On the complex formation equilibria between dioxouranium(VI) and sulfate ions

The complexation equilibria between UO2(2+) and SO4(2-) ions have been studied at 25 degrees C in the ionic medium 3 M NaClO4 by potentiometry, by spectrophotometry and by solubility measurements of UO2(IO3)2. The potentiometric investigation was carried out with the Hg-Hg2SO4(s)-SO4(2-) half-cell and glass electrode in the sulfate concentration range 0.005 to 0.07 M. The optical absorbances in the UV-visible region and the solubility data cover the ligand concentration range 0.005 to 0.3 M. The data could be explained by assuming the complexes and equilibrium constants [Table: see text]. The constants in the infinite dilution reference state, log beta1o = 3.08 +/- 0.15 and log beta2o = 4.28 +/- 0.15, estimated by assuming the validity of the specific interaction theory, are practically coincident with literature data.     

UV-photocrosslinking of inulin derivatives to produce hydrogels for drug delivery application

In this work, INU, a natural polysaccharide, has been chemically modified in order to obtain new photocrosslinkable derivatives. To reach this goal, INU has been derivatized with MA thus obtaining four samples (INU-MA derivatives) as a function of the temperature and time of reaction. An aqueous solution of the derivative INU-MA1 was irradiated by using a UV lamp with an emission range from 250 to 364 nm and without using photoinitiators. The obtained hydrogel showed a remarkable water affinity but it underwent a partial degradation in simulated gastric fluid. To overcome this drawback, INU-MA1 was derivatized with SA thus obtaining the INU-MA1-SA derivative designed to produce a hydrogel showing a low swelling and an increased chemical stability in acidic medium. Ibuprofen, as a model drug, was loaded by soaking into INU-MA1 and INU-MA1-SA hydrogels and its release from these matrices was evaluated in simulated gastrointestinal fluids. INU-MA1 hydrogel showed the ability to quickly release the entrapped drug thus indicating its potential as a matrix for an oral formulation. INU-MA1-SA hydrogel showed a pH-responsive drug delivery. Therefore it is a promising candidate for controlled drug release in the intestinal tract.     

Bis- and tris-(methylphosphonic) acid derivatives of a 14-membered tetraazamacrocycle containing pyridine: synthesis, protonation and complexation studies

Two N-methylphosphonic acid derivatives of a 14-membered tetraazamacrocycle containing pyridine have been synthesized, H(4)L(1) and H(6)L(2). The protonation constants of these compounds and the stability constants of complexes of both ligands with Ni(2+), Cu(2+) and Zn(2+) were determined by potentiometric methods at 298 K and ionic strength 0.10 mol dm(-3) in NMe(4)NO(3). The high overall basicity of both compounds is ascribed to the presence of the phosphonate arms. (1)H and (31)P NMR spectroscopic titrations were performed to elucidate the sequence of protonation, which were complemented by conformational analysis studies. The complexes of these ligands have stability constants of the order of or higher than those formed with ligands having the same macrocyclic backbone but acetate arms. At pH = 7 the highest pM values were found for solutions containing the compound with three acetate groups, followed immediately by those of H(6)L(2), however, as expected, the increasing pH favours the complexes of ligands containing phosphonate groups. The single-crystal structure of Na(2)[Cu(HL(1))]NO(3)x8H(2)O has shown that the coordination geometry around the copper atom is a distorted square pyramid. Three nitrogen atoms of the macrocyclic backbone and one oxygen atom from one methylphosphonate arm define the basal plane, and the apical coordination is accomplished via the nitrogen atom trans to the pyridine ring of the macrocycle. To achieve this geometric arrangement, the macrocycle adopts a folded conformation. This structure seems consistent with Uv-vis-NIR spectroscopy for the Ni(2+) and the Cu(2+) complexes and with the EPR for the latter.     

A critical evaluation of the factors determining the effect of intramolecular hydrogen bonding on the O-H bond dissociation enthalpy of catechol and of flavonoid antioxidants

New experimental results on the determination of the bond dissociation enthalpy (BDE) value of 3,5-di-tert-butylcatechol, a model compound for flavonoid antioxidants, by the EPR radical equilibration technique are reported. By measurement of the equilibrium constant for the reaction between 3,5-di-tert-butylcatechol and the 2,6-di-tert-butyl-4-methylphenoxyl radical, in UV irradiated isooctane solutions at different temperatures, it has been shown that the thermodynamic parameters for this reaction are DeltaH degrees = -2.8+/-0.1 kcal mol(-1) and DeltaS degrees = +1.3+/-0.2 cal mol(-1) K(-1). This demonstrates that the entropic variations in the hydrogen exchange reaction between phenols and the corresponding phenoxyl radicals are also negligible when one of the reacting species is a polyphenol and that the EPR radical equilibration technique also allows the determination of the Obond;H BDEs in intramolecularly hydrogen-bonded polyphenols. The BDE of 3,5-di-tert-butylcatechol (78.2 kcal mol(-1)) was determined to be identical to that of alpha-tocopherol. Through use of the group additivity rule, this piece of data was also used to calculate the strength of the intramolecular hydrogen bond between the hydroxyl proton and the oxygen radical centre in the corresponding semiquinone radical (5.6 kcal mol(-1)), which is responsible both for the excellent antioxidant properties of catechols and for the BDE of catechol (81.8 kcal mol(-1)). These values are in poor agreement with those predicted by DFT calculations reported in the literature (9.5 kcal mol(-1) and 77.6 kcal mol(-1), respectively). Extensive theoretical calculations indicate that the BDE of catechol is reproduced well (81.6 kcal mol(-1)) by use of diffuse functions on oxygen and the CCSD method.     

Towards an order-N DFT method

One of the most important steps in a Kohn-Sham (KS) type density functional theory calculation is the construction of the matrix of the KS operator (the “Fock” matrix). It is desirable to develop an algorithm for this step that scales linearly with system size. We discuss attempts to achieve linear scaling for the calculation of the matrix elements of the exchange-correlation and Coulomb potentials within a particular implementation (the Amsterdam density functional, ADF, code) of the KS method. In the ADF scheme the matrix elements are completely determined by 3D numerical integration, the value of the potentials in each grid point being determined with the help of an auxiliary function representation of the electronic density. Nearly linear scaling for building the total Fock matrix is demonstrated for systems of intermediate size (in the order of 1000 atoms). For larger systems further development is desirable for the treatment of the Coulomb potential. Received: 30 March 1998 / Accepted: 6 July 1998 / Published online: 15 September 1998     

Charge transfer complexes between I2 and 1,3-imidazolidine-2-thione and its n-mono- and n,n'-di-substituted derivatives

Spectrophotometric studies of iodine with 1,3-imidazolidine-2-thione and its N-mono- and N,N'-di-alkyl derivatives (alkyl = Me, Et) have been carried out in carbon tetrachloride and dichloromethane solutions. In each case, a new intense peak due to the charge transfer band of a 1:1 molecular complex was observed. The thermodynamic and spectral characteristics of these complexes have been determined. The well known “blue shift” of the visible band of I₂ is verified, but the analogous blue shift of the donor absorption is hidden by the solvents. To explain the very high differences between the stability constants of the N,N'-disubstituted derivatives and the others, an intramolecular H-bonding between NH and terminal iodine has been hypothesized. IR measurements carried out on N-ethyl-1,3-imidazolidine-2-thione seem to support this hypothesis.     

Cationic polymerizations in the presence of an electric field with polarity periodically reversed

A study has been made of the polymerizations of anethole, isobutylvinylether and cyclohexylvinylether by iodine in ethylene dichloride solution, at 25° under high vacuum in the presence of a constant or periodically reversed polarity d.c. field.

The influences of the reversal frequency, distance between the electrodes as well as mechanical stirring of the solution on the electric field effect (measured as R_PE/R_PO) have been investigated. Experiments were carried out to evaluate separately, in both compartments of the cell, the amounts of titratable iodine and polymer formed as well as the polymerization rate after the electric field application.

The field induced rate enhancement can be cancelled by suitable reversal frequencies and mechanical stirring, and depends on the distance between the electrodes.

The results confirm that the field effect is mainly due to concentration gradients of active species as a consequence of field induced electrolytic phenomena.     

Nitration of trans-2-styrylthiophene

The nitration of trans-2-styrylthiophene in carbon tetrachloride, nitroethane and acetic anhydride has been investigated. The nitration products were: β-nitro-2-styrylthiophene as the main product with 3-nitro-2-styrylthiophene and the 5-nitro isomer in smaller amounts. They were identified by the chromato-graphic and spectroscopic (uv and nmr) comparison with the reference compounds. The isomer percentages, determined by glc, were unchanged under different nitration conditions (time, temperature and molar ratio), but were dependent on the solvent used.     

The lagrangian approach to stochastic variational principles on curved manifolds

The classical definition of the action functional, for a dynamical system on curved manifolds, can be extended to the case of diffusion processes. For the stochastic action functional so obtained, we introduce variational principles of the type proposed by Morato. In order to generalize the class of process variations, from the flat case originally given by Morato to general curved manifolds, we introduce the notion of stochastic differential systems. These give a synthetic characterization of the process and its variations as a generalized controlled stochastic process on the tangent bundle of the manifold. The resulting programming equations are equivalent to the quantum Schrödinger equation, where the wave function is coupled to an additional vector potential, satisfying a plasma-like equation with a peculiar dissipative behavior.     

A theoretical study of the isomerization process in tris(2-thienyl)methyl radical

An INDO study has shown that the title compound exists in two diastereomeric propeller-like conformations, a cis form (C₃ symmetry), and a trans form (C₁ symmetry). The cis form is more stable than the trans form by 0·75 kcal/mole. The two forms undergo ready conformational interconversion by the two-ring flip mechanism, with activation energies of ca. 6 kcal/mole.