Water Vapor Does Not Catalyze the Reaction between Methanol and OH Radicals

Abstract

Recent reports [Jara-Toro et al.,Angew.Chem. Int. Ed. 2017,56, 2166 and PCCP 2018 suggest that the rate coefficient of OH reactions with alcohols would increase by up to two times in going from dry to high humidity.T his finding would have an impact on the budget of alcohols in the atmosphere and it may explain differences in measured and modeled methanol concentrations.T he results were based on ar elative technique carried out in as mall Teflon bag,w hich might suffer from wall reactions.The effect was reinvestigated using ad irect fluorescence probe of OH radicals,a nd no catalytic effect of H 2 Ocould be found. Experiments in aT eflon bag were also carried out, but the results of Jara-Toro et al. were not reproducible.F urther theoretical calculations show that the water-mediated reactions have negligible rates compared to the bare reaction and that even though water molecules can lower the barriers of reactions,t hey cannot make up for the entropyc ost.

Water Catalysis of the Reaction of Methanol with OH Radical in the Atmosphere is Negligible

“…In summary, our calculations support the experimental observations by Chao et al [28] that the water catalytic effect is not observable at 298 K, and our calculations extend this conclusion to the wider 200-400 K temperature range. We believe that the experimental contradiction is probably caused by experimental uncertainties in the earlier experiment.…”

“…Finally, we compute the rate constants of the title reaction as a function of temperature and RH. We compare the rate constants to the experimental measurements by Jara-Toro et al [27] at 294 K and Chao et al at 298 K. [28] Figure 5 shows that the HPL rate constants at 294 K and 298 K are almost identical, so we conclude that the disagreement of the two experiments is not caused by temperature difference. Furthermore, our calculations at different RHs agree very well with the experiments by Chao et al, [28] suggesting no observable catalytic effects of water vapor at 294 K or 298 K.…”

“…The MS-LQH treatment of low-frequency anharmonicity raises the vibrational partition functions of TSaW2 by factors of 6.3-9.0 and those of TSbW2 by factors of 2.2-3.1 (see TableS10).Finally, we compute the rate constants of the title reaction as a function of temperature and RH. We compare the rate constants to the experimental measurements by Jara-Toro et al[27] at 294 K and Chao et al at 298 K [28]. Figure5shows that the HPL rate constants at 294 K and 298 K are almost identical, so we conclude that the disagreement of the two experiments is not caused by temperature difference.…”

“…The total rate constants include both 2B and 3B reactions at the high-pressure limit. The experimental measurements are from (green •) Jara-Toro et al [27] and (black ■) Chao et al [28] Figure 6. Variation of the MS-CVT/SCT rate constants of the two-body (from the calculations by Gao et al [13] ) and three-body (present calculations) rate constants of CH3OH + OH + H2O at 200-400 K; the latter are shown for several values of the relative humidity.…”

“…They found that the rate constant rises quadratically with the RH and concluded that water vapor can catalyze the CH3OH + OH reaction. However, Chao et al [28] studied this water-mediated reaction by probing the time-resolved OH decays at different RHs and concluded that there is no catalytic effect of water under atmospheric conditions. Chao et al supported their experimental result with quantum chemical calculations of the rate constants, but since the experiments differ and their theoretical calculation contains approximations, we endeavored to provide a more definitive theoretical result, and that is presented here.…”