Quantum-chemical predictions of absolute standard redox potentials of diverse organic molecules and free radicals in acetonitrile

A calibrated B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) method was found to be able to predict the gas-phase adiabatic ionization potentials of 160 structurally unrelated organic molecules with a precision of 0.14 eV. A PCM solvation model was benchmarked that could predict the pK(a)'s of 15 organic acids in acetonitrile with a precision of 1.0 pK(a) unit. Combining the above two methods, we developed a generally applicable protocol that could successfully predict the standard redox potentials of 270 structurally unrelated organic molecules in acetonitrile. The standard deviation of the predictions was 0.17 V. The study demonstrated that computational electrochemistry could become a powerful tool for the organic chemical community. It also confirmed that the continuum solvation theory could correctly predict the solvation energies of organic radicals. Finally, with the help of the newly developed protocol we were able to establish a scale of standard redox potentials for diverse types of organic free radicals for the first time. Knowledge about these redox potentials should be of great value for understanding the numerous electron-transfer reactions in organic and bioorganic chemistry.     

Theoretical determination of one-electron redox potentials for DNA bases, base pairs, and stacks

Electron affinities, ionization potentials, and redox potentials for DNA bases, base pairs, and N-methylated derivatives are computed at the DFT/M06-2X/6-31++G(d,p) level of theory. Redox properties of a guanine-guanine stack model are explored as well. Reduction and oxidation potentials are in good agreement with the experimental ones. Electron affinities of base pairs were found to be negative. Methylation of canonical bases affects the ionization potentials the most. Base pair formation and base stacking lower ionization potentials by 0.3 eV. Pairing of guanine with the 5-methylcytosine does not seem to influence the redox properties of this base pair much.     

Estimation of the standard reduction potentials of some 1-arylethyl radicals in acetonitrile

The redox properties of some arylethyl radicals, which are involved in the electrochemical synthesis of important anti-inflammatory agents, have been investigated in CH₃CN by an indirect electrochemical method based on the catalytic reduction of the corresponding arylethyl halides (RX) by electrogenerated aromatic or heteroaromatic anion radicals (D⁻). The reaction between RX and D⁻ leads to a radical R, which can react with D⁻ either by radical coupling (k₃) or by electron transfer (k₄). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q=k₄/(k₃+k₄), as a function of E⁰_D/D⁻ allows estimation of the reduction potentials of the arylethyl radicals. The standard reduction potentials obtained for the radicals 1-(4-isobutylphenyl)ethyl, 1-(6-methoxy-2-naphthyl)ethyl, and 1-(4-biphenyl)ethyl are −1.64, −1.62, and −1.15 V vs. SCE, respectively.     

Probing the mechanism of hypoxia selectivity of copper bis(thiosemicarbazonato) complexes: DFT calculation of redox potentials and absolute acidities in solution

Density functional theory (DFT) calculations have been performed using the uB3LYP/6-31++G(d,p) model to calculate the solution phase one-electron reduction potentials (E(calc)) and absolute pKa values of a series of copper bis(thiosemicarbazonato) complexes. The effects of solvation in water and dimethylsulfoxide (DMSO) are incorporated as a self-consistent reaction field (SCRF) using the integral equation formalism polarisable continuum model (IEFPCM) and are found to be essential for quantitative agreement with an average error in E(calc) of -0.02 V compared to experiment. The bonding and spin densities are examined through the use of Natural Bond Order analysis and the results used to rationalise the calculated and observed reduction potentials. Calculated estimates of pKa values of several copper(II) species are presented and their implications for the mechanisms of transport and trapping within hypoxic cells are considered. Reduction is found to be a prerequisite for protonation of the complexes which suggests their transport in the blood stream as neutral species, and the mechanistic sequence is identified as a sequential electrochemical-chemical (EC) process. The complex equilibria of protonation, reoxidation and dissociation are discussed and the copper(I) diprotonated, cationic complex of diacetyl bis(4-methyl-3-thiosemicarbazonato)copper(II), Cu(I)ATSMH2(+), is identified as a possible candidate for the initial species trapped in hypoxic cells.     

Microwave-promoted solvent-free synthesis of para-quinone methides (p-QMs) derivatives

A rapid and efficient methodology for the synthesis of polysubstituted para-quinone methides (p-QMs) from aldehyde and 2,6-di-tert-butylphenol has been achieved in solvent-free and microwave irradiation condition within 33 min. This strategy displays the advantages including high atom economy, good functional group tolerance, and environmentally friendly operation.     

Accurate redox potentials of mononuclear iron, manganese, and nickel model complexes*

Density functional theory (DFT) was combined with solution of the Poisson equation for continuum dielectric media to compute accurate redox potentials for several mononuclear transition metal complexes (TMCs) involving iron, manganese, and nickel. Progress was achieved by altering the B3LYP DFT functional (B4(XQ3)LYP-approach) and supplementing it with an empirical correction term G(X) having three additional adjustable parameters, which is applied after the quantum-chemical DFT computations. This method was used to compute 58 redox potentials of 48 different TMCs involving different pairs of redox states solvated in both protic and aprotic solvents. For the 58 redox potentials the root mean square deviation (RMSD) from experimental values is 65 mV. The reliability of the present approach is also supported by the observation that the energetic order of the spin multiplicities of the electronic ground states is fulfilled for all studied TMCs, if the influence from the solvent is considered as well.     

Relative pKa of some anilinium derivatives in methanol,acetonitrile, and tetrahydrofurane solvents

In this study, the relative pK_a values of nine anilinium derivatives in methanol (MeOH), acetonitrile (AN), and tetrahydrofurane (THF) solutions were successfully calculated with mean absolute deviations of 0.63, 0.68, and 0.75 pK_a units, respectively. To this aim, their gas‐phase basicities were computed using the CBS‐QB3 composite method. Also, conductor‐like polarizable continuum model (CPCM) with UAHF, UAKS and UA0 cavities and SM8 solvation models at HF/6‐31+G(d) level of theory were applied for the calculation of the solvation Gibbs free energies. The obtained results indicate that there is reliable correlation between the experimental and computed pK_a values in the studied solutions. Therefore, to extend the pK_a database for anilines, correlation equations were used to predict the pK_a values in the investigated solvents.     

The redox properties of organotransition-metal complexes IV. The activation of cyclic polyolefiniron tricarbonyl derivatives by one-electron oxidation

Cyclic polyolefiniron tricarbonyl derivatives [Fe(CO)₂L(C_nH_m)] (L = CO or phosphorus donor) and oxidising agents such as Ag⁺ or [NO]⁺ in CH₂Cl₂ give reactive paramagnetic cations [Fe(CO)₂L(C_nH_m]⁺ which can abstract hydrogen from the solvent to give [Fe(CO)₂L(C_nH_{m + 1})]⁺.     

Characterization of the dimeric one-electron electrolytic reduction products of 1-alkylpyridinium ions in acetonitrile

A chemical-electrochemical approach for the determination of the positions of coupling in the dimeric one-electron reduction products of 1-alkylpyridinium ions in acetonitrile is described. The procedure involves the treatment of a solution of the dimeric species with an appropriate quinone followed by electrochemical examination of the resulting solution. For 1-alkylpyridinium ions without ring substituents, the dimer is concluded to be the 4,4′ coupled isomer, based on the formation in the cases of 1-methyl-, 1-ethyl-, and 1-benzylpyridinium ions of the 1,1′-dialkyl-4,4′-bipyridinium monocation radical species on treatment of the dimeric species with 1,4-benzoquinone. The characteristics, particularly the quantitative nature, of this tetrahydrobipyridine-quinone reaction have been examined.     

Calixarenes: Structure of an acetonitrile inclusion complex and some transition metal rimmed derivatives

The hostt-butylcalix[4]arene,1a, forms a 11 inclusion compound with acetonitrile as guest. The inclusion compound has been isolated and characterised by X-ray analysis of a twinned crystal at 123 K. The acetonitrile guest lies on a crystallographic four-fold symmetry axis passing through the centre of the bowl of1a which adopts a regular cone conformation. A known tetradentate and a new tridentate phosphinitocalix[4]arene derivative,2a and2c respectively, have been synthesized from1a and Ph₂Cl. Both2a and2c show a strong ability to coordinate with late transition metals and new complexes of gold(I), palladium(II) and platinum(II) are reported. Supplementary Data relating to this paper have been deposited with the British Library as Supplementary Publication No. SUP 92171 (5 pp.)This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

Experimental and computed absolute redox potentials of polycyclic aromatic hydrocarbons are highly linearly correlated over a wide range of structures and potentials

A more rigorous theoretical treatment of methods previously used to correlate computed energy values with experimental redox potentials, combined with the availability of well-developed computational solvation methods, results in a shift away from computing ionization potentials/electron affinities in favor of computing absolute reduction potentials. Seventy-nine literature redox potentials measured under comparable conditions from 51 alternant and nonalternant polycyclic aromatic hydrocarbons are linearly correlated with their absolute reduction potentials computed by density functional theory (B3LYP/6-31+G(d)) with SMD/IEF-PCM solvation. The resulting correlation is very strong (R(2) = 0.9981, MAD = 0.056 eV). When extrapolated to the x-intercept, the correlation results in an estimate of 5.17 ± 0.01 eV for the absolute potential of the ferrocene-ferrocenium redox couple in acetonitrile at 25 °C, indicating that this simple method can be used reliably for both calculating absolute redox potentials and for predicting relative redox potentials. When oxidation and reduction data are evaluated separately, the overall MAD value is improved by 50% to 0.028 eV, which improves relative potential predictions, but the computed values do not extrapolate to a reasonable estimate of the absolute potential of the ferrocene-ferrocenium ion reference.     

Substituent effects in the one-electron reduction of anthraquinone derivatives

Polarographic reduction against (C₂H₅)₄NBr as background in dimenthyformamide has been applied to 17 anthraquinones with - and -substituents. There are two pronounced half-waves, which correspond to two one-electron degrees of reduction, a semiquinone anion being formed as intermediate. An ESR signal is produced when this radical is formed by electrolysis with a cathode potential equal to E_1/2. The observed E_1/2 fit Hammett's equation closely. The relation E_1/2 to donor-acceptor aspects of the substituents is discussed.     

Basicity of some organic superbases in acetonitrile

[structure: see text] It is shown that the pK(a) values of strong neutral organic (super)bases in acetonitrile are well described by the density functional theory (DFT) employing the isodensity polarization continuum model (IPCM) for treating solvent-solute interactions. High pK(a) values are predicted for two model compounds, and their synthesis is strongly recommended.     

The electrochemistry of some ferrocene derivatives: redox potential and substituent effects

Fifteen ferrocene derivatives I–IX (four of which have been prepared for the first time: II, IIIf, V and VIII) have been prepared by Friedel–Crafts acetylation, Claisen condensation, Michael reaction, and ring closure by hydrazine hydrate. The anodic behaviour of these compounds has been studied by cyclic voltammetry at a platinum electrode in an aprotic solvent. All these substituted ferrocenes exhibit a reversible one‐electron oxidation reduction centred at each iron centre, and the effect of substituents on the half‐wave oxidation potential is discussed in terms of their electronic properties. Linear correlations have been observed between these potentials and the Hammett σ_x constant for the substituents. Cyclic voltammetry has been carried out for ferrocene derivatives IIIa, V and IX. Copyright © 2003 John Wiley & Sons, Ltd.     

Total reorganization energy and its components in processes of one-electron oxidation of phosphorus compounds in acetonitrile

The ionization processes of phosphorus(III) and (IV) compounds oxidized in the potential range of 1.8–4.0 Vvs. Ag/0.01M AgNO₃ in MeCN were studied by chronovoltammetry on a Pt ultramicroelectrode in acetonitrile and by photoelectron spectroscopy in the gas phase. A relationship between the half-wave potential (E _1/2) and vertical ionization potential (IP _v)E _1/2=0.89IP _v–6.27 is fulfilled in a wide potential range from−0.37 to 3.98 V. The total reorganization energy of the system (1.45–2.50 V) and the energy of reorganization of the solvate shell (0.9–1.9 eV) were estimated. Translated fromIzvestiya Akademii Nauk. seriya Khimicheskaya, No. 4, pp. 682–689, April, 1999.     

Evaluation of multicenter one-electron integrals of noninteger u screened Coulomb type potentials and their derivatives over noninteger n Slater orbitals

Multicenter integrals over noninteger n Slater type orbitals with integer and noninteger values of indices u of screened Coulomb type potentials, f(u)(eta,r)=r(u-1)e(-etar), and their first and second derivatives with respect to Cartesian coordinates of the nuclei of a molecule are described. Using complete orthonormal sets of Psi(alpha) exponential type orbitals and rotation transformation of two-center overlap integrals, these integrals are expressed through the noncentral potential functions depending on the molecular auxiliary functions A(k) and B(k). The series expansion formulas derived for molecular integrals of screened Coulomb potentials and their derivatives are especially useful for the computation of multicenter electronic attraction, electric field, and electric field gradient integrals. The convergence of series is tested for arbitrary values of parameters of potentials and orbitals.