bluuebunny/crossref_metadata_2025 huggingface dataset:
Stereoelectronic effects in ring closure reactions: the 2′-hydroxychalcone – flavanone equilibrium, and related systems
Authors: Brennan Colin M. , Hunt Ian , Jarvis Terence C. , Johnson C. David , McDonnell Peter D.
Published: May 2006

The 2′-hydroxychalcone (2-HOC6H4COCH=CHC6H4X)–flavanone equilibrium in trifluoroacetic acid (TFA) has been examined. The influence of substituents X on the rate of attainment of equilibrium shows that the 6-endo-trig mode of ring closure by Michael addition is disallowed, by demonstrating a negative ρ value for the reaction rate when X is varied. Reaction therefore proceeds either on the carbonyl-protonated form, which allows twisting about the 2,3 bond, its double bond character being reduced by resonance, or through direct rate-limiting protonation on the 2,3 double bond. Either pathway permits the allowed 6-exo-trig mode of ring closure to be followed. Alternative mechanisms involving intermolecular Michael addition of trifluoroacetate, followed by intramolecular 6-exo-tet displacement are considered. Such Michael adducts can be detected in the ring closures of 2-crotonyl-4-methylphenol and 4,4-dimethyl-1-(2-hydroxyphenyl)-2-penten-1-one in TFA, but they do not appear to lie on the main pathway, because the reactions proceed with equal facility in methanesulphonic acid/chloroform medium, which does not contain a suitable nucleophile for such a mechanism. Further important mechanistic information is given by studying the reactions in TFA-d, together with measurements on the (E)-2-methyl-3-oxo-5-arylpent-4-en-2-ol and 3′-methoxychalcone systems. These isotope effect studies, which yield kH/kD values of about 3, indicate that the proton is in flight during the rate-limiting step, and provide evidence against the mechanism involving a preequilibrium carbonyl protonation, such as in the Nazarov rearrangement of 3′-methoxychalcones, where kH/kD is ca. 0.7. Some results are also reported for ring closure of the 2-aminochalcones in TFA. Keywords: 2′-hydroxychalcones, flavanones, 3′methoxychalcones, 3-aryl-6-methoxyindanones, 2′-aminochalcones, 2-aryl-1, 2, 3, 4-tetrahydroquinolines, Baldwin's rules.
Hypoglucaemic triterpenoid saponins from Boussingaultiabaselloides
Authors: Espada Alfonso , Rodriguez Jaime , Villaverde M. Carmen , Riguera Ricardo
Published: May 2006

New saponins with hypoglucaemic activity (1–4) and momordin Ic (5) have been isolated from the methanolic extract of the leaves of Boussingaultiabaselloides. They were shown by spectroscopy and chemical transformation to be derivatives of 12,20(29)-dien-30-noroleanic acid. Boussingoside A1 (1) exhibits very strong hypoglucaemic activity in rats. Acid hydrolysis of the saponin 1 yielded five transformation products (6–10), though one of them (larreagenin A (6), which has been reported in the literature as a genuine plant metabolite) is considered to be an artifact formed by the acid treatment. Keywords: Boussingaultiabaselloides, Basellaceae, boussingosides, triterpenoid saponins, hypoglucaemic activity.
The metabolites of Talaromycesflavus: Part 1. Metabolites of the organic extracts
Authors: Ayer William A. , Racok Julie S.
Published: May 2006

The soil fungus Talaromycesflavus (Klöcker) Stolk and Samson is an effective biological control for Verticillium wilt of eggplant (Solanummelongena L.), a disease caused by the fungus Verticilliumdahliae Kleb. The organic soluble metabolites produced when the fungus is grown in liquid still culture include D-glucono-1,4-lactone (1), 5-hydroxymethylfurfural (5), the previously unreported tetraketides, 4,6-dihydroxy-5-methylphthalide (3) and methyl 4-carboxy-5-hydroxyphthalaldehydate (8), a hexaketide, 7-hydroxy-2,5-dimethylchromone (6), and a metabolite believed to be 3-hydroxymethyl-6,8-dimethoxycoumarin (10). A number of higher order polyketides have been isolated including altenusin (11), dehydroaltenusin (12), and the previously unreported desmethyldehydroaltenusin (14). A structurally and biosynthetically interesting spirocyclic metabolite, which we have named talaroflavone (16), as well as a related compound, deoxytalaroflavone (18), have also been obtained. Keywords: Talaromycesflavus, fungal metabolites, talaroflavone, 4,6-dihydroxy-5-methylphthalide.
Shelf-life prediction from induction period calorimetric measurements on materials undergoing autocatalytic decomposition
Authors: Hansen Lee D. , Eatough Delbert J. , Lewis Edwin A. , Bergstrom Robert G. , Degraft-Johnson Damaris , Cassidy-Thompson Karen
Published: May 2006

The shelf- or use-life of a material which decomposes by an autocatalytic reaction is shown to be inversely proportional to the rate of heat generation during the induction period of the degradation reaction. Calorimetric measurement of the rate of heat generation during the induction period takes only a few hours. It can often be done at or very near the actual storage or use temperature of the material. Measurements on Lovastatin (2-methylbutanoic acid 1,2,3,7,8,8a-hexahydro-3,7-dimethyl-8-[2-(tetrahydro-4-hydroxy-6-oxo-2H-pyran-2-yl)ethyl]-1-naphthalenylester) are used as an example of this application. Keywords: autocatalytic, calorimetry, oxidation, decomposition, Lovastatin, Mevacor.
Kinetics of self-exchange of bis(1,4,7-trithiacyclononane)iron(III)/(II) in acetonitrile and in acidic aqueous solution
Authors: Doine Hideo , Swaddle Thomas W.
Published: May 2006

The rate constant kex of the [Formula: see text] self-exchange reaction cannot be measured in most common solvents because of spontaneous reduction of the [Formula: see text] ion, which is also sensitive to photolysis by visible light. However, in CD3CN at −41 to −19 °C, reproducible proton-decoupled 13C NMR line broadening measurements are possible, and give kex = (5.3 ± 0.3) × 104 kg mol−1s−1 at 0 °C, ΔH* = 10.3 ± 1.8 kJ mol−1, and ΔS* = −116 ± 7 J K−1 mol−1, at ionic strength I = 0.1 mol kg−1. Proton NMR line broadening experiments are marginally practicable in very acidic water (2.0 mol kg−1 D2SO4/D2O) near 0 °C, and give kex = 3.2 × 106 kg mol−1 s−1 at 1 °C. The relative kex values of these and other low-spin/low-spin FeIII/II self-exchange reactions follow the predictions of the Marcus–Hush theory at least qualitatively. The effect of ionic strength, however, is less than predicted, probably because of the formation of less reactive anion–cation pairs. Keywords: electron transfer kinetics, crown thioether complexes.
Hydrolysis of aluminum(III) ion in sodium nitrate medium
Authors: Milić Nikola B. , Bugarčić Živadin D. , Djurdjević Predrag T.
Published: May 2006

Hydrolysis of aluminum(III) ion in 3.0 M (Na)NO3 medium was studied by emf method at 25° C. Data cover the range 1–78 mM Al(III) within the pH range of 2.0–4.5. At a definite pH, the extent of hydrolysis increases with increasing aluminium concentration, thereby indicating the presence of polynuclear hydrolysis products. The least squares treatment of the data indicates the formation of polynuclear hydrolytic complexes [Formula: see text] [Formula: see text] and [Formula: see text] With the reactions written in the form[Formula: see text]the stability constants obtained were log (β2,2 = −7.55 ± 0.21, log β2,4 = −16.41 ± 0.13, and log (β3,4 = −13.24 ± 0.08. Distribution curves indicate that the complex [Formula: see text] is predominant. Key words: hydrolysis, aluminum(III) ion, hydrolytic complexes, aluminum(III) ion hydrolysis.
Luminescence of excited fragments produced from methyl halides in noble gas flowing afterglow
Authors: Gu Yuezhu , Guo Jingzhong , Liu Chuanpu , Yin Yongjia , Cao Dezhao , Cai Jiye
Published: May 2006

The reaction of metastable He(23S) and Ne(3P0,2) atoms with methy halides has been investigated by flowing afterglow technique. Emission spectra have been observed and formation rate constants of some fragments have been measured. The rotational and vibrational temperatures of CH(A2Δ) were also estimated. Key words: flowing afterglow, methyl halides, metastable atoms.
Cyclopropylketenes: preparation and nucleophilic additions
Authors: Allen Annette D. , Baigrie Lynn M. , Gong Leyi , Tidwell Thomas T.
Published: May 2006

Phenylcyclopropylketene (4), tert-butylcyclopropylketene (5), and dicyclopropylketene (6) were formed by dehydrochlorination of the corresponding acyl chlorides by Et3N in THF, and are the first cyclopropylketenes to be isolated and purified. Reaction of 4 with n-BuLi and capture of the intermediate enolates with Me3SiCl gave the stereoisomeric silyl enol ethers c-PrCPh=C(OSiMe3)-n-Bu with a 79:21 preference for formation of the Z isomer resulting from nucleophilic attack syn to cyclopropyl, whereas the corresponding reaction of t-BuLi gave a 9:91 preference for attack anti to cyclopropyl. Some isopropyl-, cyclopentyl-, and cyclohexylketenes gave comparable results. Analyses of the relative sizes of the ketene substituents in the ground state by steric parameters, and of the product stabilities by molecular mechanics, both fail to predict the observed similarities in the results with different secondary alkyl groups. The hydration reactivities of 4 and 6 show that, in neutral H2O/CH3CN, c-PrCPh=C=O is more reactive than i-PrCPh=C=O, a result ascribed as mainly due to the smaller size of cyclopropyl. c-Pr2C=C=O has the same reactivity in neutral water as Et2C=C=O, but is 22 times less reactive with acid, a result attributed to the inability of the β-cyclopropyl groups to directly stabilize the cationic transition state for protonation. Key words: cyclopropylketenes, ketenes, nucleophilic addition, hydration kinetics.
An ab initio study of the structure and intramolecular proton transfer in tropolone
Authors: Ríos Miguel A. , Rodríguez Jesús
Published: May 2006

An ab initio study of tropolone was carried out at the 3-21G level, with full optimization of the geometry, and the intramolecular proton transfer in the ground state was analysed. The two equivalent Cs structures can be converted to each other via a C2v symmetry transition state with a double minimum potential. The splitting of the ground vibrational state generated by proton tunneling was determined by procedures of variable complexity and found to be in the range Δ = 0.1–0.15 cm−1, i.e., clearly smaller than experimental predictions. Key words: tropolone, intramolecular hydrogen bond, proton transfer, ab initio calculations, molecular structure.
Study of the protonation of simple Schiff bases in solvents of various polarity by means of Raman spectroscopy
Authors: Alex S. , Turcotte P. , Fournier R. , Vocelle D.
Published: May 2006

In this study, the protonation of simple Schiff bases by organic acids in various solvents is investigated by Raman spectroscopy and the spectral response of the C=N+ stretching mode is correlated with the macroscopic properties of the medium surrounding the imine. Upon protonation, the unperturbed C=N stretch increases in frequency due to the combination of the coupling with the C=N+H bend and a partial rehybridization of the C=N bond. Depending on the proton location relative to that of its counterion, one mechanism may prevail over the other. Intensity measurements show that protonation is mostly controlled by the hydrogen bonding properties of the solvent and by the polarity of the medium. However, whatever the solvent, full proton transfer cannot be achieved with organic acids with pKa similar to the COOH of amino acids found in visual or bacterial pigments, meaning that the protonation mechanism may depend upon particular properties of the protein. Key words: imines, protonation, Raman spectroscopy, rhodopsin, Schiff base.
A microstratigraphic investigation of iron ion migration in some ancient Egyptian wall paintings using PIXE Analysis
Authors: Burns George , Wilson-Yang K.M.
Published: May 2006

The painted murals in the tombs at Beni Hasan (circa 2100 B.C.) are obscured by a rock-like grey surface layer. Its formation involves subsurface removal of slightly soluble calcite (CaCO3) under the action of surface-condensed water. To study the mechanism of surface layer formation and the effect of mural cleaning during recent restorations, the PIXE microprobe was used. Iron depth profiles in red-painted samples taken from the murals, both uncleaned and cleaned, have been obtained over depths of 80 μm (the cleaned sample) and 100 μm (the uncleaned sample). It has been found that the uncleaned sample has more Fe at all depths than does the cleaned sample and that portions of the uncleaned pigment appear to be pure Fe2O3. The shapes of depth profiles of Fe in the cleaned and uncleaned samples are similar to the shapes of diffusion curves; this finding is in agreement with the mechanism of surface layer formation proposed in our earlier papers. Apparent diffusion coefficients for the ion migration of Fe, DFe, in the limestone painting substrate have been determined, assuming constant temperature diffusion over 4 000 years: DFe(u) = (1.4 ± 0.5) × 10−16 cm2/s for the uncleaned sample and DFe(c) = (0.7 ± 0.4) × 10−16 cm2/s for the cleaned sample, where errors are given as the 95% C.I. based on standard deviations of the slope of the fitted curve. These coefficients are consistent with the mechanism of slow migration of Fe in the painting substrate in an arid environment. The difference between DFe values secured for the cleaned and uncleaned samples is attributed to a partial removal of the paint layer in the cleaned sample during the restoration efforts. Thus, if it is assumed that either 20 or 40 μm of paint layer was removed during HCl-cleaning of murals, DFe(c) = (1.0 ± 0.4) × 10−16 and DFe(c) = (1.4 ± 0.6) × 10−16 cm2/s, respectively, are obtained. Removal of the top 40 μm of the mural corresponds to elimination, during the restoration, of the order of 60% of the uncleaned paint layer. Key words: Egypt, PIXE, ancient murals deterioration.
Monohalogénocyclopropanation des esters et des acides α-éthyléniques diversement alkylés. Activation et stéréosélection antithermodynamique
Authors: Barlet Roger , Baharmast Bahman , Vidal Michel
Published: May 2006

The halogenocyclopropanation of α,β-unsaturated acids and their esters through a reaction with dichloromethane and MeLi/LiBr proceeds with excellent yields. With the esters, the only halogenocyclopropanic adducts obtained are the hydroxylated ones while the acids lead to carbonylated adducts as major products, even when there is an excess of MeLi. With the acids, the cyclopropanation takes place selectively with a lithiated hydrate of ketone type intermediate favouring the formation of the ring. The stability of the primary product of the cycloaddition is such that it does not react massively with the methyllithium and that the halogenocyclopropane ketones can be isolated after hydrolysis. With the esters, the reaction is also activated by the primary formation of an hemiacetal; however, the cycloaddition is more equilibrated between this intermediate and the alkoxide which results from the later action of the MeLi. In both cases, a net syn stereoselectivity is observed in the unsaturated alcohols in spite of the opposite steric and stereoelectronic effects operating when the alkyl and functional groups are in a cis relation around the double bond. Key words: halogenocyclopropanation, activation, syn stereoselectivity. [Journal translation]
Stereospecific synthesis, 1H and 13C NMR spectroscopy, and X-ray crystallographic studies of 6,6-dibromo-3α-cyano-2,2-dimethylpenam-(1R)-S-oxide
Authors: Salomon Claudio J. , Mascaretti Oreste A. , Strouse Charles E. , Punte G.
Published: May 2006

A stereospecific synthesis of 6,6-dibromo-3α-cyano-2,2-dimethylpenam-(1R)-S-oxide (2) is described. The X-ray single crystal investigation of this compound shows that it crystallizes in the monoclinic system, space group P21, with cell parameters a = 6.194(1), b = 9.363(1), c = 9.869(1) Å, β = 96.55(2)°, V = 568(1) Å3, and two molecules per unit cell. Thestructure was solved, from low temperature data, by direct methods, and refined, by least-squares procedures, to a final R = 3.17%, Rw = 3.38%, for 1221 observed independent reflections. The sulfoxide configuration is R. The thiazolidine presents an envelope shape with the C(3) atom 0.56 Å above the other four members of the ring mean plane. Therefore, it exhibits the conformation with α-CH3 pseudoequatorial and β-CH3 and α-CN both pseudoaxial. This conformation is different from the one reported for other penicillin sulfoxide derivatives. The geometry of the β-lactam ring is typical of the fused bicyclic penam nucleus. The ring is non-planar. The N(4) atom displays a pyramidial arrangement. The examination of crystal packing shows that molecules along a are connected by a short C—H … O contactReported single crystal studies showed two dominant conformations for the thiazolidine ring in the fused bicyclic penam nucleus: the C(3) or 2β-3α substituents both pseudoaxial, as found in 2, and the S(1) or 2β-3α substituents both pseudoequatorial, adopted by most sulfoxides. Neither of these conformations can explain the nuclear Overhauser effect data. A conformation with the substituents C2-β-methyl-C3-α-H and C2-α-methyl-C3-α-CN fully eclipsed is in accordance with the experimental results in the solution. The 1H nuclear magnetic resonance data reflect the influence of the magnetic anisotropy of the CN triple bond on the gem-dimethyl groups, particularly in relation with the geometrical features of the molecule. Comparison of 13C nuclear magnetic resonance chemical shifts for 2 and its parent penicillin sulfide (1) indicates configurationally dependent substituent effects, the magnitude of which can be used for the stereochemical assignment of sulfoxide groups in this series. Key words: penicillin sulfoxide derivatives, di-halogen penicillanates, stereospecific synthesis, NMR studies, absolute configuration determination.
The effect of meta- and para-methoxy substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry. Part 26
Authors: Arnold Donald R. , Du Xinyao , Henseleit Kerstin M.
Published: May 2006

The effect of meta- and para-methoxy substitution on the reactivity of some radical cations has been determined. The compounds chosen for study were 1-(3-methoxyphenyl)-1-phenylethylene (7), 1-(4-methoxyphenyl)-1-phenylethylene (8), 3-(3-methoxyphenyl)indene (9), 3-(4-methoxyphenyl)indene (10), methyl 2-(3-methoxyphenyl)-2-phenylethyl ether (11), methyl 2-(4-methoxyphenyl)-2-phenylethyl ether (12), cis- and trans-2-methoxy-1-(3-methoxyphenyl)indane (13), and cis- and trans-2-methoxy-1-(4-methoxyphenyl)indane (14). The radical cations of these compounds were generated by photosensitization (electron transfer) using 1,4-dicyanobenzene (3) as the electron acceptor. The three reactions studied were: (1) The addition of nucleophiles (methanol) to the radical cation of the arylalkenes, a reaction that yields the anti-Markovnikov addition product. (2) The carbon–carbon bond cleavage of radical cations, which yields products derived from the radical and carbocation fragments. (3) The deprotonation of the radical cation, a reaction that can be used to invert the configuration at a saturated carbon centre. The mechanisms of these reactions are discussed and the factors that need to be considered in order to predict reactivity are defined. Molecular orbital calculations (UHF/STO-3G) were carried out on the radical cations of the model compounds 3- and 4-vinylanisole and 3- and 4-methylanisole. Key words: photochemistry, photosensitize (electron transfer), radical cation, radical.
13C nuclear magnetic resonance spectra of 3,6-disubstituted pyridazines
Authors: Heinisch Gottfried , Holzer Wolfgang
Published: May 2006

The 13C nuclear magnetic resonance spectra of 17 3,6-disubstituted pyridazine derivatives have been systematically analyzed. Chemical shifts and various 13C, 1H coupling constants are reported. Attempts were made to correlate these data with results obtained from semiempirical molecular orbital calculations as well as with substituent electronegativities and Taft's substituent constants σI and σR0. Key words: 3,6-disubstituted pyridazines, 13C NMR spectroscopy, 13C, 1H spin coupling constants.
Bipyridylacetylenes 1: the synthesis of some bipyridylacetylenes via the palladium-catalyzed coupling of acetylenes with 2,2′-dibromobipyridyl, and the single crystal X-ray structure of 6,6′-bisphenylethynyl-2,2′-bipyridine
Authors: Butler Ian R. , Soucy-Breau Chantal
Published: May 2006

The synthesis of a series of 6,6′-diethynyl-2,2′-bipyridyls from the palladium-catalyzed coupling of 6,6′-dibromo-2,2′-bipyridine with a series of arylacetylenes is reported. The single crystal X-ray structure of 6,6′-phenylethynyl-2,2′-bipyridyl was determined. Crystal data: monoclinic, A2/a, with cell constants a = 10.0385(4), b = 9.2166(4), c = 21.4130(8) Å, and β = 102.981(3)°; Dcalc. = 1.226 g cm−3, Z = 4, Rf = 0.045, S = 1.22, using 886 reflections of the 1443 unique reflections. The packing diagram indicates essentially that the molecules pack in a layered assembly with the nitrogen atoms of the dipyridyl groups on opposite sides and with a dihedral angle of 59.85(16)° between the pyridyl and phenyl groups. The acetylenic bond distances are normal. Key words: acetylene, palladium, catalysis, coupling, monomers, crystal structure.
Synthesis of α-(arylideneamino)alkylamines
Authors: Katritzky Alan R. , Jiang Jinlong , Harris Philip A.
Published: May 2006

α-(Arylideneamino)alkylamines 4 are prepared in high yields by the reaction of 1-(benzotriazol-1-yl)-N-triphenylphosphorylidenemethylamine (betmip) 2 with lithium amides and treatment of the resulting intermediates 3 with aryl aldehydes. Key words: benzotriazole, lithium amides, aryl aldehydes.
Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state
Authors: Gravel D. , Giasson R. , Blanchet D. , Yip R. W. , Sharma D. K.
Published: May 2006

Two rigid nitrobenzene derivatives, 5-nitro-1,2,3,4-tetrahydro-1,4-methanonaphthalene (6), and 5-nitro-1,2,3,4-tetra-hydro-1,4-ethanonaphthalene (7), have been synthesized and studied by picosecond flash absorption spectroscopy, steady-state irradiation, and MMX molecular mechanics calculations. Transients with lifetimes of 770 and 410 ps have been detected and assigned to the excited triplet states of 6 and 7, respectively. Consistent with prediction, the o-quinonoid intermediate is not detected in the time domain between the laser excitation and the end of decay of the triplet state. Intramolecular abstraction of the bridgehead benzylic hydrogen by the cited triplet state proceeds with relative rates of 1:125 for 6 and 7, respectively. Since structural constraints prohibit the formation of an orthoquinonoid intermediate in these systems, formation of the usual nitrosoalcohol 13 represents the first substantiation of the biradical route to product (3′ → 4, Scheme 1). Absorption of the intermediate triplet biradical resulting from hydrogen abstraction by the triplet excited state was not observed, and is therefore believed to be either very weak in the 400–700 nm region, or located outside the detection region. The kinetic results together with structural parameters determined by molecular mechanics show good correlation between structure and reactivity. Hydrogen atom transfer can proceed at up to 53° deviation from linearity of the C—H—O nuclei, with transfer rates decreasing sharply as the [Formula: see text] distance increases from 1.3 to 1.6 Å. It is suggested that the conditions of the reaction from the excited triplet state meet those required for a hydrogen tunneling mechanism. Key words: o-nitrotoluene photorearrangement, mechanism of; o-nitrobenzyl systems, mechanism of photorearrangement; [Formula: see text] distance, influence of, in hydrogen abstraction mechanism; picosecond flash absorption spectroscopy study of o-nitrobenzyl photorearrangement; radical pathway to product in o-nitrotoluene photorearrangement.
Boric acid: a new regiospecific decarboxylating agent. Syntheses of cyclic imines, β-enaminones, and β-enaminodiketones from β-enaminoesters
Authors: Delbecq Philippe , Bacos Daniel , Celerier Jean Pierre , Lhommet Gérard
Published: May 2006

The synthesis of cyclic imines 2, β-enaminones 6, and β-enaminodiketones 7 is described. Regio- and stereospecific thermolysis of β-enaminoesters 4 with boric acid permit these preparations in generally good yields.Key words: boric acid, cyclic β-enaminoesters, decarboxylation, cyclic imines, cyclic β-enaminones.
Structural studies on the polysaccharide portion of "A-band" lipopolysaccharide from a mutant (AK1401) of Pseudomonas aeruginosa strain PAO1
Authors: Arsenault Todd L. , Hughes Donald W. , MacLean David B. , Szarek Walter A. , Kropinski Andrew M. B. , Lam Joseph S.
Published: May 2006

AK1401 is a mutant of Pseudomonas aeruginosa strain PAO1 (serotype 05) that does not express O-antigen, but does express "A-band" lipopolysaccharide (LPS). The polysaccharide portion of the A-band LPS (A-PS) from AK1401 was found to consist mainly of D-rhamnose, with smaller amounts of 3-O-methylrhamnose, ribose, mannose, glucose, and a 3-O-methylhexose. 1H nuclear magnetic resonance spectra of the intact A-PS indicated that the main structural feature was a repeating trisaccharide of α-D-rhamnose having the following structure:[Formula: see text]The 1H NMR spectrum of the repeating unit was completely assigned through the use of 2D shift-correlated and relayed coherence transfer NMR spectroscopy. The linkage positions and sequence of residues were found by nuclear Overhauser enhancement difference spectroscopy. Key words: Pseudomonas aeruginosa, lipopolysaccharide, 1H NMR.
Crystal and molecular structures of two polymorphs of 2,2-di(p-nitrophenyl)-1-picrylhydrazine dichloromethane, C18H11N7O10•CH2Cl2
Authors: Wang Hong , Barton Richard J. , Robertson Beverly E. , Weil John A. , Brown Keith C.
Published: May 2006

Two polymorphic crystal forms (yellow and orange) of 2,2-di(p-nitrophenyl)-1-picrylhydrazine were crystallized from the same dichloromethane solution, and their crystal structures determined. The yellow form is triclinic, P1, a = 9.055(3), b = 10.396(4), c = 13.3187(3) Å, α = 89.83(2), β = 79.89(3), γ = 70.83(3)° at 228 K; R = 0.060, and Rw = 0.057 for 3511 reflections with I > 2σI. The orange form is monoclinic, P21/c, a = 8.999(3), b = 25.041(11), c = 12.207(5) Å, β = 121.66(2)° at 223 K; R = 0.053, and Rw = 0.058 for 3651 reflections with I > 2σI. The molecular structures in the two crystal forms are very similar. Both forms contain one solvent molecule per molecule of 2,2-di(p-nitrophenyl)-1-picrylhydrazine. In each, the two hydrazine nitrogen atoms have strong resonance interactions with the vicinal aromatic groups and both are sp2 hybridized. The molecular structure around the N—N linkage shows a staggered conformation. Thus the resonance interaction between the picryl group and the other two p-nitrophenyl groups is negligible. The difference in color between the two crystal forms is attributed to a difference in molecular packing. Key words: crystal structure, polymorphism, picrylhydrazine, conformation.
Kadsulignan C and D, two novel lignans from Kadsura longipedunculata
Authors: Liu Jia-Sen , Huang Mei-Fen , Zhou Hong-Xing
Published: May 2006

Two novel lignans, kadsulignan C (4) and D (5), were isolated from the root bark of Kadsura longipedunculata. Their structures, including absolute configurations, were elucidated by 2D 1H–13C long-range COSY experiments and chemical transformation to the known compound 10. Key words: isolation and structure, absolute configuration, kadsulignan C, kadsulignan D, lignans, Kadsura longipedunculata.
Nucleic acid related compounds. 66. Improved syntheses of 5′-chloro-5′-deoxy- and 5′-S-aryl(or alkyl)-5′-thionucleosides
Authors: Robins Morris J. , Hansske Fritz , Wnuk Stanislaw F. , Kanai Tadashi
Published: May 2006

Treatment of ribonucleosides with thionyl chloride/pyridine/acetonitrile (0 °C to ambient temperature) resulted in essentially quantitative formation of 5′-chloro-5′-deoxy-2′,3′-O-sulfinylnucleoside derivatives. These diastereomeric sulfite esters underwent deprotection readily with aqueous methanolic ammonia. This gave 5′-chloro-5′-deoxynucleosides without use of the suspected carcinogen, hexamethylphosphoramide (HMPA). Nucleophilic displacement with sodium thiolates in dimethylformamide (−30 °C to ambient temperature) gave 5′-S-aryl(or alkyl)-5′-thionucleosides in high yields. Treatment of ribonucleosides with thionyl chloride/acetonitrile without pyridine followed by aqueous work-up gave diastereomeric 2′,3′-O-sulfinylnucleosides with an unmodified 5′-hydroxyl group. Diagnostic NMR shifts for cyclic sulfite ester stereochemistry are noted. Key words: adenosine, 5′-S-aryl(or alkyl)-5′-thionucleosides, 5′-chloro-5′-deoxynucleosides, uridine, nucleosides.
Structural and surface aspects of thermally treated copper aluminium mixed hydroxides
Authors: Ahmed Awad I. , Samra S. E. , El-Hakam S. A.
Published: May 2006

CuO–Al2O3 catalysts containing various amounts of copper oxide have been prepared by precipitation. The phase changes were studied by X-ray diffraction. The results obtained revealed that the thermal treatment of solid CuO–Al2O3 at 700 °C produced only crystalline CuO. Heating to 900 °C led to the formation of copper alumina spinel together with unreacted CuO and γ-Al2O3. The spinel content was found to increase with increasing copper content. Nitrogen adsorption–desorption isotherms on the calcined samples have been measured. Surface areas have been calculated and the pore structure analysed. The textural properties of the system were found to depend on both the copper content and the calcination temperature. Key words: CuO, Al2O3 catalysts, structure, surface area, pore structure.
Ternary mutual diffusion coefficients of MgCl2 + MgSO4 + H2O and Na2SO4 + MgSO4 + H2O from Taylor dispersion profiles
Authors: Deng Zhipeng , Leaist Derek G.
Published: May 2006

The Taylor dispersion (peak-broadening) technique is used to measure the ternary mutual diffusion coefficients of the mixed salt solutions MgCl2 + MgSO4 + H2O and Na2SO4 + MgSO4 + H2O at 25 °C. The dispersion of the solutes is followed by differential refractometry. A simple least-squares procedure is developed to calculate the ternary diffusion coefficients from the refractive index profiles. The coefficients are measured at 0.04, 0.20, or 0.40 mol dm−3 total salt for the molar salt ratios 1:3, 1:1, or 3:1. The measured diffusion coefficients are compared with the limiting values for the completely dissociated salts. Key words: mixed salt solutions, multicomponent diffusion, Taylor dispersion, ternary diffusion coefficients.
Energy transfer between Langmuir–Blodgett monolayers of organic dyes
Authors: Johnson E. , Aroca R.
Published: May 2006

The Langmuir–Blodgett technique has been used to prepare multilayer structures in order to investigate the distance dependence of the nonradiative transfer of electronic energy from a donor plane of molecules to an acceptor plane. The distance between well-separated donor molecules and acceptor molecules was carefully controlled by spacer layers of arachidic acid. New systems for energy transfer studies are considered that use N-hexyl-N′-ethyl-3,4:9,10-perylenetetracarboxyldiimide (HPTCDE) and N-hexyl-3,4:9,10-perylenetetracarboxylmonoimide (HPTCO) as donors and lutetium diphthalocyanine (LuPc2) as acceptor. The limitations of the Förster dipole theory of energy transfer from a donor monolayer of point dipoles to an acceptor layer are discussed. Key words: energy transfer, Langmuir–Blodgett, diphthalocyanine, perylene.
Conformational stability, barriers to internal rotation, ab initio calculations, and vibrational assignment of 2-butanone
Authors: Durig J. R. , Feng F. S. , Wang Aiying , Phan H. V.
Published: May 2006

The Raman (3200–10 cm−1) and infrared spectra (3200–30 cm−1) have been recorded for 2-butanone (ethyl methyl ketone), CH3CH2C(O)CH3, for the gas and solid. Additionally, the Raman spectrum and qualitative depolarization ratios have been obtained for the liquid. Evidence for the existence of two conformations in the gas and liquid is observed, with the trans form (the two CH3 moieties oriented trans about the central C—C bond) much more abundant than the high-energy form, presently identified tentatively to be the gauche rotamer. The trans form is the only form remaining in the spectrum of the solid. A very weak infrared band at 65 cm−1 has been assigned to the asymmetric torsion of the trans conformer. Using this frequency, ΔH, and the dihedral angle of the gauche rotamer, a potential function is proposed for the conformational interchange. Spectral features observed at 195.5 and 181.0 cm−1 in the far infrared spectrum of the gas have been assigned to the [Formula: see text] and [Formula: see text] of the methyl torsion of the ethyl group of the trans form, from which a barrier of 868 ± 3 cm−1 (2.48 ± 0.01 kcal/mol) is calculated. Similarly, the fundamental CH3—C(O) torsion has been observed at 94.5 cm−1 for the trans conformer from which a threefold barrier to internal rotation has been calculated to be 233 cm−1 (666 cal/mol). Utilizing the rotational constants for two isotopic species, which were reported in an earlier microwave study, the heavy atom r0 structural parameters have been determined. A complete vibrational assignment is proposed, which is supported with the results from a normal coordinate analysis performed with the force fields calculated with the 3-21G basis set. Full geometry optimizations have been carried out at the 6-31G* basis set for both the trans and gauche conformations as well as for the transition states to determine the structural parameters, energy difference, and barriers to internal rotation about the central C—C bond. Key words: 2-butanone, vibrational spectrum; ab initio calculations, 2-butanone; conformational stability, 2-butanone.
A comparison of acetone and poly(propylene glycol) as solvents for lithium triflate and lithium perchlorate
Authors: Stevens James R. , Jacobsson Per
Published: May 2006

Solutions of LiCF3SO3 and LiClO4 in acetone and in poly(propylene glycol) (PPG 400 and PPG 4000) have been compared by studying the nondegenerate, symmetric stretch (A1,SO3) and (A1,ClO4) Raman modes. The Raman spectra contain bands due to the symmetric stretching motion of the "free" anion and due to the symmetric stretching motion of anions in ion aggregates. It is concluded that "free" ions, ion pairs, triplets, and aggregates are present. Although PPG has a much lower dielectric constant, it is a better solvent for these salts than the dipolar aprotic acetone. These findings have ramifications on the current controversy of whether "free" ions are present at all in polyether–salt complexes such as PPG 4000/LiCF3SO3. Key words: acetone, poly(propylene glycol), lithium triflate, lithium perchlorate, Raman.
Solvation of the proton and the trifluoromethanesulfonate ion in eight solvents
Authors: Benoit Robert L. , Figeys Daniel
Published: May 2006

The enthalpies of solution of a sterically hindered aromatic diamine, 1,8-bis(dimethylamino)naphthalene (proton sponge, PS) and of its trifluoromethanesulfonate salt, and of trifluoromethanesulfonic acid have been determined in water, methanol (MeOH), ethanol (EtOH), pyridine (Py), dimethyl sulfoxide (Me2SO), dimethylformamide (DMF), acetonitrile (MeCN), and sulfolane (Sul). Single ion enthalpies of transfer for CF3SO3− and H+, from Me2SO to the other solvents, have been calculated using two extrathermodynamic assumptions, namely the tetraphenylarsonium–tetraphenylborate one and the recently proposed assumption based on the equality of transfers of uncharged PS and its conjugated cation PSH+. While both sets of transfer enthalpies using each assumption, for CF3SO3− and H+, differ by some 12 kJ mol−1 for water and MeOH, the values are in satisfactory agreement for Py, DMF, Me2SO, and Sul. These results support the validity of both extrathermodynamic assumptions for the nonprotogenic solvents, thus putting on firm ground the solvent basicity scale based on the enthalpy of transfer of H+ (in kJ mol−1): Py(−63) > DMF(−32) > Me2SO(−27) > EtOH(−1) ~ H2O(0) ~ MeOH(+2) > PC(+44) > MeCN(+55) > Sul(+68). These values correlate well with the enthalpies of complex formation with boron trifluoride. Key words: solvation, proton, trifluoromethanesulfonate ion, proton sponge, extrathermodynamic assumption.
Structural studies of organoboron compounds. XLVIII. 1,4-Diphenyl-3,3:6,6-bis(tetramethylene)-2,5,7-trioxa-3,6-diazonia-1,4-diboratabicyclo[2.2.1]heptane
Authors: Kliegel Wolfgang , Riebe Ulf , Rettig Steven J. , Trotter James
Published: May 2006

not available
On the role of the parent cation in the dynamics of formation of laser-induced hydrated electrons
Authors: Keszei Ernö , Jay-Gerin Jean-Paul
Published: May 2006

A short account is given of the continuum and molecular descriptions of the formation of hydrated electrons. Starting with a scheme of hydration dynamics of laser-induced electrons that accounts for the formation of hydrated electrons without invoking the "direct" ionization of water, a new model explaining the dynamics of electron hydration in terms of a molecular description is proposed. According to this model, the parent cation plays an active role in the trapping of electrons, deepening electron traps that preexist in the liquid before excitation. Consequences of this description to the transient absorption spectra are briefly discussed. Keywords: laser-induced electron – cation pairs, hydrated electron formation, liquid water.
The importance of conformation in the reactivity of radical cations. Changing configuration at saturated carbon centres
Authors: Perrott Allyson L. , Arnold Donald R.
Published: May 2006

Irradiation of an acetonitrile solution of cis 1-methyl-2-phenylcyclopentane (1bcis); 1,4-dicyanobenzene (2), an electron-accepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, leads to configurational isomerization of the cyclopentane; the photostationary state lies > 99% in favour of the trans isomer. The mechanism proposed for this reaction involves formation of the radical cation of 1bcis by photoinduced electron transfer to the singlet excited state of 2, deprotonation of the radical cation assisted by the base 3, reduction of the resulting benzylic radical by the radical anion [Formula: see text], and reprotonation of the benzylic anion to give both the cis and the trans isomers of 1b. The photostationary state is controlled by the relative rates of deprotonation of the radical cations of 1bcis and trans; these rates are dependent upon the extent of overlap of the SOMO of the radical cation, which is largely associated with the phenyl ring, and the benzylic carbon–hydrogen bond. Molecular mechanics calculations (MM3 and MMP2) are used to calculate the preferred conformations of the isomers. The required orbital overlap is 31% effective with the global minimum conformation of the cis isomer and essentially ineffective for the low-lying conformations of the trans isomer. This proposed mechanism is supported by Stem–Volmer quenching studies, which indicate that both isomers quench the singlet excited state of 2 at the diffusion-controlled rate, and by deuterium incorporation studies. When irradiation of the cis isomer is carried out in acetonitrile–methanol-O-d as solvent, isomerization is accompanied by deuterium exchange at the benzylic position; the trans isomer is stable under these conditions. Keywords: photosensitized electron transfer, radical cation, deprotonation, configurational isomerization, conformation, molecular mechanics (MM3).
Molecular orbital calculation using spectral representation technique
Authors: Katsuki Shinichi
Published: May 2006

The spectral representation technique as a tool for molecular orbital calculation is introduced. The procedure for getting a potential in spectral representation is simple and straightforward. The technique is also shown to be versatile to apply to the so-called effective core-potential case, to the relativistic case, to the non-spherical frozen-area case, to the case of approximate estimation of the electronic structures of molecules without the self-consistent-field process, and to the case of incorporating external effect into the cluster by environment potential. Keywords: spectral representation, molecular orbital calculation, electronic structure, relativistic potential, environment potential.
A configuration interaction study of the ground and first excited 1∑+ states of the NaI molecule
Authors: Sakai Yoshiko , Miyoshi Eisaku , Anno Tosinobu
Published: May 2006

Multireference singly and doubly excited CI (MRSDCI) calculations are performed on the NaI molecule by using a model potential method. The potential energy curves of the ground and first excited 1∑+ states are generated over a wide range of internuclear distance R. The curves yield the avoided crossing, which is expected to arise by the mixing of the ionic and covalent configurations, and thus the curve of the first excited state shows a shallow minimum. The geometrical parameters, dipole moment, and dissociation energy are calculated. The agreement of those with the experimental results is satisfactory. Keywords: NaI, MRSDCI calculation, model potential method, potential curve, excited state.
On the allowed symmetries of all distorted forms of conformers, molecules, and transition structures
Authors: Mezey Paul G.
Published: May 2006

Based on the catchment region point symmetry theorem, a general framework and some new results are presented on symmetry constraints of deformations preserving chemical identity. The point symmetries of the unique critical point (equilibrium) nuclear arrangement corresponding to a conformer, molecule, or a transition structure and those of a family of nuclear arrangements with an arbitrarily small limit on the allowed distortions provide a complete symmetry characterization of the entire potential surface catchment region of the chemical species. Consequently, the point symmetries of all distorted molecular forms that preserve chemical identity can be deduced by testing symmetry conditions only within an arbitrarily small domain of the potential surface, in the vicinity of the equilibrium arrangement. Additional constraints are deduced on the point symmetries of transition structures obtained directly from the given conformer. Keywords: molecular distortions, symmetry, catchment regions, new symmetry theorems, framework groups, point groups.
The abinitio model potential method. Cowan–Griffin relativistic core potentials and valence basis sets from Li (Z = 3) to La (Z = 57)
Authors: Barandiarán Zoila , Seijo Luis
Published: May 2006

A comprehensive tabulation of Cowan–Griffin relativistic abinitio core model potentials and valence basis sets corresponding to the Cowan–Griffin abinitio model potential method is presented. It includes those for the elements from Li (Z = 3) to La (Z = 57), the alkaline M+ and alkaline earth M2+ cations, and for the halogen X− anions. Molecular results are presented in order to test the potentials and basis sets and to estimate the extent of the mass–velocity and Darwin relativistic effects considered within the method, which lie within the expected margins. Keywords: relativistic, model potential, effective core potential, basis sets, abinitio.
Analytic derivative theory based on the Hellmann–Feynman theorem
Authors: Hirao K.
Published: May 2006

General formulae for the second, third, and fourth derivatives of the energy with respect to the nuclear coordinates of a molecule are derived from the Hellmann–Feynman theorem. Hurley's condition is used to obtain approximations to the first-order wavefunction, from which the second, third, and fourth energies can be obtained, leading to quadratic, cubic, and quartic force constants. The procedure is equivalent to minimizing the derivative energy by perturbed variation techniques. The expressions for these higher energy derivatives are much simpler than those of the direct analytic derivative method. The electrostatic calculation involves only one-electron integrals. The coupled Hartree–Fock equations to obtain the wavefunction derivatives become much simpler. The present theory provides a great conceptual simplification. However, the theory is correct only if the basis set is complete or basis functions are independent of the perturbation. Keywords: analytic derivative theory, Hellmann–Feynman theorem, force constants, the curvature theorem.
The infrared spectrum of 2-methylaziridine from scaled abinitio force fields
Authors: Rauk Arvi , Eggimann T. , Wieser H. , Yang Danya
Published: May 2006

The structures of the cis and trans invertomers of 2-methylazirdine were optimized at the RHF/6-31G*(0.3) level of theory and the composition of the equilibrium mixture was determined to be trans/cis = 0.71:0.29, in good agreement with the experimental value, 0.66:0.35. The abinitio force field was scaled and a complete assignment of the vibrational spectra of the two conformers is presented. The IR spectrum of 2-methylazirdine was recorded in the vapor phase and in solution (cyclohexane and CCl4). The predicted IR spectrum of the equilibrium mixture agrees very well with the experimental spectrum. The 6-31G*(0.3) basis set is found to provide better geometries and more uniformly scalable force fields than the conventional 6-31G* basis set. Keywords: abinitio force fields, 2-methylaziridine.
Dynamical barriers to transport in pulsed spin systems: quantum–classical correspondence in the kicked top
Authors: Boorstein J. L. , Uzer T.
Published: May 2006

Classical phase space is not homogeneous, but contains barriers (such as KAM tori) that, whether intact or broken, affect the temporal evolution of dynamical systems. In this article, we study the quantal manifestations of these classical-mechanical structures. Here, the particular system is the kicked top, which consists of an angular momentum vector precessing about one direction and experiencing periodic sudden kicks around another direction. We find that a suitably defined probability distribution function (constructed from the time-dependent state vector) shows transitions from classically allowed regions to inaccessible regions at the locations where the classical dynamics places the KAM tori. Keywords: bottlenecks, phase space, transport, spin systems, Correspondence Principle.
Optimization of Gaussian-type basis sets for local spin density functional calculations. Part I. Boron through neon, optimization technique and validation
Authors: Godbout Nathalie , Salahub Dennis R. , Andzelm Jan , Wimmer Erich
Published: May 2006

Gaussian-type orbital and auxiliary basis sets have been optimized for local spin density functional calculations. This first paper deals with the atoms boron through neon. Subsequent papers will provide a list through xenon. The basis sets have been tested for their ability to give equilibrium geometries, bond dissociation energies, hydrogenation energies, and dipole moments. These results indicate that the present optimization technique yields reliable basis sets for molecular calculations. Keywords: Gaussian basis sets, density functional theory, boron–neon, geometries, energies of reactions.
Theoretical study of nitro–nitrite rearrangement of CH3NO2
Authors: Saxon Roberta P. , Yoshimine Megumu
Published: May 2006

Calculations designed to characterize the transition state and determine the barrier height for rearrangement of nitromethane to methyl nitrite are reported. Structures of CH3NO2, CH3ONO, dissociation products, CH3 + NO2, and CH3O + NO, and the transition state for nitro–nitrite rearrangement have been optimized at the MCSCF/4-31G level. The geometry of the transition state may be approximately described as separated CH3 and NO2 species with extremely long C—N and C—O bond lengths, 3.396 and 3.654 Å, respectively. Energies have been obtained by large-scale multireference single- and double-excitation CI calculations (6-31G* basis). The transition state is calculated to lie 56.7 kcal/mol above nitromethane (with zero-point energy). A C—N bond dissociation energy of 51.7 kcal/mol is obtained. Results are compared with the infrared multiphoton dissociation experiment of Wodtke, Hintsa, and Lee. Keywords: nitromethane, abinitio calculations, transition state, rearrangement, dissociation.