Despite the widespread importance of aqueous bicarbonate chemistry, its conjugate acid, carbonic acid, has remained uncharacterized in solution. Here we report the generation of deuterated carbonic acid in deuterium oxide solution by ultrafast protonation of bicarbonate and its persistence for nanoseconds. We follow the reaction dynamics upon photoexcitation of a photoacid by monitoring infrared-active marker modes with femtosecond time resolution. By fitting a kinetic model to the experimental data, we directly obtain the on-contact proton-transfer rate to bicarbonate, previously inaccessible with the use of indirect methods. A Marcus free-energy correlation supports an associated pKa (Ka is the acid dissociation constant) of 3.45 +/- 0.15, which is substantially lower than the value of 6.35 that is commonly assumed on the basis of the overall carbon dioxide-to-bicarbonate equilibrium. This result should spur further exploration of acid-base reactivity in carbon dioxide-rich aqueous environments such as those anticipated under sequestration schemes.

https://science.sciencemag.org/content/sci/326/5960/1690.full.pdf

Real-Time Observation of Carbonic Acid Formation in Aqueous Solution

The accumulation of proline (Pro) and overproduction of reactive oxygen species (ROS) by plants exposed to stress is well-documented. In vitro assays show that enzyme inactivation by hydroxyl radicals ((•)OH) can be avoided in the presence of Pro, suggesting this amino acid might act as a (•)OH scavenger. Although production of hydroxyproline (Hyp) has been hypothesized in connection with such antioxidant activity, no evidence on the detailed mechanism of scavenging has been reported. To elucidate whether and how Hyp might be produced, we used density functional theory calculations coupled to a polarizable continuum model to explore 27 reaction channels including H-abstraction by (•)OH and (•)OH/H2O addition. The structure and energetics of stable species and transition states for each reaction channel were characterized at the PCM-(U)M06/6-31G(d,p) level in aqueous solution. Evidence is found for a main pathway in which Pro scavenges (•)OH by successive H-abstractions (ΔG(‡,298) = 4.1 and 7.5 kcal mol(-1)) to yield 3,4-Δ-Pro. A companion pathway with low barriers yielding Δ(1)-pyrroline-5-carboxylate (P5C) is also supported, linking with 5-Hyp through hydration. However, this connection remains unlikely in stressed plants because P5C would be efficiently recycled to Pro (contributing to its accumulation) by P5C reductase, hypothesis coined here as the "Pro-Pro cycle".

Molecular mechanisms for the reaction between (˙)OH radicals and proline: insights on the role as reactive oxygen species scavenger in plant stress.

On the hydration and hydrolysis of carbon dioxide

Efficient chemical conversion of biomass is essential to produce sustainable energy and industrial chemicals. Industrial level conversion of glucose to useful chemicals, such as furfural, hydroxyme...

Comparison of Sugar Molecule Decomposition through Glucose and Fructose: A High Level Quantum Chemical Study

The gas-phase geometries, binding energies (BEs), vibrational spectra, and electron density topological features of methanol (M), water (W), and methanol−water mixed clusters (MmWn, where m = 0−4 and n = 0−4; m + n ≤ 4) have been calculated using Hartree−Fock, second-order Moller−Plesset perturbation, and density functional theory with Becke three-parameter hybrid functional combined with Lee−Yang−Parr correlation functional methods. Bader’s “atoms in molecules” theory has been used to analyze the hydrogen bonding network. To understand the effect of cooperativity, we have performed natural bond orbital analysis and reduced variational space decomposition analysis. The results show that BEs of methanol and mixed clusters are higher than those of water clusters due to the electron-donating nature of the methyl group. These findings are in accordance with the role of cooperative polarization and cooperative charge transfer in the methanol and mixed clusters. As the size of the cluster increases, the contrib...

Ab initio and DFT studies on methanol-water clusters.

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.

Myrna H Matus

Papers

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation.