Article pubs.acs.org/JPCA
Impact of OH Radical-Initiated H2CO3 Degradation in the Earth’s Atmosphere via Proton-Coupled Electron Transfer Mechanism Sourav Ghoshal and Montu K. Hazra* Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata-700 064, India S Supporting Information *
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.
1. INTRODUCTION
troposphere and lower stratosphere, on the surface of Mars and Venus, as well as in comets and the Galilean satellites.1−4,8,24,33 Indeed, as noted by Kohl et al., a comparison of some spectra on Mars with the infrared spectrum of the βH2CO3 suggest that the β-H2CO3 is present on the Martine
Carbonic acid (H2CO3) is a molecule of profound environmental and astrophysical significance as both of its constituents carbon dioxide (CO2) and water (H2O) molecules coexist in various environments.1−33 This molecule is known as the key species in the dissolution of carbonate compounds and is of fundamental importance for the regulation of blood pH and acidification of the oceans.1−3,8,26−42 Moreover, and until now, this molecule is believed to be present in the Earth’s © XXXX American Chemical Society
Received: September 9, 2015 Revised: January 1, 2016
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DOI: 10.1021/acs.jpca.5b08805 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A surface.4 It is to be noted here that the α- and β-H2CO3 were considered in the past as the two distinct polymorphs of H2CO3, especially, when the H2CO3 has been synthesized by acid−base chemistry at cryogenic temperatures.1−4,8,23,26,28,33 Moreover, these two polymorphs of H2CO3 were considered to be structurally different with respect to two different binding patterns present within their respective basic-building-block units that consist of only and exclusively tthe most stable cis−cis [(cc)] conformer of H2CO3 (Figure 1).2−4,8,10,28,33,43 However,
channels, it is also equally important to investigate the energetics of hydroxyl (OH) radical-initiated carbonic acid degradation (H2CO3 + OH → HCO3 + H2O) reaction in the gas-phase, especially, to understand the atmospheric loss of the H2CO3 molecule in the presence of OH radical detected in the Earth’s atmosphere. This is because the OH radical is known as the atmospheric detergent55,56 due to its high reactivity toward the hydrogen atom of the pollutant molecules present in the Earth’s atmosphere.56,57 Indeed, it is believed that every oxygenated hydrocarbons and especially the acids present in the Earth’s atmosphere are expected to react more-or-less rapidly with the OH radical;56−66 nevertheless, it is surprising that the OH radical-initiated H2CO3 degradation reaction has never got the attention to account for the atmospheric loss of the H2CO3 molecule. Thus, to understand the atmospheric degradation or loss of the gaseous H2CO3 molecule in the presence of the OH radical, especially, in the Earth’s troposphere and stratosphere, here we focus upon the potential energy diagram as well as the kinetics for the OH radical-initiated H2CO3 degradation reaction. Moreover, to find the potential impact of the OH radicalinitiated H2CO3 degradation reaction in the Earth’s troposphere and stratosphere, we also compare the reaction rate of the OH radical-initiated H2CO3 degradation with the reaction rates of the water (H2O), formic acid (FA), acetic acid (AA), and sulfuric acid (SA) assisted H2CO3 → CO2 + H2O decompositions on equal footing. It is to be noted here that the SA, an inorganic acid that has been detected in the Earth’s atmosphere, is also known to be an effective catalyst like FA or AA, especially in the OH bond breaking and making process via the intermolecular hydrogen transfer process.67−75 Moreover, the source chemistry of the H2CO3 molecule has been reported until date only for the troposphere and lower stratosphere but not for the upper stratosphere of the Earth’s atmosphere.1−3,32 However, and additionally, we preferred to extend our work relevant to the upper stratosphere of the Earth’s atmosphere in case any source chemistry of the H2CO3 molecule, similar to that especially exist for the HNO3 and H2SO4, is established in the near future.76−80 It is worth noting here that the constituents of the H2CO3 molecule are the CO2 and H2O molecules those coexist in the upper stratosphere of the Earth’s atmosphere81 and the H2CO3 is a molecule that belongs right at the interface between inorganic and organic chemistry.31 Moreover, we have not focused here upon the UV photodissociation of the H2CO3 molecule and its reaction with oxygen atom, which may be the potentially important channels in comparison to the OH radical-initiated H2CO3 degradation reaction, especially, to understand the daytime atmospheric loss of the H2CO3 molecule in the Earth’s upper stratosphere if the source chemistry of the H2CO3 molecule, as mentioned above, is established in the near future.
Figure 1. M06-2X-/6-311++G(3df,3pd) level optimized geometries of the two most stable “cis−cis” [(cc)] and “cis−trans” [(ct)] conformers of carbonic acid (H2CO3).
the recent work from Reisenauer44 and Bucher et al. groups45 suggests that the β-H2CO3 is the only distinct polymorph of carbonic acid and the α-H2CO3 is actually the monomethyl ester of the β-H2CO3. In the last three decades, carbonic acid has also been characterized in the gas phase by means of its microwave, infrared and mass spectra.2,3,5−7,44 In 1987, Terlouw et al.5 were the first to detect free H2CO3 molecule in the gas-phase from the thermolysis of ammonium bicarbonate (NH4HCO3) molecule via mass spectrometry. In addition, the H2CO3 molecule has also been synthesized in the laboratory under various experimental conditions similar to those encountered in extraterrestrial space.11−19,46 However, it is surprising that the H2CO3 molecule has not been detected yet in the Earth’s atmosphere and/or in outer space.2,8,19 Indeed, we note what has already been emphasized by Hudson et al.,9 Bernard et al.,2 and Huber et al.8 that the detection of gas-phase H2CO3 molecule in the Earth’s atmosphere as well as in outer space has become a challenging goal for a new generation of scientists.2,8,9 Moreover, it is hoped that one day, in the near future, it will be detected as scientists achieve success in measuring the infrared spectra of the vapor phase H2CO3 resulting from its βpolymorph (at present) via the sublimation at cold temperatures.2 It is worth noting here that, although the H2CO3 molecule was long believed to be an unstable and elusive species,5,47,48 nevertheless, it is seen from the theoretical calculations that the isolated H2CO3 molecule is kinetically very stable species due to the high barrier height (37−40 kcal/mol) associated with its unimolecular H2CO3 → CO2 + H2O decomposition reaction.49−52 Recently, the stability of the gasphase H2CO3 molecule in the Earth’s troposphere and lower stratosphere has been explored in detail by considering its decomposition into its constituents (H2CO3 → CO2 + H2O) in the presence of various atmospheric species including the H2CO3 itself.52−54 From these studies, it is seen that formic acid (FA) and acetic acid (AA) assisted H2CO3 → CO2 + H2O decomposition reactions are of potential atmospheric significance in making the gas-phase H2CO3 molecule an unstable species; although the H2O-assisted H2CO3 decomposition reaction can not be neglected completely, especially, in the clean environments of the Earth’s surface.53 Given that the gas-phase H2CO3 molecule is an unstable species in the Earth’s atmosphere via the H2O-, FA-, and AAassisted H2CO3 → CO2 + H2O decomposition reaction
2. COMPUTATIONAL METHODS Gaussian-09 suite of program has been used to carry out all the quantum chemistry calculations presented here.82 Both the geometry optimizations and frequency calculations of the monomers and complexes have been performed using the M062X level of theory in conjunction with the aug-cc-pVDZ, augcc-pVTZ and 6-311++G(3df,3pd) basis sets. The calculations have been performed with spin-unrestricted (open-shell) approximation in the case of OH radical-initiated H2CO3 degradation reaction and in the case of H2O and acids-assisted H2CO3 → CO2 + H2O decomposition reactions; the B
DOI: 10.1021/acs.jpca.5b08805 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
most effective pathway by which atmospheric degradations of FA, NA, and SA are expected to occur is the proton-coupled electron transfer (PCET) mechanism.61−66 It is believed that the hydrogen atom transfer (HAT) is another mechanism that has been identified in the OH radical-initiated degradations of FA, NA, and SA, nevertheless, it is seen from the literature that the HAT mechanism61−66 is not effective like the PCET mechanism, as the effective barrier height for the HAT mechanism is higher than the effective barrier via PCET mechanism.62−66 For an understanding the degradation of H2CO3 molecule in the presence of the OH radical, the pictorial representation of the PCET and HAT mechanisms has been shown in Figure 2 using the (ct)-H2CO3 conformer as the
calculations have been performed with spin-restricted (closedshell) approximation. Moreover, the geometry optimizations were performed using Schlegel’s method83 with tolerances of better than 0.001 A for bond lengths and 0.01° for angles and with a self-consistent field convergence of at least 10−9 of the density matrix. The residual root-mean-square (rms) forces were
