Atmospheric Decomposition of Trifluoromethanol Catalyzed by Formic Acid.

Abstract

Quantum chemistry calculations are used to investigate the energetics and kinetics of CF3OH decomposition catalyzed by a single formic acid (FA) molecule acting alone and in conjunction with a single water (H2O) molecule to form the products carbonyl fluoride (CF2O) and hydrofluoric acid (HF). While the uncatalyzed reaction has a barrier of ∼44.7 kcal/mol, the presence of a FA molecule reduces the barrier to 6.4 kcal/mol, while the presence of both a FA and H2O molecule acting in unison decreases the barrier to -1.6 kcal/mol measured relative to the separated reactants. For comparison, we have also examined the decomposition of CF3OH catalyzed by HO2 and HO2 + H2O, which have been suggested in the literature to be an important atmospheric catalyst for CF3OH decomposition. In addition, we have also examined the loss of CF3OH via its bimolecular reaction with OH radicals. The rate constants for these various reactions were calculated using canonical variational transition state theory coupled with small curvature tunneling corrections over the temperature range between 200 and 300 K. Our results show that the rates for the CF3OH + FA and CF3OH + FA + H2O reactions are ∼104 times faster compared, respectively, to the corresponding reactions involving CF3OH + HO2 and CF3OH + HO2 + H2O at 300 K. Further, we find that, although the CF3OH + FA reaction has a higher barrier compared to CF3OH + FA + H2O, measured relative to the separated reagents, its effective first order rate for CF3OH decomposition is significantly faster for temperatures above 240 K compared to that of CF3OH + FA + H2O. This trend arises from the higher unimolecular reaction barrier for the reactant complex associated with the CF3OH + H2O + FA reaction compared to that for CF3OH + FA, as well as the lower concentration of reactant dimer complexes for CF3OH + H2O + FA compared to the concentration of the monomer FA reactant in the CF3OH + FA reaction. Finally, our calculations show that the rate for CF3OH decomposition catalyzed by FA is ∼104 times faster relative to the loss of CF3OH via its bimolecular reaction with OH radicals over the 200-300 K temperature range. Thus, the present study suggests that, among the various known loss mechanisms, a unimolecular reaction catalyzed by FA is likely the dominant gas phase decomposition pathway for CF3OH in the troposphere.