bluuebunny/crossref_metadata_2025 huggingface dataset:
The Py scale of solvent polarities
Authors: Dong Dao Cong , Winnik Mitchell A.
Published: May 2006

The relative intensities I1/I3 of the vibronic bands of pyrene fluorescence in 94 solvents and the vapor phase are reported. These values, ranging from 0.47 (vapor) to 1.95 (dimethyl sulfoxide) form the Py scale of solvent polarities. This scale is compared to other scales of solvent polarity, particularly the π* scale of Kamlet and Taft in order to assess the contributions of various factors (dipolarity/polarizability, hydrogen bonding, etc.) to the solute–solvent interaction.
Hydrogen bonding of n-alcohols of different chain lengths
Authors: Wilson Lucie , Alencastro R. Bicca de , Sandorfy C.
Published: May 2006

The anesthetic potency of n-alcohols exhibits a somewhat irregular dependence on the length of the hydrocarbon chain. An attempt has therefore been made to ascertain if this is related to the relative tendency for hydrogen bond formation by these alcohols. No such relationship was found. The result was rather that the degree of association by hydrogen bond formation of dissolved alcohols appears to be independent of the chain length, that is of the extent of other interactions that exist in these solutions.
Niobium(IV) methoxide complexes with phosphorus donor ligands
Authors: Melník Milan , Sharrock Patrick
Published: May 2006

Niobium(IV) methoxide was synthesized by prolonged electroreduction of niobium pentachloride in methanol. Electron paramagnetic resonance spectra at room temperature and 77 K show the expected 10-line patterns of 93Nb at [Formula: see text] and ΔMs = 2 absorptions indicating the presence of dimer species. The spectroscopic parameters obtained in the presence of a series of phosphorus donor ligands are presented and discussed in terms of mononuclear and dinuclear structures.
Conformational features of bisphenol-A polycarbonate
Authors: Sundararajan P. R.
Published: May 2006

Conformational energies have been estimated for the segments of the bisphenol polycarbonate chain, using the Lennard–Jones and Hill's empirical force field type of functions. It is found that the conformation of the carbonate group, defined by the torsion angle ζ, is restricted to the range of 45° to 65°. The rotations χ and χ′ of the methyl groups also show similar limited flexibility. However, accessible conformations of the diphenyl propane (DPP) segment, defined by torsion angles [Formula: see text] and ψ, span a wide area of the [Formula: see text] surface, with the restriction that the rotations [Formula: see text] and ψ be synchronized such that [Formula: see text] or 270°. These features explain the slow thermal crystallization behaviour of the polycarbonate chains. The variability of the conformations of the repeat unit is illustrated with a series of figures.
Mechanisms and models for homogeneous copper mediated ligand exchange reactions of the type: CuNu + ArX → ArNu + CuX
Authors: Couture Christiane , Paine Anthony James
Published: May 2006

The title reactions are an important class of copper mediated nucleophilic aromatic substitution processes, which constitute a useful tool in the molecular design and synthesis of small molecules. We report the results of extensive investigation of these processes, primarily focussing on cyanodeiodination (ArI + CuCN → CuI + ArCN). Among the interesting features of these processes are: (a) an unusual rate equation involving autocatalysis by CuI product; (b) retardation by both excess nucleophile (as KCN) and excess leaving group (as KI), which compete with ArX to complex with CuNu; (c) only cuprous nucleophiles are active (ligand exchanged products from cupric salts arise from prior redox equilibria which form CuNu); (d) the halogen effect is large (kI ~ 40–100 kBr ~ 300–5000kCl) but the Hammett ρ value is zero; (e) ortho-alkyl groups do not hinder the reaction (and actually cause mild acceleration by relief of steric strain). Finally, the introduction of an ortho-COO− group accelerates the reaction by a factor of 104–105, but the general features of the accelerated reactions are also the same, again indicating a common mechanism, with entropic acceleration by ortho-carboxylate. Both kinetic and thermodynamic factors were considered in detail, the latter apparently for the first time. Applications to practical syntheses are considered, and novel mechanistic models for these interesting processes are discussed.
Photoconductivity studies of perylene tetracarboxyl-diimides
Authors: Popovic Zoran D. , Loutfy Rafik O. , Hor Ah-Mee
Published: May 2006

Photoconductivity of a series of perylene tetracarboxyl-dimides has been studied by delayed-collection-field and electric field induced fluorescence quenching techniques. It has been shown that these materials exhibit both extrinsic (impurity controlled) and intrinsic carrier generation originating from the first excited singlet state. Perylene tetracarboxyl-diimides are photoactive in the visible spectral range and their carrier generation efficiencies compare well with generation efficiencies in the best known organic photoconductors. This makes them suitable for xerographic applications using visible light.
IV. Order–disorder transitions: solid state kinetics, thermal analyses, X-ray diffraction and electrical conductivity studies in the Ag2SO4–K2SO4 system
Authors: Sunitha Kumari M. , Secco Etalo A.
Published: May 2006

Order–disorder transitions occurring in the Ag2SO4–K2SO4 system were investigated by reaction kinetics, thermal analyses, X-ray diffraction, and electrical conductivity techniques. Solid–liquid and solid–solid phase diagrams are reported.The conductivity data in the high temperature phase of the solid resemble superionic conductivity behavior. The higher conductivity of Ag2SO4 with K+ presence relative to pure Ag2SO4 and Ag2−xNaxSO4 compositions support a lattice expansion facilitating higher mobility of ions.The reaction kinetics, X-ray diffraction, and electroconductivity results suggest a relatively open periodic [Formula: see text] sublattice in the high-temperature phase of the sulfate-based systems studied in this series.
A simple reason for non-linear mixture rules in chemical kinetics. Part 1. Vibrational relaxation of diatomic molecules
Authors: Carruthers Chris , Teitelbaum Heshel
Published: May 2006

The generalized rate law for the vibrational relaxation of diatomic molecules is extended to include inert collision partners. V–V energy transfer processes are accounted for explicitly as are thermal effects. The molecules are treated as Morse oscillators as far as energetics are concerned; however, the microscopic rate constants are Landau–Teller type. It is found that the phenomenon of non-linear mixture rules arises when experimental data are forced to fit a first-order rate law. The persistence of V–V processes at times well-advanced into the relaxation zone is responsible for deviations from linearity. The non-linearities are most pronounced at high temperatures, and can be avoided only by using extremely dilute mixtures. Several sources of ambiguity are pointed out. The type of excitation method influences the initial deviation from a Boltzmann distribution and plays a crucial role in determining the importance of V–V processes and hence the degree of non-linearity. Thus, when the initial distribution is Boltzmann as in shock waves, the mixture rule is found to be absolutely linear for the vibrational relaxation of diatomic molecules.Several examples, heretofore not recognized as such, are pointed out in the literature.
Syntheses and X-ray structural analyses of [(C6H5)4As][Sn(EC6H5)3], E = S and Se
Authors: Dean Philip A. W. , Vittal Jagadese J. , Payne Nicholas C.
Published: May 2006

Stannous chloride, in solutions containing at least three mole equivalents of NaEC6H5, E = S and Se, forms triligated anions which crystallize readily as the tetraphenylarsonium salts, [(C6H5)4As[Sn(SC6H5)3], 1, and [(C6H5)4As][Sn(SeC6H5)3], 2. The crystals so formed are isomorphous, and their structures have been determined by single crystal X-ray diffractometry techniques. The salts crystallize in the monoclinic space group P21/c, with four formula units in unit cells of dimensions a = 10.760(1), b = 17.515(2), c = 20.247(3) Å, β = 104.71(1)° for 1, and a = 10.885(2), b = 17.587(2), c = 20.612(2) Å, β = 105.47(1)°, for 2. The structures have been refined by full-matrix least-squares techniques on F to agreement factors R = 0.024 (4780 observations with Fo > 3σ(Fo)) for 1, and R = 0.030 (3442 observations with Fo > 3σ(Fo)) for 2. The discrete [Sn(EC6H5)3]− anions are isostructural; the three S or Se atoms form the base of a triangular pyramid whose apex is occupied by the Sn atom. Two Sn—S distances in 1 are 2.532(1) and the third 2.552(1) Å, while in 2 the Sn—Se distances are 2.649(1), 2.650(1), and 2.671(1) Å. Angles at the Sn atom are 89.91(3), 90.55(3), and 96.73(3)° in 1, and 88.74(3), 89.72(3), and 97.27(3)° in 2. In both anions all three phenyl groups adopt a propeller-like conformation and are disposed in equatorial positions above the basal plane of chalcogen atoms. These salts represent the first thiolato- and sele-nolatostannates(II) to be characterized structurally. Tin-119 nmr spectra confirm that the [Sn(EC6H5)3]− ions are the same anionic Sn(II) species recently prepared insitu in solution.
Identification, par spectroscopie optique, des dihydrodiols formés lors de la métabolisation des hydrocarbures polybenzéniques. Application dans le cas de la métabolisation in vitro du dibenzo[a,e]pyrène
Authors: Chalvet Odilon , Gourvest Jean-François , Strapelias Hélène , Duquesne Maurice
Published: May 2006

The first step in the metabolization of a polycyclic hydrocarbon molecule is the saturation of a double bond, the formation of an epoxide, then of a dihydrodiol. Once separation has been achieved by chromatography, the absorption spectra of these dihydrodiols can be established and directly compared with theoretical predictions which are reasonably easy to elaborate in the case of plane molecules. The initial site of enzyme attack can then be identified without using model molecules.
The gas-phase photochemistry and thermal decomposition of glyoxylic acid
Authors: Back R. A. , Yamamoto S.
Published: May 2006

The photolysis of glyoxylic acid vapour has been studied at five wavelengths, 382, 366, 346, 275, and 239 nm, and pressures from about 1 to 6 Torr, at a temperature of 355 K. Major products were CO2 and CH2O, initially formed in almost equal amounts, while minor products were CO and H2. Except at 382 nm, the system was complicated by the rapid secondary photolysis of CH2O. Three primary processes are suggested, each involving internal H-atom transfer followed by dissociation.The absorption spectrum is reported and shows the three distinct absorption systems. A finely-structured spectrum from about 320 to 400 nm is attributed to a transition to the first excited π* ← n+ singlet state; a more diffuse absorption ranging from about 290 nm to a maximum at 239 nm is assigned to the π* ← n− state, while a much stronger absorption beginning below 230 nm is attributed to the π* ← π transition. Product ratios vary with wavelength and depend on which excited state is involved.The thermal decomposition was studied briefly in a static system at temperatures from 470 to 710 K and pressures from 0.4 to 8 Torr. Major products were again CO2 and CH2O, but the latter was always less than stoichiometric. First-order rate constants for the apparently homogeneous formation of CO2 are described by Arrhenius parameters log A (s−1) = 7.80 and E = 30.8 kcal/mol. Carbon monoxide and H2 were minor products, and the CO/CO2 ratio increased with increasing temperature and showed some surface enhancement at lower temperatures. The SF6-sensitized thermal decomposition of glyoxylic acid, induced by a pulsed CO2 laser, was briefly studied, with temperatures estimated to be in the 1100–1600 K range, and the CO/CO2 ratio increased with increasing temperature, continuing the trend observed in the static system.
Kinetics of the reduction of 1-methylquinolinium cations by 1-benzyl-1,4-dihydronicotinamide
Authors: Bunting John W. , Fitzgerald Norman P.
Published: May 2006

The reduction of a series of 3-W-1-methylquinolinium cations (1: W = H, Br, CONH2, CO2CH3, CN, NO2) by 1-benzyl-1,4-dihydronicotinamide has been investigated. In all cases the kinetically controlled product from these reactions is the appropriate 3-W-1,4-dihydro-1-methylquinoline. Only for W = Br is any significant amount of the 1,2-dihydro isomer obtained (15% in this case). This kinetic preference for C-4 attack over C-2 attack in dihydronicotinamide reductions contrasts with the kinetically preferred attack at C-2 by hydroxide ion and in borohydride reductions. Rates of reduction were measured for each 1 and also 1,2-dimethyl- and 1,4-dimethylquinolinium cations in 20% CH3CN – 80% H2O, ionic strength 1.0 at 25 °C, under pseudo-first-order conditions. Kinetic saturation due to nonproductive 1:1 complex formation was observed for several cations at high concentrations (> 0.1 M). Second-order rate constants [Formula: see text] were evaluated for each W, and also kinetic isotope effects from second-order rate constants [Formula: see text] for reduction by 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide. Second-order rate constants are correlated with σp− for W with ρ = 4.5, and are also closely correlated with [Formula: see text] for pseudobase formation at C-4 of these quinolinium cations by: [Formula: see text]. Values of [Formula: see text] vs. [Formula: see text] describe a Westheimer curve reaching a maximum of 5.8 for W = Br and falling to 1.5 for W = NO2 and 4.2 for W = H. These data are consistent with an intrinsic barrier of 2.9 ± 0.5 kcal/mol for hydride transfer between this 1,4-dihydronicotinamide and quinolinium cations. However, quinolinium cations display a dramatically enhanced rate of dihydronicotinamide reduction relative to hydroxide ion attack when compared with isoquinolinium cations. This observation, and the predominance of C-4 rather than C-2 reduction, suggests that these reactions may not be simple one-step hydride transfer processes.
The range of in anisole derivatives. An indicator of torsion and compression of the methoxy group
Authors: Schaefer Ted , Salman Salman R. , Wildman Timothy A. , Penner Glenn H.
Published: May 2006

In a series of anisole derivatives, [Formula: see text], the spin–spin coupling constant between the methyl protons and the ring proton in the ortho position, ranges from −0.23 to −0.38 Hz when the methyl group lies cis to the ortho C—H bond. 5J, as a proximate coupling, is sensitive to the average distance between the coupled protons. Its variation with substituent can be rationalized in terms of torsion about the Csp2—O bond and changes in the bond angles near the methoxy moiety. The theoretical 5J numbers can be empirically reproduced by a cos4 ψ function, where ψ is the angle by which the methoxy group twists out of the benzene plane. In general, large ortho substituents cause an increase in the magnitude of 5J (bond angle changes), strong π electron donors in the para position cause a decrease in the magnitude of 5J (increased torsional freedom), and π electron acceptors do the opposite (decreased torsions).
The distribution of branched octanols between dodecane and water
Authors: Sagert Norman H. , Lau Danny W. P.
Published: May 2006

The distribution of four branched chain octanols, 3-ethyl-3-hexanol, 4-ethyl-3-hexanol, 2-ethyl-4-methylpentanol, and 4-octanol, has been measured between dodecane and water. Measurements were made at alcohol concentrations in the dodecane of less than 0.1 mol/dm3, and as a function of temperature from 10 °C to 35 °C. From these distribution data, standard thermodynamic functions for transfer were calculated. Standard Gibbs energies of transfer from water to dodecane at 25 °C were in the range −14.1 to −15.1 kJ/mol, whereas the standard enthalpies of transfer at 25 °C varied from 29 to 39 kJ/mol. Thus, the change in the standard enthalpy tends to inhibit transfer, but a large standard entropy of transfer results in dodecane being the favoured phase.
n–π Electron donor–acceptor complexes. II. Aliphatic amines with dinitrobenzenes
Authors: Singh Joaquín O. , Anunziata Jorge D. , Silber Juana J.
Published: May 2006

The interaction of several aliphatic amines as n-donors and dinitrobenzenes (DNB) as π-acceptors has been studied in n-hexane. The formation of electron donor – acceptor (EDA) complexes is proposed to explain the spectroscopic behaviour of the mixtures. The stability constants (Ks) for these complexes have been calculated by an iterative procedure. For a given acceptor, the donor strength of RNH2 > R2NH > R3N was found. This order is explained by considering the role that steric effect may play in the EDA complex formation. On the other hand, the fact that for a given donor Ks follows the order 1,2-DNB > 1,3-DNB > 1,4-DNB, and that 1,2-DNB reacts with primary amines, led to the proposal of orientational complexes. These EDA complexes may be considered intermediates in aromatic nucleophilic substitution reactions.
Synthesis of novel, highly reactive 1-oxyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine derivatives
Authors: Csekö József , Hankovszky H. Olga , Hideg Kálmán
Published: May 2006

New multipurpose spin labels and synthons can be synthesized from 1-oxyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-4-carboxylic acid (1). Either 1, its pentafluorophenylester (2), or acylcarbonate (3) (prepared from 1 with ethyl chloroformate) can be reduced to 1-oxyl-2,2,6,6-tetramethyl-4-hydroxymethyl-1,2,5,6-tetrahydropyridine (4). The methanesulfonate (6) and the bromo compound (7) are highly reactive alkylating reagents due, in large part, to the allylic linkage in the nitroxide ring. The latter can be reacted with KSSO2CH3 to give methanethiolsulfonate (9). A novel side chain spin-labelled α-amino acid ester (11) is made by a phase-transfer alkylation of a Schiff base with 7. Compound 11 can be coupled with the protected L-phenylalanine in the presence of dicyclohexylcarbodiimide (DCC) to produce the nitroxide dipeptide (12).
Identification of the product from the reduction of permanganate ion by trimethylamine in aqueous phosphate buffers
Authors: Mata-Perez Fernando , Perez-Benito Joaquin F.
Published: May 2006

The product obtained from the reduction of potassium permanganate by trimethylamine in aqueous phosphate buffers has been identified as a soluble form of colloidal manganese dioxide which is stabilized in solution by adsorption of phosphate ions on its surface. The dependence of the rate of flocculation on several experimental variables has been studied. Phosphate ions have been found to increase the solubility of the colloid by increasing the apparent energy of activation of the flocculation process. This could explain the well-known capacity of those ions for retarding the precipitation of manganese dioxide.
Microbial hydroxylation of steroids. 11. Hydroxylation of A-nor-, B-homo-Δ1-, and Δ1-testosterone acetates by Rhizopusarrhizus
Authors: Holland Herbert L. , Chenchaiah P. Chinna
Published: May 2006

The testosterone derivatives A-nortestosterone acetate and B-homo-Δ1-testosterone acetate have been incubated with Rhizopusarrhizus, a fungus which hydroxylates testosterone at the C-6β and C-11 α positions. These modes of hydroxylation were also observed for the A-nor steroid. The B-homo steroid, however, was oxidized at C-2β and concurrently hydroxylated at C-6β and C-7aβ (but not at C-11) by R. arrhizus. Δ1 steroids of conventional skeleton were hydroxylated at C-11α by R. arrhizus; in addition, Δ1-androstenedione was epoxidized at the 1,2β position. No C-6 hydroxylation of conventional Δ1,4-3-ketosteroids was observed.
Surface tension and density of the molten PbCl2–KCl–NaCl ternary system
Authors: Fujisawa T. , Utigard T. , Toguri J. M.
Published: May 2006

The maximum bubble pressure method was used to determine the surface tension and density of melts within the PbCl2–KCl–NaCl system. The temperature range of this study was from 450 to 800 °C. In all cases, the surface tension was found to decrease with increasing temperature. At constant molar ratios of KCl to NaCl, a minimum in the surface tension was observed at approximately 40 mol% PbCl2. The ternary surface tension values were found to obey the simple additivity expression of the binary surface tensions of PbCl2–KCl and PbCl2–NaCl. Based on these findings, constant surface tension contours have been drawn.The density obtained in the present study agree well with the previously determined densities using a bottom-balance Archimedean technique reported by this laboratory.
Kinetics of the reduction of nicotinonitrile cations by 1,4-dihydronicotinamides
Authors: Bunting John W. , Brewer John C.
Published: May 2006

The rates of reduction of a series of 1-(Z-benzyl)nicotinonitrile cations by a series of 1-(X-benzyl)-1,4-dihydronicotinamides have been studied at 25 °C in 20% CH3CN – 80% H2O (pH 7.0 (5 mM phosphate), ionic strength 1.0 (KCl)). Spectral studies indicate the formation of 1,4-dihydronicotinonitrile products, without the formation of the isomeric 1,2-dihydro- or 1,6-dihydro-nicotinamide intermediates. Second-order rate constants (k2) for these reductions are closely correlated with the Hammett σ constants for X and Z. Thus, for X = H, log k2 = 0.63σz − 1.05, while for Z = 4-CN, log k2 = −0.64σx − 0.65. The close correspondence between these ρx and ρz values indicates that charge neutralization on the nicotinonitrile cation exactly balances charge generation on the nicotinamide cation product in the rate-determining transition state. Thus the migrating hydrogen species is electrically neutral in the rate-determining transition state, which contrasts with the hydridic transition states previously reported in the reduction of isoquinolinium cations by 1,4-dihydronicotinamides. When 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide is used as the reductant, primary kinetic isotope effects of 3.0 and 2.7 are observed for the reduction of the 1-methylnicotinonitrile and 1-(4-cyanobenzyl)-nicotinonitrile cations, respectively. These data are evaluated in terms of the various mechanistic possibilities for hydride transfer.
The reactions of some α-halo-β-ketosulfones with hard and soft nucleophiles. A preparation of sulfinate esters
Authors: Grossert J. Stuart , Dubey Pramod K. , Elwood Tom
Published: May 2006

The α-halo-β-ketosulfonyl functionality was attacked by ethanethiolate at the halogen which, in the presence of a proton source, usually resulted in reduction to the β-ketosulfone. A range of other soft nucleophiles did not react, whereas hard nucleophiles invariably attacked at the carbonyl, with subsequent cleavage to α-halosulfones. Ambident arylsulfinate anions usually attacked at halogen to form the sulfonyl halide, which reacted with excess sulfinate to give an intermediate that is readily attacked by alcohols. This reaction resulted in the formation of alkyl arylsulfinate esters in reasonable yields. The same reaction occurred when sulfonyl chlorides were reacted with sulfinate salts and it was rationalized by using HSAB principles. Spectral properties of some sulfinate esters are presented.
Photon harvesting polymers: intracoil energy transfer in anthryl- and fluorescein-tagged polystyrene
Authors: Hargreaves J. S. , Webber S. E.
Published: May 2006

Polystyrene containing small mole fractions of pendent 9,10-diphenylanthracene (DPA) and fluorescein (F) was prepared. Absorption of light by DPA results in very efficient intracoil sensitization of the 1F* state. The quantum efficiency (χ) of this process has been estimated as 0.75 in dimethylformamide (DMF) and 0.95 in 5:1 DMF/H2O. The increase in χ in this latter solvent mixture is ascribed to the diminished polymer coil size in this poorer solvent. Analysis of the time dependence of fuorescence suggests a combined static and dynamic quenching of 1(DPA)* by energy transfer. Studies of 1(DPA)* and 1(F)* fluorescence quenching by DABCO corroborate this model.
Substituent effects in alkynes and cyanides: a momentum density perspective
Authors: Simas Alfredo M. , Smith Jr Vedene H. , Thakkar Ajit J.
Published: May 2006

Apriori quantum mechanical calculations are made for the alkynes XCCH (X = H, Li, F, Cl, and NC), and the cyanides XCN (X = H, F, Cl, O−, S−, and NC) in order to examine how substituent effects in these homologous series are reflected by changes in the three-dimensional one-electron momentum density. Our qualitative analysis is based on a partial wave decomposition of the momentum density, and is in harmony with analogous studies of the position space charge density. Our quantitative analysis is based on various properties of the isotropic momentum density and on the kinetic energy anisotropy. Values of the latter quantity relative to the parent unsubstituted molecule of the pertinent homologous series are found to correlate with the Taft resonance and Dewar–Grisdale mesomeric substituent constants.
Étude théorique des réactions d'abstraction d'hydrogène , avec R, X≡H, CH3, NH2,OH et F
Authors: Leroy Georges , Sana Michel , Tinant Anne
Published: May 2006

Hydrogen abstraction reactions [Formula: see text] with R, X≡H, CH3, NH2,OH, and F have been studied at the abinitio 6-31G – UHF level. However, energetic properties were computed at the CI level. Rate constants and Arrhenius parameters have been obtained using the transition state theory formalism with Eckart's tunneling correction. The discrepancy between theoretical and experimental results led us to elaborate a semi-empirical procedure to calculate activation barriers, in which the bonds R—H and X—H are represented by Morse curves. Thus, the agreement between theory and experiment is much better. Moreover, the results obtained by this procedure demonstrate the non-Arrhenius behavior of all the reactions under consideration and allow us to rationalize a large number of experimental facts.
Photofrins: a spectral study
Authors: Pottier Roy , Laplante Jean Pierre , Chow Ying-Fun Ada , Kennedy James
Published: May 2006

The absorbance and fluorescence spectra of Hematoporphyrin IX, Photofrin, and Photofrin II are measured as a function of pH, concentration, and temperature, in order to isolate the ionic equilibria from the monomer/dimer/aggregate equilibria. Based on photometric and volumetric titrations, the distribution diagram for four ionic species is presented, with pKa, values of 2.7, 5.2, and 9.1. Protonation sites for these pKa's are assigned.
Étude d'un modèle simplifié de la rhodopsine: la toute trans-rétinylidène tert-butylamine en présence de divers acides carboxyliques
Authors: Bissonnette Martine , Thanh Hoa Le , Vocelle Daniel
Published: May 2006

A model system of the chromophore of rhodopsin consisting of a Schiff base (all-trans-retinylidene tert-butylamine) and several carboxylic acids has been studied by uv–vis and 1H nmr spectroscopy. In methanol, a polar solvent, results indicate that weak acids are able to protonate fully the imine function, that all the double bonds are perturbed by protonation, that a leveling effect is present, and, finally, that a proton exchange between the protonated Schiff base and the solvent is also present. In chloroform, results indicate that weak acids cannot fully protonate the Schiff base and that an equilibrium exists between an ion pair [Formula: see text] and a neutral species [Formula: see text]. Furthermore, an interaction similar to Nakanishi's external point charge seems to exist between the polyenic chain of the Schiff base and a polar function on the acid. Finally, two comparisons are established: one with a similar ScHiff base – the trans,trans-2,4-heptadienylidene tert-butylamine – the other with rhodopsine.
Étude, par spectroscopie de vibration, de l'autoprotolyse de l'acide nitrique absolu. Nouvelle interprétation
Authors: Herzog-Cance Marie-Hélène , Potier Antoine , Potier Jacqueline
Published: May 2006

The ir and Raman spectra of solid and liquid anhydrous HNO3 have been investigated for the purpose of solving the self-dissociation problem. Identified species are: the oligomer (HNO3), (chains), NO2+ and (NO3•3HNO3)− ions, two monohydrate forms: the ionic pair H2OH+… ONO2− and the molecular association H2O … HONO2, and, finally, some free H3O+ ions.
Photoélectrochimie des phtalocyanines sans métal, de cuivre et de fer
Authors: Marsan Benoît , Bélanger Guy , Piron Dominique-Louis
Published: May 2006

The cathodic and photocathodic behavior of electrodes of phthalocyanines without metal or complexed to copper or iron (H2Pc, CuPc, and FePc) has been studied in acidic, neutral, and alkaline media. In the dark, the polarization curve of the electrode of FePc shows two peaks of reduction which are not observed with the other phthalocyanines; these are associated with the reduction of the central cation. Lighting the semiconducting electrodes does not produce any effect on the reaction of production of hydrogen. An analysis of the curves of capacity vs. potential indicates the presence of intermediate levels localized within the forbidden band of the semiconductors and covering about 1% of their sites on the surface. The results can be explained if the phthalocyanines are considered to be semiconducting electrodes of type p. The energy of the bands and of the surface states can be depicted with a semiconductor–electrolyte interphase model and the nature of the charge transfers inhibited by the presence of these interfacial states is discussed. [Journal translation]
Un problème toujours d'actualité: trois électrons – trois centres
Authors: Berthier G.
Published: May 2006

The present status of theoretical calculations on free radicals is discussed, taking allyl as a benchmark. The nature of the atomic dissociation products included in a symmetry-adapted determinantal wave function gives a useful, heuristic criterion for symmetry-breaking cases.
Fourier transform infrared study on black soap films
Authors: Umemura Junzo , Matsumoto Mutsuo , Kawai Takeshi , Takenaka Tohru
Published: May 2006

Fourier transform infrared (FT-IR) spectra in the range 3800–2800 cm−1 have been recorded of the first and second black films in air prepared from aqueous solutions of sodium dodecylsulfate (SDS) without and with the addition of sodium chloride, respectively. It is found that in both films the methylene chains of SDS, which are located outside the aqueous core of the sandwich structure, are in an oriented liquid crystalline state. In a first black film, the thickness of the aqueous core is determined to be 9.6 nm, by recording polarized infrared spectra with various angles of incidence to the film surface, and by analyzing them with the aid of thin layer optics. It is found that this value varies in the range 5 ~ 10 nm from film to film. The extinction coefficients of the aqueous core are also examined. In the second black film, the thickness of the aqueous core is found to be about 1 nm. The OH stretching band of water in the second black film appears at appreciably higher wavenumbers (by 60 cm−1) than those of the first black film, indicating a high ionic strength in the aqueous core. In both black films, there is no sign of molecular orientation or rigidified structure of water.
The well-tempered GTF basis sets and their applications in the SCF calculations on N2, CO, Na2, and P2
Authors: Huzinaga S. , Klobukowski M. , Tatewaki H.
Published: May 2006

We have started to prepare a new family of high-quality Gaussian-type function basis sets capable of producing near Hartree–Fock atomic and molecular wave functions. A conspicuous feature of the family of basis sets is that many of the exponent parameters are shared among s-, p-, d-, and f-basis functions. In the present paper we report the first phase of the work covering atoms from Li to Ar, together with near Hartree–Fock calculations on N2, CO, Na2, and P2 as the applications. A modified form of Raffenetti's contraction scheme is used.
Photoreluctant azoalkanes: 185-nm photolysis of E- and Z-diazacyclooct-2-ene
Authors: Adam Waldemar , Oppenländer Thomas
Published: May 2006

The l85-nm photolyses off E- and Z-1,2-diazacyclooct-1-enes (1) result mainly in denitrogenation, affording 1-hexene and cyclohexane in a 2:1 ratio. In comparison to the direct 350-nm photolyses the denitrogenation efficiencies are enhanced by 42- and 26-fold for the E- and Z-isomers, respectively. It is proposed that the photoisomerization (350 nm) takes place from the S1(n−,π*)-state and the photodenitrogenation (185 nm) out of the S2-state.
Two-photon spectroscopy of the Schiff base of all-trans-retinal. Nature of the low-lying π π* singlet states
Authors: Murray Lionel P. , Birge Robert R.
Published: May 2006

The two-photon excitation spectrum of all-trans-N-retinylidene-n-butylamine (ATRSB) in EPA at 77 K is obtained over the wavelength region from 370 to 455 nm (λex/2) using linearly polarized light. The two-photon excitation maximum is observed at ~422 nm (λex/2) and is red shifted ~2800 cm−1 from the one-photon absorption maximum at ~377 nm. We assign the two-photon excitation spectrum to the "1Ag*−" ← S0 transition which indicates that the "1Ag*−" π π* state lies below the "1Bu*+" π π* state in ATRSB. Comparisons of the one-photon absorption, two-photon excitation, and fluorescence spectra of ATRSB with the corresponding spectra of all-trans-retinal are presented. PPP-CISD calculations correctly predict the directions but not the magnitudes of the blue shifts of the π π* excited state transition energies in going from all-trans-retinal to ATRSB. We postulate that the "1Ag*−" π π* state is preferentially stabilized relative to both the ground state and the nearby "1Bu*+" state by hydrogen bonding to solvent molecules. Comparison of the spectra reported here with the two-photon spectra of rhodopsin provides further evidence that the chromophore in rhodopsin is protonated.
Structure of bacteriorhodopsin in the acidified membrane and at high ionic strength: resonance Raman study
Authors: Massig G. , Stockburger M. , Alshuth Th.
Published: May 2006

Resonance Raman (RR) spectra of the retinylidene Schiffs base (SB) chromophore of bacteriorhodopsin (BR) were obtained from purple membrane (PM) suspensions of Halobacteriumhalobium in three different equilibrium states: (1) the native state at neutral pH; (2) BR-605, the acidified membrane at pH 2; (3) BR-565(Cl−) or BR-545 (F−), at low pH and high ionic strength. In the native state the retinyl chromophore is stabilized by an ion-pair complex of the protonated SB group with a negative carboxylate counterion (A−) of the protein, giving [Formula: see text]. In the acidic form, BR-605, it is inferred from the RR spectra that the chromophore is immobilized. This is ascribed to a breakdown of the ionic interaction by protonation of A−. In the two (3) forms the RR spectra show that the original structure of the chromophore in the native state is reestablished, which is attributed to the formation of [Formula: see text] complexes with the anions. It thus turns out from RR spectroscopic evidence that the retinyl chromophore in BR is largely governed by the [Formula: see text] ion-pair formation, which is stabilized by surrounding water molecules.
Antibonding σ* orbitals in molecular spectroscopy
Authors: Robin M. B.
Published: May 2006

This paper discusses the role of σ* antibonding valence MO's in the electronic spectra of large molecules. Using the term value and antishielding concepts, molecular absorption patterns terminating at σ* MO's are systematized and rationalized; the frequencies of such excitations correlate with the ground-state bond strengths of the bonds giving rise to the σ* MO's. In unsaturated systems, the π → σ* and np → σ* excitations can be distinguished on the basis of their term values. Confusion in the past regarding the existence of excitations to σ* relates to the fact that σ* MO's often can be totally mixed into the surrounding sea of Rydberg levels of the same symmetry, and for all practical purposes can cease to exist except as perturbations on the Rydberg levels. For nonpenetrating Rydberg manifolds there is little or no mixing with σ* MO's of the same symmetry, setting the stage for the appearance of valence-shell excitations to σ* in high-symmetry molecules which are analogous to the d → f inner-well giant resonances of atomic spectroscopy.
1H and 13C nmr study of exchange and isomerism in dimethylsulphoxide solutions of neutral methylmercury–xanthosine complexes
Authors: Allaire François , Simard Michel , Beauchamp André L.
Published: May 2006

The xanthosine (H2Xan) complexes (CH3Hg)(HXan) and (CH3Hg)2(Xan) have been studied in (CD3)2SO solution at room temperature by 1H (90 MHz) and 13C nmr (20.1 MHz) spectroscopy. For (CH3Hg)2(Xan), where the N1-H and N3-H protons have been substituted by CH3Hg+ groups, individual 1H and 13C resonances from the unequivalent CH3Hg+ groups indicate that these cations do not exchange rapidly between the two sites. For solutions of (CH3Hg)(HXan), in which only one of the N—H protons has been substituted, both types of spectra show two sets of resonances for the purine unit as well as for the CH3Hg+ groups. This indicates that the 1:1 complex exists as two linkage isomers (N3—H, N1—Hg) and (N3—Hg, N1—H) in slow exchange. These two isomers are formed in equal amounts. When free xanthosine is added, the N3-mercurated complex exchanges rapidly with the free ligand, whereas the N1-mercurated complex does not exchange.
Magnesium – nucleotide interaction. Synthesis, structure, 1H, 13C nuclear magnetic resonance and Fourier transform infrared studies of Mg-inosine-5′-monophosphate complexes
Authors: Tajmir-Riari H. A. , Theophanides T.
Published: May 2006

The reaction of inosine-5′-monophosphate acid (IMPH2) with hydrated magnesium chloride MgCl2•6H2O gives complexes of the type Mg(IMP)•5H2O and Mg2(IMP)3•15H2O. The FT-IR spectra of Mg(IMP)•5H2O in the region 1800–400 cm−1 showed marked similarities with that of the structurally characterized N7-bound Ni(IMP)•5H2O and Co(IMP)•5H2O complexes, while Mg2(IMP)3•15H2O exhibited distinct spectral similarities with the known Cd2(IMP)3•12H2O and Ca(IMP)•6.5H2O compounds, where a direct metal–phosphate and metal–sugar binding as well as metal–N7-coordination has been shown crystallographically. The Mg(II) ion in Mg(IMP)•5H2O is N7-bound with an indirect metal–phosphate and metal–carbonyl interaction through a coordinated water molecule and in the Mg2(IMP)3•15H2O compound it binds directly to the phosphate and to the sugar moiety as well as to the N7-site of the purine ring system.
Branched-chain sugars in asymmetric synthesis. Total synthesis of marine antibiotic (−)-malyngolide
Authors: Ho Pak-Tsun , Wong Simmie
Published: May 2006

Marine antibiotic (−)-malyngolide has been stereospecifically synthesized in about 25% overall yield from 2,3-O-isopropyl-idene-D-apiose, easily prepared from D-mannose.
A nuclear magnetic resonance study of the equilibria and kinetics of the reaction of penicillamine with cystine and related disulfides
Authors: Theriault Yvon , Rabenstein Dallas L.
Published: May 2006

The thiol/disulfide exchange reactions of penicillamine (PSH) with cystine and several related disulfides (RSSR) have been studied by 1H nmr. The reactions take place in two steps:[Formula: see text]The equilibria and kinetics of the reactions of PSH with cystine were characterized over the pH range 5–8, while the reactions with the disulfides of cysteamine, homocysteine, 2-mercaptoethanol, mercaptoacetic acid, 3-mercaptopropionic acid, and mercaptosuccinic acid were studied at neutral pH. From the pH dependence of the rate of the reaction of PSH with cystine, the reactive species are identified as penicillamine with its amino group protonated and its thiol group deprotonated and cystine and penicillamine–cysteine mixed disulfide with their amino groups protonated. For all the disulfides studied, the extent to which the first reaction occurs is within a factor of 2–3 of that predicted by a random distribution, while the extent to which the second reaction occurs is considerably less than for a random distribution. This is attributed to steric effects due to the two methyl groups next to the sulfur of penicillamine.
A Raman spectroscopic study of the equilibria in aqueous solutions of Hg(II) chloride
Authors: Bentham Patrice B. , Romak Cynthia G. , Shurvell Herbert F.
Published: May 2006

The techniques of factor analysis (principal component analysis) and band contour resolution have been applied to Raman spectra obtained from seven aqueous solutions of HgCl2 with various amounts of HCl added to increase the chloride to mercury ratio. The digitized spectra of the Hg—Cl stretching region (200–380 cm−1) were subjected to baseline fitting and removal, and normalization using the v2 band of ClO4− (380–530 cm−1) as an internal intensity standard. Factor analysis indicated the presence of three scattering components in the Hg—Cl stretching band envelope. Band resolution gave the positions of peaks assigned to HgCl2, HgCl3−, and HgCl42− at 320, 290, and 269 cm−1, respectively. Approximate values for equilibrium constants for the equilibria present in the system have been calculated from the resolved band areas.
Multicomponent diffusion of aqueous sodium carbonate and aqueous sodium bicarbonate
Authors: Leaist Derek G. , Noulty Robert A.
Published: May 2006

Diffusion coefficients of Na2CO3 + H2O and NaHCO3 + H2O have been measured by a conductimetric technique at 25 °C. Above 0.1 mol L−1, diffusion of aqueous Na2CO3 is typical of a simple binary electrolyte. At lower concentrations a significant proportion of the Na2CO3 component diffuses as NaHCO3 and NaOH produced by hydrolysis: [Formula: see text]. Because OH− is more mobile than HCO3−, binary Na2CO3 + H2O solutions undergoing diffusion separate spontaneously into two ternary solutions: Na2CO3 + NaOH + H2O ahead of the diffusion boundary and Na2CO3 + NaHCO3 + H2O left behind the boundary. A procedure is developed for measuring multicomponent diffusion coefficients of hydrolyzing salts. Results for Na2CO3 are in satisfactory agreement with predicted ternary values. As a consequence of the disproportionation reaction: [Formula: see text], diffusion of aqueous NaHCO3 is formally a ternary process. However, because reactants and products have similar mobilities and because only about 1% of the salt disporportionates, diffusion of aqueous NaHCO3 is effectively binary.
The stereochemistry of some 7-hydroxyindene dimers: applications of 1H nuclear magnetic resonance relaxation pathway analysis
Authors: Chazin Walter J. , Colebrook Lawrence D. , Davis Brian R. , McCormick I. Ross N. , Johnson Stephen J.
Published: May 2006

1H spin-lattice relaxation rates and nuclear Overhauser effect difference spectra have been measured (at 400 MHz) for six dimers derived from methyl-substituted 7-hydroxyindenes. Used in combination, these measurements identify the relaxation pathways available to all of the protons in the molecules. Analysis of these pathways has permitted assignment of all chemical shifts, and identification of the relative stereochemistry at all chiral centres.
Bridged-ring steroids. III. The synthesis of bridged steroids with a bicyclo[2.2.1]heptane ring B system
Authors: Yates Peter , Winnik Françoise M.
Published: May 2006

Reaction of cholesta-5,7-dien-3β-yl 2-tetrahydropyranyl ether with bromoform and potassium tert-butoxide gave 3′,3′-dibromo-3′,6β-dihydrocyclopropa[5,6]-5a-cholest-7-en-3β-yl 2-tetrahydropyranyl ether (1c), which on treatment with lithium aluminum hydride and water gave 5,8α-methano-5α-cholest-6-en-3β-yl 2-tetrahydropyranyl ether (2c). This has been converted to 3β-acetoxy-5,8α-methano-5α-cholestan-6-one (16c) and 3β-acetoxy-5,8α-methano-5α-cholestan-7-one (17c). A preliminary investigation of the photochemistry of 16c and 17c has shown that the incorporation of the bicyclo[2.2.1]heptan-2-one system in the B ring of a steroid results in photoreactions of this system that are different from those observed in the case of simple bicyclo[2.2.1]heptan-2-ones.
Molecular recognition. V. The binding of the B human blood group determinant by hybridoma monoclonal antibodies
Authors: Lemieux Raymond U. , Venot André P. , Spohr Ulrike , Bird Paul , Mandal Gaurhari , Morishima Naohiko , Hindsgaul Ole , Bundle David R.
Published: May 2006

A systematic study was made of the effect of changes in the structure of αLFuc(1→2)[αDGal(1→3)]βDGal-OMe (1) on the inhibition of the binding of an artificial antigen by two monoclonal anti-B antibodies. Both OH-4 of the βDGal unit and OH-2 of the αDGal unit are essential for binding by one of the antibodies. For the other antibody, OH-3 of the αDGal unit also forms part of the key polar grouping. Both the antibodies recognize a large lipophilic surface extending from the CH3-6 group of the αLFuc unit along the α-sides of this group and the βDGal unit to include its CH2OH-6 group, most likely intramolecularly hydrogen bonded to the neighboring O-5. The lipophilic surface then extends to include the CH2OH-6 group of the αDGal unit intramolecularly hydrogen bonded to the O-5 atom. The stability of the complex is attributed to nonpolar interaction of this surface within a hydrophobic cleft-like combining site which offers a polar grouping at its periphery for interaction with the key polar grouping of the trisaccharide. The solution conformer for 1, as determined by 1H nmr, is present in its crystal structure.
Evaluation of homogeneity of marine biological tissues for lead and cadmium by GFAAS
Authors: Kratochvil Byron , Thapa Ram S. , Motkosky Norine
Published: May 2006

Several marine biological tissues were tested for homogeneity with respect to lead and cadmium. The variance for the certified reference material TORT-1 is approximately 1 μg/g for each metal. This is of the same order as the heterogeneity of spray-dried dogfish and swordfish tissue. Scallop adductor muscle was of the same level of heterogeneity for cadmium; the lead content was below the limit of detection of the procedure used.
Introduction of boundary conditions in the Pelton–Flengas Gibbs–Duhem integration for ternary solutions
Authors: Karakaya Ishak , Thompson William T.
Published: May 2006

A multiple linear regression technique to represent partial excess property data for one component (A) in a ternary A–B–C system is presented. The procedure, following the methodology of Pelton and Flengas, recognizes the need to satisfy Henry's law for components B and C in solutions rich in component A as well as independent experimental knowledge which may be available for the B–C binary.
Characterization of thymidine ultraviolet photoproducts. Cyclobutane dimers and 5,6-dihydrothymidines
Authors: Cadet Jean , Voituriez Lucienne , Hruska Frank E. , Kan Lou-Sing , Leeuw Frank A. A. M. de , Altona Cornelis
Published: May 2006

We describe the preparation of the six configurationally distinct cyclobutane-type photodimers generated by the acetone-sensitized uv irradiation of thymidine in aqueous solution. Also prepared as minor photoproducts are the 5R and 5S di-astereoisomers of 5,6-dihydrothymidine, and two novel molecules, the 5R and 5S diastereoisomers of 5-acetonyl-5,6-dihydrothymidine. The purification of the ten molecules by chromatographic techniques (hplc, tlc) is described, along with their extensive characterization by uv, ir, cd, FAB-ms and 1H nmr. The proton chemical shifts and coupling constants are discussed in terms of the geometry of the pyrimidine, cyclobutane, and sugar rings.
The thermal decomposition of cyclobutane-1,2-dione
Authors: Cao J.-R. , Back R. A.
Published: May 2006

The thermal decomposition of cyclobutane-1,2-dione has been studied in the gas phase at temperatures from 120 to 250 °C and pressures from 0.2 to 1.5 Torr. Products were C2H4 + 2CO, apparently formed in a simple unimolecular process. The first-order rate constant was strongly pressure dependent, and values of k∞ were obtained by extrapolation of plots of 1/k vs. 1/p to1/p = 0. Experiments in a packed reaction vessel showed that the reaction was enhanced by surface at the lower temperatures. Arrhenius parameters for k∞, corrected for surface reaction, were log A (s−1) = 15.07(±0.3) and E = 39.3(±2) kcal/mol. This activation energy seems too low for a biradical mechanism, and it is suggested that the decomposition is probably a concerted process. The vapor pressure of solid cyclobutane-1,2-dione was measured at temperatures from 22 to 62 °C and a heat of sublimation of 13.1 kcal/mol was estimated.