New Copper(II) Complexes Containing 2-Furoic Hydrazide and 5-Nitro-2-Furoic Hydrazide Ligands: Synthesis,Thermal, Magnetic and Spectroscopic Characterization

Two new copper(II) complexes, [Cu₂(L¹)₄(H₂O)₂](SO₄)₄· 2H₂O and [Cu(L²)₂(H₂O)₂]SO₄, were isolated containing 2-furoic hydrazide and 5-nitro-2-furoic hydrazide ligands, respectively. The complexes were characterized by thermal, magnetic and spectroscopic techniques, showing a distorted tetragonal environment around the metal ion. Compound (1), containing 2-furoic hydrazide as the ligand, appears to be dimeric in the solid state, with substituted hydrazine acting as a bidentate bridging ligand. On the contrary, a monomeric species was observed with the 5-nitro-2-furoic hydrazide ligand, probably in the cis configuration, for compound (2). Magnetic measurements for the binuclear copper(II) complex (1) were carried out at low temperatures, in the 2–300 K range, and a magnetic field of 500 G, indicating that besides an intramolecular ferromagnetic interaction between the two Cu(II) centers, for which J/k = 1.07 K, further weak antiferromagnetic interactions between adjacent dimers, with Jz/k =–0.95 K, should be taken into account. However, in MeOH/H₂O solution, evidence of equilibria involving the dimer (1) and the corresponding mononuclear cis and trans species was obtained from e.p.r. spectra.     

Model studies of DNA C5' radicals. Selective generation and reactivity of 2'-deoxyadenosin-5'-yl radical

The reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine has been investigated by radiolytic methods coupled with product studies and addressed computationally by means of DFT-B3LYP calculations. Pulse radiolysis revealed that this reaction was complete in approximately 0.3 mus, and, at this time, no significant absorption was detected. The spectrum of a transient developed in 20 mus has an absorbance in the range 300-500 nm (epsilon(max) congruent with 9600 M(-1) cm(-1) at 360 nm), and it was assigned to aromatic aminyl radical 3. Computed vertical transitions (TD-UB3LYP/6-311+G) are in good agreement with the experimental observations. Radical 3 is obtained by the following reaction sequence: one-electron reductive cleavage of the C-Br bond that gives the C8 radical, a fast radical translocation from the C8 to C5' position, and an intramolecular attack of the C5' radical at the C8,N7 double bond of the adenine moiety. The rate constant for the cyclization is 1.6 x 10(5) s(-1). On the basis of the theoretical findings, the cyclization step is highly stereospecific. The rate constants for the reactions of C5' and aminyl 3 radicals with different oxidants were determined by pulse radiolysis methods. The respective rate constants for the reaction of 2'-deoxyadenosin-5'-yl radical with dioxygen, Fe(CN)(6)(3)(-), and MV(2+) in water at ambient temperature are 1.9 x 10(9), 4.2 x 10(9), and 2.2 x 10(8) M(-1) s(-1). The value for the reaction of aminyl radical 3 with Fe(CN)(6)(3-) is 8.3 x 10(8) M(-1) s(-1), whereas the reaction with dioxygen is reversible. Tailored experiments allowed the reaction mechanism to be defined in some detail. A synthetically useful radical cascade process has also been developed that allows in a one-pot procedure the conversion of 8-bromo-2'-deoxyadenosine to 5',8-cyclo-2'-deoxyadenosine in a diastereoisomeric ratio (5'R):(5'S) = 6:1 and in high yield, by reaction with hydrated electrons in the presence of K(4)Fe(CN)(6).     

A clear-cut experimental method to discriminate between in-plane and out-of-plane molecular transition moments

A new method to orient organic molecules, based on their absorption as guest of the crystalline nanoporous delta phase of uniaxially stretched syndiotactic polystyrene films, is presented. This molecular alignment method not only allows high degrees of guest orientation to be attained but also orients the planar guest molecules with their smallest cross section nearly parallel to the stretching direction (B) rather than perpendicular (A, as usual for absorption in amorphous polymeric phases). As a consequence, in-plane and out-of-plane transition moment vectors maximize their absorption intensities for light polarization nearly perpendicular and parallel to the stretching direction, respectively. Hence, simple linear dichroism measurements by polarized spectra can allow an easy and clear-cut discrimination between in-plane and out-of-plane guest transition moment vectors.     

Imaging metallic multilayer structures through ultrafast optically driven excited electron transport

Received: 20 September 1998 / Revised version: 21 December 1998     

Numerical Modeling of a He–N2 Capillary Surface Wave Discharge at Atmospheric Pressure

A self-consistent steady-state spatially averaged collisional-radiativemodel in which the rate coefficients involving electrons are calculatedfrom the solution to the electron Boltzmann equation has been developedfor describing an atmospheric pressure plasma in helium–nitrogen(He–N₂) mixtures. The influence of small nitrogenconcentrations (typically less than 1%) on the discharge characteristicsis studied and compared with available experimental data. It is foundthat nitrogen is highly dissociated and that the density of metastablehelium atoms is considerably reduced by the presence of nitrogen, evenat such low concentrations.     

The Ising--Sherrington-Kirpatrick Model in a Magnetic Field at High Temperature

We study a spin system on a large box with both Ising interaction and Sherrington-Kirpatrick couplings, in the presence of an external field. Our results are: (i) existence of the pressure in the limit of an infinite box. When both Ising and Sherrington-Kirpatrick temperatures are high enough, we prove that: (ii) the value of the pressure is given by a suitable replica symmetric solution, and (iii) the fluctuations of the pressure are of order of the inverse of the square of the volume with a normal distribution in the limit. In this regime, the pressure can be expressed in terms of random field Ising models.     

The Compressible to Incompressible Limit of One Dimensional Euler Equations: The Non Smooth Case

We prove a rigorous convergence result for the compressible to incompressible limit of weak entropy solutions to the isothermal one dimensional Euler equations.     

Adsorption and thermodynamic studies of Cu(II) and Zn(II) on organofunctionalized-kaolinite

Kaolinite-bearing clay samples from Perus, São Paulo state, Brazil, were used for chemical modification process with dimethyl sulfoxide and organofunctionalized with the silyating agent (RO)₃Si(CH₂)₃NH(CH₂)₂NH₂ in the present study. The resulting material and natural kaolinite were subjected adsorpion process with Cu(II) and Zn(II) from aqueous solution at pH 6.0 and controlated temperature of 298 K. The Langmuir adsorption isotherm model has been applied to fit the experimental data. The results showed that the chemical modification process increases the basal spacing of the natural kaolinite from 0.711 to 0.955 nm. The energetic effects caused by Cu(II) and Zn(II) interactions were determined through calorimetric titration at the solid–liquid interface and gave a net thermal effect that enabled the calculation of the exothermic values and the equilibrium constant.