Vibration-mediated long-wavelength photolysis of electronegative bonds beyond S0-S1 and S0-T1 transitions

Photolysis is an attractive method in organic synthesis to produce free radicals through direct bond cleavage. However, in this method, specific irradiation wavelengths of light have been considered indispensable for excitation through S0-Sn or S0-Tn transitions. Here we report the photoinduced homolysis of electronegative interelement bonds using light at wavelengths much longer than theoretically and spectroscopically predicted for the S0-Sn or S0-Tn transitions. This long-wavelength photolysis proceeds in N-Cl, N-F, and O-Cl bonds at room temperature under blue, green, and red LED irradiation, initiating diverse radical reactions. Through experimental, spectroscopic, and computational studies, we propose that this "hidden" absorption is accessible via electronic excitations from naturally occurring vibrationally excited ground states to unbonded excited states and is due to the electron-pair repulsion between electronegative atoms.

Multistate Dynamics and Kinetics of CO2 Activation by Ta+ in the Gas Phase: Insights into Single-Atom Catalysis

The activation of carbon dioxide (CO2) by a transition-metal cation in the gas phase is a unique model system for understanding single-atom catalysis. The mechanism of such reactions is often attributed to a "two-state reactivity" model in which the high-energy barrier of a spin state correlating with ground-state reactants is avoided by intersystem crossing (ISC) to a different spin state with a lower barrier. However, such a "spin-forbidden" mechanism, along with the corresponding dynamics, has seldom been rigorously examined theoretically, due to the lack of global potential energy surfaces (PESs). In this work, we report full-dimensional PESs of the lowest-lying quintet, triplet, and singlet states of the TaCO2+ system, machine-learned from first-principles data. These PESs and the corresponding spin-orbit couplings enable us to provide an extensive theoretical characterization of the dynamics and kinetics of the reaction between the tantalum cation (Ta+) and CO2, which have recently been investigated experimentally at high collision energies using crossed beams and velocity map imaging, as well as at thermal energies using a selected-ion flow tube apparatus. The multistate quasi-classical trajectory simulations with surface hopping reproduce most of the measured product translational and angular distributions, shedding valuable light on the nonadiabatic reaction dynamics. The calculated rate coefficients from 200 to 600 K are also in good agreement with the latest experimental measurements. More importantly, these calculations revealed that the reaction is controlled by intersystem crossing, rather than potential barriers.

Iron-Catalyzed Allylic C(sp3)-H Silylation: Spin-Crossover-Efficiency-Determined Chemoselectivity

The nuanced role of spin effects remains a critical gap in designing proficient open-shell catalysts. This study elucidates an iron-catalyzed allylic C(sp3)-H silylation/alkyne hydrosilylation reaction, in which the spin state of the open-shell iron catalyst dictates the reaction kinetics and pathway. Specifically, spin crossover led to alkyne hydrosilylation, whereas spin conservation resulted in a novel allylic C(sp3)-H silylation reaction. This chemoselectivity, governed by the spin-crossover efficiency, reveals an unexpected dimension in spin effects and a first in the realm of transition-metal-catalyzed in situ silylation of allylic C(sp3)-H bonds, which had been previously inhibited by the heightened reactivity of alkenes in hydrosilylation reactions. Furthermore, this spin crossover can either accelerate or hinder the reaction at different stages within a single catalytic reaction, a phenomenon scarcely documented. Moreover, we identify a substrate-assisted C-H activation mechanism, a departure from known ligand-assisted processes, offering a fresh perspective on C-H activation strategies.

A Novel Abiotic Pathway for Phosphine Synthesis over Acidic Dust in Venus' Atmosphere

Recent ground-based observations of Venus have detected a single spectral feature consistent with phosphine (PH3) in the middle atmosphere, a gas which has been suggested as a biosignature on rocky planets. The presence of PH3 in the oxidized atmosphere of Venus has not yet been explained by any abiotic process. However, state-of-the-art experimental and theoretical research published in previous works demonstrated a photochemical origin of another potential biosignature-the hydride methane-from carbon dioxide over acidic mineral surfaces on Mars. The production of methane includes formation of the HC · O radical. Our density functional theory (DFT) calculations predict an energetically plausible reaction network leading to PH3, involving either HC · O or H· radicals. We suggest that, similarly to the photochemical formation of methane over acidic minerals already discussed for Mars, the origin of PH3 in Venus' atmosphere could be explained by radical chemistry starting with the reaction of ·PO with HC·O, the latter being produced by reduction of CO2 over acidic dust in upper atmospheric layers of Venus by ultraviolet radiation. HPO, H2P·O, and H3P·OH have been identified as key intermediate species in our model pathway for phosphine synthesis.

Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the RuVI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the RuIV mono-oxo intermediate, and is involved in the regeneration of the RuVI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.

Unravelling the Effect of Acid-Driven Electron Transfer in High-Valent FeIV =O/MnIV =O Species and Its Implications for Reactivity

The electron transfer (ET) step is one of the crucial processes in biochemical redox reactions that occur in nature and has been established as a key step in dictating the reactivity of high-valent metal-oxo species. Although metalloenzymes possessing metal-oxo units at their active site are typically associated with outer-sphere electron transfer (OSET) processes, biomimetic models, in contrast, have been found to manifest either an inner-sphere electron transfer (ISET) or OSET mechanism. This distinction is clearly illustrated through the behaviour of [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) complexes, where complex 1 showcases an OSET mechanism, while complex 2 exhibits an ISET mechanism, especially evident in their reactions involving C-H bond activation and oxygen atom transfer reactions in the presence of a Lewis/Bronsted acid. However, the precise reason for this puzzling difference remains elusive. This work unveils the origin of the perplexing inner-sphere vs outer-sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) species in the presence of Bronsted acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1-HOTf and 2-HOTf, respectively), proton transfer from one of the HOTf molecules to the metal-oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. Interestingly, the preference for OSET by 1-HOTf is found to originate from the choice of MnIV =O centre to abstract spin-down (β) electron from toluene to its δ(dxy ) orbital. On the other hand, in 2-HOTf, a spin state inversion from triplet to quintet state takes place during the proton (from HOTf) coupled electron transfer (from toluene) preferring a spin-up (α) electron abstraction to its σ* (dz 2 ) orbital mediated by HOTf giving rise to ISET. In addition, 2-HOTf was calculated to possess a larger reorganisation energy, which facilitates the ISET process via the acid. The absence of spin-inversion and smaller reorganisation energy switch the mechanism to OSET for 1-HOTf. Therefore, for the first time, the significance of spin-state and spin-inversion in the electron transfer process has been identified and demonstrated within the realm of high-valent metal-oxo chemistry. This discovery holds implications for the potential involvement of high-valent Mn-oxo species in performing similar transformative processes within Photosystem II.

On the performance of the M-C (M = Fe, Ru, Os) unit toward methane activation

Gas-phase reactions of [MC]+ (M = Os and Ru) with methane at ambient temperature have been studied by using quadrupole-ion trap (Q-IT) mass spectrometry combined with quantum chemical calculations. Theoretical calculations reveal the influence of electronic signatures and that it is the energy gap of the associated frontier molecular orbitals that dominates the ability of the cluster in the initial H3C-H bond breaking. By extension, a theoretical consideration upon changing the ligand from carbide to carbyne and eventually to carbene reveals that the reactivities of the M-complex (M = Os, Ru and Fe) are determined by the energy gap of the involved orbitals. In addition, a few factors like the dipole moment, spin density and charge distributions influence the orbital energy gap to different extents. Thus, altering the local structure of the active center to modulate the orbital distribution may be a possible means of regulation of the activity.

Two state reactivity (TSR) and hydrogen tunneling reaction kinetics measured in the Co+ mediated decomposition of CH3CHO

The electronic structure of a transition metal atom allows it to act as a catalytic active site by providing lower energy alternative pathways in chemical transformations. We have identified and kinetically characterized three such pathways in the title reaction. One is an adiabatic pathway that occurs on a single potential energy surface described within the Born-Oppenheimer approximation. A second pathway opens microseconds into the reaction as a portion of the reacting population competitively transitions from triplet to singlet multiplicity to circumvent energetic barriers on the triplet surface. These pathways are single- and two-state reactive (SSR and TSR) where the Co+ cation mediates an oxidative addition/reductive elimination sequence of the CH3CHO molecule. The third observed reaction pathway is the aldehyde hydrogen tunneling through an Eyring barrier to form high-spin products. First-order rate constants for the adiabatic and nonadiabatic energy lowered pathways, and the hydrogen tunneling pathway, are each measured using the single photon initiated dissociative rearrangement reaction (SPIDRR) experimental technique. We believe that this is the first experimental study where such disparate dynamic features (SSR, TSR, and H-tunneling) are disentangled in a system's chemistry, attributing specific rate constant values to each effect and quantifying the various competitions. Moreover, multi-reference CASSCF/CASPT2 calculations indicate that structures with covalent Co-H bonds are present exclusively along the excited singlet surface. This phenomenon significantly reduces these structures' energy relative to their triplet counterparts, thus enabling the surface crossing and spin inversion that cause the observed two-state reactivity.

Hydroformylation of pyrolysis oils to aldehydes and alcohols from polyolefin waste

Waste plastics are an abundant feedstock for the production of renewable chemicals. Pyrolysis of waste plastics produces pyrolysis oils with high concentrations of olefins (>50 weight %). The traditional petrochemical industry uses several energy-intensive steps to produce olefins from fossil feedstocks such as naphtha, natural gas, and crude oil. In this work, we demonstrate that pyrolysis oil can be used to produce aldehydes through hydroformylation, taking advantage of the olefin functionality. These aldehydes can then be reduced to mono- and dialcohols, oxidized to mono- and dicarboxylic acids, or aminated to mono- and diamines by using homogeneous and heterogeneous catalysis. This route produces high-value oxygenated chemicals from low-value postconsumer recycled polyethylene. We project that the chemicals produced by this route could lower greenhouse gas emissions ~60% compared with their production through petroleum feedstocks.

A general method for locating stationary points on the mixed-spin surface of spin-forbidden reaction with multiple spin states

Some chemical reactions proceed on multiple potential energy surfaces and are often accompanied by a change in spin multiplicity, being called spin-forbidden reactions, where the spin-orbit coupling (SOC) effects play a crucial role. In order to efficiently investigate spin-forbidden reactions with two spin states, Yang et al. [Phys. Chem. Chem. Phys. 20, 4129-4136 (2018)] proposed a two-state spin-mixing (TSSM) model, where the SOC effects between the two spin states are simulated by a geometry-independent constant. Inspired by the TSSM model, we suggest a multiple-state spin-mixing (MSSM) model in this paper for the general case with any number of spin states, and its analytic first and second derivatives have been developed for locating stationary points on the mixed-spin potential energy surface and estimating thermochemical energies. To demonstrate the performance of the MSSM model, some spin-forbidden reactions involving 5d transition elements are calculated using the density functional theory (DFT), and the results are compared with the two-component relativistic ones. It is found that MSSM DFT and two-component DFT calculations may provide very similar stationary-point information on the lowest mixed-spin/spinor energy surface, including structures, vibrational frequencies, and zero-point energies. For the reactions containing saturated 5d elements, the reaction energies by MSSM DFT and two-component DFT agree very well within 3 kcal/mol. As for the two reactions OsO+ + CH4 → OOs(CH2)+ + H2 and W + CH4 → WCH2 + H2 involving unsaturated 5d elements, MSSM DFT may also yield good reaction energies of similar accuracy but with some counterexamples. Nevertheless, the energies may be remarkably improved by a posteriori single point energy calculations using two-component DFT at the MSSM DFT optimized geometries, and the maximum error of about 1 kcal/mol is almost independent of the SOC constant used. The MSSM method as well as the developed computer program provides an effective utility for studying spin-forbidden reactions.

Methane Activation by [OsC3]+: Implications for Catalyst Design

Gas-phase reactions of [OsC₃]⁺ with methane at ambient temperature have been studied by using quadrupole-ion trap mass spectrometry combined with quantum chemical calculations. The comparison of [OsC₃]⁺ with the product clusters revealed significant changes in cluster reactivity. In particular, with different ligands, the cluster may produce multiple products or, alternatively, just a single product. Theoretical calculations reveal the influence of electronic features such as molecular polarity index, charge and spin distribution, and HOMO-LUMO gap on the reactivity of the Os complexes. Fundamentally, it is the polarity of the clusters that leads to the cluster reactivity in the methane activation. Furthermore, reducing the local polarity of the catalyst active site may be one means of reducing the number of byproducts in the reaction.

Spin-Forbidden Addition of Molecular Oxygen to Stable Enol Intermediates-Decarboxylation of 2-Methyl-1-tetralone-2-carboxylic Acid

The deprotonation of an organic substrate is a common preactivation step for the enzymatic cofactorless addition of O2 to this substrate, as it promotes charge-transfer between the two partners, inducing intersystem crossing between the triplet and singlet states involved in the process. Nevertheless, the spin-forbidden addition of O2 to uncharged ligands has also been observed in the laboratory, and the detailed mechanism of how the system circumvents the spin-forbiddenness of the reaction is still unknown. One of these examples is the cofactorless peroxidation of 2-methyl-3,4-dihydro-1-naphthol, which will be studied computationally using single and multi-reference electronic structure calculations. Our results show that the preferred mechanism is that in which O2 picks a proton from the substrate in the triplet state, and subsequently hops to the singlet state in which the product is stable. For this reaction, the formation of the radical pair is associated with a higher barrier than that associated with the intersystem crossing, even though the absence of the negative charge leads to relatively small values of the spin-orbit coupling.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand

The reaction of TaMe₃ Cl₂ with the rigid acridane-derived trisamine H₃ NNN yields the tantalum(V) complex [TaCl₂ (NNN^cat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O₂ yielding the oxido-bridged bimetallic complex [{TaCl₂ (NNN^sq )}₂ O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O₂ activation was investigated by DFT calculations revealing initial binding of O₂ to the tantalum(V) center followed by complete O₂ scission to produce a terminal oxido-complex.

Oxidation and Reduction Pathways in the Knowles Hydroamination via a Photoredox-Catalyzed Radical Reaction

Systematic reaction path exploration revealed the entire mechanism of Knowles's light-promoted catalytic intramolecular hydroamination. Bond formation/cleavage competes with single electron transfer (SET) between the catalyst and substrate. These processes are described by adiabatic processes through transition states in an electronic state and non-radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable SET path by introducing a practical computational model representing SET as non-adiabatic transitions via SXs between substrate's potential energy surfaces for different charge states adjusted based on the catalyst's redox potential. Calculations showed that the reduction and proton shuttle process proceeded concertedly. Also, the relative importance of SET paths (giving the product and leading back to the reactant) varies depending on the catalyst's redox potential, affecting the yield.

Solving the Azobenzene Entropy Puzzle: Direct Evidence for Multi-State Reactivity

A solution to the azobenzene "entropy puzzle" [ J. Phys.: Condens. Matter, 2017, 29, 314002] is provided. Previous computational studies of the thermal Z → E (back-)isomerization of azobenzene could not describe the experimentally observed large negative activation entropies. Here it is shown that the experimental results are only compatible with a more complicated multistate rotation mechanism that involves a triplet excited state. Using nonadiabatic transition state theory, close to perfect agreement is achieved between all calculated and experimental Eyring parameters. We also provide new experiments that indicate the presence of a noticeable external heavy-atom effect, which is a direct result of spin-orbit coupling effects being important in the proposed mechanism. These results suggest a reexamination of the mechanisms of related thermal double bond isomerizations in other systems in cases when an excited state of triplet (or other) multiplicity becomes thermally accessible during a rotation process.

Microscopic Reactivity of Phenylferrate Ions toward Organyl Halides

Despite its practical importance, organoiron chemistry remains poorly understood due to its mechanistic complexity. Here, we focus on the oxidative addition of organyl halides to phenylferrate anions in the gas phase. By mass-selecting individual phenylferrate anions, we can determine the effect of the oxidation state, the ligation, and the nuclearity of the iron complex on its reactions with a series of organyl halides RX. We find that Ph₂ Fe(I)^- and other low-valent ferrates are more reactive than Ph₃ Fe(II)^- ; Ph₄ Fe(III)^- is inert. The coordination of a PPh₃ ligand or the presence of a second iron center lower the reactivity. Besides direct cross-coupling reactions resulting in the formation of RPh, we also observe the abstraction of halogen atoms. This reaction channel shows the readiness of organoiron species to undergo radical-type processes. Complementary DFT calculations afford further insight and rationalize the high reactivity of the Ph₂ Fe(I)^- complex by the exothermicity of the oxidative addition and the low barriers associated with this reaction step. At the same time, they point to the importance of changes of the spin state in the reactions of Ph₃ Fe(II)^- .

Computational Evaluation of Potential Molecular Catalysts for Nitrous Oxide Decomposition

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) with limited use as a mild anesthetic and underdeveloped reactivity. Nitrous oxide splitting (decomposition) is critical to its mitigation as a GHG. Although heterogeneous catalysts for N₂O decomposition have been developed, highly efficient, long-lived solid catalysts are still needed, and the details of the catalytic pathways are not well understood. Reported herein is a computational evaluation of three potential molecular (homogeneous) catalysts for N₂O splitting, which could aid in the development of more active and robust catalysts and provide deeper mechanistic insights: one Cu(I)-based, [(CF₃O)₄Al]Cu (A-1), and two Ru(III)-based, Cl(POR)Ru (B-1) and (NTA)Ru (C-1) (POR = porphyrin, NTA = nitrilotriacetate). The structures and energetic viability of potential intermediates and key transition states are evaluated according to a two-stage reaction pathway: (A) deoxygenation (DO), during which a metal-N₂O complex undergoes N-O bond cleavage to produce N₂ and a metal-oxo species and (B) (di)oxygen evolution (OER), in which the metal-oxo species dimerizes to a dimetal-peroxo complex, followed by conversion to a metal-dioxygen species from which dioxygen dissociates. For the (F-L)Cu(I) activator (A-1), deoxygenation of N₂O is facilitated by an O-bound (F-L)Cu-O-N₂ or better by a bimetallic N,O-bonded, (F-L)Cu-NNO-Cu(F-L) complex; the resulting copper-oxyl (F-L)Cu-O is converted exergonically to (F-L)Cu-(η²,η²-O₂)-Cu(F-L), which leads to dioxygen species (F-L)Cu(η²-O₂), that favorably dissociates O₂. Key features of the DO/OER process for (POR)ClRu (B-1) include endergonic N₂O coordination, facile N₂ evolution from LR'u-N₂O-RuL to Cl(POR)RuO, moderate barrier coupling of Cl(POR)RuO to peroxo Cl(POR)Ru(O₂)Ru(POR)Cl, and eventual O₂ dissociation from Cl(POR)Ru(η¹-O₂), which is nearly thermoneutral. N₂O decomposition promoted by (NTA)Ru(III) (C-1) can proceed with exergonic N₂O coordination, facile N₂ dissociation from (NTA)Ru-ON₂ or (NTA)Ru-N₂O-Ru(NTA) to form (NTA)Ru-O; dimerization of the (NTA)Ru-oxo species is facile to produce (NTA)Ru-O-O-Ru(NTA), and subsequent OE from the peroxo species is moderately endergonic. Considering the overall energetics, (F-L)Cu and Cl(POR)Ru derivatives are deemed the best candidates for promoting facile N₂O decomposition.

Reconciling Local Coupled Cluster with Multireference Approaches for Transition Metal Spin-State Energetics

Spin-state energetics of transition metal complexes remain one of the most challenging targets for electronic structure methods. Among single-reference wave function approaches, local correlation approximations to coupled cluster theory, most notably the domain-based local pair natural orbital (DLPNO) approach, hold the promise of bringing the accuracy of coupled cluster theory with single, double, and perturbative triple excitations, CCSD(T), to molecular systems of realistic size with acceptable computational cost. However, recent studies on spin-state energetics of iron-containing systems raised doubts about the ability of the DLPNO approach to adequately and systematically approximate energetics obtained by the reference-quality complete active space second-order perturbation theory with coupled-cluster semicore correlation, CASPT2/CC. Here, we revisit this problem using a diverse set of iron complexes and examine several aspects of the application of the DLPNO approach. We show that DLPNO-CCSD(T) can accurately reproduce both CASPT2/CC and canonical CCSD(T) results if two basic principles are followed. These include the consistent use of the improved iterative (T₁) versus the semicanonical perturbative triple corrections and, most importantly, a simple two-point extrapolation to the PNO space limit. The latter practically eliminates errors arising from the default truncation of electron-pair correlation spaces and should be viewed as standard practice in applications of the method to transition metal spin-state energetics. Our results show that reference-quality results can be readily achieved with DLPNO-CCSD(T) if these principles are followed. This is important also in view of the applicability of the method to larger single-reference systems and multinuclear clusters, whose treatment of dynamic correlation would be challenging for multireference-based approaches.

Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel-Bipyridine Photoredox Catalysts

Ni 2,2'-bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited-state Ni(II)-C bond homolysis. Here, we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(^Rbpy)(^R'Ph)Cl (R = MeO, t-Bu, H, MeOOC; R' = CH₃, H, OMe, F, CF₃), to illuminate the mechanism of excited-state bond homolysis. At given excitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer (MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited-state barriers (ΔH^‡ ∼ 4 kcal mol^-1) and a minimum energy excitation threshold (∼55 kcal mol^-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in synthetic chemistry.

2H-Imidazol-2-one O-Oxide: A Criegee Intermediate from a σ0π2 Singlet Ground-State Carbene

Triplet carbenes and triplet molecular oxygen can combine to form singlet carbonyl O-oxide Criegee intermediates in an overall spin-allowed transformation. While this reaction runs at the diffusion limit in the case of triplet carbenes, singlet carbenes are commonly more reluctant to bind ³O₂. In contradiction to this customarily encountered spin selectivity, the σ⁰π² singlet ground-state carbene 2H-imidazol-2-ylidene 1 reacts extremely rapidly with ³O₂ at temperatures as low as 30 K. The product of this cryogenic reaction is singlet 2H-imidazol-2-one O-oxide 7, an N-heterocyclic Criegee intermediate. The addition reaction becomes possible through the electrophilic activation of dioxygen in the triplet state, in which O₂ binding can initially proceed without a barrier. Criegee intermediate 7 exhibits an unprecedented high O-O stretching vibration at 1105 cm^-1, which can be explained by a resonance structure with an O₂ double bond. Furthermore, 2H-imidazol-2-one 5 and spiro-dioxirane 6 could be identified as the photodecomposition products of the herein-reported carbonyl oxide.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained. Quantum effects, including tunneling, zero-point vibrational energy, and reaction path interference, can be accounted for. In the limit of an adiabatic unimolecular reaction proceeding on a single electronic state, NAST reduces to the traditional transition state theory. Because NAST requires molecular properties at only a few points on potential energy surfaces, it can be applied to large molecular systems, used with accurate high-level electronic structure methods, and employed to study slow nonadiabatic processes. The essential NAST input data include the nuclear Hessian at the reactant minimum, as well as the nuclear Hessians, energy gradients, and spin-orbit coupling at the minimum energy crossing point (MECP) between two states. The additional computational tools included in the NAST package can be used to extract the required input data from the output files of electronic structure packages, calculate the effective Hessian at the MECP, and fit the reaction coordinate for more advanced NAST calculations. We describe the theory, its implementation, and three examples of application to different molecular systems.

Spin Crossover of Thiophosgene via Multidimensional Heavy-Atom Quantum Tunneling

The spin-crossover reaction of thiophosgene has drawn broad attention from both experimenters and theoreticians as a prime example of radiationless intramolecular decay via intersystem crossing. Despite multiple attempts over 20 years, theoretical predictions have typically been orders of magnitude in error relative to the experimentally measured triplet lifetime. We address the T₁ → S₀ transition by the first application of semiclassical golden-rule instanton theory in conjunction with on-the-fly electronic-structure calculations based on multireference perturbation theory. Our first-principles approach provides excellent agreement with the experimental rates. This was only possible because instanton theory goes beyond previous methods by locating the optimal tunneling pathway in full dimensionality and thus captures "corner cutting" effects. Since the reaction is situated in the Marcus inverted regime, the tunneling mechanism can be interpreted in terms of two classical trajectories, one traveling forward and one backward in imaginary time, which are connected by particle-antiparticle creation and annihilation events. The calculated mechanism indicates that the spin crossover is sped up by many orders of magnitude due to multidimensional quantum tunneling of the carbon atom even at room temperature.

Quantum Chemical Investigation of Perchloric Acid Decomposition Releasing Oxygen

The primary objectives of this study are to identify the initiation steps of perchloric acid (HClO₄) decomposition and to validate and provide insights into the reaction pathways of O₂ formation. To this end, we have performed quantum chemical calculations using the Gaussian 09 program package to identify new reaction pathways and species formed during decomposition. The thermodynamic quantities of the species, such as Gibbs free energy and enthalpy, are calculated using a double-hybrid density functional theory method, B2PLYP, with Jensen's basis set, aug-pc2. For heavy atoms, such as chlorine, the basis set is augmented by adding 2 d functions with a stride factor of 2.5. To incorporate the solvation effect, the conductor-like polarizable continuum model, which is an implicit solvation model, is used. Numerical simulations using a control-volume analysis of an experiment are also performed using the proposed mechanism. In these simulations, rates of the reactions are calculated using transition state theory, incorporating diffusion effects on the rate constants. In order to consider the nonideal behavior of a concentrated HClO₄ solution, activity coefficients are used to calculate the effective concentration of acid in solution. The activity coefficient of HClO₄ plays a critical role in the calculation of the induction period involved in the HClO₄ decomposition. A comparison of numerically predicted O₂ evolution and duration of the induction period with experimental data shows that the numerical simulation using the proposed mechanism predicts both the three-stage decomposition characteristics and the induction period observed during HClO₄ decomposition, thus validating the proposed mechanism.

Theoretical Study of Radical Inactivation, LOX Inhibition, and Iron Chelation: The Role of Ferulic Acid in Skin Protection against UVA Induced Oxidative Stress

Ferulic acid (FA) is used in skin formulations for protection against the damaging actions of the reactive oxygen species (ROS) produced by UVA radiation. Possible underlying protective mechanisms are not fully elucidated. By considering the kinetics of proton-coupled electron transfer (PCET) and radical-radical coupling (RRC) mechanisms, it appears that direct scavenging could be operative, providing that a high local concentration of FA is present at the place of ^•OH generation. The resulting FA phenoxyl radical, after the scavenging of a second ^•OH and keto-enol tautomerization of the intermediate, produces 5-hydroxyferulic acid (5OHFA). Inhibition of the lipoxygenase (LOX) enzyme, one of the enzymes that catalyse free radical production, by FA and 5OHFA were analysed. Results of molecular docking calculations indicate favourable binding interactions of FA and 5OHFA with the LOX active site. The exergonicity of chelation reactions of the catalytic Fe²⁺ ion with FA and 5OHFA indicate the potency of these chelators to prevent the formation of ^•OH radicals via Fenton-like reactions. The inhibition of the prooxidant LOX enzyme could be more relevant mechanism of skin protection against UVA induced oxidative stress than iron chelation and assumed direct scavenging of ROS.

DFT and TDDFT Study of the Reaction Pathway for Double Intramolecular C-H Activation and Functionalization by Iron, Cobalt, and Nickel-Nitridyl Complexes

Previous work was successful in synthesizing a nickel amine, [Cz^tBu(Pyr^iPr)(NH₂-Pyr^iPr)], by double C-H activation and functionalization via irradiating a disphenoidal Ni(II) azido complex, [Cz^tBu(Pyr^iPr)₂NiN₃]. The present work seeks to expand upon the earlier research and to substitute the metal with iron or cobalt. Density functional theory (DFT)-B3LYP/6-31+G(d') and APFD/Def2TZVP-was used to simulate the generation of an intermediate with significant nitridyl radical character after the loss of N₂ from the starting azido complex. DFT and time-dependent density functional theory (TDDFT) were also used to propose a detailed pathway comprised of intermediates of low, intermediate, or high spin multiplicity and photogenerated excited states for the reaction of the azido complex, [Cz^tBu(Pyr^iPr)₂MN₃], to form the amine complex [Cz^tBu(Pyr^iPr)M(NH₂-Pyr^iPr)], M = Co, Ni, or Fe.

Role of Triplet States in the Photodynamics of Aniline

The dynamics of excited heteroaromatic molecules is a key to understanding the photoprotective properties of many biologically relevant chromophores that dissipate their excitation energy nonreactively and thereby prevent the detrimental effects of ultraviolet radiation. Despite their structural variability, most substituted aromatic compounds share a common feature of a repulsive ¹πσ* potential energy surface. This surface can lead to photoproducts, and it can also facilitate the population transfer back to the ground electronic state by means of a ¹πσ*/S₀ conical intersection. Here, we explore a hidden relaxation route involving the triplet electronic state of aniline, which has recently been discovered by means of time-selected photofragment translational spectroscopy [J. Chem. Phys. 2019, 151, 141101]. By using the recently available analytical gradients for multiconfiguration pair-density functional theory, it is now possible to locate the minimum-energy crossing points between states of different spin and therefore compute the intersystem crossing rates with a multireference method, rather than with the less reliable single-reference methods. Using such calculations, we demonstrate that the population loss of aniline in the T₁(³ππ*) state is dominated by C₆H₅NH₂ → C₆H₅NH· + H· dissociation, and we explain the long nonradiative lifetimes of the T₁(³ππ*) state at the excitation wavelengths of 294-264 nm.

Modeling Spin-Crossover Dynamics

In this article, we review nonadiabatic molecular dynamics (NAMD) methods for modeling spin-crossover transitions. First, we discuss different representations of electronic states employed in the grid-based and direct NAMD simulations. The nature of interstate couplings in different representations is highlighted, with the main focus on nonadiabatic and spin-orbit couplings. Second, we describe three NAMD methods that have been used to simulate spin-crossover dynamics, including trajectory surface hopping, ab initio multiple spawning, and multiconfiguration time-dependent Hartree. Some aspects of employing different electronic structure methods to obtain information about potential energy surfaces and interstate couplings for NAMD simulations are also discussed. Third, representative applications of NAMD to spin crossovers in molecular systems of different sizes and complexities are highlighted. Finally, we pose several fundamental questions related to spin-dependent processes. These questions should be possible to address with future methodological developments in NAMD.

Mechanism for the Reaction of White Phosphorus with Cp2Cr2(CO)6 Leading Ultimately to the Triple-Decker Sandwich Cp2Cr2(μ-η5,η5-P5): A Theoretical Study

The experimentally known reaction of Cp₂Cr₂(CO)₆ with white phosphorus (P₄) to give CpCr(CO)₂(η³-P₃), Cp₂Cr₂(CO)₄(μ-η,²η²-P₂), and the triple-decker sandwich Cp₂Cr₂(μ-η,⁵η⁵-P₅) is of interest since the P₄ reactant having a tetrahedral cluster of four phosphorus atoms is converted to products having P₂, P₃, and P₅ ligands. The mechanism of this obviously complicated reaction can be dissected into three stages using a coupled cluster theoretical method that has been benchmarked with the P₂, Mn(CO)₅, and CpCr(CO)₃ dimerization processes. The first stage of the Cp₂Cr₂(CO)₆/P₄ reaction mechanism generates the unsaturated singlet intermediate Cp₂Cr₂(CO)₅ that combines with the P₄ reactant. Decarbonylation of the resulting Cp₂Cr₂(CO)₅(P₄) complex provides a singlet tetracarbonyl readily fragmenting into the stable triphosphacyclopropenyl complex CpCr(CO)₂(η³-P₃) and the chromium phosphide CpCr(CO)₂(P). The isomeric triplet tetracarbonyl Cp₂Cr₂(CO)₄(P₄), readily fragments into CpCr(CO)₂(η²-P₂), which can generate the stable diphosphaacetylene complex Cp₂Cr₂(CO)₄(η,²η²-P₂) as well as the pentamer [CpCr(CO)₂]₅(P₁₀). Combination of the coordinately unsaturated CpCr(CO)(η³-P₃) with CpCr(CO)₂(η²-P₂) can lead to a ring expansion. This generates the P₅ pentagonal ligand in a Cp₂Cr₂(CO)₃(P₅) precursor to the experimentally observed carbonyl-free triple-decker sandwich Cp₂Cr₂(μ-η,⁵η⁵-P₅) after three successive decarbonylations.

Multi- and single-reference methods for the analysis of multi-state peroxidation of enolates

In spite of being spin-forbidden, some enzymes are capable of catalyzing the incorporation of O₂(Σg-3) to organic substrates without needing any cofactor. It has been established that the process followed by these enzymes starts with the deprotonation of the substrate forming an enolate. In a second stage, the peroxidation of the enolate formation occurs, a process in which the system changes its spin multiplicity from a triplet state to a singlet state. In this article, we study the addition of O₂ to enolates using state-of-the-art multi-reference and single-reference methods. Our results confirm that intersystem crossing is promoted by stabilization of the singlet state along the reaction path. When multi-reference methods are used, large active spaces are required, and in this situation, semistochastic heat-bath configuration interaction emerges as a powerful method to study these multi-configurational systems and is in good agreement with PNO-LCCSD(T) when the system is well-represented by a single-configuration.

Mechanism-Based Inactivation of Cytochrome P450 Enzymes: Computational Insights

Mechanism-based inactivation (MBI) refers to the metabolic bioactivation of a xenobiotic by cytochrome P450s to a highly reactive intermediate which subsequently binds to the enzyme and leads to the quasi-irreversible or irreversible inhibition. Xenobiotics, mainly drugs with specific functional units, are the major sources of MBI. Two possible consequences of MBI by medicinal compounds are drug-drug interaction and severe toxicity that are observed and highlighted by clinical experiments. Today almost all of these latent functional groups (e.g., thiophene, furan, alkylamines, etc.) are known, and their features and mechanisms of action, owing to the vast experimental and theoretical studies, are determined. In the past decade, molecular modeling techniques, mostly density functional theory, have revealed the most feasible mechanism that a drug undergoes by P450 enzymes to generate a highly reactive intermediate. In this review, we provide a comprehensive and detailed picture of computational advances toward the elucidation of the activation mechanisms of various known groups with MBI activity. To this aim, we briefly describe the computational concepts to carry out and analyze the mechanistic investigations, and then, we summarize the studies on compounds with known inhibition activity including thiophene, furan, alkylamines, terminal acetylene, etc. This study can be reference literature for both theoretical and experimental (bio)chemists in several different fields including rational drug design, the process of toxicity prevention, and the discovery of novel inhibitors and catalysts.

Zeolites at the Molecular Level: What Can Be Learned from Molecular Modeling

This review puts the development of molecular modeling methods in the context of their applications to zeolitic active sites. We attempt to highlight the utmost necessity of close cooperation between theory and experiment, resulting both in advances in computational methods and in progress in experimental techniques.

New Photochromic α-Methylchalcones Are Highly Photostable, Even under Singlet Oxygen Conditions: Breaking the α-Methyl Michael-System Reactivity by Reversible Peroxybiradical Formation

The α-methylated chalcones 7a–7e behave as P-type photochromic substances with photo-stationary states (PSS) as high as 15:85 when irradiated at 350 nm. These compounds are easily accessible in pure E-configuration by aldol condensation or by oxidative coupling/elimination. The α-methyl groups make these compounds potentially reactive with singlet oxygen following the gem-rule that predicts ¹O₂ regioselectivity. Even after long irradiations times in the presence of the singlet oxygen sensitizer tetraphenylporphyrin (TPP) and oxygen, however, no oxygenation products were detected. Under these conditions, all substrates were converted into 9:1 E/Z-mixtures despite the use of low-energy light that does not allow direct or sensitized excitation of the substrates 7. Additionally, chalcone 7a reduced the singlet oxygen reactivity of the tiglic ketone 3a by about a factor of two, indicating substantial physical quenching of singlet oxygen by the α-methylated chalcones 7a–7e. Thus, a singlet oxygen-induced E/Z-isomerization involving 1,2-dioxatetra-methylene biradicals that leads to triplet oxygen and thermodynamic E/Z-mixtures is postulated and supported by quantum chemical (DFT)-calculations.

DpgC-Catalyzed Peroxidation of 3,5-Dihydroxyphenylacetyl-CoA (DPA-CoA): Insights into the Spin-Forbidden Transition and Charge Transfer Mechanisms*

Despite being a very strong oxidizing agent, most organic molecules are not oxidized in the presence of O₂ at room temperature because O₂ is a diradical whereas most organic molecules are closed-shell. Oxidation then requires a change in the spin state of the system, which is forbidden according to non-relativistic quantum theory. To overcome this limitation, oxygenases usually rely on metal or redox cofactors to catalyze the incorporation of, at least, one oxygen atom into an organic substrate. However, some oxygenases do not require any cofactor, and the detailed mechanism followed by these enzymes remains elusive. To fill this gap, here the mechanism for the enzymatic cofactor-independent oxidation of 3,5-dihydroxyphenylacetyl-CoA (DPA-CoA) is studied by combining multireference calculations on a model system with QM/MM calculations. Our results reveal that intersystem crossing takes place without requiring the previous protonation of molecular oxygen. The characterization of the electronic states reveals that electron transfer is concomitant with the triplet-singlet transition. The enzyme plays a passive role in promoting the intersystem crossing, although spontaneous reorganization of the water wire connecting the active site with the bulk presets the substrate for subsequent chemical transformations. The results show that the stabilization of the singlet radical-pair between dioxygen and enolate is enough to promote spin-forbidden reaction without the need for neither metal cofactors nor basic residues in the active site.

Extending nudged elastic band method to reaction pathways involving multiple spin states

There are diverse reactions including spin-state crossing, especially the reactions catalyzed by transition metal compounds. To figure out the mechanisms of such reactions, the discussion of minimum energy intersystem crossing (MEISC) points cannot be avoided. These points may be the bottleneck of the reaction or inversely accelerate the reactions by providing a better pathway. It is of great importance to reveal their role in the reactions by computationally locating the position of the MEISC points together with the reaction pathway. However, providing a proper initial guess for the structure of the MEISC point is not as easy as that of the transition state. In this work, we extended the nudged elastic band (NEB) method for multiple spin systems, which is named the multiple spin-state NEB method, and it is successfully applied to find the MEISC points while optimizing the reaction pathway. For more precisely locating the MEISC point, a revised approach was adopted. Meanwhile, our examples also suggest that special attention should be paid to the criterion to define an image optimized as the MEISC point.

Spin-inversion mechanisms in O2 binding to a model heme compound: A perspective from nonadiabatic wave packet calculations

Spin-inversion dynamics in O₂ binding to a model heme complex, which consisted of Fe(II)-porphyrin and imidazole, were studied using nonadiabatic wave packet dynamics calculations. We considered three active nuclear degrees of freedom in the dynamics, including the motions along the Fe-O distance, Fe-O-O angle, and Fe out-of-plane distance. Spin-free potential energy surfaces for the singlet, triplet, quintet, and septet states were developed using density functional theory calculations, and spin-orbit coupling elements were obtained from CASSCF-level electronic structure calculations. The spin-inversion mainly occurred between the singlet state and one of the triplet states due to large spin-orbit couplings and the contributions of other states were extremely small. The present quantum dynamics calculations suggested that the narrow crossing region model plays a dominant role in the O₂ binding dynamics. In addition, the one-dimensional Landau-Zener model underestimated the nonadiabatic transition probability.