Solutions of citric acid and Na2HPO4 were studied with the dynamical approach to multiequilibria systems. This widely employed buffer has a well-defined pH profile and allows for the study of the distribution of phosphate species over a wide pH range. The dynamical approach is a flexible and accurate method for the calculation of all species concentrations in multiequilibria considering ionic strength (I) via Debye–Hückel theory. The agreement between the computed pH profiles and experiment is excellent. The equilibrium concentrations of the non-hydrogen species are reported for over 30 buffer mixtures across the entire pH range. These new concentration data enable researchers to lookup the equilibrium distribution of species at any pH. The data highlight the dramatic effects of ionic strength, and for example, the position of maximal H2PO4 – concentration is shifted by almost an entire pH unit! From a more general perspective, the study allows for a discussion of the dependence of concentration quotients Q xy on ionic strength, pQ xy = f(I), and for the numerical demonstration that the thermodynamic equilibrium constants K xy,act(I) = K xy . The analysis emphasizes the need for measurements of the concentrations of several species in complex multiequilibria systems over a broad pH range to advance multiequilibria simulations.

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In theoretical studies of chemical reactions the reaction thermochemistry is usually reported for the stoichiometric reaction at standard conditions (∆G˝, ∆H˝, ∆S˝). We describe the computation of the equilibrium concentrations of the CO 2-adducts for the general capture reaction CO 2 + Capture System Õ CO 2-adduct (GCR) and the rubisco-type capture reaction CO 2 + Capture System Õ CO 2-adduct + H 2 O (RCR) with consideration of the reaction CO 2 (g) Õ CO 2 (aq) via Henry's law. The resulting equations are evaluated and graphically illustrated as a function of atmospheric CO 2 concentration and as a function of temperature. The equations were applied to the thermochemistry of small molecule rubisco-model reactions and series of additional model reactions to illustrate the range of the Gibbs free enthalpy for the effective reversible capture and of the reaction entropy for economic CO 2 release at elevated temperature. A favorable capture of free enthalpy is of course a design necessity, but not all exergonic reactions are suitable CO 2 capture systems. Successful CO 2 capture systems must allow for effective release as well, and this feature is controlled by the reaction entropy. The principle of using a two-pronged capture system to ensure a large negative capture entropy is explained and highlighted in the graphical abstract. It is hoped that the presentation of the numerical examples provides useful guidelines for the design of more efficient capture systems.

We have been interested in the development of rubisco-based biomimetic systems for reversible CO 2 capture from air. Our design of the chemical CO 2 capture and release (CCR) system is informed by the understanding of the binding of the activator CO 2 ( A CO 2 ) in rubisco (ribulose-1,5-bisphosphate carboxylase/ oxygenase). The active site consists of the tetrapeptide sequence Lys-Asp-Asp-Glu (or KDDE) and the Lys sidechain amine is responsible for the CO 2 capture reaction. We are studying the structural chemistry and the thermodynamics of CO 2 capture based on the tetrapeptide CH 3 COÀ KDDEÀ NH 2 ("KDDE") in aqueous solution to develop rubisco mimetic CCR systems. Here, we report the results of 1 H NMR and 13 C NMR analyses of CO 2 capture by butylamine and by KDDE. The carbamylation of butylamine was studied to develop the NMR method and with the protocol established, we were able to quantify the oligopeptide carbamylation at much lower concentration. We performed a pH profile in the multi equilibrium system and measured amine species and carbamic acid/ carbamate species by the integration of 1 H NMR signals as a function of pH in the range 8 � pH � 11. The determination of ΔG 1 (R) for the reaction RÀ NH 2 + CO 2 !RÀ NHÀ COOH requires the solution of a multi-equilibrium equation system, which accounts for the dissociation constants K 2 and K 3 controlling carbonate and bicarbonate concentrations, the acid dissociation constant K 4 of the conjugated acid of the amine, and the acid dissociation constant K 5 of the alkylcarbamic acid. We show how the multi-equilibrium equation system can be solved with the measurements of the daughter/parent ratio X, the knowledge of the pH values, and the initial concentrations [HCO 3 À ] 0 and [R-NH 2 ] 0 . For the reaction energies of the carbamylations of butylamine and KDDE, our best values are ΔG 1 (Bu) = À 1.57 kcal/mol and ΔG 1 (KDDE) = À 1.17 kcal/mol. Both CO 2 capture reactions are modestly exergonic and thereby ensure reversibility in an energy-efficient manner. These results validate the hypothesis that KDDE-type oligopeptides may serve as reversible CCR systems in aqueous solution and guide designs for their improvement.