Chemiluminometric determination of the pesticide 3-indolyl acetic acid by a flow injection analysis assembly

A new method is proposed for the chemiluminescent determination of the pesticide 3-indolyl acetic acid by means of an flow injection analysis system. The chemiluminescence emission is obtained by oxidation of the analyte with Ce (IV) in nitric acid and presence of β-cyclodextrine.The continuous-flow method allows the determination of 159 samples h⁻¹ of 3-indolyl acetic acid in an interval of concentrations over the range 0.5-15.0 mg l⁻¹. The limit of detection was 0.1 μg l⁻¹ and the R.S.D. (n, 17) at 2.0 mg l⁻¹ of the pesticide level was 2.7%. The method was applied to water samples.     

Solid-phase extraction system for Pb (II) ions enrichment based on multiwall carbon nanotubes coupled on-line to flame atomic absorption spectrometry

The present paper proposes the application of multiwall carbon nanotubes (MWCNTs) as a solid sorbent for lead preconcentration using a flow system coupled to flame atomic absorption spectrometry. The method comprises the preconcentration of Pb (II) ions at a buffered solution (pH 4.7) onto 30 mg of MWCNTs previously oxidized with concentrated HNO₃. The elution step is carried out with 1.0 mol L⁻¹ HNO₃. The effect of the experimental parameters, including sample pH, sampling flow rate, buffer and eluent concentrations were investigated by means of a 2⁴ full factorial design, while for the final optimization a Doehlert design was employed. Under the best experimental conditions the preconcentration system provided detection and quantification limits of 2.6 and 8.6 μg L⁻¹, respectively. A wide linear range varying from 8.6 up to 775 μg L⁻¹ (r > 0.999) and the respective precision (relative standard deviation) of 7.7 and 1.4% for the 15 and 200 μg L⁻¹ levels were obtained. The characteristics obtained for the performance of the flow preconcentration system were a preconcentration factor of 44.2, preconcentration efficiency of 11 min⁻¹, consumptive index of 0.45 mL and sampling frequency estimated as 14 h⁻¹. Preconcentration studies of Pb (II) ions in the presence of the majority foreign ions tested did not show interference, attesting the good performance of MWCNTs. The accuracy of the method was assessed from analysis of water samples (tap, mineral, physiological serum and synthetic seawater) and common medicinal herbs submitted to the acid decomposition (garlic and Ginkgo Biloba). The satisfactory recovery values obtained without using analyte addition method confirms the feasibility of this method for Pb (II) ions determination in different type of samples.     

Water-soluble polyesters from long chain alkylesters of citric acid and poly(ethylene glycol)

Long chain aliphatic alcohols have been used as model compounds to develop a preparative method for a water-soluble material, which could be a carrier for triacontanol, a highly hydrophobic plant growth regulator. New polyesters from long chain aliphatic (C = 12, 18 and 22) mono-1-alkyl citrates and poly(ethylene glycol) were synthesized and characterized by NMR spectroscopy. The polyester containing the triacontyl moiety was obtained from mono-1-triacontyl citrate, which was synthesized from the corresponding alcohol extracted from the Agave fourcroydes. The molecular weight of the polyesters depends on experimental conditions during synthesis such as reaction time, atmosphere, catalyst concentration and temperature. The reaction is second order in the early stage of the polyester synthesis. The reaction rate constant is independent of the length of the aliphatic chain, but it decreases with increasing of the poly(ethylene glycol) employed. Turbidity measurements have been used to study the polyester solubility. Solubility characteristics were found to depend on the of poly(ethylene glycol), the aliphatic-chain length and the value of for the polyester. These preparations could potentially be used to release triacontanol.     

Gutmann's Donor Numbers Correctly Assess the Effect of the Solvent on the Kinetics of SNAr Reactions in Ionic Liquids

We report an experimental study on the effect of solvents on the model S_NAr reaction between 1‐chloro‐2,4‐dinitrobenzene and morpholine in a series of pure ionic liquids (IL). A significant catalytic effect is observed with reference to the same reaction run in water, acetonitrile, and other conventional solvents. The series of IL considered include the anions, NTf₂^?, DCN^?, SCN^?, CF₃SO₃^?, PF₆^?, and FAP^? with the series of cations 1‐butyl‐3‐methyl‐imidazolium ([BMIM]⁺), 1‐ethyl‐3‐methyl‐imidazolium ([EMIM]⁺), 1‐butyl‐2,3‐dimethyl‐imidazolium ([BM₂IM]⁺), and 1‐butyl‐1‐methyl‐pyrrolidinium ([BMPyr]⁺). The observed solvent effects can be attributed to an “anion effect”. The anion effect appears related to the anion size (polarizability) and their hydrogen‐bonding (HB) abilities to the substrate. These results have been confirmed by performing a comparison of the rate constants with Gutmann's donicity numbers (DNs). The good correlation between rate constants and DN emphasizes the major role of charge transfer from the anion to the substrate.     

Dynamical Bifurcation in Gas‐Phase XH− + CH3Y SN2 Reactions: The Role of Energy Flow and Redistribution in Avoiding the Minimum Energy Path

The gas‐phase reactions of XH^? (X=O, S) + CH₃Y (Y=F, Cl, Br) span nearly the whole range of S_N2 pathways, and show an intrinsic reaction coordinate (IRC) (minimum energy path) with a deep well owing to the CH₃XH???Y^? (or CH₃S^????HF) hydrogen‐bonded postreaction complex. MP2 quasiclassical‐type direct dynamics starting at the [HX???CH₃???Y]^? transition‐state (TS) structure reveal distinct mechanistic behaviors. Trajectories that yield the separated CH₃XH+Y^? (or CH₃S^?+HF) products directly are non‐IRC, whereas those that sample the CH₃XH???Y^? (or CH₃S^????HF) complex are IRC. The IRCIRC/non‐IRC ratios of 90:10, 40:60, 25:75, 2:98, 0:100, and 0:100 are obtained for (X, Y)=(S, F), (O, F), (S, Cl), (S, Br), (O, Cl), and (O, Br), respectively. The properties of the energy profiles after the TS cannot provide a rationalization of these results. Analysis of the energy flow in dynamics shows that the trajectories cross a dynamical bifurcation, and that the inability to follow the minimum energy path arises from long vibration periods of the X?C???Y bending mode. The partition of the available energy to the products into vibrational, rotational, and translational energies reveals that if the vibrational contribution is more than 80 %, non‐IRC behavior dominates, unless the relative fraction of the rotational and translational components is similar, in which case a richer dynamical mechanism is shown, with an IRC/non‐IRC ratio that correlates to this relative fraction.     

Reversible Silylene Insertion Reactions into Si−H and P−H σ‐Bonds at Room Temperature

Phosphine‐stabilized silylenes react with silanes and a phosphine by silylene insertion into E?H σ‐bonds (E=Si,P) at room temperature to give the corresponding silanes. Of special interest, the process occurs reversibly at room temperature. These results demonstrate that both the oxidative addition (typical reaction for transient silylenes) and the reductive elimination processes can proceed at the silicon center under mild reaction conditions. DFT calculations provide insight into the importance of the coordination of the silicon center to achieve the reductive elimination step.     

Fluorinated Musk Fragrances: The CF2 Group as a Conformational Bias Influencing the Odour of Civetone and (R)‐Muscone

The difluoromethylene (CF₂) group has a strong tendency to adopt corner over edge locations in aliphatic macrocycles. In this study, the CF₂ group has been introduced into musk relevant macrocyclic ketones. Nine civetone and five muscone analogues have been prepared by synthesis for structure and odour comparisons. X‐ray studies indeed show that the CF₂ groups influence ring structure and they give some insight into the preferred ring conformations, triggering a musk odour as determined in a professional perfumery environment. The historical conformational model of Bersuker and co‐workers for musk fragrance generally holds, and structures that become distorted from this consensus, by the particular placement of the CF₂ groups, lose their musk fragrance and become less pleasant.     

Distinct reaction pathways followed upon reduction of oxy-heme oxygenase and oxy-myoglobin as characterized by Mössbauer spectroscopy

Activation of O(2) by heme-containing monooxygenases generally commences with the common initial steps of reduction to the ferrous heme and binding of O(2) followed by a one-electron reduction of the O(2)-bound heme. Subsequent steps that generate reactive oxygen intermediates diverge and reflect the effects of protein control on the reaction pathway. In this study, M?ssbauer and EPR spectroscopies were used to characterize the electronic states and reaction pathways of reactive oxygen intermediates generated by 77 K radiolytic cryoreduction and subsequent annealing of oxy-heme oxygenase (HO) and oxy-myoglobin (Mb). The results confirm that one-electron reduction of (Fe(II)-O(2))HO is accompanied by protonation of the bound O(2) to generate a low-spin (Fe(III)-O(2)H(-))HO that undergoes self-hydroxylation to form the alpha-meso-hydroxyhemin-HO product. In contrast, one-electron reduction of (Fe(II)-O(2))Mb yields a low-spin (Fe(III)-O(2)(2-))Mb. Protonation of this intermediate generates (Fe(III)-O(2)H(-))Mb, which then decays to a ferryl complex, (Fe(IV)=O(2-))Mb, that exhibits magnetic properties characteristic of the compound II species generated in the reactions of peroxide with heme peroxidases and with Mb. Generation of reactive high-valent states with ferryl species via hydroperoxo intermediates is believed to be the key oxygen-activation steps involved in the catalytic cycles of P450-type monooxygenases. The M?ssbauer data presented here provide direct spectroscopic evidence supporting the idea that ferric-hydroperoxo hemes are indeed the precursors of the reactive ferryl intermediates. The fact that a ferryl intermediate does not accumulate in HO underscores the determining role played by protein structure in controlling the reactivity of reaction intermediates.