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

Abstract

In this work, we report the first detailed theoretical comparative conformational investigation between two different classes of hydrofluoropolyethers: dihydro-and dimethoxyfluoropolyethers.The main objective was to determine a cost-effective computational methodology that could accurately reproduce the energetic rankings and thermal weight factors of the simplest examples of those two classes calculated with M08-HX/triple-f//M08-HX/double-f benchmark model chemistries. Between the tested methodologies, M08-HX/aug-pcseg-2//M08-HX/pcseg-1 was found to be the most appropriate, exhibiting a good accuracy and considerable reduction in computational cost with respect to the benchmark, being more than three times faster than M08-HX/aug-pcseg-2//M08-HX/aug-pcseg-1. This cost-effective approach will be essential in future work when studying larger hydrofluoropolyethers, where the computational complexity associated with increasing number of conformers will require such approximations. K E Y W O R D S atmospheric chemistry, conformational sampling, density functional theory, hydrofluoropolyethers, model chemistry Int J Quantum Chem. 2017;117:e25381.

Multiconformer transition state theory rate constants for the reaction between OH and ‐dimethoxyfluoropolyethers

“…Judging for these two cases alone, one can say that the similarity between the rate constants is accidental. It should also be reminded that was/is being used for benchmark reasons, and it is not a part of the patents for DM‐FPEs, since these should have and . In addition, its simplicity is not representative of typical conformational variability and so care must be taken while extracting conclusions from calculations involving this system in terms of real reactivity and atmospheric impact.…”

“…One of the difficulties in studying reaction is the fact that DM‐FPEs (and, in general, HFPEs) have a rich conformational variety . Such complexity should be included in the rate constant formula, and if one chooses transition state theory as the method of calculation, then it will assume the form of MC‐TST .…”

“…The electronic structure calculations for the final set of reactant conformers then follow. These were performed with the GAMESS package, with the choice of model chemistry being made according to our cost‐effective philosophy . The M08‐HX/apcseg‐2//M08‐HX/pcseg‐1 model chemistry was utilized, with the optimizations and single‐point energies being performed with M08‐HX from the Minnesota family of functionals (good accuracy for hydrogen‐transfer barrier heights) and with Jensen's new segmented polarization consistent double‐ and triple‐ζ basis sets, which have been optimized for Density Functional Theory (DFT) methods.…”

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 2, which includes large molecular systems with many conformers. 22,29,30,44,47 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.…”

“…Such internal rotations are usually characterized by having low barriers, which allow a rapid interconversion between rotamers at room temperature. DH-FPEs, for example, are known to possess a rich conformational variety. − Within the framework of transition-state theory (TST), the inclusion of conformational effects leads to what is generally called as multiconformer transition-state theory (MC-TST) ,,− or to very robust and detailed approaches based on improvements and extensions − to variational TST (VTST), such as multistructural VTST (MS-VTST) and multipath VTST (MP-VTST), which include variational and recrossing effects, multidimensional tunneling, multistructural and coupled torsional potential anharmonicity. The latter approaches would become prohibitively expensive for our broad spectrum investigation, so at the time the first version of this protocol was being devised, we turned our attention to the then most recent MC-TST protocols. , Although disclosing very few technical details, Petit and Harvey’s protocol for bimolecular reactions between HO • and VOCs (test set of 34 VOCs, including four fluorinated compounds with 11 atoms) yielded very good results, with the calculated rate constants agreeing “with the experimental value to within better than an order of magnitude, and to within better than a factor of three for all but six cases” …”