Salt effects on the protonation of imidazole in aqueous solution at different ionic strengths: A tentative explanation by a complex formation model

Imidazole protonation constants were determined potentiometrically, using a (H⁺)-glass electrode, in LiCl, NaCl, KCl, CaCl₂, MgCl₂, tetramethylammonium (Me₄N-) iodide and chloride, Et₄N-, Pr₄N- and Bu₄N-iodides. Salt effects were tentatively explained by assuming that complexes [H(Im)X]^o (Im = imidazole, X^–=Cl^– or I^–), [M(Im)]²⁺ (M²⁺=Mg²⁺ or Ca²⁺) and [A(Im)]⁺ (A⁺ = tetraalkylammonium cation) were formed in solution. Calcium(II)-selective electrode measurements confirmed our hypothesis about the formation of the Ca²⁺-imidazole complex. The reliability of the complex formation model is discussed on the basis of its self consistency and in light of previous results.     

Distribution and speciation of metals in surface sediments of Taranto [corrected] gulf (Ionian Sea, Southern Italy)

Concentrations of metals (Al, Fe, Ti, Ni, Mn, Cr, Zn, Cu, Pb) were determined in surface sediments from Taranto gulf (Ionian Sea, Southern Italy) in order to assess the distribution and the bioavailability of metals. Total metal content and speciation of Fe, Ni, Mn, Zn, Cu and Pb were determined. Sediment cores were also sampled in some stations of particular interest. The analytical results show that metals distribution does not show significant variations in relation to depth. Metal distribution in the Taranto gulf is mainly influenced by industrial wastes from Taranto town, by rivers of the Basilicata region and by prevailing anticlockwise marine currents. These factors cause an accumulation of metals in the sediments near to the coasts of Calabria. The statistical analysis, effected by HCA and PCA methods, can be considered a good starting point in order to classify sediment samples and to locate accumulation areas. Metal speciation shows that Fe, Ni and Zn are present in sediments mainly as sulphides, Mn is present principally in an ion-exchange form or bound to carbonate, while Cu and Pb are mainly present in the fraction bounds to humic acids.     

Synthesis of mixed Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine). Measurement of the coordination strength of hemilabile ligands with respect to their symmetric counterparts

The synthesis of Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine) having different aryl groups bound to the two nitrogen atoms is reported for the first time. The ligands were obtained by two different strategies: (i) by a transimination reaction starting from a symmetric Ar,Ar-BIAN ligand having aryl groups bearing strongly electron-withdrawing substituents or (ii) by a two-step-one-pot sequence. The ligands synthesized have been chosen so that the electronic difference between the two aryl groups is very large, but the steric difference is variable and, in one case, the ligand is almost sterically symmetric. The coordination strength of the new ligands towards three palladium complexes has been measured by a competition experiment following a protocol previously described by us. The coordination strength of the mixed ligands is the mean of those of the corresponding symmetric counterparts. The X-ray crystal structure of a palladium pi-allyl complex of the electronically asymmetric-sterically symmetric ligand (3,5-(CF(3))(2)C(6)H(3)),(3,5-Me(2)C(6)H(3))-BIAN has been solved, together with those of the two symmetric analogues to allow a comparison. Despite the fact that the dodecafluorinated ligand has a K(eq) value about three orders of magnitude lower than the non-fluorinated counterpart, no notable difference is observed in the N-Pd and Pd-C(allyl) distances in the three complexes. Calculations at the density functional level confirm that Pd-BIAN distances are not strictly correlated to the coordination energies, which are in qualitative agreement with the spectroscopic evidence. The bond length is thus not a good indication of the bond strength in these cases.     

Potential energy functions for an intramolecular proton transfer reaction in the ground and excited state

We describe the development of empirical potential functions for the study of the excited state intramolecular proton transfer reaction in 1-(trifuloroacetylamino)-naphtaquinone (TFNQ). The potential is a combination of the standard CHARMM27 force field for the backbone structure of TFNQ and an empirical valence bond formalism for the proton transfer reaction. The latter is parameterized to reproduce the potential energies both in the ground and the excited state, determined at the CASPT2 level of theory. Parameters describing intermolecular interactions are fitted to reproduce molecular dipole moments computed at the CASSCF level of theory and to reproduce ab initio hydrogen bonding energies and geometries for TFNQ-water bimolecular complexes. The utility of this potential energy function was examined by computing the potentials of mean force for the proton transfer reactions in the gas phase and in water, in both electronic states. The ground state PMF exhibits little solvent effects, whereas computed potential of mean force shows a solvent stabilization of 2.5 kcal mol⁻¹ in the product state region, suggesting proton transfer is more pronounced in polar solvents, consistent with experimental findings. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

Synthesis and Structure of η4-Enone Complexes of Ruthenium(0)

A variety of [Ru(CO)₂L(η⁴enone)] complexes (L = phosphines, phosphites, and arsines, enone = (E)-4-phenylbut-3-en-2-one) have been synthesized. ¹H-, ¹³C-, and ³¹P-NMR spectra are reported and the X-ray structures of two Ru complexes with L ? Ph₃P(7), Et₃P ( 10 ) and one Fe complex with L ? Ph₃P ( 14 ) are presented. All three compounds crystallize in the same monoclinic space group P2₁/n with a = 10.575(2) Å, b =9.213(2) Å, and c = 27.608(5) Å, β = 100.04(2)°, Z = 4 for 7 , a = 10.276(3) Å, b = 12.935(3) Å, and c = 14.854(2) Å, β = 96.96(2)°, Z = 4 for 10 , and a = 10.492(2) Å, b = 9.232(3) Å, and c = 27.129(3) Å, β = 98.67(2)°, Z = 4 for 14 . The structures of the Ru complexes are compared with the Fe analogues. In the case of M ? Ru and L ? (EtO)₃P, (MeO)₃P, and (i-PrO)₃P ( 9 , 11 , and 13 , respectively) stereoisomers could be detected by ³¹P-NMR at room temperature, wich arise from rotation at the coordinated metal centre.     

Heterotricyclic systems. Part I. Synthesis of new dipyridopyrazines

Pursuing our previous research on azaquinoxalinones (1,2-dihydropyrido[2,3-b]/[3,4-b]pyrazinones) we prepared, through a reductive cyclization of N-(3′-nitropyridin-2′-yl)piperidine-2-carboxylic acids 3a-c , a set of derivatives of a new tricyclic structure, 7,8,9,10-tetrahydro-5H-dipyrido[1,2-a:3′,2′-e]pyrazin-6(6aH)-ones 4a-c. Starting from these compounds we obtained substituted amides 5a-c and, from 4a , the amidines 6a-c. In the synthesis of 6 , a dehydrogenation reaction occurred giving rise to 7. The compounds 9 and 10 , characterized by a different ring-fusion between the pyridine and pyrazine rings, were synthesized in a similar manner.     

Multicomponent analysis of electrochemical signals in the wavelet domain

Successful applications of multivariate calibration in the field of electrochemistry have been recently reported, using various approaches such as multilinear regression (MLR), continuum regression, partial least squares regression (PLS) and artificial neural networks (ANN). Despite the good performance of these methods, it is nowadays accepted that they can benefit from data transformations aiming at removing baseline effects, reducing noise and compressing the data. In this context the wavelet transform seems a very promising tool. Here, we propose a methodology, based on the fast wavelet transform, for feature selection prior to calibration. As a benchmark, a data set consisting of lead and thallium mixtures measured by differential pulse anodic stripping voltammetry and giving seriously overlapped responses has been used. Three regression techniques are compared: MLR, PLS and ANN. Good predictive and effective models are obtained. Through inspection of the reconstructed signals, identification and interpretation of significant regions in the voltammograms are possible.     

Thermodynamics of formation of magnesium(II), calcium(II), strontium(II) and barium(II)—succinate complexes in aqueous solution

The formation constants for the complexes Mg²⁺ -, Ca²⁺ -, Sr²⁺ - and Ba²⁺ - succinate (succ²⁻) have been determined by potentiometric measurements, in aqueous solution, at different temperatures and ionic strengths. The species [M(succ)]⁰ and [M(succ)H]⁺ were found for all systems. For the stability constant the ionic strength dependence has been found, and general parameters for the relation log β = f(I) have been obtained. From the temperature dependence of stability constants ΔH values have been deduced. The procedure adopted in calculating all the thermodynamic parameters for the systems under study, where weak complexes are formed, is discussed. The stability of the complexes follows the order Mg < Ca Sr ≈ Ba.     

Use of multivariate analysis of MIR spectra to study bread staling

Different kinds of bread, stored at constant temperature and at controlled humidity conditions for a week since their manufacturing date, were analysed by Attenuated Total Reflectance-Fourier Transform InfraRed (ATR-FTIR) spectroscopy. The collected spectra were processed by Principal Component Analysis (PCA), in order to evaluate the changes occurring during bread ageing. For the sake of comparison, the 1060-950 cm spectral window has been also investigated by curve-fitting methods. It was observed that the first PC increases monotonically with ageing of samples. Furthermore, the more influential variables on PC1 correspond to spectral regions where are located stretching and bending bands, which are mainly attributed to typical starch bonds vibrations.     

Stereochemical non-rigidity of a metal polyhedron; carbon-13 and platinum-195 fourier transform nuclear magnetic resonance spectra of [Ptn(CO)2n]2- (n = 3, 6, 9, 12 or 15)

Platinum-195 spectra are reported for [Pt_n(CO)_2n]^2- (n = 3, 6, 9, 12 and 15) and carbon-13 spectra are reported for n = 6, 9 and 12 over a range of temperatures. The spectra provide evidence for (a) intramolecular rotation of the Pt₃-triangles about the principal three-fold axis, (b) inter-exchange of Pt₃-triangles, (c) lack of terminal/edge carbonyl exchange within the Pt₃(CO)₃(μ-CO)₃ group. Evidence is also presented for the formation of [Ni₃Pt₃(CO)₁₂]^2- on mixing [Pt₆(CO)₁₂]^2- [Ni₆(CO)₁₂]^2-.