Half-wave Oxidation Research Articles

Charge transfer to a solvent state. VIII. Localization of charge during the transition state of the solute: The solute cation localization energy

A good linear correlation between the polarization energy P + of the solute cation and the corresponding gas-phase ionization energy I g suggested recently, is now obtained for various hydrocarbons in solution. The radius r of the solute cation is calculated from a solute-solvent cage charge-transfer model and P + is derived from the Born formula: P + = −( e 2/2 rX1 - D op −1). In contrast to the non-linear plot of − P +′ versus I g (where − P −′ is the polarization energy with r calculated from the molar volume), the linear plot of − P + versus I g gives a slope which deviates from the slope of unity obtained for the well-known linear correlation of E 1/2 ox versus I g( E 1/2 ox is the half-wave oxidation potential). During the transition state, there is localization of charge. The electron transfer from the excited solute to the electrophilic solvent cage is through the rth C atom where the cation omega localization energy L r w is lowest, r corresponds to the position of the most active C atom in the electrophilic aromatic substitution reaction. When applying the solute-solvent cage CT complex model to aqueous micelle systems and taking into account the large difference in photoionization energies between the gas and condensed phase (Δ I = 2.5 eV), the oxidation potential of perylene and ferrocene seems to vary very little with the charge surfactant of the micelles. The energy V 0 of the conduction band of the aqueous solvent and the hydrophilic character of the solute cation seem to be the main factors which cause the lowering in photoionization energy of the solute. The present electrochemical results do not permit us to conclude that the charge of the aqueous micellle surfactant affects the yield in ionic species, an observation which has been well noted in the photoexcitation case.

Structure-resonance theory and electrochemical oxidation potentials of π-electron hydrocarbons

The polarographic half-wave oxidation potentials of 51 benzenoid hydrocarbons are correlated with the logarithms of the structure counts of the neutral molecule and the radical cation, ln SC R and ln SC R + , respectively. For example, for 25 unsubstituted hydrocarbons and 26 methylsubstituted hydrocarbons, the following relationship was obtained: E 1 2 (ox) = 4.018 + 1.607 ln SC R −1.027 ln SC R + ; r = 0.955, s = ± 0.07. Such an excellent correlation indicates that effects associated with delocalization of π-electrons and the π-electron charge distribution in the neutral and charged molecular species, respectively, are significant in electrochemical oxidation, contrary to an earlier literature report which claimed that they are not. The results also show that the structure counts provide a simple working method for correlations of the electrochemical reactivities of polynuclear aromatic hydrocarbons and their derivatives.

Fluorescence and triplet yield quenching of β-apo-14′-carotenal (C 22 aldehyde) by aromatic molecules

The fluorescence and triplet yields of β-apo-14′-carotenal in cyclohexane at room temperature are quenched in the presence of submolar concentrations of aromatic molecules predominantly through a dynamic interaction. The observed Stern-Volmer constants K SV F for fluorescence quenching vary from 0.06 M −1 for mesitylene to 45 M −1 for indole, giving excited state quenching rate constants in the range of 6 × 10 7 − 4.5 × 10 10 M −1 s −1. With methoxy-substituted benzene derivatives as quenchers, the K SV F data correlate well with the half-wave oxidation potentials of the quenchers, suggesting a modest involvement of charge transfer. Interestingly, the Stern-Volmer constants K SV T for triplet yield quenching are significantly higher than the corresponding K SV F values. This result is explained in terms of scheme(s) where the singlet excited state species that acts as precursor for triplet is assumed to be kinetically distinguishable from that responsible for fluorescence.

Merostabilization in radical ions, triplets, and biradicals. 6. The excited state behaviour of para-substituted tetraphenylethylenes

The half-wave oxidation and reduction potentials, measured by cyclic voltammetry, of tetraphenylethylene and a series of para-substituted (cyano, methyl, and methoxy) derivatives are found to correlate with σ+ and σ− values respectively. The deviations from these correlations that are observed for one of the derivatives (unsymmetrically substituted) are attributed to merostabilization.The nature of the first excited singlet state of these compounds is investigated by spectroscopic and photochemical means. Quantum yields for the direct cis–trans photoisomerization demonstrate that this provides the dominant pathway for decay of the singlet state. The π,π* triplet energies are estimated from a qualitative investigation of the relative efficiencies of triplet-sensitized cis–trans isomerization as a function of sensitizer triplet energy.

A study of the initial act of the oxidation of Lignin and compounds modeling it

The initial stage of the oxidation of lignin and of compounds modeling it has been studied by differential oscillographic voltammetry. The half-wave oxidation potentials of the compounds are given, and it is shown that the phenoxyl radicals formed are extremely unstable and take part immediately in subsequent chemical reactions.

Ultraviolet photoelectron spectroscopy and oxidative electrochemistry of 8-hydroxyquinoline and its derivatives

The gas-phase ionization potentials of 8-hydroxyquinoline and several of its alkyl derivatives in the region 8–12 eV are assigned. The energies of the highest occupied molecular orbitals and the half-wave oxidation potentials at a rotating platinum electrode are used to derive empirical values for differential solvation energies for the compounds in acetonitrile and dichloromethane solution.

Influence of the composition of the chelate ring and the structure of the ligand on the inhibiting activity of metal thiooxinates in the oxidation of hydrocarbons

1. The polarographic half-wave oxidation potentials (E1/2) of the thiooxinates and 5-S-alkylthiooxinates of Cu(II), Ni(II), Pb(II) and Co (III), as well as Zn(II) thiooxinate and Cu(II) acetylacetonate, have been determined. A correlation has been established between the ease of the oxidation of the sulfur-containing chelates of the divalent metals and their inhibiting activity in the model reaction of the low-temperature oxidation of ethylbenzene: The more easily is the chelate ring oxidized, the more actively it inhibits. 2. The introduction of an S-alkyl substituent into position 5 of the mercaptoquinoline ring of the ligand slightly alters E1/2 and the inhibiting activity of the thiooxinates. 3. The nonlinear nature of the dependence of the induction period for the oxidation of ethylbenzene on the initial concentration Co of the inhibitor CuTox2 and the corresponding extreme-value dependence of the stoichiometric inhibition coefficient f on Co are attributed to a reaction in the induction period between the conversion products of CuTox2 and 2-cyanopropyl hydroperoxide, which is formed as a result of the dissociation of the initiator in the medium of the oxidized hydrocarbon. 4. The rate constant of the reaction of the conversion products of CuTox2 with the hydroperoxide of ethylbenzene has been evaluated.

Correlation of the photoelectron and electronic spectra of thiochromones and thiochromanones with their electrochemical data

The gas phase ionization potentials, electrochemical redox potentials and spectroscopic properties of a series of thiochroman-4-one and thiochromone derivatives have been studied. A dramatic shift in the energies of the lowest vacant and highest occupied molecular orbitals of the parent thiochromanone as a function of the addition of a double bond and/or oxidation of the sulphur atom was observed. This shift in energy of the molecular orbitals was reflected in their spectroscopic characteristics. The lowest singlet (and triplet) state of compounds 1–3 in solution is π,π* in nature, while that of compounds 4–6 is n,π*. These results are best explained in terms of substituent effects on the energetics of the acetophenone moiety. The change in the nature of the lowest excited state from π,π* (1–3) to n,π* (4–6) should result in quite different types of photochemistry for the two series.A linear free-energy relationship between the singlet and triplet energies and the absolute difference between the oxidation and reduction potentials of the two series was found. These correlations have been utilized to estimate the half-wave oxidation potentials of compounds 3–6. A correlation was found to exist between the gas phase ionization potential and the solution electrochemical oxidation potential.

CHEMISTRY OF SINGLET OXYGEN—XXVI. PHOTOOXYGENATION OF PHENOLSy†

Abstract— The aerobic dye‐sensitized photooxygenation of monohydric phenols proceeds by way of singlet oxygen under the conditions studied. Various phenols give different proportions of reaction with and quenching of singlet oxygen. Para‐substituted 2,6‐di‐t‐butylphenols show a linear correlation between the log of the total rate of singlet oxygen removal and their halfwave oxidation potentials; the same correlation is given for certain phenol methyl ethers. A Hammett plot using s̀+ gives ρ ‐ 1.72 ± 0.12, consistent with development of some charge in the quenching step. Reaction of photo‐chemically generated singlet oxygen with 2,4,6‐triphenylphenol gives 2,4,6‐triphenylphenoxy radical as an intermediate in singlet oxygen quenching, although no overall reaction occurs. Kinetic analysis indicates that the radical is derived exclusively from the interaction of 2,4,6‐triphenylphenol with singlet oxygen. A charge‐transfer mechanism for quenching of singlet oxygen by phenols is proposed.

Half-wave potentials and LCAO-SCF-MIO calculations for carcinogenic benz [c]acridines.

Half-wave reduction potential and half-peak oxidation potential have been measured for 11 methyl-substituted benz [c] acridines and unsubstituted benz [c] acridine in acetonitrile with tetraethylammonium perchlorate electrolyte. A linear correlation was obtained between the energy of the lowest unoccupied molecular orbital calculated by LCAO-SCF method and the half-wave reduction potential, and also between the energy of the highest occupied molecular orbital and the half-peak oxidation potential. The correlations between carcinogenic activity and both half-wave oxidation and reduction potentials were interpreted in connection with the electron charge density of the K-region. The results of LCAO-SCF calculation for the electron charge density and the electrophilic superdelocalizability index indicated that the electrophilic reactivity of the K-region or the ring nitrogen atom of methyl-substituted benz [c] acridines may be essential for their carcinogenic activity.

Correlation E ox de carbanions/pK a de leurs acides conjugues, domaine d'existence stable des carbanions en solution aprotique ou partiellement p

pK a values of 44 enolizable compounds have been determined and collated with the half-wave oxidation potentials of the corresponding anions. A correlation between the half-wave oxidation potential of an enolate anion and the pK a of his conjugated acid was established. Generalization of this correlation allows to define a stability range of the carbanions in solution, as much with regard to oxidation-reduction as to their acidity.

Determination of the absolute energy levels of organic molecules in the gas phase and in condensed media. I. Solution and gas phase ionization energies and electron affinities of substituted pyridine-N-oxides

The basic principle for the determination and interrelation of the absolute energy levels of organic molecules in the gas phase and condensed media are discussed. The electrochemical and gas phase data for a series of substituted pyridine-N-oxides were used to examine the validity of Born’s formula, which approximate the chemical solvation energies of ions. A linear free-energy relationship between the gas phase ionization potentials (I.P.) and the electrochemical half-wave oxidation potentials (Eox1/2) for substituted pyridine-N-oxides was found. This relationship was utilized to predict the ionization potentials of other heterocyclic amine-N-oxides whose I.P.’s have not been measured. The estimated effective ionic radius reff for pyridine-N-oxide derivatives was found to be highly dependent on the nature of the substituent and linearly related to the electrophilic substituent constant σ+. Substituted pyridine-N-oxides have moderately low ionization energies and form stable positive radical cations. Their use as hole traps when immersed in insulating fluid in contact with photoconductors will be discussed.

The interrelation between polarographic half-wave potentials and the energies of electronic excited states

The polarographic half-wave oxidation and reduction potentials, singlet and triplet energies of a series of (i) monocyclic aza-aromatics, (ii) polycyclic aza-aromatics and their N-oxides, (iii) condensed aromatic hydrocarbons, and (iv) substituted benzene derivatives are reported. The interrelation between the spectroscopic and electrochemical data is developed and discussed in regard to theoretical and experimental parameters. A linear free-energy relationship between the triplet energies and the absolute difference between the oxidation and reduction potentials of the first three series was found. These correlations were utilized to estimate the triplet energies of other molecules in the series, to predict the true half-wave oxidation potential of some heteroaromatic N-oxides and to compute the coulomb repulsion integral. No correlation between ET(π, π*) and (E1/2°x − E1/2R) for substituted benzenes was found. The various reasons for this deviation were discussed.

The structure and redox properties of some planar [M IIN 4] chelate compounds of cobalt and nickel. Part II. 1The effect on molecular properties of substituents on the quadridentate ligands

The preparations are reported of a series of cobalt(II) and nickel(II) compounds with quadridentate dianionic ligands related to 1,2-bis-(o-iminobenzylideneamino) ethane. This new series of planar [M IIN 4] compounds includes species with electron-withdrawing and electron-donating substituents, and various bridging central chelate rings in place of the dimethylene bridge of the parent compound {5-Cl on the o-iminobenzaldehydo moiety; Me or Ph on the azomethine carbon; and CH2CHMe, z.sbnd;CH 2CMe 2, (CH 2) 2−, (CH 2) 3−, and 3-NO 2 and 4-NO 2-o-phenylene for the chelate ”bridge“}. Half-wave oxidation potentials have been measured for most of the compounds and these show that electron-withdrawing substituents raise E 1 2 while electron-donating ones lower it - apparently independently of the position of the substituent on the ligand. All solid compounds were characterised by their X-ray powder diffraction patterns (and i.r. spectra), a few were dimorphic, and several also gave crystalline solvates when recrystallised from DMF. About 60% of the solids can be classified into two structural groups from the X-ray diffraction patterns. The cobalt(II) compounds have a low intensity band at ∼ 10 x 10 3 cm −1 which in one case moves by 1.5 x 103 cm −1 between the solid state and solution. This change is associated with conformational changes in the molecule. In addition to the monomeric species, a cationic nickel(II) dimer of a mono-de-protonated ligand has been characterised.

Photoinduced reactions—LIX: Reactivity of singlet oxygen toward methoxybenzenes

The dye-sensitized photooxygenation of a series of methoxybenzenes (Ia-Ie) in methanol has been investigated. Highly substituted methoxybenzenes (Ig, Ii-Il) were susceptible to photooxygenation, while less substituted methoxybenzenes (Ia-Ig, Ih) were found to be unreactive under similar conditions. The participation of singlet oxygen in the reaction was elucidated by photooxygenation using sensitizers having different triplet energies and by quenching experiments. Relative reaction rates of methoxybenzenes with singlet oxygen were shown to correlate with their half-wave oxidation potentials (E 1 2 ) and with their charge-transfer maxima ( v c.t. ) with tetracyanoethylene.

Photoinduced reactions. XLIV. Reactivity of singlet oxygen toward methoxybenzenes.

Abstract The dye-sensitized photooxygenation of a series of methoxybenzenes (Ia-Ie) in methanol has been investigated. Highly substituted methoxybenzenes (Ig, Ii-Il) were susceptible to photooxygenation, while less substituted methoxybenzenes (Ia-Ig, Ih) were found to be unreactive under similar conditions. The participation of singlet oxygen in the reaction was elucidated by photooxygenation using sensitizers having different triplet energies and by quenching experiments. Relative reaction rates of methoxybenzenes with singlet oxygen were shown to correlate with their half-wave oxidation potentials (E 1 2 ) and with their charge-transfer maxima ( v c.t.) with tetracyanoethylene.