General theory of photochemical reaction kinetics. I. Quantum-statistical description

By methods of statistical mechanics a consistent derivation of photochemical kinetic equations has been carried out. The method of density matrix and projection operator formalism was used. A self-consistent set of kinetic equations describing the combined behaviour of field and medium under the conditions (1) interaction of field with optically active scribing the combined behavior of field and medium under the conditions of (I) interaction of field with optically active molecule transition, (2) chemical reaction in the medium, and (3) effects of relaxation processes caused by collisions has been obtained. These equations, unlike the balance equations widely used in photochemistry, correctly take into account the role of medium polarization in a photochemical process.     

The mass action law theory of micellar solutions

Theories of micellization based on the application of the mass action law to aggregation processes in surfactant solutions are reviewed. The rigorous thermodynamic justification of the approach, explanation of the critical micelle concentration, inter-relations between the main micellization parameters, and an analysis of the surface tension isotherm of a micellar solution are given. Properties of ionic micellar systems, including counterion binding and the behavior of free monomeric ions, are discussed in detail with illustrative estimations for sodium dodecylsulfate and other surfactants.     

非线性化学反应与质量作用定律: BZ-CSTR反应体系的 周期调制

以流速为扰动参数,对连续搅拌釜式反应器(CSTR)中的Belousov-Zhabotinsky(BZ)反应体系进行周期扰动,改变扰动周期或扰动强度,在某些条件下,本征振荡可以扰动控制为另一类周期振荡形式,以BZ反应的四变量Montanator模型为基础,对相应流速区域的分叉图进行了周期扰动,计算结果与实验基本定性一致,表明对本体系而言,在分叉点附近质量作用定律依然具有一定的适用性。     

Synthesis and kinetics of photochemical reactions of novel bifunctional salicylideneiminospironaphthoxazines

A series of novel photochemically bifunctional compounds was synthesized. Their molecules combine the photochromic spironaphthoxazine fragment with a salicylideneimine fragment containing various substituents. The latter can undergo intramolecular proton transfer in the electron-excited state. Photolysis products of three types, namely, open merocyanine forms having an enol or cis-keto form of the salicylideneimine moiety and a trans-keto form with the closed spiro fragment, were detected using pulse photolysis technique in toluene and methanol solutions. The spectral kinetic characteristics of the photoproducts were studied, and their quantum yields were measured. The effects of substituents in the salicylideneimine fragment and the solvent nature were discussed.     

The kinetics of photochemical deconjugation reactions of methyl geranate

The photochemical transformations of methyl geranate are analyzed in terms of the dependence of quantum yield upon base, 1,2-dimethylimidazole, concentration. The dependence of quantum yield of deconjugated esters methyl (3Z)-3,7-dimethy1-3,6-octadienoate (12), methyl 3-methy1ene-7-methyl-6-octenoate (13) and methyl (3E)-3,7-dimethyl-3,6-octadienoate (14) and the ratio of (13)/(12) and (13)/(14) upon base concentration, as well as the dependence of the ratios (methyl 2-isopropenyl-5-methy1cyc1opentanecarboxy1ate (15): deconjugated ester) (10)/(12), (10)/(13) and (10)/(14) upon the reciprocal of the base concentration, are consistent with relative rate constant ratios for ([1,5] sigmatropic shift)/(dienol deprotonation) for photodienols 15, (precursor of 13), 17 (precursor of 12) and 19 (precursor of 14) of 72, 1.0 and 85.     

Formal kinetics of nonlinear photochemical reactions in solid solutions

Analytic expressions are derived for the rate constants of some two-quantum photochemical reactions. The limits of application of the approximations are discussed. It is shown that the quantum yield in two-photon sensitization (second photon absorbed by a molecule in a triplet state) is independent of the sensitizer when the substrate concentration is high. The concept of quantum yield for a nonlinear reaction is discussed.We are indebted to V. E. Kholmogorov for guidance in this work and to our colleagues for discussion of the results.     

Refining of nonionic surfactant micellization theory based on the law of mass action

Two approaches to determining critical micelle concentration (CMC) are assessed, i.e., from the inflection point in the curve for the concentration dependence of the degree of micellization and as K^1/(1–n), where K is the constant of the law of mass action and n is the aggregation number. The latter approach makes the theory simpler, while the former explicitly expresses the critical degree of micellization via the aggregation number. The concentrations of monomers and micelles are analyzed as functions of the overall concentration of a surfactant in a micellar solution. These functions look much simpler in the graphical form as compared with their complex exact analytical representation. This has resulted in derivation of simple analytical approximations for these functions, with these approximations being useful for calculations. The concentration dependence of the surfactant diffusion coefficient has been considered based on these approximations. It turned out that this dependence not only provides the known method for determining the diffusion coefficient of micelles, but also gives the possibility in principle to determine the aggregation number from the slope of the dependence of the diffusion coefficient on the inverse concentration (counted from the CMC in the CMC units). This new method for determining the aggregation number has been tested using the literature data on the diffusion coefficient of penta(ethylene glycol)-1-hexyl ether in an aqueous solution.     

A convolution approach to the kinetics of chemical and photochemical reactions

A new method for writing kinetic equations directly in the integrated form is presented. This method applies to any species that is consumed solely through first order steps, regardless of the complexity of its formation pathways.     

Micellization theory based on the law of mass action with a variable aggregation number

Variations in the aggregation number of spherical micelles are considered within the micellization theory based on the law of mass action. The mechanism of micellization in a polydisperse aggregated system and the transition to a monodisperse model are explained. A relation between aggregation numbers and chemical potentials of molecules or ions is determined using the curve for equilibrium distribution of aggregates over the aggregation numbers. It is shown that the aggregation numbers of nonionic surfactants unambiguously grow with concentration; however, such a conclusion cannot be drawn for ionic surfactants. For the explicit concentration dependence of the aggregation number, two versions of an analog of the Langmuir equation are proposed to be used, i.e., versions comprising the total surfactant concentration and the concentration of monomers. Comparison with experimental data is carried out by the example of conventional surfactants, namely, sodium dodecyl sulfate and hexadecyltrimethylammonium bromide.     

Dendrimers as photochemical reaction media. Photochemical behavior of unimolecular and bimolecular reactions in water-soluble dendrimers

Synthesis and studies of poly(alkyl aryl ether) dendrimers, possessing carboxylic acid functionalities at their peripheries, are reported. 5-Bromopentyloxy methylisophthalate was utilized as the monomer to O-alkylate the phenolic hydroxyl groups of poly(alkyl aryl ether) dendrimers. Dendrimers of first, second, and third generations, possessing 6, 12, and 24 carboxylic acids, respectively, were thus prepared. These dendrimers were soluble in alkaline aqueous solutions, and the ensuing microenvironmental properties of the aqueous solutions were assessed by pyrene solubilization studies. Upon establishing the presence of nonpolar microenvironments within the dendritic structures, solubilizations of few organic substrates were conducted and their photochemical behaviors were assessed. Specifically, the photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether and photodimerization of acenaphthylene were conducted. These studies revealed that the product distribution and the "cage effect" were more distinct and efficient for the third generation dendrimer, than for the first and second generation dendrimers. The photochemical studies of carboxylic acid functionalized dendrimers were compared to that of hydroxyl group terminated poly(alkyl aryl ether) dendrimers.     

The theory and practice of thermoanalytical kinetics of solid-state reactions

on kinetics have become a feature of ICTAC Congresses and are generally well-attended and result in lots of useful discussion. The Workshop at ICTAC 13 continued the tradition with about 50 attendees. The workshop was chaired by Haruhiko Tanaka and Michael Brown. Six introductory presentations were made on the subject, followed by the discussion summarized below. The six introductory speakers were: (1) Ranjit K. Verma; (2) Mike Reading; (3) Crisan Popescu; (4) Erwin Kaiserberger; (5) Mike Brown; (6) Haruhiko Tanaka.     

Statistical theory for the kinetics and dynamics of roaming reactions

We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments.     

Thermal,photochemical and mass spectral reactions of 2-ethyl-5-methyl-1-pyrroline

Pyrolysis of 2-ethyl-5-methylpyrroline ( 1 ) results in formation of 2,5-dimethylpyrrole, 2-ethyl-5-methylpyrrole, 2-ethylpyrrole and other minor products. Loss of a hydrogen atom or methyl radicals is suggested as the first step in these reactions. Support for the former comes from hydrogen atom abstraction by photo-excited benzophenone. The mass spectral reactions of 1 are briefly considered.     

Corrections to the law of mass action and properties of the asymptotic t=infinity state for reversible diffusion-limited reactions

For diffusion-limited reversible A+A<==>B reactions we reexamine two fundamental concepts of classical chemical kinetics-the notion of "chemical equilibrium" and the "law of mass action." We consider a general model with distance-dependent reaction rates, such that any pair of A particles, performing standard random walks on sites of a d-dimensional lattice and being at a distance mu apart of each other at time moment t, may associate forming a B particle at the rate k+(mu). In turn, any randomly moving B particle may spontaneously dissociate at the rate k-(lambda) into a geminate pair of As "born" at a distance lambda apart of each other. Within a formally exact approach based on Gardiner's Poisson representation method we show that the asymptotic t=infinity state attained by such diffusion-limited reactions is generally not a true thermodynamic equilibrium, but rather a nonequilibrium steady state, and that the law of mass action is invalid. The classical concepts hold only in case when the ratio k+(mu)k-(mu) does not depend on mu for any mu.