Solvent influence on the viscosity-temperature relationship for dilute polybutadiene solutions

The temperature dependence of the viscosity of dilute polybutadiene solutions has been described by an Arrhenius expression according to Moore's treatment. The activation energy of flow was found to depend on both concentration and molecular weight through an empirical parameter, K_e. This parameter was positive in toluene, zero in decalin and negative in dioxane. This behaviour is discussed in connection with the temperature dependence of the expansion coefficient (x_η) and the excluded volume parameter (z). A recent Yamakawa-Tanaka expression relating x_η and z was used in the theoretical calculation of K_e.     

Neutron scattering from liquid crystal mesophases: Form of the elastic incoherent structure factor for “monocrystal” aligned nematogen with specific Q to n orientation

The elastic incoherent structure function (ElSF), A_o(Q), is derived for specific experimental orientations of the momentum transfer vector, Q, with respect to the nematic director, n, of an aligned “monocrystal” mesophase. The particular uniaxial model first considered is that in which the initial probability component of the orientational correlation function departs from uniformity to become a peaked function. The analytical EISF for Q||n is derived on the basis of the microscopic scattering symmetry and an effectively static principal axis. Brief explicit consideration is given to the EISF for the more general “dynamic principal axis” model with specific Q to an experimental configuration. An alternative closed form is given for S_k (δ′), the dynamical equivalent of an order parameter.     

Stereoselective synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via intramolecular hetero-Diels-Alder reaction

The synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via an intramolecular hetero-Diels-Alder reaction of easily accessible α,β-unsaturated γ-ketoesters was investigated. The reaction was found to proceed in a highly stereoselective way leading to single, cis-configured product isomers. The same diastereomer is formed, independently of the configuration of the enone double bond of the precursor. The respective E- and Z-isomers react either through an endo-E-syn or an exo-Z-syn transition state.     

Asymmetric synthesis of (4S,5S)-2-oxo-4-phenyloxazolidine-5-carboxylic acid using a 1,2-addition of PhMgBr to an N-sulfinimine derived from (R)-glyceraldehyde acetonide and (S)-t-BSA

We report an asymmetric synthesis of (4S,5S)-2-oxo-4-phenyloxazolidine-5-carboxylic acid via stereoselective addition of phenylmagnesium bromide (PhMgBr) to an N-sulfinimine derived from (R)-glyceraldehyde acetonide. (S)- and (R)-Glyceraldehyde acetonides, important chiral synthons in synthetic organic chemistry, are prepared from the corresponding epichlorohydrin using an identical synthetic methodology.     

Two-photon ionization of an inner shell electron of the Cl atom

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine forE=1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a 1s electron moves to an emptyp-orbital, and those in which an electron from an occupiedp-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of ap-electron into the continuum. The second class of intermediate states is followed by the transfer of a 1s electron into an emptyp-state. The largest contribution comes from 3p →d-continuum followed by 1s → 3p transition. Our result is σ⁽²⁾/I=2.06×10⁴¹ cm⁴/W where σ⁽²⁾ is the two photon ionization cross-section andI is the light intensity.     

Geometric parameterization of the YBaCo4O7 structure type: Implications for stability of the hexagonal form and oxygen uptake

We explore the stability of the hexagonal form of MBaCo₄O₇ cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M^3+/2+ and the three-dimensional network of CoO₄ tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo₄O₇ structure.     

Cycloaddition of a chiral nitrone to allylic motifs: an access to enantiopure sugar-based amino acids displaying a stable glycosidic bond and to 4(S)-4-hydroxy-l-ornithine

A (?)-menthone-derived nitrone and various allyl O-, and S-glycosides reacted at 110 °C to afford the corresponding cycloadducts in good yields. For an N-acetylated allyl N-glycoside, an N-glycoside-based product was formed in poor yield with loss of the N-acetyl residue, while the major product 4 (60%), in which the sugar moiety was absent, arose from cleavage of the N-glycosidic bond, under the cycloaddition conditions. The cycloadducts of the O-, and S-glycoside type were ring-opened and subjected to acidic and basic hydrolysis, for removing the chiral auxiliary. This resulted in glycosidic bond cleavage for O-glycosides and loss of material, while an S-glycoside amino acid was isolated in 78% yield, indicating a higher resistance of the S-glycosidic bond. N–O Bond cleavage and hydrolytic treatments applied to 4 afforded 4(S)-4-hydroxy-l-ornithine in high yield. Use of the nitrone derived from (+)-menthone should afford the enantiomer of 4, both precursors of 4-hydroxy arginine derivatives by guanidination.     

A generalised friction model for the evaluation of angular momentum auto correlation functions

A theory of brownian motion where the friction coefficient is a finite correlation function of molecular torque is used to derive an expression for the angular velocity autocorrelation function, ψ(r). This is related to the orientational a.c.f., φ(t), using an analysis of bandshapes due to Kubo and extended by Shimizu. Both functions φ(t) and ψ(t) contain inherent weaknesses since ψ(t) has a MacLaurin expansion even up to t⁴ only, and π(t) does not reduce to the Kummer function for an ensemble of free rotors in the so-called limit of vanishing friction. Reasons for this behaviour are discussed.     

Stereospecific enzymatic hydrolysis of racemic epoxide: a process for making chiral epoxide

Among various microbial cultures evaluated, Rhodotorula glutinis SC 16293 and Aspergillus niger SC 16311 catalyzed the stereospecific hydrolysis of the racemic epoxide, RS-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-1,2-oxirane, 1 to the corresponding R-diol, R-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-ethane-1,2-diol, 3. The S-epoxide, S-1-{2′,3′-dihydrobenzo[b]furan-4′-yl}-1,2-oxirane, 2 remained unreacted in the reaction mixture. A reaction yield of 45–50% (theoretical maximum yield is 50%) and an enantiomeric excess (ee) of >95% were obtained for unreacted S-epoxide 2 using each culture. Addition of 10% methyl tert-butyl ether to an aqueous reaction mixture during hydrolysis by R. glutinis improved the ee of the unreacted S-epoxide 2 to >99% (yield 48%) and that of the R-diol 3 to 79%. Unlike R. glutinis, hydrolysis of racemic epoxide 1 in the presence of 10% methyl tert-butyl ether by A. niger showed an adverse effect and gave S-epoxide 2 in 54% yield and 49% ee.     

Generation of trans-π-pentadienyliron tricarbonyl cations from protonation of (trans,trans-2,4-hexadienal)iron tricarbonyl

Protonation of (trans,trans-2,4-hexadienal)iron tricarbonyl, (V) has been shown to yield two trans-pentadienyliron cations, an s-cis form, (VIa) and an s-trans form (VIb), in an approximately equal ratio. This ratio represents a kinetically controlled product distribution and indicates that the original unprotonated aldehyde complex was present in equally populated s-cis and s-trans forms. Both trans ions, (VIa) and (VIb) isomerize to the cis ion (VIIb), ΔF^ne = 18.5 kcal/mol. From an analysis of the kinetics and NMR line widths it is clear that the barrier to rotation about the C₁C₂ bond in (VI) is at least 15 kcal/mol and probably considerably exceeds this number. At temperatures below −100° the OH protons of (VIa) and (VIb) can be observed owing to slow exchange at this temperature. Chemical shifts of these protons indicate extensive charge delocalization.     

A general equation correlating intramolecular rates with ‘attack’ parameters: distance and angle

Using density functional theory (DFT) at the B3LYP level with the 6-31G(d,p) basis set, a general equation is derived relating activation energy with the distance between the two reactive centers (r_GM), and the hydrogen-bonding angle (α_GM) in an intramolecular proton transfer process. The strong correlation between the values of r_GM and α_GM with the activation energy, ΔG^‡, which reflects the experimental reaction rate, provides an excellent tool to predict reaction rate based on calculated geometrical parameters for a certain system (ΔH^‡, ΔG^‡ vs r_GM and α_GM). The slope of the equation can be used as an indicator of the mode by which the two reacting centers orchestrate in an intramolecular process.     

Approximate analytic expression for the electrophoretic mobility of a cylindrical colloidal particle. Relaxation effect

An approximate expression is derived for the electrophoretic mobility μ _⊥ of an infinitely long cylindrical particle of radius a and zeta potential ζ oriented perpendicular to an applied electric field in an electrolyte solution. The obtained expression is applicable for cylinders with large radii such that κa≥ca. 30, where κ is the Debye–Hückel parameter. It is shown that the obtained mobility expression μ _⊥, which consists of Smoluchowski’s equation and the correction term of the order of exp(ze∣ζ∣/2kT)/κa (where z is the valence of counterions in the electrolyte solution, e is the elementary electric charge, k is Boltzmann’s constant, and T is the absolute temperature), coincides with the mobility expression for a sphere of radius a and zeta potential ζ correct to the order of exp(ze∣ζ∣/2kT)/κa.     

Mössbauer study of LiFeVPO x as a new cathode material for lithium-ion battery

The Mössbauer spectrum of LiFeVPO _x , LiFeV_0.5PO _x and LiFePO _x glasses prepared by conventional melt-quenching method for cathode active material is composed of a doublet due to distorted Fe^IIIO₄ tetrahedra. The Mössbauer spectrum of LiFePO _x glass has an additional doublet due to distorted Fe^IIO₆ octahedra. Heat treatment of LiFeVPO _x and LiFeV_0.5PO _x glasses at a given temperature close to each crystallization temperature causes a marked decrease in the value of Δ, reflecting a decrease in the distortion or an increase in the local symmetry of distorted Fe^IIIO₄ tetrahedra. Heat treatment of LiFeVPO _x glass causes an increase in the electric conductivity from the order of 10^?7 to 10^?3 S·cm^?1, together with an increase in the specific discharge-and charge-capacity of a coin-type Li-ion cell from 50 to 150 mAh·g^?1. These results prove that structural relaxation of the glass network causes an increase in the electric conductivity and an increase in the energy density of the Li-ion cell.     

Transcrystallization in nanofiber bundle/isotactic polypropylene composites: effect of matrix molecular weight

Polyamide 66 (PA 66) nanofiber bundles were first electrospun and then introduced into isotactic polypropylene (iPP) melts to prepare nanofiber bundle/iPP composites. To reveal the influences of matrix molecular weight (M _n ) on the transcrystalline layer, three kinds of iPP with different M _n were adopted. Polarized optical microscope was employed to investigate the transcrystallinity. In the presence of PA 66 nanofiber bundle, the heterogeneous nucleation distinctly happened in iPP melts. Moreover, the higher the iPP M _n , the denser the nuclei. Both a decrease in matrix M _n and an increase in isothermal crystallization temperature led to an increase in the induction time. The maximum temperature at which the transcrystalline layer can be optically observed increased with the increase of M _n . The growth rate of transcrystallinity decreased with the increasing M _n and crystallization temperature. Moreover, selective melting of the transcrystalline layers confirmed that it was merely composed of α form crystal for all composites.     

An improved model of the fringing fields of a quadrupole mass filter

The fringing field region of a quadrupole mass filter is modelled using an iterative finite difference technique to solve Laplace's equation for the electrostatic potential. The results are used to formulate an expression f(z) such that the electrostatic potential of the fringing field, F(x, y, z, t) may be written in the form F(x, y, z, t) = f(z) φ(x, y, t), where φ(x, y, t) is the hyperbolic field of the quadrupole. The function f(z) is expressed in the form f(z) = 1 - exp {- az - bz²}), where a and b are constants and z is in units of r_O, where 2r_O is the spacing of the quadrupole rods. The effect of the distance from the quandrupole rods to the end plate of the quadrupole mass filter, d on f(z) is investigated and the results presented show that for d < 0.125 r_O the function f(z) does not alter significantly.     

C2-Symmetric diphenylphosphoramide and diphenylthiophosphoramide derived from (1R,2R)-1,2-diaminocyclohexane as ligands for the titanium(IV) alkoxide-promoted addition of diethylzinc to aldehydes

Chiral C₂-symmetric diphenylphosphoramide 4 and diphenylthiophosphoramide 5 were prepared from the reaction of diphenylphosphinic chloride and diphenylthiophosphinic chloride with (1R,2R)-(−)-1,2-diaminocyclohexane in the presence of diisopropylethylamine in high yields. They were used as chiral ligands in the catalytic asymmetric addition reaction of diethylzinc to aldehydes in the presence of titanium(IV) isopropoxide to give the corresponding sec-alcohols in 70–83% ee with an (R)-configuration and in 40–50% ee with an (S)-configuration, respectively.     

Synthesis of an O-acyl isopeptide by using native chemical ligation to efficiently construct a hydrophobic polypeptide

By using dimethylformamide to suppress the O-to-N acyl migration, we efficiently synthesized an O-acyl isopeptide by native chemical ligation of a peptide-thioester and a Cys-O-acyl isopeptide. The reaction mixture was then loaded onto an octadecylsilane reverse-phase HPLC column, and the isopeptide was purified by using a linear gradient of CH₃CN in 0.1% aqueous trifluoroacetic acid. The recovery rate of the O-acyl isopeptide was considerably higher than that of the corresponding native polypeptide. Synthesis of O-acyl isopeptides via native chemical ligation, with O-to-N acyl migration as the final step to give the native form, has potential as an efficient method of constructing hydrophobic polypeptides.     

Development of a convenient route for the preparation of the N-Cbz-protected guaninyl synthon required for Boc-mediated PNA synthesis

An efficient, high yielding, chemo- and regioselective, five-step synthetic route to N²-Cbz-guanin-9-yl acetic acid has been developed, which avoids the use of triphosgene. After formation of the N²-Boc protected purine from 2-amino-6-chloropurine, two successive base-controlled alkylations allowed an N⁹-tert-butyl acetate function followed by an N²-Cbz moiety to be installed. The selectivities of these reactions were confirmed through an X-ray crystallographic study of the 6-(2-nitrophenoxy) analogue. Final hydrolytic dechlorination and removal of the Boc and tert-butyl ester protecting groups were accomplished concomitantly under acidic conditions to afford the guanin-9-yl PNA monomer synthon in an overall yield of 53%.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.