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A common misconception about the Eyring equation

Linearization and direct fitting to the Eyring equation both give the entropy of activation with the same reliability as that of the enthalpy of activation.

One- versus two-electron oxidation with peroxomonosulfate ion: reactions with iron(II), vanadium(IV), halide ions, and photoreaction with cerium(III).

The kinetics of the redox reactions of the peroxomonosulfate ion (HSO(5)(-)) with iron(II), vanadium(IV), cerium(III), chloride, bromide, and iodide ions were studied. Cerium(III) is only oxidized upon illumination by UV light and cerium(IV) is produced in a photoreaction with a quantum yield of 0.33 +/- 0.03. Iron(II) and vanadium(IV) are most probably oxidized through one-electron transfer producing sulfate ion radicals as intermediates. The halide ions are oxidized in a formally two-electron process, which most likely includes oxygen-atom transfer. Comparison with literature data suggests that the activation entropies might be used as indicators distinguishing between heterolytic and homolytic cleavage of the peroxo bond in the redox reactions of HSO(5)(-).

The Role of Stochastic Models in Interpreting the Origins of Biological Chirality

This review summarizes recent stochastic modeling efforts in the theoretical research aimed at interpreting the origins of biological chirality. Stochastic kinetic models, especially those based on the continuous time discrete state approach, have great potential in modeling absolute asymmetric reactions, experimental examples of which have been reported in the past decade. An overview of the relevant mathematical background is given and several examples are presented to show how the significant numerical problems characteristic of the use of stochastic models can be overcome by non-trivial, but elementary algebra. In these stochastic models, a particulate view of matter is used rather than the concentration-based view of traditional chemical kinetics using continuous functions to describe the properties system. This has the advantage of giving adequate description of single-molecule events, which were probably important in the origin of biological chirality. The presented models can interpret and predict the random distribution of enantiomeric excess among repetitive experiments, which is the most striking feature of absolute asymmetric reactions. It is argued that the use of the stochastic kinetic approach should be much more widespread in the relevant literature.

Deterministic Kinetics in Chemistry and Systems Biology

This bookgives a concise overview of the mathematical foundations of kinetics used in chemistry and systems biology. The analytical and numerical methods used to solve complex rate equations with the widely used deterministic approach will be described, with primary focus on practical aspects important in designing experimental studies and the evaluation of data. The introduction of personal computers transformed scientific attitudes in the last two decades considerably as computational power ceased to be a limiting factor. Despite this improvement, certain time-honored approximations in solving rate equations such as the pre-equilibrium or the steady-state approach are still valid and necessary as they concern the information content of measured kinetic traces. The bookshows the role of these approximations in modern kinetics and will also describe some common misconceptions in this field

Mechanism-Based Chemical Understanding of Chiral Symmetry Breaking in the Soai Reaction. A Combined Probabilistic and Deterministic Description of Chemical Reactions

The experimentally observed distribution of enantiomers in the Soai reaction is interpreted in this Article on the basis of a chemical mechanism using a newly developed stochastic kinetic method, accelerated Monte Carlo simulation combined with deterministic continuation and symmetrization. The method is in principle suitable for handling large mechanisms with realistic particle numbers and could be useful for any case where the kinetics of a process shows inherent random fluctuations. The mechanism shows how a slow initial reaction combined with efficient and highly enantioselective autocatalysis can give rise to chiral symmetry breaking under completely nonchiral external conditions.

Light-induced multistep redox reactions: The diode-array spectrophotometer as a photoreactor

The light source of a photometer may induce chemical reactions in photosensitive reactive systems. Diode-array spectrophotometers are particularly suitable for producing such phenomena. This paper provides an overview on how this equipment can be used as a photoreactor. The principles of various techniques to control the intensity and spectral region of the illuminating light are discussed in detail. It will be shown that the quantum yields of various photochemically induced redox reactions can be determined by exploiting specific features of diode-array spectrophotometers. Kinetic coupling between primary photo-chemical and secondary thermally activated reaction steps are utilized to explore intimate details of composite redox reactions. Key aspects of the method applied are demonstrated via the photoreactions of 2,6-dichloro-1,4-benzoquinone (DCQ), the photoinduced autoxidation of S(IV) and a photochemically activated redox reaction of the chlorate ion.

Structure–reactivity relationships and substituent effect additivity in the aqueous oxidation of chlorophenols by cerium(IV)

The oxidation of all 19 chlorophenols and unsubstituted phenol by cerium(IV) was studied in an acidic aqueous solution in order to carry out a systematic test of chlorine substituent effects on the reactivity. All reactions were found to show 2 ∶ 1 cerium(IV)  ∶ phenol stoichiometry and a simple second-order rate equation. Rate constants did not correlate well with characteristic parameters such as pK values, carbon-13 NMR chemical shifts or Hammett substituent constants. Nevertheless, a strict additivity of chlorine substituent effects was found in both characteristic and reactivity parameters. The data suggest that a proton-coupled electron transfer mechanism could be operative. 2,4,6-Trichlorophenol was found to show exceptionally high reactivity towards cerium(IV).

Stochastic chemical kinetics

Stochastic chemical kinetics : , Stochastic chemical kinetics : , کتابخانه مرکزی دانشگاه علوم پزشکی ایران

The effect of parity violation on kinetic models of enantioselective autocatalysis

A detailed analysis of enantioselective autocatalytic reaction models is carried out using both deterministic and stochastic approaches. The models include the small differences between the total energies of enantiomers (DeltaE(PV) approximately 10(-13) J mol(-1)) that arise due to parity violation. Different possible orders of autocatalysis are considered, and the reasons why the predictions of the stochastic and deterministic approaches are different under certain conditions are explored. The continuous time discrete state (CDS) stochastic approach is clearly superior to the deterministic approach. It is concluded that the small difference between enantiomers caused by DeltaE(PV) cannot be amplified under conditions reasonable for the generation of biological chirality. It also seems highly unlikely that biological chirality was determined by any intrinsic difference between enantiomers.

Central role of phenanthroline mono- N -oxide in the decomposition reactions of tris(1,10-phenanthroline)iron(II) and -iron(III) complexes

1,10-Phenanthroline mono-N-oxide (phenO) is a product of the decomposition of tris(1,10-phenanthroline)iron(III), Fe(phen)(3)(3+), and has a slight autocatalytic effect on the overall reaction. The mechanism is proposed to involve Fe(phen)(3)(4+) as a minor intermediate. The addition of phenO significantly influences the kinetic features of the decomposition of Fe(phen)(3)(3+) and the oxidation of Fe(phen)(3)(2+) by HSO(5)(-). The autocatalytic decomposition explains the difficulties in the preparation of Fe(phen)(3)(3+) and may contribute to exotic kinetic phenomena studied using Fe(phen)(3)(3+)/Fe(phen)(3)(3+) as a supposedly innocent indicator.