Testing Noncollinear Spin-Flip, Collinear Spin-Flip, and Conventional Time-Dependent Density Functional Theory for Predicting Electronic Excitation Energies of Closed-Shell Atoms

Conventional time-dependent density functional theory (TDDFT) is based on a closed-shell Kohn–Sham (KS) singlet ground state with the adiabatic approximation, using either linear response (KS-LR) or the Tamm–Dancoff approximation (KS-TDA); these methods can only directly predict singly excited states. This deficiency can be overcome by using a triplet state as the reference in the KS-TDA approximation and “exciting” the singlet by a spin flip (SF) from the triplet; this is the method suggested by Krylov and co-workers, and we abbreviate this procedure as SF-KS-TDA. SF-KS-TDA can be applied either with the original collinear kernel of Krylov and co-workers or with a noncollinear kernel, as suggested by Wang and Ziegler. The SF-KS-TDA method does bring some new practical difficulties into play, but it can at least formally model doubly excited states and states with double-excitation character, so it might be more useful than conventional TDDFT (both KS-LR and KS-TDA) for photochemistry if these additional difficulties can be surmounted and if it is accurate with existing approximate exchange–correlation functionals. In the present work, we carried out calculations specifically designed to understand better the accuracy and limitations of the conventional TDDFT and SF-KS-TDA methods; we did this by studying closed-shell atoms and closed-shell monatomic cations because they provide a simple but challenging testing ground for what we might expect in studying the photochemistry of molecules with closed-shell ground states. To test their accuracy, we applied conventional KS-LR and KS-TDA and 18 versions of SF-KS-TDA (nine collinear and nine noncollinear) to the same set of vertical excitation energies (including both Rydberg and valence excitations) of Be, B⁺, Ne, Na⁺, Mg, and Al⁺. We did this for 10 exchange–correlation functionals of various types, both local and nonlocal. We found that the GVWN5 and M06 functionals with nonlocal kernels in spin-flip calculations can both have accuracy competitive to CASPT2 calculations. When the results were averaged over all 36 test energy differences, seven (GVWN5, M06, B3PW91, LRC-ωPBE, LRC-ωPBEh, PBE, and M06-2X) of the 10 studied density functionals had smaller mean unsigned errors for noncollinear calculations than the mean unsigned error of the best functional (M06-2X) for either conventional KS-TDA or KS-LR

Anchor Points Reactive Potential for Bond-Breaking Reactions

We present a new method for fitting potential energy surfaces in molecular-mechanics-like internal coordinates based on data from electronic structure calculations. The method should be applicable to chemical reactions involving either bond dissociation or isomerization and is illustrated here for bond dissociation, in particular the breaking of an O–H bond in methanol and the breaking of an N–H bond in dimethylamine. As compared to previously available systematic methods for fitting global potential energy surfaces, it extends the maximum size of the system than can be treated by at least an order of magnitude

Photodissociation Dynamics of Phenol: Multistate Trajectory Simulations including Tunneling

We report multistate trajectory simulations, including coherence, decoherence, and multidimensional tunneling, of phenol photodissociation dynamics. The calculations are based on full-dimensional anchor-points reactive potential surfaces and state couplings fit to electronic structure calculations including dynamical correlation with an augmented correlation-consistent polarized valence double-ζ basis set. The calculations successfully reproduce the experimentally observed bimodal character of the total kinetic energy release spectra and confirm the interpretation of the most recent experiments that the photodissociation process is dominated by tunneling. Analysis of the trajectories uncovers an unexpected dissociation pathway for one quantum excitation of the O–H stretching mode of the S₁ state, namely, tunneling in a coherent mixture of states starting in a smaller <i>R</i>_OH (∼0.9–1.0 Å) region than has previously been invoked. The simulations also show that most trajectories do not pass close to the S₁–S₂ conical intersection (they have a minimum gap greater than 0.6 eV), they provide statistics on the out-of-plane angles at the locations of the minimum energy adiabatic gap, and they reveal information about which vibrational modes are most highly activated in the products

Which Ab Initio Wave Function Methods Are Adequate for Quantitative Calculations of the Energies of Biradicals? The Performance of Coupled-Cluster and Multi-Reference Methods Along a Single-Bond Dissociation Coordinate

We examine the accuracy of single-reference and multireference correlated wave function methods for predicting accurate energies and potential energy curves of biradicals. The biradicals considered are intermediate species along the bond dissociation coordinates for breaking the F–F bond in F₂, the O–O bond in H₂O₂, and the C–C bond in CH₃CH₃. We apply a host of single-reference and multireference approximations in a consistent way to the same cases to provide a better assessment of their relative accuracies than was previously possible. The most accurate method studied is coupled cluster theory with all connected excitations through quadruples, CCSDTQ. Without explicit quadruple excitations, the most accurate potential energy curves are obtained by the single-reference RCCSDt method, followed, in order of decreasing accuracy, by UCCSDT, RCCSDT, UCCSDt, seven multireference methods, including perturbation theory, configuration interaction, and coupled-cluster methods (with MRCI+Q being the best and Mk-MR-CCSD the least accurate), four CCSD­(T) methods, and then CCSD

Mechanism of Manganese-Catalyzed Oxygen Evolution from Experimental and Theoretical Analyses of ¹⁸O Kinetic Isotope Effects

The biomimetic oxomanganese complex [Mn^III/IV₂(μ-O)₂(terpy)₂(OH₂)₂]­(NO₃)₃ (<b>1</b>; terpy = 2,2′:6′,2″-terpyridine) catalyzes O₂ evolution from water when activated by oxidants, such as oxone (2KHSO₅·KHSO₄·K₂SO₄). The mechanism of this reaction has never been characterized, due to the fleeting nature of the intermediates. In the present study, we elucidate the underlying reaction mechanism through experimental and theoretical analyses of competitive kinetic oxygen isotope effects (KIEs) during catalytic turnover conditions. The experimental ¹⁸O KIE is a sensitive probe of the highest transition state in the O₂-evolution mechanism and provides a strict constraint for calculated mechanisms. The ¹⁸O kinetic isotope effect of 1.013 ± 0.003 measured using <i>natural abundance</i> reactants is consistent with the calculated isotope effect of peroxymonosulfate binding to the complex, as described by density functional theory (DFT). This provides strong evidence for peroxymonosulfate binding being both the first irreversible and rate-determining step during turnover, in contrast to the previously held assumption that formation of a high-valent Mn-oxo/oxyl species is the highest barrier step that controls the rate of O₂ evolution by this complex. The comparison of the measured and calculated KIEs supplements previous kinetic studies, enabling us to describe the complete mechanism of O₂ evolution, starting from when the oxidant first binds to the manganese complex to when O₂ is released. The reported findings lay the groundwork for understanding O₂ evolution catalyzed by other biomimetic oxomanganese complexes, with features common to those of the O₂-evolving complex of photosystem II, providing experimental and theoretical diagnostics of oxygen isotope effects that could reveal the nature of elusive reaction intermediates

Mechanistic Insights into Surface Chemical Interactions between Lithium Polysulfides and Transition Metal Oxides

The design and development of materials for electrochemical energy storage and conversion devices requires fundamental understanding of chemical interactions at electrode/electrolyte interfaces. For Li–S batteries that hold the promise for outperforming the current generation of Li ion batteries, the interactions of lithium polysulfide (LPS) intermediates with the electrode surface strongly influence the efficiency and cycle life of the sulfur cathode. While metal oxides have been demonstrated to be useful in trapping LPS, the actual binding modes of LPS on 3d transition metal oxides and their dependence on the metal element identity across the periodic table remain poorly understood. Here, we investigate the chemical interactions between LPS and oxides of Mn, Fe, Co, and Cu by combining X-ray photoelectron spectroscopy and density functional theory calculations. We find that Li–O interactions dominate LPS binding to the oxides (Mn₃O₄, Fe₂O₃, and Co₃O₄), with increasing strength from Mn to Fe to Co. For Co₃O₄, LPS binding also involves metal–sulfur interactions. We also find that the metal oxides exhibit different binding preferences for different LPS, with Co₃O₄ binding shorter-chain LPS more strongly than Mn₃O₄. In contrast to the other oxides, CuO undergoes intense reduction and dissolution reactions upon interaction with LPS. The reported findings are thus particularly relevant to the design of LPS/oxide interfaces for high-performance Li–S batteries

Hydrophobic CuO Nanosheets Functionalized with Organic Adsorbates

A new class of hydrophobic CuO nanosheets is introduced by functionalization of the cupric oxide surface with <i>p</i>-xylene, toluene, hexane, methylcyclohexane, and chlorobenzene. The resulting nanosheets exhibit a wide range of contact angles from 146° (<i>p</i>-xylene) to 27° (chlorobenzene) due to significant changes in surface composition induced by functionalization, as revealed by XPS and ATR-FTIR spectroscopies and computational modeling. Aromatic adsorbates are stable even up to 250–350 °C since they covalently bind to the surface as alkoxides, upon reaction with the surface as shown by DFT calculations and FTIR and ¹H NMR spectroscopy. The resulting hydrophobicity correlates with H₂ temperature-programmed reduction (H₂-TPR) stability, which therefore provides a practical gauge of hydrophobicity

Facet-Dependent Kinetics and Energetics of Hematite for Solar Water Oxidation Reactions

The performance of a photoelectrochemical (PEC) system is highly dependent on the charge separation, transport and transfer characteristics at the photoelectrode|electrolyte interface. Of the factors that influence the charge behaviors, the crystalline facets of the semiconductor in contact with the electrolyte play an important role but has been poorly studied previously. Here, we present a study aimed at understanding how the different facets of hematite affect the charge separation and transfer behaviors in a solar water oxidation reaction. Specifically, hematite crystallites with predominantly {012} and {001} facets exposed were synthesized. Density functional theory (DFT) calculations revealed that hematite {012} surfaces feature higher OH coverage, which was confirmed by X-ray photoelectron spectroscopy (XPS). These surface OH groups act as active sites to mediate water oxidation reactions, which plays a positive role for the PEC system. These surface OH groups also facilitate charge recombination, which compromises the charge separation capabilities of hematite. Indeed, intensity modulated photocurrent spectroscopy (IMPS) confirmed that hematite {012} surfaces exhibit higher rate constants for both charge transfer and recombination. Open circuit potential (OCP) measurements revealed that the hematite {012} surface exhibits a greater degree of Fermi level pinning effect. Our results shed light on how different surface crystal structures may change surface kinetics and energetics. The information is expected to contribute to efforts on optimizing PEC performance for practical solar fuel synthesis

Photoelectrochemical Urea Synthesis from Nitrate and Carbon Dioxide on GaN Nanowires

Semiconductor photoelectrodes can be used to synthesize urea from carbon dioxide and nitrate under solar light. We find that GaN nanowires (NWs) have inherent catalytic activity for nitrate conversion to nitrite, while Ag cocatalysts loaded onto GaN NWs further promote the performance of photoelectrochemical urea synthesis. Under optimized conditions, a high faradaic efficiency of 75.6 ± 2.6% was achieved at a potential of −0.3 vs reversible hydrogen electrode. Control experiments and theoretical calculations suggest that the high selectivity of urea originates from the facilitated C–N coupling between key intermediates of NO2 and COO– at an early stage of the reduction reaction. This work demonstrates the potential of GaN NWs with loaded Ag cocatalysts to achieve solar-powered urea synthesis with an efficiency higher than that of previously reported methods

New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of γ‑Ketohydroperoxides

We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from γ-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic O–O dissociation in existing kinetic models. Our <i>ab initio</i> calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products. Geometries and frequencies of all stationary points are obtained using the M06-2X/MG3S DFT model chemistry, and energies are refined using RCCSD­(T)-F12a/cc-pVTZ-F12 single-point calculations. Thermal rate coefficients are computed using variational transition-state theory (VTST) calculations with multidimensional tunneling contributions based on small-curvature tunneling (SCT). These are combined with multistructural partition functions (Q^MS–T) to obtain direct dynamics multipath (MP-VTST/SCT) gas-phase rate coefficients. For comparison with liquid-phase measurements, solvent effects are included using continuum dielectric solvation models. The predicted rate coefficients are found to be in excellent agreement with experiment when due consideration is made for acid-catalyzed isomerization. This work provides theoretical confirmation of the 30-year-old hypothesis of Korcek and co-workers that KHPs are precursors to carboxylic acid formation, resolving an open problem in the kinetics of liquid-phase autoxidation. The significance of the new pathways in atmospheric chemistry, low-temperature combustion, and oxidation of biological lipids are discussed