors in the context of a QSAR approach are used to codify the relationship between molecular structure and rate constants. Unique experimental data is collected from the literature for the reaction , where are diverse radicals. The four-parameter QSAR model (n = 34, r 2 = 0.81, = 0.74, = 0.12, s 2 = 0.19) is presented for the bimolecular rate constants, accompanied with model diagnostics and analysis of descriptors in the model.

On the possible catalysis by single water molecules of gas-phase hydrogen abstraction reactions by OH radicals

The gas phase hydrogen abstraction reaction between OH and CY(2)XH, where X = H, F, OH, or NH(2) and Y = H, CH(3) or F, in the absence and presence of a single water molecule is investigated using both density function theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. We find that a single water molecule could have a catalytic effect at low temperatures possible in laboratory experiments, but does not seem to catalyze these reactions at 298 K, and will not play a role under relevant atmospheric conditions.

Water-catalyzed gas-phase hydrogen abstraction reactions of CH3O2 and HO2 with HO2: a computational investigation

The gas-phase hydrogen abstraction reactions of CH(3)O(2) and HO(2) with HO(2) in the presence and absence of a single water molecule have been studied at the CCSD(T)/6-311++G(3d,2p)//B3LYP/6-311G(2d,2p) level of theory. The calculated results show that the process for O(3) formation is much faster than that for (1)O(2) and (3)O(2) formation in the water-catalyzed CH(3)O(2) + HO(2) reaction. This is different from the results for the non-catalytic reaction of CH(3)O(2) + HO(2), in which almost only the process for (3)O(2) formation takes place. Unlike CH(3)O(2) + HO(2) reaction in which the preferred process is different in the catalytic and non-catalytic conditions, the channel for (3)O(2) formation is the dominant in both catalytic and non-catalytic HO(2) + HO(2) reactions. Furthermore, the calculated total CVT/SCT rate constants for water-catalyzed and non-catalytic title reactions show that the water molecule doesn't contribute to the rate of CH(3)O(2) + HO(2) reaction though the channel for O(3) formation in this water-catalyzed reaction is more kinetically favorable than its non-catalytic process. Meanwhile, the water molecule plays an important positive role in increasing the rate of HO(2) + HO(2) reaction. These results are in good agreement with available experiments.

Rate constants of gas-phase hydrogen abstraction reactions: a balance between the association and the abstraction dynamical bottlenecks

The OH radical is the principal daytime oxidant agent in the Earth's troposphere and its concentration determines the atmospheric lifetimes of many of the biogenic and anthropogenic organic compounds, such as acetone or methanethiol. Most of the OH-initiated oxidations are thought to take place mainly via a H-abstraction process, that is, the OH radical abstracts a hydrogen atom from the organic molecule to yield a water molecule and the corresponding organic radical. The mechanism normally proceeds via the formation of an intermolecular hydrogen-bonded complex in the entrance channel. Due to the formation of those reactant complexes, the reaction pathway at the entrance channel may be divided in two zones: an association region, where usually there is neither classical nor adiabatic potential energy barrier, and a H-abstraction region where, in contrast, a saddle point structure is usually located. Then, the bimolecular reaction takes place through two dynamical bottlenecks in the sense of the variational transition state theory: one in the association region and the other in the abstraction region. The calculation of the contribution of the association dynamical bottleneck to the overall hydrogen abstraction rate constant is the main objective of the present work. This study has been carried out for two OH-initiated oxidations: the OH+CH 3C(O)CH 3 process and the OH+CH 3SH reaction.