Constants are defined in Scheme 2. Equation 9 can, therefore, be rewritten as:. In order to increase the ratio between the number of experimental points and the number of fitted parameters, values for each solvent composition and different oxygen concentrations were fitted together. Correlation coefficients obtained range from 0. The experimental data and the fitted curves are shown in Figures 3 and 4 , for three different O 2 concentrations in solvent mixtures with different water or deuterated water contents.
In this way, the unknown rate constants 3MI and 3MI were obtained by fitting as shown in Figures 3 and 4. All the rate constants present in the kinetic Scheme 2 are listed in Table 2. The correlation obtained between the logarithm of a given rate constant with those solvent empirical parameters is indicative of the mechanism nature of the process characterized by that constant rate. The following Equation 11 was used for that:. The fitting of the total and the chemical rates to the solvent parameters of Table 3 yield h and b virtually zero:.
These results may be interpreted as a mechanism for the chemical reaction involving a hydrogen ion abstraction from water since the relative weight of parameter a is larger in H 2 O than in D 2 O.
It was already reported in the literature the role of such complexes in the substrate oxidation by singlet oxygen. Formation of such a complex is entropically unfavoured. This assumption is required to explain the pseudo-first order experimentally verified. Indeed, the 3MI concentration decrease, as a function of irradiation time, always follows an exponential law, i. The physical deactivation of singlet oxygen by 3MI is negligible when compared to the reactive one.
This last result confirms that an abstraction of a proton is involved in the mechanism. However, the inclusion of these paths in Scheme 2 demands such a large number of kinetically parameters that the fitting procedure becomes practically impossible. We are grateful to Prof. Costa for the flash photolysis facilities. The same kind of relation can also be found for other aqueous solvent mixtures. The fitted curves are shown in Figures. The only bad fit is explainable by an isotopic exchange between deuterated water and hydrogenated acetonitrile already reported in the literature.
The rate constants for collision with water deuterated or not are independent of the organic pure solvent used in the mixture. Abrir menu Brasil. Abrir menu. Keywords: singlet oxygen; psoralen; solvent effects. Davies, M. Photobiol , 30 , Foote, C. H15, p. Wilkinson, F. Data , 24 , Zebger, I. Bilski, P. Lemp, E. Palumbo, M. Aubry, J. Carreno, M. Murov, S. Bensasson, R. Kamlet, M. Casassas, E.
Wentworth, P. Zhu, X; Larsen, N. Pilling, M. Bardez, E. A , , Xu, X. Publication Dates Publication in this collection 10 Oct Date of issue History Accepted 17 Dec Received 18 July Cristina Sousa. Figures 9 Formulas Google Google Scholar. We describe a model for the latter process that includes competition with precursor destruction and the effect of sample structure. This allows us to put the ultraviolet photon, low-energy electron, and fast-ion experiments on a common footing for the first time.
The formation of the trapped oxygen atom precursor is favored by the preferential loss of molecular hydrogen and is quenched by reactions with mobile H. The presence of impurity scavengers can limit the trapping of O, leading to the formation of oxygen-rich molecules in ice. Rate equations that include these reactions are given and integrated to obtain an analytic approximation for describing the experimental results on the production and loss of molecular oxygen from ice samples.
In the proposed model, the loss rate varies, roughly, inversely with solid-state defect density at low temperatures, leading to a yield that increases with increasing temperature as observed.
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