Beziehungen zwischen chemischer Verschiebung und Ladungsdichte

This work is concerned with the relationship between chemical shifts (δ) of protons and their charge densities (q). It can be shown that the relation δ^H = a + bq^H + cq^C (q^C = charge density at the corresponding C-atom) fits best for a prediction of δ-values from calculated charge densities. The smallest standard error for the prediction of δ-values is obtained if the charge densities are calculated by the CNDO- or INDO-method.     

Glass transitions in ionic polymers

The glass transitions of an ionic polymer in bulk have been studied as a function of the counterion for various homo- and copolymers. It is shown that the glass transition temperature can vary by as much as 530°C., from ?10°C. for the nonionic material to +520 for Ca²⁺ or Zn²⁺ substituted polymer. From simple electrostatic considerations, it is shown that a linear correlation should exist beween the glass transition and the ratio of the cation charge, q, to the internuclear distance, a, between cation and chain anion; for this particular material (the polyphosphate chain) the relation is T_g = 625(q/a) ?12 where q is in units of one electron and a is in A. If the data are interpreted in the light of the Gibbs-DiMarzio theory, it is seen that both the chain stiffness and the intermolecular energy increase in an approximately parallel manner as q/a increases.     

Polymerization processes at conditions of moderately short kinetic chains by calorimetry

A method has been developed for the determination of the heat of the acts of chain growth q_g and of the summary heats of the acts of chain initiation and termination q^** for the radical polymerization of vinyl monomers at the conditions of moderately short kinetic chains, when the length of the kinetic chain v < 100, but the degree of polymerization is ¯P > 20. The analysis of the experimental data, obtained by investigating the kinetics of polymerization of vinyl monomers in solution by calorimetry has led to the conclusion, that at v > 100 an approximation of the long material chains can be used (when the contribution of q^** is negligibly small) with a permissible experimental error; however, in the region of moderately short chains a correction for q^** must be introduced, in order to obtain the correct kinetics of the process.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 695–701, November–December, 1988.     

Unusual finite size effects in the Monte Carlo simulation of microphase formation of block copolymer melts

Extensive Monte Carlo simulations are presented for the Fried-Binder model of block copolymer melts, where polymer chains are represented as self and mutually avoiding walks on a simple cubic lattice, and monomer units of different kind (A, B) repel each other if they are nearest neighbors (ε_AB > 0). Choosing a chain length N = 20, vacancy concentration Φ_v = 0,2, composition ƒ = 3/4, and a L × L × L geometry with periodic boundary conditions and 8 ≤ L ≤ 32, finite size effects on the collective structure factor S(q) and the gyration radii are investigated. It is shown that already above the microphase separation transition, namely when the correlation length ξ(T) of concentration fluctuations becomes comparable with L, a nonmonotonic variation of both S(q) and the radii with L sets in. This variation is due to the fact that the wavelength λ^*(T) of the ordering (defined from the wavenumber q^* where S(q) is maximal at λ^* = 2 π/q^*) in general is incommensurable with the box. The competition of two nontrivial lengths ξ(T), λ^* (T) with L makes the straigthforward application of finite size scaling techniques impossible, unlike the case of polymer blends. Since also the specific heat is found to have a broad rounded peak near the transition only, locating the transition accurately from Monte Carlo simulations remains an unsolved problem.     

Interaction of blockcopolymer micelles as probed by small-angle neutron and by small-angle X-ray scattering

 The analysis of the interaction of micelles formed by a blockcopolymer is given by means of small-angle X-ray (SAXS) and small-angle neutron scattering (SANS). The blockcopolymer consists of poly(styrene) and poly(ethylene oxide) (molecular weight of each block: 1000 g/mol) and forms well-defined micelles (weight-association number: 400, weight-average diameter: 15.4 nm) in water. The internal structure has been studied previously (Macromolecules 29:4006 (1996)) by SAXS. There it has been shown that the micelles are spherical objects. The structure factor S(q) as a function of the scattering vector q (q=(4π/λ) sin (θ/2); λ: wavelength of the radiation in the medium; θ: scattering angle) can be extracted from both sets of small-angle scattering data (SANS: q≤0.4 nm^-1; SAXS: q≤0.6 nm^-1). It is shown that particle interaction in the present system can be described by assuming soft interaction which is modeled by a square-step potential. Received: 12 May 1997 Accepted: 9 July 1997     

A quasielastic scattering approximation for target ionisation by heavy structured projectiles

A modification of the impulse approximation for capture to continuum is used to describe the ejection of loosely bound target electrons by heavy ions. In contrast to the currently applied elastic scattering models based on the binary encounter approximation, the present model treats only the short-range part of the ionic field within the elastic scattering theory, but uses the more sophisticated impulse approximation for the Coulomb tail of the interaction potential. Calculations have been performed for B ^q+, Fe ^q+, Au ^q+ and U ^q+ colliding with helium at energies between 0.5 and 6 MeV/amu. The comparison with absolute experimental spectra shows that theory works well for light projectiles, being able to describe the intensity and its variations with ionic chargeq _ion in the binary encounter peak region. For very heavy low-energy projectile ions the intensity is, however, seriously under-predicted at small emission angles, indicating a breakdown of perturbative treatments.     

Light,Small Angle Neutron and X-Ray Scattering from Gels

This paper gives two examples of experiments that demonstrate the power of small angle scattering techniques in the study of swollen polymer networks. First, it is shown how the partly ergodic character of these systems is directly detected by neutron spin echo experiments. The observed total field correlation function of the intensity scattered from a neutral gel allows the ergodic contribution to be directly distinguished from the non ergodic part, at values of transfer wave vector q that lie well beyond the range of dynamic light scattering. The results can be compared with those obtained at much lower q from visible light scattering. Second, a recent application of small angle X-ray (SAXS) and neutron (SANS) scattering is described for a polyelectrolyte molecule, DNA, in semi-dilute solutions under near-physiological conditions. For these observations, the divalent ion normally present, calcium, is replaced by an equivalent ion, strontium. The comparison between SANS and SAXS yields a quantitative picture of the cloud of divalent counter-ions around the central DNA core. At physiological conditions, the cloud is thinner than that predicted on the basis of the Debye screening length but thicker than if the counter-ions were condensed on the DNA chain.     

Homogeneous Fermi liquid with ‘artificial’ repulsive inverse square law interparticle potential energy

A good deal is known by now on the so-called jellium model of the homogeneous electron liquid. However, much of the quantitative progress at experimentally realizable densities has come from quantal computer simulation. Therefore, we here consider a homogeneous Fermion liquid with ‘artificial’ repulsive interaction λ/(r_ij )² between Fermions i and j at separation r_ij . We discuss first of all the way the static structure function S(q), essentially the Fourier transform of the pair correlation function, is changed because of non-zero λ from the ‘Fermi hole’ form due entirely to Pauli principle effects between parallel spin Fermions. Unlike jellium with e ²/r_ij repulsive interactions, S(q) is proportional to q at long wavelengths, whereas the plasmon in jellium annulls the q term and S(q) is quadratic in q as q tends to zero. However for λ/(r_ij )² interactions, the coefficient of q appearing in the Fermi hole structure factor, is renormalized by particle repulsions. Then some discussion is given of Fermion quasiparticle lifetimes τ as the Fermi surface is approached. Arguments are presented that τ^?1 is proportional to |E???E _F| as E tends to the Fermi energy. This is already interesting, in fact, in connection with the jellium model and therefore an approximate analytic form of τ is finally derived.     

Determination of chain stiffness and polydispersity from static light-scattering

Chain stiffness is often difficult to distinguish from molecular polydisperity. Both effects cause a downturn of the angular dependence at large q² (q = (4π/λ)sin θ/2) in a Zimm plot. A quick estimation of polydisperity becomes possible from a bending rod (BR) plot in which lim (c → 0) qRθ/Kc is plotted against q(〈S²〉_z)^1/2 = u. Flexible and semiflexible chains show a maximum whose position is shifted from u_max = 1.41 for monodisperse chains towards larger values as polydispersity is increased, while simultaneously, the maximum height is lowered. Stiff chains display a constant plateau at large q, its value is πM_L where M_L is the linear mass density. Using Koyama's theory, the number of Kuhn segments can be determined from the ratio of the maximum height to the plateau height, if the polydispersity index z = (M_w/M_n ? 1)^?1 is known. Thus, if the weight-average molecular weight M_w, is known, the contour length L_w, the number of Kuhn segments (N_k)_w, the Kuhn segment length l_k and the polydispersity of the stiff chains can be determined. The influence of excluded volume is shown to have no effect on this set of data. The reliability of this set can be cross-checked with the mean-square radius of gyration 〈s²〉_z which can be calculated from the Benoit-Doty equation for polydisperse chains. Rigid and slightly bending rods exhibit no maximum in the BR plot, and the effect of polydispersity can no longer be distinguished from a slight flexibility if only static scattering techniques are applied.     

Fully relativistic study on electron impact elastic scattering from Nq+ (q = 1–3), Na+, Arq+ (q = 1–3, 7–8), and Xeq+ (q = 2–6, 8)

A study is presented on the elastic scattering of electrons from N^q+ (q = 1–3), Na⁺, Ar^q+ (q = 1–3, 7–8), Xe^q+ (q = 2–6, 8) to understand the available experimental differential cross section results. A model potential approach has been utilized to describe the scattering process. The model potential includes the static, exchange, polarization and absorption potentials. The static potentialis obtained through the charge density calculated by obtaining ionic wave functions using multi-configuration Dirac-Fock (MCDF) approximation. Thereafter, the static potential is added to the suitable exchange, polarisation and absorption potentials to construct the spherically averaged complex optical potential. Using the obtained potential in the Dirac equations,these are solved with the partial wave phase shift analysis method and the differential cross sections are calculated. Results for different ions exhibit prominent interference structures in the energy versus cross section curves and show good agreement on comparison with the experimental results available in the selected energy ranges.     

Ligament-like tough double-network hydrogel based on bacterial cellulose

Using the double-network (DN) method, bacterial cellulose/polyacrylamide (BC/PAAm) DN gels able to sustain not only high elongation but also high compression have been synthesized by combining BC gel as the first network with PAAm as the second network in the presence of N,N′-methylene bisacrylamide (MBAA) as a cross-linker. This DN gel was obtained by modifying the monomer concentration of the second network, acrylamide monomer (AAm) and MBAA, and by controlling the water content of the first network, BC gel. The mechanical properties are discussed in term of the swelling degree (q), which is independent of the concentration of AAm and MBAA. It was found that, for BC/PAAm DN gels with the first network formed from BC gel with high q (BC _q=120), the tensile and compressive modulus (E) scales with q as E μ q ^{- 2} E \propto q^{ - 2} . The tensile fracture stress, σ _F, of this DN gel was almost independent of q, that is s_\textF μ q⁰, \sigma_{\text{F}} \propto q^{0}, but the compressive fracture stress, σ _F, scaled with q as E μ q ^{- 2} E \propto q^{ - 2} . Meanwhile, the tensile and compressive fracture strain (ε _F) of the gel is almost independent of q, which is caused by AAm concentration change, but linearly increased with q, which is caused by MBAA concentration change. Furthermore, by decreasing the water content of the BC gel prior to polymerization of the second (PAAm) network, a ligament-like tough BC/PAAm DN gel could be obtained with tensile strength of 40 MPa.     

Extended molecular symmetry groups

The group theoretical analysis of Longuet-Higgins molecular symmetry groupsG(m, n) andGB(m) and their torsional extensionsG ^q (m, n) andGB ²(m) of molecules consisting of two coaxially rotated parts is presented. The structure of the groups is described in terms of direct and semi-direct products. The groupsG ^q (m, n) andGB ²(m) are shown to be group extensions of the groupsG(m,n) andGB(m), respectively. All irreducible representations of the groupsG(m,n),GB(m), andGB ²(m) are derived using standard techniques of the extension or induction. A restriction method is proposed for derivation of irreducible representations of the groupG ^q (m,n). The class structure of the groups is determined and the character tables are given in the most general case.     

A new algorithm to evaluate bielectronic integrals with 1s Slater‐type orbitals obtained by the integral transforms

This article presents a variation of the integral transform method to evaluate multicenter bielectronic integrals (12|34), with 1s Slater‐type orbitals. It is proved that it is possible to define, out of the expression of (12|34) given by the integral transform method, a function F(q) that has the property of having a unique Q, such that F(Q) = (12|34). Therefore, F(q) may be used to calculate (12|34). It is shown that the evaluation of F(Q) turns out to be simpler than the three‐dimensional integral involved in the calculation of (12|34), and an algorithm is presented to calculate Q. The results show that relative errors on the order of 10^?3 or lower are obtained very efficiently. In addition, it is shown that the proposed algorithm is very stable. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Molecular dimensions and chain conformation in glassy syndiotactic polystyrene

For the first time the small-angle neutron scattering (SANS) from mixtures of protonated and totally deuterated syndiotactic polystyrene (sPS) has been studied. Two amorphous samples with similar molecular weights have been measured at various concentrations of the protonated part. All measurements were performed at room temperature using the scattering equipment of two different laboratories. The molecular weight M_w evaluated from SANS data agreed with those obtained by gel permeation analysis (GPC). In the Kratky representation the scattering contribution due to the contrast scattering shows a plateau behavior up to q = 0.45 Å^?1, where q is magnitude of the scattering vector. This observation is in evident contrast to what is expected from the rotational isomeric state (RIS) model. In addition the characteristic ratios C_∞, derived either from the plateau height or from radii of gyration of the Zimm regime and being in reasonable agreement with each other, show strong deviations from the predictions of the RIS model. © 1994 John Wiley & Sons, Inc.     

Solvent Isotope Effect on the Rate Constants of Singlet-Oxygen Quenching by edta and Its Metal Complexes

The reactivity of singlet oxygen (O₂(¹Δ_g)) with edta and its metal complexes with Al³⁺, Cu²⁺, Fe³⁺, and Mn²⁺ was investigated. The emission of singlet oxygen at 1270 nm in D₂O was measured in order to determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constant (k_r + k_q) of the chemical reaction between singlet oxygen and the acceptor (k_r) and of the physical quenching of singlet oxygen by the acceptor (k_q) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H₂O were used to determine quantum yields of the sensitized photooxidation, and the combined results of these experiments allowed the determination of k_r and k_q separately. A strong isotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical procedure used. The isotope effect, as well as the reactivity of edta and its metal complexes, depend markedly on the complexed metal ion.     

Length scale hierarchy in sol,gel, and coacervate phases of gelatin

Length scale hierarchy in gelatin sol, gel, and coacervate (induced by ethanol) phases, having same concentration of gelatin in aqueous medium (13% w/v), has been investigated through small angle neutron scattering and rheology measurements. The static structure factor profile, I(q) versus wave vector q, was found to be remarkably similar for all these samples. This data could be split into three distinct q‐regimes: the low‐q regime, I_ex(q) = I_ex(0)/(1+q²ζ²)² valid for q < 3R_g^?1; the intermediate q‐regime, I(q) = I(0)/(1+q²ξ²) for 3R_g^?1 < q < ξ^?1; and the asymptotic regime, I(q) = (c/q) exp(?R_c²q²/2) for q > ξ^?1. Consequently, three distinct length scales could be deduced from structure factor data: (a) inhomogeneity of size, ζ = 20 ± 1 nm for all the three phases; (b) average mesh size, ξ₀ = 2.6 ± 0.2 nm for sol and gel, and smaller mesh size, ξ_os = 1.2 ± 0.2 nm for coacervate; and (c) cross section of gelatin chains, R_c = 0.35 ± 0.04 nm. In addition, the structure factor data obtained from coacervating solution analyzed in the Guinier region, I(q) = exp(?q²R_g²/3), yielded value of typical radius of gyration of clusters, R_g ≈ 69 nm that indicated existence of triple‐helices of length, L ≈ 239 nm; (d) Frequency and temperature sweep measurements conducted on coacervate samples revealed two other length scales: (e) viscoelastic length, ξ_ve = 14 ± 2 nm and (f) correlation length at melting, ξ_T = 500 ± 70 nm. Thus, existence of six distinct length scales, (a–f), ranging from 1.2 to 500 nm has been established in the coacervate phase of gelatin–ethanol–water system. Results are discussed within the framework of Landau‐Ginzburg treatment of dynamically asymmetric systems (Prog Theor Phys 1977, 57, 826; Phys Rev A 1991, 44, R817; J Phys II (France) 1992, 2, 1631). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1653–1667, 2006     

Photochemical reaction and diffusion of caged calcium studied by the transient grating

A photoreaction of caged calcium (Ca²⁺) was investigated by using the time-resolved transient grating (TG) method. The q²-dependent feature of the TG signal was analyzed based on a model that there are two parallel dissociation steps with different rates and Ca²⁺ is released promptly after the fracture of the caged compound. The TG signal representing the presence of Ca²⁺ was appeared by the volume contribution. Although the diffusion coefficients of Cg and the decomposed product are different without Ca²⁺, only one diffusion component was observed after the dissociation of CgCa²⁺, indicating that the caged compounds and Ca²⁺ diffuse together.     

Creep recovery of acrylate urethane oligomer/acrylate networks. 2. Investigation of different modes of molecular motion

The creep recovery and dynamic mechanical properties of acrylate urethane oligomer/acrylate networks were investigated. The retardation spectra L_CR obtained from the creep recovery experiments were significantly different from the corresponding retardation spectra L_DMA obtained from the dynamic mechanical measurements. The reduced frequency ω^* dependence of L_DMA and the relaxation spectra H_DMA in the higher ω^* region were approximately represented as L_DMA ∝_ω^*−p and H_DMA ∝_ω^*q, although L_CR decreased faster than L_DMA with an increase in ω^*. The exponents q were close to ½ characterizing the Rouse modes in the systems containing an acrylate urethane oligomer of M_w = 5000 but less than ½ in the system containing an acrylate urethane oligomer of M_w = 12,000. For the latter systems, significant thermorheological complexity was observed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2543–2550, 1997     

QSAR models of cytochrome P450 enzyme 1A2 inhibitors using CoMFA,CoMSIA and HQSAR

Quantitative structure–activity relationship (QSAR) studies were conducted on an in-house database of cytochrome P450 enzyme 1A2 inhibitors using the comparative molecular field analysis (CoMFA), comparative molecular similarity analysis (CoMSIA) and hologram QSAR (HQSAR) approaches. The database consisted of 36 active molecules featuring varied core structures. The model based on the naphthalene substructure alignment incorporating 19 molecules yielded the best model with a CoMFA cross validation value q² of 0.667 and a Pearson correlation coefficient r² of 0.976; a CoMSIA q² value of 0.616 and r² value of 0.985; and a HQSAR q² value of 0.652 and r² value of 0.917. A second model incorporating 34 molecules aligned using the benzene substructure yielded an acceptable CoMFA model with q² value of 0.5 and r² value of 0.991. Depending on the core structure of the molecule under consideration, new CYP1A2 inhibitors will be designed based on the results from these models.