Chemistry of 11-vertex rhodathiaboranes: reactions with monodentate phosphines

The reaction of [8,8-(PPh(3))(2)-nido-8,7-RhSB(9)H(10)] (1) with PR(3) in a 1:2 ratio affords mixtures that contain the mono-substituted bis-PR(3)-ligated rhodathiaboranes [8,8-(PPh(3))(L)-nido-8,7-RhSB(9)H(10)] [L = PMe(2)Ph (5), PMe(3) (6)] and the corresponding tris-PR(3)-ligated compounds [8,8,8-(L)(3)-nido-8,7-RhSB(9)H(10)] [L = PMe(2)Ph (7), PMe(3) (8)]. These latter species are more conveniently prepared from the reaction of 1 with three equivalents of the monodentate phosphines, PMe(2)Ph and PMe(3). Reaction between 1 and PMePh(2) in a 1:2 ratio yields the disubstituted rhodathiaborane [8,8-(PMePh(2))(2)-nido-8,7-RhSB(9)H(10)] (4), whereas the use of three equivalents of phosphine leads to the formation of B-ligated eleven-vertex [8,8,8-(PMePh(2))(2)(H)-nido-8,7-RhSB(9)H(9)-9-(PMePh(2))] (9). Compounds 4-9 have been characterized by NMR spectroscopy, and the structures of 8 and 9 confirmed by X-ray diffraction analyses. The characterization of the cluster compounds has been aided by the use of DFT calculations on some of the species. Variable-temperature NMR studies have demonstrated a lability of the PMePh(2) ligands in compounds 4 and 9, providing mechanistic insights about the ligand substitutional chemistry in these eleven-vertex rhodathiaboranes.     

Separation of enantiomers of ephedrine by capillary electrophoresis using cyclodextrins as chiral selectors: comparative CE, NMR and high resolution MS studies

The enantiomer migration order (EMO) of ephedrine was investigated in the presence of various CDs in CE. The molecular mechanisms of chiral recognition were followed for the ephedrine complexes with native α- and β-CD and heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD) by CE, NMR spectroscopy and high-resolution MS. Minor structural differences were observed between the complexes of ephedrine with α- and β-CD although the migration order of enantiomers was opposite when these two CDs were applied as chiral selectors in CE. The EMO was also opposite between β-CD and HDAS-β-CD. Significant structural differences were observed between ephedrine complexes with the native CDs and HDAS-β-CD. The latter CD was advantageous as chiral CE selector not only due to its opposite electrophoretic mobility compared with that of the cationic chiral analyte, but also primarily due to its enhanced chiral recognition ability towards the enantiomers of ephedrine.     

Electrochemistry of Os(2,2'-bpy)2ClPyCH2NHCOPh tethered to Au electrodes by S-Au and C-Au junctions

Osmium pyridine-bipyridine redox centers have been tethered to Au electrodes by chemical modification through Au-S and Au-C bonds respectively. 4-Mercapto benzoic acid and the reduction product of the aryl diazonium salt of 4-amino benzoic acid were reacted on Au surfaces, with further post-functionalization by chemical reaction of the osmium complex amino-pyridine derivative with the surface carboxylates. The resulting modified Au surfaces were characterized by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), resonant raman spectroscopy and cyclic voltammetry.     

Electrochemical gating of single osmium molecules tethered to Au surfaces

The electrochemical study of electron transport between Au electrodes and the redox molecule Os[(bpy)₂(PyCH₂ NH₂CO-]ClO₄ tethered to molecular linkers of different length (1.3 to 2.9 nm) to Au surfaces has shown an exponential decay of the rate constant k _ET ⁰ with a slope β = 0.53 consistent with through bond tunneling to the redox center. Electrochemical gating of single osmium molecules in an asymmetric tunneling nano-gap between a Au(111) substrate electrode modified with the redox molecules and a Pt-Ir tip of a scanning tunneling microscope was achieved by independent control of the reference electrode potential in the electrolyte, E _ref ? E _s, and the tip-substrate bias potential, E _bias. Enhanced tunneling current at the osmium complex redox potential was observed as compared to the off resonance set point tunneling current with a linear dependence of the overpotential at maximum current vs. the E _bias. This corresponds to a sequential two-step electron transfer with partial vibration relaxation from the substrate Au(111) to the redox molecule in the nano-gap and from this redox state to the Pt-Ir tip according to the model of Kuznetsov and Ulstrup (J Phys Chem A 104: 11531, 2000). Comparison of short and long linkers of the osmium complex has shown the same two-step ET (electron transfer) behavior due to the long time scale in the complete reduction-oxidation cycle in the electrochemical tunneling spectroscopy (EC-STS) experiment as compared to the time constants for electron transfer for all linker distances, k _ET ⁰.     

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions

We report an experimental study of the rotovibrational fundamental PQR-band shapes in the IR absorption spectra of HCl dissolved in condensed rare gases in a wide range of temperatures. The effective vibrational frequencies are determined from analysis of the fine rotational structure partially resolved in the band wings. The central Q-branch components appear redshifted with respect to the effective vibrational frequencies, their shifts in different solvents found to match the HCl stretching mode shifts in binary Rg...HCl van der Waals heterodimers. Theoretical quasi-free rotor and modified rotor models are applied to describe evolution of the band profiles at changing thermodynamic conditions. Both models are shown to reproduce equally well the observed spectral density distributions in the band wings. However, the modified rotor formalism that accounts for depopulation of the lower-energy rotational solute states provides better agreement with the experiment in the range of the P- and R-branch maxima. We surmise that the Q branches separated from the measured spectral profiles are formed by transitions between rotationally hindered states of diatomic molecules coupled to the solvent by the local anisotropy of the interaction potential.     

Random marginal and random removal values

We propose two variations of the non-cooperative bargaining model for games in coalitional form, introduced by Hart and Mas-Colell (Econometrica 64:357–380, 1996a). These strategic games implement, in the limit, two new NTU-values: the random marginal and the random removal values. Their main characteristic is that they always select a unique payoff allocation in NTU-games. The random marginal value coincides with the Consistent NTU-value (Maschler and Owen in Int J Game Theory 18:389–407, 1989) for hyperplane games, and with the Shapley value for TU games (Shapley in In: Contributions to the theory of Games II. Princeton University Press, Princeton, pp 307–317, 1953). The random removal value coincides with the solidarity value (Nowak and Radzik in Int J Game Theory 23:43–48, 1994) in TU-games. In large games we show that, in the special class of market games, the random marginal value coincides with the Shapley NTU-value (Shapley in In: La Décision. Editions du CNRS, Paris, 1969), and that the random removal value coincides with the equal split value.      

Polarographic behaviour of orotic acid (vitamin B13). Cathodic processes

The electroanalytical behaviour of orotic acid in cathodic processes has been studied in HClO₄, at several pH values in Britton-Robinson buffers, using DPP and CV techniques. Orotic acid undergoes three cathodic waves over the entire pH range considered. Optimum conditions for determination of orotic acid using the DPP technique are also studied.     

Structure of sodiated octa-glycine: IRMPD spectroscopy and molecular modeling

The structure of the sodiated peptide GGGGGGGG-Na⁺ or G₈-Na⁺ was investigated by infrared multiple photon dissociation (IRMPD) spectroscopy and a combination of theoretical methods. IRMPD was carried out in both the fingerprint and N—H/O—H stretching regions. Modeling used the polarizable force field AMOEBA in conjunction with the replica-exchange molecular dynamics (REMD) method, allowing an efficient exploration of the potential energy surface. Geometries and energetics were further refined at B3LYP-D and MP2 quantum chemical levels. The IRMPD spectra indicate that there is no free C-terminus OH and that several N—Hs are free of hydrogen bonding, while several others are bound, however not very strongly. The structure must then be either of the charge solvation (CS) type with a hydrogen-bound acidic OH, or a salt bridge (SB). Extensive REMD searches generated several low-energy structures of both types. The most stable structures of each type are computed to be very close in energy. The computed energy barrier separating these structures is small enough that G₈-Na⁺ is likely fluxional with easy proton transfer between the two peptide termini. There is, however, good agreement between experiment and computations in the entire spectral range for the CS isomer only, which thus appears to be the most likely structure of G₈-Na⁺ at room temperature.