Thermochemistry of SinOx(OH)y Vapor Species from Quantum Chemical Calculations.

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.

Modeling Singlet Oxygen-Induced Degradation Pathways Including Environmental Effects of 1,2-Dimethoxyethane in Li-O2 Batteries through Density Functional Theory.

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.

Quantum chemical and experimental thermodynamic studies of HfO(g).

Hafnium dioxide vaporizes primarily to HfO(g) in a reducing environment. The thermochemistry of HfO(g) is calculated from quantum methods and measured via Knudsen effusion mass spectrometry. For the computations, all-electron and relativistic effective core potential calculations are used. The calculation of an accurate dissociation energy and an entire potential energy curve is reported. These calculations lead to ΔfH°(298) = 63.19 ± 10 kJ/mol, S°(298) = 235.52 J/mol K, and Cp(298-2500 K) = (2.741 × 10-9)T3 - (9.853 × 10-6)T2 + (1.295 × 10-2)T + 2.761 × 10-1 J/mol K. Experimentally, HfO(g) is generated from the reaction of Hf(s) and HfO2(s) in a specially made Hf Knudsen cell. A third law treatment of the data leads to ΔfH°(298) of 58.4 ± 12.3 kJ/mol, in good agreement with the calculated value.

Computational Chemistry Derivation of Cr, Mn, and La Hydroxide and Oxyhydroxide Thermodynamics.

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).

Thermochemistry of Gaseous Ytterbium and Gadolinium Hydroxides and Oxyhydroxides.

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.

Reaction of Singlet Oxygen with the Ethylene Group: Implications for Electrolyte Stability in Li-Ion and Li-O2 Batteries.

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.

Proton Abstraction from DME n···X+ by OH-, O2-, and XO2-, for X = Li, Na, and K: Implications for Li-O2 Batteries.

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.

POLYCYCLIC AROMATIC HYDROCARBONS WITH STRAIGHT EDGES AND THE 7.6/6.2 AND 8.6/6.2 INTENSITY RATIOS IN REFLECTION NEBULAE.

We have investigated the mid-infrared spectral characteristics of a series of polycyclic aromatic hydrocarbons (PAHs) with straight edges and containing an even or odd number of carbons using density functional theory (DFT). For several even and odd-carbon PAHs, the 8.6/6.2 and 7.6/6.2 intensity ratios computed in emission after the absorption of a 8 eV photon match the observed ratios obtained for three reflection nebulae (RNe), namely NGC 1333, NGC 7023, and NGC 2023. Odd-carbon PAHs are favored, particularly for NGC 1333. Both cations and anions are present with the cations being predominant. Relevant PAHs span sizes ranging from 46 to 103-113 carbons for NGC 7023 and NGC 2023 and from 38 to 127 carbons for NGC 1333 and have symmetries ranging from D2h to C s . Our work suggests that even and odd-carbon PAHs with straight edges are viable candidates for the PAH emission seen towards irradiated Photo-Dissociation Regions (PDRs).

Li+-ligand binding energies and the effect of ligand fluorination on the binding energies.

The Li+-ligand binding energies are computed for seven ligands and their perfluoro analogs using Density Functional Theory. The bonding is mostly electrostatic in origin. Thus the size of the binding energy tends to correlate with the ligand dipole moment, however, the charge-induced dipole contribution can be sufficiently large to affect the dipolebinding energy correlation. The perfluoro species are significantly less strongly bound than their parents, because the electron withdrawing power of the fluorine reduces the ligand dipole moment.

Phenolic Polymer Solvation in Water and Ethylene Glycol, I: Molecular Dynamics Simulations.

Interactions between pre-cured phenolic polymer chains and a solvent have a significant impact on the structure and properties of the final postcured phenolic resin. Developing an understanding of the nature of these interactions is important and will aid in the selection of the proper solvent that will lead to the desired final product. Here, we investigate the role of the phenolic chain structure and the solvent type on the overall solvation performance of the system through molecular dynamics simulations. Two types of solvents are considered: ethylene glycol (EGL) and H2O. In addition, three phenolic chain structures are considered, including two novolac-type chains with either an ortho-ortho (OON) or an ortho-para (OPN) backbone network and a resole-type (RES) chain with an ortho-ortho network. Each system is characterized through a structural analysis of the solvation shell and the hydrogen-bonding environment as well as through a quantification of the solvation free energy along with partitioned interaction energies between specific molecular species. The combination of simulations and the analyses indicate that EGL provides a higher solvation free energy than H2O due to more energetically favorable hydrophilic interactions as well as favorable hydrophobic interactions between CH element groups. In addition, the phenolic chain structure significantly affects the solvation performance, with OON having limited intermolecular hydrogen-bond formations, while OPN and RES interact more favorably with the solvent molecules. The results suggest that a resole-type phenolic chain with an ortho-para network should have the best solvation performance in EGL, H2O, and other similar solvents.

Phenolic Polymer Solvation in Water and Ethylene Glycol, II: Ab Initio Computations.

Ab initio techniques are used to study the interaction of ethylene glycol and water with a phenolic polymer. The water bonds more strongly with the phenolic OH than with the ring. The phenolic OH groups can form hydrogen bonds between themselves. For more than one water molecule, there is a competition between water-water and water-phenolic interactions. Ethylene glycol shows the same effects as those of water, but the potential energy surface is further complicated by CH2-phenolic interactions, different conformers of ethylene glycol, and two OH groups on each molecule. Thus, the ethylene glycol-phenolic potential is more complicated than the water-phenolic potential. The results of the ab initio calculations are compared to those obtained using a force field. These calibration studies show that the water system is easier to describe than the ethylene glycol system. The calibration studies confirm the reliability of force fields used in our companion molecular dynamics study of a phenolic polymer in water and ethylene solutions.

The low-lying electronic states of MgO.

The low-lying singlet and triplet states of MgO have been studied using a SA-CASCF/IC-MRCI approach using the aug-cc-pV5Z basis set. The spectroscopic constants (r e , w e , and T e ) are in good agreement with the available experimental data. The computed lifetime for the B state is in excellent agreement with two of the three experimental results. The d state lifetime is in good agreement with experiment, while the computed D state lifetime is about twice as long as experiment.

Ab Initio Simulations and Electronic Structure of Lithium-Doped Ionic Liquids: Structure, Transport, and Electrochemical Stability.

Density functional theory (DFT), density functional theory molecular dynamics (DFT-MD), and classical molecular dynamics using polarizable force fields (PFF-MD) are employed to evaluate the influence of Li(+) on the structure, transport, and electrochemical stability of three potential ionic liquid electrolytes: N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]). We characterize the Li(+) solvation shell through DFT computations of [Li(Anion)n]((n-1)-) clusters, DFT-MD simulations of isolated Li(+) in small ionic liquid systems, and PFF-MD simulations with high Li-doping levels in large ionic liquid systems. At low levels of Li-salt doping, highly stable solvation shells having two to three anions are seen in both [pyr14][TFSI] and [pyr13][FSI], whereas solvation shells with four anions dominate in [EMIM][BF4]. At higher levels of doping, we find the formation of complex Li-network structures that increase the frequency of four anion-coordinated solvation shells. A comparison of computational and experimental Raman spectra for a wide range of [Li(Anion)n]((n-1)-) clusters shows that our proposed structures are consistent with experiment. We then compute the ion diffusion coefficients and find measures from small-cell DFT-MD simulations to be the correct order of magnitude, but influenced by small system size and short simulation length. Correcting for these errors with complementary PFF-MD simulations, we find DFT-MD measures to be in close agreement with experiment. Finally, we compute electrochemical windows from DFT computations on isolated ions, interacting cation/anion pairs, and liquid-phase systems with Li-doping. For the molecular-level computations, we generally find the difference between ionization energy and electron affinity from isolated ions and interacting cation/anion pairs to provide upper and lower bounds, respectively, to experiment. In the liquid phase, we find the difference between the lowest unoccupied and highest occupied electronic levels in pure and hybrid functionals to provide lower and upper bounds, respectively, to experiment. Li-doping in the liquid-phase systems results in electrochemical windows little changed from the neat systems.

Structure and energetics of Li(+)-(BF4(-))n, Li(+)-(FSI(-))n, and Li(+)-(TFSI(-))n: ab initio and polarizable force field approaches.

The Li(+)-BF4(-) and BF4(-)-BF4(-) interactions are studied using second order perturbation theory (MP2) and coupled cluster singles and doubles approach, including the effect of connected triples, CCSD(T). The MP2 and CCSD(T) results are in excellent agreement. Using only the MP2 approach, the interactions of Li(+) with bis(trifluoromethane)sulfonimide anion (TFSI) and Li(+) with bis(fluorosulfonyl)imide anion (FSI) are studied. The results of these high level calculations are compared with density functional theory (DFT) calculations for a variety of functionals and with the APPLE&P force field. The B3LYP approach well reproduces the accurate calculations using both a small and large basis set. The M06 and M06L functionals in the larger basis set are in good agreement with the high level calculations. While the APPLE&P force field does not outperform the best functionals, the APPLE&P results agree better with the accurate results than do some of the functionals tested.

Computational and experimental investigation of Li-doped ionic liquid electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4].

We employ molecular dynamics (MD) simulation and experiment to investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrrolidinium bis(trifluoromethylsufonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]), as a function of Li-salt mole fraction (0.05 ≤ xLi(+) ≤ 0.33) and temperature (298 K ≤ T ≤ 393 K). Structurally, Li(+) is shown to be solvated by three anion neighbors in [pyr14][TFSI] and four anion neighbors in both [pyr13][FSI] and [EMIM][BF4], and at all levels of xLi(+) we find the presence of lithium aggregates. Pulsed field gradient spin-echo NMR measurements of diffusion and electrochemical impedance spectroscopy measurements of ionic conductivity are made for the neat ionic liquids as well as 0.5 molal solutions of Li-salt in the ionic liquids. Bulk ionic liquid properties (density, diffusion, viscosity, and ionic conductivity) are obtained with MD simulations and show excellent agreement with experiment. While the diffusion exhibits a systematic decrease with increasing xLi(+), the contribution of Li(+) to ionic conductivity increases until reaching a saturation doping level of xLi(+) = 0.10. Comparatively, the Li(+) conductivity of [pyr14][TFSI] is an order of magnitude lower than that of the other liquids, which range between 0.1 and 0.3 mS/cm. Our transport results also demonstrate the necessity of long MD simulation runs (∼200 ns) to converge transport properties at room temperature. The differences in Li(+) transport are reflected in the residence times of Li(+) with the anions (τ(Li/-)), which are revealed to be much larger for [pyr14][TFSI] (up to 100 ns at the highest doping levels) than in either [EMIM][BF4] or [pyr13][FSI]. Finally, to comment on the relative kinetics of Li(+) transport in each liquid, we find that while the net motion of Li(+) with its solvation shell (vehicular) significantly contributes to net diffusion in all liquids, the importance of transport through anion exchange increases at high xLi(+) and in liquids with large anions.

Infrared vibrational and electronic transitions in the dibenzopolyacene family.

We report experimental spectra in the mid-infrared (IR) and near-IR for a series of dibenzoacenes isolated in Ar matrices. The experiments are supported by Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations with both vibrational and electronic transitions studied. For the neutrals, we find good agreement between the experimental and B3LYP and BP86 results for all species studied. The band at about 1440 cm(-1) carries more intensity than in typical PAHs and increases in intensity with the size of the dibenzoacene molecule. For the ions the B3LYP approach fails to yield reasonable IR spectra for most systems and the BP86 approach is used. Electronic transitions dominate the vibrational bands in the mid-IR region for the large dibenzoacene ions. In spite of the very strong electronic transitions, there is still reasonable agreement between theory and experiment for the vibrational band positions. The experimental and theoretical results for the dibenzoacenes are also compared with those for the polyacenes.

Comparison of ReaxFF, DFTB, and DFT for phenolic pyrolysis. 2. Elementary reaction paths.

Reaction paths for the loss of CO, H2, and H2O from atomistic models of phenolic resin are determined using the hybrid B3LYP approach. B3LYP energetics are confirmed using CCSD(T). The energetics along the B3LYP paths are also evaluated using the PW91 generalized gradient approximation (GGA), the more approximate self-consistent charge density functional tight binding (SCC-DFTB), and the reactive force field (ReaxFF). Compared with the CCSD(T)/cc-pVTZ level for bond and reaction energies and barrier heights, the B3LYP, PW91, DFTB(mio), DFTB(pbc), and ReaxFF have average absolute errors of 3.8, 5.1, 17.4, 13.2, and 19.6 kcal/mol, respectively. The PW91 is only slightly less accurate than the B3LYP approach, while the more approximate approaches yield somewhat larger errors. The SCC-DFTB paths are in better agreement with B3LYP than are those obtained with ReaxFF.

Comparison of ReaxFF, DFTB, and DFT for phenolic pyrolysis. 1. Molecular dynamics simulations.

A systematic comparison of atomistic modeling methods including density functional theory (DFT), the self-consistent charge density-functional tight-binding (SCC-DFTB), and ReaxFF is presented for simulating the initial stages of phenolic polymer pyrolysis. A phenolic polymer system is simulated for several hundred picoseconds within a temperature range of 2500 to 3500 K. The time evolution of major pyrolysis products including small-molecule species and char is examined. Two temperature zones are observed which demark cross-linking versus fragmentation. The dominant chemical products for all methods are similar, but the yields for each product differ. At 3500 K, DFTB overestimates CO production (300-400%) and underestimates free H (~30%) and small C(m)H(n)O molecules (~70%) compared with DFT. At 3500 K, ReaxFF underestimates free H (~60%) and fused carbon rings (~70%) relative to DFT. Heterocyclic oxygen-containing five- and six-membered carbon rings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassical method for incorporating quantum nuclear energies of molecules were also performed. These results suggest that chemical equilibrium can be affected by quantum nuclear effects at temperatures of 2500 K and below. Pyrolysis reaction mechanisms and energetics are examined in detail in a companion manuscript.

Reactions of group 3 metals with OF2: infrared spectroscopic and theoretical investigations of the group 3 oxydifluoride OMF2 and oxyfluoride OMF molecules.

The oxidifluoride molecules, OYF(2) and OLaF(2), are produced via the reactions of laser ablated metal atoms with OF(2) in solid argon. The product structures are characterized using matrix isolation infrared spectroscopy as well as theoretical calculations. Similar to the very recently characterized OScF(2) molecule, OYF(2) is predicted to have a (2)B(2) ground state with C(2v) symmetry while the heavier OLaF(2) has a (2)A″ ground state with near C(2v) symmetry. The unpaired electron is mainly located on the terminal oxygen atom, suggesting radical character for the group 3 OMF(2) molecules. In addition, the closed shell singlet OMF molecules with bent geometries are also observed, and they are found to have triple metal-oxygen bonds with higher stretching frequencies and shorter bond lengths than their OMF(2) counterparts. α-Fluorine transfer from OF(2) to metal centers is predicted to be highly exothermic, which is very favorable for the formation of new OMF(2) and OMF species.

Infrared spectroscopic and theoretical studies of the OTiF2, OZrF2 and OHfF2 molecules with terminal oxo ligands.

The isolated group 4 metal oxydifluoride molecules OMF(2) (M = Ti, Zr, Hf) with terminal oxo groups are produced specifically on the spontaneous reactions of metal atoms with OF(2) through annealing in solid argon. The product structures and vibrational spectra are characterized using matrix isolation infrared spectroscopy as well as B3LYP density functional and CCSD(T) frequency calculations. OTiF(2) is predicted to have a planar structure while both OZrF(2) and OHfF(2) possess pyramidal structures, all with singlet ground states. Three infrared absorptions are observed for each product molecule, one M-O and two M-F stretching modes, and assignments of these molecules are further supported by the corresponding (18)O shifts. The molecular orbitals of the group 4 OMF(2) molecules show triple bond character for the terminal oxo groups, which are also supported by an NBO analysis. These molecular orbitals include a σ bond (O(2p) + Ti(sd hybrid)), a normal electron pair π bond (O(2p) + Ti(d)), and a dative π bond arising from O lone pair donation to the overlapping Ti d orbital. The M-O bond dissociation energies for OMF(2) are comparable to those in the diatomic oxide molecules. The OTiF intermediate is also observed through two slightly lower frequency bond stretching modes, and its yield is increased in complementary TiO + F(2) experiments. Finally, the formation of group 4 OMF(2) molecules is highly exothermic due to the weak O-F bonds in OF(2) as well as the strong new MO and M-F bonds formed.