Interaction of Ca2+ with uracil and its thio derivatives in the gas phase

The structures and relative stabilities of the complexes formed by uracil and its sulfur derivatives, namely, 2-thio-, 4-thio, and 2,4-dithio-uracil when interacting with Ca(2+) in the gas phase have been analyzed by means of density functional theory (DFT) calculations carried out at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) level. For uracil and 2,4-dithiouracil, where the two basic sites are the same, Ca(2+) attachment to the heteroatom at position 4 is preferred. However, for the systems where both types of basic centers, a carbonyl or a thiocarbonyl group, are present, Ca(2+)-oxygen association is favored. The most stable complexes correspond to structures with Ca(2+) bridging between the heteroatom at position 2 of the 4-enol (or the 4-enethiol) tautomer and the dehydrogenated ring nitrogen, N3. The enhanced stability of these enolic forms is two-fold, on the one hand Ca(2+) interacts with two basic sites and on the other triggers a significant aromatization of the ring. Besides, Ca(2+) association has a clear catalytic effect on the tautomerization processes which connect the oxo-thione forms with the enol-enethiol tautomers. Hence, although the enol-enethiol tautomers of uracil and its thio derivatives should not be observed in the gas phase, the corresponding Ca(2+) complexes are the most stable species and should be accessible, because the tautomerization barriers are smaller than the Ca(2+) binding energies.     

Theoretical and electrochemical studies on organometallic symmetrical schiff base complexes of Zn(II), Cu(II), Ni(II) and Co(II)

The electronic communication between two redox centres through a Schiff base complex has been investigated in a series of ethylenediimine-bis(1-ferrocenyl-1,3-butanedionate) complexes of Zn(II) 1, Cu(II) 2, Ni(II) 3 and Co(II) 4. Cyclic voltammetry experiments of 1 and 2 exhibit a unique two-electron reversible oxidation wave, whereas in the case of 3 and 4 two and three one-electron oxidation processes are, respectively, observed. These results suggest some electronic interaction between the iron atoms of the ferrocenyl groups. DFT calculations carried out on model complexes show that for all the studied compounds the removal of the first two electrons corresponds to the oxidation processes of the iron centres in the weakly coupled ferrocenyl termini. The electronic communication between the two iron centres increases on going from 1 to 4. Finally, a re-indexation of the bands observed in the UV-Visible spectra has been carried out using TDDFT calculations.     

Catalytic Alkyne Semihydrogenation with Polyhydride Ni/Ga Clusters

The bimetallic, decanuclear Ni₃Ga₇-cluster of the formula [Ni₃(GaTMP)₃(μ²-GaTMP)₃(μ³-GaTMP)] ( 1 , TMP=2,2,6,6-tetramethylpiperidinyl) reacts reversibly with dihydrogen under the formation of a series of (poly-)hydride clusters 2 . Low-temperature 2D NMR experiments at −80 °C show that 2 consist of a mixture of a di- ( 2_Di ), tetra- ( 2_Tetra ) and hexahydride species ( 2_Hexa ). The structures of 2_Di and 2_Tetra are assessed by a combination of 2D NMR spectroscopy and DFT calculations. The cooperation of both metals is essential for the high hydrogen uptake of the cluster. Polyhydrides 2 are catalytically active in the semihydrogenation of 4-octyne to 4-octene with good selectivity. The example is the first of its kind and conceptually relates properties of molecular, atom-precise transition metal/main group metal clusters to the respective solid-state phase in catalysis.     

Computational study on the kinetics of the reaction between Ca and urea

A computational study, in the framework of statistical kinetic theories, of the reaction of Ca²⁺ with urea has been carried out. The kinetically preferred products are NH₃ + [CaOCNH]²⁺, which are the fifth products in order of stability. The second kinetically preferred products are , followed by [CaNH₃]²⁺+HNCO, whereas the most stable ones, and , appear only in residual quantities. These estimates are in agreement with the experimental evidence and provide a suitable mechanism to understand the competition between Coulomb explosion and neutral loss processes.     

Universal Components of Random Nodal Sets

We give, as L grows to infinity, an explicit lower bound of order \({L^{\frac{n}{m}}}\) for the expected Betti numbers of the vanishing locus of a random linear combination of eigenvectors of P with eigenvalues below L. Here, P denotes an elliptic self-adjoint pseudo-differential operator of order \({m > 0}\), bounded from below and acting on the sections of a Riemannian line bundle over a smooth closed n-dimensional manifold M equipped with some Lebesgue measure. In fact, for every closed hypersurface \({\Sigma}\) of \({\mathbb{R}^n}\), we prove that there exists a positive constant \({p_\Sigma}\) depending only on \({\Sigma}\), such that for every large enough L and every \({x \in M}\), a component diffeomorphic to \({\Sigma}\) appears with probability at least \({p_\Sigma}\) in the vanishing locus of a random section and in the ball of radius \({L^{-\frac{1}{m}}}\) centered at x. These results apply in particular to Laplace–Beltrami and Dirichlet-to-Neumann operators.     

Stabilization of acepentalene by coordination to transition metals: a DFT investigation

The possibility of stabilizing the unstable acepentalene (acp) molecule through coordination to transition metals is investigated by means of DFT calculations. Comparison with related experimentally known pentalene complexes indicate that their acp homologues are just slightly less stable, suggesting that they could be synthesized and isolated. Other original mono- and trinuclear species, such as M(acp)2 (M = Zr, Hf), Co3(acp)2-, and Nb3(acp)2+, are also predicted to be stable.     

Electron-sponge behavior and electronic structures in cobalt-centered pentagonal prismatic Co11Te7(CO)10 and Co11Te5(CO)15 cluster anions

The novel cluster anion [Co(11)Te(5)(CO)15]- ([3]-) has been isolated and structurally characterized as part of the salt [Cp*(2)Nb(CO)2][3] (Cp* = C(5)Me(5)). The cobalt-centered Co10 pentagonal prism is surrounded by a shell of two mu5-Te, three mu4-Te ligands, and 15 CO groups in terminal, symmetrical, and sigma-semibridging bonding modes. The hybrid carbonyl-telluride character of the ligand shell is reflected in the solid state by a one-dimensional assembly of polyhedral prisms along a backbone of [Cp*(2)Nb(CO)2]+ cations. Electrochemical studies reveal the presence of four redox couples of [3]n (n = -1 to -5). The electronic structures of various metal-centered and empty pentagonal-prismatic (PP) M10 clusters (M = Co, Ni) are calculated and compared to those of pentagonal-antiprismatic (PA) M10 structures. Closed-shells of 152 and 156 metal valence electrons, respectively, are found to determine the electronic structures and chemical properties of these geometries. From these considerations, magnetic properties have been predicted. They have been verified for the [Co(11)Te(7)(CO)10]- cluster anion, which exhibits a singlet-triplet gap of 0.318 kcal/mol.     

Alkynyl ruthenium colorimetric sensors: optimizing the selectivity toward fluoride anion

We report on the synthesis of alkynyl ruthenium colorimetric sensors whose receptors are constituted by thiazolidinedione, rhodanine, or barbituric heads as recognition centers for anions. As modifications in the charge density at these recognition centers affect the whole molecule, through the alkynyl ligand acting as a communicating wire, the effects of hydrogen-bonding interactions with the anions were observed with the naked eye and monitored by UV-vis absorption spectrometry. The selectivity of the sensors was improved through electronic modifications of the alkynyl ruthenium subunit: the higher the electron density at the receptor head, the higher the selectivity is. TD-DFT calculations rationalize the long-range electronic communication as a main characteristic of the alkynyl ruthenium species and as a key to improve the selectivity of alkynyl ruthenium-based sensors toward anions.     

Development of miniaturized pH biosensors based on electrosynthesized polymer films

A new type of pH biosensor was developed for biological applications. This biosensor was fabricated using silicon microsystem technology and consists in two platinum microelectrodes. The first microelectrode was coated by an electrosynthesized polymer and acted as the pH sensitive electrode when the second one was coated by a silver layer and was used as the reference electrode. Then, this potentiometric pH miniaturized biosensor based on electrosynthesized polypyrrole or electrosynthesized linear polyethylenimine films was tested. The potentiometric responses appeared reversible and linear to pH changes in the range from pH 4 to 9. More, the responses were fast (less than 1 min for all sensors), they were stable in time since PPy/PEI films were stable during more than 30 days, and no interference was observed. The influence of the polymer thickness was also studied.     

Study of luminescent defects in hafnia thin films made with different deposition techniques

Hafnia thin films for high-power optical coatings have been characterized by photoluminescence pumped by 4.66 eV photons and photothermal deflection measurements. These data are compared to the statistical laser damage behavior in order to find correlations between destructive and non-destructive characterizations. Thin films have been produced at two thicknesses and using different thin-film deposition techniques typically employed for optical coating fabrication: EBD (HfO₂ target), EBD (Hf target), RLVIP and DIBS. The photoluminescence spectra show significant differences depending on the deposition techniques and thicknesses. EBD films show significant luminescence but the luminescence of ion-assisted films could not be distinguished from the uncoated substrate. All EBD coating spectra could be described by a linear combination of four bands. Further, XRD measurements show that the 255-nm-thick films had a relatively high crystallinity: EBD films contained the monoclinic phase and the ion-assisted films contained oriented nanocrystals of orthorhombic hafnia. The presence of orthorhombic phases indicates high compressive strain quenching the photoluminescence of these samples.