bluuebunny/crossref_metadata_2025 huggingface dataset:
The triple helical structure of lentinan, a linear β-(1 → 3)-D-glucan
Authors: Bluhm Terry L. , Sarko Anatole
Published: May 2006

Probable models for the crystalline structure of lentinan, the β-(l → 3)-D-glucan of the fungus Lentinus elodes, were evaluated by X-ray fiber diffraction and theoretical conformational analysis of single and multiple helices. The X-ray diagram is consistent with a hexagonal unit cell with dimensions a = b = 15.8 Å and c (fiber repeat) = 6 Å. Conformational analysis predicts five probable models for this fiber repeat, of which one is a single helix, two are double helices, and two are triple helices. On the basis of packing of these helices into the unit cell and a comparison of their calculated Fourier transforms with the observed X-ray diffraction intensity distribution, all but the two triple helical models are eliminated from consideration. The two triple helices differ only in chirality and both left- and right-handed models are stabilized by interstrand [Formula: see text] hydrogen bonds, such as are found in the triple helical structure of β-(1 → 3)-D-xylan. The right-handed triple helix may be the more probable structure of the two, by analogy with the known structure of the β-(1 → 3)-D-xylan. It is likely that the β-(1 → 3)-D-glucan of curdlan and those of Armillaria mellea and Lilium longiflorum also possess the same structure.
Trajectory study of dissociation reactions. Br2 in Ar at 3500 K
Authors: Chang D. T. , Burns George
Published: May 2006

Dissociation of Br2 in Ar was studied at 3500 K using classical 3-D trajectory technique, and compared with earlier trajectory calculations. Some of the assumptions used previously were eliminated, while others were studied in some detail. The one-way flux, equilibrium rate coefficient, obtained from over 8400 trajectories, was found to be over an order of magnitude larger than the experimental rate constant. This was taken as an indication that at high temperatures the nonequilibrium effects are important in dissociation reactions. In order to understand these effects better, additional calculations using an improved set of assumptions were performed. The calculated dissociation rate constant for Br2 + Ar → 2Br + Ar reaction, which accounted for nonequilibrium effects, agrees reasonably well with experimental results.
On the involvement of diazonium ions in the photorearrangement of azoxybenzene
Authors: Bunce Nigel J.
Published: May 2006

Photolysis of aromatic azoxy compounds in non-basic solvents in the presence of 2-naphthol leads quite generally to azonaphthols in addition to the normal photorearrangement product. Free diazonium ions are implicated as the precursors of the azonaphthols, both the latter and the usual photorearrangement product being derived from the same excited state of the azoxy compound. Variation of the solvent, light intensity, substrate concentration, and percent reaction all cause changes in the product distribution. Such changes can be accommodated by postulating an intermediate which can either afford the normal rearrangement product by loss of a proton to a weak base, or expel a diazonium ion when no base is present.
Vinyl sulfide hydrolysis
Authors: McClelland Robert A.
Published: May 2006

The hydrolysis of vinyl sulfides in acid solutions is shown to proceed via a mechanism analogous to that of vinyl ethers, with slow proton transfer to the carbon–carbon double bond to produce a sulfur stabilized carbonium ion. Relative rates of hydrolysis of compounds RXCH=CH2 are CH3S—, 41; CH3O—, 1500; C6H5S—, 1; and C6H5O—, 10.5. This order is compared with that of other systems which produce similar stabilized carbonium ions.
sHigh resolution 1H nuclear magnetic resonance studies of a flavine and its product with MoCl4
Authors: Roberie T. , Bhacca N. S. , Selbin J.
Published: May 2006

The high resolution 1H nmr spectra of the substituted flavine, 3-N-methyltetraacetylriboflavine (3-Me-TARF), and its non-aqueous solution complexes with Gd(fod)3, Eu(fod)3, MoCl4, and MoCl4•2CH3(CH2)2CN, were studied in order to try to discern the binding sites of the flavine as it attaches to the molybdenum. Evidence was found that all three metal atoms, Gd(III), Eu(III), and Mo(IV), are attached in solution not only by the primary binding (chelating) sites of the flavine, viz., the O-4 and N-5 atoms, but also by an acetyl oxygen atom, at the C-4′ site of the ribityl side chain. 300 MHz spectra of the 3-Me-TARF have permitted the coupling constants for the side chain methine and methylene protons to be obtained.
General base catalysis in acidic solutions. Acceleration of intramolecular phosphonate-assisted amide hydrolysis
Authors: Kluger Ronald , Lam C.-H.
Published: May 2006

The hydrolysis of the amide linkage of 2′-phosphonobenzanilide, 1, occurs with nucleophilic assistance from the adjacent phosphonic acid moiety. The basic component of buffers in acidic solutions provides additional rate acceleration. A Brønsted plot yields a straight line for buffers of phosphoric acid, cyanoacetic acid, chloroacetic acid, and dichloroacetic acid with [Formula: see text]. It is suggested that the second acidic group of the phosphonic acid of 1 dissociates upon formation of an addition intermediate, leading to a zwitterion (T±). This zwitterionic intermediate decomposes in the rate-determining step, with added base or water removing a proton from a hydroxyl group of the intermediate. The product is a mixed phosphonic–carboxylic anhydride which hydrolyzes rapidly. It is concluded that a phosphoric acid derivative is a particularly good catalyst for this type of reaction since a stabilized intermediate can be internally generated along the reaction coordinate.
The chromous chloride promoted addition of N-haloamides to olefins. IV. Mechanistic aspects
Authors: Driguez Hughes , Lessard Jean
Published: May 2006

A discussion on the mechanism of the chromous chloride promoted addition of N-haloamides to olefins is presented on the basis of the results already reported and some additional evidence. The mechanism proposed involves a radical chain addition in which the N-haloamide acts as the transfer agent, the termination steps being chromium(II) reduction of the amido radical and chromium(II) reduction of the intermediate adduct radical. The addition of N-chloro- and N-bromoacetamide to norbornadiene is described. The use of chromous chloride to initiate the reaction of N-chloropiperidine with ethyl vinyl ether and cyclohexene led to low yields of addition.
Oxidation with chromium(VI) oxide - pyridine complex. A study of reaction parameters using cholesterol as substrate
Authors: Piers Edward , Worster Paul M.
Published: May 2006

Oxidation of cholesterol with chromium(VI) oxide – pyridine complex under various conditions produced cholest-5-en-3-one accompanied by varying amounts of cholest-4-ene-3,6-dione. The efficiency of the oxidation and the product distribution were found to be dependent on the atmosphere, temperature, molar ratio of oxidant to substrate, molar ratio of pyridine to chromium trioxide, the heterocyclic amine used, and the proton source employed. The yield of cholest-5-en-3-one was optimized by oxidation of cholesterol with a ten-fold excess of the monopyridine – chromium(VI) oxide complex in dichloromethane under a nitrogen atmosphere at 0 °C.
Synthesis of 4,4′-bis(dicyanomethylene)bicyclohexylidine
Authors: Pedersen Larry D. , Weiler Larry
Published: May 2006

4,4′-Bis(dicyanomethylene)bicyclohexylidine and its dihydro derivative, 4,4′-bis(dicyanomethylene)bicyclohexyl were prepared from the condensation of malononitrile with the corresponding diketone. Bicyclohexylidene-4,4′-dione was synthesized from the monoketal of cyclohexanedione via the symmetric azosulfide.The uv spectra of the 4,4′-bis(dicyanomethylene)bicyclohexylidine and related model compounds indicate the presence of a long range interaction between the dicyanomethylene groups of the title compound.
A synthetic approach to resistomycin
Authors: Kingston John F. , Weiler Larry
Published: May 2006

An outline of a synthetic route to the antibiotic resistomycin is presented. We report the successful construction of an intermediate containing all of the carbon atoms in resistomycin with requisite functionalities. The key steps involved an aldol type condensation of the dianion from 2,4,6-trimethoxybenzoylacetone and a suitable acetophenone. This was followed by a novel acid-catalyzed cyclization–condensation to form the naphthalene intermediate required in our scheme.
Comment: A problem in extracting structure information from the temperature of maximum density for aqueous solutions
Authors: Kuppers James R.
Published: May 2006

The shift in the temperature of maximum density for aqueous solutions customarily has been dissected into an ideal and a structural contribution. This analysis demonstrates that the dissection leads to the formulation of an ideal state which is attained by a fortuitous superposition of temperature dependent properties of pure components and, hence, cannot serve as a reliable reference state for comparing the influence of different solutes upon solution structure.
Synthetic application of cyclobutanes V. α-Carbalkoxymethylation of α, β-unsaturated ketones
Authors: Liu Hsing-Jang , Yao Patrick Chi-Lin
Published: May 2006

Two general methods for α-carbalkoxymethylation of both enolizable and nonenolizable (towards the γ-position) α,β-unsaturated ketones have been developed. Method A involves three synthetic steps: photocycloaddition of the starting enone to 1,1-dimethoxyethylene, hydrolysis–oxidation of the adduct with acetic acid and 30% hydrogen peroxide, and O-alkylation of the resulting mixture of lactone and acid using anhydrous potassium carbonate and an alkyl iodide, e.g., 13 → 17 → 21 + 22 → 23. Method B differs from method A in the means of securing the required cyclobutanone intermediate. Thus, photocycloaddition of 13 to vinyl acetate followed by hydrolysis of the adduct gave two epimeric keto alcohols 39 whose oxidation with dimethyl sulfoxide and acetic anhydride afforded diketone 40. Baeyer–Villiger oxidation of 40 followed by methylation of the products 21 and 22 completed the overall α-carbomethoxymethylation process to give keto ester 23.
Cobalt(II) and zinc(II) complexes of the 'tripod' ligand tris(2-benzimidazylmethyl)amine. Some five-coordinate derivatives and some with mixed stereochemistries
Authors: Thompson Laurence K. , Ramaswamy Baratham S. , Seymour Elizabeth A.
Published: May 2006

A series of cobalt(II) and zinc(II) complexes of the title ligand are reported which contain mixed stereochemistries, e.g., five-coordinate trigonal bipyramidal cations [M(ligand)X]+, and four-coordinate anions, [MX4]2− (M = Co, Zn; X = Cl, Br, NCS). The blue cobalt complexes, [Co(ligand)X]2[CoX4], are readily converted to purple five-coordinate complexes, [Co(ligand)X]X, by recrystallisation from a solvent containing water, e.g. acetone. Other five-coordinate cobalt and zinc derivatives were also prepared, [Co(ligand)X]X (X = NO3, ClO4), [Co(ligand)X]BPh4 (X = Cl, Br, NCS, NO3, ClO4), [Zn(ligand)X]BPh4 (X = Cl, Br, I, NCS). Nuclear magnetic resonance data for all the zinc complexes indicate a significant deshielding of one benzene ring proton on each coordinated benzimidazole group. Conductance data suggest the possibility of cationic association through pseudo-hydrogen bonding in both the cobalt and zinc derivatives involving Cl, Br, NCS anions.
Crystal structure of Sb2Tl7
Authors: Stokhuyzen Rolf , Chieh Chung , Pearson William B.
Published: May 2006

The single-crystal study of Sb2Tl7 confirms the work of Morral and Westgren. The crystal is cubic with a = 11.618(6) Å and space group of Im3m. There are 54 atoms per unit cell. It is the only known example of this structure type which can be considered as a true superlattice of CsCl. The structure was determined using 106 observed reflections measured on a G.E. XRD-6 diffractometer. The final R and Rw were 0.097 and 0.060, respectively. The Tl–Tl distances range from 3.21 to 3.55 Å and the mean Sb-Tl distance is 3.27 Å. There is no Sb–Sb contact in the structure
Nucleic acid related compounds. 24. Transformation of tubercidin 2′,3′-O-orthoacetate into halo, deoxy, epoxide, and unsaturated sugar nucleosides
Authors: Robins Morris J. , Jones Roger A. , Mengel Rudolf
Published: May 2006

Treatment of tubercidin (4-amino-7-β-D-ribofuranosylpyrrolo[2,3-d]pyrimidine) (1) with methyl orthoacetate gave the 2′,3′-O-orthoester, 2. Pivalic acid chloride in refluxing pyridine converted 2 into a mixture containing 4-N-pivalamido-7-(3-chloro-3-deoxy-2-O-acetyl-5-O-pivalyl-β-D-xylofuranosyl)pyrrolo[2,3-d)pyrimidine (3a) and the corresponding 2′-O-(4,4-dimethyl-3-pivaloxypent-2-enoyl) (DMPP) compound (3b) via acetoxonium ion intermediates. Treatment of 2 with sodium iodide/pivalyl chloride/pyridine gave the iodo analog (3c) of DMPP derivative 3b plus the 3′,4′-unsaturated nucleoside (5a). Treatment of 3a–c with methanolic sodium methoxide gave the ribo-epoxide 4, which underwent N1 → 3′ intramolecular cyclization readily. Dehalogenation of 3 and deprotection gave 3′-deoxytubercidin (8). Deblocking of 5a gave 5b which was hydrogenated to give 8 plus its 4′-epimer 9. Heating of 3c or 5a produced 4-N-pivalamido-7-(5-pivaloxymethylfuran-2-yl)pyrrolo[2,3-d]pyrimidine (10a). Deblocking of 10a gave 10d which was hydrogenated to give racemic 4-amino-7-(2,3-dideoxy-β-D,L-glycero-pentofuranosyl)pyrrolo[2,3-d]pyrimidine (7,11). The 2′,3′-unsaturated nucleoside (6) was obtained and hydrogenated to produce 2′,3′-dideoxytubercidin (7). Spectroscopic identification of products, epoxide instability, and comparison with other procedures are discussed.
Polymorphism in phenol under pressure: dielectric properties and molar polarizations of polycrystalline phenol I and II
Authors: Bertie John E. , Tremaine Peter R.
Published: May 2006

The dielectric properties of phenol I have been measured as isothermal functions of pressure between 140 and 1520 bar and between +35 and −10 °C. No dielectric relaxation was observed. The static (50 kHz) permittivity at 10 °C is given with a precision of 0.3% by[Formula: see text]where P is in bar. The extrapolated value at 1 bar is 2.882 ± 0.009 which compares with literature values between 2.74 and 2.84. The 50 kHz molar polarization, [Formula: see text], at 10 °C is given by[Formula: see text]where V and Vo are the molar volumes at pressures P and 1 bar, respectively. At 10 °C and 1 bar, the electronic and atomic polarizations are estimated to be 26.9 ± 0.3 cm3 and 5.1 ± 0.4 cm3, respectively. Expressions for the isobaric temperature dependence of εo′ are reported for several pressures. Below 1500 bar, (∂εo′/∂T)P is negative, as expected from the density change but in contrast with previous results and with results obtained while varying the temperature under a constant applied pressure of 1 kbar. The accuracy of the temperature dependence at 1 bar is not high, judging from the thermal expansivity calculated from it.εo′ of phenol II at 10 °C and an estimated pressure of 2000 bar is 3.10 ± 0.06, with the corresponding molar polarization 30.75 ± 0.5 cm3. No intrinsic dielectric relaxation was observed in phenol II and the molar polarization shows no marked discontinuity at the transition. Phenol II is, therefore, like phenol I, a hydrogen-bonded solid in which the hydroxyl hydrogen atoms are ordered, either fully or in chains. The dielectric parameters and Arrhenius activation energies of two transient dispersions which appeared whenever phenol II formed are discussed.
C-Alkylation of 1,3-dihydroxyanthraquinones. Total syntheses of (±)-averufin and (±)-bipolarin
Authors: Castonguay André , Brassard Paul
Published: May 2006

Various methods of substituting 1,3-dihydroxyanthraquinones in the 2-position were investigated. It was eventually found that direct hydroxyalkylation in the presence of definite amounts of sodium bicarbonate could be carried out. The reaction of 1,3,6,8-tetrahydroxyanthraquinone with 5-oxohexanal or 4-hydroxybutanal gave, respectively, among other products, (±)-averufin and (±)-bipolarin.
Synthesis, ion-exchange properties, and analytical applications of thermally stable tin(IV) vanadate
Authors: Qureshi Mohsin , Nabi Syed Ashfaq , Zehra Nighat
Published: May 2006

Samples of stannic vanadate have been synthesized by varying the mixing ratio of the reagents and pH of the mixture. The most stable sample has been synthesized by mixing 0.25 M solutions of stannic chloride and sodium metavanadate in the ratio of 2:3 at pH 2.5. The material behaves as a weak cation exchanger. This sample has been critically studied for its ion-exchange, chemical, and thermal behaviour. The analytical utility of the material has been demonstrated by achieving some binary separations of metal ions on its column. Arsenate has also been quantitatively removed from a mixture of tungstate, phosphate, and antimonate.
Crystal structure of vanadium(III) tris(metaphosphate)
Authors: Middlemiss Nora , Hawthorne Frank , Calvo Crispin
Published: May 2006

Vanadium(III) tris(metaphosphate), V(PO3)3, crystallizes in the monoclinic space group Ic with lattice parameters a = 10.615(2), b = 19.095(4), c = 9.432(1) Å, β = 97.94(1)° with Z = 12. The equivalent parameters in the standard space group Cc are a = 13.189(1), b = 19.095(4), c = 9.432(1) Å, and β = 127.15(1)°. The structure was refined by full-matrix least-squares to an R = 0.091 (Rω = 0.065) utilizing 2467 reflections with the atomic positions and their isotropic vibration amplitudes as parameters. The structure consists of infinite chains of PO4 tetrahedra sharing corners with each other and bridged by VO6 octahedra. All oxygen atoms are shared between just two cations. The average [Formula: see text] bond is 1.581 Å while the average of those shared with vanadium is 1.483 Å. The VO6 group is moderately distorted, with differences of less than 0.06 Å between the longest and shortest V—O bond lengths in any of the three distinct VO6 groups. The average V—O bond lengths for the three VO6 groups are 1.995, 1.991, and 1.987 Å. A marked superlattice effect based on a cell with b/3 is noted.
Studies with inorganic precipitate membranes: Part XIV. Evaluation of effective fixed charge densities
Authors: Beg M. Nasim , Siddiqi Fasih A. , Shyam Radhey
Published: May 2006

Effective fixed charge densities of cobalt and nickel sulphide (parchment supported) membranes in contact with various 1:1 electrolyte solutions have been evaluated from membrane potential measurements. The methods used for the estimation of charge densities were: (a) the Teorell–Meyer–Sievers method (TMS) and (b) the methods developed recently by Kobatake and co-workers. The two limiting forms of Kobatake's equation for dilute and concentrated solutions gave identical values of charge densities. The theoretical predictions for membrane potential were borne out quite satisfactorily by experimental results obtained with both the membranes. Apparent transference numbers of coions and permselectivities of the membranes for electrolytes have also been calculated. A method based on permselectivity values for the determination of charge density was also used. It was interesting to note that the charge densities evaluated from different methods of Kobatake and co-workers gave identical values and that the results were comparable to those derived from the TMS method.
Open-chain nitrogen compounds. Part II. Preparation, characterization, and degradation of 1(3)-Aryl-3(1)-methyltriazenes; the effect of substituents on the reaction of diazonium salts with methylamine
Authors: Ahern T. Patrick , Fong Handrick , Vaughan Keith
Published: May 2006

Treatment of the diazonium salts, X•C6H4N2+, with aqueous methylamine affords good yields of the monomethyltriazenes, X•C6H4•N=N•NHMe, when the substituent is a strongly electron-withdrawing group (X = o-, m-, and p-NO2; o-, m-, and p-CO2R; p-CN and p-COCH3). Preparation of the triazene from the p-bromobenzene diazonium salt was accompanied by formation of a pentaazadiene. Monomethyltriazenes were not obtained when diazonium salts containing other substituents (X = H, p-CH3, o-CF3, p-Cl, p-F, p-NMe2, p-OH, p-OCH3, p-Ph, p-NHCOCH3) were treated with methylamine. In these cases the products were either pentaazadiene, or 1,3-diaryltriazenes or unstable materials. The monomethyltriazenes vary considerably in stability and give rise to a number of different degradation products, which were either diaryltriazenes or 3-alkyl-1,3-diaryltriazenes or simply arylamines. 1(3)-(p-Nitrophenyl)-3(1)-methyltriazene was found to be a moderately effective methylating agent.
Photoionization energies: liquids and solutions versus gases (Extended Abstract)
Authors: Nemec Ladislav , Chia Lucille , Delahay Paul
Published: May 2006

not available
Experimental detection of collective modes In a polar liquid: application to the case of the solvated electron in H2O and NH3
Authors: Ascarelli Gianni
Published: May 2006

We present experimental data that confirm the predicted existence of a collective mode in a liquid corresponding to the longitudinal optical mode in an ionic crystal. The experimental investigation was carried out in nitromethane, and the results bear out all the calculated properties of this collective mode: the dipolar plasmon. The calculated frequency of the dipolar plasmon, as well as the dielectric constant at high and low frequencies, are then used to calculate without adjustable parameters the polaron coupling constant of the solvated electron in NH3 and H2O. A comparison of the calculated and measured properties of the solvated electron indicates that in either case a polaron-like continuum theory can at most account for only a fraction of the energy of the observed optical absorption.
Experimental evidence from liquid semiconductors
Authors: Enderby J. E.
Published: May 2006

Two broad types of liquid semiconducting alloys will be discussed, namely those involving alkali metals (e.g., the Li–Pb and the Cs–Au system) and those in which a chalcogen is involved (e.g., Cu–Te or Ni–Te). It will be argued that relatively simple ionic bonding schemes in alkali metal systems must be replaced by more complicated ones in chalcogen based alloys. The close interaction between atomic structure on one hand, and the electronic structure on the other will be emphasized.
Electron in cold alcohols: a pulse radiolysis study in ethanol
Authors: Gilles L. , Bono M. R. , Schmidt M.
Published: May 2006

The pulse radiolysis of cold ethanol near its freezing point shows 'partly solvated' states of the electron with transient spectra whose maxima vary with time. It is shown that the determination of the solvation times of the electron can depend strongly on the wavelength considered; thus an explanation could be given for the different values obtained by Chase and Hunt at λ = 1300 and λ = 1050 nm.
Picosecond molecular relaxations: the role of the fluid in electron solvation
Authors: Kenney-Wallace G. A.
Published: May 2006

The role of the fluid is considered for a model of electron solvation in liquid alcohols. The events that lead to trapping and solvation of excess electrons are reconstructed from experimental data on the time and frequency-dependent electron absorptions (kmol) and the appropriate molecular rotational relaxations (τ2, τc), local liquid structure, viscosity (η), and orientational polarization (β) of the supporting fluid. The quasifree electron is captured at subpicosecond times in a preexisting trap in the liquid, which in alcohols is identified as an alcohol cluster whose local configurational fluctuations will be frozen in on this timescale. Rapid configurational relaxation of the cluster molecules, including multi-phonon processes, then occurs in the field of the excess electron at times ≤10−12 s. Finally, the molecules in the fluid layer adjacent to the initial trapping site align in the now screened field of the localized electron in a manner comparable to solvation of an ion embedded in a polar fluid. The relaxation may occur in competition with electron migration and reaction. The observed rate constant of the final step kmol is shown to be proportional to βη−1 for a range of alcohols at room temperature. The implications of this model for photobleaching experiments in liquids are briefly discussed.
Electric field perturbed electron tunnelling in nonpolar organic glasses
Authors: Doheny A. J. , Albrecht A. C.
Published: May 2006

Isothermoluminescence (ITL) and electrophotoluminescence (EPL) resulting from electron–cation recombination are measured in a 2-methylpentane–methylcyclohexane glass. The ITL is more characteristic of quantum-mechanical tunnelling and the EPL signal is markedly stronger in this new glass than previous measurements in 3-methylpentane. Quantum-mechanical tunnelling theory is used to predict recombination rates of electrons in the potential field of a cation plus an applied field. Numerical integration of the nonhomogeneous kinetic equations resulting from a distribution of cation–electron separations leads to qualitative and quantitative predictions of the EPL signal that are observed experimentally. Fitting of the theory to experiment supports the conclusions that the angular distribution of the photoelectrons about the cations is close to isotropic, that the electrons active in ITL and EPL on the time scale of minutes are separated about 50 Å from their parent cation, and that the trap ionization potential in this nonpolar hydrocarbon glass is in the range of 0.5 to 0.7 eV.
Temperature and solvent dependence of electron scavenging efficiency in polar liquids: water and alcohols
Authors: Hunt John W. , Chase W. John
Published: May 2006

High concentrations of electron scavengers reduce the initial yield of esol− and its precursor, et−. The scavenging efficiency of a solute is inversely proportional to its C37 value, the solute concentration required to reduce the initial electron yield to 37% of the yield with no scavenger present. Over a range of 40 to −60 °C the C37 value for a given scavenger is almost constant, and is the same for esol− and et−. In water and alcohols, the C37 value is not correlated with the time required for solvation of the electron, τs, or the static dielectric constant, εs, but rather with the concentration of oxygen atoms in the solvent. These results support the model in which the scavenging of a mobile electron occurs before the electron is trapped in the solvent.
Numerical investigation of tunnelling contributions to electron scavenging reactions in liquids at short times
Authors: Butler P. Robin , Pilling Michael J. , Rice Stephen A. , Stone Timothy J.
Published: May 2006

Fick's second diffusion equation, with an added exponential sink term, is integrated numerically to simulate the decay of electrons at short times in the presence of scavengers. The time dependence of the scavenger concentration profile, the scavenging rate constant, and the electron concentration are illustrated graphically. Using the experimental results of Buxton et al. and Jonah et al. It is shown that the Smoluchowski equation is valid within their experimental time ranges provided the cage encounter distance is replaced by Reff, where Reff can be evaluated explicitly in terms of reaction parameters. It is also shown that tunnelling from relaxed traps may make a significant contribution to ultra-short time electron scavenging.
The internal coordinate Morse oscillator as a means to simplify the anharmonicity problem in the thermodynamics of systems of polyatomic molecules
Authors: Bron Jan , Wallace R.
Published: May 2006

Two postulates simplify the evaluation and algebraic complexity of the expression for the anharmonic correction to the ground rotational–vibrational energy of a polyatomic molecule. The two postulates are: (1) anharmonicity of vibration is associated with bond stretching coordinates only; all other internal coordinates can be represented by harmonic potentials; (2) bonds vibrate according to a Morse potential. The result is the replacement of tedious computer calculations by a 'back of an envelope' type of computation. Relationships between anharmonic corrections to the ground state vibrational energies of isotopically different molecules are derived. Good agreement with exact results is obtained. Quantitative and qualitative applications are given.
Two types of localized excess electrons in crystalline D2O ice
Authors: Buxton George V. , Gillis Hugh A. , Klassen Norman V.
Published: May 2006

In a pulse radiolysis study of crystalline D2O ice, an intense infrared absorption band with λmax > 2350 nm has been found at low temperatures, in addition to the well-known visible absorption band of the trapped electron. The infrared band is also attributed to trapped electrons, partly because of its similarity to the electron absorption band found recently in some D2O glasses at low temperatures. The effects of temperature, dose per pulse, accumulated dose, and added NH4F, HF, and ND3 on the yields and decay kinetics of both bands have been investigated. It is concluded that the electron trap giving rise to the visible band is a vacancy which at low temperatures is radiation-produced by a two-step spur process. At temperatures close to the melting point the vacancy-trap probably exists before the radiation pulse at equilibrium concentration. The electron trap which gives rise to the infrared band is thought to be a cavity that occurs naturally in the perfect lattice. For previously unirradiated samples the infrared band decays by a second order process which is remarkably fast [Formula: see text] The decay reaction is probably neutralization by D2O+. Doping with NH4F increases the yield of the infrared absorption and greatly decreases its decay rate. The total yield of localized electrons in irradiated crystalline D2O is higher than has been generally recognized.
A relationship between correlation energies and sizes: the series of beryllium- and neon-like ions
Authors: Bernardi Fernando , Mezey Paul G. , Csizmadia Imre G.
Published: May 2006

A simple empirical relation for the estimation of the correlation energy (Ecorr) is found to be valid for the isoelectronic series of Be- and Ne-like ions to a very good accuracy. Although originally such a relation was proposed for the He-like ions, the present results indicate that it is applicable for layer systems as well. Thus the interelectron pair correlation in systems such as Be or Ne does not affect adversely the accuracy of the relation:[Formula: see text]where the denominator is the size [Formula: see text] weighted by the square of the effective nuclear charge Zeff.
The intramolecular hydrogen bond in some β-nitroalcohols. Chemistry of nitroalkanes. Part CXIX
Authors: Lipczyńska-Kochany E. , Urbański T.
Published: May 2006

New experimental evidence is given on the existence of an intramolecular hydrogen bond in β-nitroalcohols between the nitro and hydroxylic groups. The conclusion is based on examination of na → π* and infrared absorption bands and nmr spectra.
Crystal and molecular structure of hexadecamethylcyclooctaphosphazene, (NPMe2)8
Authors: Oakley Richard T. , Paddock Norman L. , Rettig Steven J. , Trotter James
Published: May 2006

Crystals of hexadecamethylcyclooctaphosphazene are tetragonal, a = 13.637(1), c = 8.215(1) Å, Z = 2, space group P4/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to a final R of 0.033 and Rw of 0.032 for 1306 reflections with I ≥ 4σ(I) The molecule has crystallographic fourfold (C4) symmetry with weighted mean bond lengths P—N, 1.590(13), P—C, 1.811(2), and C—H, 0.95(2) Å (those not involving hydrogen have been corrected for libration, rms deviations from the mean are given in parentheses). Angles in the 16-membered ring are 119.2(1) and 115.1(1)° at P and 131.5(1) and 148.2(1)° at N.
1H and 13C dynamic nuclear magnetic resonance study of hindered rotation in thiobenzoylpiperidines and thiobenzoylmorpholines. Correlation between barrier heights of amides and thioamides
Authors: Piccinni-Leopardi Carla , Fabre Omer , Zimmermann Daniel , Reisse Jacques , Cornea F. , Fulea C.
Published: May 2006

The free energies of activation for hindered rotation around the C—N bond have been determined for a series of N,N-disubstituted thioamides by means of 13C and 1H dynamic nmr.A comparison between barriers for 20 amides and the corresponding thioamides, studied under similar conditions, has been drawn up using both our results and data obtained from the literature. An excellent linear correlation has been obtained. The same correlation also holds for primary (thio)amides and seems to be of general significance. Substituent effects on the barrier heights of thioamides and amides are discussed in connection with some particular cases.
Hydride transfer reactions. Oxidation of N-methylacridan by π acceptors
Authors: Colter Allan K. , Saito Gunzi , Sharom Frances J.
Published: May 2006

The oxidation of N-methylacridan (1) to N-methylacridinium ion (2) by the π acceptors p-benzoquinone (BQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ), p-chloranil (CA), tetracyanoethylene (TCNE), and 2,3-dicyano-1,4-benzoquinone (DCBQ) has been investigated. Second-order rate constants in acetonitrile (AN) vary by a factor of more than 107 for this series. The rate of reaction of 1 with BQ increases 45-fold as the solvent composition is varied from AN to 50% (v/v) AN–water, but is insensitive to changes in buffer ratio or concentration in 75% AN. Spectroscopic evidence for charge-transfer complexing between the reactants was obtained with BQ and CA, and kinetic evidence for complexing was obtained with BQ in 75% AN. The primary isotope effect, p, calculated from the rates of oxidation of 1, 1-9-d, and 1-9,9-d2 (1(HH), 1(HD), and 1(DD)) in AN varied from 4.5 (TCNE) to 13 (BQ), while the secondary isotope effect, s, was approximately constant (∼1.1). Values of the isotope partitioning ratio, ipr (the ratio of 2(D) to 2(H) formed in reaction of 1(HD)) were determined for BQ in three AN-water mixtures and for CA, TCNE, and DCBQ in AN. For all systems studied, except BQ in 90% AN, where determination of the ipr is complicated by isotopic exchange between unreacted 1 and product (2), the ipr agrees with p/s from the kinetic measurements. These results are discussed in terms of mechanism and compared with those of other hydride transfer reactions involving dihydronicotinamide donors.
Mechanism of L-ascorbic acid oxidation and dehydro-L-ascorbic acid reduction on a mercury electrode. I. Acid medium
Authors: Ruiz Juan José , Aldaz Antonio , Dominguez Manuel
Published: May 2006

A polarographic study of the oxidation mechanism of L-ascorbic acid and of the reduction mechanism of dehydro-L-ascorbic acid was carried out in an acid medium.For L-ascorbic acid, the oxidation process involves a two electron transfer and obeys the overall reaction[Formula: see text]The polarographic curve shows that the limiting current is governed by diffusion. On the rising portion of the wave, the two electron oxidation process consists of two consecutive one electron transfers, the second being the rate determining step (rds). The reaction orders, together with the Tafel slopes, were calculated.The reduction of dehydro-L-ascorbic acid at the limiting current is kinetically controlled and involves a two electron transfer. The reaction kinetic pathways were studied and the reaction orders and Tafel slope were calculated. It is deduced that, for low overvoltages, the second one electron transfer is the rate determining step.
Lichen substances. X. Physciosporin, a new chlorinated depsidone
Authors: Maass W. S. G. , McInnes A. G. , Smith D. G. , Taylor A.
Published: May 2006

Physciosporin, a new chlorinated depsidone, has been isolated from the lichen Pseudocyphellaria physciospora and identified as methyl 2-chloro-4-formyl-3,8-dihydroxy-1,6,9-trimethyl-11-oxo-11H-dibenzo[b,e][1,4]dioxepin-7-carboxylate (methyl 5-chlorovirensate), 1.
σ-Complexes as biophysical and biochemical probes. Part II. Transformations of σ-complex intermediates from reaction of 4-nitrobenzofuroxan and methoxide ion
Authors: Buncel Erwin , Chuaqui-Offermanns Noemi , Hunter Brian K. , Norris Albert R.
Published: May 2006

Reaction of 4-nitrobenzofuroxan with methoxide ion in methanolic dimethyl sulphoxide solution gives rise to 7-hydroxy-4-nitrobenzofurazan (as the anion) and 7-methoxy-4-nitrobenzofurazan (as the methoxide C-7 adduct); the proposed reaction mechanism invokes the formation of several transient intermediates. The results bear on the postulated mechanism for the antileukemic activity of 4-nitrobenzofuroxan.
Transition state determination by the X-method
Authors: Mezey Paul G. , Peterson Michael R. , Csizmadia Imre G.
Published: May 2006

A simple direct procedure to locate saddle points (transition states) on energy surfaces is described. The advantage of the method is that it may utilize effective unconstrained optimization techniques while convergence may occur only to saddle points and not to minima. Thus no further tests are needed to decide the nature of the critical point located. Both the simplex and conjugate gradients optimization techniques were applied within the framework of the proposed method (the X-method) and numerical tests were carried out on both 'mathematical' and 'chemical' model surfaces.
Reaction of the fluoroantimony(V) and fluoroarsenic(V) salts of the polyatomic cations of sulphur with tetrafluoroethylene and hexafluoropropylene. Reaction of tetraselenium bis(hexafluoroarsenate(V)) with tetrafluoroethylene
Authors: Desjardins C. David , Passmore Jack
Published: May 2006

S8(AsF6)2 reacts with C2F4 in SO2 to form (C2F5)2Sx (x = 2 → 6), C2F5SXCF2C(O)F (x = 2 → 5), Sx(CF2C(O)F)2 (x = 2 → 4), CF3SxCF2C(O)F (x = 3, 4), CF3SxC2F5 (x = 3 → 5), and (CF3)2S3. Solid S8(AsF6)2 reacts with C3F6 to form (C3F7)2Sx (x = 2 → 5). In SO2 solution, the same reactants yield the additional products[Formula: see text]and related species incorporating the C(O)F moiety. Solid S4(Sb2F11)2 reacts with C2F4 to yield (C2F5)2Sx (x = 1 → 4),[Formula: see text]and with C3F6 to give (C3F7)2Sx(x = 1 → 3), and[Formula: see text]Solid Se4(AsF6)2 reacts with C2F4 to form (C2F5)2Sex(x = 1 → 3) and in SO2 solution, the species C2F5Se2CF2C(O)F, Se2(CF2C(O)F)2, CF3Se2CF2C(O)F, C2F5Se2CF3, and CF3Se2CF3 are also formed.Products were identified by mass spectroscopy and the major species were isolated by vpc and further characterized by 19F nmr. The mechanisms of these reactions are discussed.
Arrhenius parameters for the reactions of O(3P) atoms with several aldehydes and the trend in aldehydic C—H bond dissociation energies
Authors: Singleton D. L. , Irwin R. S. , Cvetanović R. J.
Published: May 2006

The phase-shift technique has been used to determine the temperature dependence of the reaction of ground state oxygen atoms with several aldehydes. Oxygen atoms were generated by modulated photosensitized decomposition of nitrous oxide and were monitored by the chemiluminescence from their reaction with nitric oxide. The Arrhenius expressions determined over the temperature interval 298–472 K are: k1 (acetaldehyde) = (7.21 ± 1.49) × 109 exp (−1960 ± 153/RT); k1(propionaldehyde) = (7.78 ± 0.75) × 109 exp (−1727 ± 66/RT); k1(butyralde-hyde) = (9.99 ± 0.56) × 109 exp (−1702 ± 40/RT); k1(isobutyraldehyde) = (7.92 ± 1.02) × 109 exp (−1445 ± 91/RT), where the units are ℓ mol−1 s−1 and cal mol−1. The indicated uncertainties are one standard deviation. After small corrections were made for the potential abstraction of alkyl hydrogens, the activation energies of aldehydic hydrogen abstraction were used to estimate the aldehydic C—H bond dissociation energies, D(RCO—H). The trend of slightly decreasing values of D(RCO—H) thus obtained for the sequence H2CO, CH3CHO, C2H5CHO, n-C3H7CHO, i-C3H7CHO was also indicated by the aldehydic C—H stretching frequencies.
Metal-ion oxidations in solution. Part XIX. Redox pathways in the oxidation of penicillamine and glutathione by chromium(VI)
Authors: McAuley Alexander , Olatunji M. Adegboyega
Published: May 2006

Three moles of penicillamine or glutathione are required to reduce chromium(VI) to chromium(III). The kinetics and mechanism of the redox reaction have been studied using the stopped-flow method. The reaction proceeds via the formation of a transient intermediate (K1) which decomposes either in a proton-catalyzed pathway or by reaction with a second mole of thiol. The rate law[Formula: see text]where n = 1 for penicillamine and 2 for glutathione has been shown to hold over a range of thiol and hydrogen-ion concentrations. At 25 °C k2 = 14.3 ± 1.0 M−1 s−1 for penicillamine (ΔH≠ = 9 ± 2 kcal mol−1, ΔS≠ = −33 ± 6 cal K−1 mol−1) and 12.1 ± 0.4 M−1 s−1 for glutathione (ΔH≠ = 7 ± 2 kcal mol−1, ΔS≠ = −40 ± 5 cal K−1 mol−1). Several chromium(III) products have been identified by ion-exchange methods. The significance of the second-order pathways in these reactions is discussed.
Heat capacities, volumes, and expansibilities of sodium phenyl carboxylates in water
Authors: Ostiguy Claude , Ahluwalia Jagdish C. , Perron Gerald , Desnoyers Jacques E.
Published: May 2006

The heat capacities per unit volume at 25 °C and densities from 10 to 55 °C of sodium phenyl carboxylates were measured with a flow microcalorimeter and a flow densimeter. The apparent molal heat capacities and volumes were used to derive standard partial molal heat capacities [Formula: see text], volumes [Formula: see text], and expansibilities [Formula: see text]. The CH2 contribution to [Formula: see text] is the same as that usually observed for aliphatic solutes but the contribution to [Formula: see text] and [Formula: see text] is not constant and significantly different. No simple explanation can be offered for this anomaly.
Characterization of natural gas hydrates by nuclear magnetic resonance and dielectric relaxation
Authors: Davidson D. W. , Garg S. K. , Gough S. R. , Hawkins R. E. , Ripmeester J. A.
Published: May 2006

Continuous-wave proton nmr spectra of the clathrate hydrates and/or deuteriohydrates of methane, ethane, propane, isobutane, and neopentane–D2S have been recorded down to 2 K. Between 50 and 200 K each H2O hydrate spectrum consists of a line 3 to 4 G wide from reorienting guest molecules and a broader band from rigid water molecules. Line shapes characteristic of non-rotating guests are obtained in D2O hydrates at low temperatures, except for methane which gives a narrow line to 2 K. Neopentane, shown for the first time to be capable of enclathration, exhibits a Resing effect and other features related to its tetrahedral symmetry. Low-temperature dielectric absorption from reorienting guest-molecule dipoles has been measured in H2S, propane, isobutane, and n-butane–H2S hydrates. For steric reasons n-butane is encaged as a gauche rather than the trans isomer. Average barriers to reorientation estimated from nmr and dielectric data are 1.2 kcal/mol for ethane in type I hydrate and 0.6, 1.2, 1.4, and 0.8 kcal/mol for propane, isobutane, n-butane, and neopentane in type II.
Application de la spectrométrie infrarouge, Raman et résonance magnétique nucléaire à l'étude des complexes en solution aqueuse. Al3+–H2C2O4
Authors: Jaber M. , Bertin F. , Thomas-David G.
Published: May 2006

A study by Raman spectrometry of the system Al3+–H2C2O4 in aqueous solution as a function of the acidity of the solution and of the ratio t = [metal]/[ligand] shows very clearly the successive formation of the complexes Al(C2O4)+, Al(C2O4)2−, and Al(C2O4)33−. The formation of the species Al(C2O4)+, which had been rejected by some authors, is confirmed and it was easily possible to identify its characteristic frequencies as well as those of the species Al(C2O4)2−. The 27Al and 13C nmr spectra confirm these results and allow the identification of each complex by means of a clearly characteristic signal. Moreover, the results of this study permit the rejection of the hypothesis of the formation of hydroxylated complexes, at least in important quantities.It also was possible to use the intensities of the Raman scattering bands at eight selected characteristic frequencies of the species Al(C2O4)+, Al(C2O4)2−, and Al(C2O4)33− to determine their distribution as a function of the acidity of the solution. [Journal translation]
Raman spectrum of solid CH4 in phase II
Authors: Cabana A. , Thé Nguyen Dinh
Published: May 2006

The Raman spectrum of crystalline CH4 in phase II has been obtained. The v1, v2, and v3 bands have been observed as well as two weak bands in the lattice vibration region. The spectrum is found to be consistent with the structure originally proposed by James and Keenan. There is excellent agreement between a theoretical model proposed by Yamamoto and co-workers and the spectra presented here.
Studies of reagents for spectrophotometric measurement of ionic magnesium
Authors: McMinn Dennis G. , Kratochvil Byron
Published: May 2006

Four mono-azo aryl dyes having the o-carboxy-o′-hydroxy functionality were investigated as reagents for the measurement of ionic Mg2+ in solutions containing complexing ligands. 2-(2′-Carboxyl-1′-benzeneazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid came closest to the ideal of having absorbance unaffected by pH over the pH range of interest and of differentiating between Ca2+ and Mg2+ complex formation. Ligand acid dissociation constants and formation constants for the metal–ligand complexes were measured.
Synthetic applications of the photochemically induced addition of oxycarbinyl species to α-enones. Part I. The addition of simple alcohols
Authors: Fraser-Reid Bert , Holder Neville Lewis , Hicks David Roy , Walker David Louis
Published: May 2006

The photochemically induced conjugate addition of simple alcohols to a variety of α-enones has been investigated. With hex-2-enopyranosid-4-uloses (1), the alkylations occur from the less-hindered side and are completely stereo- and regioselective. The resulting 1,4-ketoalcohols (2) are readily cyclized to α-cyclopropyl ketones which have been prepared by alternative, less desirable routes. Alkylative additions of hex-1-enopyran-3-uloses occur also although less readily and without stereospecificity, giving C-glycosides. 2-Cyclohexenone and carvone are also photoalkylated by methanol giving moderate yields of the 1,4-ketols. In the case of carvone, carvone camphor was not detected as a by-product. Steroidal α-enones and mesityl oxide both fail to be photoalkylated by methanol.
Ligand effects upon the reactions of Ni(II) with sodium tetrahydroborate: Ni(I) complexes of bipyridyl and 1,10-phenanthroline
Authors: Holah David G. , Hughes Alan N. , Hui Benjamin C.
Published: May 2006

The reactions between NaBH4 and Ni(II) have been studied in the presence of a variety of ligands in an effort to determine (a) the conditions under which reduction occurs, (b) the extent of the reduction (e.g. to Ni(I), Ni(0) complexes, or to Ni metal or boride), and (c) whether intermediate Ni complexes can be isolated.With ligands having no π-bonding capabilities (NH3, ethylenediamine, edta, citrate), reduction depends upon the Ni:ligand ratio and, in the presence of an excess of the ligand, reduction of Ni(II) is very slow. When vacant coordination sites exist on the metal through dissociation of, for example, Ni(NH3)62+, allowing for the interaction of the BH4 group with the metal, then rapid reduction to the metal (or boride) takes place.With the π-bonding N-donor ligands 1,10-phenanthroline (phen) and 2,2′-bipyridyl (bipy) reduction of the stable ML32+ complexes probably occurs via an outer-sphere electron transfer mechanism but, in these cases, new species of the type NiL2X (L = phen, bipy; X = BH4, PF6, BPh4), which formally contain Ni(I), have been isolated.