Photolytic rate coefficient

The formation and decay of OH was observed following flash photolysis of mixtures containing 0.1-0.4 Torr NO2, 2.5 Torr H2 and 20-300 Torr He. A typical oscilloscope trace is shown in fig. 2. In this system OH radicals are produced as a result of processes (1) to (3). The rate coefficients for reactions (2) and (3) are /t2 = 2x lO" cm molecule" s and 3 = 5x 10 cm molecule s With the NO2 and H2 concentrations used in these experiments, the half-life of H atoms was 6 s, that of 0( D) atoms much shorter, and OH radicals were essentially formed completely within the life-time of the photolytic flash. The chlorine filter almost completely prevented long wavelength photolysis of NO2 and even if 0( P) atoms were produced, either by photolysis or by quenching of 0( /)) atoms, they would react quite rapidly with NO2... 

Typical thermal and photolytic techniques may be used but a common method is to fill an ampoule, fitted with a break-seal, with the reactant or reactants, dose, quench in liquid nitrogen, and reattach to the vacuum. This is followed by conventional analysis. Facilities for heating the sample at the radiation source can be employed. RV s are constructed from Pyrex or metal " and may in some cases be coated with graphite. However, with different surfaces, different products can result . Thin-walled RV s have to be used for a-particle-induced reactions . In the radon-sensitised reaction of Hj and O2, Lind has shown that the rate coefficient is inversely proportional to the square of the diameter of a spherical RV. 

The overwhelming majority of results listed in Table 16 have been obtained by the methods outlined above and sometimes described as low-temperature photolysis experiments, by which is meant that the radical source is photolytic and the mean experimental temperature (T ) is below 700°K. Quite recently, higher temperature data (T, — 1000°K) have been obtained for methyl [220] and ethyl [221] H-abstrac-tion reactions from alkanes from pyrolysis studies of the alkanes in flow systems. The rate coefficients so obtained for the reaction... 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

Holloway, A.L., J. Treacy, H. Sidebottom, A. Mellouki, V. Daele, G. Le Bras, and I. Barnes (2005), Rate coefficients for the reactions of OH radicals with the keto/enol tautomers of 2,4-pentanedione and 3-methyl-2,4-pentanedione, allyl alcohol and methyl vinyl ketone using the enols and methyl nitrite as photolytic sources of OH, J. Photochem. Photobiol A, Ghem., 176, 183-190. 

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

Partiele size, shape, surfaee area Density Hygroscopicity Aqueous solubility as a flmetion of pH Solubility in organie solvents Solubility in presenee of surfaetants (e.g., bile acids) Dissolution rate Wettability Partition coefficient (octanol-water) Chemical stability in solid state Photolytic stability Oxidative stability Incompatibility with formulation additives Complexation with formulation additives Systemic metabolism... 

There are two distinct types of experiments with caged compounds to be considered here those concerned with the development and assessment of new compounds and those connected with specific applications to different physiological processes. We shall discuss the two problems in succession. Detailed descriptions of methods for assessing the rates and quantities of the photolytic formation of active substrates from different caged compounds are presented by Walker et al. (1988) and Milbum et al. (1989). Some aspects of the decomposition of npe-ATP will serve as an example for such investigations. The broad absorption band of l-(2-nitrophenyl)ethyl phosphate in the near UV, allows the use of a wavelength for photolysis which results in minimum interference with that used for monitoring the reaction. The rapid ( > 10 s ) initial formation of the presumed acinitro intermediates (with extinction coefficients of 6.3 x 10 cm and 9.1 X 10 M cm at 380 nm and 406 nm respectively) and the concomi-... 

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