bluuebunny/crossref_metadata_2025 huggingface dataset:
1H and 13C NMR study of the pyrazolo[1,5-a]pyrimidine system
Authors: Chimichi Stefano , Cosimelli Barbara , Bruni Fabrizio , Selleri Silvia
Published: May 2006

The 13C and 1H NMR spectra of some pyrazolo[1,5-a]pyrimidine derivatives are discussed. All 13C resonances were unambiguously assigned by means of both 2D experiments and gated decoupled spectra from which one-bond and long-range 13C–1H coupling constants were determined. The literature assignments for H-5 and H-7 in the parent system have been revised and a simple method for distinguishing between 5-methyl and 7-methyl compounds is suggested, based on the carbon chemical shift of the methyl group or on its fine structure in the 1H NMR spectrum. Regioselective syntheses for all the reported methyl derivatives are also described.
The synthesis and structural characterization of a series of pentacoordinate diorganotin(IV) N-arylidene-α-amino acid complexes
Authors: Smith F. E. , Hynes Rosemary C. , Ang T. T. , Khoo L. E. , Eng G.
Published: May 2006

Eight new diorganotin complexes formulated as R2Sn(OArCR″=NCHR′COO) were prepared and characterized by 1H and 13 C NMR, IR, and elemental analysis. A single crystal X-ray diffraction study of the dibutyltin N-salicylidenevaline complex, Bu2Sn(OC6H4CH=NCH(i-Pr)COO), determines the molecular structure. The crystals are orthorhombic, space group P212121 with a = 9.187(2) Å, b = 10.003(2) Å, c = 23.482(4) Å, V = 2157.8(6) Å3, Z = 4, and Dc = 1.392 g cm−3. The final discrepancy factors are RF = 0.040, and Rw = 0.021 for 1131 significant reflections. The tin atom has a distorted trigonal bipyramidal coordination, with no short intermolecular contacts. The two axial Sn—O bonds of 2.078(10) and 2.151(8) Å and the equatorial Sn—N bond of 2.158(8) Å are among the shortest found in related complexes. The fact that the closest intermolecular Sn—O distance is 4.511(8) Å indicates a nonbridging carbonyl group.
Metal pyrazolate polymers. Part 3. Synthesis and study of Cu(I) and Cu(II) complexes of 4-Xdmpz (where X = H, Cl, Br, I, and CH3 for Cu(I) and X = H, Cl, Br, and CH3 for Cu(II); dmpz = 3,5-dimethylpyrazolate)
Authors: Ehlert Martin K. , Storr Alan , Thompson Robert C.
Published: May 2006

The copper(I) complexes [Cu(4-Xdmpz)]3 (where X = H, Cl, Br, I and CH3; dmpz = 3,5-dimethylpyrazolate) and the copper(II) complexes [Cu(4-Xdmpz)2]x have been synthesized and characterized. Qualitative solubility, infrared spectroscopic, and differential scanning calorimetric studies are reported for all complexes. Mass spectra support trimeric formulations for the copper(I) complexes. Scanning electron micrographs and powder X-ray diffractograms have been recorded for the copper(II) compounds. Electronic and EPR spectroscopic studies as well as magnetic susceptibility studies from 2 to 300 K are also reported for the copper(II) compounds, which are proposed to have polymeric chain structures. The magnetic data reveal strong antiferromagnetic interactions in all four of the copper(II) compounds. The data have been analysed employing an isotropic Heisenberg model for antiferromagnetic coupling in extended chain polymers. Values of the exchange coupling constant, J, are determined as −58, −61, −66, and −66 cm−1 for the X = H, CH3, Cl, and Br complexes respectively. The X = Cl compound exhibits an abrupt decrease in magnetic susceptibility below 40 K and possible causes of this anomalous behaviour are discussed.
Studies with tissue cultures of the Chinese herbal plant, Tripterygiumwilfordii. Isolation of metabolites of interest in rheumatoid arthritis, immunosuppression, and male contraceptive activity
Authors: Kutney James P. , Hewitt Gary M. , Lee Gin , Piotrowska Krystyna , Roberts Malcolm , Rettig Steven J.
Published: May 2006

A detailed study of metabolites produced by the plant cell culture line of Tripterygiumwilfordii, a Chinese herbal plant, is presented. Eighteen compounds within the diterpene and triterpene families have been isolated and fully characterized. Of these, five are novel compounds, and their structures were determined by a combination of spectral analysis, chemical correlation and single crystal X-ray diffraction. The interest of these compounds in the treatment of rheumatoid arthritis, skin allergies, and for male contraception is noted.
The sodium perchlorate complex of cis-syn-cis dicyclohexano-24-crown-8 ether. X-ray crystal structure and solution conformations as studied by variable temperature 13C NMR spectroscopy at 125 MHz. An example of eight-coordinate sodium
Authors: Buchanan G. W. , Lear Y. , Bensimon C.
Published: May 2006

The major isomer obtained by hydrogenation of dibenzo-24-crown-8 ether is shown to have the cis-syn-cis configuration as determined from the crystal structure of its sodium perchlorate complex. This material possesses eight-coordinate sodium with Na—O distances ranging from 2.54 to 2.78 Å. The complex is shown to exist in a single asymmetric conformation below 240 K as determined by 13C NMR at 125 MHz. From NMR coalescence temperature measurement, the free energy of activation for cyclohexane ring inversion in this complex has been determined to be 51.0 ± 0.6 kJ/mol.
Reactions of lignin model compounds with pulp bleaching chemicals. Cannizzaro-type reaction of tetrachloro-o-benzoquinone with alkali
Authors: McKague A. Bruce , Reeve Douglas W. , Rettig Steven J. , Trotter James
Published: May 2006

Reaction of tetrachloro-o-benzoquinone 1 with aqueous sodium hydroxide resulted in a Cannizzaro-type disproportionation into tetrachloromuconic acid and tetrachlorocatechol. The tetrachloromuconic acid was isolated and identified by X-ray crystallography as the methyl ester of the cyclized lactone 3. Crystals of C7H3Cl3O4, 4, are orthorhombic, a = 9.991 (2), b = 7.981 (1), c = 24.626(2) Å, Z = 8, space group Pna21. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.033 for 1609 reflections with I ≥ 3σ(I).
The chemistry of thujone. XVI. Versatile and efficient routes to safronitrile, β-cyclogeranonitrile, β-cyclocitral, damascones, and their analogues
Authors: Kutney James P. , Gunning Philip J. , Clewley Robin G. , Somerville John , Rettig Steven J.
Published: May 2006

Thujone, a waste by-product of the Canadian forest industry, has been utilized as a starting material to develop a versatile synthetic route to the damascones (rose oil ketones) and related analogues. The synthetic sequence provides a route to β-cyclocitral (45), the latter having been previously converted to β-damascone (2). In addition, thujone-derived intermediates are converted to β-damascenone (48) and to intermediates that can be utilized for the preparation of damascone analogues. In conjunction with the above, an efficient route to safronitrile (42), β-cyclogeranonitrile (43), and β-cyclocitral (45) from 2,6-dimethylcyclohexanone has been developed. In summary, these studies afford an attractive versatile route to these important perfumery materials.
Phytotoxic metabolites of Phomopsisconvolvulus, a host-specific pathogen of field bindweed
Authors: Tsantrizos Youla S. , Ogilvie Kelvin K. , Watson Alan K.
Published: May 2006

Phomopsisconvolvulus is a host-specific pathogen which causes leaf spots and anthracnose lesions in the important perennial weed Convolvulusarvensis (field bindweed). Investigation of the metabolites produced by the fungus led to the isolation of the known fungal metabolites ergosterol (1) and ergosterol peroxide (2), in addition to four novel phytotoxins: the phthalides convolvulanic acid A (3), convolvulanic acid B (4), convolvulol (5), and the α-pyrone convolvulopyrone (6). The latter compounds were assigned the structures of 4-carboxy-3-hydroxy-7-methoxy-6-methyl-1(3H)-isobenzofuranone (3), 4-carboxy-7-methoxy-6-methyl-1(3H)-isobenzofuranone (4), 4-(hydroxymethyl)-7-methoxy-6-methyl-1(3H)-isobenzofuranone (5), and 3-[4-methoxy-3-methyl-α-pyron-6-yl]-2-methyl-2-butenoic acid (6) respectively. Strong herbicidal activity against C. arvensis was observed with compounds 3 and 4 at concentrations of 3–5 × 10−4 M.
Influence of temperature on the liquid-to-liquid extraction of ethanol from (water + ethanol + 1,2-dichloroethane)
Authors: Sólimo Horacio N. , Zurita José L.
Published: May 2006

To evaluate the influence of temperature on the extraction of ethanol from aqueous solutions with 1,2-dichloroethane, liquid–liquid equilibrium data for the system water + ethanol + 1,2-dichloroethane were obtained at 298.15, 308.15, and 318.15 K. The binodal curves were determined by the cloud-point method, and conjugate points on the tie-lines were obtained by correlating the refractive index of these curves as a function of composition. The tie-line data at each temperature were satisfactorily correlated using the method of Othmer and Tobias, and the plait-point coordinates were estimated. The experimental data were also compared to values calculated by the UNIFAC group contribution method for the activity coefficients using the isoactivity conditions as restraint equations, and with the NRTL and UNIQUAC models. The UNIFAC method proved to be more accurate than the NRTL and UNIQUAC equations fitted to the experimental data. Under the experimental conditions used, ethanol extraction by 1,2-dichloroethane appears to be independent of the temperature.
Iron porphyrin catalyzed oxidation of lignin model compounds: the oxidation of veratryl alcohol and veratryl acetate
Authors: Cui Futong , Dolphin David
Published: May 2006

meso-Tetra(2,6-dichloro-3-sulfonatophenyl)porphyrin iron chloride catalyzes the oxidation of 3,4-dimethoxybenzyl alcohol (veratryl alcohol) in aqueous solution to give veratraldehyde along with demethoxylation and ring-cleavage products. The isolation of a direct ring-cleavage product from the oxidation of veratryl acetate in aqueous solution supports the previously proposed ring-cleavage mechanisms. The oxidation in methanol, however, does not lead to ring-cleavage products. When veratryl alcohol was oxidized in methanol, solvent was found to be incorporated into the 3-position of veratryl alcohol, giving new insight into the mechanism of oxidation.
Analysis of minor constituents in fullerene soots by LC–MS using a heated pneumatic nebulizer interface with atmospheric pressure chemical ionization
Authors: Anacleto J. F. , Boyd R. K. , Pleasance S. , Quilliam M. A. , Howard J. B. , Lafleur A. L. , Makarovsky Y.
Published: May 2006

The minor constituents of toluene extracts of three fullerene-rich materials have been characterized by on-line LC–MS techniques, incorporating both positive and negative ion mass spectra obtained by atmospheric pressure chemical ionization. Two of the materials were soots, produced by resistive heating of graphite in an inert atmosphere, from different commercial suppliers. The third material was obtained as a condensate from a controlled laminar flame, and was shown previously (Anacleto etal:, Rapid Commun. Mass Spectrom. 6, 214 (1992)) to contain significant quantities of compounds that behaved under the present LC–MS conditions as isomers of the C60 and C70 fullerenes, and which reverted to the latter upon heating. This finding was confirmed here, and extended to the higher clusters C76, C84, C90, and C94. One of the graphite-derived soots contained monoxides of the carbon clusters as the principal minor components, while the other soot contained hydrogenated species including C60H2, C60H4, C70H2, and (C60•CH4). The flame-generated material contained all of these minor constituents, together with complexes of C60 with larger aliphatic molecules as well as large quantities of polycyclic aromatic hydrocarbons and related species. Photo-oxidation of a purified C60 preparation, from the graphitic soot containing mostly monoxide impurities, was shown to lead to increased levels of the mono-, di-, and tri-oxides of C60.
Spatial and temporal visualization of a pH-dependent complexation equilibrium by nuclear magnetic resonance imaging
Authors: Balcom B. J. , Carpenter T. A. , Hall L. D.
Published: May 2006

Alteration of a chemical equilibrium, ethylenediaminetetraacetic acid chelation of aqueous copper, in response to a pH gradient that protonates the complexing agent, has been observed spatially and temporally by 1H nuclear magnetic resonance. Protonation of the complexing agent shifted the complexation equilibrium towards free copper ion. Free and bound copper ions alter the spin lattice relaxation time (T1) of water in the host polyacrylamide gel to different extents. Based on this difference, a T1 weighted (fast repetition time, short echo time) two-dimensional spin-warp imaging sequence mapped out the distribution of the free and bound species. Addition of a pH 3 aqueous solution to the gel was insufficient to alter the equilibrium; a pH 1 aqueous solution liberated the complexed copper. Quantitative one-dimensional experiments gave a T1 weighted profile that showed the reaction front is displaced a distance proportional to the square root of time.
The synthesis of esters under microwave irradiation using dry-media conditions
Authors: Loupy André , Petit Alain , Ramdani Mohamed , Yvanaeff Celine , Majdoub Mustapha , Labiad Bouchta , Villemin Didier
Published: May 2006

Practical and simple techniques are described for using nonmodified domestic microwave ovens as safe and convenient laboratory devices to obtain numerous esters. High pressures are avoided by conducting reactions with reactants impregnated on solid mineral supports in "dry media" or by phase transfer catalysis (PTC) in the absence of organic solvents. Two kinds of microwave effects are involved: (1) displacement of the equilibrium by evaporation of volatile polar molecules (water or alcohols) in esterifications and transesterifications; (2) acceleration of ionic reactions in carboxylate alkylations. As solvents are avoided, there is no need for sealed vessels and water separators.
A new synthesis of castasterone and brassinolide from stigmasterol. A concise and stereoselective elaboration of the side chain from a C-22 aldehyde
Authors: Back Thomas G. , Blazecka Peter G. , Krishna M. Vijaya
Published: May 2006

Brassinolide (1) and castasterone (2) were prepared from aldehyde 7, in turn available from stigmasterol (3). The addition of (E)-1-propenyllithium to 7, followed by diastereoselective Sharpless epoxidation of allylic alcohol 8 using (L)-(+)-diethyl tartrate, and regioselective epoxide-opening of (threo) epoxy alcohol 15 with isopropylmagnesium chloride in the presence of a catalytic amount of cuprous cyanide afforded diol 17. The epoxidation and cuprate addition steps were investigated in greater detail with the simpler model aldehyde 4. Hydrolysis of 17 provided 2, which was converted preferentially into 1 or its regioisomer 22 by Baeyer–Villiger oxidation with trifluoroperoxyacetic acid or peroxyseleninic acids, respectively.
The dipole oscillator strength distribution and predicted dipole properties for ammonia
Authors: Burton Gordon R. , Chan Wing Fat , Cooper Glyn , Brion C. E. , Kumar Ashok , Meath William J.
Published: May 2006

Newly available and highly accurate oscillator strength data, extending over the continuous energy range from the first excitation threshold to 200 eV, are used together with mixture rule estimates and other photoabsorption data to construct a refined dipole oscillator strength distribution to infinite photon energy using Thomas–Reiche–Kuhn sum rule and molar refractivity constraints. This constrained dipole oscillator strength distribution has been used to calculate a wide range of related dipole properties (Sk, k = 2, 1, 0, −1/2, −1, −3/2, −2, −5/2, −3, −4, −5, −6, −8, −10, −12; Lk and Ik, k = 2, 1, 0, −1, −2). The theoretical analysis and associated consistency checks support the high accuracy of the newly available absolute oscillator strengths for the photoabsorption of NH3.
An 1H NMR conformational analysis of 3-phenylpentane in solution
Authors: Schaefer Ted , Lee Lina B.-L.
Published: May 2006

Some 30 proton chemical shifts and proton–proton coupling constants are reported for a 4.7 mol% solution of 3-phenylpentane in a CS2/C6D12/TMS solvent mixture at 300 K. The long-range coupling constant over six formal bonds between the methine and para protons is used to deduce an apparent twofold barrier of 15.0 ± 0.3 kJ/mol to rotation about the Csp2—Csp3 bond, at least twice as large as that for isopropylbenzene in solution. AM1 computations agree with experiment in finding the conformation of lowest energy as that in which the methine C—H bond is situated in the phenyl plane, but predict a barrier height of only 13.9 kJ/mol. The vicinal coupling constants are consistent with a fractional population, 0.38(2), of the TT conformer, that in which all the carbon atoms of the alkyl chain lie in a plane. A doubly degenerate conformer, TG+(G−T), in which one methyl group is twisted away from the phenyl substituent, then has a fractional population of 0.62(2). The assumption that only these three conformers are present is tested with the signs and magnitudes of the four different coupling constants over four bonds. These coupling constants are consistent with the absence of significant proportions of the other six all-staggered conformers. These six are characterized by a close approach of the methyl groups (1,5 interactions) or by proximity of the methyl and phenyl moieties.
2-Acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles and related compounds: a question of hypervalent S … O interactions
Authors: Pandya Naresh , Basile Anthony J. , Gupta Ajay K. , Hand Patrick , MacLaurin Cindy L. , Mohammad Taj , Ratemi Elaref S. , Gibson Martin S. , Richardson Mary Frances
Published: May 2006

A new synthesis of 1 by selective deacetylation of 2 is reported. The acylation step implied in earlier syntheses of 1 and related compounds is exemplified by direct acylations of 3 to give 4a and 4b. Several new 2-acylidene-3-(2,4-dibromophenyl)-5-phenyl-2,3-dihydro-1,3,4-thiadiazoles (11) and thioacylidene analogues (12) are described. The crystal structures of 11a, 11b, 11c, and 11d reveal a hypervalent interaction, ca. 2.45–2.7 Å long, between the sulfur and carbonyl oxygen atoms. The dibromophenyl ring is nearly perpendicular to the thiadiazole ring plane in the crystal structures, and NMR data suggest that this conformation is maintained in solution. Molecular mechanics calculations show that the S … O interaction need only be a few kilocalories in order to stabilize the observed acylidene side chain configuration over other possible isomers. Crystals of 11a are triclinic, [Formula: see text] with cell dimensions a = 12.937(2), b = 13.429(2), c = 13.489(2) Å, α = 60.14(1)°, β = 74.59(1)°, γ = 58.70(1)°, Z = 4, and R = 0.044 for 3287 observed reflections. Crystals of 11b are monoclinic, P21/c, with cell dimensions a = 10.512(1), b = 12.084(2), c = 16.268(4) Å, β = 96.91(1)°, Z = 4, R = 0.050 for 1938 observed reflections. Crystals of 11c are monoclinic, P21/c, with cell dimensions a = 17.492(4), b = 16.979(1), c = 14.962(1) Å, β = 106.46(1)°, Z = 8, R = 0.057 for 3112 observed reflections. Crystals of 11d are monoclinic, P21/c, with cell dimensions a = 11.749(1), b = 8.533(1), c = 22.670(4) Å, β = 100.17(1)°, Z = 4, and R = 0.059 for 2265 observed reflections.
High-resolution NMR spectroscopic analysis of the C-polysaccharide of Streptococcus pneumoniae
Authors: Kulakowska Malgorzata , Brisson Jean-Robert , Griffith Douglas W. , Young N. Martin , Jennings Harold J.
Published: May 2006

The complete structure of the complex cell-wall C-polysaccharide from Streptococcus pneumoniae R36A was elucidated using a combination of high-resolution NMR spectroscopy and theoretical calculations. The C-polysaccharide is composed of the following repeating unit β-D-Glcp1 → 3-α-D-AATp1 → 4-α-D-GalpNAc1 → 3-β-D-GalpNAc1 → 1-ribitol-5-phosphate where AAT is 2-acetamido-4-amino-2,4,6-trideoxygalactopyranoside. The C-polysaccharide contains two phosphorylcholine substituents per repeating unit, which are situated on O-6 of each of the two 2-acetamido-2-deoxy-galactopyranose residues. The repeating units are linked through a diphosphate ester from ribitol to O-6 of the β-D-glucopyranosyl residue.
Micellar catalysis of organic reactions. Part 33. Amide hydrolysis in neutral solution in the presence of a copper-containing micelle
Authors: Broxton Trevor J. , Coa Robin A.
Published: May 2006

The hydrolysis of 5-nitro-2-(trifluoroacetylamino)benzoic acid (1) has been studied at pH 7 in water and in the presence of micelles of cetyltrimethylammonium bromide (ctab) and of copper-containing micelles formed from the reaction of N,N,N′-trimethyl-N′-hexadecylethylenediamine and cupric chloride. It has been found that the hydrolysis of 1 is inhibited by micelles of ctab but strongly catalysed by the copper-containing micelle at this pH. At a higher pH where the hydroxide ion reaction becomes important the reaction is catalysed by micelles of ctab as well, but the catalysis is stronger by the copper-containing micelle. The effect of added sodium chloride on the rate of reaction is shown to be larger for reaction in the presence of ctab than for reaction in the presence of the copper micelles. Also reported are the effects of the buffer concentration on the rate of reaction at various pH for both micelles. It is concluded that the mechanism of reaction in the copper-containing micelle involves a metal-bound hydroxyl rather than a free hydroxide ion loosely associated with the cationic micelle surface. It is interesting that the catalysis of this reaction by the copper-containing micelle is large enough to allow amide hydrolysis at a reasonable rate at neutral pH at ambient temperature.
Thallimetric oxidations Part VII: Titrimetric and spectrophotometric methods for the determination of hypophosphite and ternary mixtures of hypophosphite, phosphite, and oxalate
Authors: Rao M.S. Prasada , Rao A. Rama Mohan , Ramana Karri V. , Sagi S.R.
Published: May 2006

Titrimetric and spectrophotometric methods are developed for the estimation of hypophosphite (PO23−) at mmol and micromol ranges, respectively. In these methods thallium(III) is used as an oxidant and the reduced thallium(I) is determined either oxidimetrically by potassium bromate in the titrimetric method or by measurement of the optical density of thallium(III) at 260 nm in the presence of 0.1 M NaCl and 1 M HClO4 in the spectrophotometric method. Convenient methods are described for the determination of ternary mixtures of hypophosphite (PO23−), phosphite (PO33−), and oxalate (C2O42−). A method is also described for the estimation of thallium(III) using hypophosphite as reductant, and the method is applied to the determination of mixtures of thallium(I) and thallium(III).
Design and synthesis of a cephalosporin–carboplatinum prodrug activatable by a β-lactamase
Authors: Hanessian Stephen , Wang Jianguo
Published: May 2006

The design and syntheses of two cephalosporin–carboplatinum prodrugs that can be released by a β-lactamase are described. The hydrolysis of cephalosporins catalyzed by a β-lactamase with acetyl or DACCP as 3′-leaving groups is studied by 1H nuclear magnetic resonance in deuterated buffer solutions. These notions provide a new approach to the use of platinum complexes for antitumor therapy.
Coordination-induced keto–enol tautomerization in the formation of Ni2+ and Cu2+complexes of a fluorinated keto-diol
Authors: Vittal Jagadese J. , Willis Christopher J.
Published: May 2006

The fluorinated keto-diol (CF3)2C(OH)CH2C(=O)CH2C(CF3)2OH forms stable, neutral, 1:1 complexes of Ni2+ (6a) and Cu2+ (6b) with tetramethylethylenediamine (TMED) as coligand. The structure of each complex has been determined crystallographically; they are isomorphous and isostructural, monoclinic with space group Cc (No. 9). Complex 6a, C15H20F12N2NiO3, has a = 11.495(2), b = 15.516(2), c = 12.323(2) Å, β = 99.88(1)°, V = 2165.4(6) Å3Z = 4. Complex 6b, C15H20CuF12N2O3, has a = 11.603(2) b = 15.661(2), c = 12.276(2) Å, β = 99.75(2)°, V = 2198.5(6) Å Z = 4. In each, the metal is in square-planar geometry bonded to two alkoxide groups and two N atoms of TMED. A previous assignment of a zwitterionic structure to these complexes has been shown to be incorrect. Instead, the ligand is coordinated as the di-ionized enol tautomer (CF3)2C(–O−)CH2C(–O−)=CHC(CF3)2OH, the C=C double bond being external to the six-membered chelate ring. The remaining OH group of the ligand forms part of a second six-membered ring held together by hydrogen bonding.
Studies on nitracrine and its nitro isomers devoted to tautomeric phenomena, structural and physicochemical features, as well as surrounding electrostatic potential
Authors: Rak Janusz , Byażejowski Jerzy , Zauhar Randy J.
Published: May 2006

Relative stabilities and features of tautomers of N,N-dimethyl-N′-(1-nitro-9-acridinyl)-1,3-propanediamine (known as nitracrine by the World Health Organization (WHO) or as Ledakrin in Poland), which exhibits antitumour activity, and its three nitro isomers were examined by AM1 and, in some cases, by PM3 and ab initio (STO-3G) methods. The lowest energy form of three compounds was always an imino tautomer, while the energy of the amino tautomers was a few kcal mol−1 higher and that of the aci forms (in the case of the 2- and 4-nitro isomers) was more than 25 kcal mol−1 higher. Each tautomer can further exist in several structural forms (conformers or stereoisomers). In some of them internal hydrogen bonding interactions are feasible. The barriers to rotation of the -NHR group, as well as to inversion of the -NHR group (in amino tautomers of nitracrine and its 2-nitro isomer) and of the whole molecule (in the case of non-planar imino structures), were also evaluated, as were enthalpies of formation (total energies), entropies, free energies, dipole moments, and energies of HOMO and LUMO orbitals. Molecular electrostatic potential maps reveal that only in nitracrine are both -NO2 and -NHR groups surrounded by a negative electrostatic potential, whereas in other isomers the -NHR fragment is surrounded by a positive electrostatic potential while the -NO2 group remains in the region of a negative electrostatic potential. The possible influence of physicochemical features on the biological activity of the molecules is also considered.
Formation of 2,2-disubstituted 1,3-cyclopentanediones from ketals with 1,2-bis(trimethylsilyloxy)cyclobutene
Authors: Wu Yong-Jin , Strickland Dean W. , Jenkins Tracy J. , Liu Pei-Ying , Burnell D. Jean
Published: May 2006

The direct formation of 2,2-disubstituted 1,3-cyclopentanedione compounds by a Lewis acid catalysed reaction with 1,2-bis(trimethylsilyloxy)cyclobutene proceeds in good to excellent yields with unhindered ketals, but steric hindrance reduces the yields considerably. A carbonyl group α or β to the ketal, or a carbon–carbon double bond α to the ketal, stops the reaction completely. Orthoesters do not give geminally acylated products in synthetically useful yields.
Fourier transform infrared spectroscopy studies of cyanide ion solutions of dimethylformamide and aqueous dimethylformamide
Authors: Jobe David J. , Westaway Kenneth Charles
Published: May 2006

Fourier transform infrared spectroscopy has been used to investigate the behaviour of HCN and alkali metal and tetraethylammonium cyanides in DMF and aqueous DMF. The cyanide ion exists as free ions and ion pairs in pure DMF but as water-solvated and DMF-solvated ions in aqueous DMF. The aqueous DMF solutions can be treated as a pseudo two-phase system with a distribution coefficient of 0.018 between the aqueous and DMF-rich pseudo phases. A kinetic analysis of the cyanide ion – benzyl chloride reaction in DMF and aqueous DMF has shown that the rate constant for the SN2 reaction is markedly decreased with the addition of as little as 2.5% water. This decrease is primarily due to changes in transition state solvation. The SN2 reactions between cyanide ion and benzylmethyl-4-substituted phenylsulphonium perchlorates in 20% aqueous DMF have also been investigated.
Étude thermodynamique des trois isomères de l'acide hydroxybenzoïque
Authors: Sabbah Raphaël , Le Thi Huy Duc
Published: May 2006

A thermodynamic study of the three hydroxybenzoic acids was carried out by combustion and sublimation calorimetry, heat capacity measurements, and differential thermal analysis. The experimental results (in kJ mol−1) are summarized as:[Formula: see text]From these experimental results, it was possible to determine for the three isomers (i) the resonance energies. From their comparison, the ortho isomer seems to be the most stable. This result is discussed using a structural consideration; (ii) the enthalpies of atomization. These values are in good agreement with that calculated using a contribution method.
Order–disorder transitions in adamantane derivatives: vibrational spectroscopic and 13C NMR studies of 1-chloroadamantane
Authors: Huang Yining , Paroli Ralph M. , Gilson D.F.R. , Butler I.S.
Published: May 2006

The order–disorder behaviour of 1-chloroadamantane (1-C10H15Cl) has been investigated by differential scanning calorimetry, and variable-temperature vibrational and 13C NMR spectroscopy. The factor group splittings in the vibrational spectra are in accord with the known crystal structures of the two phases. The 13C spin-lattice relaxation times and dipolar dephasing measurements have been analysed to give the barriers to rotation in both phases and to determine the nature of the rotations in each phase. In the ordered phase, the motion is limited to rotation about the molecular axis. In the disordered phase, additional motions occur about axes through the tertiary carbon atoms.
Inhibitors of endo-α-mannosidase. Part II. 1-Deoxy-3-O-(α-D-glucopyranosyl)-mannojirimycin and congeners modified in the mannojirimycin unit
Authors: Spohr Ulrike , Bach Mimi , Spiro Robert G.
Published: May 2006

The syntheses of 1-deoxy-3-O-(α-D-glucopyranosyl)-mannojirimycin (9) and its 2-deoxy, 2-O-methyl, 4-deoxy, 4-O-methyl, 6-deoxy, 6-O-methyl, N-methyl, and N-propyl congeners are described. Since 9 was previously shown to effectively inhibit endo-α-D-mannosidase, a glycoprotein-processing hydrolase, these chemical modifications were designed to assist in the assessment of intermolecular hydrogen bonds of the inhibitor–enzyme complex. The previously reported data require that all hydroxyl groups of the deoxymannojirimycin unit of 9, namely, OH-2, OH-4, OH-6, and also the NH-5 group, interact with charged and polar groupings of the enzyme, since deoxygenations and alkylations abolished or significantly reduced activities. Conformational analysis of 9 and some of its congeners based on NMR chemical shifts, experimental and theoretical nuclear Overhauser enhancements, and HSEA calculations were performed. The chemical modifications of the glucose unit of 9 are described in the accompanying paper.
Metal-thiolate chemistry: The structure of the first adamantane-type cation, [(μ-SPh)6(HgPPh3)4]2+
Authors: Vittal JAGADESE J. , Dean Philip A.W. , Payne Nicholas C.
Published: May 2006

The cosmpound [(μ-SC6H5)6{HgP(C6H5)3}4][ClO4]2 crystallizes in the triclinic space group [Formula: see text] as a bis(chloroform) solvate with cell dimensions a = 15.799(4), b = 26.813(5), c = 15.040(5) Å, α = 96.35(2), β = 114.52(2), γ = 100.45(2)° and two formula units per cell. The crystal and molecular structures of this salt were solved by X-ray diffraction techniques using 9898 counter data with I > 2.5σ(I) recorded at −50(2) °C. Refinement of 477 variables by full-matrix least-squares converged at an agreement factor R = 0.046. The cation has adamantane-type stereochemistry with a (μ-SPh)6Hg4 cage and four terminal triphenylphosphine ligands. The S—Cα vectors of the bridging SC6H5 groups adopt the configuration [aae, aae, aee, aee] with the minimum number of two 1,3-axial, axial substituent interactions. There is a significant flattening of the two Hg3S3 rings with two axial substituents into an envelope conformation; each has two torsion angles less than 25°, instead of the ideal values of ±60° in adamantane. One perchlorate ion is doubly solvated by chloroform molecules in an unusual, weakly hydrogen bonded, [ClO4•2HCCl3]− complex.
A simple apparatus for the study of experimental fractal structures in gaseous dielectric breakdown
Authors: Logan J. , Treurniet J. , Bawagan A.D.O. , Rizzetto S. , Braun J.M. , Aguda B.D.
Published: May 2006

An apparatus for the study of the discharge patterns generated in gaseous dielectric breakdown is described. The discharge patterns are imaged using a fast optical imaging system (≈ 40 ns exposure time) and are found to be fractals. The experimental fractal dimension obtained using a +17 kV pulse in air at atmospheric pressure is Dexpt ≈ 1.8 ± 0.1, which is in very good agreement with previous studies.
Synthesis and physicochemical properties of bolaamphiphiles from dimeric acid
Authors: Bosch Ma Pilar , Parra José Luis , Sánchez-Baeza Francisco
Published: May 2006

The physicochemical behaviour of bolaamphiphile derivatives with an anionic, a cationic, or a non-ionic polar group and their mixtures is studied mainly by surface tension measurements, quasi-elastic light-scattering studies, 1H NMR spectroscopy, and surface monolayers. Bolaamphiphiles are synthesized by classical methods from a non-expensive mixture of α,ω-dicarboxylic acids, obtained by thermal Diels–Alder type cyclization of methyl 10-E,12-E linoleate, obtained from tall oil. The anionic and cationic bolaamphiphiles synthesized are water soluble and form stable monolayers when they are compressed. The anionic and the cationic bolaamphiphiles with an iodide atom as counter-anion show a behaviour between liquid-condensed and liquid-expanded. The other synthesized cationic bolaamphiphiles and the (1:1) mixtures of anionic–cationic bolaamphiphiles behave as liquid-condensed.
Rotamers of dimethyl(acetyl)(η5-cyclopentadienyl)platinum(IV) in the gas phase
Authors: Jennings M.C. , Puddephatt R.J.
Published: May 2006

The platinum(IV) complex, [CpPt(CH3)2(COCH3)], was examimed by IR spectroscopy in the gas phase and found to exist as an equilibrium mixture of two rotational conformers. The equilibrium was examined as a function of temperature by using gas phase IR and the energy difference between the rotamers was determined. Molecular modelling studies were also carried out and are consistent with a model in which the conformers correspond to different rotational positions of the acetyl group.
Optimization of pyrolysis–desorption chemical ionization mass spectrometry and tandem mass spectrometry of polysaccharides
Authors: Helleur R.J. , Thibault Pierre
Published: May 2006

The operating conditions for pyrolysis–desorption ammonia chemical ionization mass spectrometry and tandem mass spectrometry have been optimized and the technique evaluated for the production and analysis of structurally-informative pyrolytic fragmentation ions corresponding to intact anhydrohexose oligosaccharides, using amylose as the model polysaccharide. Among the various parameters examined it was found that the nature of the solvent used to adhere the sample to the emitter coil and the configuration of the emitter and the rate at which it is heated all play important roles in determining the efficiency of the pyrolytic process and the production of high mass fragment ions. Adjustment of reagent gas pressure together with source temperature also influence the chemical integrity of high mass oligomeric pyrolysis products. Under optimal operating conditions using ammonia reagent gas, the analyses of cellulose, laminarin, agars, and chitin gave relatively abundant ions corresponding to ammonium (or protonated) adducts of up to anhydrohexose tetrasaccharide. More importantly, the generation of such higher mass fragment ions provided a sustained ionic current of sufficient duration to perform tandem mass spectrometric analyses.
Chemistry of the shellfish toxin domoic acid: characterization of related compounds
Authors: Walter John A. , Falk Michael , Wright Jeffrey L.C.
Published: May 2006

Five new compounds related to the neurotoxin domoic acid have been characterized by spectroscopic methods. They include a diastereoisomer that occurs naturally with domoic acid in mussels and four hydrogenation products. The stereochemistry of the two epimeric tetrahydro derivatives could not be assigned on the basis of the NMR data alone because of high conformational flexibility. However, a clear-cut assignment was possible by molecular modelling in combination with the NMR data.
Dynamics of the reaction of nascent ethynyl with hydrogen
Authors: Tasaki Shintaro , Satyapal Sunita , Bersohn Richard
Published: May 2006

At room temperature ethynyl radicals, C2H, react only slowly with hydrogen molecules in spite of the fact that the reaction C2H + H2 → C2H2 + H is exothermic by 26 kcal/mol. Nascent C2H (C2D) radicals produced by photodissociation of C2H2(C2D2) at 193 nm are shown to be as reactive with D2 (H2) as fluorine atoms with rate constants of (2.3 ± 0.4) × 10−11 and (3.2 ± 0.5) × 10−11 cm3 molecule−1 s−1 respectively. The H and D atoms have average kinetic energies of 7.6 ± 0.7 and 7.8 ± 1.0 kcal/mol, respectively. About 70% of the available energy is therefore released as internal energy. The enhanced reactivity of the nascent ethynyl is ascribed to partial mixing of the vibrationally excited X state with the A state. This reactivity is rapidly lost in collisions with N2. Overall the reaction of excited C2H with H2 is shown to be remarkably similar to the reaction of F atoms with H2.
Trajectory study of the effect of energy barrier location on the energy disposal for the reaction O(3P) + Cl2 → OCl + Cl
Authors: Kosmas Agnie M.
Published: May 2006

The mode of partitioning of total energy available in the reaction O(3P) + Cl2 → OCl + Cl is investigated as a function of the initial collision energy, in correlation with Polanyi's adiabaticity rules, using a series of related LEPS-type potential energy functions. All surfaces possess a classical energy barrier of similar height located in the entrance valley of the reaction, but present a small, gradual displacement of the top of the barrier to later positions along the approach coordinate. Quasiclassical trajectory calculations on these surfaces indicate marked differences in the disposal of total energy and the dynamical behaviour of the reaction. Also the effect of excess reagent vibration is investigated.
Near ultraviolet photolysis of phosphine studied by H atom photofragment translational spectroscopy
Authors: Lambert Ian R. , Morley Gregory P. , Mordaunt David H. , Ashfold Michael N.R. , Dixon Richard N.
Published: May 2006

We report time-of-flight measurements of the H atom fragments arising in the photodissociation of phosphine at three near ultraviolet excitation wavelengths—218.2 nm, 221.6 nm, and 228.0 nm. For these wavelengths, energetic considerations dictate that ground state [Formula: see text] fragments must be the partner product of the primary photolysis event. The measured H atom kinetic energy distributions indicate that internal excitation of these partner fragments accounts for the major part [Formula: see text] of the available energy, in excess of that required to break the H—PH2 bond, provided by the photon. Inspection of the poorly resolved structure evident near the peak of the H atom kinetic energy distribution suggests the presence of substantial bending excitation and a-axis rotational excitation in the PH2 partner fragment. The H atom kinetic energy distribution also shows clear evidence of atoms arising from secondary photolysis of the primary [Formula: see text] fragments. This secondary photo-dissociation is observed to show a marked anisotropy, consistent with a perpendicular photo-excitation of the [Formula: see text] fragment. Arguments are presented which indicate that the secondary photolysis leads both to the formation of PH(X) fragments, carrying high levels of rotational excitation, and to the three-body atomisation process.
Crystal chemistry of tetraradial species. Part 4. Hydrogen bonding to aromatic π systems: crystal structures of fifteen tetraphenylborates with organic ammonium cations
Authors: Bakshi Pradip K. , Linden Antony , Vincent Beverly R. , Roe Stephen P. , Adhikesavalu D. , Cameron T. Stanley , Knop Osvald
Published: May 2006

The aim of this investigation is to provide a classification and examples of N—H …π (and also O—H …π) bonds to the aromatic π systems in organic ammonium tetraphenylborates that would serve as reference for X—H …π(arene) bonds in general. To this end the crystal structures of the tetraphenylborates of the following cations have been determined: Me3NH+, Et3NH+, quinuclidinium, DabcoH+, Et(iso-Pr)2NH+ (monohydrate), (Ph3B)NH[—(CH2)2—]2NHMe+ (Me2CO solvate), Me2NH2+ (MeCN and Et2CO solvates), Et2NH2+, (iso-Pr)2NH2+, azoniacycloheptane, guanidinium (monohydrate), MeNH3+, EtNH3+, and 1-adamantammonium (monohydrate). These structures contain a variety of normal, bifurcated, and trifurcated N—H …π bonds as well as normal O—H …π bonds to the phenyl groups of the anion. The X—H …π bonds will form whenever opportunity arises, even though the result may be unfavourable bonding geometry. Branched bonds and orientational disorder represent compromise solutions in situations where the H(X) hydrogens are presented with competing phenyl acceptors or where the general organization of the crystal structure offers unfavourable bonding conditions to these hydrogens. The distributions of the distances from X or H(X) to the centre of the phenyl-ring skeleton are analyzed in detail, as are also those of the mean X … C and H(X)… C distances to the ring carbons.
LATTICE FOKKER–PLANCK EQUATION
Authors: SUCCI S. , MELCHIONNA S. , HANSEN J.-P.
Published: May 2006

A lattice version of the Fokker–Planck equation is introduced. The resulting numerical method is illustrated through the calculation of the electric conductivity of a one-dimensional charged fluid at zero and finite-temperature.
Image retrieval and perceptual similarity
Authors: Neumann Dirk , Gegenfurtner Karl R.
Published: May 2006

Simple, low-level visual features are extensively used for content-based image retrieval. Our goal was to evaluate an image-indexing system based on some of the known properties of the early stages of human vision. We quantitatively measured the relationship between the similarity order induced by the indexes and perceived similarity. In contrast to previous evaluation approaches, we objectively measured similarity both for the few best-matching images and also for relatively distinct images. The results show that, to a large degree, the rank orders induced by the indexes predict the perceived similarity between images. The highest index concordance employing a single index was obtained using the chromaticity histogram. Combining different information sources substantially improved the correspondence with the observers. We conclude that image-indexing systems can provide useful measures for perceptual image similarity. The methods presented here can be used to evaluate and compare different image-retrieval systems.
Object prefetching using semantic links
Authors: Pons Alexander P.
Published: May 2006

To date the most common means of gaining access to the Internet continues to be via dial-up modem connections. These slow communication channels significantly affect the rendering of the majority of web pages. Higher speed communications channels can alleviate rendering latency but based on the web page's content, delays still are incurred. The technique of web object prefetching can expedite the presentation of web pages by utilizing the current web page's view time to acquire the web objects of future web pages. The proposed Semantic Link Prefetcher utilizes information associated with the current web page's hyperlink set to predict which web objects to prefetch during the limited view time interval of the current web page. The results presented in this paper show that this proposed method can be effective in improving a web browser's cache-hit percentage, while significantly lowering web page rendering latency.
Managing availability
Authors: Wiberg Mikael , Whittaker Steve
Published: May 2006

Interruptions are a central aspect of working life. The prevalence of remote co-workers and the use of mobile technology mean that interruptions are more prevalent, and workers have to learn to manage availability. To understand general issues in availability management, we carried out a naturalistic study of how interruptions are handled in face-to-face situations. We found that availability management requires negotiation, that it is also highly dependent on awareness about the availability of others, and that it demands cognitive effort to shift attention to the interruption. On the basis of these observations, we developed a technology, named. The Negotiator, that embodies three main design requirements: (a) support for negotiation, (b) contextual information about when a recipient is available for a call, (c) lightweightness to reduce attention overhead. We carried out an experimental study of interruption management using this technology. The interface satisfied the original design requirements, that is, people, were able to use it effectively to negotiate times to talk, while successfully carrying out an intellectually demanding activity. Contrary to our expectations, however, people preferred to take responsibility for returning calls rather than delegating them, and they preferred to schedule calls as soon as possible rather than deferring them. We suggest that there are social reasons why people do this. They feel a social obligation to return calls as soon as possible so as not to inconvenience others and also to be responsible for making these calls themselves. They also take calls sooner to avoid having to remember future conversational commitments. We discuss the theoretical and technical implications of these findings.
Hierarchical RLE level set
Authors: Houston Ben , Nielsen Michael B. , Batty Christopher , Nilsson Ola , Museth Ken
Published: May 2006

This article introduces the Hierarchical Run-Length Encoded (H-RLE) Level Set data structure. This novel data structure combines the best features of the DT-Grid (of Nielsen and Museth [2004]) and the RLE Sparse Level Set (of Houston et al. [2004]) to provide both optimal efficiency and extreme versatility. In brief, the H-RLE level set employs an RLE in a dimensionally recursive fashion. The RLE scheme allows the compact storage of sequential nonnarrowband regions while the dimensionally recursive encoding along each axis efficiently compacts nonnarrowband planes and volumes. Consequently, this new structure can store and process level sets with effective voxel resolutions exceeding 5000 × 3000 × 3000 (45 billion voxels) on commodity PCs with only 1 GB of memory. This article, besides introducing the H-RLE level set data structure and its efficient core algorithms, also describes numerous applications that have benefited from our use of this structure: our unified implicit object representation, efficient and robust mesh to level set conversion, rapid ray tracing, level set metamorphosis, collision detection, and fully sparse fluid simulation (including RLE vector and matrix representations.) Our comparisons of the popular octree level set and Peng level set structures to the H-RLE level set indicate that the latter is superior in both narrowband sequential access speed and overall memory usage.
Language performance at high school and success in first year computer science
Authors: Rauchas Sarah , Rosman Benjamin , Konidaris George , Sanders Ian
Published: May 2006

We describe the first part of a study investigating the usefulness of high school language results as a predictor of success in first year computer science courses at a university where students have widely varying English language skills. Our results indicate that contrary to the generally accepted view that achievement in high school mathematics courses is the best individual predictor of success in undergraduate computer science, success in English at the first-language level in high school correlates better with actual performance. We discuss the implications of this for universities whose medium of teaching is English, operating in social contexts where many students are not native English speakers.