Impact of OH Radical-Initiated H2CO3 Degradation in the Earth's Atmosphere via Proton-Coupled Electron Transfer Mechanism.

Abstract

The decomposition of isolated carbonic acid (H2CO3) molecule into CO2 and H2O (H2CO3 → CO2 + H2O) is prevented by a large activation barrier (>35 kcal/mol). Nevertheless, it is surprising that the detection of the H2CO3 molecule has not been possible yet, and the hunt for the free H2CO3 molecule has become challenging not only in the Earth's atmosphere but also on Mars. In view of this fact, we report here the high levels of quantum chemistry calculations investigating both the energetics and kinetics of the OH radical-initiated H2CO3 degradation reaction to interpret the loss of the H2CO3 molecule in the Earth's atmosphere. It is seen from our study that proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) are the two mechanisms by which the OH radical initiates the degradation of the H2CO3 molecule. Moreover, the PCET mechanism is potentially the important one, as the effective barrier, defined as the difference between the zero point vibrational energy (ZPE) corrected energy of the transition state and the total energy of the isolated starting reactants in terms of bimolecular encounters, for the PCET mechanism at the CCSD(T)/6-311++G(3df,3pd) level of theory is ∼3 to 4 kcal/mol lower than the effective barrier height associated with the HAT mechanism. The CCSD(T)/6-311++G(3df,3pd) level predicted effective barrier heights for the degradations of the two most stable conformers of H2CO3 molecule via the PCET mechanism are only ∼2.7 and 4.3 kcal/mol. A comparative reaction rate analysis at the CCSD(T)/6-311++G(3df,3pd) level of theory has also been carried out to explore the potential impact of the OH radical-initiated H2CO3 degradation relative to that from water (H2O), formic acid (FA), acetic acid (AA) and sulfuric acid (SA) assisted H2CO3 → CO2 + H2O decomposition reactions in both the Earth's troposphere and stratosphere. The comparison of the reaction rates reveals that, although the atmospheric concentration of the OH radical is substantially lower than the concentrations of the H2O, FA, AA in the Earth's atmosphere, nevertheless, the OH radical-initiated H2CO3 degradation reaction has significant impact, especially toward the loss of the H2CO3 molecule in the Earth's atmosphere. In clean environments, which exist in greater numbers in comparison to the polluted environments of the Earth's atmosphere, the impact of the OH radical-initiated H2CO3 degradation reaction is seen to be comparable to that from a competing pathway which utilizes hydrogen bonded molecules such as H2O, FA or AA to catalyze the H2CO3 decomposition. Similarly, in the polluted environments, and especially in the Earth's troposphere, although the reactions rates for the OH radical-initiated H2CO3 degradation and FA-assisted H2CO3 decomposition are comparable within a factor of ∼15, nevertheless, the AA-assisted H2CO3 decomposition reaction is appeared to be the dominant channel. This follows only because of slightly greater catalytic efficiency of the AA over FA upon the H2CO3 → CO2 + H2O decomposition reaction. In contrary, although the catalytic efficiencies of SA, FA, and AA upon the H2CO3 decomposition reaction are similar to each other and the concentrations of both the SA and OH radical in the Earth's atmosphere are more-or-less equal to each other, but nevertheless, the SA-assisted H2CO3 decomposition reaction cannot compete with the OH radical-initiated H2CO3 degradation reaction.