Dramatic changes in geometry after ionization: experimental and theoretical studies on the electronic properties of fluorocarbonyl (mono-, di-, and tri-) sulfur compounds

In this work, we present a complete study on He I photoelectron spectroscopy (PES) for the fluorocarbonyl mono-, di-, and trisulfur compounds FC(O)SCl, FC(O)SSCH(3), and FC(O)SSSC(O)F. After optimizations of the structure for stable conformers at different levels of theory, a complete theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) was performed. Calculations of radical-cationic forms were carried out in order to compare their properties with those of the neutral molecules. The first IP values are 10.7, 9.0, and 10.5 eV for FC(O)SCl, FC(O)SSCH(3), and FC(O)SSSC(O)F, respectively. The groups bonded to the S atom mainly influence the ionizations originating from the sulfur lone pairs. A wide electronic delocalization in the FC(O)S moiety can be deduced from experimental and theoretical results, which leads to a strong energetic stabilization of the n' '(S) (sulfur lone pair pi orbital). Other conclusions relate to effects on the substituents attached to the S atom and the importance of the molecular planarity in the orbital stabilization of the FC(O)S moiety for the neutral molecules. It is worthwhile mentioning that FC(O)SCl retains its planar structure after ionization, but drastic changes occur in the geometry of both FC(O)SSCH(3) and FC(O)SSSC(O)F. The FC(O)SSCH(3) molecule adopts a heavy atom planar structure after ionization. The FC(O)SSS moiety becomes a planar form after the ionization of the FC(O)SSSC(O)F molecule, whereas the second C(O)F group maintains its original conformation with respect to the SSS group.     

Electronic and Steric Influence of Axial and Equatorial Ligands in Vitamin B12 Model Complexes Derived from Cobaloxime: Electrochemical and 59Co-NMR Studies

In four series of strictly related organocobalt complexes, derived from cobaloximes by replacement of the O…H…O with O…-BF₂…O and/or (CH₂)₃ groups, the trends of ⁵⁹Co-NMR shielding and electrochemical data are discussed. A largely parallel behaviour of the plots of E_1/2(I) values for the first Co(III)/Co(II) electron transfer vs. the ₅₉Co chemical shifts reflects the similar sensitivity of the two parameters to a change in electron affinity of the central metal ion due to a variation of the organic group R. E_1/2(II) values for the second Co(II)/CO(I) electron transfer are less sensitive to the change of R, but the trend of the plot vs. δ(₅₉Co) is still parallel in the four series. Consistent deviations from a roughly linear dependence of E_1/2(I) on pK_a of the hydrocarbon acid corresponding to R, on Taft constant s?_* and on ⁵⁹Co shielding are noticed for the isopropyl derivatives and attributed to a steric effect. This was confirmed in a series of R? Co(DMG)pyridine complexes in which ⁵⁹Co shielding decreases steadily with increasing steric parameter E_s (Taft) of the alkyl group. There is experimental evidence from X-ray data that δ(⁵⁹Co) decreases with an increase of the Co? C bond length, illustrating steric hindrance in alkyl coordination to be responsible for the decreased shielding of the ⁵⁹Co nucleus. The relative displacements of the graphic displays for the different series reflect the effect of changes in electron affinity of the redox center, due to the equatorial ligand, which, in turn, is caused by variations in the electron-withdrawing power due to the introduction of the BF₂ group and by the change from ?2 to ?1 valence of the (CH₂)₃-capped ligands.     

The effects of oxygenation on the optical properties of dimethyl-dithienothiophenes: comparison between experiments and first-principles calculations

Modifications of the optical properties of dimethyl-dithienothiophenes due to the oxygen functionalization of the central sulfur atom are investigated. We have measured the absorption, photoluminescence (PL) and PL excitation spectra, the PL quantum efficiencies, and the PL decay times. These experimental results are interpreted and compared with first-principles time-dependent density-functional theory calculations, which predict, for the considered systems, excitation and emission energies with an accuracy of 0.1 eV. It is found that the oxygenation strongly changes optical and photophysical properties. These effects are related to the modifications of the energetically lowest-unoccupied molecular orbital and the energetically second highest occupied one, which change the relative position of the two lowest singlet and triplet excited states.     

Catalytic oxidation of thiols to disulfides using iodine and CeCl3·7H2O in graphite

A very simple procedure for the efficient oxidation of thiols to disulfides catalyzed by I₂/CeCl₃·7H₂O in graphite and ethyl acetate as the solvent, in an open system at room temperature is described. The reaction proceeds cleanly under mild conditions and was performed with aromatic, aliphatic, and heterocyclic thiols.     

Preparation, characterization and thiol-thione tautomeric studies of 2-thiono-4-methyl-5-(2,2,2-trifluoro-1-trifluoromethylethyl)-1,3-thiazoline

The synthesis of a new trifluoromethyl 2-mercapto-1,3-thiadiazole derivative from 5,5,5-trifluoro-4-(trifluoromethyl)-3-penten-2-one, its structural study in the solid state and in the liquid phase and the thiol-thione tautomeric equilibrium study of the title compound are reported. The crystalline structure shows short intermolecular FF contacts. Theoretical calculations using HF and DFT methods were performed. Vibrational spectra were calculated for both tautomers and compared with experimental data. The experimental and theoretical results can be interpreted in terms of the existence of a thione tautomer in the solid and in solution.     

Sterols with Cyclopropane-Containing Side Chains: Synthesis and Acid Isomerization

To test the eventual involvement of sterols with a cyclopropane containing side chain in bio-methylation processes, 24,25-methanocholest-5-en-3β-ol ( 5 ) and 24,25-methano-26-methylcholest-5-en-3β-ol ( 6 ) were synthesized. Together with the naturally occurring cyclopropane petrosterol ( 1 ), they were submitted to acid isomerization to provide chemical models for some of their potential metabolites.     

Polychloroprene rubbers used for adhesives: a small angle X-ray scattering preliminary investigation under stretching

Summary Three types of polychloroprene rubbers (Du Pont de Nemours Co. Neoprenes AD, WHV and W) used for adhesives and having different crystallization rates have been investigated.The mechanical properties of the Neoprenes samples were determined and the films were examined under stretching by SAXS for a preliminary investigation on their different superreticular order.

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Zusammenfassung Drei Typen von Polychloropren-Kautschuk mit verschiedener Kristallisationsgeschwindigkeit (Neoprene AD, WHV und W von Du Pont de Nemours), die als Klebemittel dienen, wurden als gedehnte Filme mechanisch und röntgenographisch (SAXS) untersucht. Sie zeigen Unterschiede in der Überstruktur.

     

In situ disorder-order transformation in synthetic gallosilicate zeolites with the NAT topology

Here, we report that synthetic gallosilicate molecular sieves with the NAT topology and Si/Ga ratios close to but slightly higher than 1.50 undergo an in situ transformation under their crystallization conditions. The materials have been studied ex situ by using powder X-ray diffraction, elemental and thermal analyses, and multinuclear MAS NMR. The transformation is characterized by a change in the distribution of Si and Ga of the NAT framework, from a quite (but not completely) disordered phase to a very highly (but not completely) ordered one, accompanied by a change from tetragonal to orthorhombic symmetry. During most of the solution-mediated transformation, no noticeable signs of fresh precipitation, phase segregation, or changes in the chemical composition are detected. Intermediate materials show variations in the degree of Si-Ga ordering and orthorhombic distortion and are not physical mixtures of the disordered and ordered phases. Ab initio calculations strongly suggest a preferential siting of Si in the tetrahedral sites involved in a smaller number of 4-rings in the NAT topology (i.e., the low multiplicity site). The cost of violations of Loewenstein's rule has also been calculated. For this topology and chemical composition the preferential siting and Loewenstein's rule drive together the system to the ordered configuration. A Monte Carlo sampling procedure affords a reasonable model for the initial, mainly disordered state, which fits well within the experimental disorder-order series.     

New Factorial Designs to Evaluate Chemisorption of Divalent Metals on Aminated Silicas

Adsorption processes of Cu(II), Co(II), and Hg(II) on two aminated silica gel surfaces with immobilized ethylenediamine and diethylenediamine groups were studied by a new 2(3) full factorial design. Two metal quantity levels, temperatures of 25 and 50 degrees C, and silica amounts of 100 and 200 mg were employed. Our study indicates that higher factorial design levels increase adsorption for all metals evaluated, with the exception of the mass parameter. Adsorption is also significantly affected by important antagonistic and synergistic effects involving all factors. Both functionalized silica gel surfaces present higher interactions and good perspectives in preconcentration studies for mercury. The factorial design results are also discussed in terms of some solvation properties for each of the metals studied. Copyright 2001 Academic Press.