Developing a sequential injection-square wave voltammetry (SI-SWV) method for determination of atrazine using a hanging mercury drop electrode

This paper describes the development of a sequential injection analysis method to automate the determination of atrazine by square wave voltammetry exploiting the concept of monosegmented flow analysis to perform in-line sample conditioning and standard addition. To perform these tasks, an 800 μL monosegment is formed, composed by 400 μL of sample and 400 μL of buffer/standard solution. To obtain an efficient homogenization, the sample solution is divided in five zones intercalated by four zones of the Britton-Robinson buffer (pH 2.0) in presence of appropriate concentration of NaNO₃ and varying atrazine standard concentrations. This mixture zone is isolated from the carrier solution by two 100 μL air bubbles. After homogenization in an auxiliary reaction coil the mixture zone is injected toward the flow cell, which is adapted to the capillary of a hanging drop mercury electrode, at a flow rate of 50 μL s⁻¹. After a suitable delay time, the potential is scanned from −0.5 to −1.2 V versus Ag/AgCl using a frequency of 300 Hz and pulse height of 25 mV. The linear dynamic range is observed for atrazine concentrations between 1.16 × 10⁻⁷ and 2.32 × 10⁻⁶ mol L⁻¹, obeying the linear equation i_p = (−6.91 ± 0.07) × 10⁸[atrazine] + (4 ± 8), with r² = 0.9996, for which the slope is given in nA L mol⁻¹. The detection and quantification limits of the method are 2.1 × 10⁻⁸ and 7.0 × 10⁻⁸ mol L⁻¹, respectively. The sampling frequency is 37 h⁻¹, when the standard addition protocol is followed. This frequency can be increased to 42 h⁻¹ if the protocol to obtain in-line calibration curve is used for quantification. The method was applied for determination of atrazine in spiked river water samples and its accuracy was evaluated by comparison with the batch standard addition approach, which revealed that there is no evidence of statistically significant differences between the two methods.     

The effect of the "inert" counteranions in the deprotonation of the dihydrogen complex trans-[FeH(eta 2-H2)(dppe)2]+: kinetic and theoretical studies

Kinetic studies indicate that trans-[FeH(H2)(dppe)2]+ reacts with an excess of NEt3 to form cis-[FeH2(dppe)2] in a single kinetic step. The second-order rate constant is strongly affected by the presence of added salts, an acceleration being observed with BF4- and PF6- salts and a deceleration with BPh4-. Theoretical calculations indicate that the role of the accelerating anions consists of the formation of ion pairs that provide a more effective reaction pathway for deprotonation. However, for the ion pair with the bulky BPh4- anion, steric crowding in the proximities of the dihydrogen ligand hinders the approach of the base, and the reaction is decelerated.     

Synthesis of donor-acceptor alkynylcyclopropanes by diastereoselective cyclopropanation of electron-deficient alkenes with alkoxyalkynyl Fischer carbene complexes

The reaction of electron-deficient alkenes with alkoxyalkynyl Fischer carbene complexes (FCCs) represents a straightforward route to a new type of captodative (donor-acceptor) alkynylcyclopropanes, which have been prepared in moderate to high yields and in a diastereoselective manner. Some studies regarding the employment of additives to facilitate the recovery of the metal moiety after the reaction are also described. Finally, the first example of a cyclopropanation reaction through employing Fischer carbene complexes under microwave irradiation is presented; this method proved to be an advantageous alternative to the thermal reaction.     

Structure and Dynamics of Methyl-substituted Beryllocene: [Be(C5Me5)2]

In this paper we study theoretically the molecular structure of [Be(C₅Me₅)₂], with special interest in similarities and differences found in the computed geometric parameters, depending on the treatment of the electron correlation used. Given the low energy differences found between the different configurations studied (less than 4 kcal mol⁻¹), and the high fluxionality found in experimental studies for this compound, we analyzed the dynamics of the system by means of first principles molecular dynamics calculations. A complex dynamics is found and analyzed in terms of two molecular rearrangement processes: 1,2-sigmatropic intraring rearrangement and a ring inversion mechanism that interchanges the roles (with regard to their coordination to the central Be atom) of the two rings.Dedicated to Dr. Jeal Paul Malrieu on occasion of his 65th birthday.     

Static light scattering from microgel particles: model of variable dielectric permittivity

We perform static light scattering experiments on a dilute suspension of microgel particles and model the resultant form factors Pq by assuming an exponentially decaying dielectric permittivity. The result is that Pq is a Lorentzian function of the scattering wavevector q for length scales greater than the particle size; the width approximately corresponding to twice the particle radius. This simple model reasonably accounts for scattered light from both swollen and shrunken microgel phases.     

Accurate intermolecular ground state potential of the Ne-N2 van der Waals complex

Ab initio ground state potential energy surfaces are obtained from interaction energies calculated with the coupled cluster singles and doubles model including connected triples corrections [CCSD(T)] and the aug-cc-pVXZ (X=5,Q,T,D) basis sets augmented with two different sets of midbond functions (denoted 33221 and 33211). The aug-cc-pV5Z-33221 surface is characterized by a T-shaped 49.5 cm(-1) minimum at Re=3.38 Angstroms and a linear saddle point at 3.95 Angstroms with De=36.6 cm(-1). These results agree well with the values provided by the accurate semiempirical potentials available. The rovibronic spectroscopic properties are determined and compared to the available experimental data and previous theoretical results. We study the basis set convergence of the intermolecular potentials and the rotational frequencies. The aug-cc-pVTZ basis sets provide reasonable binding parameters, but seem not to be converged enough for the evaluation of the microwave spectra. The aug-cc-pVQZ basis sets considerably improve the triple zeta results. The differences between the results obtained with the aug-cc-pVTZ-33221 basis set surface and those with the aug-cc-pVQZ-33221 are smaller than those of the corresponding bases with the set of 33211 midbond functions. The aug-cc-pVQZ surfaces are close to the aug-cc-pV5Z, that are expected to be close to convergence. With our best surfaces the errors in the frequencies with respect to the accurate experimental results go down to 0.6%.     

Electron impact mass spectrometry of some macrocyclic polyether lactones

Electron impact mass spectrometry has been used together with linked scans, exact mass measurements and mass-analysed ion kinetic energy spectrometry to study nine inacrocyclic polyether lactones in detail. The presence of abundant hydrogenated oxirane ions has been observed for all the compounds examined, and confirmed by kinetic energy release measurements.     

Micellar-stabilized room-temperature phosphorimetric determination of the fungicide thiabendazole in canned pineapple samples

A method for the determination of the fungicide thiabendazole (TBZ) by micellar-stabilized room-temperature phosphorescence is described. It does not require any separation step and allows the direct determination of the fungicide in canned pineapple samples. The effect of various experimental conditions is discussed in detail. The analytical curve of thiabendazole gives a linear dynamic range of 23.8–500.0 ng mL^–1 and a detection limit of 23.8 ng mL^–1. Recoveries of 103.9 and 89.2% for syrup and canned pineapple pulp, respectively, were obtained for 250 ng mL^–1 thiabendazole.     

Cascade alkenyl amination/Heck reaction promoted by a bifunctional palladium catalyst: a novel one-pot synthesis of indoles from o-haloanilines and alkenyl halides

A novel approach for the synthesis of the important indole ring is described. Indoles are obtained from o-bromoanilines and alkenyl halides in a Pd-catalyzed cascade process that involves an alkenyl amination followed by an intramolecular Heck reaction. The overall process represents the first example of the participation of alkenyl amination reactions in Pd-catalyzed cascade reactions. Initially, the relative reactivity of aryl and alkenyl bromides and chlorides towards Pd-catalyzed amination was investigated. Competition experiments were carried out in the presence of primary and secondary amines, and these revealed the reactivity order alkenyl bromides > aryl bromides > alkenyl chlorides > aryl chlorides, as well as very high chemoselectivity; the more reactive halide was always favored. Thereafter, optimized reaction conditions for the sequential alkenyl amination/Heck cyclization to give indoles were investigated with the model reaction of o-bromoaniline with alpha-bromostyrene. An extensive screening of ligands, bases, and reaction conditions revealed that the [Pd2(dba)3]/DavePhos, NaOtBu, toluene combination at 100 degrees C were the optimized reaction conditions to carry out the cascade process (dba=dibenzylideneacetone, DavePhos=2-dicyclohexylphosphino-2'-N,N-dimethylaminobiphenyl). The reaction proceeds with aryl, alkyl, and functionalized substitutents in both starting reactants. The cyclization was also studied with N-substituted o-bromoanilines (which would give rise to N-substituted indoles); however, in this case, indole formation occurred only with 1-substituted-2-bromoalkenes. Finally, the application of this methodology to o-chloroanilines required further optimization. Although the catalyst based on DavePhos failed to promote the cascade process, a catalytic combination based on [Pd2(dba)3]/X-Phos promoted the formation of the indole ring also from the less reactive chloroanilines.     

Group contraction and the nine Cayley-Klein geometries

A one-to-one correspondence between the nine Cayley-Klein geometries and the so-called kinematic groups is presented. As is well known, the kinematic groups are related to each other through group contraction. The pattern of contraction is explained by relating each kinematic group to a specific Cayley-Klein geometry. The very meaning of group contraction is deeply rooted in the relationship between the nine geometries. Lie algebras of those geometries are explicitly constructed.In part from the Tesina de LicenciaturaM. A. Fernandez Sanjuan, Tesina de Licenciatura: Contracciones anisötropas del grupo de Poincar, Universidad de Valladolid, 1981.