Assessment of theoretical methods for the study of hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation (IUPAC Technical Report)

Abstract

Global climate change is a major concern as it leads to an increase in the average temperature of the earth's atmosphere. The existence and persistence of some gaseous species in the atmosphere contribute to global warming. Experimental techniques are used to study the kinetics and degradation of global warming gases. However, quantum mechanical methods are also useful for the kinetic and radiative forcing study of global warming species and can precede experimental investigations. Research has also been targeted to develop more adapted procedures using ab initio and density functional theory (DFT) methods. This report provides a global perspective, in simplified manner, of the theoretical studies of the degradation of gas species in the atmosphere with an emphasis on the hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation. En route, the results obtained from these studies are analysed and compared with experimental data where available. Our analyses indicate that the theoretical predictions are in agreement with experimental findings but the predicted parameters are dependent on the method being used. Theoretical methods are used to predict the thermodynamic parameters of reactions, and, with relevance to this report, the global warming potential (GWP) index can also be calculated. This report can be useful for future investigations involving global warming gaseous species while providing suggestions on how computations can fill in data gaps when experimental data are unavailable.

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

“…The choice of DFT is motivated by its favorable compromise between accuracy and computational cost, a property of utmost importance in the study of reaction , which includes large molecular systems with many conformers. ,,,, Specifically, it is well-known that the accuracy of the barrier heights for hydrogen abstraction will depend on the choice of the exchange–correlation functional. The Minnesota family of functionals are known to have a good performance in describing the reactivity of VOCs. ,,,− ,,,− Although in this particular context M06-2X is generally used more often than M08-HX, the latter functional has recently been employed in accurate MS-VTST and MP-VTST rate constant calculations ,,− of atmospheric reactions. M08-HX has mean unsigned errors (MUEs) of 1.00 and 0.73 kcal mol –1 for the DBH76 and HTBH38 data sets involving hydrogen-transfer barrier heights, respectively .…”

Reactivity of α,ω-Dihydrofluoropolyethers toward OH Predicted by Multiconformer Transition State Theory and the Interacting Quantum Atoms Approach

“…The choice of density functional theory (DFT) is motivated by the favorable compromise between accuracy and computational cost, the latter being important for performing calculations on a large number of structures in the present work. The accuracy for calculating hydrogen-abstraction barrier heights depends on the choice of the exchange-correlation functional, and the Minnesota family of functionals has in previous works been found to perform well for describing the reactivity of volatile organic compounds. ,,,− Although M06-2X has been used more frequently than M08-HX, the latter has mean unsigned errors of 1.00 and 0.73 kcal mol –1 for the DBH76 and HTBH38 data sets involving hydrogen-transfer barrier heights, respectively . The corresponding values for M06-2X are 0.2 − and 0.4 kcal mol –1 higher. , The more recent M11 functional has a mean unsigned error for the HTBH38 data set almost 0.6 kcal mol –1 higher than that for M08-HX.…”

Assessment of model chemistries for hydrofluoropolyethers: A DFT/M08‐HX benchmark study

“…By taking these factors into account we opted for an electronic structure method with low CPU time scaling, such as KS‐DFT (which in many cases scales approximately as K 4 , where K is a measure of the size of the molecule, typically related to the number of basis functions). Based on published research concerning the reactivity of a variety of volatile organic compounds and on our own preliminary calculations on HG‐ and ‐ , KS‐DFT (employing M06‐2X and M08‐HX functionals) and MP2 appear to be promising theoretical approaches. In the previously cited list of references M06‐2X is used extensively while M08‐HX is only used once, but because this functional is an improvement on M06‐2X, we opted to make it our benchmark functional in our calculations, which will also include a comparison between the two functionals.…”