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

A method to calculate the one-electron reduction potentials for nitroaromatic compounds based on gas-phase quantum mechanics

Nitroaromatic compounds (NACs) are widespread environmental contaminants, and the one-electron reduction potential (E oH) is an important parameter used in modeling their environmental fate. We have identified a method that is both accurate and efficient to predict E oH values for NACs, using gas-phase quantum mechanics (QM) calculations combined with empirical correlations. First, the adiabatic electron affinity (EA) at 0 K is calculated using the B98/MG3S method, and the predictions are scaled by a factor of 0.802 to account for systematic errors in the density functional calculations. Second, the E oH values are predicted from a linear correlation between E oH and EA. Using this method, E oH values were predicted with a mean absolute deviation from measured values of 0.021 V for the 14 NACs used to obtain the correlation and 0.029 V for six additional NACs. This represents a substantial improvement in accuracy over predictions by other QM methods, which are affected by large errors in solvation or aqueous-phase calculations for some compounds.

Scales of oxidation potentials, pK(a), and BDE of various hydroquinones and catechols in DMSO

The one-electron oxidation potentials [E(ox)(NHE)(H(2)Q)], pK(a) (pK(a1) and pK(a2)) values, and bond dissociation energies (BDE(1) and BDE(2)) of 118 important p- and o-dihydroquinones in DMSO were systematically predicted for the first time by using DFT method and the PCM cluster continuum model. The calculated results agree well with the available experimental determinations. The study shows that all the five thermodynamic parameters correlate well with the Hammett substituent parameters σ(p) (for p-H(2)Q, E(ox)(NHE)(H(2)Q(·+)/H(2)Q) = 1.66Σσ(p) + 0.54, pK(a1) = -5.69Σσ(p) + 16.54, pK(a2) = -5.19Σσ(p) + 23.91, BDE(1) = 3.43Σσ(p) + 82.29, BDE(2) = 4.64Σσ(p) + 67.70 and for o-H(2)Q, E(ox)(NHE)(H(2)Q(·+)/H(2)Q) = 1.85Σσ(p) + 0.46, pK(a1) = -5.53Σσ(p) + 13.28, pK(a2) = -5.24Σσ(p) + 26.70, BDE(1) = 3.54Σσ(p) + 82.08, BDE(2) = 3.82Σσ(p) + 75.93), which hints that we can get these thermodynamic parameters as long as the structure of the hydroquinones were known. The comparisons of the calculated five thermodynamic parameters between p-hydroquinones and o-hydroquinones and the number of the phenyl ring effects on these thermodynamic parameters were also studied. At last, intramolecular hydrogen bond energies in hydroquinones at neutral, radical cation, radical, anion different state were systematically calculated and analyzed. Combined with the papers published in our group before, we will have a systematic thermodynamic picture of the transfer details between different kinds of quinones and corresponding hydroquinones, which strongly promote the fast development of the understanding and applications of quinones.

Accurate estimation of the one-electron reduction potentials of various substituted quinones in DMSO and CH3CN

The one-electron reduction potentials of 116 important p- and o-quinones in DMSO and CH(3)CN were predicted for the first time by using the B3LYP/DZP++ method and the PCM cluster continuum model. The calculated gas-phase electron affinities and one-electron reduction potentials agree well with the available experimental observations, respectively. The study showed the one-electron reduction potentials of the 116 quinones range from -0.949 to 1.128 V in DMSO and from -0.904 to 0.971 V in CH(3)CN. The one-electron reduction potentials of p-quinones are generally smaller than those of the o-quinones by about 0.132 V. For quinones with aromatic properties, 2-substituted-1,4-naphthquinones have the largest one-electron reduction potentials, followed by substituted-1,4-anthraquinones and then by substituted-9,10-anthraquinones. The study also showed that the one-electron reduction potentials of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma(p): E(NHE)(p-Q/p-Q(*-)) = 0.45Sigma sigma(p) - 0.194 (V) and E(NHE)(o-Q/o-Q(*-)) = 0.45Sigma sigma(p) - 0.059 (V). Combined with the hydride affinities of quinones in the former paper [DeltaG(H)-(A)(p-Q) = -16.0Sigma sigma(p) - 70.5 (kcal/mol) and DeltaG(H)-(A)(o-Q) = -16.2Sigma sigma(p) - 81.5 (kcal/mol)] and the one-electron reduction potentials of quinones estimated in this work, we obtained the homolytic bond dissociation energies of the hydroquinone anions (QH(-)) and found that these thermodynamic parameters also have linear correlations against the sum of the Hammett substituent parameters sigma(p) if only the substituents have no larger electrostatic inductive force and no large steric hindrance: BDE(p-QH(-)) = 5.05Sigma sigma(p) + 63.18 (kcal/mol) and BDE(o-QH(-)) = 5.33Sigma sigma(p) + 71.30 (kcal/mol). Knowledge about the redox potentials of the quinones should be of great value for the understanding of the nature of chemical reactions of quinones, the designing of new electronic materials of quinones, and the examining of biological activities of quinones.

Modification of carbon substrates by aryl and alkynyl iodonium salt reduction

Different carbon materials were modified using iodonium ion reduction creating radicals, which after reaction with carbon surfaces formed grafted layers of molecules. Several molecules (4-bromophenyl, 4-fluorophenyl, 6-chlorohexyne, and 4-bromobutyne) were grafted on glassy carbon and Vulcan XC72 carbon substrates. Carbon substrates were shown to be free of halogen atoms; therefore, the quantification of the grafted groups containing halogen atoms was facilitated. The grafting of the different molecules was first electrochemically studied on glassy carbon electrodes using cyclic voltammetry, in order to determine the reduction potential of the corresponding iodonium ions. Voltammetric study using Fe(CN)(6)(4-) and Fe(CN)(6)(3-) probe molecules and XPS characterization were also used to evidence the effectiveness of grafting from iodonium ion reduction reaction. Reduction potentials were found in the range from -0.9 V vs SCE to -1.0 V vs SCE, lower than those for corresponding diazonium ion reduction reaction on glassy carbon (close to -0.3 V vs SCE). Therefore, grafted layers from iodonium ions were carried out on carbon Vulcan XC72 powder using NaBH(4) as reducing agent. Functionalized carbon powders were characterized by elemental analysis, thermogravimetric analysis, and X-ray photoelectron spectroscopy to evidence the presence of grafted molecules on the materials. However, low grafting yields were obtained. Then, several synthesis parameters were studied to optimize the grafting reactions, such as the control of the addition of reactants and their concentrations, leading to increase the surface concentration by a factor 2. At last, according to XPS measurements the grafting of alkinyliodonium ions led to very low surface concentrations (0.5 wt % for 6-chlorohexyne), whereas elemental analysis and TGA indicate ca. 2.4 wt % and ca. 5 wt %, respectively.

Benchmark Calculations of Absolute Reduction Potential of Ferricinium/Ferrocene Couple in Nonaqueous Solutions

High-level ab initio molecular orbital theory is used to obtain benchmark values for the ferricenium/ferrocene (Fc(+)/Fc) couple, the IUPAC recommended reference electrode for nonaqueous solution. The gas-phase ionization energy of ferrocene is calculated using the high-level composite method, G3(MP2)-RAD, and two higher-level variants of this method. These latter methods incorporate corrections for core correlation and, in the case of the highest level considered, use (RO)CCSD(T)/6-311+G(d,p) in place of (RO)CCSD(T)/6-31G(d) as the base level of theory. All methods provide good agreement with one another and the corresponding experimental values. Solvation energies have been calculated using PCM, CPCM, SMD, and COSMO-RS. Using G3(MP2)-RAD-Full-TZ gas-phase energies and COSMO-RS solvation energies, the absolute redox potentials of the Fc(+)/Fc couple have been calculated as 4.988, 4.927, and 5.043 V in acetonitrile, 1,2-dichloroethane, and dimethylsulfoxide solutions, respectively.

Redox-coupled proton transfer in the active site of cytochrome cbb3

Cytochrome cbb3 is a distinct member of the superfamily of respiratory heme-copper oxidases, and is responsible for driving the respiratory chain in many pathogenic bacteria. Like the canonical heme-copper oxidases, cytochrome cbb3 reduces oxygen to water and couples the released energy to pump protons across the bacterial membrane. Homology modeling and recent electron paramagnetic resonance (EPR) studies on wild type and a mutant cbb3 enzyme [V. Rauhamäki et al. J. Biol. Chem. 284 (2009) 11301-11308] have led us to perform high-level quantum chemical calculations on the active site. These calculations bring molecular insight into the unique hydrogen bonding between the proximal histidine ligand of heme b3 and a conserved glutamate, and indicate that the catalytic mechanism involves redox-coupled proton transfer between these residues. The calculated spin densities give insight in the difference in EPR spectra for the wild type and a recently studied E383Q-mutant cbb3-enzyme. Furthermore, we show that the redox-coupled proton movement in the proximal cavity of cbb3-enzymes contributes to the low redox potential of heme b3, and suggest its potential implications for the high apparent oxygen affinity of these enzymes.

Mechanistic Analysis of Oxidative C-H Cleavages Using Inter- and Intramolecular Kinetic Isotope Effects

A series of monodeuterated benzylic and allylic ethers were subjected to oxidative carbon-hydrogen bond cleavage to determine the impact of structural variation on intramolecular kinetic isotope effects in DDQ-mediated cyclization reactions. These values are compared to the corresponding intermolecular kinetic isotope effects that were accessed through subjecting mixtures of non-deuterated and dideuterated substrates to the reaction conditions. The results indicate that carbon-hydrogen bond cleavage is rate determining and that a radical cation is most likely a key intermediate in the reaction mechanism.

Oxidation of amines by flavoproteins

Many flavoproteins catalyze the oxidation of primary and secondary amines, with the transfer of a hydride equivalent from a carbon-nitrogen bond to the flavin cofactor. Most of these amine oxidases can be classified into two structural families, the D-amino acid oxidase/sarcosine oxidase family and the monoamine oxidase family. This review discusses the present understanding of the mechanisms of amine and amino acid oxidation by flavoproteins, focusing on these two structural families.

The correlation of cathodic peak potentials of vitamin K(3) derivatives and their calculated electron affinities. The role of hydrogen bonding and conformational changes

2-methyl-1,4-naphtoquinone 1 (vitamin K(3), menadione) derivatives with different substituents at the 3-position were synthesized to tune their electrochemical properties. The thermodynamic midpoint potential (E(1/2)) of the naphthoquinone derivatives yielding a semi radical naphthoquinone anion were measured by cyclic voltammetry in the aprotic solvent dimethoxyethane (DME). Using quantum chemical methods, a clear correlation was found between the thermodynamic midpoint potentials and the calculated electron affinities (E(A)). Comparison of calculated and experimental values allowed delineation of additional factors such as the conformational dependence of quinone substituents and hydrogen bonding which can influence the electron affinities (E(A)) of the quinone. This information can be used as a model to gain insight into enzyme-cofactor interactions, particularly for enzyme quinone binding modes and the electrochemical adjustment of the quinone motif.