Kinetics Of Photochemical Reactions Research Articles

Rate of initiation in photodegradation of poly(methyl methacrylate)

Abstract The quantum yield in the formation of free radicals upon irradiation of PMMA with 254 nm light has been measured by the inhibition method, using substituted piperidine- and pyrroline-1-oxyls as radical acceptors. Kinetics of photochemical reactions of nitroxide radicals in this polymer has been investigated and the quantum yield has been determined.

Kinetics of photochemical reactions with excitation by polychromatic radiation

For quantitative studies of photochemical reactions with low quantum yields the paper suggests application of polychromatic radiation sources whose overall intensity is by orders higher than that of currently available monochromatic sources. The more complicated and laborious calculation methods suggested for treatment of the experimental data can be solved by usual numerical procedures.

Industrial photochemistry II: influence of the stirring of unstable radical species on the kinetics of long chain photochemical reactions

The experimental rate of a photochemically initiated long chain reaction seems to be a function of the absorption profile of the light, the mean lifetime of the free radical chain carriers and finally of all the processes of homogenization of the medium. The influence of these different factors is defined theoretically and illustrated with the aid of gas—liquid-type reactions, i.e. photooxidation of heptanal and benzaldehyde and photochlorination of benzyl chloride.

Kinetics of Triplet Sublevel Selective Photochemical Reactions in the Solid State

Abstract A detailed description of the overall kinetics of photochemical reactions has to deal with photo-physical activation and de-activation rates as well as with true photochemical rates. Based on the hypothesis that for photoreactions involving the lowest excited triplet state the chemical reaction rates of the individual triplet zero-field levels have different values, a method is presented for the evaluation of these rates from bulk measurements under steady state illumination conditions. The complications arising from the detection of solid state reactions are discussed, and a simple solution is given, illustrated by a numerical example.

The effect of temperature on ozone formation in the propene/nitrogen dioxide/air system

Abstract A series of laboratory experiments were conducted to determine whether the positive correlation observed in the ambient atmosphere between ozone and temperature could, in part, be due to a temperature dependency of the photochemical smog reaction kinetics. The experiments were conducted using the propene/NO2/air system in General Motors Research Laboratories’ dynamic irradiation chamber. Over the range of temperatures investigated, 10°C to 50°C, the maximum observed ozone concentration increased with increasing temperature from 10°C to 40°C. Between 40°C and 50°C there was no further increase in ozone. In contrast, it was observed that the amount of peroxyacetylnitrate formed decreased with increasing temperature. These observations can be attributed to the temperature dependence of the thermal decomposition of peroxyacetylnitrate. At low temperatures, the increased stability of this compound makes it a sink for nitrogen dioxide; conversely, at higher temperatures, peroxyacetylnitrate is too unst...

Kinetics of photochemical reactions with elemental fluorine

Abstract The rate of the reaction of xenon to form xenon difluoride was measured. The influence of hydrogen fluoride, oxygen and nickel difluoride on the rate of this reaction was also determined. Results showing that the mechanism of the photochemical reaction between xenon and fluorine is rather complex will be discussed in detail.

General theory of photochemical reaction kinetics. II. Generalized mass action law and formal photochemical reactions kinetics

On the basis of a consistent set of photochemical kinetic equations previously obtained by the authors the effect of a resonant optical field on chemical equilibrium and on uni- and bimolecular chemical reaction kinetics is studied. We established, in particular, the relationship between steady-state concentrations of reactants in a quasicontinuous light pulse field, that is, the generalized mass action law. Dependences of reaction product yields on the intensity of coherent light were found, also the relationships between effective reaction rare constants and coherent optical field parameters as well as relaxation interactions. The results are compared with those obtained from a phenomenological description widely used in pratical photochemistry, based on elementary kinetics equations of balance type for reactant concentrations. It is shown that photochemical processes is coherent optical fields cannot be described correctly by rate (or balance) equations.

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 kinetics of photochemical reactions. Part IV. Photoreduction of carbonyl triplets by bonds other than C–H. Semiempirical calculations

A modified B.E.B.O. (Bond Energy–Bond Order) method has been used to calculate kinetic parameters for the reactions of carbonyl triplets with various substrates having X–H bonds (where X is not carbon). The results are compared with experiment where this is possible. The possibility of abstraction of hydrogen atoms bonded to oxygen or nitrogen should be taken into account rather than disregarded a priori.

Kinetics of photochemical reactions of triphenylverdazyl radicals

1. The basic pathways of photochemical destruction of an electronically excited triphenylverdazyl radical (TPV) are stripping of hydrogen from the solvent with the formation of the corresponding leuco-base, as well as its dissociation at the N-N bond with the formation, in particular, of aniline and 1,3-diphenyl-1,2,4-triazole. 2. From the concentration dependence of the rate of quenching of the radical it was found that the product of the rate constant of concentration quenching (kq) by the lifetime of the excited state (Τ) is equal to 5·103 liters/mole. 3. The dipole moment of the radical (2.94 D in the ground state) increases to 9 ± l D upon excitation.

The kinetics of photochemical reactions. Part II. Calculation of kinetic parameters for the intermolecular hydrogen abstraction reactions of the triplet state of carbonyl compounds

By application of a novel method based on the Bond-Energy–Bond-Order method, a series of kinetic parameters for reactions involving hydrogen abstraction by carbonyl triplets have been calculated. In principle the method is only applicable in the gas-phase; however, a good agreement with experiment is generally observed in reactions involving n–π* triplets in non-polar solvents. The possibility of using the method for carbonyl compounds having low lying π–π* triplets is discussed. A series of pre-calculated kinetic parameters are reported in order to allow the fast estimation of rate constants avoiding detailed calculations.

The kinetics of photochemical reactions. Part I. Application of a modified bond-energy–bond-order method to the atom abstraction reactions of excited carbonyl compounds

A modification of the Bond-Energy–Bond-Order method has been applied to the intermolecular photoreduction of carbonyl compounds. It provides a way of predicting the kinetics of these reactions as well as an insight into the role of the different thermodynamic parameters and the shape of the potential energy surface. The results strongly support the biradical model for the carbonyl triplet, in which the oxygen atom is formally considered as a free radical centre. A comparison with alternative models is also carried out.

Flash-photolysis of chloranil as studied by electron spin resonance spectroscopy.

THE KINETICS of photochemical reactions of quinones in alcoholic solutions has been a subject of considerable interest in past years[l]. In these'studies, the photolysis of quinones has been shown by optical methods to yield two different short-lived radical intermediates, the semiquinone radical anion (Q;) and the monoprotonated neutral semiquinone radical (QH.). The relative concentrations of these two radicals are strongly pH dependent, presumably due to the equilibrium Q H . * Q T + H +

Kinetics of Photochemical Reactions in RNA Molecules

The reaction kinetics of irradiated RNA molecules is analyzed for cases in which dimer formations and hydrations of uracil residues occur simultaneously. The interferences between the reactions on adjacent reactant sites and the effect of the base sequences of RNA molecules are taken into account. These correlations lead to an infinite hierarchy of kinetic equations and in a first approximation a set of second order kinetic equations is obtained. Numerical solutions are presented compared with those of usual linearized kinetic equations.

The Effect of Oxygen on the Photoconductivity of Anthracene. II

The photoconductivity in the surface layers of crystalline anthracene has been found to increase considerably over its vacuum level when it is measured in the presence of oxygen or air. Replacing the oxygen by nitrogen or argon causes no such increase. The increased photoconductivity is attributed to the photooxidation of the surface, both the light and the oxygen being necessary to produce the effect. No reaction takes place in the dark. The oxide formed is fairly unstable and dissociates in the dark. Its rate of dissociation is considerably increased when the crystal is illuminated in the absence of oxygen. The experiments indicate that the peroxide is formed by a two-stage process, the highly unstable intermediate product being formed very rapidly while the final peroxide is formed much more slowly. The nature of the intermediate product is not known for certain. The end product is very likely the oxide formed by an oxygen bridge across the 9, 10 carbon atoms of the anthracene molecule. Apparently, both the oxidation products modulate the photocurrent solely by their effect on the mean free path of the carriers; the intermediate product producing a relatively large increase while the end product produces a smaller decrease in the mean free path. It is suggested that the technique of photoconductivity measurements might prove to be useful in the study of the kinetics of certain photochemical reactions.

Diffusion Effects in the Photochemistry of Solid Films

Little work has been done, either experimentally or theoretically, on the problem of photochemical reaction kinetics in the solid phase, although this field is of practical importance in connection with the weathering of polymeric films and sheets. Reaction kinetics in solid films are greatly influenced by diffusion phenomena. The Laplace transformation technique offers a convenient method of solving many problems concerning the interaction of diffusion behavior with reaction kinetics. The method is illustrated for the diffusion of a gaseous product from an irradiated film, with and without reaction rate dependence on product concentration, and for the diffusion of a gaseous reactant into an irradiated film, with and without reaction rate dependence on reactant concentration. In each case the expression for dissolved gas concentration across the thickness of the film is computed, and the quantum yield for the reaction involved is expressed as a function of film thickness, diffusion coefficient, rate of light absorption, and initial conditions. Equations for the effect of temperature variation and of intermittent light are derived in a few instances.

The kinetics of the reaction between hydrogen and nitrous oxide.─I

Until about five years ago, the theoretical treatment of the mechanism of the oxidation of simple molecules had been comparatively neglected. Prior to this, however, considerable progress had been made in the study of the kinetics of thermal and photochemical gas reactions. That knowledge has now been successfully applied and extended to solve some of the major problems in combustion chemistry, and thereby has given rise to the development of the theory of thermal chain reactions. Hitherto, the investigation of these reactions has been confined almost entirely to oxidations by molecular oxygen. It is known, however, that many gases ignite in:nitrous oxide at about the same temperature as they do in oxygen, and it might be anticipated that here, too, a chain process is in operation. The object of studying the interaction of hydrogen and nitrous oxide was to determine whether it is a chain reaction, and if so, to make a detailed analysis of its mechanism by the kinetic method. One of the first criteria in looking for the possibility of the propagation of chains in a gaseous mixture is that the reaction must be exothermic. This condition is amply fulfilled in the present instance, for 75 k. cal. are liberated per mole of water formed. Indeed, the reaction is even more exothermic than the formation of one mole of water from hydrogen and oxygen, when only 50 k. cal. are evolved. This greater exothermicity is due to the fact that 45 k. cal. required to dissociate 1 mole of N 2 O N 2 and O, whereas the production of 1 mole of O atoms from O 2 required about 60 k. cal.