Facile interfacial electron transfer through n-alkyl monolayers formed on silicon (111) surfaces

Electron transfer (ET) kinetics through alkyl monolayers, formed on n-type Si(111) surface by the direct reaction of alkylmagnesium bromide (n-C_nH_2n+1MgBr, n=2, 6, 10, and 15) with hydrogen-terminated Si(111), was investigated in acetonitrile (MeCN) with anthraquinone (AQ) as the electrochemical probe. Cyclic voltammetric measurements indicate that the ability of the monolayer to block interfacial electron transfer increases with increasing alkyl chain length. In particular, the voltammetric behavior changes from non-rectifying (i.e., chemically reversible redox couple), to rectifying (i.e., diode-like when the reverse wave is pushed into the gap) with increasing chain length. The dependence of the logarithm of the electron transfer rate constant as a function on the number of carbons in the alkyl chain is not consistent with electron tunneling through the full thickness of the film. In fact, the measured constant, 0.05 ± 0.03 per methylene, is much smaller than the well-established tunneling constant, ∼1.0/CH₂ in the closely packed alkanethiol monolayers on gold suggesting permeation of the AQ into the film.     

35Cl Nqr spectroscopy of the [PO2Cl2]− and POCl3 groups in Me(PO2Cl2)2·2D (Me = Mg,Ca, Mn; D = POCl3,CH3COOC2H5,CH3COCH3)

³⁵Cl NQR spectra of dichlorophosphates Me(PO₂Cl₂)₂ · 2D (Me = Mg, Ca, Mn; D = CH₃COOC₂H₅, CH₃COCH₃, POCl₃) are studied in the temperature range 77 ? T (K) ? 305. It is shown that the three compounds with CH₃COOC₂H₅ as donor are isomorphic at 77 K, the crystal structure of Mn(PO₂Cl₂)₂· 2CH₃COOC₂H₅. The structure of Mg(PO₂Cl₂)_2^?· 2CH₃COCH₃ and of Mg(PO₂Cl₂)₂ · 2POCl₃ probably consists of infinite chains as found for Mn(PO₂Cl₂)₂· 2CH₃COOC₂H₅. Mg(PO₂Cl₂)₂· 2CH₃COOC₂H₅ shows phase transformations and a complicated dynamical behaviour leading to strong deviations from a Bayertype NQR function v = f(T). The donor capacity of POCl₃ in Mg(PO₂Cl₂)₂· 2POCl₃ is comparable with the donor strength in AsCl₃ · POCl₃ · A d_π-p_π overlap of the P-O bond influences the P-Cl bond.     

Effect of addends in C<Subscript>60</Subscript> derivatives on the electrochemical behavior of the compounds in salt matrices of artificial lipids

Effect of addends in fullerene compounds on the fullerenes’ redox behavior in the matrix of tetraoctylammonium bromide, an artificial cationic lipid, deposited on conducting glass is studied by example of di- and trisubstituted fullerenopyrrolidines and a methanofullerene derivative. It is shown by cyclic voltammetry that in all cases the hydrophobic interaction of fullerenes with alkyl radicals in the matrix, as well as electrostatic interaction of the fullerene reduction products (anions), shifts the fullerene reduction potentials toward positive values. This allows performing quasireversible redox-transformation in neutral aqueous media. It is shown that the rate of electron transfer to and from fullerene in the fullerenopyrrolidines is higher than in methanofullerene, which presumably can be explained by violation of the symmetry and conjugation partial lack due to the introducing of the methane fragment.     

Emulsion polymerization of butyl acrylate: Spin trapping and EPR study

The propagating radical in the emulsion polymerization reaction of butyl acrylate was detected by Electron Paramagnetic Resonance (EPR) spectroscopy using two spin-trapping agents, 2-methyl-2-nitrosopropane (MNP) and α-(4-pyridyl 1-oxide)-N-tert-butylnitrone (PyOBN). Through analysis of hyperfine structure of the spectra obtainedfrom the trapped radicals, the propagating radical is inferred to be the well known acrylate radical, ? [CH₂? CH(COOC₄H₉)]_n? CH₂? CH(COOC₄H₉)? . © 1994 John Wiley & Sons, Inc.     

1,2-dithiolium salts

The reaction of aryl-1,2-dithiol-3-thiones with alkyl iodides and with ICH₂COOH and ICH₂COOC₂H₅ has been studied. This gives 3-alkylithio-S-carboxymethylthio-, and 3-ethoxycarbonylmethylthio-1, 2-dithiolium iodides. Compounds of the first type are formed only with lower unbranched alkyl iodides (CH₃I, C₂H₅I, and n-C₃H₇I), and salts of the last two types were previously unknown.     

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl₃, CH₃NO₂, CH₃CN, (CH₃)₂CO, CH₃COOC₂H₅, and (C₂H₅)₂O on MgO at 50–100°C. Differential molar enthalpy changes (?ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into ?ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.     

Effect of tetra-n-alkylammonium bromides on the volumetric properties of glycine,l-alanine and glycylglycine at T=298.15 K

The effect of tetra-n-alkylammonium bromides, R₄NBr (R=CH₃, C₂H₅, C₄H₉) on the densities, ρ, of glycine, l-alanine and glycylglycine are reported at T=298.15 K. The apparent molar volumes of amino acids in aqueous tetra-n-alkylammonium salts, φ_VAJW, and of tetra-n-alkylammonium bromides in aqueous amino acids and peptide, φ_VJAW, are calculated from the measured densities. Both φ_VAJW and φ_VJAW have been analysed accurately using a simple equation. Positive transfer volumes are observed for glycine, l-alanine and glycylglycine in the presence of R₄NBr. Tetra-n-butylammonium bromide shows almost double increase in the transfer volumes of amino acids or peptide than tetramethyl- or tetraethylammonium bromides. Negative transfer volumes for the tetra-n-alkylammonium bromide salts are noted in aqueous amino acids or peptide due to large tetra-n-alkylammonium cation undergoing hydrophobic hydration.     

Synthese d'un nouveau monomere hydrofluorocarboxylique le difluoro-2,2-butene-3-oate d'ethyle

The reaction of bromine with chlorotrifluoroethylene yields an addition product BrCF₂CFClBr, which can be further transformed into BrCF₂COOC₂H₅ by hydrolysis with oleum and esterification. The mono adduct ester, BrCH₂CH₂CF₂COOC₂H₅, is selectively obtained by ethylene telomerization in the presence of a radical initiator and ethyl bromodifluoroacetate as telogen; side-addition of two ethylene molecules cannot be completely avoided. The ester was dehydrobrominated with sodium ethanolate, yielding ethyl 2,2 difluoro 3-butenoate, CH₂CH-CF₂COOC₂H₅     

Effect of a Carboxylic Group on the Conformational Properties of Some Gemini Surfactants

Molar volumes of derivatives of N,N-bisdimethyl-1,2-ethanediamine of general formula [C _n H_2n+1OOCCH₂(CH₃)₂N⁺CH₂CH₂N⁺(CH₃)₂CH₂COOC _n H_2n+1]2Cl⁻ (bis-C _n BEC), and betaine ester derivatives of general formula N⁺(CH₃)₃CH₂COOC _n H_2n+1Cl⁻ (C _n BEC), were calculated by means of molecular connectivity indices and an additivity scheme. The COO group contribution in the β-position to the molar volume of bis-C _n BEC was found from experimental data to be significantly lower from that for C _n BEC and was estimated to be 13.5 cm³⋅mol⁻¹. It is suggested that this effect is due to hydrogen bonding between the carboxyl groups via water molecules. Molecular dynamics simulations of bis-C₁₂BEC and 14–2–14 molecules in water were performed and suggest that the upper part of the bis-C₁₂BEC, containing the carboxyl groups, is stiffened by its hydration shell.     

Electrochemical materials science: tailoring intrinsically conducting polymers. The example: substituted thiophenes

A series of 3-(p-X-phenyl) thiophene monomers (X= –H, –CH₃, –OCH₃, –COCH₃, –COOC₂H₅, –NO₂) was electrochemically polymerized to furnish polymer films that could be reversibly reduced and oxidized (n- and p-doped). The oxidation potentials of the monomers and formal potentials of the n- and p-doping processes of polymers were correlated with resonance and inductive effects of the substituents on the phenyl ring as well as the semiempirically calculated heat of formation of the monomer radical cations. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13-16, 2005     

Infrared and Raman spectra of N-n-alkylthioureas

The i.r. and Raman spectra of solid alkyl thioureas H₂NCSNH(CH₂)nCH₃ (n = 0–15), TUR′, and their urea analogue H₂NCONH(CH₂)nCH₃ (n = 0–9), UR′, are presented. The vibrational properties of N-monosubstituted TUR′and UR′are studied on the basis of the spectral comparison between the members of the series and compared with the properties of N,N′-di-substituted MTUR′ and MUR′, correspondingly. Some features of the methylene ν₄ and ν₇ band progressions were found to shift from the corresponding features of solid n-alkanes by vibrational coupling. The observed wavenumber of the longitudinal accordion-like skeletal motion (LAM-1) of TUR′ was found to correspond to that of the n-alkane with the same number of carbon atoms as TUR′ has skeletal atoms.     

Rotational and vibrational distributions of N2(C 3Πu) excited by state-selected Ar(3P2) and Ar(3P0) metastable atoms

The ESR method is used to study the oxidation kinetics of the CH₃, C₂H₅, n-C₄H₉, i-C₄H₉, s-C₄H₉, t-C₄H₉, n-C₆H₁₃, C₆H₁₁, C₆H₅CH₂, CH₃C₆H₄CH₂, and C₆H₅CH₂CH₂ radicals in methanol matrix at 87 K. The reaction kinetics are shown to be describable in terms of a time-dependent rate constant k(t). The contribution from the matrix relaxation to k(t) has been determined. The oxidation rate and the shape of the kinetic curve are independent of the type of the radical. Models interpreting the experimental data are discussed.     

Swelling of crosslinked poly-4-vinylpyridine

Polyvinylpyridine of molecular weight 0.77 × 10⁶ was crosslinked by 2.5, 5, and 10% BrCH₂C₆H₄CO(CH₂)₈COC₆H₄CH₂Br and by 10% ClCH₂C₆H₄CH₂Cl; quaternization of the gel was completed with n-butyl bromide. Swelling ratios in aqueous solutions of lithium, sodium, and 4-isopropyl-N-n-butylpyridinium bromide and in methanolic lithium bromide were determined. Selectivity increased in the sequence Li⁺ <Na⁺ <PrN⁺?C₅H₅. The Flory parameter χ₁, which measures interaction between solvent and polymer, decreased from 1.5 kT to nearly zero with increasing density of crosslinks in the aqueous solutions, and from 3 kT to 0.5 kT in the methanol solutions. The inverse proportionality of q^2/3 (q = swelling ratio) to the crosslinking density was approximately verified for swelling of the resins in water, methanol, and dimethylformamide.     

Intercalation of primary diamines in the lamellar niobate HNb3O8 · H2O

The intercalation of alkyl diamines in the protonic oxide HNb₃O₈ · H₂O is quantitative for the diamines H₂N(CH₂)_nNH₂ with n ranging from 2 to 10. All the intercalated oxides [H₃N(CH₂)_nNH₃]_0.5Nb₃O₈ · yH₂O are hydrated at room temperature; they can be easily and reversibly dehydrated to the oxides [H₃N(CH₂)_nNH₃]_0.5Nb₃O₈. The structural behavior of those compounds is compared to that of the alkyldiammonium titanoniobates [H₃N(CH₂)_nNH₃]_0.5TiNbO₅. An interpretation of their structural properties is given which takes into account the tendency of amines to assume an orientation transverse to oxide layers, the conformation of the amine chains, and the tendency to form dense organic layers.     

Micelle hydration by1H-NMR

¹H-NMR studies were carried out for solution of amphiphilic betaine ester derivatives (of the general formula (CH₃)₃N⁺CH₂COOC _n H_2n+1Cl^– (V-n), wheren=10, 12, 14, and 16) andn-dodecyltrimethylammonium chloride (I-12). The spectra were taken at concentrations above and below critical micelle concentrations and chemical shifts were analyzed. It was stated that micelles are hydrated at the depth of the two CH₂ groups in the case ofV-n and the CH₂COO group in the case ofI-12. Therefore, the CH₂COO group during the micellization behaves as if it were CH₂CH₂ group.     

Solubility in the water-Catamine AB-(alkali metal or ammonium chloride) systems

Solubility in ternary aqueous stratifying systems containing Catamine AB (alkylbenzyldimethylammonium chloride [C _n H_{2n + 1}N⁺(CH₃)₂CH₂C₆H₅] · Cl, a cationic surfactant, where n = 10–18) and LiCl, NaCl, KCl, and NH₄Cl inorganic salts was studied for the first time at 25°C. The boundaries of two-phase liquid equilibrium fields were determined. The studied stratifying systems were proposed for use in the liquid extraction of metal ions.     

The relationship between properties of fluorinated graphite intercalates and matrix composition 7

Inclusion compounds (intercalates) of fluorinated graphite matrix with ethyl acetate (C₂F_xBr_z·yCH₃COOC₂H₅, x = 0.49, 0.69, 0.87, 0.92, z = 0.01) were prepared by guest substitution from acetonitrile to ethyl acetate. The kinetics of the thermal decomposition (the first stage of filling → the second stage of filling) was studied under isothermal conditions at 291–307 K. The relationship of the host matrices’ structure with inclusion compounds’ thermal properties and kinetic parameters is discussed.     

ANODIC OXIDATION OF 1,n-HALO(ALKYLTHIO)ALKANES AND 1,n-CHLORO(ALKYLSULFINYL)ALKANES

Abstract

Anodic oxidation of 1,n-halo(alkylthio)alkanes [X-(CH₂)_n-S-R, X = Cl, Br, I] and l,nhalo(alkylsulfinyl)alkanes [Cl-(CH₂)_n-S(O)-R] was studied by cyclic voltammetry in anhydrous acetonitrile and by controlled potential electrolyses. The ease of sulfur oxidation was not affected by the alkyl substituents R or the number of methylene groups n in compounds with n > 2. The oxidation of the 1,2-halo(alkylthio)ethanes (n = 2) occurred at slightly higher potentials. The peak potentials decreased slightly in the order Cl > Br > I which is probably due to the electronegativity of the halogen atoms. The investigated anodic oxidation was shown to be a two electron irreversible process. Electrolyses in aqueous acetronitrile led to the corresponding sulfoxides and sulfones in good yields.     

Radical ions : XXVIII. Tris(trimethylsilylmethyl)aminium,N(CH2Si(CH3)3)3: A stable fluxional aminium radical cation

The first ionization potentials of 11 silylmethyl-substituted amines R_3?nN(CH₂Si(CH₃)₃)_n as determined photoelectron spectroscopy range from 9.07 eV to 7.66 eV. The most easily ionized molecule, N(CH₂Si(CH₃)₃)₃, can also be oxidized with AlCl₃ in H₂CCl₂ solution to its aminium radical cation. The ESR spectra recorded between 180 K and 310 K display a strong temperature dependence due to rotations around the >NCH₂bonds.