Thermal studies on metal complexes of 5-nitroso-pyrimidine derivatives

The following cadmium(II) complexes of deprotonated 6-amino-5-nitroso-uracil (AH), deprotonated 6-amino-3-methyl-5-nitroso-uracil (BH) and neutral 6-amino-1-methyl-5--nitroso-uracil (CH) have been prepared and their thermal behaviour studied by TG and DSC techniques: CdA₂ · 3 H₂O, CdB₂ · 2 H₂O, CdB₂ and CdCl₂(CH)₂ · 2 H₂O.Dehydration processes of these complexes take place in only one step and the enthalpy values associated with them are comprised in the 42.7–80.5 kJ · mole^–1 H₂O range. The pyrolytic reactions finish in the 630–900 C temperature range, remaining as final residue in the case of CdA₂ · 3 H₂O, CdB₂ · 2 H₂O and CdB₂ complexes, cadmium oxide. The pyrolytic decomposition of CdCl₂(CH)₂ · 2 H₂O complex does not yield any residue, due to the elimination of cadmium chloride.

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Zusammenfassung Folgende Cadmium(II)-Komplexe von deprotoniertem 6-Amino-5-nitrosouracil (AH), deprotoniertem 6-Amino-3-methyl-5-nitroso-uracil (BH) und neutralem 6-Amino-1-methyl-5-nitroso-uracil (CH) wurden hergestellt: CdA₂ · 3 H₂O, CdB₂· 2 H₂O, CdB₂ und CdCl₂(CH)₂ · 2 H₂O. Das thermische Verhalten dieser Komplexe wurde mittels TG und DSC untersucht. Die Dehydratisierung der Komplexe verlÄuft in nur einem Schritt. Die Enthalpiewerte für diese Prozesse liegen im Bereich von 42.7–80.5 kJ · mol^–1 H₂O. Die pyrolytischen Reaktionen verlaufen im Temperaturbereich von 630–900C zu Ende, wobei im Falle der Komplexe CdA₂ · 3 H₂O, CdB₂ · 2 H₂O und CdB₂ Cadmiumoxid als Rückstand zurückbleibt. Die pyroytische Zersetzung von CdCl₂(CH)₂ · 2 H₂O verlÄuft wegen der Flüchtigkeit von CdCl₂ rückstandslos.

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CdA₂ · 3 H₂O, CdB₂ · 2 H₂O, CdB₂ CdCl₂(CH)₂ · 2 H₂O, A — 6--5-, B — 6--3--5- CH — 6--1--5- . . 42.7–80.5 · ^–1. 630–900, . CdCl₂(CH)₂ · 2 H₂O - , .

     

Extraction of palladium(II) by the hexachloroderivate of cobalt dicarbolide from nitric acid medium

Liquid—liquid extraction of divalent palladium by solutions of the hexachloroderivate of cobalt dicarbolide (HBCl₆) in the mixture of solvents (30 v/v % nitrobenzene+20 v/v % n-dodecane +50 v/v/ % toluene) from nitric acid medium has been studied. Besides HBCl₆ the organic phase contained also 2,2′-dipyridyl (dipy). The yield of palladium extraction from 0.5M HNO₃ is greater than 99.0%. The species extracted into the organic phase corresponds to the formula [Pd(dipy)₂] (BCl₆)₂.     

Ab initio analytical potential energy surface and quasiclassical trajectory study of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+))-->OH(+)(X (3)Sigma(-))+H((2)S) reaction and isotopic variants

An analytical potential energy surface (PES) representation of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+)) system was developed by fitting around 600 CCSD(T)/cc-pVQZ ab initio points. Rate constant calculations for this reaction and its isotopic variants (D(2) and HD) were performed using the quasiclassical trajectory (QCT) method, obtaining a good agreement with experimental data. Calculations conducted to determine the cross section of the title reaction, considering collision energies (E(T)) below 0.3 eV, also led to good accord with experiments. This PES appears to be suitable for kinetics and dynamics studies. Moreover, the QCT results show that, although the hypotheses of a widely used capture model are not satisfied, the resulting expression for the cross section can be applied within a suitable E(T) interval, due to errors cancellation. This could be a general situation regarding the application of this simple model to ion-molecule processes.     

Electrodeposition of mercury film on electrodes modified with clay minerals

A new type of carbon paste electrode modified with clay mineral and covered with a mercury film is presented in this work. Electrodeposition of the mercury film was performed on the carbon paste electrode modified with montmorillonite. The mercury film was deposited by both electrodeposition in situ and a preliminary electrodeposition. The pre-deposited film of mercury showed to be suitable for anodic stripping voltammetry. An open-circuit sorption of Cd, Pb, and Cu with subsequent anodic stripping voltammetry exhibited higher current responses of metals. Besides the enhanced sensitivity superior separation of the current responses during a simultaneous stripping of metals is expected to be achieved by means of the newly prepared electrode. Presented at the 57th Congress of Chemical Societies, Tatranské Matliare, 4–8 September 2005.     

Mechanism for basic hydrolysis of N-nitrosoguanidines in aqueous solution

A kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl-3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H(2)PO(4)(-)/HPO(4)(2)(-), H(3)BO(3)/H(2)BO(3)(-), and HCO(3)(-)/CO(3)(2)(-) regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of p = 11.5. The reaction rate for the process through the monoanion, k(2), decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k(3) = 10(10) s(-)(1) to estimate the value of p for the formation of the dianion of the N-nitrosoguanidine, obtaining values of p < 24.     

A universal temperature controlled membrane interface for the analysis of volatile and semi-volatile organic compounds

A universal temperature controlled membrane interface (TCMI) has been constructed for hollow-fibre membranes. The membrane temperature is controllable in the range -70 to 250 degrees C using an electric heater and a flow of cooled nitrogen or helium gas. Volatile and semi-volatile organic compounds may be detected either by continuous diffusion across the membrane or by in-membrane pre-concentration followed by thermal desorption into the detector. The TCMI interface is demonstrated in combination with mass spectrometry and GC-MS, for the determination of VOCs and SVOCs in aqueous and air samples and for the on-line monitoring of a bioreactor.     

Pentanuclear Ba(II) complex of a macrocyclic ligand

We report here the first pentanuclear Ba(II) complex of a new tri-aza, tri-oxa macrocycle with two carboxymethyl "arms" pending from two N atoms, H2L2. The crystal structure corresponds to the formula [Ba5(H0.375L2)4(ClO4)(CH3CH2OH)(H2O)2](ClO4)2.5 x 9.5H2O and reveals the presence of four molecules of the ligand surrounding five Ba(II) ions, giving rise to an unusual structure with the metal ions inside a spherical organic cavity.     

Cross sections of the O+ + H2 --> OH+ + H ion-molecule reaction and isotopic variants (D2, HD): quasiclassical trajectory study and comparison with experiments

A dynamics study [cross section and microscopic mechanism versus collision energy (E(T))] of the reaction O+ + H2 --> OH+ + H, which plays an important role in Earth's ionosphere and interstellar chemistry, was conducted using the quasiclassical trajectory method, employing an analytical potential energy surface (PES) recently derived by our group [R. Martinez et al., J. Chem. Phys. 120, 4705 (2004)]. Experimental excitation functions for the title reaction, as well as its isotopic variants with D2 and HD, were near-quantitatively reproduced in the calculations in the very broad collision energy range explored (E(T) = 0.01-6.0 eV). Intramolecular and intermolecular isotopic effects were also examined, yielding data in good agreement with experimental results. The reaction occurs via two microscopic mechanisms (direct and nondirect abstraction). The results were satisfactorily interpreted based on the reaction probability and the maximum impact parameter dependences with E(T), and considering the influence of the collinear [OHH]+ absolute minimum of the PES on the evolution from reactants to products. The agreement between theory and experiment suggests that the reaction mainly occurs through the lowest energy PES and nonadiabatic processes are not very important in the wide collision energy range analyzed. Hence, the PES used to describe this reaction is suitable for both kinetics and dynamics studies.     

All-solid-state potentiometric sensors sensitive to nonionic surfactants based on ionophores containing ethoxylate units

Earlier work of potentiometric Ion-selective electrodes (ISEs) sensitive to nonionic surfactants of the polyethoxylate type is further extended. The ISEs constructed were all-solid-state sensors with plasticized PVC membranes. The sensing material was a tetraphenylborate salt of the barium complex with a polyethoxylate nonionic surfactant. As membrane component, the combinations of two polyethoxylates of the nonylphenoxy type, which differed in the number of oxyethylene units (5 or 12), and two different plasticizers, (o-nitrophenyloctyl ether and o-nitrophenylphenyl ether), were tested. The response of these electrodes to different nonionic surfactants and the interference effect of several species has been evaluated. For all the types of tested electrodes, the sensitivities shown were ca. 30.0 mV dec(-1) and the limit of detection, ca. 10(-5) M, when a nonylphenoxyde with 12 oxyethylene units was used as standard. The membrane with the best response characteristics was then applied in potentiometric titrations of this kind of surfactants in the presence of Ba(2+) ion and using tetraphenylborate as the titrant.     

Modelling and prediction of retention in high-performance liquid chromatography by using neural networks

Summary Multi-layer feed-forward neural networks trained with an error back-propagation algorithm have been used to model retention behaviour of liquid chromatography as a function of the composition of the mobile phases. Conventional hydro-organic and micellar mobile phases were considered. Accurate retention modelling and prediction have been achieved using mobile phases defined by two, three and four parameters. With micellar mobile phases, the parameters involved included the concentrations of surfactant and organic modifier, pH and temperature. It is shown that neural networks provide a competitive tool to model varied inherent nonlinear relationships of retention behaviour with respect to the mobile phase parameters. The soft models defined by the weights of the networks are capable of accommodating all types of linear and nonlinear relationships, neural networks being specially useful when the relationships between retention behaviour and the mobile phase parameters are unknown. However, to train neural networks more experimental points than with hard-modelling methods are required, hence the use of the networks is recommended only for those cases where adequate theoretical or empirical models do not exist.