bluuebunny/crossref_metadata_2025 huggingface dataset:
On N-acetylcysteine. Part I. Experimental and theoretical approaches of the N-acetylcysteine/H2O2complexation
Authors: Arroub Jlil , Bergès Jacqueline , Abedinzadeh Zohreh , Langlet Jacqueline , Gardès-Albert Monique
Published: May 2006

The complexation of N-acetylcysteine (RSH) with hydrogen peroxide has been studied experimentally and theoretically. Experimentally we have measured the evolution of RSH, H2O2, and RSSR (N-acetylcystine) as a function of time. Surprisingly, H2O2 decays by a biphasic process, which is not the case for RSH and RSSR. In the first stage of the kinetics, H2O2 disappears without oxidizing the thiol function of RSH. By analogy with glutathione (GSH), the formation of a complex between RSH and H2O2 has been proposed. The thermodynamic equilibrium constant of complex formation has been determined. Theoretical calculations were performed within the SIBFA method to pinpoint the sites of complexation in isolated and hydrated states. A mixed "discrete–continuum" model was used to evaluate the solvent effect. The two stable complexes found in isolated state have different behaviour under the influence of the solvent. Comparison with complexed GSH is discussed.
Spin diffusion in a triblock thermoplastic elastomer
Authors: Cho Gyunggoo , Natansohn Almeria
Published: May 2006

A triblock copolymer thermoplastic elastomer (SEBS) consisting of an ethylene–butylene random-copolymer rubbery domain flanked by rigid polystyrene short blocks has been analyzed using novel solid-state NMR techniques.13C NMR relaxation measurements suggest that at the T1(H) level (minimum observable phase size of about 18 nm) the material appears homogeneous, but at the T1ρ(H) level (minimum observable phase size of about 3 nm) the expected two distinct phases are noted. Two-dimensional WISE spectra (high-resolution 13C – broadline 1H) show the different mobility in the two phases. A broad component associated with the highly mobile rubbery phase indicates the presence of an interface. The mobile phase is selected using a dipolar filter pulse sequence and the magnetization is allowed to transfer to the rigid polystyrene phase during a period called "mixing time." From the growth of the polystyrene magnetization during the mixing period, the size of the dispersed polystyrene phase can be calculated to be 16 ± 2 nm.
Preparation and characterization of azaindolyl-azaindole and structure of its halogen-free dicationic cluster containing the µ4-oxotetracopper(II) core
Authors: Poitras Jacques , Beauchamp André L.
Published: May 2006

Refluxing NbCl5 and excess 7-azaindole (Haza) in benzene yielded a solid mixture containing NbCl5(Haza), NbCl4(Haza)2, the azaindolium ion (H2aza)+, and the azaindolyl-azaindolium ion (H2aza-aza)+. The neutral (Haza-aza) molecule was obtained from the hydrolysed mixture and shown by X-ray diffraction (monoclinic, P21/c, a = 10.025, b = 13.758, c = 8.416 Å, β = 102.89°, Z = 4, R = 0.035) to result from the coupling of two azaindole units via N7—C6′. This compound was the only oxidation product detected, but concurrent formation of other niobium- and (or) azaindole-containing products keeps the yield of Haza-aza low. Dark green crystals of [Cu4O(aza-aza)4]Cl2•6.5H2O were obtained from (Haza-aza) and CuCl2 in wet methanol. The crystal structure (monoclinic, C2/c, a = 17.704, b = 25.240, c = 14.457 Å, β = 116.14°, Z = 4, R = 0.051) shows the presence of a tetranuclear dicationic cluster consisting of a µ4-oxide ion surrounded by a tetrahedron of Cu(II) atoms. A distorted square-planar coordination is achieved about each copper atom by (aza-aza)− ligands each bridging two copper atoms and providing a third nitrogen donor to one of them. For each such cation, the unit cell also includes two chloride ions and 6.5 disordered lattice water molecules. The 1H NMR and infrared spectroscopy data are discussed.
A detailed shape characterization of regular polypeptide conformations
Authors: Arteca Gustavo A.
Published: May 2006

Regular polypeptide conformations include secondary structural motifs such as α-helices and β-strands. The occurrence of some regular conformation is usually deduced from a local analysis of dihedral angles. However, the value of a dihedral angle in itself does not provide any information on the conformation's "shape." This drawback can be circumvented with global, rather than local, macromolecular shape descriptors. Recently, fractal exponents have been proposed as a source of such descriptors. Yet, this approach does not fully capture all essential shape features, since protein backbones are not fractal. In this work, we deal instead with a more "natural" characterization of the polymer's global shape that uses both the chain's geometry and "topology." For the geometry, we study the behaviour of molecular size and anisometry. For the chain's folding features, we study the self-entanglements in a polymer fold. We compute these descriptors for all relevant secondary structural motifs. By using self-entaglements and molecular geometry, we provide a view of secondary structure that is both conceptually appealing and also more discriminating than previous ones in the literature. Keywords: molecular shape analysis, protein secondary structure, self-entanglements.
Preparation of diversely protected 2-azido-2-deoxyglycopyranoses from glycals
Authors: Czernecki Stanislas , Ayadi Ebtissam
Published: May 2006

A new and efficient preparation of diversely protected 2-azido-2-deoxyglycopyranosides from the corresponding glycals is described. The glycals are first transformed into protected phenyl 2-azido-2-deoxy-selenoglycopyranosides by azido-phenylselenylation. Two procedures were employed according to the protecting groups present: sodium azide and diphenyldiselenide in the presence of (diacetoxyiodo)benzene for peracetylated glycals (Procedure A) or trimethylsilyl azide and tetra-n-butylammonium fluoride in the presence of N-phenylselenophthalimide for perbenzylated glycals (Procedure B). A gluco–manno mixture (90%) is obtained from protected d-glucal whereas only the galacto isomer is formed from protected d-galactal (75%). The compatibility of the second procedure with one free hydroxyl group and a variety of protecting groups was verified with 1,5-anhydro-2-deoxy-3,4-O-isopropylidene-d-lyxo-hex-1-enitol and its 6-O-acetyl, 6-O-allyl, 6-O-benzyl, and 6-O-tert-butyldimethylsilyl derivatives as well as with 1,5-anhydro-4,6-O-benzylidene-2-deoxy-d-lyxo-hex-1-enitol and its 3-O-acetyl and 3-O-benzyl derivatives, which were transformed into phenyl 2-azido-2-deoxy-α-d-selenogalactopyranoside derivatives in good yield. In the second step, hydrolysis of these selenoglycosides afforded diversely protected glycopyranoses in high yield. Peracetylated derivatives were hydrolyzed in the presence of N-iodosuccinimide, whereas mercury trifluoroacetate was employed for 3,4-O-isopropylidene, 4,6-O-benzylidene, and perbenzylated derivatives. In some cases the two steps can be carried out without isolation of the intermediate selenoglycoside. Keywords: glycals, 2-azido-2-deoxygalactopyranose, 2-azido-2-deoxyglucopyranose, selenoglycosides.
Manifestations of noncovalent bonding in the solid state. 6. [H4(cyclam)]4+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) as a template for crystal engineering of network hydrogen-bonded solids
Authors: Subramanian S. , Zaworotko Michael J.
Published: May 2006

A series of salts of composition [H4(cyclam)]X•nH2O, where cyclam = 1,4,8,11-tetraazacyclotetradecane and X = 4Cl− (1−, n = 4), 4Br− (2, n = 4), 4I− (3, n = x), 4ClO4−(4, n = 2), 4CNS− (5, n = 2), 2SO42−(6, n = 6), and (p-CH3C6H4SO3−) (7, n = 0) have been prepared and crystal structures of 1, 2, 4, 5, 6, and 7 have been determined by single crystal X-ray crystallography. 1, 2, 3, 4, and 7 were prepared by crystallizing cyclam with a large excess of the corresponding acid whereas the thiocyanate, 5, and the sulphate, 6, salts were prepared by metathesis of the perchlorate salt, 4, with KCNS and K2SO4, respectively. Crystal data: 1: orthorhombic, Pnnb, a = 7.7962(16), b = 14.278(4), c = 17.787(4) Å, V = 1979.9(8) Å3, Z = 4, Dc = 1.39 Mg m−3, Rf = 0.051, and Rw = 0.069 for 1176 reflections with I > 3σ(I). 2: triclinic. [Formula: see text]a = 7.6860(8), b = 7.8848(9), c = 9.6385(11) Å, α = 105.111(16)°, β = 95.495(18)°, γ = 102,464(17)°, V = 543.43(12) Å3, Z = 1, Dc = 1 1.82 Mg m−3,Rf = 0.034, and Rw = 0.041 for 1530 reflections with I > 3σ(I). 4: monoclinic, P21/n, a = 8.4519(11), b = 15.9248(12), c = 9.2981(17) Å, β = 106.243(18)°, V = 1201.5(3) Å3, Z = 2, Dc, = 1.76 Mg m−3, Rf = 0.052, and Rw = 0.068 for 1902 reflections with I > 3.0σ(I). 5: triclinic, [Formula: see text] a = 8.2996(7), b = 9.0061(15), c = 9.2015(15) Å, α = 107.60(3)°, β = 97.738(14)°, γ = 108.863(9)°, V = 599.44(15) Å3, Z = 1, Dc = 1.309 Mg m−3Rf = 0.038 and Rw = 0.049 for 1856 reflections with I > 4σ(I). 6: triclinic, [Formula: see text] a = 7.8687(5), b = 8.8396(12), c = 9.0405(25) Å, α = 68.547(16), β = 85.332(9), γ = 78.473(7)°, V = 573.43(18) Å3, Z = 1, Dc = 1.693 Mg m−3, Rf = 0.053 and Rw = 0.076 for 1441 reflections with I > 3σ(I). 7: triclinic, [Formula: see text] a = 8.533(3), b = 9.110(3), c = 15.3605(21) Å, α = 82.475(17)°, β = 74.307(20)°, γ = 73.26(3)°, V = 1099.0(5) Å3, Z = 1, Dc = 1.343 Mg m−3, Rf = 0.042 and Rw = 0.052 for 2460 reflections with I > 2.5σ(I). 1, 2, 4, 5, 6, and 7 exhibit extensive network hydrogen bonding between the anions, cations, and, if appropriate, water molecules of crystallization. The dimensionality of the hydrogen-bonded networks is dependent upon the nature of the anion and the number of water molecules of crystallization since the cation adapts a centrosymmetric exodentate conformation in all six salts. Keywords: crystal engineering, cyclams, network hydrogen bonding.
Condensation reactions between vanadate and small functionalized peptides in aqueous solution
Authors: Tracey Alan S. , Jaswal Jaswinder S. , Nxumalo Fikile , Angus-Dunne Sarah J.
Published: May 2006

The role that histidyl, seryl, tyrosyl, and other side chains of small peptides play in the formation of vanadate complexes has been investigated using 1H, 13C, and 51V nuclear magnetic resonance spectroscopy. Formation constants of product complexes in aqueous solution have been determined and the influence of pH on the equilibria established. The results revealed that, in histidine-containing compounds, the protonation state of the imidazole ring of the free ligand strongly influences product formation. The results unequivocally showed that no vanadium-to-imidazole bond was formed for any of the histidine-containing complexes studied. However, imidazole and other aromatic side-chain residues have an effect on product stability via their influence on the protonation state (pKa) of the products. Aliphatic side-chains, such as those in asparagine or glutamate, do not appear to play a significant role in the formation of products. This is not true, however, of serine or threonine where the side-chains readily generate vanadium–oxygen bonds. Keywords: vanadate, vanadium, 51-V NMR, peptides, complexes, aqueous equilibria.
The heterogeneity of two Canadian kraft lignins
Authors: Thring R.W. , Griffin S.L.
Published: May 2006

Two kraft lignins, one precipitated with carbon dioxide and the other with sulphuric acid, have been fractionated into three distinct fractions by sequential extraction using organic solvents of increasing hydrogen-bonding capacity. The lignins and fractions were comparatively characterized in terms of yield, methoxyl group content, molecular weight distribution, and 13C NMR spectroscopy. For a given lignin, the fractions showed differences in yield, composition, and chemical structure. A significant portion (21%) of the carbon dioxide precipitated kraft lignin is composed of ethyl acetate soluble material of low molecular weight. This fraction, which is richer in guaiacyl moieties than the other fractions, is virtually absent in the sulphuric acid precipitated kraft lignin. The occurrence of "tails" in the high molecular weight fraction, especially from the carbon dioxide precipitated kraft lignin, suggests the presence of material of very high molecular weight. Data from 13C NMR spectroscopy does little to elucidate the nature or origin of the material causing these tails. Keywords: kraft lignins, heterogeneity, fractionation, characterization.
Excess molar volumes and excess viscosities of ternary mixtures (2-butanol + 1-chlorobutane + 1-butylamine) and (2-methyl-2-propanol + 1-chlorobutane + 1-butylamine) at 298. 15 K
Authors: Domínguez M. , Camacho A. , López M. C. , Royo F. M. , Urieta J. S.
Published: May 2006

Excess molar volumes and excess viscosities, at 298.15 K, of the ternary mixtures (2-butanol + 1-chlorobutane + 1-butylamine) and (2-methyl-2-propanol + 1-chlorobutane + 1-butylamine) and of the binary mixtures (2-butanol + 1-butylamine), (2-methyl-2-propanol + 1-butylamine), and (1-chlorobutane + 1-butylamine) have been measured. The ternary excess properties were fitted to the polynomial equation of Cibulka. The experimental results were also compared with those predicted by the empirical equations of Redlich–Kister, Tsao–Smith, Kohler, and Colinet. Keywords: binary and ternary mixtures, excess properties, 2-butanol, 2-methyl-2-propanol, 1-chlorobutane, 1-butylamine.
Cyclometallation of bis(diphenylphos-phino)methane ligands on a binuclear ruthenium(0) carbonyl complex
Authors: Mirza Hameed A. , Vittal Jagadese J. , Puddephatt Richard J.
Published: May 2006

The first example of simple orthometallation of a bis(diphenylphosphino)methane ligand at a binuclear centre is reported. Both dppm ligands of the binuclear ruthenium(0) complex [Ru2(CO)5(µ-dppm)2], 1, are orthometallated when the complex is heated in benzene solution to yield the unusual complex [Ru2(CO)4-(Ph2PCH2PPhC6H4)2], 2. The structure of 2 was determined crystallographically (monoclinic, P21/n, a = 21.349(4), b = 12.209(5), c = 22.457(5) Å, β = 104.34(2)°, Z = 4, R, Rw = 0.0658, 0.0561) and the unusual cyclometallated complex was also characterized spectroscopically. The cyclometallation occurs in such a way that each ligand gives rise to two fused five-membered rings, RuRuPCP and RuRuPCC, fused at the Ru—Ru and Ru—P bonds. Keywords: ruthenium, bis(diphenylphosphino)methane, metallation.
A novel PC-based gravimetric autotitrator with a multi-solution delivery system
Authors: Hetherington Mark A. , Chen Min , Reid R. Stephen
Published: May 2006

The development of a novel PC-based gravimetric autotitrator is described. A single electronic balance serves as the weight buret for the delivery of three different solutions. It is therefore possible to add titrand and titrant under automated control. Investigation of the standard nickel(II)–glycinate–proton equilibrium system was used to evaluate performance. Keywords: titrator, titration, potentiometry, automated control.
The enols of acetic acid and methyl acetate
Authors: Guthrie J. Peter , Liu Zhi
Published: May 2006

We have determined the heat of formation of 1,1-dimethoxyethene, ΔHf(l) = −75.56 ± 0.87 kcal mol−1, by measuring the heat of hydrolysis. The heat of vaporization, 8.47 ± 0.10 kcal mol−1, was estimated from the boiling points at various pressures. The standard entropy of gaseous 1,1-dimethoxyethene, 82.02 ± 2. cal deg−1 mol−1, was calculated using frequencies obtained by MO calculations at the 6-31G** level. The free energy of transfer, 1.02 ± 1, was estimated by additivity with allowance for a distant polar interaction. Thus the free energy of formation in aqueous solution, ΔGf(aq) = −37.07 ± 1.48 kcal mol−1, could be calculated. The free energies of hydrolysis for enol ethers appear to be insensitive to structure; we used an average value and so calculated the values of the free energies of formation of the enols of acetic acid and methyl acetate, −67.90 ± 1.67 and −52.48 ± 1.53 kcal mol−1, respectively. These values are in good agreement with other recent evaluations. This method should be applicable to related enols. Keywords: enols, thermodynamics, heat of formation, entropy, ketene acetal.
Synthesis of (S)-4-hydroxy-α-lapachone and biotransformation of some 4-chromanones by Mortierellaisabellina ATCC 42613
Authors: Holland Herbert L. , Qi Jia , Manoharan T. Samuel
Published: May 2006

1-Methoxy-2-naphthol was converted by MOM ether formation, lithiation at C-3, reaction with 3,3-dimethylacryloyl chloride, cyclization, and oxidation using cerium ammonium nitrate to 4-keto-α-lapachone (2,2-dimethyl-4-keto-1-oxa-1,2,3,4-tetrahydroanthra-5,10-quinone). Reduction by biotransformation using Mortierellaisabellina ATCC 42613 gave (S)-4-hydroxy-α-lapachone in high (> 95%) optical purity. Reduction of several 2,2-dimethyl-4-chromanones to the corresponding (S)-alcohols by M. isabellina is also reported. Keywords: biotransformation, chemoenzymatic, 4-hydroxy-α-lapachone, Mortierellaisabellina.
Investigations of aluminum hydroxyphosphates and activated sludge by 27Al and 31P MAS NMR
Authors: Duffy Stephen J. , vanLoon Gary W.
Published: May 2006

High-resolution solid-state 27Al and 31P MAS NMR has been used to investigate the nature of aluminium- and phosphorus-containing solids formed during wastewater treatment. Although difficult to characterize by conventional techniques, these amorphous solids can be characterized by MAS NMR, providing information about their short-range ordering. In the present study, most of the solids were precipitated under conditions of high alkalinity, similar to those encountered during wastewater treatment. The ageing time of the aluminum hydroxide prior to the addition of phosphate (t1), the ageing time of the aluminum hydroxyphosphates after the addition of phosphate (t2), and the phosphorus to aluminum molar ratio were controlled while the effects of changing the parameters were examined. It was found that the aluminum MAS NMR chemical shift was related to the amount of phosphate present in the solid, which in turn was related to t1, t2, and the P:Al molar ratio. The results also lend support to the hypothesis that phosphate removal occurs through an adsorption process onto amorphous aluminum hydroxide, rather than through direct precipitation of aluminum phosphate. The increased understanding of aluminum and phosphate chemistry and the species formed during wastewater treatment will be useful in optimizing wastewater treatment processes. Keywords: aluminum, phosphate, NMR, activated sludge, wastewater treatment.
Azo polymers for reversible optical storage. 8. The effect of polarity of the azobenzene groups
Authors: Ho Mei-Sing , Natansohn A. , Barrett C. , Rochon P.
Published: May 2006

Poly[4′-[[(2-methacryloyloxy)ethyl]ethylamino]azobenzene] (pMAEA) was prepared. Its reversible optical storage properties were studied and compared with the properties of previously reported poly[4-(2-methacryloyloxy)ethylazobenzene] (pMEA) and poly[4-nitrophenyl-4′-[[2-(methacryloyloxy)ethyl]ethyl-amino]phenyldiazene] (pDR1M). The effect of the dipole moment of the azobenzene moiety on the optical storage properties was studied by comparing the photochemically induced birefringence "writing" and "relaxation" behaviors. The writing and relaxation rates are both in the order pDR1M > pMAEA > pMEA. In addition, pDR1M, which contains donor/acceptor substituents in the azobenzene side chain, exhibits the highest saturation level of optically induced birefringence. The fraction of birefringence conserved after relaxation is lowest for pMEA, which has no donor/acceptor substituents in the azobenzene side chain, and highest for pDR1M. Keywords: polymers, azobenzene type, birefringence, photochemically induced orientation, reversible optical storage.
β-Trichlorosilylstyrene oligomers
Authors: Brook Michael A. , Sebastian Thomas , Hülser Peter , Jüschke Ralf , Wenzel Stefan , Townsend Jennifer A. , Falletta Patricia R.
Published: May 2006

Under cationic conditions using triflic acid as the initiator, it is possible to oligomerize β-trichlorosilylstyrene 2 to low molecular weight oligomers 14 with a maximum degree of polymerization of about 9. Termination of the process was shown to occur by an intramolecular Friedel–Crafts reaction, leading to highly functionalized, indane-terminated oligomers 9, 12, etc. At lower temperatures, the reaction is diastereoselective. The oligomerization process was shown to require electron-withdrawing groups on silicon; the replacement of Cl spectator ligands with alkoxy or alkyl groups led to protiodesilylation. The mechanism for formation of the indane-terminated oligomers is discussed. Keywords: cationic polymerization, β-silylstyrene, stereoselective oligomerization, indene termination.
Activation barriers for oxygen diffusion in polystyrene and polycarbonate glasses: effects of codissolved argon, helium, and nitrogen
Authors: Wang Bojie , Ogilby Peter R.
Published: May 2006

A recently developed spectroscopic technique was used to determine oxygen diffusion coefficients as a function of temperature for polystyrene and polycarbonate films. Data were recorded at total pressures <300 Torr over the temperature range 5–45 °C under conditions in which argon, helium, and nitrogen, respectively, were copenetrants. In all cases, the presence of the additional gas caused an increase in the oxygen diffusion coefficient. Arrhenius plots of the data yield (a) a diffusion activation barrier, Eact, and (b) a diffusion coefficient, D0, that represents the condition of "barrier-free" gas transport for the temperature domain over which the Arrhenius plot is linear. For all cases examined in both polystyrene and polycarbonate, D0 increased with an increase in the partial pressure of added gas. In polystyrene, the presence of an additional gas did not change Eact. In polycarbonate, Eact obtained in the presence of helium and argon likewise did not differ from that obtained in the absence of the copenetrant. When nitrogen was the added gas, however, a larger value of Eact was obtained. This latter observation is interpreted to reflect the plasticization of polycarbonate by nitrogen. Eact and D0 data are discussed within the context of a model that distinguishes between dynamic and static elements of free volume in the polymer matrix. Keywords: oxygen diffusion, polystyrene, polycarbonate, activation barrier.
Sequence specificities for lysozyme depolymerization of partially N-acetylated chitosans
Authors: Stokke Bjørn T. , Vårum Kjell M. , Holme Hilde K. , Hjerde Ragnhild J.N. , Smidsrød Olav
Published: May 2006

The influence of sugar residue sequence in partially N-acetylated chitosans on relative hydrolysis rate catalyzed by lysozyme was studied. The relative rates were modelled assuming an Arrhenius-type relation for the relative rate constants. The apparent activation energy was assumed to consist of additive contributions from GlcN or GlcNAc residues within the polymer chain interacting with sites A–F of the active cleft of lysozyme. This model accounted well for the relative hydrolysis rates reported for well-defined oligomers. Calculated and experimental data for the dependence of the initial relative hydrolysis rates on fraction of acetylated units, FA, showed an FA3,6 dependence. A fully water-soluble highly N-acetylated chitosan with FA = 0.68 was depolymerized using lysozyme for further testing of the model. Analyses of the 13C nuclear magnetic resonance spectra of the diad sequences at the new reducing and nonreducing ends formed by lysozyme showed that this enzymatic depolymerization was dominated by chitosan sequences presenting GlcNAc residues to sites C, D, and E of the active cleft. In contrast, there was no selectivity between GlcNAc and GlcN residues interacting with site F. These selectivities were confirmed by the calculated contributions to the apparent activation energy of these sites. The experimentally determined depletion in the diad and triad frequencies of GlcNAc during the course of lysozyme hydrolysis was in good agreement with the model calculations. Keywords: lysozyme, chitosan, chitin, sequence specificity, subsite model.
Radical-induced degradation of a lignin model compound. Decomposition of 1-phenyl-2-phenoxyethanol
Authors: Huang Y. , Pagé D. , Wayner D.D.M. , Mulder P.
Published: May 2006

Reaction of 1-phenyl-2-phenoxyethanol (1) with thermally or photochemically generated tert-butoxyl radicals leads, via the intermediate ketyl radical, to the formation of the corresponding ketone, α-phenoxyacetophenone (4), as the only product at low conversion under an inert atmosphere. An approximately twofold increase in the product yield is observed when the reactions are carried out under oxygen. Under the photochemical conditions it is shown that 4 is the primary product and that acetophenone and phenol are formed as a result of secondary photolysis of 4. These data suggest that the rate constant for fragmentation of the ketyl radical derived from 1 is on the order of 10 s−1 at 298 K and contradict a report in the literature that suggests a rate constant of >106 s−1. The relevance of this study to the photodegradation of lignin and consequent photoyellowing is discussed and a revised mechanism for the photoyellowing of pulp is proposed. Keywords: ketyl radical, photochemical degradation, thermal degradation, lignin.
The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 10: intramolecular reactions involving alk-4-enols and 1,4-dicyanobenzene
Authors: McManus Kimberly A. , Arnold Donald R.
Published: May 2006

Our study of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include alk-4-enols. Irradiation of acetonitrile solutions of the alk-4-enols, 6-methyl-5-hepten-2-ol (16) and 5-methyl-5-hexen-2-ol (17), and the aromatic, 1,4-dicyanobenzene (1), leads to cyclized 1:1 (alk-4-enol:aromatic) adducts. The addition of biphenyl (5) to the irradiation mixture, serving as a codonor, increases the yields and the efficiency of formation of the adducts. The structures assigned to the products trans-2-(isopropyl 4-cyanophenyl)-5-methyltetrahydrofuran (18, 29%) and cis-2-(isopropyl 4-cyanophenyl)-5-methyltetrahydrofuran (19, 24%) from 16 (reaction [5]), and r-2-(methyl 4-cyanophenyl)-2,t-5-dimethyltetrahydrofuran (20, 13%), r-2-(methyl 4-cyanophenyl)-2,c-5-dimethyltetrahydrofuran (21, 7%), r-3-(4-cyanophenyl)-3,t-6-dimethyltetrahydropyran (22, 11%), and r-3-(4-cyanophenyl)-3,c-6-dimethyltetrahydropyran (23, 2%), from 17 (reaction [6]), rests mainly upon analysis of the 1H and 13C nuclear magnetic resonance spectra. The structures of 19, 22, and 23 were firmly established by X-ray crystallography. The observed ratio of regioisomers indicates a strong preference for 1,5-exo-trig, relative to 1,6-endo-trig, cyclization of the intermediate alk-4-enol radical cation. The mechanistic implication of these results is discussed. Keywords: photoinduced electron transfer, radical ions, cyclization of radical cations, intramolecular reactions of radical cations.
Effect of concentration, pH, and ionic strength on the viscosity of solutions of a soil fulvic acid
Authors: Rey F. , Ferreira M.A. , Facal P. , Machado A.A.S.C.
Published: May 2006

The influence of pH (3.0–8.0), ionic strength (0.005–0.10 M), and concentration (ca. 50–300 mg L−1) on the viscosimetric behaviour of solutions of a soil fulvic acid has been determined. The results show a complex influence of the concentration on the specific viscosity with maxima at about 80–100 mg L−1 and a minimum at 140–160 mg L−1. The viscosity is minimum at pH ca. 6 and decreases when the ionic strength increases. The results are interpreted by a rod-type model from which the dimensions of fulvic acid molecules are calculated. Key words: soil fulvic acids, viscosity, configurational changes.
The metabolites of Trichoderma longibrachiatum. Part II The structures of trichodermolide and sorbiquinol
Authors: Andrade Romano , Ayer William A. , Trifonov Latchezar S.
Published: May 2006

Trichoderma longibrachiatum Rifai aggr. is a fungus reported to be antagonistic to the fungus Mycena citricolor, the causative agent of the American leaf spot disease of coffee. We have investigated the metabolites produced when T. longibrachiatum is grown in liquid culture and have isolated several new compounds. The structures of trichodermolide (2) and sorbiquinol (3) were determined by a combination of spectroscopic techniques, including 1H and 13C NMR, IR, UV, CD, ORD, and mass spectrometry, and by preparation of derivatives. Key words: Trichoderma longibrachiatum, leaf spot disease, coffee, fungal metabolites.
Synthèse et analyse structurale de nouvelles méso-arylporphyrines glycosylées en vue de l'application en photothérapie des cancers
Authors: Gaud Olivier , Granet Robert , Kaouadji Mourad , Krausz Pierre , Blais Jean Claude , Bolbach Gerard
Published: May 2006

The synthesis of 13 novel meso-glycosylarylporphyrins where the carbohydrate moiety is separated from the aryl substituent by a spacer arm is described. These compounds were synthesized by different methods, either by direct glycosylation of the ortho- or para-hydroxyalkoxyarylporphyrin or by condensation of glycosylated aldehyde with pyrrole or meso-(p-tolyl)dipyrromethane. In all cases, a β configuration was observed. Deprotection of the sugar then followed in a basic medium. The compounds were characterized by a variety of means. A detailed 1H and 13C NMR study allowed complete structural determination. The UV–visible and laser desorption mass spectra are presented. Due to their sensitizing abilities, these resultant compounds are of considerable interest for photodynamic therapy. Key words: porphyrins, glycosylations, phototherapy, cancer.
An AM1 calculational study of the protonation and reactions of 3,4-dihydro-2-oxo-1,4-ethanoquinoline, 3,4-dihydro-2-oxo-1,4-propanoquinoline, 3,3,4,5-tetrahydro-2-oxo-1,5-ethanobenzazepine, 3,3,4,5-tetrahydro-2-oxo-1,5-propanobenzazepine, and N-methyl-4-bromo-2-methylacetanilide
Authors: Werstiuk N.H. , Brown R.S. , Wang Q.
Published: May 2006

The gas phase N- and O-protonation of distorted amides 3,4-dihydro-2-oxo-1,4-ethanoquinoline (1a), 3,4-dihydro-2-oxo-1,4-propanoquinoline (1b), 3,3,4,5-tetrahydro-2-oxo-1,5-ethanobenzazepine (1c), and 3,3,4,5-tetrahydro-2-oxo-1,5-propanobenzazepine (1d) and normal amides 4-bromo-2,N-dimethylacetanilide (5a) and 2,N-dimethylacetanilide (5b), along with the N-protonation of benzoquinuclidine (7), has been studied calculationally at the semiempirical level of theory with AM1. Calculated enthalpies of N- and O-protonation (proton affinities) indicate that 1a, 1b, and 1c should protonate on nitrogen and 1d, 5a, and 5b on oxygen. A satisfactory straight-line relationship between the calculated proton affinities of 1a, 1b, 1c, and 7 with experimental solution pKa’s is used to estimate the solution pKa of 1d. Results of a calculational study on the gas phase N- and O-methylation of 1b, 1c, and 1d are nicely in accord with the findings of an experimental solution phase study of the methylation–hydrolysis of 1b and 1d: while 1b methylates on nitrogen, 1d methylates on oxygen. A calculational study of possible pathways of hydrolysis of the N-protonated amides indicates that solvolysis of the hydrates of these species represents a viable route for their conversion to ring-opened products. Key words: protonation, methylation, reactions, distorted amides, AM1 calculations.
N10-(2′-Mercaptoethanoyl)-2,2,5,5-tetramethyl-3,4-diathia-7,10-diaza-bicyclo[5.3.0]decane and its reaction with oxotrichlorobis(triphenylphosphine)rhenium(V)
Authors: Alarabi H. , Bell R.A. , Howard-Lock H.E. , Kowanetz J. , Lock C.J.L.
Published: May 2006

The ligand molecule N10-(2′-mercaptoethanoyl)-2,2,5,5-tetramethyl-3,4-dithia-7,10-diazabicyclo[5.3.0]decane has been prepared and characterized by 1H and 13C NMR spectroscopy and by mass spectrometry. The protected analogue, N10-[(2′-triphenylmethylthio)ethanoyl]2,2,5,5-tetramethyl-3,4-dithia-7,10-diazabicyclo[5.3.0]decane dimethanol hemihydrate, was examined by the same techniques and also by X-ray crystallography. Crystals were triclinic, P-1, a = 11.125(2), b = 11.986(2), c = 13.562(3) Å, α = 103.54(3)°, β = 90.29(3)°, γ = 107.11(3)°, and Z = 2. The crystal was unstable in air at room temperature, so measurements were made on a crystal sealed in a tube that contained methanol vapour Intensities were measured with a Rigaku AFC6R diffractometer and monochromated CuKα radiation (λ = 1.54178 Å). The structure was solved by direct methods and refined to R = 0.1497, wR = 0.0655 based on 5000 independent reflections. The high residuals were caused by solvent disorder. Bond lengths and angles were normal. The reaction of the ligand with oxotrichlorobis(triphenylphosphine)rhenium(V) yielded an unexpected asymmetric complex, oxo(1,1-dimethyl-1,8-dimercapto-3,6-diazaoctan-7-onato-N3,N6,S1,S8) rhenium(V). Crystals were monoclinic, P21/n, a = 10.633(2), b = 11.221(2), c = 11.678(1) Å, β = 116.10(1)°, Z = 4. Intensities were measured with a Siemens P4 diffractometer and monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by the heavy atom method and refined to R = 0.0471, wR = 0.0340 based on 2866 unique reflections. Most bond lengths and angles were normal. The Re≡O distance of 1.681(5) Å was longer than normal. It is postulated that this was caused by competitive π bonding between the deprotonated amidic nitrogen atom and the rhenium atom, as shown by the short Re—N distance (1.997(6) Å) compared to the equivalent distance for the amine nitrogen atom (Re—N, 2.151(4) Å). Key words: N2S2 ligands, rhenium, crystal structure.
On the differential geometry of interatomic surfaces
Authors: Popelier Paul L.A.
Published: May 2006

Using differential geometry, we propose the total curvature of interatomic surfaces to characterize bonds. In this way visual interpretations of interatomic surfaces are now rigorously quantified. The analysis of the intrinsic geometry of an interatomic surface is implemented in the program MORPHY 2.0. It is shown that the total curvature depends on anionic polarizability, electronegativity differences, and steric effects determined by the total chemical environment of the bonded atoms in question. In general the proposed index measures the external chemical distortion of an atom in a molecule. It can be used in the context of uniform electric fields and in conformational studies. Key words: interatomic surfaces, differential geometry, total curvature.
Calculation of static molecular properties in the framework of the unitary group based coupled cluster approach
Authors: Paldus Josef , Li Xiangzhu
Published: May 2006

The recently developed and implemented state selective, fully spin-adapted coupled cluster (CC) method that employs a single, yet effectively multiconfigurational, spin-free reference and the formalism of the unitary group approach (UGA) to the many-electron correlation problem, has been employed to calculate static electric properties of various open-shell (OS) systems using the finite field (FF) technique. Starting with the lithium atom, the method was applied at the first-order interacting space single and double excitation level (CCSD(is)) to several first- and second-row hydrides having OS ground state, namely to the CH, NH, OH, SiH, PH, and SH radicals. In the case of NH we also considered three OS excited states. In all cases the dipole moment and polarizability were determined using a high quality basis set and compared with the experiment, whenever available, as well as with various configuration interaction results and other theoretical results that are available from the literature. The agreement of our CCSD(is) values with experiment is very satisfactory except for the 3Σ− ground state of the NH radical, where the experimentally determined dipole moment is too small. No experimental data are available for the corresponding polarizabilities. It is also shown that the FF technique is not suitable for calculations of higher order static properties, such as the hyperpolarizability β and γ tensors. For this reason we formulate the linear response version of our UGA-based CCSD approach and discuss the aspects of its future implementation. Key words: static molecular properties, dipole moments, polarizabilities, free radicals, unitary group based coupled cluster method, linear response theory, finite field technique.
Topologically partitioned dynamic polarizabilities using the theory of atoms in molecules
Authors: Hättig Christof , Hebβ Bernd A. , Jansen Georg , Ángyán János G.
Published: May 2006

Frequency-dependent distributed polarizabilities have been determined from time-dependent Hartree–Fock calculations, using the partitioning of the molecular space suggested by Bader's topological theory of atoms in molecules. The basis set dependence of the distributed dynamic polarizabilities is analyzed in terms of the first few Cauchy moments, for the carbon monoxide, water, cyanogen, urea and benzene molecules. Two alternative relocalization schemes have been considered in order to reduce the number of distributed dynamic polarizability parameters. The first one, closely related to the atomic polarizability model of Bader, leads to atomic charge–dipole and dipole–dipole polarizabilities, describing the response of the molecular charge distribution to a uniform external field, in terms of atomic charges and dipoles. The second scheme, similar to that suggested by Stone, retains the fully distributed description of the dynamic charge-flow polarizabilities, while all two-center dipole–dipole and charge–dipole contributions are condensed in one-center dynamic dipole–dipole polarizabilities. Key words: Bader-partitioning, distributed dynamic polarizabilities, Cauchy-moments, benzene, polarizability of; urea, polarizability of.
Current-density dependent exchange-correlation functionals
Authors: Becke Axel D.
Published: May 2006

Previous models for exchange (Becke and Roussel, Phys. Rev. A: 39, 3761 (1989)) and for correlation (Becke, J. Chem. Phys. 88, 1053 (1988)) are, in a simple and natural way, generalized to include explicit dependence on current density J. First-principles incorporation of J into exchange-correlation density functionals, as proposed here, is crucial for further progress in the study of magnetic effects in density-functional theory. Key words: density-functional theory, exchange-correlation functionals, current density.
On the accuracy of density functional theory for ion–molecule clusters. A case study of PLn+ clusters of the first and second row hydrides
Authors: Lopez Xabier , Ugalde Jesus M. , Sarasola Cecilia , Cossio Fernando P.
Published: May 2006

PLn+ clusters (n = 1, 2 and L = NH3, OH2, FH, PH3, SH2, CIH) in both their triplet and singlet states have been characterized by common approximate density functional methods, SVWN, BVWN, BLYP, and B3LYP. The phosphorus–ligand distances (R), dissociation energies (D0), triplet–singlet gaps (Δt−s), and several bond properties, such as the electron density (ρ(rc)), the Laplacian [Formula: see text] and the local energy density H(rc) at the bond critical point, were compared with those obtained by accurate ab initio molecular orbital theory, namely, second-order Møller–Plesset (MP2) and G2 theory. In general, it is observed that the local spin density approximation (SVWN) yields stronger bonds than ab initio molecular orbital theory. However, addition of gradient corrections to the exchange functional (BVWN) yields ion–molecule bonds that are too weak. Finally, taking account also of gradient corrections to the correlation functional (BLYP) leads to very close agreement with ab initio results. Among these functional, Becke's hybrid functional, B3LYP, best fit the second-order Møller–Plesset and G2 data, reproducing the qualitative trends observed for the above-mentioned properties of phosphorus clusters, except for [Formula: see text] This fit is particularly good for distances, dissociation energies, and electron densities at the bond critical point, and both methods show similar deviations of the values of binding energies and triplet–singlet gap with respect to the G2 data. Compared with our most accurate ab initio molecular orbital data, namely G2, significant overbinding for the singlets, larger for one-ligand than for two-ligand complexes, and significant overestimation of the triplet–singlet gap for one-ligand complexes is observed for both methods, namely, B3LYP and MP2. The deviations at the second-order Møller–Plesset level of theory are mainly due to the lack of quadratic configuration interaction (QCI) corrections, and this deficiency is also present to some extent in B3LYP. However, for larger clusters these corrections are smaller, therefore the B3LYP functional is expected to lead to accurate descriptions. Key words: DFT, Bader analysis, G2, ion–molecule complexes, phosphorus.
Proton and electron transfers in O•H•O and C•H•O hydrogen-bridged ions: their role in the dissociation chemistry of ionized acetol, CH3C(=O)CH2OH•+
Authors: Ruttink Paul J.A. , Burgers Peter C. , Terlouw Johan K.
Published: May 2006

Low-energy acetol ions CH3C(=O)CH2OH•+, 1, dissociate to CH3C(H)OH+ and HC=O• by a double hydrogen transfer (DHT), a common reaction among oxygen-containing radical cations. Recent experimental work has shown that the isotopologue CH3C(=O)CH2OD•+ specifically loses HC=O• to produce CH3C(D)OH+. This finding refutes an earlier postulated attractive mechanism based on the behaviour of 1 in ion-molecule reactions. Using ab initio MO calculations (at the CEPA//RHF/DZP level of theory complemented with valence bond (VB) methods), a low-energy pathway was traced that may explain all of the available experimental observations. It is shown that the unimolecular chemistry of 1 can be understood in terms of two proton transfers, taking place in intermediate O•H•O and C•H•O bonded hydrogen-bridged radical cations. The two protons originate from the same moiety and a charge transfer complex is therefore implicated and shown to be involved. These concepts of proton and charge transfer may well be more generally applicable and they do correctly predict the unimolecular chemistry of ionized acetoin, CH3C(=O)CH(CH3)OH•+ and related α-ketols. Key words: ab initio calculations, hydrogen-bridged ions.
Origin of depressed dipole moments in five-membered, unsaturated heterocycles
Authors: Laidig Keith E. , Speers Peter , Streitwieser Andrew
Published: May 2006

The smaller molecular dipole moments of cyclopentadiene, pyrrole, furan, and thiophene, relative to their saturated analogs, are investigated using the HF/6-311++G**//HF/6-311++G** level of theory. Investigation of the atomic origins of the molecular dipole moments demonstrates that the decreased dipole moments of the unsaturated heterocycles result from simple perturbations of the butadiene fragment, rather than a delocalization of hetero-atom lone pairs into the π system. Comparison with the origins of the molecular dipole moments of the saturated analogs shows that a special delocalization mechanism is not necessary. Key words: ab initio, atoms-in-molecule, electronic effects, delocalization, polarization.
Bis(pyridine)difluoroboron, tris(pyridine)fluoroboron, and other (pyridine)haloboron cations. A systematic NMR study
Authors: Farquharson Melvin J. , Hartman J. Stephen
Published: May 2006

The adducts pyr•BF2Br and pyr•BFBr2 (pyr = pyridine) form fluoroboron cations by displacement of Br− by excess pyridine, the ease of cation formation being pyr2BF2+ » pyr2BFBr+ » pyr3BF2+•Cl− can be displaced from pyr•BF2Cl and pyr•BFCl2, but much less readily, to form pyr2BF2+, pyr2BFCl+, and, under forcing conditions, a few percent of pyr3BF2+. Non-fluorine-containing mixed boron trihalide adducts of pyridine also form haloboron cations by heaviest-halide-ion displacement, for example pyr•BClI2 giving pyr2BClI+, the ease of displacement always being I− > Br− > Cl−, and displacement always occurring more readily from mixed boron trihalide adducts than from unmixed-halogen adducts. The mechanistic implications of this are discussed. ortho Substituents greatly reduce the ability of pyridine to displace heavy halide ion, so 2-methylpyridine gives 2-Mepyr2BF2+ and 2-Mepyr2BFBr+ but not 2-Mepyr2BFCl+ or 2-Mepyr3BF2+, while 2,6-dimethylpyridine does not form any haloboron cations. 19F spin-lattice relaxation times of the fluoroboron cations are much shorter than those of neutral boron trihalide adducts in the same solution, and provide a further diagnostic test for their presence. Key words: fluoroboron cations, pyridines, mixed boron trihalide adducts, fluorine-19 NMR, boron-11 NMR.
Acid-catalyzed hydrolysis of 4-diazo-3-isochromanone: the effect of coplanarity on the carbon protonation of α-phenyl-α-carbonyldiazo compounds
Authors: Jefferson E.A. , Kresge A.J. , Paine S.W.
Published: May 2006

Hydrolysis of the cyclic α-phenyl-α-carbonyl-diazo compound, 4-diazo-3-isochromanone, in dilute aqueous perchloric acid solutions was found to give the hydronium ion isotope effect [Formula: see text] which shows that this reaction occurs by rate-determining hydronation of the substrate on the carbon atom α to its diazo group. Comparison of the rate constant obtained, [Formula: see text] with that for the corresponding acyclic analog, methyl phenyldiazoacetate, indicates that the cyclic compound is 57 times less reactive. Semi-empirical AM1 molecular orbital calculations suggest that this difference in reactivity is caused by enforced near-coplanarity of the diazo and phenyl groups in the cyclic substrate, as opposed to a staggered arrangement of these groups in the acyclic analog; this coplanarity then enhances delocalization of negative charge from the diazo α-carbon atom into the phenyl group, which reduces the negative charge density on the α-carbon atom and slows the rate of reaction. Key words: hydrolysis, diazoalkanes, charge delocalization, AM1 calculations.
The synthesis and high-resolution NMR spectroscopy of ethyl N-(2-triphenylmethylthio)ethanoyl-O-{4′-[4″-(1″-bis(2″′-chloroethyl)amino)phenyl]butanoyl}-L-seryl-S-benzyl-L-cysteine: a chelate–chlorambucil complex for use as a ligand for 99mTc radio-imaging
Authors: Bell Russell A. , Hughes Donald W. , Lock Colin J.L. , Valliant John F.
Published: May 2006

A potential agent for the imaging of tumours has been synthesized from the antineoplastic agent chlorambucil. Standard peptide coupling techniques were used to synthesize a tripeptide covalently coupled to chlorambucil in 10 steps. The final product was characterized by high-resolution NMR spectroscopy. Key words: ethyl N-triphenylmethylthioethanoyl-O-{4′-[4″-(1"-bis(2″′-chloroethyl)amino)phenyl]butanoyl}-L-seryl-S-benzyl-L-cysteine, NMR spectroscopy, synthesis, radio-imaging agent.
The chemistry of cyclohexenediones produced by the blackleg fungus
Authors: Pedras M. Soledade C.
Published: May 2006

Several oxidation products of metabolites produced by avirulent isolates of Phoma lingam, the blackleg fungus, are reported; in addition, the profile of metabolites common to P. lingam and P. wasabiae is clarified. Key words: cyclohexenedione, Phoma lingam, P. wasabiae.
The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile
Authors: Cox Robin A.
Published: May 2006

In aqueous sulfuric acid, aliphatic N-nitro amines decompose to N2O and alcohols. An excess acidity analysis of the observed rate constants for the reaction shows that free carbocations are not formed. The reaction is an acid-catalyzed SN2 displacement from the protonated aci-nitro tautomer, the nucleophile being a water molecule at acidities below 82–85% H2SO4, and a bisulfate ion at higher acidities. Bisulfate is the poorer nucleophile by a factor of about 1000. Twelve compounds were studied, of which results obtained for nine at several different temperatures enabled calculation of activation parameters for both nucleophiles. The reaction appears to be mainly enthalpy controlled. The intercept standard-state rate constants are well correlated by the σ* values for the alkyl groups; the slopes are negative, with a more negative value for the slower bisulfate reaction. Interestingly the m≠m* slopes also correlate with σ*, although the scatter is bad. Key words: N-nitro amines, excess acidity, bisulfate, nucleophiles, acid-catalyzed, kinetics.