Association behavior of one-end hydrophobically modified poly[<Emphasis Type="Italic">N</Emphasis><Superscript>5</Superscript>-(2-hydroxyethyl) <Emphasis Type="SmallCaps">l</Emphasis>-glutamine] in water/ethylene glycol mixed solvent

Amphiphilic polymers Cn-PHEG consisting of water-soluble poly[N ⁵-2-(hydroxyethyl) l-glutamine] (PHEG) and hydrophobic alkyl chain (carbon number n = 12, 14, 16, or 18) attached at the PHEG terminal was prepared, and association behavior and structure of associate for Cn-PHEG in selective solvent (water/ethylene glycol mixed solvent) have been investigated. α-Helix content of PHEG block for all the polymers increased with weight fraction of ethylene glycol in the mixed solvent (W _EG). By light scattering measurements, formation of a small micelle was suggested for C14-, C16-, and C18-PHEG when W _EG = 0. With the increase in W _EG, appearance of a larger associate was revealed for C16- and C18-PHEG. Evaluated molecular weight and radius of gyration suggested that the micelle is star-like sphere when W _EG = 0 and worm-like cylinder when W _EG = 0.7. C12-PHEG did not demonstrate any distinct micellization behavior because of the weak hydrophobicity of C12 chain.     

Density functional study of ethylene oxidation on an Ag(111) surface

The mechanism of ethylene epoxidation on Ag surfaces has been investigated using the density functional method and Ag _n clusters (n = 3 to 10) modeling the Ag(111) surface. The adsorption energy of O₂ to the Ag clusters was strongly dependent on the HOMO level of the cluster, and the clusters with higher HOMO levels afforded larger O₂ adsorption energies. The energetics was investigated for both the molecular and atomic oxygen epoxidation mechanisms. For the atomic oxygen mechanism, epoxidation was found to proceed without an activation energy, whereas a small amount of activation energy (about 5 kcal/mol) was calculated for the molecular oxygen mechanism. Received: 2 July 1998 / Accepted: 9 September 1998 / Published online: 8 February 1999     

Structural and calorimetric investigations on nonaqueous liquid crystals and gel phases in the binary system K-stearate/glycerol

The K-stearate/glycerol (KC₁₈/Gl) binary system was studied at mole fractions of stearate of x _KC18 = 0.10, 0.25, 0.30 and 0.50. Small- and wide-angle X-ray diffraction (XRD) measurements were combined with differential scanning calorimetry (DSC) measurements at different temperatures. The investigations were intended to verify the previously published phase diagram and were targeted at the confirmation of the gel-like (G₁) phase and the isotropic (I) phase. The XRD and DSC measurements lead to the conclusion that the G₁ phase as well as the I phase, the existence of which had been proposed from texture observations, do in fact not exist. Consequently, a correction of the preliminary phase diagram is given. This corrected phase diagram reveals the crystalline phase (C) ⇆ gel phase (G) ⇆ hexagonal phase (H_α) ⇆ isotropic, micellar phase phase transitions for low KC₁₈ concentrations of x _KC18 = 0.15–0.3 and the C ⇆ G ⇆ lamellar phase (L_α) phase transitions for concentrations about or higher than x _KC18 = 0.35. The C, G, L_α and H_α phases have been further characterized by structural parameters (characteristic d values) as a function of temperature. The phase transitions C ⇆ G, G ⇆ L_α and G ⇆ H_α correlate with sharp shifts in the d value of the first small-angle reflections. Received: 20 April 1999 Accepted: 28 July 1999     

Acetylene-insertion reactions into Pt(II)–H and Pt(II)–SiH3 bonds. An ab initio MO study and analysis based on the vibronic coupling model

Acetylene insertion into Pt(II)–H and Pt(II)SiH₃ bonds of PtH(SiH₃)(PH₃) was investigated using ab initio molecular orbital and M?ller-Plesset perturbation theory methods. The insertion into PtH was predicted to proceed with a smaller activation energy (E _a =12.8 kcal/mol) than that into PtSiH₃ (E _a =20.9 kcal/mol). The reaction energy (ΔE) of the insertion into PtH is 10 kcal/mol smaller than that for the insertion into PtSiH₃, which reflects differences in bond energies between CH and CSi and between PtH and PtSiH₃. A comparison with ethylene insertion revealed that the acetylene insertion occurs more easily, and the latter reaction is more exothermic. A simple vibronic coupling model combined with Toyozawa's interaction mode analysis was used to examine interesting differences in E _a and ΔE between insertions into PtH and PtSiH₃, and between acetylene and ethylene insertions. This analysis suggests that the factors determining E _a are the stiffness of the PtH and PtSiH₃ bonds and the vibronic coupling strength of acetylene and ethylene. Received: 13 August 1998 / Accepted: 2 September 1998 / Published online: 15 February 1999     

Temperature dependency of the intrinsic carrier density of hydrogenated amorphous silicon in MOS structures

The admittance versus frequency of a hydrogenated amorphous silicon metal oxide semiconductor capacitor is measured at a fixed bias in inversion and for temperatures in the range of 20–50 °C. The data are fitted to theoretical capacitance and conductance curves where the time constant of inversion is the result of the fit. In turn, the time constant can be converted to the (minority) carrier lifetime so that a lifetime value for each measurement temperature is available. The conversion from the time constant to the minority carrier lifetime requires the knowledge of the temperature-dependent intrinsic carrier density or rather its activation energy. The criterion for the correct choice is a temperature-independent carrier lifetime. Three published room temperature values of the intrinsic carrier density have been tested. The carrier lifetime activation energy is E _a = 0.70 ± 0.03 eV. Received: 17 June 1998 / Accepted: 23 October 1998     

Electrocatalytic oxidation of aspirin and acetaminophen on a cobalt hydroxide nanoparticles modified glassy carbon electrode

The electrocatalytic oxidation of aspirin and acetaminophen on nanoparticles of cobalt hydroxide electrodeposited on the surface of a glassy carbon electrode in alkaline solution was investigated. The process of oxidation and the kinetics have been investigated using cyclic voltammetry, chronoamperometry, and steady-state polarization measurements. Voltammetric studies have indicated that in the presence of drugs, the anodic peak current of low valence cobalt species increases, followed by a decrease in the corresponding cathodic current. This indicates that drugs are oxidized on the redox mediator which is immobilized on the electrode surface via an electrocatalytic mechanism. With the use of Laviron’s equation, the values of anodic and cathodic electron-transfer coefficients and charge-transfer rate constant for the immobilized redox species were determined as α _s,a = 0.72, α _s,c = 0.30, and k _s = 0.22 s⁻¹. The rate constant, the electron transfer coefficient, and the diffusion coefficient involved in the electrocatalytic oxidation of drugs were reported. It was shown that by using the modified electrode, aspirin and acetaminophen can be determined by amperometric technique with detection limits of 1.88 × 10⁻⁶ and 1.83 × 10⁻⁶ M, respectively. By analyzing the content of acetaminophen and aspirin in bulk forms using chronoamperometric and amperometric techniques, the analytical utility of the modified electrode was achieved. The method was also proven to be valid for analyzing these drugs in urine samples.     

Structure and stability of hypervalent NLi n Na2 (n = 1–4) and related species at density functional theory level

A number of configurations of NLi _n Na₂ (n = 1–4) species were optimized using the B3LYP–density functional theory method; the 6-31G^* basis set was used in this calculation. In order to study all possible dissociation energies, some related species such as NLi₂Na, NLi _n (n = 1–4), Li _n (n = 1, 2) and Na _n (n = 1, 2) were also considered. Optimizations of these species were followed by fundamental frequency calculations at the same level. Global minima of these species were shown to adopt C _{2 v} (NLi₄Na₂, NLi₂Na₂), D _{3 h} (NLi₃Na₂) and C _s (NLiNa₂ and NLi₂Na) configurations. All possible dissociation energies were obtained. Received: 30 November 1998 / Accepted: 15 October 1999 / Published online: 14 March 2000     

Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids

The initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene) have been studied with a high-speed camera. For drops of 1–4 μL volume, the increase in contact radius r can be described by a power law r μ t^a r \propto {t^{\alpha }} , with the spreading exponent α = 0.50 and for the first ≈8 ms. Thereafter, the three-phase contact line was pinned leading to a macroscopic static contact angle of Θ₀ = 12–15°. The insoluble liquids ethanol (α = 0.47, Θ₀ = 0) and water (α = 0.35, Θ₀ = 90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer, like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport of toluene through the vapor phase plays a significant role.     

Small-angle neutron-scattering study of bis(quaternaryammonium bromide) surfactant micelles in water. Effect of the long spacer chain on micellar structure

The microstructure of the normal micelles formed by dimeric surfactants with long spacers, [Br⁻(CH₃)₂N⁺(C _m H_{2 m +1})-(CH₂) _S  -(C _m H_{2 m +1})N⁺(CH₃)₂Br⁻, m = 10 and s = 8, 10 and 12], has been investigated by small-angle neutron scattering and compared with previously reported results for micelles of the same dimeric surfactants with shorter spacers (m = 10 and s = 2, 3, 4 and 6). It was found that for dimeric surfactants with long spacers (s = 8 and 10), both micellar growth and variation in shape occur to only a small extent, if at all, compared with dimeric surfactants with short spacers. However, for the dimeric surfactant with the longest spacer, s = 12, the extent of micellar growth and shape variation is also large. These results are due to the differences in conformation of dimeric surfactants with short spacers (s = 2–6) compared with that of the surfactants with long spacers (s = 8–12). Received: 15 June 1998 Accepted: 22 July 1998     

The saddle point of the nucleophilic substitution reaction Cl− + CH3Cl: results of large-scale coupled cluster calculations

On the basis of large-scale coupled cluster calculations including connectedz triple substitutions in a perturbative way, the geometrical parameters of the D _{3 h} saddle point of the Walden inversion reaction Cl⁻ + CH₃Cl′→ ClCH₃ + Cl′⁻ are predicted to be R _s (C—Cl) = 2.301 ? and r _s (C—H) = 1.069 ?. The barrier height with respect to the reactants is recommended to be 11.5 ± 1.0 kJ mol⁻¹. Connected triple substitutions lower the barrier height by almost a factor of 2, but have very little influence on the geometric structure of the saddle point. Received: 26 June 1998 / Accepted: 15 July 1998 / Published online: 28 September 1998     

An electroconductivity study on degree of counterion binding or dissociation of a-sulfonatomyristic acid methyl ester micelles in water as a function of temperature

 For a sodium salt of α-sulfonatomyristic acid methyl ester (14SFNa), one of the α-SFMe series surfactants, the differential conductivity (∂κ/∂C) _T , _P vs. square root of concentration (√C) was employed in order to determine not only CMC but also the limiting molar conductance (Λ⁰) and the molar conductance of micellar species (Λ^M). Based on the data of the degree of counterion binding to micelles (β) determined previously at different temperatures ranging 15–50 °C at every 5 °C, the experimental values of the degree of dissociation (ionization) of a micelle (α_EX) were calculated by regarding as α_EX=1−β. The ratio Λ^M/Λ⁰ corresponding to the ratio of slopes below and above CMC in the curve of specific conductivity (κ) vs. concentration (C), which has been often assumed to be the degree of ionization of micelles (α), was compared with the present α_EX. However, the ratio Λ^M/Λ⁰ (=α) was found to have a correlationship with α_EX (=1−β) as α_EX≈0.40×(Λ^M/Λ⁰), or strictly, α_EX=0.40 (Λ^M/Λ⁰)+0.08, indicating that the simple ratio of the slopes below and above CMC in κ vs. C curve is not true for α_EX=1−β. On the other hand, the method proposed by Evans gave a value closer to α_EX compared with the simple ratio. Received: 17 September 1996 Accepted: 8 April 1997     

Prediction of 13C NMR chemical shifts in substituted naphthalenes

The prediction of the ¹³C NMR signals for derivatives of naphthalene has been investigated using mathematical modeling techniques. Two empirical multiple regression models which utilize the field, resonance, and Charton's steric parameters together with molar refractivity were developed, one for α- and the other for β-substituted naphthalene derivatives. In the α case the model had a correlation coefficient of observed versus predicted line positions of r = 0.973 with a standard deviation of 2.2 ppm while in the β case r = 0.979 with the standard deviation being 2.3 ppm. The database consisted of 3152 signals from 394 naphthalene derivatives. We also report the use of the Taft steric parameter in place of the Charton steric parameter in the above- mentioned prediction equations. Received: 19 June 1998 / Accepted: 20 October 1998 / Published online: 16 March 1999     

Photocurrent and differential capacity measurements at polybithienyl and poly(3-butylthiophene) films

Photocurrent and differential capacity measurements have been carried out at polybithienyl (PBT) and poly(3-butylthiophene) (PBuT) films on platinum. The photocurrents are cathodic, similar to inorganic p-type semiconductors. The band gap energy was determined from the photocurrent spectra (E _g=1.7 eV for PBT and E _g=1.9 eV for PBuT). The dependence of the differential capacity on the potential could be presented as Mott-Schottky plot, at least in a limited potential region. The flatband potential was determined (E _fb= 0.67 V for PBT and E _fb=0.58 V for PBuT). Received: 9 June 1998 / Accepted: 22 August 1998     

Kinetics and equilibrium of the complexation of Al3+ with poly(maleic, acrylic) acid

Kinetics and equilibrium of the complexation of Al³⁺ with a polycarboxylic acid (PCA, random copolymer of maleic and acrylic acid with a mean molecular weight of 92 kDa) are investigated by the stopped flow technique and potentiometric titration. The complexation proceeds according to the Eigen–Tamm mechanism, i.e. in first diffusion-controlled step an outer sphere complex is formed. The second rate determining step is the formation of the inner sphere complex, controlled by the exchange rate of hydration water. For this second step the rate constant is k ₁=3 s^-1. It is in the order of magnitude of the water exchange at the Al³⁺ ion as expected for the Eigen–Tamm mechanism. The activation parameters are also determined. Parallel to this direct reaction path a base catalyzed path is found, typical for complexation reactions of hydrolyzable metal ions. Stable complexes are formed for which the overall association constant K _ass=Q _o(1+K _i) is determined by two parts: a chemical (intrinsic) part, described by the inner sphere association constant K _i=3 and an electrostatically controlled part described by the outer-sphere association quotient Q _o. The evaluation of the kinetic experiments allows to determine the value of log(Q _o) as a function of pH: 3.3<log Q _o<4.6. From these data the potential is calculated in the range −67 to ∝93 mV at pH values between 2 and 4. For comparison, analogous experiments with the monomeric subunits of the polyacid, glutarate (GA), and tricarballylate (TCA), are performed. The complexation with the monomeric subunits glutaric- and tricarballylic acid can be explained within the classical view of a discrete outer sphere association constant Q _o. Received: 13 November 1997 Accepted: 24 March 1998     

Density functional study on zerovalent lanthanide bis(arene)-sandwich complexes

Several zerovalent lanthanide bis(arene)-sandwich complexes, Ln(η⁶-C₆H₆)₂, Ln = La, Ce, Eu, Gd and Lu, have been studied by means of density functional theory. The calculated geometries are in good agreement with experiment. The calculated dissociation energies of the bond Ln-(η⁶-C₆H₆) may be considerably underestimated, but they correctly reveal the variation regularity. The bonding in these molecules can be described in terms of a relatively weak π-electron donation from benzene to Ln and a stronger electron back-donation from Ln 5d to the benzene π* orbitals. During bond formation, there is electron promotion from Ln 6s to 5d instead of from 4f to 5d, in opposition to the proposal of Anderson et al. The relativistic effect only slightly influences the molecular geometry, but decreases the bonding energy considerably through lowering the Ln 6s level and raising the 5d level. It enhances the trend of the bonding energy to decrease along the lanthanide series. Received: 22 June 1998 / Accepted: 9 September 1998 / Published online: 17 December 1998     

Cage Structure Formation of Singly Doped Aluminum Cluster Cations Al<Subscript><Emphasis Type="BoldItalic">n</Emphasis></Subscript><Emphasis Type="BoldItalic">TM</Emphasis><Superscript>+</Superscript> (<Emphasis Type="BoldItalic">TM</Emphasis> = Ti,V, Cr)

Structural information on free transition metal doped aluminum clusters, Al _n TM ⁺ (TM = Ti, V, Cr), was obtained by studying their ability for argon physisorption. Systematic size (n = 5 – 35) and temperature (T = 145 – 300 K) dependent investigations reveal that bare Al _n ⁺ clusters are inert toward argon, while Al _n TM ⁺ clusters attach one argon atom up to a critical cluster size. This size is interpreted as the geometrical transition from surface-located dopant atoms to endohedrally doped aluminum clusters with the transition metal atom residing in an aluminum cage. The critical size, n _crit , is found to be surprisingly large, namely n _crit = 16 and n _crit = 19 – 21 for TM = V, Cr, and TM = Ti, respectively. Experimental cluster–argon bond dissociation energies have been derived as function of cluster size from equilibrium mass spectra and are in the 0.1–0.3 eV range.     

Influence of the chain length of K-soaps on the phase diagram and structural parameters of nonaqueous liquid crystals and the gel phase in K-soap/glycerol binary systems

 Analogously to the aqueous K-soap/water systems already examined, the five glycerol · (Gl)-containing systems KC _n /Gl (n = 12, 14, 16, 18, 22) also built up hexagonal (H_α), lamellar (L_α), isotropic micellar (S), gel-like (G) and crystalline phases (C). These phases were identified by texture observations with a polarizing microscope, by differential scanning calorimetry measurements and by X-ray diffraction investigations. The appertaining phase regions were plotted in the binary phase diagram. Binary Gl-containing K-soap systems have the following properties. The H_α phase is built up at low soap concentration. The L_α phase is formed at high soap concentrations. The temperature of the phase transition H_α ⇆ S runs through a maximum. Increasing the chain lengths of the soaps shifts the formation of the H_α phase to lower soap concentration. A strong correlation between the chain length of K-soaps and the d values of L_α, H_α, G and C phases is found. Based on the comparison of the X-ray diffractograms of the G phase a structural model is proposed. The G phase consists of two groups of domains with two different dimensions. Received: 9 August 1999/Accepted in revised form: 20 September 1999     

Boronic acid-facilitated α-hydroxy-carboxylate anion transfer at liquid/liquid electrode systems: the EICrev mechanism

The transfer of the α-hydroxy-carboxylates of glycolic, lactic, mandelic and gluconic acid from the aqueous electrolyte phase into an organic 4-(3-phenylpropyl)-pyridine (PPP) phase is studied at a triple-phase boundary electrode system. The tetraphenylporphyrinato complex MnTPP dissolved in PPP is employed to drive the anion transfer reaction and naphthalene-2-boronic acid (NBA) is employed as a facilitator. In the absence of a facilitator, the ability of α-hydroxy-carboxylates to transfer into the organic phase improves, consistent with hydrophobicity considerations giving relative transfer potentials (for aqueous 0.1 M solution) of gluconate>glycolate>lactate>mandelate. In the presence of NBA, a shift of the reversible transfer potential to more negative values is indicating fast reversible binding (the mechanism for the electrode process is EIC_rev) and the binding constants are determined as K _glycolate = 2 M⁻¹, K _mandelate = 60 M⁻¹, K _lactate = 130 M⁻¹ and K _gluconate = 2,000 M⁻¹. The surprisingly strong interaction for gluconate is rationalised based on secondary interactions between the gluconate anion and NBA.     

The Thermal Dehydration of Magnesium Aluminum Pentafluoride Dihydrate: Crystal Structures of MgAlF5(H2O)2 and MgAlF5

Summary.  Small plate-like single crystals of MgAlF₅(H₂O)₂ have been obtained during hydrothermal treatment (270°C) of microcrystalline material prepared by precipitation of stoichiometric solutions of Al₂(SO₄)₃ ·  18H₂O and Mg(NO₃)₂ · 6H₂O with diluted hydrofluoric acid. The crystal structure of MgAlF₅(H₂O)₂ has been refined from single crystal data (Imma (# 74), Z = 4, a = 7.0637(7), b = 10.1308(10), c = 6.7745(7) ?, 398 structure factors, 33 parameters, R(F² > σ(F ²)) = 0.0245, wR(F² all) = 0.0525). Main features of the inverse weberite type structure are infinite chains of trans-bridged [AlF₆] octahedra which are connected via common fluorine atoms by isolated [MgF₄(H₂O)₂] octahedra. MgAlF₅(H₂O)₂ dehydrates at temperatures above 300°C to give MgAlF₅. XRPD analysis of this phase has revealed isotypism with FeAlF₅. The crystal structure of MgAlF₅ (Immm (# 71), Z = 2, a = 7.268(1), b = 6.123(2), c = 3.543(1) ?) is built of infinite chains of edge-sharing [MgF₆] octahedra and chains of corner-sharing [AlF₆] octahedra along [001]. Upon further heating to temperatures above 500°C, MgAlF₅ decomposes to MgF₂ and α − AlF₃. Received January 15, 2001. Accepted February 12, 2001     

Buffer Standards for the Physiological pH of the Zwitterionic Compound,DIPSO, from 5 to 55 °C

The values of the second dissociation constant, pK ₂, and related thermodynamic quantities of 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) have already been reported over the temperature range 5 to 55 °C including 37 °C. This paper reports the pH values of four NaCl-free buffer solutions and four buffer composition containing NaCl salt at I=0.16 mol⋅kg⁻¹. Conventional pa _H values are reported for all eight buffer solutions. The operational pH values have been calculated for four buffer solutions recommended as pH standards, at 25 and 37 °C after correcting the liquid junction potentials with the flowing junction cell.