Silver-water interactions: Part I. Model of the inner layer at the metal/water interface

Silver-water interactions, as expressed by the surface potential of the water molecules and the modification of the surface potential of the metal, are determined, at the potential of zero charge, as a function of the superficial structure of the electrode by means of the two more significant approaches, that based on the potential of zero charge-work function relation and that using the differential capacity of the inner-layer and its different components. The surface potential of water g^s(dip) is estimated from the inner-layer capacity while the modification of the surface potential of the metal δ_χ^M is obtained from theoretical calculations. The comparison of the [δ_gc^M − g^S(dip)] values with those deduced from the experimental pzc and work function is more than satisfactory. g^S(dip) is proposed to be equal to 0.19, 0.17 and 0.15 V, and −δ_χ^M equal to 0.41, 0.35 and 0.33 V, respectively for the (111), (100) and (110) faces of silver. Consequently, the strength of the silver-water interactions decreases from (111) to (110).The proposition of a large but electrode charge σ-independent capacitance contribution of the metal to the differential capacity of the inner layer Cⁱ is advanced from theoretical estimates and from the quantitative analysis of the Cⁱ(σ) curve for the mercury/water interface. The Cⁱ(σ) curve continues to represent the σ dependence of g^S(dip) as for the model with a σ- and metal-independent δ_χ^M. The basic change is that the capacity at fixed orientation of the water molecules C(ion) can no longer be identified with the minimum value of Cⁱ at high negative σ. A distance of 0.05 nm between the metal and the water molecules is proposed in order to interpret the low value of C(ion) equal to 8 μF cm⁻².The Cⁱ maximum located at the potential of zero charge for the three low-index faces of silver, is attributed to a maximum value of ∂g^S(dip)/∂σ, whatever the value of g^S(dip) for σ = 0 may be. On the other hand, the proposed estimates of the capacitance contribution of silver lead to an identical value of C(ion) and consequently to an identical structure of the inner layer at fixed orientation of the water molecules for the (111) face of silver as for mercury and the other sp metals. The same close-packed arrangement of the metal atoms at the surface of the electrode would be responsible for this identity.     

Analysis of the differential capacity at the silver / lithium perchlorate aqueous solution interface

Analysis of the differential capacity of ideally polarized Ag electrodes without specific adsorption is presented. The capacity was measured for three crystal faces of Ag [(111), (100) and (110)] and for polycrystalline Ag in aqueous solutions at different concentrations of LiClO₄. The inner layer contribution C _i was analysed for all the systems studied according to Grahame's concept. Moreover, using the procedure proposed by Amokrane and Badiali, several contributions to C _i, i.e. the metal (C _m), the solvent (C _s) and dipole orientation (C _dip), were calculated. The influence of the kind of crystal face of the Ag electrode on the values of these contributions is discussed. Received: 21 May 1999 / Accepted: 19 November 1999     

Reaction gas chromatography: Study of the photodecomposition of selected substances

Summary A simple apparatus that permits to carry out photolytic reactions in direct connection with a gas chromatograph has been designed. The photodecomposition of C₅–C₇ aliphatic alkanes, C₁–C₅ primary, secondary and tertiary aliphatic alcohols and of some other substances was studied using this apparatus. The degradation products are characteristic of the individual types of alcohols. The identification of the individual types of alcohols. The identification of the degradation products confirms the proposed schemes for the photodegradation reactions. The apparatus described can also be used for the study of photolysis kinetics, as it permits the easy and rapid variation of the reaction conditions.     

The stereochemistry of the dichlorocobalt(III) complexes with (2S,5S,9S)- and (2S,5R,9S)-trimethyltriethylenetetraamines

Dichlorocobalt(III) complexes of (2S,5S,9S)-trimethyltriethylenetetraamine (L₁) and (2S,5R,9S)-trimethyltriethylenetetraamine (L₂) have been prepared. Both L₁ and L₂ coordinate to the cobalt(III) ion to give three isomers: Λ-cis-α, Δ-cis-β, trans isomers for L₁ and Δ-cis-α, Δ-cis-β, trans isomers for L₂. Each of the trans-dichloro complexes of the two ligands have been isomerized stereospecifically to the cis-α-dichloro complex in methanol, and each of the cis-α-dichloro complexes stereospecifically to the trans-diaqua complex in water. Both the geometrical and optical inversions took place at the same time in the observed stereospecific isomerizations.     

Accurate Structure and Bonding Description of the Transition Metal‐Disulfur Monoxide Complexes [(PMe3)2M(S2O)] (M = Ni,Pd, Pt): Grimme Dispersion Corrected DFT Study

Geometry and bonding energy analysis of M–S₂O bonds in the metal‐disulfur monoxide complexes [(PMe₃)₂M(S₂O)] of nickel, palladium, and platinum were investigated at DFT, DFT‐D3, and DFT‐D3(BJ) methods using three different functionals (BP86, PBE, and TPSS). The TPSS/DFT‐D3(BJ) yields better geometry, while the BP86 geometry is least accurate for studied complexes. The geometry of platinum complex optimized at TPSS/DFT‐D3(BJ) level is in excellent agreement with the available experimental values. The M–S bonds are shorter than the M–S(O) bonds. The Mayer bond orders suggest the presence of M–S and M–S(O) single bonds. Both the M–S and M–S(O) bond lengths vary with the density functionals as TPSS‐D3(BJ) < TPSS < PBE < BP86. The Hirshfeld charge distribution indicates that the overall charge flows from metal fragment to [S₂O]. The Ni–S₂O bond has greater degree of covalent character than the ionic. The contribution of dispersion interactions is large in computing accurate bond dissociation energies between the interacting fragments. The BDEs are largest for the functional TPSS and smallest for the functional BP86. The DFT‐D3 dispersion corrections to the BDEs between the metal fragments [(PMe₃)₂M] and ligand fragment [(S₂O)] for the TPSS functional are in the range 7.1–7.3 kcal · mol^–1, which are smaller than the corresponding DFT‐D3(BJ) dispersion corrections (9.4–10.6 kcal · mol^–1).     

The Neutral Part of the Bark of Pinus Taiwanensis Hayata

The neutral part of the acetone extract from the bark of Pinus taiwanensis Hayata has been Investigated and found to consist of alkanes (C₁₄-C₃₃), longifolene, aliphatic esters (C₄₂, C₄₄, C₄₉), aliphatic alcohols (C₂₄, C₂₆), 3β-21α-dimethoxyserrat-14-enc, 3β-methoxyserrat-14-en-21-one, β-sitosterol, 3β-methoxyserrat-14-en-21ã-ol and 3β-hydroxyserrat-14-en-21-one.     

A Quantum-Chemical Study on the Photochemical Properties of Azides of a Series of Heteroarenes from Pyridine to Dibenzoacridine in Neutral and Protonated Forms

Molecules of a series of heteroaromatic azides in the ground (S ₀) and the lowest excited singlet (S ₁) states were calculated by the PM3 semiempirical method. It was shown that in the S ₀ state, the azide group in all the azides has quasi-linear geometry and a significant positive charge on the two terminal nitrogen atoms. The azide photoactivity is determined by the population of the σ _NN ^* orbital in the excited state, which is unoccupied in the ground state. The population of this orbital was found to depend on the size and charge of the aromatic π system. For the initial members of this azide series, the σ _NN ^* orbital is populated in both neutral and protonated forms. This is consistent with the experimental data and means that these azides are photoactive. With an increase in the size of the aromatic system, the energetic gap between the σ _NN ^* orbital and LUMO increases. As a result, the σ _NN ^* orbital is not populated in the S ₁ state when a particular threshold size of the π system is achieved, and the azide becomes photo-inactive.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 4, 2005, pp. 259–266.Original Russian Text Copyright © 2005 by Budyka, Oshkin.     

Temperature Dependence of the Isobutane Chemical Ionization Mass Spectra of Open-chain and Cyclic Alcohols; Structural,Stereochemical and Molecular-Size Effects. A reevaluation of the isobutane chemical ionization of alcohols

The temperature dependence of the isobutane chemical ionization (CI.) mass spectra of 54 open-chain, cyclic and unsaturated C₅- to C₁₀-alcohols was studied at temperatures ranging from 60 to 250°, and enthalpy changes were calculated for the corresponding main reactions of typical alcohols. The CI. reactivity is controlled by the temperature and the substrate structure as usual, and in addition, by the molecular size. The combination of thermal, structural and substrate-size effects leads to the following main conclusions. At low-reactivity conditions, i.e. at 150° or less, the alcohols with less than 11 C-atoms give four distinct types of spectra, with (M – OH)⁺ usually as the base peak. The characteristic ions are MC₄H₉⁺ and (M – H)⁺ for primary, MH⁺ and (MC₄H₉ – H₂O)⁺ for secondary, (MC₄H₉ – H₂O)⁺ for tertiary and allyl-type alcohols. Configurational assignments of stereoisomeric alcohols are also possible, by means of steric compression and shielding effects. The MH⁺/(M – OH)⁺ ratio in the spectra of epimeric methylcyclohexanols is at least 3 to 4 times higher for the isomers with mainly axial OH-group conformation compared to the equatorial isomers. Stereospecific (M - H)⁺ ions are apparently formed from trans-2-methylcyclopentanol and endo-norbornan-2-ol by a favorable abstraction of the unshielded H(α)-atoms versus normal behavior of the other epimers. While the spectra recorded at 200° show almost exclusively (M – OH)⁺ ions, those at 250° give nevertheless some C-skeleton information through the temperature dependent decomposition of the (M – OH)⁺ ions.     

Mixed Monolayers of Amphiphilic Cyclodextrins and Phospholipids: II. Surface Potential–Area Behavior of Per-(6-amino-2,3-di-O-hexyl) β-CD Hydrochloride Salt and 1,2 Dipalmitoyl, 3-sn-Phosphatidic Acid Mixed Monolayers

The dielectric properties of mixed monolayers of per-(6-amino-2,3-di-O-hexyl) β-CD hydrochloride (NH₃-β-CD-OC6) and 1,2 dipalmitoyl, 3-sn-phosphatidic acid (DPPA) have been assessed using surface potential measurements at constant area. From the comparison of these surface potential (ΔV) versus surface density (δ) relationships with those of surface pressure (π) against surface density (δ) it was apparent that the increase in the NH₃-β-CD-OC6 content in mixed films gave rise to a gradual increase in the saturation value of the surface potential (ΔV_max). This potential for pure DPPA was found to be equal to 396 mV and for pure CD 554 mV. The ΔV_maxvalues reflect the onset of reorientation effects that arrive at molar areas before the collapse of these films. Independently of reorientation effects, the obtained results strongly indicate that the dipolar term contributing to the overall ΔVvalue was for NH₃-β-CD-OC6 due to the hydration of its NH⁺₃group. For both DPPA and NH₃-β-CD-OC6 molecules the contribution of the electric double layer (Ψ) was calculated and was found for DPPA and NH₃-β-CD-OC6 to be equal to −249 and +252 mV, respectively. These calculated Ψ values made possible the evaluation of dipole moments for NH₃-β-CD-OC6 and DPPA monolayers which revealed a marked difference in dipolar properties between these two film forming components. In contrast to DPPA which exhibited a decrease in the surface dipole moment (μ) with the decrease inA, NH₃-β-CD-OC6 displayed an increase in μwith the decrease inAforAvalues above 580 Ų. Below this value μdecreases with decreasing molecular area and this variation arises from a change in the polarity of the electric double layer arising from interactions with the complementary anion. The differences in dielectric properties between the two film forming molecules have been attributed to modification, during compression, in the structure of the interfacial water bound to the cyclodextrin.     

A complex of buckminsterfullerene with sulfur,C60·2S8: synthesis and crystal structure

The C₆₀·2S₈ complex was prepared by reaction of buckminsterfullerene C₆₀ with sulfur in trichloroethylene and its single-crystal X-ray structure was studied at room temperature. Crystals of this compound are monoclinic, space groupC 2/c, a=20.90(1),b=21.10(1),c=10.537(9) Å, =111.29(7)°,Z=4,d _calc=1.89 g·cm^–3. The crystal structure of the C₆₀·2S₈ complex consists of packed fullerene molecules that form hexagonal channels along thec axis with eight-membered crown-shaped S₈ cyclic molecules inside the channels. The distances between the centers of neighboring fullerene molecules are 10.036(7), 10.636(7), and 10.537(9) Å. Each C₆₀ molecule is linked to eight S₈ molecules with ten shortened intermolecular contacts C...S 3.41(1)–3.52(2) Å. The average values of the C=C and C-C bond lengths are 1.32(3) and 1.47(3) Å, which attest to a significant degree of localization of electron density in the c₆₀ molecule.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 262–266, February, 1994.     

Natural abundance C REDOR coupled to a singly N-labeled nucleus: simultaneous determination of interatomic distances in crystalline ammonium [N] -glutamate monohydrate

REDOR technique was applied to natural abundance ¹³C nuclei coupled to a singly labeled ¹⁵N nucleus to determine the ¹³C, ¹⁵N interatomic distances simultaneously in crystalline ammonium [¹⁵N] -glutamate monohydrate (1). Consequently, the interatomic C–N distances between ¹⁵N and ¹³C=O, ¹³C_α, ¹³C_β, ¹³C_γ, and ¹³C_δ carbon nuclei for 1 were determined with a precision of ±0.15 Å, after the experimental conditions such as the location of samples in the rotor, length of π pulse etc. were carefully optimized. ¹³C-REDOR factors for three spin system, (ΔS/S₀)_CN1N2, and the sum of two isolated 2-spin system, (ΔS/S₀)^*=(ΔS/S₀)_CN1+(ΔS/S₀)_CN2, were further evaluated by the REDOR measurements on isotopically diluted 1 in a controlled manner. Subsequently, the intra- and intermolecular C–N distances were separated by searching the minima in the contour map of root mean square deviation (RMSD) between the theoretically and experimentally obtained (ΔS/S₀)^* values against two interatomic distances, r_C–N1 and r_C–N2. When the intramolecular C–N distance (r_C–N1) of the particular carbon nucleus is substantially shorter than the intermolecular one (r_C–N2), C–N distances within a single molecule were obtained with an accuracy of ±0.06 Å as in the cases of C=O, C_α and C_β carbon nuclei. C–N distances between the molecule in question and the nearest neighboring molecules can be also obtained, although accuracy was lower. On the contrary, it was difficult to determine the interatomic distances in the same molecule when the intermolecular dipolar contribution is larger than the intramolecular one as in the case of C_δ carbon nucleus.     

The influence of cyclic alcohols on the temperature of maximum density of water

The effect of a homologous series of cyclic monohydric alcohols (C₄−C₈) on the temperature of maximum density of water has been examined. The structural contribution to the shift in the temperature of maximum density (TMD) is positive (increased structuredness) for alcohols up to C₆ but becomes negative at C₈. The behavior is compared with that of linear monohydric alcohols and Ω-diols.     

Polymerisations of -caprolactone and β-butyrolactone with Zn-, Al- and Mg-based organometallic complexes

The reaction of EtAlCl₂ with 1,2-{LiN(PMes₂)}₂C₆H₄ (Mes = 2,4,6-Me₃C₆H₂) and of butyloctylmagnesium with 1,2-{NH(PPh₂)}₂C₆H₄ gave [AlEt(1,2-{N(PMes₂)}₂C₆H₄-κ²N,N′)(THF)] (1) and [Mg(1,2-{N(PPh₂)}₂C₆H₄-κ²N,N′)(THF)₂] (2), respectively. Complexes 1 and 2 were fully characterised by NMR (¹H, ¹³C, ³¹P) and IR spectroscopy and mass spectrometry. Complexes 1 and 2 were employed as catalysts in the polymerisation of -caprolactone, which produced polymers with a narrow molecular weight distribution. For comparison the polymerisations of -caprolactone and β-butyrolactone were carried out with the Zn complex [ZnPr{1-N(PMes₂)-2-N(PHMes₂)C₆H₄-κ²N,N′}] (3) as catalyst, which produced polymers with narrow molecular weight distributions and high molecular weights.     

Self-assembly of the pre-formed inclusion complexes between cyclodextrins and alkanethiols on gold electrodes

A new kind of self-assembled monolayer (SAM) formed in aqueous solution through the pre-formed inclusion complexes (abbreviated CD · C_n) between α, β-cyclodextrins (CDs) and alkanethiols (CH₃(CH₂)_n−1)SH, n = 10, 14 and 18) was prepared successfully on gold electrodes. High-resolution ¹H NMR was used to confirm the formation of CD · C_n. X-ray photoelectron spectroscopy, cyclic voltammetry and chronoamperometry were used to characterize the resulting SAMs (denoted as M_CD·Cn). It was found that M_CD·Cn were more stable against repeated potential cycling in 0.5 M H₂SO₄ than SAMs of CH₃(CH₂)_n−1SH (denoted as M_Cn), with a relative sequence of M_β−CD·Cn > M_α−CD·Cn > M_Cn. In addition, an order of blocking the electron transfer between gold electrodes and redox couples (both Fe(CN)³₆ and Ru(NH₃)³⁴₆) in solution, M_CD·C10 > M_CD·C14 > M_CD·C18, was observed. A plausible explanation is provided to elucidate some of the observations.     

Design variations of a polymer–enzyme composite biosensor for glucose: Enhanced analyte sensitivity without increased oxygen dependence

The glucose sensitivity and oxygen dependence of a variety of implantable biosensors based on glucose oxidase (GOx), incorporating an electrosynthesized poly-o-phenylenediamine (PPD) permselective barrier on 125-μm diameter Pt disks (Pt_D) and cylinders (Pt_C, 1-mm length), were measured and compared. Full glucose calibrations and experimental monitoring of solution oxygen concentration allowed us to determine apparent Michaelis–Menten parameters for glucose and oxygen. In the linear region of glucose response, the most sensitive biosensor design studied was Pt_D/PPD/GOx (enzyme deposited over polymer) that was 20 times more sensitive than the more widely used Pt_C/GOx/PPD (enzyme immobilized before polymer deposition) configuration. The oxygen dependence, quantified as K_M(O₂), of both active and less active designs was surprisingly similar, a finding that could be rationalized in terms of an increase in K_M(G) with increased enzyme loading. The Pt_D/PPD/GOx design will now enable us to explore glucose concentration dynamics in smaller and layered brain regions with good sensitivity and minimal interference from fluctuations in tissue pO₂.     

Ammonia chemical ionization mass spectrometry of alcohols: Structural,stereochemical, molecular-size and temperature effects

Ammonia chemical ionization (CI) mass spectra of various open-chain, cyclic and unsaturated C₅- to C₁₀-alcohols were obtained at source temperatures ranging from 60° to 250°C. The reactivity of the ammonia adduct ion MNH and its fragmentation channels are characteristic for substrate structure. Although strongly temperature-dependent, the spectra give nevertheless information on the OH-group environment as well as on the C-skeleton at any source temperature. Primary, secondary and tertiary alcohols as well as allylic and simple olefinic alcohols can be distinguished by their spectra, which show ammonium adduct ions [MNH₄]⁺, adduct dehydrogenation ions [MNH₄-H₂], ammonium substitution ions [MNH₄-H₂O]⁺ and [M-OH]⁺-ions as the main characteristic peaks. Moreover, konfigurational assignments of stereoisomeric alcohols are possible for larger substrate-size and source-temperature ranges than with isobutane CI mass spectrometry. Homologous M NH-ions show molecular-size control of fragmentation and linear MNH-ions are less stable than branched isomers due to incomplete energy randomization.     

Highly enantioselective reduction of achiral ketones with NaBH4/Me3SiCl catalyzed by (S)-α,α-diphenylpyrrolidinemethanol

The reducing agent prepared from sodium borohydride, trimethylsilyl chloride and a catalytic amount of (S)-α,α-diphenylpyrrolidinemethanol has been successfully applied to the enantioselective reduction of ketones. The optically active secondary alcohols were obtained in excellent enantiomeric excess and almost quantitative chemical yield.     

Oxidation of primary and secondary alkanols with the CeIII-LiBr-H2O2 system

Action of a novel oxidation system, Ce(NO₃)₃·6H₂O (cat.)-LiBr (cat.)-H₂O₂ (stoichiometric oxidant) on primary aliphatic C₆–C₉ alcohols gives selectively esters, whereas secondary aliphatic C₅–C₉ alcohols are converted into ketones. Selectivity of these transformations is provided by slow addition of H₂O₂ to the other reactants.     

Reactions of Neutral and Charged Molybdenum Oxide Clusters with Low-Molecular-Weight Alcohols in a Gas Phase Studied by Ion Cyclotron Resonance

A set of oxygen-containing molybdenum oxide clusters Mo _x O _y (x = 1–3; y = 1–9) was obtained with the use of a combination of a Knudsen cell and an ion trap cell. The reactions of positively charged clusters with C₁–C₄ alcohols were studied using ion cyclotron resonance. The formation of a number of organometallic ions, the products of initial insertion of molybdenum oxide ions into the C–O and C–H bonds of alcohols, and polycondensation products of methanol and ethanol were found. The reactions of neutral molybdenum oxide clusters Mo _x O _y (x = 1–3; y = 1–9) with protonated C₁–C₄ alcohols and an ammonium ion were studied. The following limits of proton affinity (PA) were found for neutral oxygen-containing molybdenum clusters: (MoO) < 180, (Mo₂O₄, Mo₂O₅, and Mo₃O₈) = 188 ± 8, PA(MoO₂) = 202 ± 5, PA(MoO₃, Mo₂O₆, and Mo₃O₉) > 207 kcal/mol.