Polarographic Half-wave Reduction Potentials Research Articles

Structure Resonance Theory. Correlations of Electron Affinities and Polarographic Reduction Potentials of Polycyclic Aromatic Hydrocarbons

The electron affinities and the polarographic half-wave reduction potentials of aromatic hydrocarbons were correlated with the logarithms of the structure counts of the neutral molecules and the radical anions, ln SC(R) and ln SC(R°), respectively. Regioselectivity in the electrochemical reduction of these hydrocarbons is also discussed. The results obtained in this work indicate that the structure resonance theory can be successfully used for correlations of this type, and that effects due to the delocalization of π-electrons in both the neutral and charged species are important factors in the electro-chemical reduction of aromatic hydrocarbons.

QSPR models for prediction of half wave potentials of some chlorinated organic compounds using SR-PLS and GA-PLS methods

The half-wave reduction potential is an important electrochemical property used for the characterization of organic compounds. This property, which is a characteristic constant for a reversible oxidation–reduction system, can be useful for predicting electrochemical properties of other organic compounds. In this work, quantitative structure–property relationship (QSPR) models have been introduced for estimating polarographic half-wave reduction potentials of 21 chlorinated organic compounds. Two QSPR models have been developed based on genetic algorithm–partial least squares (GA-PLS) and stepwise regression–partial least squares (SR-PLS) to predict half-wave potentials (E1/2) of some chlorinated organic compounds. Variable selection is very important for QSPR modelling. In the present study, two selection variables methods were compared to choose molecular descriptors for the construction of a model by the PLS method. Both GA-PLS and SR-PLS methods resulted in accurate prediction, with more accurate results obtained by the GA-PLS model. The respective root mean square error of the prediction set obtained by the GA-PLS and SR-PLS models were 0.082 and 0.1302.

QSPR correlations of half-wave reduction potentials of cata-condensed benzenoid hydrocarbons

Polarographic half-wave reduction potentials of 31 cata-condensed benzenoid hydrocarbons were estimated by using lowest unoccupied molecular orbital energies (ELUMO), an indicator of the number of linearly-fused benzenoid rings (n), and distance-based topological indices. The first two parameters account for most of the variance. A parallelism between experimentally measured measured Diels-Alder reactivity and polarographic half-wave reduction potentials is illustrated by a correlation between these data.

Structure-activity relationships for benzotriazine DI-N-oxides

SR 4233 (3-amino-.1,2,4-benzotriazine 1,4-dioxide) is a bioreductive agent that selectively kills and radiosensitizes hypoxic mammalian cells in vitro and murine tumors in vivo. In an attempt to better understand the mechanism of action of the drug, and to determine whether a superior analog may exist, 15 benzotriazine-di-N-oxide analogs of SR 4233 have been evaluated to date for the following properties: hypoxic and aerobic toxicity toward CHO cells in vitro, drug-induced stimulation of oxygen consumption by incubation with respiration-inhibited cells, and acute LD 50 evaluated in BALB c mice. We noted several correlations between these biological properties of the drugs and some of their physicochemical characteristics. Both the hypoxic cytotoxicity and stimulation of oxygen consumption by respiration-inhibited cells were positively correlated with E 1 2 , the polarographic half-wave reduction potential, and a measure of electron affinity. The air-to-nitrogen differential cytotoxicity reached a maximum (corresponding to SR 4233) and then declined with increasing E 1 2 . The acute LD5o of each analog in mice decreased with increasing E 1 2 . One new compound, SR 4482, was found to be more toxic to hypoxic cells in vitro, but less toxic to mice, than SR 4233. It is similar in structure to SR 4233, but lacks any substituent in the 3-position of the triazine ring. This promising drug may represent a member of a new subseries of 1,2,4-benzotriazines with different structure-activity relationships.

Antibacterial activity and polarographic half-wave reduction potential of 2-nitrobenzo[b]furans.

The antibacterial activities of a series of 2-nitrobenzo [b] furan derivatives against St. aureus, B. subtilis, E. coli, Sal. typhimurium, Sal. enteritidis, Sh. flexneri, Pr. vulgaris or Ps. aeruginosa were determined in vitro. Most of the compounds showed considerable activities against the bacteria except Pr. vulgaris and Ps. aeruginosa and one of them was about 30 times as active as nitrofurantoin against St. aureus. Mono- and dimethoxy derivatives (2a, 3a, 4a, 5f) were the most active. The polarographic half-wave potentials (E1/2) of the 2-nitrobenzo [b] furans at pH 7 were in a narrow range of -0.450±0.04 V, whereas the E1/2 values of regioisomeric nitrobenzo [b] furans were more negative (-0.560--0.726 V). In the case of 2-nitrobenzo [b] furans, substituent (s) on the benzene ring had little influence on the reduction potential of the 2-nitro group, whereas the antibacterial activity depended markedly on the substituent group (s).

New description of the substituent effect on electronic spectra by means of substituent constants. II. n-.PI.* and .PI.-.PI.* Transitions.

In order to confirm and to extend the scope of the equation 1, 3EUVho→lu=aF+bR+c, presented by us in the foregoing paper, the polarographic half-wave reduction (Ered1/2) and oxidation (Eoxd1/2) potentials in nonaqueous media and the electronic spectra of 4-substituted azobenzenes have been analyzed. Although the equation was originally derived for π-π* type HOMO→LUMO transitions with the same character in a series of substances, in this paper we have theoretically verified that the equation is also applicable to the n-π* bands of conjugated systems. The singlet n-π* and π-π* bands of 4-substituted azobenzenes and the triplet n-π* bands of 4, 4'-disubstituted benzophenones were employed to examine the above equation. The results were reasonable, and are discussed from the viewpoint of molecular orbital theory.

Mutual correlation between nonaqueous oxidation-reduction potentials and electronic spectra, and its extension to describe the substituent effect on electronic spectra by means of substituent constants.

In order to confirm and to extend the scope of the equation (Eoxd1/2-Ered1/2)=k11, 3 EUVho-lu+k2, presented previously by us, the polarographic half-wave reduction (Ered1/2) and oxidation (Eoxd1/2) potentials in nonaqueous media and the electronic spectra of 4, 4'-disubstituted stilbenes have been measured and analyzed by means of the equation with reasonable results. The substituent effect on Eoxd1/2 and Ered1/2 values has been well described by using the Hammett-type substituent constants. This result has been combined with the above equation, and thus a general equation describing the substituent effect on electronic spectra in terms of substituent constants has been derived as EUVho-lu=aF+bR+c, F and R being Swain and Lupton's substituent constants. This equation has been experimentally examined using the strong longest-wavelength π-π* bands of 4, 4'-disubstituted stibenes and azoxybenzenes. The results were quite reasonable. Finally, the description of the substituent effect on electronic spectra in terms of substituent constants is discussed on the basis of molecular orbital theory.

Spectroscopic and electrochemical properties of para-methoxyazobenzene derivatives

Ultraviolet photoelectron spectroscopy, gas-phase absorption spectroscopy, polarographic half-wave reduction potentials and INDO/S-CI molecular-orbital calculations have been employed to investigate the electronic structure of some p-methoxyazobenzene derivatives. The photoelectron bands in the low-energy region of the spectrum have been assigned. The ag(n–) and the au(π1) ionizations display almost the same sensibility towards the substituent. The observed trends are qualitatively predicted by calculation. The electronic transitions are interpreted with the aid of INDO/S-CI calculations. Use is also made of the approach of Haselbach and Schmelzer approach. Reversible half-wave reduction potentials are used as a measure of the solution electron affinity of the molecule. Good linear relations are found between E1/2 and ELUMO and the Hammett σ constant. Correlations between electrochemical and spectroscopic data provide evidence that variations in the experimental transition energies along the series, with the exception of the nitro-derivatives, originate from changes in the (–J+ 2K) and ΔGsolv terms.

Relative Electron-capture Efficiency of Disubstituted Benzene in γ-Irradiated MTHF Glass at 77 K

Abstract The relative electron-capture efficiencies of disubstituted benzenes were measured by a competitive method. It was found that the efficiencies of the disubstituted benzenes are correlated with the reactivity towards nucleophilic species as expressed by Hammett’s substituent constant. The logarithmic values of the efficiency are roughly proportional to the polarographic half-wave reduction potentials and to the electron affinities.

Studies on halogen quenching through the Stern-Volmer plot (author's transl)

The quenching effect for halogenated benzenes, methanes and ethanes have been investigated. The halogen quenching was accurately measured using the internal conversion electrons emitted from 113Sn-113mIn. From the quenching constants determined by the Stern-Volmer plots with respect to various halogen quenchers, the following results have been obtained. (1) The quenching constants increase with the number of halogen substituents, so as linearly in halogenated benzenes and exponentially in halogenated methanes and ehtanes. Even the isomers of halogenides have different quenching constants. (2) There is a linearity between logarithm of the quenching constant and a polarographic half-wave reduction potential. (3) Electron excitation provides larger quenching constants than UV excitation for halogenated methames. Based on these results, the mechanism of halogen quenching have been discussed in connection with the exciplex formation.

Rhodium(III) complexes containing unidentate amines

Convenient syntheses of [Rh(NRH2)5X]2+(R = Me, Et, Prn, Bun, or Bui; X = Cl or Br), trans-[Rh(NR′H2)4Cl2]+(R′= Pri, cyclohexyl, or Ph), and trans-[Rh(pyr)4Cl2]+(pyr = pyrrolidine) are reported and [Rh(NRH2)5Cl]2+ is shown to be a convenient starting material for the synthesis of trans-[Rh(NRH2)4Cl2]+. Polarographic half-wave reduction potentials are reported together with physical and spectroscopic properties. The rate of exchange of amino-protons in trans-[Rh(NMeH2)4Cl2]+ and the rate constants and activation parameters for the substitution of chloride by bromide in [Rh(NRH2)5Cl]2+ and trans-[Rh(NRH2)4Cl2]+(R = Me, Et, or Prn) have been determined and are compared with data on related complexes of RhIII, CoIII, and CrIII.

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.

Presumptive Structures of Activated Complexes of Polarographic Redox Reactions of Unsaturated Organic Compounds. I. Polarographic Reductions of Aromatic Hydrocarbons, Aromatic Aldehydes, and Quinones

Abstract Activated complexes of the polarographic reductions of unsaturated organic compounds were assumed to be formed by the interaction of the π-molecular orbital of each compound with a hypothetical atomic orbital of the electrode employed. Linear correlations were obtained between the mutual differences in the polarographic half-wave reduction potentials of a series of unsaturated organic compounds and the respective differences in the π-stabilization energies calculated on the basis of the presumed structures of activated complexes. These linear relationships were considered to lend experimental support to the propriety of the presumed structures of the activated complexes of the polarographic reductions of unsaturated organic compounds.

CORRELATION BETWEEN THE REACTION OF KETONES WITH ZINC AND THEIR POLAROGRAPHIC HALF-WAVE REDUCTION POTENTIAL

Abstract Correlation between the result on the reaction of ketones with zinc dust in acetic acid and their polarographic half-wave reduction potential in organic solvents is described.

Radical-anions of cyclopentadithiophenones and dithienothiophen 7,7-dioxides. Electron spin resonance spectra and polarographic halfwave reduction potentials

The e.s.r. spectra of the radical-anions of cyclopentadithiophenones, generated by electrolysis in acetonitrile and (or) dimethylformamide, and of three dithienothiophen 7,7-dioxides, produced by potassium reduction in dimethoxyethane, have been recorded. Assignment of the hyperfine splitting constants is based on the hyperfine splitting constants of deuteriated derivatives. The polarographic half-wave reduction potentials of cyclopentadithiophenones were determined in dimethylformamide. Cyclopentadithiophenones appear to be reduced by a reversible one-electron transfer in the first reduction step. An attempt to relate the hyperfine splitting constants to computed spin densities by Huckel–McLachlan calculations is only partially successful.

Conjugation in sulphones. Electronic spectra and polarographic behaviour of thiophen dioxides

The interaction of the sulphone group with an adjacent unsaturated centre has been treated on the basis of HMO theory. The sulphur atom of the SO2 group is assumed to form π-bonds by means of its 3d orbitals. Using the same set of empirical parameters [αS(SO2)=α– 1·4β, α0=α+β, βSO= 0·9β, and βC–SO2= 0·7β], a good correlation has been obtained between the longest-wavelength absorption band and the calculated N→V1 transition energy for 18 sulphones. The energy of the lowest unoccupied molecular orbital shows a good correlation with the polarographic half-wave reduction potential. The SO bond of sulphones is shown to be hardly affected by mesomeric interactions.

Studies of the charge-transfer complexes of 1,4-naphthoquinone acceptors

The naphthoquinone acceptors form stable charge-transfer complexes in solutions of aprotic solvents with aromatic hydrocarbons as donors. From the charge-transfer transition energies of the complexes as well as from the polarographic half-wave reduction potentials of the acceptors relative electron affinities of the acceptors are determined. In addition, the association constant, molar extinction coefficients, oscillator strengths, and enthalpies of formation of the complexes were obtained from charge-transfer spectral studies with hexamethylbenzene as donor. The average electron affinities of 2,3-dichloro-(0·90 eV), 2,3-dichloro-5-nitro-(1·18 eV), 2,3,5,6-tetrachloro-7-nitro-(1·30 eV), 2,3-dicyano-(1·53 eV), 2,3-dicyano-5-nitro-(1·68 eV), and 2,3-dicyano-5,6-dichloro-7-nitro-1,4-naphthoquinone (1·75 eV) obtained from the charge-transfer spectral studies clearly show the cumulative effects of electron-withdrawing substituents on the naphthoquinone π-system. The π-acid character of 1,4-naphtho-quinone aceptors are also correlated with the calculated molecular orbital energies and with the experimental polarographic half-wave reduction potential.

Estimation of Work Function of Photosensitive Dyes from their Polarograms, Electronic Spectra and HMO Calculations

WE have previously defined the quasi-Fermi level eq−F and the hyper-redox potential Ehyper of a molecule as follows1 where elv and eho are the lowest vacant and the highest occupied electronic energy levels of the molecule calculated by the HMO method (Huckel approximation method)2 E½ and Emax are its polarographic half-wave reduction potential and excitation energy, and C is constant. It was found that the difference in power of spectral sensitization of the photographic process among various dyes could be explained by the values of their eq−F and .

An electron spin resonance and polarographic study of the sulphone group

The electron spin resonance spectra of the radical anions of diphenyl sulphone, dibenzothiophene S, S-dioxide and thianthrene S, S, S′, S′-tetroxide have been measured and the proton hyperfine splitting constants assigned to the various protons by measurement of the spectra of deuterio and methyl derivatives. The experimentally determined spin populations were correlated by molecular orbital calculations, using the formalism of Koch and Moffit to describe the sulphone group, and it is shown that irrespective of the mutual orientation of this group and the conjugated system it may be described as far as the electron spin resonance spectra are concerned as contributing a vacant symmetric orbital to the conjugated system whose coulomb integral (αSO2 ) and sulphur carbon resonance integral (βC-SO2 ) are given by αSO2 =αC – 2·0β, βC-SO2 =0·8β where α and β are the carbon coulomb integral and carbon-carbon resonance integrals respectively. The polarographic half wave reduction potentials of these compounds and some of their methyl derivatives dissolved in anhydrous dimethylformamide were also determined and the effect of methyl substitution on this quanttiy is also well described by the above parameters for the sulphone group. Finally a difference between the results obtained here and those previously obtained by Vincow from the electron spin resonance spectrum of thioxanthone S, S-dioxide radical anion is noted and discussed.

Thermal Stability of Polynuclear Aromatic Hydrocarbons and the Polarographic Half-wave Reduction Potential

WE have noted a relation of the thermal stability of certain polynuclear aromatic hydrocarbons to the polarographic half-wave potential. In attempting to find some theoretical justification for this relation, it was found that Bergman1 had related the E1/2 to the ‘p’ ultra-violet band as determined by Clar2. Also, Basu3 has related the E1/2 to the lowest unoccupied energy orbital. Thermal stabilities were determined by Madison4 by pyrolysis followed by mass spectrometric analysis of the gas phase products. Table 1 presents the composite data: there the compounds are grouped into classes on the basis of Bergman's classification. The groups have theoretical justification in so far as that compounds of the same class plot linearly with respect to E1/2 versus ‘p’ band frequency. Further, comparisons are valid: (a) if the compounds are planar molecules of the same class; and/or (b) if the compounds are members of an anellation series (for example, naphthalene, anthracene, naphthacene).