Hydrogen Abstraction from Fluorinated Ethyl Methyl Ether Systems by OH Radicals

Curtis W White,Jaime M Martell

doi:10.1155/2016/3740278

Abstract

A systematic computational investigation of hydrogen abstraction by OH from the full series of fluorinated ethyl methyl ethers (EME) containing at least one H and one F, C2HnX5-nOCHmX3-m (n=0–5, m=0–3; and n=m=0 not allowed), including 147 reactants and 469 transition states, has been carried out, employing the MP2/6-31G(d) level of theory. Results for optimized geometries, including evidence of intramolecular hydrogen bonding in transition states, and barrier heights are presented. Trends pertaining to the number of fluorines substituted, key bond lengths, barrier heights, and key bond angles were found with good correlations and were investigated. An increase in the number of F increases the barrier height of the reaction. An increase in some parameters such as C–H length of TS, relative change in C–H from reactants to TS, ∠COC of reactants, ∠HOH in the TS, and relative change in ∠HOH between TS and free water bond angle also correlates with increased barrier height. An increase in other parameters like C–H length in the reactants and hydrogen bonding can decrease the barrier height.

Highlights

Hydrofluorocarbons (HFCs) are among the leading environmentally acceptable alternatives to ozone destroying chlorofluorocarbons (CFCs)
Fluorinated ethers have been proposed as second-generation HFC replacements [3, 4] because of their low ozone depletion potentials and their suspected shorter atmospheric lifetimes as compared to analogous HFCs, which reduces their global warming potential
If we look at plots of hydrogen bonding to oxygen and F versus the number of F we see that as the percent of hydrogen bonding to oxygen increases the barrier height decreases and as the percent hydrogen bonding to F increases barrier height increases

Summary

Introduction

Hydrofluorocarbons (HFCs) are among the leading environmentally acceptable alternatives to ozone destroying chlorofluorocarbons (CFCs). The determination of atmospheric lifetimes of HFCs is a key in assessing their environmental impact They initially degrade in the troposphere through hydrogen atom abstraction by relatively abundant hydroxyl radicals. Hsu and DeMore [8] used a relative rate technique for the reactions of ∙OH radicals with CF3OCH3, CF2HOCF2H, and CF3OCF2H They found significantly slower reaction rates for the latter two, which translates to significantly longer atmospheric lifetimes than previously estimated [9]. The BMK functional uses about 50 percent [20] While this may improve calculated barrier heights, B3LYP is still more widely employed, as it provides reasonably accurate results for a greater array of chemical problems. This research covered every possibility for H abstraction on each molecule and every structural isomer

Calculations

Nomenclature

Results and Discussion

Conclusions