Abílio M. Da Silva

Aqueous photochemistry of pesticides triadimefon and triadimenol

The photochemistry of pesticides triadimefon and triadimenol was studied in aqueous solution and in methanol/water mixtures, in controlled and natural conditions. The photodegradation kinetics and product distribution are strongly dependent on the solvent and on the irradiation wavelength. The degradation rates are faster at 254 nm than at 313 nm. The kinetics is faster in water than methanol. Direct photoreaction is an important dissipation pathway of triadimefon in natural water systems while triadimenol is stable in these conditions. 1,2,4-Triazole and 4-chlorophenol are two of the major photodegradation products. The formation of the 4-chlorophenoxyl radical was detected for both pesticides in methanol and methanol/water mixtures. In methanol/water mixtures the reaction of both pesticides also occurs with 4-chlorophenolate formation, which increases with the water content. The photochemical studies of pesticides and other pollutants should be made in conditions as similar as possible to those observed in environmental systems.

Environmental and laboratory studies of the photodegradation of the pesticide fenarimol

Abstract The photophysics and photochemistry of the pesticide fenarimol were studied. From the absorption and fluorescence spectra, quantum yields and lifetimes, it is suggested that the lowest excited singlet state has predominatly n,π* character, and is localized on the pyrimidine ring. Phosphorescence measurements suggest a small singlet—triplet splitting. Halide ions are found to quench fenarimol flourescence. Photodecomposition of fenarimol in a variety of solvents was studied by gas chromatography—mass spectrometry. Although the products have not yet been characterized, degradation of this compound appears to involve cleavage of bonds to the quaternary carbon without significant dechlorination. Preliminary studies of the kinetics and relative quantum yields of photodegration show that the reaction proceeds via a first-order process, which is independent of pH, but which involves a photoactive intermediate. Chloride and bromide ions inhibit the photolysis,whereas the triplet quencher sorbic acid has no effect.

Structure-efficiency relationships in hydrogen photoabstraction reactions by ketones: thermal activation versus nuclear tunnelling

Abstract The tunnel effect theory is applied to a study of the efficiency of hydrogen photoabstractions by excited states of carbonyl compounds. Efficiency depends on the energy of the crossing point ω of the potential energy curves of S0 and of the radical intermediate state I and consequently depends on the CH bond strengths, the oscillator displacements, the nature of the electronic states, the ionization energy of the substrates, the affinity of the ketones and the solvent polarity. The theory shows that the dependence of the efficiency on the molecular structure provides a good criterium to distinguish between the mechanisms of tunnelling and thermal activation. Experimental data support the view that hydrogen photoabstractions occur via a tunnelling mechanism and not via a thermal activation process. Qualitative rules of structure—efficiency relationships are presented.

Dissipation of triadimefon on the solid/gas interface.

The dissipation of triadimefon, as pure solid and in the Bayleton 5 commercial formulation, was studied under controlled and natural conditions. Volatilization and photodegradation were shown to be the main dissipation processes. The volatilization results can be described by an empirical model assuming exponential decay of the volatilization rate. The filler of the commercial formulation is determinant for the volatilization but has little effect on the photodegradation rates. The main photoproducts were identified and a reaction mechanism proposed.

Fenarimol solar degradation pathways and photoproducts in aqueous solution

The degradation photoproducts of the fungicide fenarimol obtained from irradiation of aqueous solutions with sunlight were characterised. The photoproducts resulting from samples with different exposure times were extracted and separated using chromatographic techniques. Seven main photoproducts were detected using high performance liquid chromatography with a photodiode array detector, gas chromatography with mass spectrometry detector and Fourier transform infrared spectroscopy. Structures are suggested for possible photoproducts based on the characterisation results, minimum energy geometry of the parent compound, and the mass spectral behaviour of fenarimol. These correspond to the compounds with m/z 328 (three structural isomers (a), (b) and (c)), m/z 294 (two structural isomers (a) and (b)), m/z 292, 278 and 190. Of the various major products detected, the isomer 328(a) is seen to be particularly unstable under the action of sunlight. The most stable photoproducts are found to be those with m/z 294(a), 278 and 190. However, upon prolonged solar irradiation all of these break down to produce polar, low molecular weight compounds. Comparison with our own and other results on fenarimol photolysis indicate significant solvent effects on the process. The combination of these structural characterisation results and previous data from spectroscopic and photodegradation kinetics studies allows us to suggest some possible mechanisms for the photodegradation of fenarimol under sunlight.

Comparative study of the dissipation of triadimefon in greenhouse and field conditions

The dissipation of triadimefon, {1‐(4‐chlorophenoxy)‐3,3‐dimethyl‐1‐(1H‐1,2,4‐triazol‐l‐yl)butan‐one}, was studied after its application to melon leaves, glass and paper, both in greenhouse and field conditions. The dissipation rate of triadimefon in its commercial formulation Bayleton 5 was found to be lower in greenhouse than field. The results for different samples in the same conditions show that the dissipation of triadimefon was found to be biphasic. This result can be accounted by a semi‐empirical model which assumes an initial fast decline of the dissipation rate, attributed to an exponential decay of the volatilization rates, followed by a second phase where the dissipation is due to a first order degradation processes.

UV-visible absorption spectra and luminescence of the pesticide fenarimol

Abstract The UV-visible absorption and emission spectra have been measured of the pesticide fenarimol ((±)-2,4′-dichloro-α-(pyrimidin-5-yl)-benzhydryl alcohol) in solution. From comparison with the spectra of chlorotoluenes and pyrimidine, and from the effect of solvent polarity on the absorption spectrum, it is shown that the lowest excited singlet state is localized on the pyrimidine ring, and has n,π* character. Higher excited π,π* states are localized on both chlorotoluene and pyrimidine rings. Fenarimol shows a weak, fluorescence from the n,π* state, with a quantum yield which is strongly dependent on solvent. It is shown that this is due to changes in the nonradiative decay rate, particularly in protic solvents, due to increased intersystem crossing. Phosphorescence is observed in low temperature glasses. Although this shows two decay components, it is suggested that only one triplet state is involved, and that this has predominantly π,π* character.

The quenching of the fluorescence of aromatic hydrocarbons by tertiary derivatives of group V elements

Abstract A study is made of the quenching of fluorescence of a number of aromatic compounds by triphenyl derivatives of group V elements and by triethylamine in both non-polar and polar solvents. Whilst quenching is observed in many cases, the lack of correlation of the quenching rate with the ionization potential of the quencher suggests that this does not simply involve electron or charge transfer. Evidence is presented that a heavy atom effect involving the group V element is also important. Further, a search has been made for evidence of exciplex formation in the quenching of azulene S2 fluorescence by a variety of compounds. However, no new emissions attributable to such a species are observed under the conditions studied. In addition, in polar solvents there is no evidence for an overall electron-transfer reaction in the quenching suggesting that if an intermediate charge-transfer or ion pair state is formed this may be of higher energy than the lowest singlet or triplet state, and may decay to yield one of these levels.

The use of barium (II) to promote triplet←singlet transitions in organic compounds; an external heavy-atom effect

Abstract The possibility of using barium (II) to promote spin forbidden transitions is examined. Methanolic solutions of both quinoline and naphthalene in the presence of perchlorate reveal weak bands in their electronic absorption spectra which are assigned to the lowest triplet←singlet transitions. Barium (II) is also found to quench the fluorescence of naphthalene, probably as a result of a heavy atom effect.

Intersystem crossing and internal conversion in benzene vapour at low pressures

The pressure dependence of triplet formation in C6H6 and C6D6 excited at 253.7 nm was studied using both but-2-ene sensitized isomerization and biacetyl phosphorescence. Pressures were varied between 0.1 and 20 Torr by addition of argon. The triplet yield decreases as the pressure decreases from 0.70 at high pressures ( 8 Torr) to 0.18 at 0.1 Torr in C6H6 and from 0.65 to 0.11 in C6D6. The results are interpreted in terms of a collisional assisted intersystem crossing S1 [graphic omitted] → T2 in the low pressure region. For pressures above 1 Torr the observed pressure dependence is attributed to a reversible intersystem crossing process S1 [graphic omitted] → T2. It is shown that a decrease in triplet lifetime with an increase in vibrational energy does not explain the present observations even at the lowest pressures used. The results show also that internal conversion S1 [graphic omitted] → S0 is an important channel of nonradiative decay in the lowest excited singlet state of C6H6 and C6D6.