Electrochemical reactions with protonations at equilibrium Part V. THE 1 e, 1 H+ homogeneous isotopic exchange reaction

A theoretical analysis is presented of the homogeneous isotopic electron exchange reaction when a proton is involved in the reaction. The protonations are assumed to be much more rapid than the electron exchanges, i.e. they can be considered at equilibrium. It is shown that the reaction behaves as a single monoelectronic process. An apparent pH-dependent exchange rate constant is defined; its value becomes smaller than the actual exchange rate constants in the pH interval between the pK_a's of the system. Applications to redox polymer modified electrodes are discussed.     

Electrochemical reactions with protonations at equilibrium: Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode

The kinetics of the p-benzoquinone/hydroquinone Q/QH₂ couple on a platinum electrode are analysed on the basis of the theory presented earlier (E. Laviron, J. Electroanal. Chem., 146 (1983) 15) for the nine-member square scheme when the protonations are assumed to be at equilibrium, using experimental data from the literature. The square scheme is of the NN type. The Tafel plots and the variations of the experimental apparent rate constants between pH 0 and 7 are in good agreement with the theoretical predictions. The heterogeneous rate constants found for the elemental electrochemical steps are as follow: Q Q^?, k_h3=1/6×10^?3 cm s^?1; QH^.QH^?, k_h5=0.11 cm s^?1; QH⁺QH^., k_h2?160 cm s^?1; k_h4 for the reaction QH₂^+.QH₂ is in the range 0.5–4 cm s^?1. Between pH 0 and 7, the reaction sequence during the reduction is, for the most part, successively H⁺e^?H⁺e^?, e^?H⁺H⁺e^?, and e^?H⁺e^?H⁺ (reverse sequence during the oxidation).     

Redox properties of ruthenium(III/II)–edta complexes with o-phenylenediamine and o-benzoquinone diimine ligands

The reaction of [Ru^III(edta)(H₂O)]⁻ with o-phenylenediamine (opda) in water, under aerobic conditions, affords the diamagnetic [Ru^II(edta)(bqdi)]²⁻ product (where edta stands for the ethylenediaminetetraacetate co-ligand, and bqdi represents the non-innocent o-benzoquinone α,α^′-diimine ligand). In the current communication, the redox chemistry of this system in aqueous solution is described in details on the basis of electrochemical and spectroelectrochemical studies. The electrochemical behavior of “free” opda is rather complicated with further chemical reactions following the irreversible two-proton/two-electron oxidation (opda→bqdi+2e⁻+2H⁺), whereas its complex is electrochemically well-behaved with two chemically reversible redox processes: the monoelectronic couple associated with the metal ion (Ru^III/Ru^II) and another bielectronic step centered on the coordinated ligand (bqdi/opda). The set of UV–Vis electronic spectra were obtained by electrolytical generation, in situ, of all the redox species accessible in the CV working conditions (i.e., the starting [Ru^II(edta)(bqdi)]²⁻, the fully oxidized [Ru^III(edta)(bqdi)]⁻, and the fully reduced [Ru^II(edta)(opda)]²⁻ species), which are stable and totally interconvertible. The electrochemistry and absorption spectroscopy of these complexes in water were found to be comparable with the tetraammine counterparts. A remarkable difference in redox behavior between the diimine- and the analogous dioxolene-complexes was also revealed by comparison of the system reported herein with the one derived from catechol, and rationalized in terms of the quite efficient π-accepting electronic nature of the bqdi ligand.     

Theoretical study of the low-lying electronic states of ZnO and ZnS

Theoretical spectroscopic constants (r_e, D_e) and dipole moments (μ, ∂μ/∂r) are determined for the ¹σ⁺, ^1,3Π and ³σ⁺ states of ZnO and ZnS, using extended Gaussian basis sets and incorporating correlation using both configuration- interaction and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are relatively small. At the CPF level, both ZnO and ZnS have ¹Σ⁺ ground states, with the ³Π state lying 209 and 2075 cm⁻¹ higher, respectively. The ³σ⁺ state lies about 1.5 eV higher in ZnO and 2.1 eV higher in ZnS. The ^1,3Π states are relatively close together since the exchange splitting is small with the σ electron localized on Zn and the π electron on oxygen (or sulfur).     

b1Σ+ Emissions from group V–VII diatomic molecules. b 0+ → X1 0+, X2 1 emissions of AsI and SbI

Chemiluminescence from the b 0⁺ → X₁ 0⁺ band system of AsI and of the b 0⁺ → X₁ 0⁺, X₂ 1 systems of SbI in the near-infrared spectral region has been observed in a discharge flow system. Analysis of the spectra led to the spectroscopic constants (in cm^?1) of AsI: ω_e(X₁, X₂) = 257 ± 2, ω_ex_e(X₁, X₂) = 0.82 ± 0.2, T_e(b 0⁺) = 11738 ± 5, ω_e(b 0⁺) = 271 ± 2, ω_ex_e(b 0⁺) = 0.66 ± 0.2, and of SbI: T_e(X₂ 1) = 965 ± 10, ω_e(X₁, X₂) = 206 ± 6, T_e(b 0⁺) = 12328 ± 10, ω_e(b 0⁺) = 211 ± 6. The intensity ratio of the two sub-systems b 0⁺ → X₂ 1 and b 0⁺→ X₁ 0⁺ was found to be ≈0.013 in the case of SbI and ? 0.01 for AsI.     

524—NAD/NADH as a model redox system: Mechanism,mediation, modification by the environment

The biologically important redox couple, β-nicotinamide adenine dinucleotide/1,4,β-dihydronicotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e^?) transfer steps coupled with (a) dimerization of an intermediate free radical product, (b) protonation—deprotonation of an intermediate product, (c) other chemical reactions, (d) adsorption of reactant, intermediate and product species, and (e) mediation by electrode surface species. Cathodic reduction of NAD⁺ proceeds through two 1 e^? steps well separated in potential; protonation of the free radical produced on the first step occurs prior to the second electron-transfer; a first-order chemical reaction coupled to the latter may involve rearrangement of an initial dihydro product to 1,4-NADH (and some 1,6-NADH). In the apparently single stage 2 e^? anodic oxidation of NADH, the initial step is an irreversible heterogeneous electron transfer, which proceeds to at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH^+?, loses a proton (first order reaction) to form a neutral radical, NAD^?, which may participate in a second heterogeneous electron transfer (ECE mechanism) or may react with NAD^+? (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD⁺.     

The scavenging of the precursors of the solvated electron from the positron lifetime spectroscopy and the doppler broadening of annihilation line shape technique

The electron scavenging properties of aqueous solutions of two series of solutes are investigated, using the positron as a probe. For a better interpretation of the data, both lifetime spectroscopy and the Doppler broadening of annihilation line shape technique are used. All solutes inhibit the positronium (Ps) formation, by the scavenging of electrons. The first series consists of the halate ions, that should follow the Hunt linear relation between the rate constant for reaction with the solvated electrons, k_(e^?aq+S), and that for its precursor(s), 1/C₃₇. The Ps inhibition constants, k, are 0.14, 1.44 and 3.45 M^?1 for ClO^?₃, BrO^?₃ and IO^?₃, respectively. This sequence is quantitatively consistent with that of the respective k_(e^?eq+S). The second series includes the SeO⁼₄, Te(OH)₆ and BrO^?₄ species, and the Ps inhibition constants are 5.62, 10.5 and 14.3 M^?1 respectively. These values are much higher than expected from the k_(e^?aq+S) constants, on basis of the Hunt relation, in agreement with previous results from pulse radiolysis experimets.     

b 1Σ+ Emissions from group V–VII diatomic molecules: bO+ → X1 O+ emission of Pl

Chemilluminescence of the bO⁺ → X₁O⁺ band system of P1 has been observed in a discharge flow system. Thirty-eight bands of the sequences, δν = +2, +1, 0, ?1, ?2 and ?3 were recorded photoelectrically at medium resolution. Evidence is presented that the vibrational numering assigned to the bands in the recently published first analysis of this system has to be modified. The re-analysis leads to the new constants (in cm ^?1) T_e = 11135 ± 5, ω′_e 400 ± 2, ω_e′_e = 1.4 ± 0.3, ω″_e = 372 ±2 and ωχ″_e, 1.4 ± 0.3 for the bO⁺ and χ₁ states, respectively. An upper limit of 0.01 was found for the ratio of the (0.0) band intensifies of the two sub-systems bO⁺ → χ₂ 1 and bO⁺ → χ₁O⁺.     

Kinetics and mechanism of the electrochemical reduction of 2,2′-bipyridine: Part II. Consecutive isomerization processes of dihydrobipyridine

2,2′-Bipyridine is reduced by electrolysis at constant potential at a Hg electrode in the pH range 12<pH<14. A reversible 2e^?-, 2 H⁺-step was found. The first product Y is transformed in a fast irreversible reaction into a substance A, which undergoes slow consecutive reactions according to the scheme A agB agC. The mechanism was simulated by a computer program and the rate constants optimized. B is further reducible in a 2 e^?-, 2 H⁺-step, forming D. Therefore at long times of electrolysis only D remains. The substances Y, A, B, C are isomeric dihydrobipyridines. D is a tetrahydrobipyridine. The substances A, B, C and D could be isolated in solution, and partially identified by u.v.-spectroscopy, but polymerized if they were concentrated. The stable substance C is 2,5-dihydrobipy, Y probably N,N′-dihydrobipy. A reaction mechanism is proposed and it is shown that only two reaction paths remain possible, A being either the 1,4- or the 1,6-dihydrobipy.     

Luminescence in near-thermal charge exchange. III. CH(+) and CD(+) A → X emission from He+ C2H2 and He+ + C2D2

A pulsed ICR cell fitted with synchronous photon counting equipment is used to investigate the emission produced between 185 and 500 nm by near-thermal charge exchange between He⁺ and C₂H₂ (C₂D₂). The emission bands observed are A ²Δ → X²π and (weakly) B²Σ^? → X²π in CH(CD) and A ¹π → X¹Σ in CH⁺(CD⁺). Wavelength measurements on the bandheads of the (0,0) and (0,1) bands of CD⁺ A → X are used to evaluate vibrational constants of CH⁺(CD⁺) X¹Σ⁺. The results are (in cm^?1): ω_e = 2869 ± 27 (2106 ± 20); ω_eχ_e = 65 ± 13 (35 ± 7). These constants are used to calculate Morse-potential Franck—Condon factors and vibrational branching ratios for CH⁺ and CD⁺ A → X emission. The spectral distributions and the (relatively low) absolute emission rates produced by He⁺/C₂H₂(C₂D₂) charge exchange are briefly discussed in the light of presently available information on the charge transfer reaction and on the excited states of C₂H_2?⁺     

Kinetics and mechanism of reduction of oxo-chromium(V) salen by vanadium(IV)

The reduction of oxo-chromium(V) salen with a 40–160-fold excess of oxovanadium(IV) ([H⁺] = 0.02–0.1 M) at 25 °C has been investigated. The observed absorbance changes fitted a pseudo-first-order process. The nature of the intermediate, final product and reaction mechanism have been proposed on the basis of reaction conditions and observed rate constants. E.s.r. data support 1:1 stoichiometry with VO²⁺ in a deficiency. With an excess of VO²⁺ a Cr^III product corresponding to a two electron reduction process has been obtained. The spectral and ion exchange properties of the chromium product correspond to that of the N,N-ethylene-bis(salicylideneimine) derivative of Cr^III. The rate of formation of the final product increases with decreasing [H⁺]. The observed kinetic behavior is consistent with a mechanism involving the formation of a Cr^IV—V^V intermediate in an equilibrium step prior to the electron transfer step. The equilibrium constant for the formation of the intermediate has been estimated to be 11.2 ± 0.8 M^–1. The second-order-rate constants for the reduction of Cr^V species have been estimated to be 0.14 × 10², 0.10 × 10² and 0.05 × 10² M^–1 S^–1 at [H⁺] = 0.02, 0.05 and 0.1 M respectively. Like the Fe^II—Cr^V redox couple, the V^IV—Cr^V redox reaction also follows an inner-sphere process.     

A Study of picosecond dehalogenation of chlorobenzene anions in liquids by positronium inhibition measurements

Radiation chemistry and results of Ps yields indicate that the following processes occur in the positron spur in solution of halogen-substituted hydrocarbons, RX_n: e⁺ + e^? → Ps, e^? + RX _n → (RX_n)^? → RX_n?1 + X^?, e⁺ + (RX_n)^? → Ps + RX_n, e⁺ + X^? → [X^?, e⁺]. Hence the trapped electron can form Ps only if (RX _n)^? is stable or has a lifetime that is longer than o comparable to the Ps formation time. Previous studies have shown that some of the strongly chlorinated benzenes (n = 4.5 give reasonable inhibition in benzene but not in linear hydrocarbons. The reason is very probably that the dechlorination time is much shorter in benzene than in saturated hydrocarbons because Cl^? is more strongly solvated in benzene than in non-aromatic hydrocarbons. To test those ideas further we have begun detailed studies of solutions of the possible “intermediate” inhibitors, viz. 1,2,3,5- and 1,2,4,5-C₆H₂Cl₄, in mixtures of C₆H₆/C₆H₁₄ different methyl-substituted benzene aniline, anisole, dioxane and ethylbenzene. The results are discussed and interpreted in terms of the spur model. The Ps inhibition efficiency of the two isomeric forms of tetrachlorobenzene studied, appears most probably to depend on intramolecular electron transfer with subsequent dehalogenation of the molecular anion on a picosecond timescale. The divergence in inhibitor efficiency obtained for the chlorobenzenes when dissolved in aromatic solvents compared to the same solutes when dissolved in a saturated alkane appears most probably to be caused by complex formation between the initially formed chlorobenzene anion and benzene molecules, which permits a rapid relaxation of the molecular anion with subsequent bond stretching and expulsion of the chloride anion.     

Pulse radiolytic studies of metallic clusters in a quaternary microemulsion system

The radiation-chemical reduction of Tl⁺, Co²⁺ and Ni²⁺ ions in water-in-oil microemulsion has been described. Rate constants for the reduction of the metal ions by e^- _aq were found to be much lower compared to those in aqueous solution. The absorption spectrum and the rate constants of formation of the transient intermediates of metal clusters were correlated with the water-pool size, surrounding environment and compared with data in aqueous solution. Tl₂ ⁺ reacts with Cd²⁺ with k = 1.1 × 10⁸ M^-1 s^-1. Reactions of Tl₂ ⁺ and Co⁺ with Ag⁺ leads to the formation of Ag metal particles. No reaction was observed from Cd⁺ to Tl⁺ or Co²⁺. The results are discussed in terms of dynamic exchange of materials between the polar cores of microemulsion. This exchange facilitates formation of silver particles.     

The first two excited 1Σg+ states of Cs2

Laser-induced fluorescence Of Cs₂ molecules in the infrared region (4000–9000 cm^?1) has been observed using several exciting wavelengths from an argon-ion laser and from a ring dye laser. Accurate molecular constants for the first two excited ¹Σ_g⁺ electronic states are derived from spectra recorded at high resolution by Fourier transform spectroscopy. Main molecular constants are: (2)¹Σ_g⁺: T_c = 12114.090 cm^?1, ω_e = 23.350 cm^?1, B_c = 7.4.5 × 10^?3 cm^?1, R_c = 5.8316 Å; (3)¹Σ_g⁺: T_e = 15975.450 cm^?1, ω_e = 22.423 cm^?1 , B_e = 8.23 × 10^?3 cm^?1, R_c = 5.5569 Å.     

In-beam electron impact mass spectrometry of aliphatic alcohols

The characteristics of the in-beam electron impact mass spectra of six isomers of undecanol as well as several 1-alkanols have been examined. In addition to the characteristic ions observed in the conventional electron impact spectra, the [2M+1]⁺, [2M+1-H₂O]⁺, [2M+1-2H₂O]⁺, [2M-R or R′]⁺, [2M-H₂O? R or R′]⁺, [2M? 2H₂O? R or R′]⁺ and [M+1? H₂O]⁺ peaks are common in the in-beam electron impact mass spectra of the undecanol isomers of structure RCH(OH)R′. Deuterium labelling experiments have shown that the extra proton in the protonated dimer ions, [2M+1]⁺, originates from the hydroxy group. The processes which produce the important peaks in the high m/e regions are discussed.     

Collisional relaxation of metastable electronic states of Fe+

The overall rate constants for collisional relaxation of metastable excited states of Fe⁺ by He, Ar, Kr, H₂, ²H₂, CO, N₂, NO, CH₄, and CH₃OH have been studied by using charge-exchange ion-molecule reaction chemistry. The rate constants vary according to the nature of the quenching reagent as well as the energy level and electron configuration of the Fe⁺ ions. In general, NO, CH₄, and CH₃OH are the most efficient quenching reagents with rate constants that approach the Langevin collision rate, whereas the reaction rates for the rare gas atoms are slow and vary depending upon the specific electron configuration of the Fe⁺ ion. The mechanism of collisional relaxation is discussed with emphasis on a curve-crossing. mechanism for the rare gas atoms. An electron-transfer mechanism is described for the relaxation of high lying (Fe⁺)*.     

11-R-dibenzo[b,e][1,4]diazepin-1-ones, the chemosensors for transition metal cations

A series of 11-R-dibenzo[b,e][1,4]diazepin-1-ones was synthesized. The luminescence, complex formation, and redox properties of these compounds were investigated. The 11-(anthracen-9-yl)-3,3-dimethyl-2,3,4,5,10,11-hexahydro-1H-dibenzo[b,e][1,4]diazepin-1-one was found to be an effective and selective fluorescent chemosensor for Cd²⁺ cations.     

The yields of1 P and1 D resonances in the He(e, 2e)He+ reaction

In the first Born approximation the dependence of the yields of the¹ P and¹ D resonances in the He(e, 2e)He⁺ reaction on the momentum transfer in the recoil peak region at incident energiesE ₀=1000 eV is studied. It is shown that in a certain range of the ejection angle and for the large momentum transfer the yield of the¹ D resonance dominates over the¹ P resonance one.     

A direct interaction between the H+-F1F0-ATPase and the K+ transport within the membrane of anaerobically grown bacteria

An N,N′-dicyclohexylcarbodiimide (DCCD) sensitive 2H⁺-K⁺ exchange with fixed stoichiometry is found in anaerobically grown bacteria and requires the F₁F₀-ATPase and the Trk or similar system. These are assumed to interact with each other directly within the membrane and to form a supercomplex, functioning as an H⁺-K⁺ pump. The findings of reversal of 2H⁺-K⁺ exchange to drive coupled synthesis of ATP as well as of DCCD-sensitive ATPase activity stimulated by K⁺ have confirmed such a “supercomplex model” for the H⁺-K⁺ pump. The phenomenon is absent in bacterial grown anaerobically in the presence of nitrate or aerobically. Under these conditions these transport systems are assumed to operate separately and the Trk or similar system works as an ionophore. Conditions have been studied under which the F₁F₀-ATPase and the Trk or similar system work cooperatively as a supercomplex with local transduction of energy, or separately via energetic mediation by Δμ_H, the transmembranedifference in transport systems within the membrane without mediation by Δμ_H during low effective energetic processes (glycolysis) may be carried out by dithiol-disulphide interchange. A source of reducing equivalents and/or Δμ_H is required. Direct interaction between transport systems with formation of supercomplexes for regulation of cell metabolism could be a general feature among membrane proteins in bacteria.     

Single‐Electron Self‐Exchange between Cage Hydrocarbons and Their Radical Cations in the Gas Phase

We show that the radical cations of adamantane (C₁₀H₁₆^.+, 1 H^.+) and perdeuteroadamantane (C₁₀D₁₆^.+, 1 D^.+) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C₂₀H₂₈^.+, 2 H^.+) is also stable (as in solution). By using the natural ¹³C abundances of the ions, we determine the rate constants for the reversible isergonic single‐electron transfer (SET) processes involving the dyads 1 H^.+/ 1 H, 1 D^.+/ 1 D and 2 H^.+/ 2 H. Rate constants for the reaction 1 H^.++ 1 D? 1 H+ 1 D^.+ are also determined and Marcus’ cross‐term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision‐controlled. Ab initio computations of the electronic coupling (H_DA) and the reorganization energy (λ) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.