Showing papers in "Faraday Discussions of The Chemical Society in 1973"
TL;DR: In this paper, the effect of reagent energy on the rotational dependence of reaction rate was investigated using LEPS (London, Eyring, Polanyi, Sato) potential energy hypersurfaces.
Abstract: Experiment and theory have been used to ascertain the effect of various forms of reagent energy on the dynamics of three reactions; (a) F + HCl → HF + Cl, (b) F + D2→ DF + D and (c) H + Cl2→ HCl + Cl. The principal experimental technique was the arrested relaxation form of the infra-red chemiluminescence method, coupled with a quite wide variation in the temperature of the atomic or the molecular reagent. In addition, a novel approach was employed in which the “fluorescence depletion” of specified vibrational-rotational (v, J) states was measured, and hence the relative rate of reaction of these quantum states was obtained. The theoretical technique was the 3D classical trajectory method using LEPS (London, Eyring, Polanyi, Sato) potential-energy hypersurfaces. Some support was obtained from experiment and theory for the following generalisations. (1)ΔT and ΔV enhance the reaction rate-constant, but the former is more effective for these substantially-exothermic reactions. (ΔT and ΔV denote reagent translation and vibration enhancements, in excess of the activation-barrier). (2) On the average ΔT→ΔT′+ΔR′(the primed energies refer to reaction products; ΔR′ is enhancement in product rotational energy). (3) On the average ΔV→ΔV′. (1)–(3) are in accord with findings from earlier trajectory studies. The effect of substantial increase in collision-energy can be understood in terms of a contribution from “induced repulsive energy-release”, and that of increased reagent vibrational energy in terms of both induced repulsive energy-release and (more important)“induced attractive energy-release”. For the reaction F + HCl the increase in rate constant with reagent vibrational quantum number, k(v), was obtained for ν= 0–2, experimentally and theoretically. The fluorescence-depletion method was used for a preliminary measurement of the rotational dependence of reaction rate, k(J), in F + HCl(ν= 1). An estimate was obtained for the variation in threshold collision-energy, Ev′J′0, with the v′J′ state being formed in the reaction F + HCl(v= 0).
220 citations
TL;DR: In this article, the use of laser-induced fluorescence as a molecular beam detector for the measurement of internal state distributions of reaction products is presented and applied to the reactions of barium with the hydrogen halides.
Abstract: The use of laser-induced fluorescence as a molecular beam detector for the measurement of internal state distributions of reaction products is presented and applied to the reactions of barium with the hydrogen halides. It is found that most of the reaction exoergicity appears as translational energy of the products and that the total reactive cross section is positively correlated with the average fraction of the exoergicity appearing as vibrational excitation.
144 citations
TL;DR: Theoretical models for the dissolution of iron, the cupri-cuproloride reduction on an inert electrode and the reversible transition from active to passive state of iron were given and checked by experiment as discussed by the authors.
Abstract: When an intermediate species is involved in an electrochemical reaction and is adsorbed on the metal surface, the coverage most often depends on the potential. In this case, impendance measurements allow for the overall reaction to be split into elementary processes on the basis of different relaxation times. Theoretical models are given and checked by experiment for the dissolution of iron, the cupri-cuproloride reduction on an inert electrode and the reversible transition from active to passive state of iron.
123 citations
TL;DR: In this article, the authors employed specular reflectance spectroscopy to identify reaction intermediates and study the kinetics of their decay reactions, and found that solution-free anion radical CO[graphic omitted] was the intermediate in the reduction of CO2 in aqueous solution and in a number of aprotic solvents.
Abstract: Modulated specular reflectance spectroscopy was employed to identify reaction intermediates and to study the kinetics of their decay reactions. The solution-free anion radical CO[graphic omitted] was found to be the intermediate in the reduction of CO2 in aqueous solution and in a number of aprotic solvents. In the former case, the rate constant for protonation was found to be 5.5 1. mol–1 s–1. In aprotic systems, the mechanism for formation of oxalate involved coupling of CO[graphic omitted] with CO2 and not a dimerisation of the former, the rate constant being 7.5 × 103 l. mol–1 s–1. The decay reaction of the thianthrene cation radical generated anodically in acetonitrile containing controlled concentrations of water was found to be pseudo first order; possible mechanisms are discussed.
108 citations
68 citations
TL;DR: In this paper, the a.c. impedance for a reaction involving an intermediate species is discussed and compared with other work in this area, and the authors compare their work with other works in the literature.
Abstract: The a.c. impedance for a reaction involving an intermediate species is discussed and compared with other work in this area.
67 citations
TL;DR: In this paper, the cross sections of hydrogen atoms scattered by rare gases have been measured in the velocity range 14.14 × 14 × 105 cm s−1(001 < Ecm < 100 eV).
Abstract: Integral elastic cross sections of hydrogen atoms scattered by rare gases have been measured in the velocity range 14–14 × 105 cm s–1(001 < Ecm < 100 eV) The experimental data have been fitted to the Lennard-Jones and exp (α,6) potential models and are found to provide information on the attractive and repulsive parts of the potential The potential well parameters Iµ, Rm and the Born-Mayer repulsive potential parameters A, α have been determined for all of the rare gases These Iµ, Rm parameters, in combination with the corresponding rare gas dimer parameters, are shown to be consistent with combining rules
65 citations
TL;DR: In this article, it was shown that the product angular and velocity distributions indicate that a statistical collision complex is formed which persists for many vibrational periods and at least a few rotational periods.
Abstract: Molecular beam experiments show that diatomic molecule exchange reactions involving an ionic bond proceed readily at thermal collision energies. Previous work found two such reaction families, alkali halide + alkali halide and dialkali + halogen. This study adds a third, alkali halide + halogen. For CsI + Cl2→ CsCl + ICl the product angular and velocity distributions indicate that a statistical collision complex is formed which persists for many vibrational periods and at least a few rotational periods. This complex presumably corresponds to the alkali trihalide salt, Cs+(ClICl)–. For the CsBr + ICl reaction only formation of CsCl + IBr has been observed, even at collision energies well above the endoergic threshold for formation of CsI + BrCl. The energy disposal is not statistical and the product angular distribution is quite asymmetric about 90°, indicating that a large fraction ([graphic omitted] ⅓ to ½) of the collision complexes break up in less than one rotational period. The preferred directions of emission are 0° for CsCl and 180° for IBr, where 0° and 180° designate the incident beam directions (c.m. system) for ICl and CsBr, respectively. These properties can be plausibly interpreted in terms of the electronic structure of the trihalide group.
56 citations
TL;DR: In this paper, the origin of the nonstatistical ratio of reactive to nonreactive product is examined and its dependence on angular momentum is pointed out and related to evidence for the participation of linear K-Cl-Na-Br.
Abstract: Some results of a classical trajectory calculation on NaBr + KCl → NaCl + KBr, a reaction which proceeds via a long-lived collision complex, are presented. The dynamics of the complex are briefly considered and complex lifetimes and the equilibration of vibrational energy are described. The origin of the nonstatistical ratio of reactive to nonreactive product is examined. Its dependence on angular momentum is pointed out and related to evidence for the participation of linear K–Cl–Na–Br.
55 citations
TL;DR: In this article, a method for calculating the eigenvalues for systems not permitting separation of variables is described, where trajectories are supplemented by interpolation to connect open ends of quasi-periodic trajectories.
Abstract: Semiclassical theory for bound states is discussed and a method is described for calculating the eigenvalues for systems not permitting separation of variables. Trajectory data are supplemented by interpolation to connect open ends of quasi-periodic trajectories. The method is also applied to quasi-bound states.Previously, semiclassical S-matrix theory has focused on “direct” reactions. Processes involving complexes (compound state resonances) are treated in the present paper and an expression is derived for the S-matrix. Use is made of the above analysis of quasi-bound states and of trajectories connecting those states with open channels. The result deduced for the S-matrix has the expected factorization property, and expressions are given for computing the quantities involved. Some extensions and applications will be described in later papers. An implication for classical trajectory calculations of complexes is noted.
51 citations
TL;DR: In this paper, an extensive set of studies of the reactions of fluorine atoms with olefins, dienes, aromatic and heterocyclic molecules by the method of crossed molecular beams is presented.
Abstract: This paper briefly reviews an extensive set of studies of the reactions of fluorine atoms with olefins, dienes, aromatic and heterocyclic molecules by the method of crossed molecular beams. From measurements of the angular distribution of products and the recoil energy spectrum of products it is inferred that the statistical theories of reaction are inadequate. Although RRKM theory seems capable of predicting the ratios of cross sections for pairs of products, the same theory is incapable of accounting for the internal energy distribution of the products. The failure of the hypothesis that internal energy exchange is more rapid than chemical reaction is discussed and a proposal advanced to explain the origin of the failure.
TL;DR: In this paper, the authors fitted a detailed 6-atom potential energy surface to molecular beam data for Rb + CH3I → RbI+CH3, and used the potential surface to predict the energy dependence of the reactive cross-section.
Abstract: Using empirical Monte Carlo trajectory procedures, we fitted a detailed 6-atom potential energy surface to molecular beam data for Rb + CH3I → RbI + CH3. Measurements at a single energy of product recoil velocity and scattering angle distributions, and of the effect of aligning the CH3I, were employed. We then used the potential surface to predict the energy dependence of the reactive cross-section, and to ascertain the consequences of omitting the CH3 hydrogens from the calculations. Experimental data with which to compare our results are expected to appear.
TL;DR: In this paper, bias-dependent electroreflectance (ER) theory is developed to account for field-induced plasma resonances, and the use of ER techniques for resolving critical points in the band structure of metals is demonstrated.
Abstract: The optical and electronic properties of Au and Pt electrode surfaces have been investigated by in situ electrochemical modulation spectroscopy (ECMS). The surface field is shown to have a pronounced effect on the dielectric function of the surface layer. A new bias-dependent electroreflectance (ER) theory is developed to account for field-induced plasma resonances, and the use of ER techniques for resolving critical points in the band structure of metals is demonstrated. In ECMS studies of the adsorbed hydrogen layer on Pt, ER effects have been taken into account and it is shown that both strongly- and weakly-bound forms induce a decrease in the reflectance. The broad, intense absorption of these species is attributed to photon-excited electron transitions from resonant bound-state levels to empty Pt conduction-band levels above the Fermi energy.
TL;DR: In this paper, individual rotational quantum transitions have been resolved in the energy loss spectra of Li+ ions after scattering by H2 molecules at the centre of mass angles between 14 and 32°.
Abstract: Individual rotational quantum transitions have been resolved in the energy loss spectra of Li+ ions after scattering by H2 molecules. Time of flight measurements at 0.6 eV centre of mass energy yield differential inelastic cross sections for the j= 0 →j= 2 and j= 1 →j= 3 rotational transitions relative to the differential elastic cross section (j= 0 →j= 0, j= 1→j= 1) at centre of mass angles between 14 and 32°.
TL;DR: In this paper, the authors address the following problems: (a) optimal means of characterization of the product (or reactant) energy state distribution, (b) compaction of the voluminous body of data (or computer-simulation thereof) comprising such distributions, and (c) development of measures of the specificity of energy release, of the selectivity of the energy consumption (and their mutual dependence).
Abstract: The role of energy in promoting chemical change and the reverse problem of the energy distribution of the reaction products are of fundamental interest in the study of chemical dynamics. Since the earliest atomic flame experiments it has been known that exoergic elementary reactions often lead to products with high internal excitation. More recently, direct evidence from molecular and ion beam experiments indicates that, for reactions with an activation barrier, reagent internal energy is often more effective in promoting reaction than is translational energy. In addition, detailed product state distributions for exoergic reactions determined by chemiluminescence, chemical laser and molecular beam methods have frequently indicated substantial population inversion. Both phenomena (obviously related, through microreversibility) can be qualitatively understood from potential surface considerations (via classical trajectory analysis), but several problems remain.We address ourselves to the following questions: (a) optimal means of characterization of the product (or reactant) energy state distribution, (b)compaction of the voluminous body of data (or computer-simulation thereof) comprising such distributions, and (c) development of measures of the specificity of the energy release, of the selectivity of the energy consumption (and their mutual dependence).As a link between the reactive scattering and the non-equilibrium statistical mechanics we use the concept of the entropy(or information) of the product (or reactant) state distribution. The required measure of specificity(or of selectivity) is provided by the concept of the surprisal of a particular outcome. This approach leads to a means of dealing with molecular beam measurements of product angular and translational energy distributions.
TL;DR: In this article, the reaction of K atoms with CF3I molecules has been studied using oriented molecules and the authors find possible explanation in two (probably related) mechanisms, such as charge transfer to a repulsive ionic state, and the ability of the K atom to fly past the CF3 group and strike the I.
Abstract: The reaction of K atoms with CF3I molecules has been studied using oriented molecules. If the I end is closest to the incoming K atoms, KI is formed and recoils backwards in the centre of mass system. If the CF3 end is closest to the incoming K atoms, KI is again formed but scatters forwards in the CM. This is in contrast to the oriented CH3I + Rb reaction in which no forward scattering is observed from configurations with CH3 closest to the incoming Rb.These observations find possible explanation in two (probably related) mechanisms. Even for CF3I oriented in what appears to be an unfavorable configuration, it is possible for the K atom to fly past the CF3 group and strike the I. This is expected to be less likely in CH3I because the CH3 shields the I more than does CF3.For CF3I (but not CH3I) charge transfer to a repulsive ionic state is postulated. The products are expected to recoil along the direction of the molecular axis and this roughly accounts for the experimental angular distribution.
TL;DR: In this paper, the reaction potential energy surfaces show a broad shallow basin at small internuclear distances, and suggest that the reaction dynamics must be direct or at most involve a short-lived complex.
Abstract: Reactive scattering measurements for the alkali atom-dimer exchange reactions M′+ M2→ MM′+ M are reported, where M′= Na, K and M2= Cs2, Rb2, K2. The total reaction cross sections are large [graphic omitted] 100 A2, indicating that a major fraction of collisions, captured by the long range van der Waals attraction, lead to reaction. Very little (∼10 %) of the available energy is disposed into product translation, consequently the MM′ dimer products are highly vibrationally excited (∼75 % of the MM′ bond energy). Only an indication of the centre-of-mass differential reaction cross sections can be deduced from the data. Calculations of the reaction potential energy surfaces show a broad shallow basin at small internuclear distances, and suggest that the reaction dynamics must be direct or at most involve a short-lived complex.
TL;DR: In this paper, the electrochemical behavior of the cis-trans-pairs: diethyl maleate (DEM), Diethyl fumarate (DEF) and cis- and trans-dibenzoylethylene (CDBE and TDBE) in N,N-dimethylformamide solutions was investigated using cyclic and rotating ringdisc electrode (RRDE) voltammetry and bulk electrolysis.
Abstract: The electrochemical behaviour of the cis-trans-pairs: diethyl maleate (DEM), diethyl fumarate (DEF) and cis- and trans-dibenzoylethylene (CDBE and TDBE) in N,N-dimethylformamide solutions was investigated using cyclic and rotating ring-disc electrode (RRDE) voltammetry and bulk electrolysis. In both cases the cis-isomer is reduced at more negative potentials than the trans-isomer. Although the radical anion formed from the trans-isomer is fairly stable, that of the cis-isomer undergoes a rapid isomerization to form the trans-radical anion and also reacts more rapidly in either self- or cross-coupling reactions. The electron spin resonance (e.s.r.) spectra obtained by electrolytic reduction of either DEM or DEF has been re-interpreted as being caused by different conformations of the trans-radical anion. Identical e.s.r. spectra are also obtained from reduction of CDBE and TDBE; this is attributed to a single conformation of TDBE radical anion.
TL;DR: In this paper, a general semiclassical theory that combines exact classical dynamics and quantum superposition is presented, where the quantum-like degrees of freedom are quantized semiclassically via use of double-ended boundary conditions, while the unquantized classical-like degree of freedom enter only through a phase space average over their initial coordinates and momenta.
Abstract: Within the framework of a general semiclassical theory that combines exact classical dynamics and quantum superposition it is shown how a certain averaging procedure allows one to treat some degrees of freedom in a strictly classical sense while others are quantized semiclassically. This enormously simplifies the application of the theory to three-dimensional collision systems and also leads to an interesting formal structure in the theory: the quantum-like degrees of freedom are quantized semiclassically via use of double-ended boundary conditions, while the unquantized classical-like degrees of freedom enter only through a phase space average over their initial coordinates and momenta. Preliminary results for vibrational excitation of H2 by He are presented and compared with available quantum mechanical calculations.
TL;DR: Differential elastic scattering cross sections for the systems H2+ O2, SF6, NH3, CO, and CH4 have been obtained from crossed beam studies as mentioned in this paper, and it is concluded that anisotropy effects do not seem to be important in these H2(D2) systems.
Abstract: Differential elastic scattering cross sections for the systems H2+ O2, SF6, NH3, CO, and CH4 and for D2+ O2, SF6, and NH3 have been obtained from crossed beam studies. In all cases, rapid quantum oscillations have been resolved which permit the determination of intermolecular potential parameters if a central-field assumption is adopted. These potentials were found to be independent of both the isotopic form of the hydrogen molecule, and the relative collision energy. As a result of this, and the ability of these spherical potentials quantitatively to describe the measured scattering, it is concluded that anisotropy effects do not seem to be important in these H2(D2) systems.
TL;DR: In this article, a detailed center-of-mass differential reaction cross sections have been obtained (at three energies), showing the expected predominant back-scattering of the fast-recoiling KI product.
Abstract: Molecular beam scattering (velocity-analysis) experiments have been carried out for the now-classic “rebound” reaction K + CH3I → KI + CH3 over a range of relative translational energies text-decoration:overlineET from 0.077 to 0.16 eV. Detailed centre-of-mass differential reaction cross sections have thereby been obtained (at three energies), showing the expected predominant “back-scattering” of the fast-recoiling KI product. There is very little coupling between the angular and recoil energy distributions. The differential solid-angle cross section peaks at 180°(with a half-angle close to 90°) with shape essentially independent of text-decoration:overlineET over the range of the experiments. The distribution of final translational energy ET′ is rather sharp, showing a peak at about 60 % of the total available energy. The average recoil energy text-decoration:overlineET′ increases with text-decoration:overlineET.These results are combined with previous experimental findings from this laboratory on the CH3I–K, —Rb reactions (the “alignment effect” and the ET-dependence of the total reaction cross section), and compared with observations of other groups. A current, overall, experimental summary is presented, to serve as a target for theory.
TL;DR: In this article, the photocurrents of mercury in contact with CO2, HCOOH and CH3Cl solutions are analyzed and the indirect formation of CO[graphic omitted] is inferred.
Abstract: The photocurrents of mercury in contact with CO2, HCOOH and CH3Cl solutions are analyzed. The CO[graphic omitted] radical anion is oxidized at potentials anodic to –1.3 V against N.C.E. and is completely reduced in a 1-electron process at sufficiently cathodic potentials. A proton donor is involved in the reduction process and there is a change in the reduction mechanism when going from neutral to acid solutions. The photocurrents for formic acid solutions in the presence of HCOO– ions are interpreted and the indirect formation of CO[graphic omitted] is inferred. Methyl radicals are found to be reduced in a 1-electron process at potentials cathodic to –1.5 V and cannot be oxidized on a mercury electrode. The range of electrode potential stability for these radicals is discussed in relation to electrosynthetic processes.
TL;DR: In this article, a method is proposed for resolution of processes involving more than one electrochemisorbed species when the electroactivity of, e.g., two species arises over a common potential range.
Abstract: In certain electrocatalytic reactions, co-adsorption of more than one electroactive intermediate may arise. The charge under the overall potentiodynamic current-potential profile for the adsorbed species may not then be used to evaluate coverage by the species. A method is proposed for resolution of processes involving more than one electrochemisorbed species when the electroactivity of, e.g., two species arises over a common potential range. The method is based on the different kinetic relaxation response of “reversibly” electrosorbed species from that for more “irreversibly” electrosorbed species. Applications to the case of reduction of adsorbed nitriles at Pt enables a two-stage mechanism to be formulated and in the reduction of benzene the charges for co-adsorption of H and benzene reduction can be distinguished. In the initial stages of Pt surface oxidation, the method enables the reversibly electrosorbed OH species to be distinguished from more irreversibly reactive rearranged surface oxide.The behaviour of chemisorbed CH3CN and the oxide film on Pt below a monolayer have also been investigated by a.c. modulated reflectance over a range of a.c. frequencies and wavelengths of light.
TL;DR: In this paper, the angular distribution measurements of methyl iodide reactive scattering from crossed beams of methyl radicals and halogen molecules, ICl, IBr, I2 are reported.
Abstract: Angular distribution measurements of methyl iodide reactive scattering from crossed beams of methyl radicals and halogen molecules, ICl, IBr, I2 are reported. The methyl radical beam is generated by pyrolysis of azomethane flowing through a heated tantalum tube at ∼1600 K; the halogen molecule beam issues from a nozzle beam source at ∼380 K. Reactively scattered methyl iodide is detected by an electron bombardment mass spectrometer. The differential cross sections peak sideways in centre-of-mass coordinates, showing a marked similarity to the corresponding deuterium atom–halogen molecule reactions. Qualitative molecular orbital theory offers a comprehensive rationalisation of methyl radical, deuterium atom and halogen atom reaction dynamics with halogen molecules.
TL;DR: There are a number of areas in chemical kinetics where generalizations have been helpful in interpreting and correlating a large body of experimental data in gas phase or solution reactions.
Abstract: There are a number of areas in chemical kinetics where generalizations have been helpful in interpreting and correlating a large body of experimental data in gas phase or solution reactions I am reminded here of Bronsted’s relation between rate constants and equilibrium constants, Eyring’s and Evans and Polanyi’s work on transition state theory, Rice, Ramsperger and Kassel’s work on unimolecular reaction, later augmented to RRKM, treatments of the curve crossing problems, Hammett’s σρ relation and acidity function, and the subsequent equations they stimulated, theories of three-body recombination of atoms and of electron transfers in solution and at electrodes, simple BEBO calculations on activation energies, the Woodward-Hoffman rules and their implications for activation energies, Breit-Wigner and later treatments of resonances, models for ion-molecule reactions, to name a few In the case of inelastic non-reactive collisions one would include the SSH theory, distorted wave theory for some systems, the Anderson theory of spectral line broadening and its later extensions
The interested observer, as well as the seasoned practitioner, might well ask which of these generalizations of analytical thought apply to current problems of molecular dynamics, what new ones have been developed, or what experimental generalizations are there, if any, which literally cry out for a theoretical answer He might ask, too, whether the present field is sufficiently different from the previous ones that the approximate analytical theory will be literally swept under by a Spartan-like phalanx of exact classical trajectories and their semiclassical and quantum mechanical counterparts, with much analytical thought going into this army
We shall not attempt to answer all of these questions here, but shall summarize instead some of the trends which appear to be developing in the field Calculations in the area are diverse, and some classification would be useful A possible scheme for dynamical calculations is proposed in this introductory paper
TL;DR: In this paper, the uniform asymptotic evaluation of multidimensional integrals for the S-matrix in semiclassical collision theory is considered and a non-separable two-dimensional canonical integral with four coalescing saddle points is presented.
Abstract: The uniform asymptotic evaluation of multidimensional integrals for the S-matrix in semiclassical collision theory is considered. A concrete example of a non-separable two-dimensional integral with four coalescing saddle points is chosen since it exhibits many of the features of more general cases. It is shown how a uniform asymptotic approximation can be obtained in terms of a non-separable two-dimensional canonical integral and its derivatives. This non-separable two-dimensional canonical integral plays a similar role to the Airy integral in one-dimensional integrals with two coalescing saddle points. The uniform approximation is obtained by applying to the two-dimensional case the asymptotic techniques introduced by Chester et al. for one-dimensional integrals. An exact series representation is obtained for the canonical integral by means of complex variable techniques. The series representation can be used to evaluate the canonical integral for small to moderate values of its arguments, whilst for large values of its arguments existing asymptotic techniques may be used.
TL;DR: The potential dependence for the production of propane, ethane and methane from propane adsorbate is given in this paper, where the technique of electrochemical mass spectrometry is described and applied to the oxidation of the propane at platinum.
Abstract: The technique of electrochemical mass spectrometry is described and applied to the oxidation of propane at platinum. The potential dependence for the production of propane, ethane and methane from propane adsorbate is given. Using propane-2-13C, it has been shown that all propane formed on reductive desorption originates from the carbon skeleton of the original propane and that methane arises with equal probability from all the carbon atoms in the original propane. The adsorbate formed at 0.25 V consists of a mixture of surface species whose stoichiometries could include C3H8, C3H7, C2H6, and C2H5. All adsorbate can be oxidized quantitatively to carbon dioxide.
TL;DR: In this paper, the free energies for the first and second transition states in the electrochemical reduction of anthraquinone are equal at the potential of zero photocurrent with pH.
Abstract: Light from a mercury lamp shone through a semi-transparent electrode into a solution containing a mixture of anthraquinone and its corresponding hydroanthraquinone generates semiquinones inside the diffusion layer of the electrode. The semiquinone species can either be oxidized or reduced on the electrode. The rate constants for their electrochemical reactions and for their homogeneous recombination can be measured. At the potential of zero photocurrent the electrochemical rate constants for the reduction and oxidation of the semiquinone are equal. At this point the free energies for the first and second transition states in the electrochemical reduction of anthraquinone are equal. The variation of the potential of zero photocurrent with pH determines the electrochemical mechanism of the anthraquinone.
TL;DR: In this paper, the high energy solution of the multi-curve crossing problem involving many vibronic levels is shown to reduce to that of a single curve crossing modulated by Franck-Condon factors.
Abstract: The high energy solution of the multi-curve crossing problem involving many vibronic levels is shown to reduce to that of a single curve crossing modulated by Franck–Condon factors. A quantitative validity criterion is derived.
TL;DR: The theory of alternating current experiments at a ring-disc electrode has been confirmed using the ferri-ferrocyanide system using phase-sensitive detection at higher frequencies as discussed by the authors.
Abstract: The theory of alternating current experiments at a ring-disc electrode has been confirmed using the ferri-ferrocyanide system The frequency range has been extended using phase-sensitive detection At higher frequencies the passage of current through the electrolyte resistance causes charging of the ring electrode double layer and the results have to be corrected for this Results for the Br2/Br– system show that species are adsorbed on the electrode The most probable hypothesis is that Br· is the species involved and an estimate can be obtained for its surface coverage This allows the calculation of the rate constant for e + Br·→ Br–, and the mechanistic implications of the value found are discussed