3. HOMOGENEOUS SENSITIZED SYSTEMS170 3·1 General background 170 3·1·1 Singlet oxygen as oxidant 171 3·2 Sensitized oxidation of chlorophenols by singlet oxygen 172 3·3 Homogeneous reactions of chlorophenols with metal complexes 176 4. PHOTOINDUCED DEGRADATION OF CHLOROPHENOLS 180 4·1 Free radical oxidation of chlorophenols 180 4·2 Free radical processes in semiconductor-photocatalysed degradation of chlorophenols 182 4·2·1 TiO 2 -mediated degradation of chlorophenols 183 4·3 Sensitization of chlorophenol degradation by supported catalysts 188 4·4 Radiation induced degradation of 2,4,6-trichlorophenol 191 5. REACTIONS OF CHLOROPHENOLS WITH TRANSIENT INTERMEDIATES 193 6. CONCLUSIONS 197 7. ACKNOWLEDGEMENTS 198 8. REFERENCES 198 ABSTRACTThe photodegradation of mono-, di-, tri-and pentachlorophenols in aqueous solution is surveyed from several viewpoints, namely kinetic and mechanistic, the nature of the reactive intermediates and final products, and the potential of photochemical means of treating water contaminated by chlorophenols. In direct photolysis, the roles of heterolytic and homolytic processes are considered, and the appearance of carbene, in addition to ionic and radical, intermediates noted. Sensitized photolysis deals with the roles of singlet oxygen and of a variety of metal complexes. The induced degradation of chlorophenols refers to the oxidation of chlorophenols by free radicals generated from photo-and radiolytic systems, particularly illuminated semiconductors such as titanium dioxide. The article finishes with an overview of the reactions of various types of reactive intermediates with chlorophenols. 1. dIonization potentials of 2-chlorophenol (8·66 eV) and 4-chlorophenol (8·94 eV) in organic solvents have been determined from charge transfer spectra with TCNE [51] e Values obtained for 4-chlorophenol for OH bond dissociation energies by this method, photoacoustic calorimetry [52], pulse radiolysis [53] and theoretical calculations [54] vary over the range 84·4-90·3 kcal mol -1 f Calculated in [50] g From [55] h n-butanol solution [56]

Analysis is made of reported results on the kinetics and mechanism of ascorbic acid oxidation with oxygen in the presence of cupric ions. The diversities due to methodological reasons are cleared up. A kinetic study of the mechanism of Cu2+ anaerobic reaction with ascorbic acid (DH2) is carried out. The true kinetic regularities of catalytic ascorbic acid oxidation with oxygen are established at 2.7 6 pH < 4,5 X 6 [Cu2+] d 10-3M, and Wo = K~[CU~+][DH~][O~]~.~/[H+]where ~1 (25OC) = 0.13 f 0.01 M-o.5.sec-1. The activation energy for this reaction is E1 = 22 f 1 kcal/mol. It is found by means of adding Cu+ acceptors (acetonitrile and ally1 alcohol) that the catalytic process is of a chain nature. The Cu+ ion generation at the interaction of the Cu2+ ion with ascorbic acid is the initiation step. The rate of the chain initiation at where K~J (25OC) = (1.8 f 0.3)M-1-sec-1, Ei,l = 31 f 2 kcal/mol. The reaction of the Cu+ ion with 0 2 is involved in a chain propagation, so that the rate of catalytic ascorbic acid oxidation for the system Cu2+-DHz-02 is

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PACS. 75.10.Jm -Quantized spin models. PACS. 75.40.Cx -Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.). PACS. 75.45.+j -Macroscopic quantum phenomena in magnetic systems.Abstract. -We study a system with spin-Peierls dimerization in next-nearest-neighbor (NNN) interaction and also the presence of nearest-neighbor (NN) interaction under a magnetic field. We predict the existence of different fractionally quantized magnetization plateaus with many interesting physical properties. The effect of the frustrating NNN interaction and the role of quantum fluctuations are also explored for different magnetization plateaus. We explain more physically the gapless state of spin spectrum for small values of the frustrating NNN interaction. The transition from commensurate gapped phase to incommensurate Luttinger liquid phase is of Mott-δ type.c EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2005-10183-2 c with ∆. B) We present the curves for different magnetization plateaus. m = 1/2 is independent of ∆ and J 2 J 1 in contrast with m = 1/4, 0. The curves for m = 1/4, 0 plateaus are for ∆ = 0.5, 0.4, 0.3, respectively, from top to bottom.