Insights into the reactivity of H2CO3 and its derivatives in bulk water, water clusters, and at the air/water interface: Ab initio molecular dynamics and metadynamics studies

• Author / Creator

  Galib,Mirza

• H2CO3 can decompose into CO2 in water either via the water route (H2CO3 ↔ CO2+H2O ) or the hydroxide route (H2CO3 + H2O ↔ CO2+OH−+H3O+). The water route reactions play a fundamental role in the global carbon cycle, and the hydroxide route reactions play an important role in the regulation of blood pH and the transport of CO2 during respiration. Despite the plethora of experimental work on these reactions in water, a microscopic understanding of the underlying mechanisms was lacking due to a shortage of theoretical work. In this thesis, we employed Car-Parrinello molecular dynamics and metadynamics to ascertain the microscopic mechanisms and compute the free energy changes and barriers of a series of reactions involving H2CO3, HCO3- , and CO2 in bulk water, water clusters, and at the air/water interface in order to shed much needed light on these reactions. Before delving into the reactivity of H2CO3 in water, we investigated the energetics and mechanisms of the conformational changes of the cis-cis (CC), cis-trans (CT), and trans-trans (TT) conformers of H2CO3 in water. The free energy barriers/changes for the various conformational changes via the change in one of the two dihedral angles were calculated and contrasted with the previously calculated values for the gas phase. We then investigated the energetics and mechanisms of the dissociation (H2CO3 ↔ HCO3− + H+) and hydroxide-route decomposition (HCO3- → CO2 + OH−) of all three conformers of H2CO3 in water. The CT and TT conformers were found to undergo decomposition in water via a two-step process: dissociation followed by decomposition. This is in contrast with the concerted mechanism proposed for the gas phase, which involves a dehydroxylation of one of the OH groups and a simultaneous deprotonation of the other OH group to yield CO2 and H2O. Our calculated pKa values and decomposition free energy barriers for the CT and TT conformers are consistent with the experimental values. The decomposition of H2CO3 in different-sized water clusters was investigated to determine whether the concerted or step-wise mechanism predominates in bulk water. We found that in the small clusters (containing 6 and 9 water molecules), the decomposition occurs via a concerted proton shuttle mechanism involving a cyclic transition state, whereas in the larger clusters (containing 20 and 45 water molecules), the decomposition follows a two-step mechanism involving a solvent-separated HCO3- /H3O+ ion pair intermediate. The larger clusters contain a sufficient number of water molecules to fully solvate the H3O+ intermediate, a prerequisite for the step-wise reaction. Our results demonstrated that the decomposition of H2CO3 predominantly occurs via the step-wise mechanism in bulk water. The dissociation of H2CO3 at the air/water interface was then studied. Our results indicated that H2CO3 dissociates faster at the water surface than in bulk water, in contrast to recent experiments and simulations which have shown that HNO3 has a lower propensity to dissociate at the water surface than in bulk water. We found that, at the water surface, there is a more structured solvation environment around H2CO3 than in bulk water, which leads to a decrease in the dissociation energy barrier via a stabilization of the transition state relative to the undissociated acid. We then investigated the hydration of CO2 to form HCO3- (i.e., CO2 + 2 H2O → HCO3- + H3O+) at the air/water interface. We found that CO2 is weakly solvated, even more so than in the bulk due to a deficiency of water molecules at the surface. Both the reaction mechanism and dissociation energy barrier were found to be similar to those in bulk water. Although the energy barrier was initially expected to be different, our result was not surprising given the fact that CO2 is weakly solvated in both environments. The insights gained from this thesis have implications on understanding CO2 and H2CO3 chemistry in a variety of environments encountered in atmospheric and geo- logical chemistry and set the stage for further kinetic and thermodynamic studies in a wide range of aqueous environments with different morphologies and compositions.

• Subjects / Keywords

  • Cpmd
  • Metadynamics
  • Carbonic acid

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3DF6KD8M

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.