Gas Phase Reaction of Ketene with H2S in Troposphere: Catalytic Effects of Water and Ammonia

Abstract

To get an insight of the energetics and kinetics of a reaction between ketene and H2S in troposphere which has not been studied before. Additions of water monomer (H2O) to simplest ketene i.e, H2C=C=O (mentioned as ketene,henceforth) in Earth's atmosphere result in formation of acetic acid.However, this reaction is not feasible under tropospheric conditions due to high reaction barrier amounting to nearly 40 kcal mol-1. Signicant reduction of barrier height (below 20 kcal mol-1) is achieved upon ad- dition of another H2O as catalyst. It is worth mentioning that like H2O and ammonia (NH3), H2S could also play important role in the loss mechanism of various atmospherically important species. Due to close similarity with H2O, studying sulfolysis reaction between ketene and H2S could provide some interesting insights into the nature of hydrogen bonded complexes of ketene as well as the impact of product formed under the atmospheric conditions. Water and ammonia catalyzed gas-phase addition reaction of ketene with H2S has been inves- tigated using CCSD(T)-F12a/cc-pVTZ-F12a//M06-2X/6-311++G** level of calculation. In this study, rate constants for all possible reaction channels are calculated using transition state theory. It is found that, under tropospheric conditions, the catalyzed reaction channels are signicantly faster than uncatalyzed reactions. Between the two catalysts, ammonia acts as better catalyst than water for this reaction. Since concen- tration of water is manyfold larger than ammonia, eective rate of water catalyzed reaction becomes higher than that of ammonia. Under combustion condition, ammonia catalyzed channel is faster below 1500 K, while the uncatalyzed reaction channel becomes faster above that temperature. Results from the present study show that barrier for thioacetic acid formation through uncatalyzed sulfolysis of ketene is substantially high. The extent of barrier is lowered appreciably by ammonia compared to water as catalyst. It has been observed that hydrolysis reaction is more probable than sulfolysis reaction under atmospheric condition in troposphere, but, ammonia catalysed sulfolysis is the fastest one at 298 K. Effective rate constant of water catalysed hydrolysis reaction is found to be more than ammonia catalysed reaction due to higher monomer concentration of water than ammonia.Ammonia catalyzed reaction rate increases monotonously with increasing temperature. Further rate coefficient for uncatalyzed reaction is found to be dominant under combustion conditions i.e. above 1500 K. This reaction has not been studied elsewhere though similar type of atmospherically important reactions were studied in details.