Jorge J. P. Furlong

Reactivity of hydroxyl radicals with neonicotinoid insecticides: mechanism and changes in toxicity

The reactivity of hydroxyl radicals (HO ) towards three neonicotonoid insecticides, namely imidacloprid, thiacloprid and acetamiprid was investigated. These radicals were generated by photolysis of H(2)O(2) solutions. Flash photolysis experiments were used to determine the rate constants of 5.5 x 10(10) M(-1)s(-1), 6 x 10(10) M(-1)s(-1), and 7.5 x 10(10) M(-1)s(-1), for the reactions of HO with acetamiprid, imidacloprid, and thiacloprid, respectively. Continuous irradiation experiments in the absence and presence of H(2)O(2) allowed the identification and toxicity evaluation of the primary photo- and oxidation products of the insecticides. In all cases, the less toxic 6-chloronicotinic acid was found to be the major product at higher degrees of oxidation. The results reported here indicate that the half life of the insecticides due to their reaction with HO radicals in natural aquatic reservoirs may vary between 5 h and 19 days, and therefore the hydroxyl radical-mediated oxidation may be a significant abiotic elimination route. However, elimination of the insecticide under such conditions might not improve the quality of the contaminated water, as the primary products of degradation still show considerable toxicity to Vibrio fischeri assays.

Reactivity of neonicotinoid insecticides with carbonate radicals

The reaction of three chloronicotinoid insecticides, namely Imidacloprid (IMD), Thiacloprid (THIA) and Acetamiprid (ACT), with carbonate radicals (CO·₃⁻) was investigated. The second order rate constants (4 ± 1) × 10⁶, (2.8 ± 0.5) × 10⁵, and (1.5 ± 1) × 10⁵ M⁻¹ s⁻¹ were determined for IMD, THIA and ACT, respectively. The absorption spectra of the organic intermediates formed after CO·₃⁻ attack to IMD is in line with those reported for α-aminoalkyl radicals. A reaction mechanism involving an initial charge transfer from the amidine nitrogen of the insecticides to CO·₃⁻ is proposed and further supported by the identified reaction products. The pyridine moiety of the insecticides remains unaffected until nicotinic acid is formed. CO·₃⁻ radical reactivity towards IMD, ACT, and THIA is low compared to that of HO• radicals, excited triplet states, and ¹O₂, and is therefore little effective in depleting neonicotinoid insecticides.

Tautomeric equilibrium of amides and related compounds: theoretical and spectral evidences

Abstract The tautomeric equilibrium of several amides and related compounds is studied experimentally via mass spectrometry techniques. Experimental findings are complemented with theoretical calculations in order to rationalise mass spectra results of some amides, thioamides, ureas and thioureas. Some further possible studies are pointed out in order to complement present findings. Both mass spectra and theoretical results indicate that tautomerism involves neutral species rather than the ionic analogues. Besides, thio-derivatives have a relatively higher tendency to occur as imidols.

Separation of keto–enol tautomers in β-ketoesters: a gas chromatography–mass spectrometric study

Abstract The gas chromatographic behaviour for some β-ketoesters was studied. Additionally, the feasibility of the gas chromatographic separation of the corresponding tautomer forms was examined. In this work mass spectrometric detection allowed identification of both keto and enol forms and an estimation of their relative amounts for methylacetoacetate, α-chloromethylacetoacetate, ethylacetoacetate and α-chloroethylacetoacetate. This finding demonstrates slow tautomerisation kinetics permitting the chemical identity maintenance of the tautomers.

Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals

Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr-Val, Val-Tyr and Val-Tyr-Val) mediated by singlet molecular oxygen [O(2)((1)Delta(g))], phosphate (HPO(4)(*-) and PO(4)(*2-)) and sulfate (SO(4)(*-)) radicals was studied, employing time-resolved O(2)((1)Delta(g)) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O(2)((1)Delta(g))-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O(2)((1)Delta(g)) by Tyr-derivatives (k(t)/k(r) values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr-Val, 2.8 for Val-Tyr and 1.5 for Val-Tyr-Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O(2)((1)Delta(g)) quenching process. Products analysis by LC-MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO(4)(*-) and PO(4)(*2-), generated by photolysis of the P(2)O(8)(4-) at different pH) and sulfate radicals (SO(4)(*-), produced by photolysis of the S(2)O(8)(2-)) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO(4)(*-) > or = HPO(4)(*-) > or = PO(4)(*2-). Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO(4)(*-).

Mass spectrometry as a valuable tool for the study of tautomerism of amides and thioamides

Abstract Mass spectra of some amides and thioamides under different sample introduction temperatures and ionization electron energies are reported and analysed along with the corresponding theoretical calculations. The influence of temperature on the imidol/amide selected fragments abundance ratio (for different amides and for different pair of ions of the same amide whenever possible) are studied and employed to estimate the enthalpy change for the tautomeric equilibria. The same analysis is reported for different electron energies obtaining comparable results despite the impact on relative ion fragments abundances. AM1 molecular orbital calculations not only support that tautomerism in the mass spectrometer is negligible after ionization of the selected amides but also render a means to determine the theoretical heat of reaction which are well correlated with the experimental values.

Mass Spectrometry as a Tool for the Determination of Heats of Tautomerization of Thioamides in the Gas Phase

Tautomerism of several thioamides was studied by mass spectrometry. The analysis of the corresponding mass spectra allowed some fragmentations to be assigned to specific tautomers and heats of tautomerization to be determined through temperature effects and electron energy studies. Experimental determinations were strongly supported by theoretical calculations. AM1 semi-empirical results indicate that the thioamide–thioimidol equilibrium can be studied by mass spectrometry. Also, additional evidence was obtained regarding tautomerism occurrence between neutral species.

Spectrometric and theoretical evidences for the occurrence of tautomeric structures of selected ketones

Abstract The enolisation degree of ketones is generally favoured by the increase of the steric effect exerted by the substitution α to the carbonyl group. The analysis of the corresponding mass spectra has allowed us to assign unambiguously some fragmentations to specific tautomers and to establish an acceptable correlation between the corresponding ion abundance ratios and the AM1 MO theoretical calculation of the approximate enolisation equilibrium constants.

Mass spectrometry as a tool for studying tautomerism

The review contains new data on tautomerism of organic compounds belonging to different classes, which were obtained by mass spectrometry and confirmed by quantum-chemical calculations.

Mass Spectral Study of the Occurrence of Tautomeric Forms of Thiohydantoins

Mass spectrometry is used to evaluate the occurrence of thio-enol structures among the several possible tautomers of thiohydantoins and dithiohydantoins. Mass spectra of differently-substituted thiohydantoins are examined looking for common mass spectral behaviors to be predicted. Ion fragmentations from specific tautomers allow the most stable thio-enol structure for both type of compounds. The mass spectrum of the alkylation product of 5,5-dimethyldithiohydantoin and the nuclear magnetic resonance spectra of the alkylation products of both 2-thiohydantoin and dithiohydantoin support the fact that the most likely thio-enol structure is determined by the presence of one or two thio-carbonyl groups in the hydantoin molecule.