The influence of orientation on the electrocatalytic hydrogenation of hydroquinone chemisorbed at smooth polycrystalline platinum electrodes

The influence of orientation on the electrocatalytic hydrogenation of hydroquinone (HQ) chemisorbed at smooth polycrystalline platinum electrodes in aqueous solutions has been investigated; experimental measurements, performed in the absence of bulk (unadsorbed) HQ, were based upon thin-layer electrochemical techniques. The extent of hydrogenation was characterized by (i) n_H. the average number of hydrogen atoms reacted per chemisorbed HQ molecule, and (ii) the electrolytic charge Q_ox for oxidation of chemisorbed organic which remained on the surface after the hydrogenation reaction. The measured values of n_H indicate that the extent of HQ hydrogenation is (i) dependent upon the potential E_Hyd at which hydrogenation was earned out, and (ii) a sensitive function of its initial adsorbed orientation; at a given E_Hyd, n_His larger in the flat (η⁶) than in the edge (2,3-η²) orientation. Correlation of Q_ox with n_H, indicates that an appreciable fraction of partially hydrogenated species is desorbed from the surface; this fraction, which is a function of E_Hyd, is larger in the 2.3-η² than in the η⁶ orientation.     

Orientational Preference of Long,Multicenter Bonds in Radical Anion Dimers: A Case Study of π‐[TCNB]22− and π‐[TCNP]22−

The similar shape and electronic structure of the radical anions of 1,2,4,5‐tetracyanopyrazine (TCNP) and 1,2,4,5‐tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon?carbon bond in π‐[TCNP]₂^2? and in π‐[TCNB]₂^2?, in good accord with the Maximin Principle predictions. Instead, the two known structures of π‐[TCNP]₂^2? have a D_2h(θ=0°) and a C₂(θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six‐membered rings). The only known π‐[TCNB]₂^2? structure has a C₂(θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (E_int) and the variation of the E_int components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the E_int(θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of E_int(θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the E_int(θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)^.? ??? cation⁺ interactions are also included in the model aggregate are the experimental data reproduced computationally.     

Rheoviscosimetric Study of PEG35000 and PEG6000 at pH 7 and pH 9: Conformational Transitions and Unperturbed Dimensions

These investigations may be of interest for researchers working on the determination of the hydrodynamic properties of the polyethylene glycols in various conditions. Various concentration regimes and the intrinsic viscosity variation were determined for polyethylene glycols of different molecular weights at different pH. The temperature-concentration phase diagrams were obtained and the information on the entanglement process of polyethylene glycol under the effect of pH was evaluated. The parameters [η]_θ, K_θ and the expansion factor α_η in the unperturbed state as a function of concentrations and temperatures were determined.     

Differential cross section measurements of rainbow structure in Li+-Ar,Kr and Xe

The differential elastic cross sections (θ_lab < 20°) for Li⁺-Ar, Kr, Xe have been measured for 3 <E_c.m. < 9 eV and compared with those predicted from the electron gas theory of Gordon and Kim for closed shell systems. The experimental well depths are typically deeper than predicted by theory. The largest discrepancy of 9% was found for Li⁺-Kr.     

Multiple collisions in molecular beam scattering experiments

Due to the forward peaked differential cross section for elastic atom—atom scattering the effect of multiple collisions has to be considered in the analysis of crossed beam measurements of the total cross section and especially of the small angle differential cross section at large values of the beam attenuation. At angles θ ≈ θ₀, with θ₀ the quantum mechanical scaling angle of the elastic differential cross section, the correction for the latter case amounts to 20% at beam attenuations I/I₀ = exp(?1). Firstly, a careful analysis of the probabilities for single and multiple scattering is given, resulting in an expression for the measured beam signals which is correct for all values of the beam attenuation. The probability for multiple scattering is then calculated for an inverse power potential V(r) = ?C_sr^?s, with s = 4 through s = 7, which include both the case of ion—atom scattering (s = 4) and atom—atom scattering (s = 6). The results are given as effective differential cross sections σ_n(θ) for n-fold scattering. They are described by a single, simple analytical function with four free parameters that have been determined for n = 2, 3 and 4 by a least squares method. The σ_n(θ) are normalised to the total cross section Q.     

Energy dependence of rotational and vibrational distributions of N2(C) for the Ar*(3P0,2) + N2(X) excitation transfer reaction

The Ar* + N₂(X) → N₂(C, v′, N′) + Ar excitation transfer reaction has been investigated experimentally in two different atomic beam experiments. The inelastic cross sections Q_{v′ = 0}(E) and Q_{v′ = 1}(E) to the v′ vibrational level have been measured in the energy range 0.06 ⩽ E(eV) ⩽ 6, using a crossed beam machine. Both cross sections show a behaviour typical for a curve crossing mechanism, with maximum values Q₀ = 8.0 Ų and Q₁ = 1.2 Ų at E = 0.16 eV and E = 0.13 eV, respectively. The oscillatory behaviour of the ratio Q₁(E)/Q₀(E), as first observed by Cutshall and Muschlitz, is also present in our data. Within the model of Gislason et al. the results indicate a decreasing bond stretching with increasing energy. As an alternative we discuss the possibility that the oscillation is due to a different energy dependence of the cross sections for the Ar*(³P₀) and Ar*(³P₂) fine structure states in the mixed beam of metastable Ar*. The vibrational and rotational distributions have also been measured at E = 0.065 eV in a small scale atomic beam-scattering cell experiment, which can be considered as an intermediate between a bulk experiment and a crossed beam experiment. The relative vibrational populations are n_v′ = 100, 16.0, 3.03 and 0.31 for v′ = 0 through 3, with rotational “temperatures” of T_rot,v′ = 1960, 1010, 370 and 130 K. Pronounced deviations (“hump”) of the Boltzmann rotational distributions occur at N′ ≈ 27 for v′ = 0, 1 and 2, with a fractional population of 1, 3 and 11%. For v′ = 0 the “hump” is largely obscured by overlap with the v′ = 1 bandhead. These bimodal distributions are in qualitative agreement with the results of Nguyen and Sadeghi for v′ = 0. The results are discussed within the framework of a curve crossing mechanism with the Ar⁺-N⁻₂ diabatic potential as an intermediate. By assuming equal charges on both N atoms the Coulomb potential of the collinear orientation lies lower (0.45 eV at R = 2.5 Å) than the perpendicular orientation, with the consequence of different transfer probabilities for both orientations. Within a classical model or rotational excitation the final N′ values can be calculated for both orientations, resulting in much higher N′ values for the perpendicular orientation. This mechanism supplies a qualitative explanation for the observed bimodal rotational distributions.     

The mechanical moduli of lamellar semicrystalline polymers

Bounds on the elastic constants are derived for semicrystalline polymers whose local morphology is lamellar. Local response matrices (stiffness and compliance) are formulated in three dimensions that simultaneously incorporate uniform in-plane strain and additive forces from layer to layer of crystalline and amorphous phases and uniform stress and additive displacements normal to the lamellar surfaces. Spatial averaging of the stiffness and compliance matrices under the assumption of axially symmetric orientation gives the upper and lower bounds on the longitudinal and transverse tensile moduli and the axial and transverse shear moduli as functions of the separate phase elastic constants, the volume percent crystallinity, and the moments of the orientation 〈cos²θ〉 and 〈cos⁴θ〉. The bounds are much tighter than the Voight upper and Reuss lower bounds that do not recognize phase geometry. Using the known crystal elastic constants of polyethylene, sample calculations on isotropic unoriented materials show that the divergence of bounds at high crystallinity necessitated by the extreme crystal anisotropy shows up only at very high crystallinity. At low temperature the bounds are tight enough to specify G₁, the amorphous modulus, from the measured G and the known crystal elastic constants. At higher temperatures and lower G, the bounds are not tight enough for this purpose but the shear modulus versus crystallinity and temperature data are well fitted by the lamellar lower bound using a temperature-dependent, crystallinity-independent G₁.     

Quasi-classical trajectory approach to the stereo-dynamics of the reaction F+HO→HF+O

Quasi-classical trajectory (QCT) calculations are employed for the reaction F + HO(0,0)→HF + O based on the adiabatic potential energy surface (PES) of the ground 3A″triplet state. The average rotational alignment factor P2(j′·k) as a function of collision energy and the four polarization dependent generalized differential cross sections have been calculated in the center-of-mass (CM) frame, separately. The distribution P(θr) of the angle between k and j′, the distribution P(θr) of dihedral angle denoting k-k′-j′ correlation, and the angular distribution P(θr, Φr) of product rotational vectors in the form of polar plots are calculated as well. The effect of Heavy-Light-Heavy (HLH) mass combination and atom F's relatively strong absorbability to charges on the alignment and the orientation of product molecule HF rotational angular momentum vectors j′ is revealed.     

The relationship between proton–proton NMR coupling constants and substituent electronegativities. II—conformational analysis of the sugar ring in nucleosides and nucleotides in solution using a generalized Karplus equation

The relationship between vicinal NMR proton–proton coupling constants and the pseudorotational properties of the sugar ring in nucleosides and nucleotides is reinvestigated. Compared with our earlier study several important improvements are introduced: first, a new empirical generalization of the classical Karplus equation is utilized, which allows an accurate correction for the effects of electronegativity and orientation of substituents on ³J(HH); second, empirical correlations between the parameters governing the conformation of β-D -furanosides (taken from an analysis of 178 crystal structures) were used to define proton–proton torsion angles as a function of the pseudorotation parameters P and Φ_m; and, third an iterative least-squares computer program was devised to obtain the best fit of the conformational parameters to the experimental coupling constants. NMR data for the sugar ring in the following compounds were taken from the literature and analysed: 3′,5′-cyclic nucleotides, a base-stacked ribonucleotide, 2′-anhydroarabinonucleosides, α-D -2′,2-O-cyclouridine, 2′- and 3′-aminosubstituted ribonucleosides, 2′- and 3′-deoxyribonucleosides. The present results confirm that the conformational properties found in the solid state are, on the whole, preserved in solution.     

Study of crystalline orientation in drawn ultra-high–molecular weight polyethylene films

A wide-angle x-ray diffraction (WAXD) study of the development of molecular orientation in the crystalline phase of ultra-high–molecular weight polyethylene films prepared by the gelation–crystallization method is presented. WAXD scans of the undrawn films show that the lamellae are oriented in the plane of the films. Upon drawing at 130°C, the orientation of the molecular chains changes from the direction normal to the film surface (ND) to the elongation direction. The decrease of the 200/020 intensity ratio at low draw ration (λ <10) indicates that double orientation develops during the transformation from the lamellar to the fibrillar morphology, with the a-axis oriented parallel to ND. The orientation distributions of the 110, 200, 020, and 002 planes of the orthorhombic unit cell of polyethylene were studied and characterized by the coefficients of a Legendre polynomial series. At a draw ratio of 4.5, the second-order coefficient, 〈P₂(cos χ〉, already gets close to its limiting value, but it is shown that higher order coefficients of the polynomial series can be used to describe the evolution of the orentation, even up to λ = 50. The coefficients relative to the molecular chain orientation, 〈P_n(cos χ)〉_c, can be calculated from different crystalline reflections. Curve-fitting calculations were made in order to improve the correlation between the results obtained from the orientation distribution of the 110, 020, and 002 planes. A Person VII function was found to give a better fit of the experimental curves than Gaussian or Lorentzian equations. © 1993 John Wiley & Sons, Inc.     

Electromechanical properties of single domain PZN–12%PT measured by three different methods

In this paper, we compare the electromechanical properties of tetragonal single domain PZN–12%PT single crystals obtained by different methods: resonance–antiresonance method, Brillouin scattering, and resonant ultrasound spectroscopy. The agreement between the different measurements is found satisfactory for many elastic constants within experimental uncertainties. Differences are notable for the elastic constants associated to the propagation of shear waves (c₆₆^E and c₄₄^E). This can be accounted for by imperfections of the sample in acoustic spectroscopy and specific difficulties of shear resonators used for the resonance method. Strong discrepancies are noted for the hardened elastic constant c₃₃^D associated to longitudinal waves propagating along the polarization direction; we suggest this can arise from a frequency dependence of the dielectric constant.     

Determination of the liquidus curves of phase diagrams of condensed systems by measuring their E.M.F.

The accuracy of the determination of the liquidus curves of phase diagrams of condensed systems by e.m.f. measurements of suitable cells is discussed. The accuracy of the evaluation of the experimental data may be increased by considering the activity coefficients of electroactive components of the system in the corresponding equations using a suitable method of graphical correlation of the measured isothermal data. A further method of polythermal evaluation of experimental data has also been suggested. From the dependence of E_{r, cell} vs. x′_i at a given T and of E_{r, cell} vs. T at a given x′_i for one-phase and multi-phase conditions, twice as many corresponding x_{i, sat}?T_PC pairs may be obtained from a suitably chosen set of measurements.     

Studies of the effect of orientation on the elastic constants of stretched and hydrostatically extruded polychlorotriflouroethylene: A reorientational model in semicrystalline polymers

A reorientational model of polymer solids is given. The solid is described as a collection of small, orientable units. The elastic constants of the oriented polymer are given as a combination of the elastic constants of the orientable units and of the orientation parameters 〈P₂(cosθ)〉 and 〈P₄(cosθ)〉. The elastic constants of stretched and hydrostatically extruded samples of polychlorotriflouroethylene are obtained with Brillouin light scattering. These are fitted to the model and the orientation parameters are calculated. The elastic constants from the extruded samples are fit well by the model but the fit to the stretched film is less satisfactory. Hydrostatic extrusion in polymers is thus assumed to be a reorientation process in the amorphous phase, but the stretching process is not completely reorientational.     

Studies of electrochemical nucleation by means of standard and modified pulse potentiostatic techniques

This report compares experimental data for the overpotential dependence of the stationary nucleation rate I_st and of the induction time t_o obtained in two different ways: (i) by varying the potential E_n of the working electrode at a constant concentration of metal ions (standard pulse potentiostatic technique), and (ii) by varying the concentration of metal ions at a constant electrode potential E_n (modified pulse potentiostatic technique). It is shown that the latter experimental method allows us to fix the surface state of the working electrode and to create well-defined conditions for nucleus formation. Information is obtained on the rate-determining step of the nucleation process.     

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.     

Predictability of properties in ternary n-alkane/butanone/poly(dimethylsiloxane) systems from flory-huggins binary interaction parameters and inversion point in preferential solvation

An empirical procedure has been developed, based on the Flory-Huggins theory as generalized by Pouchlý, which permits the calculation of preferential sorption coefficient, λ, and the total sorption term, Y, from previous information on the binary interaction parameters, χ_13′⁰, χ₂₃⁰ and g₁₂(φ₁₀) and of the mixture composition at which the sign of λ inverts. The obtained expressions were applied to seven cosolvent polymer systems, n-alkane/butanone/poly(dimethylsiloxane), for which experimental values of λ and Y are known. In all the studied systems, the theoretical predictions are in fair accord with the experimental data.     

Cole-Cole plot analysis of dielectric behavior of monoalkyl ethers of polyethylene glycol (CnEm)

The complex permittivity of monoalkyl ether of polyethylene glycol (C_nE_m) was measured at 336.5 K at frequencies from 0.2 to 20 GHz. The number of bonded ethylene glycol units was varied from 1 to 120 to observe the effect of the PEG-chain length on the dielectric permittivity of the whole polymer. The measured real and imaginary parts of complex permittivity of these polymers were studied by the graphical analysis of the Cole-Cole plot as proposed by Havriliak and Negami. This analysis makes us understand that, in the high-frequency region above 9.7 GHz, the Cole-Cole plot of all C_nE_m is located on one single line. This phenomenon implies the same relaxation mechanism of all C_nE_m in this frequency region. It is found that the same relaxation mechanism of the dipoles of the hydroxyl group in n-alcohol contributes to the dipole relaxation of C_nE_m. In the low-frequency region, the arc of the Cole-Cole plot has the same shape as pure PEG, but it is scaled-down linearly following the decrease of the number of PEG units bonded to the n-alkanes. This phenomenon explains the linear contribution of ether dipoles, existing in bonded PEGs, on the complex permittivity of C_nE_m in the low-frequency region.     

The interrelation between transfer and relaxation processes in aqueous solutions of electrolytes and hydration numbers and activation energies of molecular motions

The influence of transfer processes and activation energies on the electrical conductivity and nuclear magnetic relaxation rate of a reference aqueous solution of KCl and sea water at 15°C was studied. The closest agreement between the calculated and experimental conductivity values was obtained with the coordination numbers n _S of the K⁺ and Cl^? ions equal to 4 and 1, respectively, and the activation energy close to E _a for vapor (3.38 kcal/mol). According to nuclear magnetic relaxation rate, viscosity, diffusion, and self-diffusion measurements, the n _S values of these ions are 8 and 4, respectively, and E _a ≈ 4.6 kcal/mol. The main reasons for the difference in the n _S and E _a values for transfer processes in aqueous solutions of strong electrolytes are discussed. The temperature and concentration dependences of NMR relaxation rates and the other parameters related to molecular mobility are best described by a function which is the sum of exponential functions whose number depends on solution concentration.     

Infra-red and Raman spectra of N-methyl-N′-n-alkylthioureas

The i.r. and Raman spectra of solid CH₃NHCSNH(CH₂)nCH₃ (n = 0–15) MTUR′ and CH₃NDCSND(CH₂)nCH₃ (n = 0–9) MTUR′-d₂ are examined. The vibrational properties of N,N′-disubstituted thioureas are studied on the basis of the spectral comparison among the series of compounds, of the wavenumber shift by N-deuteration and of a comparison with the vibrational properties of urea analogue, MUR′. From the analysis of interactions between end-group vibrations and methylene band progression modes it is concluded that MTUR′ molecules are in a skeletal planar structure with an extended n-alkyl chain.     

Topological investigations of binary and ternary mixtures: excess isentropic compressibilities

The speed of sound, U_ij 1,3-dioxolane (D) in binary mixtures (ij) with benzene, cyclohexane, n-hexane or n-heptane and U_ijk for 1,3-dioxolane in ternary mixtures (ijk) with the same hydrocarbons have been measured as a function of composition at 298.15 K. The observed data have been utilised to evaluate excess isentropic compressibility of binary, (κ_s^E)_ij and ternary (κ_s^E)_ijk mixtures using density and speed of sound values of the binary and ternary mixtures. The Moelyn-Huggins concept of interaction between the molecular surfaces of the components of a binary mixture [Polymer 12 (1971) 389] has been extended to evaluate excess isentropic compressibility of the studied binary and ternary mixtures. It has been observed that κ_s^E values predicted by a graph-theoretical approach using connectivities of third degree for binary mixtures compare reasonably well with their corresponding experimental values and κ_s^E for ternary mixtures are of the same sign and order of magnitude.