How Many Water Molecules Are Actively Involved in the Neutral Hydration of Carbon Dioxide?

Abstract

The detailed reaction pathways for the hydration of carbon dioxide by water and water clusters containing two, three, and four water molecules (CO2 + nH2O → H2CO3 + (n − 1)H2O, n = 1−4) have been investigated in both gas phase and aqueous solution using ab initio molecular orbital (MO) theory up to the quadratic configuration interaction QCISD(T)/6-31G(d,p)//MP2/6-31G(d,p) level, both SCRF and PCM models of continuum theory, and a mixed approach based on MO calculations in conjunction with Monte Carlo and reaction field simulations. It is confirmed that the CO2 hydration constitutes a case of active solvent catalysis where solvent molecules actively participate as a catalyst in the chemical process. In aqueous solution the hydration mechanism is multimolecular, where geometric parameters of the solvent fully intervene in the reaction coordinate. The hydration reaction was found to proceed through an attack of a water oxygen to the CO2 carbon in concert with a proton transfer to a CO2 oxygen. The proton transfer is assisted by a chain of water molecules, which is necessary for a proton relay between different oxygens. Owing to a significantly larger charge separation in the transition structures, nonspecific electrostatic interactions between solute and solvent continuum also play a more important stabilizing role. Regarding the answer to the title question, our calculations suggest that although a water tetramer (n = 4) seems to be necessary for CO2 hydration in the gaseous phase, a reaction channel involving formation of a bridge containing three water molecules (n = 3) is likely to be actively involved in the neutral hydration of CO2 in aqueous solution.

Recovery Act: Molecular Simulation of Dissolved Inorganic Carbons for Underground Brine CO2 Sequestration

“…The forth forms begins a second shell by hydrogen bonding with the first water molecule. The HCO3-is completely solvated by the water shells and we although we find a minima for the HCO3 -(H2O) 4 conformer, the HCO3 -(H2O) 7 conformer and HCO3 -(H2O) 9 conformers are all within 0.5 kcal/mol (the accuracy of our method) (Figure 4). …”

“…However, when one comes to measuring DIC species using the MIR spectrum there is a great void in the available experimental and theoretical Mid-IR absorption spectra of CO2 (aq), H 2 CO 3 , HCO 3 -and CO 3 2-in water. The majority of theoretical simulations also only deal with gas phase or clusters of these CO 2 species [7]. There is MD simulation of hydrogenbonds in these CO2 species in water [8], but no spectra were given.…”

“…In the smaller clusters, we find that the water molecules form a strong hydrogen bond with one of the oxygen on CO2, while the second water molecule in CO2(H2O) 2 hydrogen bonds with the first water. The most stable structure is found to be CO2(H2O) 6 and CO2(H2O) 7 (Figure 3). For the HCO3-(H2O) n structures, the first water molecule forms a strong hydrogen bond with the O-group (water is the h-bond donor), the second water forms a hydrogen bond with the OH group (water is the h-bond acceptor), while the third water forms a weaker h-bond with the double bonded O group.…”

Cooperative effects in two-dimensional ring-like networks of three-center hydrogen bonding interactions

“…It should be mentioned that a single molecule of a carbonic acid can exist in three different minimum-energy conformations. [48][49][50][51][52][53][54][55] The most stable conformations is the trans-trans isomer, followed by the cistrans and cis-cis forms. Previous ab initio studies on the dimer of carbonic acid, with each monomer in its most stable trans-trans conformation, show that this dimer is remarkably stable.…”

Use of Local Softness for the Interpretation of Reaction Mechanisms

“…The hydration occurs by an initial nucleophilic attack of one water oxygen to one site of the double bond, followed by a proton relay within the water chain and ended up by an electrophilic attack of a proton to another site [19][20][21][22][23].…”