Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.     

Extraction--spectrophotometric determination of oxalate in urine and blood serum

An extraction--spectrophotometric method is described for the determination of oxalate, based on the formation of a mixed ligand vanadium (V)--mandelohydroxamic acid--oxalate complex. The complex was extracted into a solution of trioctylmethylammonium chloride (Adogen 446) in toluene and the absorbance measured at 535 nm. The experimental variables and interferences in this determination were studied. The detection limit is 0.5 microgram ml-1 and the range of application is between 2 and 8 micrograms ml-1. The method was applied to the determination of oxalate in urine and blood serum.     

Quelques considérations sur la structure électronique du naphtalène

Theπ electronic structure of naphtalene is studied, using a semi-empirical approximation for coulombic and resonance integrals; forH andS (H, hamiltonian;S, overlap) the complete matrices are adopted, together withKohlrausch's nuclear effective charges. A non uniform charge distribution is obtained, with charge accumulation at the tertiary carbons. Bond orders between non neighboring atoms indicate contributions to the fundamental state from other structures besides the classical ones. A comparative study of the obtained interatomic distances is carried. It is shown that [H, S]=0 is not a sufficient condition for the identity of the bond orders for the fundamental state calculated with and without overlap.     

Synthesis and structural study of ethylmethylsilanediol by quantum chemical calculations and IR and Raman spectroscopies

The intrinsic instability of small alkylsilanediols and their propensity toward self-condensation have been the main determiners of the scarce number of experimental works dealing with their synthesis and vibrational characterization. This is the case of the title compound, ethylmethylsilanediol (EMSD), which preparation and purification is, to the best of our knowledge, firstly reported in the present work. Hence, we also report the first records of the IR and Raman spectra of the molecule that have been thoroughly analyzed and completely assigned with the support of DFT calculations. Further, as a previous step of the vibrational assignment, we accomplished a thorough conformational analysis that allowed indentifying five conformations that represent minima on the potential energy surface (PES) of the molecule, depending on the different arrangement that both, the alkyl side chain and the –OH groups, can adopt. Finally, natural bond orbital (NBO) calculations were implemented to justify the stability order and the calculated geometries for the set of conformers in terms of the stabilization derived from the anomeric effect.     

Two Types of σ-Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes

The reactivity of amidinatotetrylenes of the type E(tBu₂bzm)R¹ (E=Si, Ge; tBu₂bzm=N,N′-bis(tertbutyl)benzamidinate; R¹=alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C₂R²}(CO)₅] (R²=Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C² atom (γ-attack), which leads to σ-allenyl complexes in which the original C_carbene atom maintains its attachment to the Cr(CO)₅ and OEt groups (compounds 3 ), and (b) attack of the amidinatotetrylene to the C_carbene atom (α-attack), which ends in σ-allenyl complexes in which the original C_carbene atom is not attached to the metal atom and has been inserted into an E−N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 ). It has been found that compounds 3 are thermodynamically less stable than their corresponding 4 isomers and that some of the former (E=Ge; R¹=CH₂SiMe₃) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R¹=Mes, tBu) end in the carbene-substitution products [Cr{E(tBu₂bzm)R¹}(CO)₅].     

Oxygen Barrier Properties of Waterborne Polyurethane/Silica Hybrids

The oxygen barrier properties of films obtained from waterborne polyurethane/silica hybrid dispersions were analyzed. Two different types of polyurethanes were used, based on poly(propylene glycol) and poly(1,4-butylene adipate). Three different strategies were followed in the preparation of the hybrid dispersions. In the first type of materials (series 1), the inorganic part came exclusively from the covalent incorporation of trifunctional silane groups into the polymeric chains. The other two series contained, in addition to the trifunctional silane groups, tetrafunctional silane groups either physically blended (series 2) or “in situ” generated (series 3). Materials of series 1 showed an increase of the oxygen permeability coefficient with the silane content. However, the other two types of materials presented just the opposite dependence. In this latter case, the systems containing “in situ” generated silica (series 3) presented higher permeability coefficient values, probably because of the steric hindrance imposed by the polyurethane that gave rise to silica networks containing silanol groups and free volume holes. Moreover, lower permeability coefficient values were obtained when larger size particles were added. This fact could mean that the polyurethane/silica interface effects were not totally hindered even when the organic/inorganic phases were covalently bonded.