Ekeland’s principle for vector equilibrium problems

In this paper, the authors deal with bifunctions defined on complete metric spaces and with values in locally convex spaces ordered by closed convex cones. The aim is to provide a vector version of Ekeland’s theorem related to equilibrium problems. To prove this principle, a weak notion of continuity of a vector-valued function is considered, and some of its properties are presented. Via the vector Ekeland’s principle, existence results for vector equilibria are proved in both compact and noncompact domains.     

Focusing-defocusing phenomena of radio echo signal after passage of the morning solar terminator

Summary Campaign results of high-repetition ionospheric soundings, performed at the ionospheric station of Rome using a Digisonde 128P, are presented. We introduce an index derived from run rapid ionograms that can be used to detect oscillations of isodensity surfaces ofF ionospheric region started by morning solar terminator.     

The behaviour of alkylcobalt carbonyls under “oxo” conditions

Both normal and secondary alkylcobalt carbonyls are shown not to undergo isomerization under catalytic “oxo” conditions, and the relevance of this obervation for the mechanism of the “oxo” reaction is discussed.     

Triorganotin 4-isopropylbenzoates as model transesterification catalysts for triorganotin carboxylates grafted to cross-linked polystyrene

Starting from 4-isopropylbenzoic acid, three new triorganotin carboxylates bearing methyl, butyl and phenyl substituents at tin, respectively, were prepared and fully characterized by spectroscopic and thermal techniques, with particular regard to the coordination number of tin atom, in solution as well as in the solid state. The triorganotin compounds, tested as transesterification catalysts in the reaction between ethyl acetate and primary, secondary or tertiary alcohol, respectively, displayed, as expected, a strong decrease of activity on passing from the primary to the tertiary alcohol reactant. Different activities by the tin carboxylates were also observed in the reaction between primary alcohol and ethyl acetate. The reaction mechanism, as elucidated by Sn NMR, involves coordination of both ester substrate and alcohol reactant to the triorganotin compound, the reaction conversion appearing related not only to the Lewis acidity of the tin atom, but also to the nature of the reactants. Preliminary catalytic tests were also carried out in the reaction between glyceryl tridodecanoate (as a model of natural triglyceride) and ethanol, mimicking the preparation of biodiesel fuel. Although in this case lower conversions were obtained with respect to the reactions on ethyl acetate, the catalytic activity of organotin derivatives appears considerable.     

Structure factors calculated from molecular wavefunctions. II. Analysis of the molecular charge distribution in diborane

The electron density distribution in the crystal of diborane has been determined using structure factors based on molecular densities. The libration-correction and third-cumulant terms have also been included in the expression for the temperature factor. The equilibrium r_e values for BH bond lengths obtained by this treatment are about 0.02Åshorter than the spectroscopic ones. A total librational motion of the diborane molecule is determined: the root-mean-square oscillations about inertial axis are 11.0° (8), 9.6° (5) and 7.2° (5), respectively. The dynamic theoretical deformation density shows a three-center BHB bond picture for the bridge structure.     

Acid-base equilibria in organic solvents : Part 2. Cyclic voltammetry in the study of hydrogen-bond formation

The cyclic voltammetric technique is used to study hydrogen-bond formation in some polar organic solvents (S) of electroanalytical interest (1,2-dimethoxy, tetrahydrofuran, dimethylformamide, dimethyl sulphoxide and pyridine). The cathodic shift of the proton reduction caused by stepwise addition of the solvent investigated to a solution of anhydrous perchloric acid in acetonitrile is utilized. The theoretical treatment applied produced evidence that HS⁺ and HS⁺₂ are the only acidic species involved, so that the relevant formation constants can be evaluated. The data obtained mostly compare well with those available in the literature. The features that condition the tendency to hydrogen-bonding and the effect of hydrogen bonding on solvent basicity are discussed.     

Synthesis and characterization of Cu,Ni and Zn binding capability of some amino- and imidazole hydroxamic acids: Effects of substitution of side chain amino-N for imidazole-N or hydroxamic-N-H for -N-CH3 on metal complexation

Solution equilibrium studies on Cu(II)-, Ni(II)- and Zn(II)-N-Me-β-Alaninehydroxamic acid (N-Me-β-Alaha), -N-Me-α-alaninehydroxamic acid (N-Me-α-Alaha), -Imidazole-4-carbohydroxamic acid (Im-4-Cha), -N-Me-imidazole-4-carbohydroxamic acid (N-Me-Im-4-Cha) and -Imidazole-4-acetohydroxamic acid (Im-4-Aha) systems have been performed by pH-potentiometry, UV–Vis spectrophotometry, EPR, CD, ESI-MS and ¹H NMR methods. According to the results: (i) the amino-N atoms are more basic in N-Me-α-Alaha and N-Me-β-Alaha than the hydroxamate function, but the trend is just the opposite between the imidazole-N(3) and hydroxamate. (ii) The metal ion anchor is always the hydroxamate part in the amino acid derivatives, while it is always the imidazole-N(3) in the studied imidazolehydroxamic acids. (iii) The three studied N-Me derivatives do not form metallacrowns. Only hydroxamate type chelate is formed with N-Me-β-Alaha, but with N-Me-α-Alaha a new type of coordination mode (via amino-N and hydroxamate-O) also exists. N-Me-Im-4-Cha also forms a dinuclear complex, [M₂L₃], with Cu(II) and Ni(II) (but not with Zn(II)). In this complex, one of the three ligands might bridge the two metal ions (five-membered hydroxamate-(O,O) plus five-membered (N_im, O_carb) bridging bis-chelating mode), while each of the additional two ligands binds to one metal. (iv) The two studied N–H derivatives, having dissociable proton on the hydroxamic-N, are able to form metallacrown species. A pentanuclear complex, [M₅L₄H₋₄], is exclusively formed above pH 4 between Cu(II) and Im-4-Aha. Interestingly, this 12-metallacrown-4 type complex, although together with various mononuclear binding isomers, appears also with Ni(II) and Zn(II). Unfortunately, the complexes of Im-4-Cha are not soluble in water at physiological pH at all.     

Asymmetric synthesis by chiral ruthenium complexes : VI. Enantiomer-discrimination in the hydrogen transfer from racemic alcohols to prochiral ketones in the presence of H4Ru4(CO)8[(−)-diop]2

Hydrogen transfer from racemic alcohols to prochiral ketones in the presence of H₄Ru₄(CO)₈[(?)-DIOP]₂ has been examined. The enantiomer-discrimination is influenced by the structure of the reactants, temperature and the excess of phosphine present.     

Experimental and theoretical study of the interaction of CO2 with alpha-Al2O3

Density functional molecular cluster calculations are combined with X-ray photoelectron spectroscopy (XPS), quadrupolar mass spectrometry (QMS), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to investigate the interaction of CO2 with alpha-Al2O3 and partially reduced alpha-Al2O3. The electronic structure of the stoichiometric and partially reduced substrate, adsorbate geometries, chemisorption enthalpies, and adsorbate vibrational parameters are computed and discussed. Theoretical results agree quite well with experimental data and previous theoretical investigations. As far as the adsorbate-substrate interaction is concerned, the results of our calculations indicate that CO2 forms bidentate-chelating carbonate species. The bonding scheme of this surface complex implies a significant substrate-->adsorbate transfer of charge (from the occupied dangling bond of a surface Lewis base site into one component of the CO2 2 pi u LUMO) assisted by a definitely weaker adsorbate-->substrate donation (from one component of the CO2 1 pi g HOMO into an empty dangling bond of a surface Lewis acid site). Our estimate of the chemisorption enthalpy (-15 kcal/mol) agrees quantitatively with calorimetric data reported for CO2 adsorbed on high surface area alpha-alumina (-16.0 kcal/mol). [Mao, C.-F.; Vannice, M. A. Appl. Catal. A 1994, 111, 151.] According to XPS and QMS outcomes, theoretical results predict that the interaction of CO2 with partially reduced alpha-Al2O3 gives rise to the reduction of the adsorbate to CO and to the concomitant substrate reoxidation.