Quantum-Chemical Predictions of Absolute Standard Redox Potentials of Diverse Organic Molecules and Free Radicals in Acetonitrile

Effect of Imidazole on the Electrochemistry of Zinc Porphyrins: An Electrochemical and Computational Study

In this study, the electrochemical behavior of zinc meso-substituted porphyrins in the presence of imidazole is examined by using both cyclic voltammetry (CV) and density functional theory (DFT) methods. The results show that the first half-wave oxidation potentials (1st E_1/2) of zinc porphyrins complexed with imidazole all move to the negative side, while the second ones (2nd E_1/2) move to the positive side, resulting in larger half-wave oxidation potential splittings of the two oxidation states (ΔE = second E_1/2 - first E_1/2) comparing with the zinc porphyrins. By employing DFT calculations, we have found that both sterically controlled inter π-conjugation between porphyrin rings and meso-substituted phenyl groups and deformation of porphyrin rings do play important roles in contributing to the half-wave oxidation potentials. Imidazole exhibits strong effects on the deformation of porphyrin rings which is dominant in determining the first E_1/2 while the inter π-conjugation between porphyrin rings and meso-substituted phenyl groups mainly contributes to the second E_1/2. Without imidazole, the inter π-conjugation between porphyrin rings and meso-substituted phenyl groups is the only important criterion which effects both first E_1/2 and second E_1/2 of zinc porphyrins.

Some Theoretical and Experimental Insights on the Mechanistic Routes Leading to the Spontaneous Grafting of Gold Surfaces by Diazonium Salts

The spontaneous grafting of diazonium salts on gold may involve the carbocation obtained by heterolytic dediazonation and not necessarily the radical, as usually observed on reducing surfaces. The mechanism is addressed on the basis of DFT calculations and experiments carried out under conditions where the carbocation and the radical are produced selectively. The calculations indicate that the driving force of the reaction leading from a gold cluster, used as a gold model surface, and the carbocation to the modified cluster is higher than that of the analogous reaction starting from the radical. The experiments performed under conditions of heterolytic dediazonation show the formation of thin films on the surface of gold. The grafting of a carbocation is therefore possible, but a mechanism where the cleavage of the Ar-N bond is catalyzed by the surface of gold cannot be excluded.

Properties of Intermediates in the Catalytic Cycle of Oxalate Oxidoreductase and Its Suicide Inactivation by Pyruvate

Oxalate:ferredoxin oxidoreductase (OOR) is an unusual member of the thiamine pyrophosphate (TPP)-dependent 2-oxoacid:ferredoxin oxidoreductase (OFOR) family in that it catalyzes the coenzyme A (CoA)-independent conversion of oxalate into 2 equivalents of carbon dioxide. This reaction is surprising because binding of CoA to the acyl-TPP intermediate of other OFORs results in formation of a CoA ester, and in the case of pyruvate:ferredoxin oxidoreductase (PFOR), CoA binding generates the central metabolic intermediate acetyl-CoA and promotes a 10⁵-fold acceleration of the rate of electron transfer. Here we describe kinetic, spectroscopic, and computational results to show that CoA has no effect on catalysis by OOR and describe the chemical rationale for why this cofactor is unnecessary in this enzymatic transformation. Our results demonstrate that, like PFOR, OOR binds pyruvate and catalyzes decarboxylation to form the same hydroxyethylidine-TPP (HE-TPP) intermediate and one-electron transfer to generate the HE-TPP radical. However, in OOR, this intermediate remains stranded at the active site as a covalent inhibitor. These and other results indicate that, like other OFOR family members, OOR generates an oxalate-derived adduct with TPP (oxalyl-TPP) that undergoes decarboxylation and one-electron transfer to form a radical intermediate remaining bound to TPP (dihydroxymethylidene-TPP). However, unlike in PFOR, where CoA binding drives formation of the product, in OOR, proton transfer and a conformational change in the "switch loop" alter the redox potential of the radical intermediate sufficiently to promote the transfer of an electron into the iron-sulfur cluster network, leading directly to a second decarboxylation and completing the catalytic cycle.

Theoretical study on homolytic B–B cleavages of diboron(4) compounds

The B–B BDEs of diboron(4) compounds and the Pt–B and Cu–B BDEs of their corresponding complexes were investigated by SOGGA11-X method.

In search of the best DFT functional for dealing with organic anionic species

“And the winner is…” This work assesses the ability of different Density Functional Theory (DFT) functionals for a proper treatment of organic anionic species.

Computational study on C–B homolytic bond dissociation enthalpies of organoboron compounds

The theoretical study of three hybridized C–B BDEs with different substituents can provide corresponding guidance to experimental research studies.

Benchmarking the DFT methodology for assessing antioxidant-related properties: quercetin and edaravone as case studies

The overall objective was to identify an accurate computational electronic method to virtually screen phenolic compounds through their antioxidant and free-radical scavenging activity. The impact of a key parameter of the density functional theory (DFT) approach was studied. Performances of the 21 most commonly used exchange-correlation functionals are thus detailed in the evaluation of the main energetic parameters related to the activities of two prototype antioxidants, namely quercetin and edaravone, is reported. These functionals have been chosen among those belonging to three different families of hybrid functionals, namely global, range separated, and double hybrids. Other computational parameters have also been considered, such as basis set and solvent effects. The selected parameters, namely bond dissociation enthalpy (BDE), ionization potential (IP), and proton dissociation enthalpy (PDE) allow a mechanistic evaluation of the antioxidant activities of free radical scavengers. Our results show that all the selected functionals provide a coherent picture of these properties, predicting the same order of BDEs and PDEs. However, with respect to the reference values, the errors found at CBS-Q3 level significantly vary with the functional. Although it is difficult to evidence a global trend from the reported data, it clearly appears that LC-ωPBE, M05-2X, and M06-2X are the most suitable approaches for the considered properties, giving the lowest cumulative mean absolute errors. These methods are therefore suggested for an accurate and fast evaluation of energetic parameters related to an antioxidant activity via free radical scavenging.

Synthesis of Tetrahydroisoquinoline Alkaloids and Related Compounds through the Alkylation of Anodically Prepared α-Amino Nitriles

α-Amino nitrile 2a was conveniently prepared in two individual steps from chiral hexafluorophosphate salt isoquinolinium (-)-8b including anodic cyanation as an efficient means to activate the sp(3) C1-H bond of the THIQ nucleus. The lithiation of 2a was carried out in THF at -80 °C in the presence of LDA to produce a stable α-amino carbanion which was condensed on a large variety of alkyl halides. The resulting quaternary α-amino nitriles were subjected to a stereoselective reductive decyanation in ethanol in the presence of NaBH4 as the hydride donor to yield N-Boc-1-alkyl-THIQs (+)-10a-g in up to 97:3 er's after removal of the chiral auxiliary group. Examination of the ORTEP view of THIQ (+)-1f revealed that the newly created stereogenic center had an absolute S configuration. Likewise, (-)-xylopinine was synthesized in four workup steps in an overall 63% yield from α-amino nitrile (+)-2b. In this process, crystallization of an enantioenriched mixture (90:10) of (-)-norlaudanosine with 1 equiv of (-)-N-acetyl-l-leucine afforded the leucinate salt (+)-13 (99:1 dr). Similarly, (+)-salsolidine was displaced from its (-)-DBTA salt (-)-12 in 99:1 er, which was determined by proton and carbon NMR spectroscopy in the presence of thiophosphinic acid (+)-14 as the chiral solvating agent.

Electrochemical Behavior of meso-Substituted Porphyrins: The Role of Cation Radicals to the Half-Wave Oxidation Potential Splitting

In this study, the electrochemical behavior of free base and zinc meso-substituted porphyrins is examined by cyclic voltammetry (CV) and density functional theory (DFT). The results show that the half-wave oxidation potential splitting of the two oxidation states (ΔE= second E1/2 - first E1/2) of tetraphenylporphyrin (H2TPP) and its zinc complex (ZnTPP) are higher than those of porphyrins and their zinc complexes with meso-substituted five-membered heterocylic rings. The ΔE values follow the trend of TPP > T(3'-thienyl)P > T(3'-furyl)P > T(2'-thienyl)P for both meso-porphyrins and their respective zinc complexes. By employing DFT calculations, we have found that the trend of ΔE values is consistent with that of highest spin density (HSD) distribution and HOMO-LUMO energy gaps of cationic radicals as well as the π-conjugation between central porphyrin and meso-substituted rings. Also, they exhibit the better resonance between the porphyrin ring with meso-substituted rings as moving from porphyrins and their zinc complexes with phenyl rings to five-membered heterocyclic rings. A good agreement between calculated and experimental results indicates that cationic radicals, especially their spin density distribution, do play an important role in half-wave oxidation potential splitting of meso-porphyrins and their zinc complexes.

Free Energies of Redox Half-Reactions from First-Principles Calculations

Quantitative prediction of the energetics of redox half-reactions is still a challenge for modern computational chemistry. Here, we propose a simple scheme for reliable calculations of vertical ionization and attachment energies, as well as of redox potentials of solvated molecules. The approach exploits linear response approximation in the context of explicit solvent simulations with spherical boundary conditions. It is shown that both vertical ionization energies and vertical electron affinities, and, consequently redox potentials, exhibit linear dependence on the inverse radius of the solvation sphere. The explanation of the linear dependence is provided, and an extrapolation scheme is suggested. The proposed approach accounts for the specific short-range interactions within hybrid DFT and effective fragment potential approach as well as for the asymptotic system-size effects. The computed vertical ionization energies and redox potentials are in excellent agreement with the experimental values.

Ionization Energies and Aqueous Redox Potentials of Organic Molecules: Comparison of DFT, Correlated ab Initio Theory and Pair Natural Orbital Approaches

The calculation of redox potentials involves large energetic terms arising from gas phase ionization energies, thermodynamic contributions, and solvation energies of the reduced and oxidized species. In this work we study the performance of a wide range of wave function and density functional theory methods for the prediction of ionization energies and aqueous one-electron oxidation potentials of a set of 19 organic molecules. Emphasis is placed on evaluating methods that employ the computationally efficient local pair natural orbital (LPNO) approach, as well as several implementations of coupled cluster theory and explicitly correlated F12 methods. The electronic energies are combined with implicit solvation models for the solvation energies. With the exception of MP2 and its variants, which suffer from enormous errors arising at least partially from the poor Hartree-Fock reference, ionization energies can be systematically predicted with average errors below 0.1 eV for most of the correlated wave function based methods studies here, provided basis set extrapolation is performed. LPNO methods are the most efficient way to achieve this type of accuracy. DFT methods show in general larger errors and suffer from inconsistent behavior. The only exception is the M06-2X functional which is found to be competitive with the best LPNO-based approaches for ionization energies. Importantly, the limiting factor for the calculation of accurate redox potentials is the solvation energy. The errors in the predicted solvation energies by all continuum solvation models tested in this work dominate the final computed reduction potential, resulting in average errors typically in excess of 0.3 V and hence obscuring the gains that arise from choosing a more accurate electronic structure method.

Operationally simple hydrotrifluoromethylation of alkenes with sodium triflinate enabled by Ir photoredox catalysis

We report herein a single component Ir photoredox catalyst which is capable of catalyzing the hydrotrifluoromethylation of terminal alkenes and Michael acceptors with sodium triflinate (Langlois reagent) in methanol under irradiation at room temperature. Various synthetically useful functional groups, including ester, amide, ether, aldehyde, sulfone, ketone and aryl boronate, are well tolerated in this reaction.

Discovery and mechanistic study of a photocatalytic indoline dehydrogenation for the synthesis of elbasvir

Elbasvir is a potent NS5A antagonist for the treatment of chronic hepatitis C. A seemingly trivial indoline oxidation en route to the target compound was complicated by epimerization of a stereogenic hemiaminal center under most standard oxidation conditions. To address this issue, a novel visible light photoredox process for indoline oxidation was developed involving an iridium photosensitizer and environmentally-benign perester oxidant. The reaction was discovered through a high-throughput experimentation campaign and the optimized process was demonstrated on 100 g scale in flow to afford a key intermediate towards the target compound. A battery of kinetic, electrochemical, and spectroscopic studies of this process indicates a radical chain mechanism of dehydrogenation involving selective HAT from the substrate by an alkoxy radicals. Notably, isotope effects were used to validate the chain mechanism when quantum yield data proved ambiguous.

Anodic Methods for Covalent Attachment of Ethynylferrocenes to Electrode Surfaces: Comparison of Ethynyl Activation Processes

The electrochemical oxidation of ferrocenes having an H- or Li-terminated ethynyl group has been studied, especially as it relates to their covalent anchoring to carbon surfaces. The anodic oxidation of lithioethynylferrocene (1-Li) results in rapid loss of Li(+) and formation of the ethynyl-based radical FeCp(η(5)-C5H4)(C≡C), (1, Cp = η(5)-C5H5), which reacts with the electrode. Chemically modified electrodes (CMEs) were thereby produced containing strongly bonded, ethynyl-linked monolayers and electrochemically controlled multilayers. Strong attachments of ethynylferrocenes to gold and platinum surfaces were also possible. The lithiation/anodic oxidation process is a mirror analogue of the diazonium/cathodic reduction process for preparation of aryl-modified CMEs. A second method produced an ethynylferrocene-modified electrode by direct anodic oxidation of the H-terminated ethynylferrocene (1-H) at a considerably more positive potential. Both processes produced robust modified electrodes with well-defined ferrocene-based surface cyclic voltammetry waves that remained unchanged for as many as 10(4) scans. Ferrocene derivatives in which the ethynyl moiety was separated from the cyclopentadienyl ring by an ether group showed very similar behavior. DFT calculations were performed on the relevant redox states of 1-H, 1-Li, and 1, with emphasis on the ferrocenyl vs ethynyl character of their high valence orbitals. Whereas the HOMOs of both 1-H and 1-Li have some ethynyl character, the SOMOs of the corresponding monocations are strictly ferrocenium in makeup. Predominant ethynyl character returns to the highest valence orbitals after loss of Li(+) from [1-Li](+) or loss of H(+) from [1-H](2+). These anodic processes hold promise for the controlled chemical modification of carbon and other electrode surfaces by a variety of ethynyl or alkynyl-linked organic and metal-containing systems.

Accurate Prediction of One-Electron Reduction Potentials in Aqueous Solution by Variable-Temperature H-Atom Addition/Abstraction Methodology

A robust and efficient theoretical approach for calculation of the reduction potentials of charged species in aqueous solution is presented. Within this approach, the reduction potential of a charged complex (with a charge |n| ≥ 2) is probed by means of the reduction potential of its neutralized (protonated/deprotonated) cognate, employing one or several H-atom addition/abstraction thermodynamic cycles. This includes a separation of one-electron reduction from protonation/deprotonation through the temperature dependence. The accuracy of the method has been assessed for the set of 15 transition-metal complexes that are considered as highly challenging systems for computational electrochemistry. Unlike the standard computational protocol(s), the presented approach yields results that are in excellent agreement with experimental electrochemical data. Last but not least, the applicability and limitations of the approach are thoroughly discussed.

Computational study on alkenyl/aryl C(sp2)–O homolytic cleavage of carboxylates and carbamates

The alkenyl/aryl C(sp²)–O cleavage and the substituent effect in both carboxylates/carbamates and corresponding Ni complexes were investigated in detail by wB97 method.

Computational-aided design of melatonin analogues with outstanding multifunctional antioxidant capacity

The IIcD melatonin-analogue was identified as the most promising multifunctional antioxidant from a set of 19, and better for that purpose than the parent molecule and Trolox.

Predicting Electrocatalytic Properties: Modeling Structure-Activity Relationships of Nitroxyl Radicals

Stable nitroxyl radical-containing compounds, such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and its derivatives, are capable of electrocatalytically oxidizing a wide range of alcohols under mild and environmentally friendly conditions. Herein, we examine the structure-function relationships that determine the catalytic activity of a diverse range of water-soluble nitroxyl radical compounds. A strong correlation is described between the difference in the electrochemical oxidation potentials of a compound and its electrocatalytic activity. Additionally, we construct a simple computational model that is able to accurately predict the electrochemical potential and catalytic activity of a wide range of nitroxyl radical derivatives.

Dual Catalytic Decarboxylative Allylations of α-Amino Acids and Their Divergent Mechanisms

The room temperature radical decarboxylative allylation of N-protected α-amino acids and esters has been accomplished via a combination of palladium and photoredox catalysis to provide homoallylic amines. Mechanistic investigations revealed that the stability of the α-amino radical, which is formed by decarboxylation, dictates the predominant reaction pathway between competing mechanisms.

Towards high throughput screening of electrochemical stability of battery electrolytes

High throughput screening of solvents and additives with potential applications in lithium batteries is reported. The initial test set is limited to carbonate and phosphate-based compounds and focused on their electrochemical properties. Solvent stability towards first and second reduction and oxidation is reported from density functional theory (DFT) calculations performed on isolated solvents surrounded by implicit solvent. The reorganization energy is estimated from the difference between vertical and adiabatic redox energies and found to be especially important for the accurate prediction of reduction stability. A majority of tested compounds had the second reduction potential higher than the first reduction potential indicating that the second reduction reaction might play an important role in the passivation layer formation. Similarly, the second oxidation potential was smaller for a significant subset of tested molecules than the first oxidation potential. A number of potential sources of errors introduced during screening of the electrolyte electrochemical properties were examined. The formation of lithium fluoride during reduction of semifluorinated solvents such as fluoroethylene carbonate and the H-transfer during oxidation of solvents were found to shift the electrochemical potential by 1.5-2 V and could shrink the electrochemical stability window by as much as 3.5 V when such reactions are included in the screening procedure. The initial oxidation reaction of ethylene carbonate and dimethyl carbonate at the surface of the completely de-lithiated LiNi0.5Mn1.5O4 high voltage spinel cathode was examined using DFT. Depending on the molecular orientation at the cathode surface, a carbonate molecule either exhibited deprotonation or was found bound to the transition metal via its carbonyl oxygen.

Air stable organic salt as an n-type dopant for efficient and stable organic light-emitting diodes

Air-stable and low-temperature-evaporable n-type dopants are highly desired for efficient and stable organic light-emitting diodes (OLEDs). In this work, 2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I), a thermally decomposable precursor of organic radical o-MeO-DMBI, has been employed as a novel n-type dopant in OLEDs, because of its air stability, low decomposition temperature, and lack of atom diffusion. The n-type electrical doping is evidenced by the rapid increase in current density of electron-only devices and the large improvement in conductivity, originated from increased electron concentration in electron-transport layer (ETL) and reduced electron injection barrier. A highly efficient and stable OLED is created using o-MeO-DMBI as an n-type dopant in Bphen. Compared with the control device with its high-temperature-evaporable n-type dopant cesium carbonate (Cs2CO3), o-MeO-DMBI-doped device showed an incredible boom in current efficiency from 28.6 to 42.2 cd/A. Moreover, the lifetime (T(70%)) of o-MeO-DMBI-doped device is 45 h, more than 20 times longer than that of the Cs2CO3-doped device (2 h). The enhanced efficiency and stability are attributed to the improved balance of holes and electrons in the emissive layer, and the eliminated atom diffusion of cesium.

Computational organic chemistry: bridging theory and experiment in establishing the mechanisms of chemical reactions

Understanding the mechanisms of chemical reactions, especially catalysis, has been an important and active area of computational organic chemistry, and close collaborations between experimentalists and theorists represent a growing trend. This Perspective provides examples of such productive collaborations. The understanding of various reaction mechanisms and the insight gained from these studies are emphasized. The applications of various experimental techniques in elucidation of reaction details as well as the development of various computational techniques to meet the demand of emerging synthetic methods, e.g., C-H activation, organocatalysis, and single electron transfer, are presented along with some conventional developments of mechanistic aspects. Examples of applications are selected to demonstrate the advantages and limitations of these techniques. Some challenges in the mechanistic studies and predictions of reactions are also analyzed.

Theoretical study on homolytic C(sp2)–O cleavage in ethers and phenols

The C(sp²)–O BDEs and the substituent effect of ethers/phenols were investigated in detail by the wB97 method.

Are thermodynamic cycles necessary for continuum solvent calculation of pKas and reduction potentials?

Continuum solvent calculations of pKas and reduction potentials usually entail the use of a thermodynamic cycle to express the reaction free energy in terms of gas phase energies and free energies of solvation. In this work, we present a systematic study comparing the solution phase free energy changes obtained in this manner with those directly computed within the SMD solvation model against a large test set of 117 pKas and 42 reduction potentials in water and DMSO. The inclusion of vibrational contributions in the free energy of solvation has a negligible impact on the accuracy of thermodynamic cycle predictions of pKas and reduction potentials. Additionally, when gas phase energies in the thermodynamic cycle are computed at more accurate levels of theory, very similar results (mean unsigned difference of 0.5 kcal mol(-1)) can be achieved when the high-level computations (MP2/GTMP2Large and G3(MP2)-RAD(+)) are directly carried out within the continuum model. Increasing the accuracy of the electronic structure theory may or may not improve the agreement with experiment suggesting that the error is largely in the solvation model. For amino acids where their gas and solution phase species exist as different tautomers, the direct approach provided a significant improvement in calculated pKas. These results demonstrate that direct calculation of solution phase pKas and reduction potentials within the SMD model provides a general and reliable approximation to corresponding thermodynamic cycle based protocols, and is recommended for systems where solvation induced changes in geometry are significant. Further studies are necessary to ascertain whether the results are generalisable to other continuum solvation models.