Terence J. Kemp

Photochemical transformation of polychlorinated phenols

Abstract To increase our understanding of the photocatalytic effects in the transformation of chlorinated organic compounds, the detailed mechanism of the direct photolysis of polychlorinated phenols has been established under aerobic and anaerobic conditions. The quantum yield of photolysis of 2,4,5-trichlorophenol (2,4,5-TCP) has been determined at different wavelenghts and pH values. It has been shown that direct photolysis of polychlorinated phenols is accompanied by chloride ion detachment from different positions of the benzene ring with the formation of 2,5-dichlorohydroquinone and dichlorocyclopentadiene derivatives in the case of 2,4,5-TCP, as well as polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF). Based on the 2,4,5-TCP direct photolysis products, it has been concluded that chlorophenol free radicals are formed as a result of CCl cleavage, which probably act as the origin of highly chlorinated PCDD and PCDF. To utilize visible light in the photocatalytic destruction of organic pllutants, we investigated the kinetics of transformation of polychlorinated phenols in systems involving singlet oxygen formation (dye-sensitized reactions with Rose Bengal, Methylene Blue and Eosin). It has been shown that superoxide radical is not reactive towards 2,4,5-TCP, whereas singlet oxygen results in the destructive oxidation of polychlorinated phenols without formation of PCDD and PCDF among the reaction products. The detailed mechanism of the interaction of singlet oxygen with 2,4,5-TCP has been investigated. It has been concluded that the initial trichlorophenol is dechlorinated and singlet oxygen interaction with 2,4,5-TCP proceeds via a mechanism with the formation of hydrogen peroxide without intermediate superoxide radicals.

Structural and materials properties of a polysulphide-modified epoxide resin

Abstract The nature of epoxy resins of the diglycidyl ether of bisphenol A modified by incorporation of 10 to 20 g phr of various linear polysulphides has been examined by 13 C n.m.r. spectroscopy (conventional and cross-polarization/magic angle spinning), transmission electron microscopy, scanning transmission electron microscopy and dynamic mechanical thermal analysis. Most, but not all, cured materials feature disperse rubber particles with diameters of μ m. The materials properties of the modified resins have been measured by tensile and impact testing: modification entails improvement but not necessarily either in proportion to the linear polysulphide content or dependent on the demonstrable presence of disperse particles.

Light flux and light flux density dependence of the photomineralization rate of 2,4-dichlorophenol and chloroacetic acid in the presence of TiO2

Abstract An examination of the effects of the variation of the light flux and light flux density over wide ranges on the photomineralization rates r i of 2,4-dichlorophenol and monochloroacetic acid by TiO 2 in aqueous suspension reveals that r i depends on the product of the light flux (to the first power) and the light flux density (to the power 0.0 and −0.5 at low and high light flux densities respectively).

Solvatochromic effects in the fluorescence and triplet-triplet absorption spectra of xanthone, thioxanthone and N-methylacridone

Abstract While the fluorescence of xanthone, thioxanthone and N-methylacridone shows the customary red shift with increasing solvent polarity, as measured by a polarization function, indicating an increase in dipole moment on excitation, the triplet-triplet (T1Tn) absorption spectra are strongly blue shifted, indicating a decrease in dipole moment on excitation.

Coordination polymers based on octacyanometalates(IV,V)(M = Mo, W) and aliphatic polyamine copper(II) tectons with [N3] donor atom sets

The cyano-bridged [CuII(tetrenH2)]2[WIV(CN)8]2·5H2O (tetren = tetraethylenepentaamine) (1), [CuII(tetrenH2)][CuII(tetrenH)][WV(CN)8][WIV(CN)8]·2.5H2O (2), [CuII(dien)]2[WIV(CN)8]·4H2O (dien = diethylenetriamine) (3) and its isomorphous molybdenum(IV) analogue (4) have been prepared and structurally characterised. 1 and 2 are built from the W2Cu2(μ-CN)4 squares extended into 1-D structure by cyano-bridges. 2-D 3 and 4 form a square grid pattern with tungsten atoms in the corners and –CN–Cu(dien)–NC– linkages on the edges of the squares. The magnetic behaviour of 1 and 3 indicates the presence of two isolated CuII spins S = 1/2 with a very weak antiferromagnetic coupling through the diamagnetic NC–WIV–CN bridges in the low temperatures. Assembly 2 exhibits a weak ferromagnetic interaction between CuII and WV isolated by diamagnetic [WIV(CN)8]4− spacer from another CuII centre within WV–CN–CuII–NC–WIV–CN–CuII unit and the antiferromagnetic interaction between the CuII2WVWIV units.

Mass spectral characterization of the thermal degradation of poly(propylene oxide) by electrospray and matrix-assisted laser desorption ionization

Abstract The thermal degradation of poly(propylene oxide) (PPO), M n = 2000, can be characterized by electrospray (ESI) and matrix-assisted laser desorption ionization (MALDI). ESI and MALDI spectra of partially degraded PPO provide strong support for the thermal degradation pathway previously suggested by Griffiths et al. and Lemaire et al. Although these pathways differ in detail, it is not possible to distinguish between them from the masses of the resultant degradation species. Gel permeation chromatography data indicate that both mass spectrometric methods emphasize the presence of low-mass material, particularly in the degraded samples. This is attributed to the different sensitivities of the two techniques and some in situ fragmentation during mass spectrometric analysis.

KINETICS AND MECHANISM OF PHOTODEGRADATION OF CHLOROPHENOLS

The 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.

Thermal and photodegradation of polysulfide pre-polymers: products and pathways

Abstract The thermal and photodegradation of a number of linear polysulfide (LP) pre-polymers H(SC2H4OCH2OC2H4S)nH have been studied by 1H and 13C NMR spectroscopy, IR spectroscopy, gel permeation chromatography (GPC) and particularly, by electrospray ionisation mass spectrometry (ESI). ESI results point to the important role of hydrolytic pathways leading to the elimination of CH2O and the formation of hydroxy-terminated products, followed by the reduction of disulfide groups by CH2O. This conclusion is supported by the observation of very slow degradation by heat or UV-light of an LP containing no formal group. IR and NMR data show the formation of carbonyl-containing products by an oxidative free-radical pathway: 2D NMR data confirm that a formate ester is the main carbonyl-containing product, as authenticated recently in the degradation of various polyglycols.

Degradation studies of polyurethanes based on vegetable oils.Part 2. Thermal degradation and materials properties

Polyurethanes (PUs), produced by cross-linking with a di-isocyanate hydroxylated vegetable oils (rapeseed and sunflower), have been examined from the viewpoints of their thermal degradation and their materials properties, and how both of these aspects are influenced both by the addition of TiO2 filler and by UV irradiation. The thermal decomposition of the PUs was investigated using thermal gravimetric analysis (TGA) and IR spectroscopy: the decomposition of a foam of the PU based on rapeseed oil showed complex kinetics, being apparently single-stage and of the first order at 973 K but with at least two separate stages at other temperatures. Thermolysis of PUs as films resulted in the production of carbonyl groups (v 1793 cm−1) suggesting formation of an acid anhydride. Pyrolysis in vacuo produced volatiles which could be separated and examined by IR and NMR spectroscopy. Reaction mechanisms for the thermal decomposition of the PUs are proposed. The materials properties of the PUs, with and without TiO2 filler, have been examined by differential scanning calorimetry (DSC), dynamical mechanical thermal analysis (DMTA), tensile testing and measurement of scratch resistance and Shore D hardness. Addition of TiO2 improves systematically the scratch resistance and hardness of the PUs, but the glass transition temperature Tg showed a more complicated dependence on the TiO2 loading, initially falling and then increasing. Prolonged UV irradiation of TiO2-loaded samples increased the value of Tg at all loadings, suggesting increased levels of cross-linking on irradiation. The most dramatic effect of adding TiO2 was on the shape of the stress-strain curves: at 10% loading clear ‘soft-but-tough’ behaviour is evident. The values of tan δ and the storage modulus are also strongly affected by addition of TiO2, with and without UV-irradiation. From the viewpoint of utilisation of these PUs, their thermal stability up to 350°C and the beneficial effects of adding TiO2 make them promising materials.

The roles of N- and O-coordination in the crystal and molecular structures of uranyl complexes with anthranilic and pyrazinic acids

Abstract Oxonium tris(2-aminobenzenecarboxylato) dioxouranium (VI) monohydrate crystals contain monomeric molecules composed of a UO 2 group (UO 1.755 A mean; 178.3°) and six oxygen atoms in the equatorial plane (UO mean 2.456 A) donated by three bidentate carboxylic groups of anthranilic acid molecules. The coordination polyhedron is thus a hexagonal bipyramid involving solely O-coordination. Monoaquobis(2-pyrazinecarboxylato)dioxouranium(VI) dihydrate complex molecules are monomeric. The UO 2 group (UO mean 1.770 A, 179.1°) is surrounded in the equatorial plane by two oxygen atoms (UO mean 2.318 A), two nitrogen atoms (U mean 2.583 A) donated by pyrazinic acid molecules, and one oxygen atom from a water molecule (UO 2.405 A). The coordination polyhedron around the U atom is thus a pentagonal bipyramid and reflects the propensity for ring N atoms to coordinate with uranyl ions. In the crystal, molecules of both title compounds are held together by systems of hydrogen bonds.