bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis, conformational studies, and inclusion properties of tris[(2-pyridylmethyl)oxy]hexahomotrioxacalix[3]arenes
Authors: Yamato Takehiko , Haraguchi Mitsuteru , Nishikawa Jun-Ichi , Ide Seiji , Tsuzuki Hirohisa
Published: April 2011

O-Alkylation of the flexible macrocycle 1 with 2-(chloromethyl)pyridine in the presence of Cs2CO3 resulted in the preferential formation of partial-cone-2. The cone-to-partial-cone ratio of 2 increased in the presence of K2CO3, and led to almost equal in the presence of NaH. The template effect of the alkali metal cations plays an important role in this O-alkylation reaction. The structural characterization of these products is also discussed. The two-phase solvent extraction data indicated that tris[(2-pyridylmethyl)oxy]hexahomotrioxacalix[3]arenes 2 show moderate extractability for n-butylammonium ion and high extractability for Ag+. The 1H NMR titration of cone-2 with AgSO3CF3 clearly demonstrates that a 1:1 complex is formed, which is stable on the NMR time scale. Thus, the synergism of cyclophane moiety and two or three lower-rim side chains with pyridyl groups play a significant role on the complexation of tris[(2-pyridylmethyl)oxy] derivatives cone-2 and partial-cone-2 with Ag+ ion and n-butylammonium ion.Key words: macrocycles, calixarenes, O-alkylation, ionophores, cation-N interaction.
Article
Authors: Ranger Maxime , Leclerc Mario
Published: April 2011

Electroactive and photoactive copolymers derived from fluorenes have been prepared from palladium-catalyzed Suzuki couplings. For instance, poly((4,4prime-biphenylene)-2,7-(9,9-dioctylfluorene)) and poly((2,5-thienylene)-2,7-(9,9-dioctylfluorene)) exhibit strong emission in the blue region (406 nm, phifl = 0.72) and in the green region (496 nm, phifl = 0.49), respectively. These fluorene-based pi-conjugated polymers also show reversible electroactivity upon reduction and oxidation. The good electrical transport of both p-type and n-type charge carriers combined with excellent luminescent properties should lead to the development of efficient light-emitting devices.Key words: polyfluorenes, Suzuki couplings, luminescence, electrochromism, n and p doping.
Evidence for room temperature mesomorphism in a mixed-valent diruthenium(II,III) quintapalmitoleate polymer
Authors: Caplan Jennifer F , Murphy Christopher A , Swansburg Susan , Lemieux Robert P , Cameron T Stanley , Aquino Manuel AS
Published: April 2011

The synthesis of Ru2(μ-O2CR)4(μ´-O2CR) (1), R = -CH2(CH2)6CH=CH(CH2)5CH3, has been achieved and characterization using elemental analysis and FTIR and UV-Vis spectroscopies undertaken. Strong evidence for a hexagonal discotic mesophase has been found using differential scanning calorimetry, variable-temperature polarizing optical microscopy, and X-ray powder diffraction. A solid to liquid crystal transition was found upon heating at 128°C and the mesophase is found to persist to room temperature upon cooling from 150°C. This is the first report of room temperature mesomorphism in a mixed-valent metallomesogen. Key words: ruthenium carboxylate, liquid crystal, metallomesogen, mixed valence, polymer.
A comparative theoretical study of the electronic structure of some nickel(II) azides, thiocyanates, and isothiocyanates
Authors: Abu-Eittah R H , El-Esawy M , Ghoneim N , Aly A T
Published: April 2011

The electronic structure, conformation, and molecular orbitals of some nickel(II) azides, thiocyanates, and isothiocyanates have been studied. Three different basis sets: split valence (SV), split valence with six d-Gaussians (SV6D), and double zeta (DZ) sets, were used to find the best ground state for nickel. It has been found that the combination, DZ-3F, gives results closest to the experimental values. The electronic structures of the nickel azides studied were completely different from those of the nickel thiocyanates. On the other hand, the electronic structures of the nickel thiocyanates studied were highly comparable to those of the corresponding nickel isothiocyanates. Molecular orbitals were computed for the complexes studied and the types of electronic transitions expected were identified and discussed.Key words: Ni(II) azides, thiocyanates, and isothiocyanates: ab initio SCF calculations; MO calculations on some Ni(II) complexes; theoretical treatment of some Ni(II) ions and salts; geometry and energetics of some nickel(II) azides, thiocyanates, and isothiocyanates.
Synthesis and properties of a sterically encumbered poly(thienylene vinylene): poly[E-1,2-(4,4prime-dihexyl-2,2prime-dithienyl)ethylene]
Authors: Lowe Jimmy , Bartels Carl , Holdcroft Steven
Published: April 2011

Electrically conducting poly[E-1,2-(4,4prime-dihexyl-2,2prime-dithienyl)ethylene] was synthesized and structurally characterized by UV-visible, 1H NMR, and FTIR spectroscopy. The head-to-head arrangement of the alkylthiophene dyads directly affects the UV-visible, photoluminescence, and electroluminescence maxima of the polymer. The conductivity of the doped polymer was measured to be ~2 S cm-1 and the stability of the oxidized polymer possessing various dopants was compared. UV-visible irradiation of PDHDTE rendered the films insoluble and photolithography was used to produce micron-size images. FTIR was used to monitor the photoproducts during photolysis of PDHDTE. Mechanisms of photooxidation were proposed, based on the photoproducts observed.Key words: polythiophene derivative, head-to-head dyad, photolithography, photooxidation, electroluminescence.
ipso-Nitration of tert-butyl[n.2]metacyclophanes; through-space electronic interactions between two benzene rings
Authors: Yamato Takehiko , Kamimura Hideo , Tsuzuki Hirohisa
Published: April 2011

The selective introduction of one or two nitro groups by direct replacement of tert-butyl groups via the ipso aromatic nitration of the meta-bridged aromatic compounds having two arene rings is described. The ipso-nitration at the tert-butyl groups of syn- and anti-di-tert-butyl-dimethoxy[n.2]metacyclophanes 1 is attributed to the highly activated character of the aryl ring and the increased stabilization of a σ-complex intermediate arising from the dienone-type σ-complex intermediate possible in the case of an internally methoxy substituent. However, only one tert-butyl group is ipso-nitrated in the mild reaction conditions such as copper(II) nitrate in an acetic anhydride solution because of deactivation of the second aromatic ring by the introduced nitro group. A first σ-complex intermediate would be stabilized by two types of the through-space electronic interaction, i.e., face-to-face overlapping and intra-annular interaction with the opposing benzene ring, thus accelerating the reaction. The higher yields of mononitrated product are obtained in syn-conformer than in anti-conformer. Therefore, face-to-face overlapping might be much more favourable to stabilize the first σ-complex intermediate than the intra-annular interaction. On the other hand, nitration of 1 with fuming nitric acid afforded the two-fold ipso-nitration product 3 in quantitative yield. Thus, the number of ipso-nitration at the tert-butyl groups of 1 was strongly affected by the reactivity of the nitration reagent.Key words: cyclophanes, electrophilic aromatic substitutions, ipso-nitration, σ-complex intermediates, through-space electronic interactions.
2H NMR studies of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine-cholesterol bilayers at high pressure
Authors: Bonev Boyan B , Morrow Michael R
Published: April 2011

Wideline deuterium nuclear magnetic resonance was used to investigate the phase behaviour of bi layer dispersions of chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62) and DPPC-d62 containing 28.5 mol% cholesterol for temperatures between 0°C and 90°C and pressures from ambient to 270 MPa. Spectra were obtained over a range of temperature at ambient pressure, 100 MPa, and 210 MPa and over a range of pressure at fixed temperatures of 0°C and 45°C (40°C for the sample containing cholesterol). The combination of complementary isobaric and isothermal scans produced observations that suggest two possible refinements to previously reported DPPC-d62 pressure-temperature phase diagrams. In contrast to DPPC-d62, which displays at least three transitions as temperature is lowered from 90°C to 0°C at a fixed pressure of 210 MPa, the sample containing 28.5 mol% cholesterol displayed a continuous change from fluid to ordered over a wide temperature range for 210 MPa and below. These observations suggest that pressure does not induce any substantial repositioning of cholesterol in the bi layer over the range of conditions studied and that cholesterol reduces the sensitivity of bilayer properties to pressure.Key words: deuterium nuclear magnetic resonance, model membrane, high pressure, cholesterol, phase transition.
Synthesis and characterization of polyethylene glycol polyacrylamide copolymer (PEGA) resins containing carbohydrate ligands. Evaluation as supports for affinity chromatography
Authors: Auzanneau France-Isabelle , Christensen Mette Knak , Harris Shannon L , Meldal Morten , Pinto B Mario
Published: April 2011

The PEGA resin, a beaded polyethylene glycol dimethylacrylamide copolymer, was evaluated as an affinity support for the purification of carbohydrate-binding macromolecules, namely, the cation-independent mannosyl phosphate receptor (CI-MPR) and a polyclonal antibody directed against a Streptococcus Group A oligosaccharide. Two polyethylene glycol (PEG) derivatives, a di-acryloylated PEG1900derivative or a longer di-acryloylated PEG4000derivative, were used as cross-linkers. The longer cross-linker was synthesized in four steps from polyethylene glycol 4000. The mannosyl 6-phosphate (M6P)-containing immunoaffinity columns were prepared through the inverse suspension radical copolymerization of the corresponding allyl glycoside with acrylamide and the PEG cross-linker. The resin with the shorter cross-linker (PEG1900derivative) had a 6.3% molar cross-linking while that with the longer cross-linker (PEG4000derivative) had a 3.8% molar cross-linking. For the Streptococcus Group A trisaccharide-containing immunoaffinity columns, three PEGA affinity supports bearing free amino groups were prepared and subsequently substituted with a trisaccharide activated as its squarate adduct. While one resin contained the shorter cross-linker PEG1900and had a 3% molar cross-linking, the other two resins contained the longer cross-linker PEG4000 with a molar cross-linking of 5% and 3%, respectively. In affinity chromatographic studies, the M6P-containing columns were ineffective in retaining the cation-independent mannosyl phosphate receptor (CI-MPR, ~ 215kDa), whereas antibody (~ 150kDa) retention was observed with two of the three Streptococcus Group A trisaccharide-containing immunoaffinity columns. Key words: PEGA resins, immunoaffinity supports, carbohydrate ligands, antibody purification.
vic-Diacrylic ester porphyrins as starting materials for monobenzoporphyrin and opp-dibenzoporphyrin syntheses
Authors: Boudif Arezki , Gimenez Sandrine , Loock Bernard , Momenteau Michel
Published: April 2011

Synthesis of a bis(vic-diacrylic ester) porphyrin using the "3+1" methodology is described. A two-step procedure is used to convert the vic-diacrylic ester porphyrin and the bis(vic-diacrylic ester) porphyrin to the monobenzoporphyrin and the opp-dibenzoporphyrin, respectively. Characterization of the compounds by 1H NMR and UV-visible spectroscopies is also discussed.Key words: tripyrrane, diformylpyrrole, vic-diacrylic ester porphyrin, "3+1" synthesis methodology, monobenzoporphyrin, opp-dibenzoporphyrin, photosensitizer.
Article
Authors: Heinrich B , Kowalewski M , Cochran J F
Published: April 2011

The interface magnetic energy in fcc Cu/Co/Cu(001) and Pd/Co/Pd(001) and the interlayer exchange coupling in bcc Fe/Cu/Fe(001) will be discussed and compared to recent theoretical models. The effect of interface magnetostriction and interface magnetic anisotropy will be described using a model proposed by Victora and MacClaren. The Victora-MacLaren theory of magnetic anisotropies is based on a generalized Néel pair interaction model that allows one to express the total interface energy in terms of the interface anisotropy and interface magnetoelastic contributions. It will be argued that the change in the sign of the interface anisotropy Ks as a result of replacing Cu atoms by Pd atoms is caused primarily by the enhanced role of the spin-orbit interaction in the Co-Pd pair interaction and in the tendency of Pd to acquire a significant magnetic moment in the proximity of ferromagnetic atoms. It will be shown that the interface lattice separations play an important role and the balance between these two magnetoelastic terms determines the total magnetoelastic contribution to the interface energy. The magnetoelastic interface contributions to Ks are +0.08 ergs/cm2 and +0.17 ergs/cm2 for the fcc Cu/Co/Cu(001) and Pd/Co/Pd(001) structures, respectively. The interlayer exchange coupling has been investigated in bcc trilayer structures Fe/Cu/XcCu1 - c/Cu/Fe where XcCu1 - c represents an alloyed atomic layer with X = Fe, Cr, Ag. The purpose of these studies was to determine the effect of foreign atoms inside a nonmagnetic spacer on the direct interlayer exchange coupling between the outside Fe layers mediated by the valence electrons of the Cu spacer. The experimental results will be compared with recent first principles calculations using the Layer Korringa-Kohn-Rostoker (LKKR) method. It will be shown that the local electron potential of the foreign atoms affects the spin transport across the spacer and is chiefly responsible for the observed behavior of the interlayer exchange coupling in Fe/Cu/XcCu1 - c/Cu/Fe. The interlayer e cxchange coupling depends very strongly on the magnetic state of the foreign atoms within the spacer. For foreign atoms with no long-range ferromagnetic order the exchange coupling is mainly affected by the 3d valence band electrons. Foreign atoms that are isoelectronic with the spacer atoms modify the strength of the exchange coupling only due to lattice relaxations that are associated with their atomic size.Key words: ultrathin metallic magnetic films, magnetic anisotropy, exchange interaction, Molecular Beam Epitaxy.
The electrophilic substitution reaction of the dithionitronium cation [SNS]+ with benzene
Authors: Brownridge Scott , Passmore Jack , Sun Xiaoping
Published: April 2011

The compound [SNS]+([SNS][AsF6]) reacts with benzene in liquid sulfur dioxide to give orange-, blue-, and then purple-colored solutions. The assignment of the orange color to a molecule-ion charge-transfer complex [C6H6·SNS]+ is supported by the linear dependence of the ionization potential of the arenes (C6H6, C6HMe5, C6H5But, C6HMe5) and the energy of the charge-transfer absorption of freshly prepared arene-[SNS][AsF6] mixtures in liquid SO2 solution. Variable-temperature multinuclear NMR studies of the reactions of [SNS][AsF6] and [SNS][Sb2F11] with benzene are consistent with the blue color being due to a sulfur protonated substitution product [C6H5(S2N)H]+, providing the first example of a CH electrophilic substitution reaction of SNS+. The geometries calculated at the RHF/6-31G' level for [C6H5(SNS)H]+, the isomeric [C6H5NSSH]+, and [C6H5N(S)SH]+, together with NMR data, support [C6H5(SNS)H]+(i.e., suggest S, not N, is attached to the ring) as the structure of the cation. The electrophilic aromatic substitution reaction of [SNS]+ and benzene is also supported by NMR studies of [SNS][AsF6] and other arenes (e.g., C6HMe5) in SO2 solution. The UV-visible spectrum of [SNS]+ ([SNS][AsF6]) in liquid SO2 is reported, and the absorption ( lamda = 406 nm, epsilon = 80) responsible for the yellow color is assigned to the [SNS]+ HOMO-LUMO transition. Evidence is also presented for the formation of a molecule-ion charge-transfer complex between 5-methyl-1,3,2,4-dithiadiazolium and hexamethylbenzene in liquid SO2, the first dithiadiazolium charge-transfer complex.Key words: UV-visible, charge transfer, dithionitronium, benzene, electrophilic substitution.
Method for the differentiation of leaf litter extracts and study of their interaction with Cu(II) by molecular fluorescence
Authors: Esteves da Silva Joaquim CG , Machado Adélio ASC , Ferreira Miguel A , Rey Francisco
Published: April 2011

Six leaf litter extracts (LLE) (eucalyptus (Eucaliptus globulus), fern (Pteridium aquilinium), oak (Quercus robur), chestnut (Castanea sátiva), laurel (Laurus nobilis), and ulex (Ulex europoeus) canopies) were isolated following an extraction procedure similar to that used for fulvic acids (FA) and were characterized by elemental analysis and UV-Vis, FT-IR, and synchronous molecular fluorescence (SyF) spectroscopies. Moreover, information about their interaction with the Cu(II) ion in aqueous solution (100 mg yL of LLE in 0.1 M KNO3 at pH = 6) was obtained from the measurement of SyF spectra at increasing concentrations of Cu(II). These spectral sets were treated by a self-modeling mixture analysis method (SIMPLISMA) to obtain improved quenching profiles to be used in the estimation by the method of Ryan and Weber of the conditional stability constants (Kc), concentration of binding sites, and percentage of fluorescent binding sites accessible for complexation. For comparison purposes, two samples of FA extracted from two horizons of an oak forest soil (0-5 cm and 5-15 cm) were also studied. The spectroscopic data obtained for LLE and FA were different. The results suggest that LLE are characterized by relatively high concentrations of individualized simple molecules and include reactive structures (alkene and protein residues). The ratio of aliphatic yaromatic structures was higher in LLE than for FA. Both LLE and soil FA form stable complexes with Cu(II), but the logarithm of the conditional stability constant of the 1:1 complexes is larger for LLE (about 5) than for the soil FA (about 4.5).Key words: leaf litter extracts, soil fulvic acids, synchronous fluorescence, Cu(II) complexation, SIMPLISMA, spectral pattern recognition.
Observations on the stereochemistry of reduction of 2,6-dimethylcyclohexanones
Authors: Goodwin Thomas E , Meacham Jennifer M , Smith Mark E
Published: April 2011

The reduction of cis-2,6-dimethylcyclohexanone with NaBH4 in methanol is shown to produce predominantly the axial alcohol, an unexpected result based upon prior reports and current paradigms for similar cyclohexanone reductions. This finding prompted a careful and systematic investigation of the NaBH4 and LiAlH4 reductions of cis- and trans-2,6-dimethylcyclohexanones in various solvents, with additional results contrary to literature reports. Possible explanations for these discrepancies are given, an unusual solvent effect is noted, the rate of epimerization versus reduction is examined, molecular modeling results are reported, and an important caveat is offered for future stereochemical studies of this nature.Key words: cyclic ketone reduction, stereochemistry, molecular modeling.
Article
Authors: Pizzocaro Christine , Lessard Roger A , Bolte Michèle
Published: April 2011

Dichromated poly(acrylic acid) (DCPAA) has been used as real time holographic recording material. We report here the photochemical behavior of DCPAA when irradiated under conditions representative of those used for hologram recording. Under UV-vis spectroscopy a photoredox process (CrVI –> CrIII) was observed whose efficiency depends on the presence of dimethylformamide (DMF) used as an electron donor. ESR spectroscopy upon irradiation permits detection of the formation of Cr(V) of photochemical origin. We also identified chromium(V) of thermal origin (in the dark), organic macroradicals, and chromium(III). The formation of CO2 and carboxylate ions was clearly shown by IR spectroscopy. The photoreaction is assumed to go through an acido-basic reaction between dichromate in the excited state and poly(acrylic acid). The resulting unstable chromium polyacrylate undergoes a redox process that gives rise to chromium(V) and a macroradical R-COO from which CO2 is formed. The presence of DMF (10% by volume) makes the overall reaction faster by a factor of approximately 3.5-4. The similar shape of the curves giving chromium(V) formation as a function of irradiation time and the curves giving the diffraction efficiencies of the hologram recorded in DCPAA as a function of exposure energy, with and without DMF, strongly argues in favor of a direct involvement of chromium(V) in the quality of the resulting hologram.Key words: holography, dichromated poly(acrylic acid), dimethylformamide, chromium.
Preparation, 1H and 13C NMR, and ab initio/IGLO study of mono-O-protonated deltic acid and theoretical investigation of di-, tri-, and tetra-O-protonated deltic acids
Authors: Prakash GK Surya , Rasul Golam , Olah George A , Liu Ronghua , Tidwell Thomas T
Published: April 2011

The hitherto elusive mono-O-protonated deltic acid C3O3H3+ was prepared by protolysis of di-tert-butoxy deltate in FSO3H-SO2ClF and in FSO3H:SbF5 (Magic Acid; 1:1 molar solution) in SO2ClF as solvent at -78°C and was characterized by 1H and 13C NMR spectroscopy. The structure and NMR chemical shifts were also calculated by the ab initio/IGLO method. No NMR evidence was found for persistent di-O-protonated deltic acid under these conditions, although a limited equilibrium with the mono-O-protonated species can be involved. Di-, tri-, and tetra-O-protonated deltic acids were also studied by ab initio/IGLO method.Key words: protonated deltic acid, aromaticity, superacids, NMR spectroscopy, ab initio and IGLO calculations.
31P chemical shift anisotropies of trimethyl- and triphenylphosphine-substituted Group 6 metal pentacarbonyl complexes
Authors: Wosnick Jordan H , Morin Frederick G , Gilson Denis FR
Published: April 2011

The 31P chemical shift tensor components and anisotropies of the trimethyl- and triphenylphosphine complexes of the group 6 metal pentacarbonyls, M(CO)5PR3 (M = Cr, Mo, W and R = Me, Ph), have been measured using solid-state CP-MAS 31P NMR spectroscopy. For the trimethylphosphine derivatives, the chemical shift tensors have near axial symmetry and the shift tensor components are in reasonable agreement with the calculated values for the chromium and molybdenum complexes. In the triphenylphosphine complexes, the tensors are asymmetric due to the different torsion angles of the phenyl rings. The trend to higher shielding of the isotropic 31P chemical shifts on descending group 6 arises from changes in the perpendicular components of the shift tensor. The one-bond coupling constants, 1J(95/97Mo-31P), for the trimethyl- and triphenylphosphine complexes are 129 and 133 Hz, respectively.Key words: chemical shift anisotropy, phosphines, chromium, molybdenum, tungsten.
Ketone coordination chemistry of the diborane t-BuCH=C[B(C6F5)2]2
Authors: Köhler Katrin , Piers Warren E
Published: April 2011

Reactions of the bifunctional borane t-BuCH=C[B(C6F5)2]2, 1, with simple Lewis bases were investigated to probe its Lewis acidity. Ethereal bases (Et2O, THF) bind very weakly, if at all, as no adducts were detectable in solution. Certain ketones, however, were observed to bind to 1. Acetone forms an adduct, 2, reversibly in solution (Keq at 293 K = 69.1), and the temperature dependence of this equilibrium yield thermodynamic parameters of H° = -59.5 ± 2.0 kJ mol-1; S° = -167 ± 4 J K-1 mol-1. Low temperature NMR spectroscopy suggests that the acetone binds to the more sterically accessible boron center trans to the t-Bu group. Ketone binding is strengthened when 2,3-diphenylcyclopropenone is reacted with 1, irreversibly forming an isolable adduct, 4, in high yield (95%). The availability of a resonance form incorporating the aromatic cyclopropenyl cation contributes to the added stability of this adduct in comparison to 2.Key words: organoboranes, diboranes, ketone complexes.
Dissolution behavior in water of a model hydrophobic alkali-swellable emulsion (HASE) polymer with C20H41 groups
Authors: Horiuchi Kazunaga , Rharbi Yahya , Yekta Ahmad , Winnik Mitchell A , Jenkins Richard D , Bassett David R
Published: April 2011

The dissolution behavior of a hydrophobic alkali-swellable emulsion (HASE) in water was examined by dynamic light scattering and pyrene fluorescence probe experiments. The HASE polymer was obtained by the copolymerization of ethyl acrylate (EA), methacrylic acid (MAA), and a macromonomer containing a C20H41 group separated from the polymer backbone by 32 ethylene oxide units. The behavior of the copolymer was followed as a function of the neutralization degree α = [NaOH]/[COOH] of the MAA component. Particles swell with increasing α until α = 0.4 after which polymer dissolution begins. At low α, pyrene, added as a probe to the solution, dissolves in the EA-MAA copolymer phase; whereas at high α, pyrene is largely localized in C20H41 micelles. Pyrene fluorescence measurements show that the striking viscosity increase of the solution with increasing α correlates with polymer dissolution and intermolecular network formation. Key words: associative polymer, hydrophobic substituents, polymeric micelles.
Article
Authors: Knaul Jonathan Z , Kasaai Mohammad R , Bui V Tam , Creber Katherine AM
Published: April 2011

Starting from a chitosan sample with a degree of deacetylation of 71%, three separate sample sets were generated by successive deacetylation and reacetylation processes. The degree of deacetylation of samples was determined by UV spectrometry supported by thermogravimetric analysis. The molecular weight of chitosan samples was determined in a solvent system of 0.25 M CH3COOH/0.25 M CH3COONa, using viscometry and gel permeation chromatography (GPC) with a TSK-gel column. The first set of samples had a similar degree of deacetylation (DDA) but differing molecular weights. The second set of samples had a similar molecular weight but differing degrees of deacetylation. The Mark-Houwink-Sakurada constants used for the determination of viscosity average molecular weight and the universal calibration of the GPC system were K = 1.40 × 10-4 dL/g and a = 0.83. Results showed that molecular weights determined from both techniques are in good accord only at lower degrees of deacetylation. This may be attributed to the fact that the chemical structure of chitosan samples could have been largely altered with increasing or decreasing degree of deacetylation. Nevertheless, the trend with which the molecular weights vary with the deacetylation time is consistent over a limited DDA range. A literature review of molecular weight analysis of chitosan is included.Key words: chitosan, degree of deacetylation, gel permeation chromatography, molecular weight, viscometry.
Temperature dependence of self-entanglements in protein backones near the glass-like transition
Authors: Arteca Gustavo A , Caughill Denise I
Published: April 2011

Many proteins undergo a phase change whereby catalytic activity is switched on at a glass-transition temperature T* ~ 200 K. Over T*, large anharmonic motions dominate, most particularly on atoms coupled to the hydration layer. These changes are detectable in the positional disorder, e.g., in the Debye-Waller thermal B-factors. Yet, the glass transition is not detected in descriptors of backbone mean shape, e.g., the molecular size. In this work, we show that other descriptors of mean shape may be better at detecting the transition. We suggest that a property that uses the equilibrium geometry and the backbone connectivity can exhibit larger variations with thermal fluctuations. In particular, we contrast the effects of bond length fluctuations on the mean number of overcrossings and the radius of gyration. Using the glass transition of ribonuclease-A as a test example, we show that the change in Debye-Waller factors is also reflected in the mean overcrossing numbers computed with the backbones. The approach can be used to follow other configurational transitions and to assess the effect of various external perturbations.Key words: Molecular shape fluctuations, isotropic temperature factors, anharmonic vibrations, glass phase transition, flexible molecular chains.
Reaction of carbanions generated from arylmethylphosphonates with cyclic enones. Regio- and stereoselectivity of addition
Authors: Modro Agnes M , Modro Tom A , Mphahlele Malose J , Perlikowska Wieslawa , Pienaar André , Sales Marcelo , Rooyen Petrus H
Published: April 2011

α-Lithiated benzylphosphonate esters, XC6H4CH(Li)PO3Et2, add regioselectively to cyclohex-2-enone at the carbonyl group; higher reaction temperatures lead to the reversal to the starting materials, and not to the 1,4-addition. Relative configuration of the C(1) and Cα centers in the 1,2-adducts was determined from the 13C NMR spectra. For 2-methylcyclohex-2-enone the regioselectivity of the addition is decreased. The lithioderivative of diethyl 1-naphthylmethylphosphonate, C10H7CH(Li)PO3Et2, shows reverse regioselectivity yielding the 1,4-, but not the 1,2- addition product. The crystal and molecular structure of that 1,4-adduct has been determined and demonstrates that the conformation about the C(1)-Cα bond is close to that existing in solution, deduced from the value of the 1H NMR 3JHH vicinal coupling constant of the H(1)-C(1)-Cα-H hydrogens. The same conformation was also confirmed as the most stable by molecular modeling. Key words: α-lithiated arylmethylphosphonates, 1,2- vs. 1,4-addition to cyclic enones, steric effects of the cyclohex-2-enone, molecular structure and molecular modelling of the 1,4-adduct of 1-naphthylmethylphosphonate to cyclohex-2-enone.
Article
Authors: Eriksson Leif A , Jansson C Magnus , Arnold Donald R , Boyd Russell J
Published: April 2011

The cleavage reactions of the 1-butene and 4,4-dimethyl-1-pentene molecules and their cations, to form neutral and charged hydrocarbon products, are investigated using hybrid Hartree-Fock/density functional theory. In comparison with previous theoretical results (Du et al.), the density functional cleavage and ionization energies, including zero-point vibrational energy, lie in better agreement with experimental and thermochemical data. Assuming vertical ionization processes the mean absolute deviation (MAD) compared with experiment is 3.4 kcal/mol for the reaction sequences studied. Using adiabatic ionization processes instead gives a MAD of 5.2 kcal/mol. The largest deviation from experiment occurs for the cleavage reactions of the neutral parent molecules, where the difference between theory and experiment is up to 12.8 kcal/mol. In addition to reaction energies we also report optimized ground-state structures, and for the radicals studied, isotropic hyperfine coupling constants that are compared to experimental data. It is found that the experimental hyperfine properties of the 1-butene cation can be explained by rotational averaging caused by the flat potential surface for rotation about the C2-C3 bond.Key words: density functional theory (DFT), alkenes, radical cations, bond cleavage, isotropic hyperfine coupling constants.
Reaction of ketoximes with aliphatic aldehydes and diphenylborinic acid. Crystal and molecular structure of a diphenylboron chelate derived from the pivaldehyde adduct to cyclohexanone oxime
Authors: Kliegel Wolfgang , Riebe Ulf , Rettig Steven J , Trotter James
Published: April 2011

Various ketoximes have been reacted with aliphatic aldehydes in the presence of oxybis(diphenylborane) in order to assess the influence of increasingly bulky residues on the nature of the products. Crystals of 5-tert-butyl-2-cyclohexylidene-3,3-diphenyl-1,4-dioxa-2-azonia-3-boratacyclopentane, 4c, are monoclinic, a = 14.0647(9), b = 11.416(1), c = 14.756(1) Å, β = 115.696(4)°, Z = 4, space group P21/c. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.038 (Rw = 0.038) for 2511 reflections with I >= 3σ(I). A bulky substituent at the aldehyde favors α-hydroxyalkylation at the oxime oxygen atom and subsequently leads to the formation of BOCON diphenylboron chelates, whereas formaldehyde reacts by N-alkylation of the oximes, resulting in COBON diphenylboron chelates.Key words: ketoximes, organoboron compounds, crystal structure.
The energy barrier for CO migration in Mn2(CO)6(H2PCH2PH2)2: a PM3(tm) study
Authors: Decker Stephen A , Klobukowski Mariusz
Published: April 2011

Carbonyl fluxionality in Mn2(CO)6(dhpm)2 (dhpm = bis(dihydrophosphino)-methane), serving as a model of Mn2(CO)6(dppm)2 (dppm = bis(diphenylphosphino)-methane), was studied using both the PM3(tm) semiempirical method and the DFT//PM3(tm) approach. When the PM3(tm) method is used for both geometry optimization and energy determination, the most stable conformer of Mn2(CO)6(dppm)2 is predicted to possess six planar carbonyls, two of which are symmetrically bridging the Mn atoms, in contrast to the usually assumed planar all-terminal structure of Mn2(CO)6(dppm)2. Carbonyl scrambling via the pairwise exchange mechanism in Mn2(CO)6(dppm)2 is predicted to occur as follows at the PM3(tm)//PM3(tm) level: lowest energy, planar symmetrically di-bridged conformer -–> planar, all-terminal conformer -–> lowest energy, planar symmetrically di-bridged conformer. The corresponding barrier to the exchange process is 32 kcal/mol, much higher than the experimental value of 10 kcal/mol in the related Mn2(CO)6(dppm)2 complex. Calculation of the total energies using density functional theory at the PM3-optimized geometries improves results in two ways. Firstly, the energy barrier to scrambling is reduced to only 15-19 kcal/mol depending on the functional employed. Secondly, the energy ordering of the conformers is reversed, with the planar, all-terminal structure of Mn2(CO)6(dppm)2 being more stable than the bridging structure. The resulting mechanism is identical to that found in our earlier work on Mn2(CO)10. When DFT is used to optimize the conformer geometries, the energy barrier is further reduced to 13 kcal/mol.Key words: PM3(tm) semiempirical method, density functional theory, carbonyl scrambling.
Réactions daddition conjuguée dorganolithiens aux acides carboxyliquesα, β-insaturés
Authors: Mortier Jacques , Vaultier Michel , Plunian Barbara , Sinbandhit Sourisak
Published: April 2011

In THF at -78°C, organolithium derivatives and unprotected α, β-unsaturated carboxylic acids give a conjugated addition reaction. Reactions of 1- and 2-naphthoic acids with various organolithium derivatives and electrophiles are reported. Two X-ray studies show that the reaction occurs with a good diastereoselectivity: the dilithiated enolate formed as an intermediate adds the electrophile exclusively via the face opposite to that of the organolithium. A study of the 1 H NMR spectra of the products obtained shows that these compounds adopt a preferred conformation in solution. The protonolysis of the intermediate enolate by trifluoroacetic acid in an inverted mode gives rise to the formation of 1,2-disubstituted dihydronaphthalenes. With acyclic α, β-unsaturated carboxylic acids, the diastereoselectivity of the reaction is marginal. With secondary butyllithium, the protonolysis of the intermediate enolates gives a 1,4-addition and an average diastereoselectivity. Reaction of phenylpropiolic acid with butyllithium gives rise to the formation of a product resulting from a trans addition to the triple bond; the organolithium derivative adds in α to the carboxylic group.Key words: 1,4-addition reactions, carboxylic acids, organolithium derivatives.[Journal Translation]
A versatile synthetic route to 1,5-dithiocins from o-mercapto aromatic aldehydes
Authors: Still Ian WJ , Natividad-Preyra Rosanne , Toste F Dean
Published: April 2011

An earlier procedure for the facile preparation of benzo-fused 1,5-dithiocins 2a-2c from o-mercapto- benzaldehydes has been improved and shown to be capable of extension to the preparation of several naphthalene-derived analogues. The general method also afforded several N-alkylated 1,5-dithiocins 4, 5 by replacing NH3 with the appropriate primary amine. It was found that N-acylation of the 1,5-dithiocins was successful only with methyl chloroformate. Attempted N-phenylation met with limited success but was shown to be unnecessary since even the less reactive aniline readily undergoes the general reaction of primary amines. When simple α-amino acids, or their methyl esters, were employed as the primary amine in the reaction with o-mercaptobenzaldehyde, the formation of the N-alkylated 1,5-dithiocins 4a, 17a,17b with accompanying loss of -COOH or -COOMe was observed, in preparatively useful yields. A mechanism is proposed for this interesting transformation.Key words: 1,5-dithiocins, α-amino acids, N-acylation, decarboxylation.
Porphyrins incorporating heterocyclic N-oxides: (oxidopyridyl)porphyrins, porphyrin-N-oxides, and a tirapazamine-porphyrin conjugate
Authors: Posakony Jeffrey J , Pratt Russell C , Rettig Steven J , James Brian R , Skov Kirsten A
Published: April 2011

Porphyrins containing one to four 4-pyridyl groups as meso-substituents were synthesized via a mixed aldehyde condensation, and then "N-oxidized" with m-chloroperbenzoic acid to produce five novel (oxidopyridyl) porphyrins and seven porphyrin-N-oxides, which were characterized by analysis and spectroscopic methods, especially NMR; an X-ray crystal structure of 5-(1-oxido-4-pyridyl)-10,15,20-triphenylporphyrin was also obtained. Crystals of (oxidopyridyl)triphenylporphyrin are tetragonal, a = b = 15.174(1), c = 13.709(1) Å, Z = 4, space group I2d. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.031 (Rw = 0.026) for 685 reflections with I [Formula: see text] 3σ(I). Sulfonation of two of the (oxidopyridyl)porphyrins was achieved readily with no loss of oxygen from the oxidopyridyl groups. Tirapazamine (3-amino-1,2,4-benzotriazine-1,4-di-N-oxide) was treated with triphosgene to yield the previously reported 2H-[1,2,4]oxadiazolo[3,2-c][1,2,4]benzotriazin-2-one-5-oxide (1); this reacts like an isocyanate and with 5-(4-aminophenyl)-10,15,20-triphenylporphyrin yields a tirapazamine-porphyrin conjugate (2).Key words: porphyrin-N-oxides, (oxidopyridyl)porphyrins, tirapazamine.
A capillary electrophoretic determination of the kinetics of tetradecanedisulfate hydrolysis
Authors: Walker Richard A , Marangoni D Gerrard , Smith-Palmer Truis
Published: April 2011

Capillary electrophoresis was used to follow the decrease in concentration of tetradecanedisulfate as a function of time during its hydrolysis in 0.5 M sulfuric acid solution at 60, 75, and 90°C. The hydrolyses were mostly carried out at premicellar concentrations of surfactant, from 1 to 7.5 mM. Large increases of rate occurred at higher concentrations (at least 15 mM). The reaction was found to be first order in tetradecanedisulfate. The activation energy and entropy of activation were calculated to be 97 kJ mol-1 and -55 J K-1 mol-1, respectively.Key words: capillary electrophoresis, surfactant, acid hydrolysis, tetradecanedisulfate, activation energy, entropy of activation.
Theoretical studies of heterofullerenes C68X2 (X = N, B)
Authors: Chen Zhongfang , Ma Keqin , Pan Yinming , Zhao Xuezhuang , Tang Auchin
Published: April 2011

A systematic investigation on the molecular structures of the possible energetically favored isomers of C68X2 (X = N, B) has been performed using the semiempirical MNDO, AM1, and PM3 methods. The most stable isomer of C68X2 was computed, and the static properties such as heats of atomization, ionization potentials, electron affinities, absolute electronegativities, and global hardness have been predicted theoretically. The proposition that the changes of hybridization from sp2 to sp3 are the predominant factors of the regiochemistry for both C60X2 and C58X2 are supported by the calculations, and the regioselectivity of the doped fullerenes may be deduced in light of the knowledge of the fullerene adducts based on this proposition.Key words: C68N2, C68B2, MNDO, AM1, PM3.
One-dimensional ribbons of solvated Ag sulfonates
Authors: Shimizu George KH , Enright Gary D , Rego Gabriela S , Ripmeester John A
Published: April 2011

Single crystal X-ray structure determinations of two solvated silver sulfonates have been obtained and these compounds have been shown to adopt infinite one-dimensional motifs. {AgOTs(MeCN)}[Formula: see text] (OTs = p-toluenesulfonate) crystallizes in the monoclinic space group, P21, a = 8.4278(5) Å, b = 5.7413(3) Å, c = 12.1057(7) Å, β = 109.24(1)°. {Ag(NDSA)(MeCN)2(H3O)(H2O)2}[Formula: see text] (NDSA = 1,5-naphthalenedisulfonate) crystallizes in the triclinic space group, P[Formula: see text], a = 8.3407(4) Å, b = 10.4374(5) Å, c = 12.3399(6) Å, α = 101.941(8)°, β = 109.24(1)°, γ = 102.190(8)°. Despite one compound containing a monosulfonate and the other a disulfonate, both complexes form infinite one-dimensional arrays.Key words: silver, sulfonates, coordination polymer.
Synthesis and kinetics of decomposition of some novel S-nitrosothiols
Authors: Munro Andrew P , Williams D Lyn H.
Published: April 2011

TheS-nitrosothiols 2-acetamido-2-deoxy-S-nitroso-1-thio-β-D-glucopyranose 3,4,6-triacetate (GPSNO) and S-nitroso-N-carbamyl-D,L-penicillamine (SNCP) were synthesized by S-nitrosation of the corresponding thiols, isolated, and fully characterized. The nitrosothiol (TGSNO) from 1-thioglycerol was obtained as a red gelatinous liquid, which decomposed rapidly at room temperature and so was not characterized. The kinetics of decomposition of GPSNO showed that there is a surprisingly large thermal pathway overlaid with a Cu2+/RS- catalyzed reaction. The results strongly suggest that the product disulfide complexes Cu2+ (for which there is some spectral evidence), leading to incomplete conversion by that route. Ascorbate also acts as a Cu2+ reductant. Another S-nitroso sugar, S-nitroso-1-thio-β-D-glucose (SNTG), behaved very similarly from solutions generated and used in situ. The decomposition of TGSNO shows induction periods suggesting that slow initial generation of Cu+ (the true catalyst) is taking place. There appears to be also a significant alternative pathway (analogous to that found for GPSNO), where the rate appears to be independent of [Cu2+], but very unusually this pathway is effectively halted by addition of EDTA either at the start of the reaction or at a later time. Reaction schemes are put forward to account for these unusual reaction characteristics.Key words: S-nitrosothiols, nitric oxide, ascorbate, copper catalysis.
Intermolecular proton transfer in catalysis by carbonic anhydrase V
Authors: Earnhardt J Nicole , Tu Chingkuang , Silverman David N
Published: April 2011

The dehydration of bicarbonate catalyzed by carbonic anhydrase is accompanied by the transfer of a proton from solution to the zinc-bound hydroxide. We have investigated the properties of proton transfer from donors in solution, mostly derivatives of imidazole and pyridine, to a truncated mutant of carbonic anhydrase V with replacements that render the active site cavity less sterically constrained, Tyr 64 →> Ala and Phe 65 →> Ala. Catalysis was measured by determining the rate of exchange of 18O between the CO2-HCO3- system and water, and rate constants for proton transfer were estimated as the rate-limiting step in the release of H218O from the enzyme to solution. Each proton donor enhanced catalytic activity in a saturable manner. The resulting rate constants for proton transfer when compared with the values of pKa of the donor and acceptor gave a Brønsted plot of high curvature. These data could also be described by Marcus theory which showed an intrinsic barrier for intermolecular proton transfer near 0.8 kcal/mol and a work term or thermodynamic contribution to the free energy of reaction near 10 kcal/mol. This low intrinsic kinetic barrier for proton transfer is very similar to nonenzymic bimolecular proton transfer between nitrogen and oxygen acids and bases in solution. However, the significant thermodynamic contribution suggests appreciable involvement of solvent and active-site organization prior to proton transfer. These Marcus parameters are very similar to those describing intramolecular proton transfer from His 64 in carbonic anhydrase, suggesting similarities in the intra- and intermolecular proton transfer processes.Key words: carbonic anhydrase, proton transfer, Marcus theory, carbon dioxide.
Ring opening of cyclobutene-1,2-dione and 3-trimethylsilylcyclobutene-1,2-dione. Dimerization/decarbonylation of a bisketene to a ketenylbutenolide
Authors: Allen Annette D , Tidwell Thomas T
Published: April 2011

Hydrolysis of 3-trimethylsilyl-4,4-dichlorocyclobut-2-ene-1-one (4) in H2SO4 gave small amounts of cyclobutene-1,2-dione (2a) and 3-trimethylsilylcyclobutene-1,2-dione (2d). Photolysis of 2a gave evidence for the formation of the bisketene (CH=C=O2) (1a) in solution for the first time, as identified by the IR band at 2120 cm-1 and the 1H NMR signal at δ 3.20. Thermolysis of 2d gave evidence for the intermediacy of O=C=CHC(SiMe3)=C=O (1d), with an 1H NMR signal at δ 3.43, and the isolation of the unusual ketenyl lactone 6, which could arise by formation of an unsymmetrical [2+2] ketene dimer from 1d, followed by rearrangement and decarbonylation.Key words: bisketenes, cyclobutenediones, ketene dimerization.
Structure-reactivity correlations in the reaction of 2,4-dinitrophenyl X-substituted benzoates with alicyclic secondary amines
Authors: Um Ik-Hwan , Min Ji-Sook , Lee Hye-Won
Published: April 2011

Apparent second-order rate constants (kapp) have been measured spectrophotometrically for the reaction of 2,4-dinitrophenyl X-substituted benzoates with a series of alicyclic secondary amines in H2O containing 20 mol% DMSO at 25°C. The microconstants involved in the reaction (k-1/k2, k1, and k1k2/k-1) have also been calculated. The magnitude of kapp, k1, and k1k2/k-1 values increases with increasing amine basicity and with increasing acid strengthening ability of the acyl substituent X. The k-1/k2 value decreases from ca. 6.5 to 0.3 with increasing the amine basicity, but remains almost constant upon changing the acyl substituent X for a given amine, indicating that the rate-determining step is governed by the basicity of amine but not by the electronic nature of the acyl substituent X. The Brønsted-type plots for kapp show a break at pKa = 9.1, supporting the assumption that a change in the rate-determining step occurs from rate-limiting breakdown to formation of the addition intermediate as amine basicity increases. The corresponding Brønsted-type plots for k-1/k2, k1, and k1k2/k-1 are linear but their β values are different. σ+ constants show better correlation with log kapp, log k1 and log k1k2/k-1 for the reaction with low basic amines (pKa < 9.1), while σ constants exhibit better correlation for the reaction with highly basic amines (pKa > 9.1). The magnitude of ρ1 is identical to that of ρapp and ρeq for a given amine.Key words: aminolysis, Brønsted-type plot, structure-reactivity correlations, rate-determining step.
Identity-reaction proton transfers from oxygen acids yielding localized vs. delocalized conjugate bases. An ab initio study
Authors: Van Verth James E , Saunders, Jr. William H
Published: April 2011

Identity-reaction proton transfers from a series of oxygen acids to the corresponding conjugate bases have been studied by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The acids are H3O+, CH3OH2+, CH2 = OH+, HC(O)OH2+, CH2 = CHOH2+, H2O, CH3OH, HOOH, HOCH2OH, FOH, FCH2OH, HC(O)OH, and CH2 = CHOH. Gas-phase acidities were calculated at the G2(MP2) level in order to have benchmark values for all acidities regardless of whether experimental values were available. Barriers to proton transfer relative to the separated reactants, ΔHTS, show a straight-line relation to acidity for all but two of the neutral acids and for all but one of the cationic acids. Two neutral acids, HOOH and FOH, show negative deviations that can be attributed to polarizability of the atoms attached to the proton donor oxygens. The cationic acid HC(O)OH2+ shows a large positive deviation, which probably arises from substantial heavy-atom reorganization from reactant to TS. Charges provide evidence of a lag in delocalization in the reaction of CH2 = CHOH2+, though it does not show an elevated ΔHTS.Key words: ab initio, oxygen acids, proton transfer, acidity.