Advanced Theory and Simulation to Guide the Development of CO2 Capture Solvents

Increasing atmospheric concentrations of greenhouse gases due to industrial activity have led to concerning levels of global warming. Reducing carbon dioxide (CO₂) emissions, one of the main contributors to the greenhouse effect, is key to mitigating further warming and its negative effects on the planet. CO₂ capture solvent systems are currently the only available technology deployable at scales commensurate with industrial processes. Nonetheless, designing these solvents for a given application is a daunting task requiring the optimization of both thermodynamic and transport properties. Here, we discuss the use of atomic scale modeling for computing reaction energetics and transport properties of these chemically complex solvents. Theoretical studies have shown that in many cases, one is dealing with a rich ensemble of chemical species in a coupled equilibrium that is often difficult to characterize and quantify by experiment alone. As a result, solvent design is a balancing act between multiple parameters which have optimal zones of effectiveness depending on the operating conditions of the application. Simulation of reaction mechanisms has shown that CO₂ binding and proton transfer reactions create chemical equilibrium between multiple species and that the agglomeration of resulting ions and zwitterions can have profound effects on bulk solvent properties such as viscosity. This is balanced against the solvent systems needing to perform different functions (e.g., CO₂ uptake and release) depending on the thermodynamic conditions (e.g., temperature and pressure swings). The latter constraint imposes a "Goldilocks" range of effective parameters, such as binding enthalpy and pK _a, which need to be tuned at the molecular level. The resulting picture is that solvent development requires an integrated approach where theory and simulation can provide the necessary ingredients to balance competing factors.

Effect of Photoacid Strength on Fluorescence Modulation of 2-Naphthol Derivatives inside β-Cyclodextrin Cavity: Insights from Fluorescence, Isothermal Calorimetry, and Molecular Dynamics Simulations

Fluorescence response of a photoacid inside a confined environment often differs markedly from the bulk response. Is there any correlation between the extent of fluorescence modulation and the strength of the photoacid? Here, we used three photoacids: 2-naphthol (2OH, pK_a* = 3.3), 6-sulfonate-2-naphthol (6SO₃-2OH, pK_a* = 3.06), and 6-cyano-2-naphthol (6CN-2OH, pK_a* = 0.6) with remarkably different excited-state acidities to investigate fluorescence modulation inside the nanocavity of β-cyclodextrin (β-CD). Interestingly, we found strong fluorescence modulation for 2OH and 6SO₃-2OH but almost none for 6CN-2OH. Isothermal calorimetry measurements showed that all three fluorophores form 1:1 inclusion complex with comparable binding constants (285, 420, and 580 M^-1 for 2OH, 6SO₃-2OH, and 6CN-2OH, respectively). Molecular dynamics simulation further revealed that binding modes are quite similar, and the distribution of water molecules around the proton-donating hydroxyl group of the photoacids are also comparable. Consequently, the difference in the fluorescence response should be accounted solely to the difference in the photoacidity strengths.

Quantum Mechanical Modeling of the Vibrational Spectra of Minerals with a Focus on Clays

We present an overview of how to use quantum mechanical calculations to predict vibrational frequencies of molecules and materials such as clays and silicates. Other methods of estimating vibrational frequencies are mentioned, such as classical molecular dynamics simulations; references are given for additional information on these approaches. Herein, we discuss basic vibrational theory, calculating Raman and infrared intensities, steps for creating realistic models, and applications to spectroscopy, thermodynamics, and isotopic fractionation. There are a wide variety of programs and methods that can be employed to model vibrational spectra, but this work focuses on hybrid density functional theory (DFT) approaches. Many of the principles are the same when used in other programs and DFT methods, so a novice can benefit from simple examples that illustrate key points to consider when modeling vibrational spectra. Other methods and programs are listed to give the beginner a starting point for exploring and choosing which approach will be best for a given problem. The modeler should also be aware of the numerous analytical methods available for obtaining information on vibrations of atoms in molecules and materials. In addition to traditional infrared and Raman spectroscopy, sum-frequency generation (SFG) and inelastic neutron scattering (INS) are also excellent techniques for obtaining vibrational frequency information in certain circumstances.

Ab initio molecular dynamics study of Se(iv) species in aqueous environment

An ab initio molecular dynamics investigation is carried out on various water-borne Se(iv) species, H₂SeO₃, HSeO₃^- and SeO₃^2-, in aqueous environment. Consistent with the reported acid dissociation constants, in neutral solution H₂SeO₃ exchanges protons with the surrounding water molecules establishing a dynamic equilibrium with HSeO₃^-. The SeO₃^2- species is found to be stable only in basic environment, which is emulated in the present simulation through introducing a hydroxide ion, OH^-, in the system. The hydration structure, hydrogen bonding and spectroscopic signatures of the species are comprehensively analyzed. The influence of the solute's hydration structure on the structural and dynamic response of the solvent is discussed. The correlation between the strength as well as the number of hydrogen bonds accepted by the solute on its vibrational properties are analyzed.