We consider different approximations to compute the heat capacity for Al(OH)3 and compare the computed results to the experiment. We find that the calculation of anharmonic effects for this molecule is not currently practical, and scaled harmonics, using the scaling factor for H2O(g), offer the best approach to including anharmonic effects. The Pitzer-Gwinn approach for hindered rotations is recommended.

In this study computational methods are used to derive thermochemical data for Sc, Fe, Co, Ni, and Hf hydroxides and oxyhydroxides. As done previously, molecular geometries and vibrational modes were derived with DFT methods; for the enthalpies of formation more computationally intensive coupled cluster methods were necessary. For each species Delta f H o (298), S o (298), and C p in the form A + BT + CT 2 + D/T + E/T 2 with A, B, C, D, and E fitted constants are presented. These are combined with previously reported calculations for Al, Cr, Si, Ta, Al, Zr, Y, Yb, Gd, and Mn to build a compound database for metal hydroxides and oxyhydroxides. Sample calculations for applications where high temperature water vapor is encountered are shown. The majority of the database was generated from ab initio calculations; however, experiments were critical benchmarks for many of the species.

Potential decomposition mechanisms are investigated using density functional theory (DFT) for a set of amide and urea solvents. Reaction energies and barriers are reported for proton and hydrogen abstraction, and preferred abstraction sites are identified. The N-H bond of secondary amides and the a-hydrogen atoms are more susceptible to proton abstraction than other sites in the solvent molecules. Additionally, hydrogen abstraction is more favorable at the N-alkyl substituents as well as a-hydrogen atoms that result in the formation of secondary and tertiary radicals. All proton abstraction energies are sensitive to the presence of a coordinating Li+, but the hydrogen abstraction energies do not depend on the presence of a Li+. The stabilization due to the presence of Li+ is most pronounced for sites near the carbonyl group where the Li+ interacts with the lone pair of electrons formed by proton abstraction and the carbonyl O atom. The initial steps of a Baeyer-Villiger oxidation type mechanism are also examined. Barriers for these initial steps are significantly lower if HO2- is the oxidant compared to LiO2-. A comparison to our previously reported experimental results indicates that no single reaction could be identified as the rate-limiting step that would predict the performance of this set of solvents in Li-O-2 batteries.

Nonaqueous Li-O2 batteries have the potential to aid in the electrification of our society due to their relatively high theoretical energy density. Unfortunately, the technology suffers from large degrees of irreversibility due to the aggressive chemical environment associated with the oxidation of the discharge product lithium peroxide. Herein, we present a study of a range of linear and cyclic amides and ureas as aprotic electrolyte solvents for the Li-O2 battery, some of which show slight increases in reversibility relative to the well-established pseudo-stable glymes, although we find that none provide reversibility necessary to enable a rechargeable system. Using quantitative differential electrochemical mass spectrometry, acid titrations, and isotopic labeling of O2 and carbon in the positive electrode, we provide insight into the degradation pathways for these solvents. In the companion article, we compare our experimental results presented here to solvent decomposition pathways including a Baeyer-Villiger oxidation mechanism.

The thermochemistry of the Si-O-H system has been extensively studied both experimentally and theoretically due to its importance in chemical processes, degradation of silica-protected materials in combustion, and geological processes. In this paper, we review past studies and use quantum mechanical methods to generate a new data set. Molecular geometries were generated with DFT using the B3LYP functional. Energetics were calculated with RCCSD(T) methods extrapolated to the complete basis set (CBS/45) limit. Particular attention was given to the treatment of the vibrational modes. A rigid rotor model was used, corrections for anharmonicity were applied, and the Pitzer-Gwinn treatment of the hindered rotation of the M-OH groups was applied. The generated enthalpies of formation at 298 K are compared to those of experiments and other calculations. Generally, the agreement is good. A set of thermodynamic data (enthalpy of formation at 298 K, entropy at 298 K, and heat capacity polynomial to 3000 K) is presented for SiOH, SiO(OH), Si(OH)(2), SiO(OH)(2), Si(OH)(3), Si(OH)(4), Si2O(OH)(6), and Si3O2(OH)(8). These can be added to any of the common computational thermodynamics packages. The application of these data to high-temperature corrosion and geological problems is discussed.

Thermodynamic quantities are calculated for gaseous hydroxides and oxyhydroxides of Cr, Mn, and La. These would form due to water-vapor-containing environments reacting with Cr-forming alloys or oxide components of potential fuel cell interconnects or anode materials. Structures and vibrational modes for the expected hydroxides and oxyhydroxides are calculated with the B3LYP hybrid functional. Enthalpies of formation from selected reactions for each species are calculated using the CCSD(T)/CBS approach. Results show good agreement with literature estimates, measurements, and calculations. The resultant data is reported as ΔfH°(298), S°(298), and Cp(T) and put into the database for a free-energy minimizer code. Calculations are presented to show the hydroxide and oxyhydroxide vapor pressures above H2O + Cr2O3, Mn3O4, and La2O3, as well as the anode material La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM).

We employ density functional theory (DFT) to examine reaction mechanisms involving singlet oxygen 1Δg (1O2) and 1,2-dimethoxyethane (DME) to probe potential parasitic reactions occurring in Li-O2 batteries. First, we investigate the attack of 1O2 on the ethylene group (-CH2-CH2-) to form H2O2 and a C-C double bond in a single step. Second, we look at hydroperoxide formation that occurs via a two-step mechanism. We employ an implicit solvent model, Li+ coordination, and external electric fields to model the complex electrolyte environment near the cathode of a Li-O2 battery. The initial barriers for these reactions are decreasing functions of the dielectric constant of the implicit solvent model as well as the strength of the electric field. These initial barriers range between 17 and 26 kcal mol-1 for large dielectric constants and in the presence of electric fields. We discuss the implications of these results on ether-based electrolytes for Li-O2 batteries.

The reliability of the small-core lanthanide effective core potentials (ECP) is tested using MF and MF $$_3$$ , for M=Eu, Gd, Tb, and Yb and the atomic excitation energies for Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. In some case, the ECP and all-electron (AE) results are in good agreement, while in others there are significant differences. The differences are much larger when the segmented basis set is used in conjunction with the ECP than when the atomic natural orbital (ANO) basis set is used. The study of the atoms suggests that problems for lanthanide-containing molecules are associated with poor atomic excitation energies in the ECP treatment and even using the ANO basis set does not completely solve the problem. We note that the problem appears to be more severe for density functional approaches than for traditional correlation methods. We suggest that additional studies and new effective core potentials may be required for the lanthanide atoms.

Advanced concepts for in-space propulsion require coatings that are resistant to erosion in high temperature and pressure hydrogen. The erosion of refractory carbides of interest for this application (ZrC, NbC, HfC, and TaC) is investigated using combined ab initio thermodynamic computations and equilibrium product analyses. The carbides are shown to erode through a combination of four governing reactions, the relative extent of which depend on environmental conditions. The product profiles from these reactions are complex but exhibit lower hydrogen saturation at higher temperatures and lower pressures. A metric is derived to determine the applicability of equilibrium analyses for erosion rates, based on experimental conditions. Heritage mass loss experiments on ZrC in hydrogen satisfy the equilibrium criteria, and, correspondingly, the computed equilibrium erosion rate agrees quantitatively. The results suggest that previously postulated non-equilibrium effects, namely the prolonged incongruent vaporization originating from high carbon mobility, do not drive erosion over the hours-long timescales of the experiments. For specific in-space propulsion designs, comparisons of carbide performance show TaC and HfC outperform other carbides and meet the criteria needed to close designs.

Gd2O3 and Yb2O3 are the proposed constituents of advanced coating systems in combustion environments. In such environments, they are exposed to high-temperature water vapor, which would lead to gaseous hydroxide formation. Thermodynamic parameters are reported for YbOn(OH)m and GdOn(OH)m species. We first study the MH, MO, MF, and MCl (M = Yb and Gd) species, where some experimental data exist. Structures and spectroscopic constants were calculated at the B3LYP level. For YbOn(OH)m, the B3LYP approach is used in conjunction with an effective core potential, while for GdOn(OH)m, it was necessary to use all-electron basis sets. Enthalpies of formation were calculated with a BD(T) approach. The enthalpies of formation, entropies, and heat capacities were added to a thermochemical database. Hydroxide and oxyhydroxide vapor pressures are calculated above pure Yb2O3, Gd2O3, and Y2O3 in 50% H2O/Ar from 1000 to 3000 K. Hydroxide and oxyhydroxide vapor pressures are also calculated for potential coating compositions.

Recent experimental and computational evidence indicates that singlet oxygen (1O2) attacks the ethylene group (-CH2-CH2-) in ethylene carbonate (EC) leading to degradation in Li-ion batteries employing EC as the electrolyte solvent [J. Phys. Chem. A 2018, 122, 8828-8839]. Here, we employ computational quantum chemistry to explore this mechanism in detail for a large set of organic molecules. Benchmark calculations comparing density functional theory to the complete active space second-order perturbation theory and internally contracted multireference configuration interaction indicate that the M11 functional adequately captures trends in the transition-state energies for this mechanism. Based on our results, we recommend that solvents which include the ethylene group should be avoided in Li-ion and Li-O2 batteries where 1O2 is generated unless neighboring functional groups raise the reaction barrier to avoid this decomposition pathway.

Thermodynamic parameters are reported for gaseous hydroxides and oxyhydroxides of Al, Zr, and Y. The structures and vibrational frequencies are calculated using density functional theory with the B3LYP functional. This yields entropies at 298.15 K and heat capacities. The enthalpies are calculated from appropriate reactions and the CCSD(T) (Coupled Cluster, Singles, Doubles, and perturbative Triples) approach. The hydroxide groups are treated as hindered rotors for all species. The results are compared to the limited experimental and theoretical calculations for these species. Finally, the data are put into a database for a free-energy minimizer and the vapor pressures for each species are compared.

The astronomical emission features, formerly known as the unidentified infrared bands, are now commonly ascribed to polycyclic aromatic hydrocarbons (PAHs). The laboratory experiments and computational modeling performed at NASA Ames Research Center generated a collection of PAH IR spectra that have been used to test and refine the PAH model. These data have been assembled into the NASA Ames PAH IR Spectroscopic Database (PAHdb). PAHdb’s library of computed spectra, currently at version 3.20, contains data on more than 4000 species and the library of laboratory-measured spectra, currently at version 3.00, contains data on 84 species. The spectra can be perused and are available for download at www.astrochemistry.org/pahdb/. This paper introduces the library of laboratory-measured spectra. Although it has been part of PAHdb since its inception, the library of laboratory-measured spectra lacked a proper description in the literature. Here, the experimental methods used to obtain the data are described in detail, an overview of the contents of the experimental library is given, and specific tools developed to analyze and interpret astronomical spectra with the laboratory data are discussed. In addition, updates to the website, documentation and software tools since our last reporting are presented. Software tools to work with the spectroscopic libraries are being developed actively and are available at GitHub. Lastly, a comprehensive demonstration showing how the laboratory-measured data can be applied to explore absorption features in observations toward embedded sources is presented. This demonstration suggests that PAHs very likely contribute to interstellar absorption spectra associated with dense clouds and underscores the need for further IR spectroscopic studies of PAHs trapped in water ice.

In the lithium-O-2 battery, redox mediators lower the charging overpotential while facilitating the oxidation of the discharge product (lithium peroxide, Li2O2) to molecular oxygen. Previous studies have shown that compounds such as 9,10-dimethylphenazine and 10-ethylphenothiazine are effective as redox mediators. Herein, we investigate the radical cation chemistry of thianthrene toward Li2O2 and lithium oxide (Li2O). Given the high oxidation potential of thianthrene (4.15 V vs. Li-0/Li+) several electron-rich analogs (including 2,3,6,7-tetramethox-yselenanthrene and a phenoxytellurine) were synthesized to decrease the oxidation potential toward the oxidation potential of Li2O2. Control experiments showed, in the absence of any candidate molecules, similar to 15% of the electrons were diverted to parasitic chemistries (2.3 electrons per oxygen). Surprisingly, experiments with several of the candidate molecules demonstrated that the charging of a battery (with a LiFePO4 anode) yields oxygen in a 2-electron process for most of the charging process. These 2-electron yields occurred at potentials where the thianthrene analogs could not be oxidized and thus act as redox mediators for Li2O2 oxidation. Under conditions where the mediator is oxidized to the radical cation, both the charging overpotentials and yields of oxygen were diminished. These data suggest that the thianthrene scaffold serves as a poor candidate for redox mediation but acts in a beneficial manner to suppress parasitic chemistries (e.g., parasitic reactions involving the likely formation of singlet oxygen).

A principal mode of corrosion in combustion or fuel cell environments is the formation of volatile hydroxides and oxyhydroxides from metal or oxide surfaces at high temperatures. It is important to determine the degree of volatility and accurate thermodynamic properties for these hydroxides. Significant gaseous metal hydroxides/oxyhydroxides are discussed, along with available experimental and theoretical methods of characterizing species and determining their thermodynamic properties.

The abstraction of a proton by OH-, O2-, and XO2- from DME n···X+, where X is Li, Na, or K, is studied using density functional theory. Both the gas phase and the solution phase are studied. In general, when explicit solvent molecules are added, the difference between the gas-phase and solution results becomes rather small. While the DME n···X+ binding energies differ significantly for various alkali cations, the reaction energies and transition-state energies are far less sensitive to the choice of an alkali cation. XO2- has a lower barrier height than OH-, which, in turn, has a lower barrier height than O2-. The reaction energies follow the same trends.

Tantalum pentoxide and water vapor are predicted to react at elevated temperatures to form TaO(OH)(3)(g), TaO2(OH)(g), and Ta(OH)(5)(g). The thermochemistry of these species is calculated with quantum chemistry methods. Geometries and vibrational frequencies are determined from B3LYP DFT methods. Energetics are calculated from high levels of theoryCCSD(T) and larger basis sets for Ta, O, and H. We report the enthalpies of formation at 0K and 298.15K, entropy at 298.15K, and heat capacity. These quantities are used to calculate vapor pressures at 1400-1800K and 50% water vapor. TaO(OH)(3)(g) is found to be the dominant species. The calculated vapor pressure of TaO(OH)(3)(g) is converted to a vapor flux and compared to previous experimental flux measurements from a flat plate in a slowly flowing H2O/Ar gas and also vapor flux from a steam jet experiment. These open system experiments result in lower fluxes that are within 1.12-17X of the calculated equilibrium fluxes. This suggests that the experimental measurements are near equilibrium or have a small kinetic barrier.

Recent experiments have suggested that melt flow plays a critical role in the ablation of meteoroids during atmospheric entry. Thus, modeling ablation requires knowledge of the melt properties of meteoritic constituents. These properties, however, are poorly understood and difficult to obtain with experimental techniques at entry conditions. An alternative means of obtaining high-temperature melt properties is through ab initio molecular dynamics (AIMD) simulations. Such simulations are performed here to characterize the melt properties of enstatite (MgSiO3), which is prevalent in certain types of chondrites, and its constitutive oxides (SiO2 and MgO). The structure, thermodynamic properties (density, bulk modulus, heat capacity, and coefficient of thermal expansion), and transport properties (diffusion and viscosity) are computed across the entire liquid phase and agree well with the limited number of available experiments. The high fidelity AIMD results are compared against less accurate models for melt property determination, which include classical molecular dynamics simulations and empirical mixture rules. Properties obtained from these alternative models generally show large relative errors compared to experiment, with viscosity, in particular, having errors of up to 98%. The present results highlight the potential of AIMD simulations to provide “quantitatively accurate” properties for melts of complex silicates found in meteorites and terrestrial rocks.