bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural studies of organoboron compounds. XLVI.1 5,5-Pentamethylene-2-phenyl-4,5-dihydrooxazol-N-oxide(NOB)trifluoroborane
Authors: Kliegel Wolfgang , Schumacher Ute , Rettig Steven J , Trotter James
Published: May 2006

The reaction of N-hydroxy-N-[(1-hydroxycyclohexyl)methyl]benzamide with dimethylether-boron trifluoride gives 5,5-penta-methylene-2-phenyl-4,5-dihydrooxazol-N-oxide(NOB)trifluoroborane [3-(2,2,2-trifluoro-1-oxa-2-borataethyl)-5,5- pentamethylence-2-phenyl-1-oxa-3-azonia-2-cyclopentene] in high yield. Crystals of the product are monoclinic, a = 8.1147(4), b = 15.448(1), c = 11.4144(6) Å, β = 95.133(5)°, Z = 4, space group P21/c. the structure was solved by direct mothods and was refined by full-matrix least-squares procedures to R = 0.043 and Rw = 0.058 for 2178 reflections with I ≥ 3σ(I). the structure analysis confirms that the title compound has an open-chain boron-nitrogen betaine structure and, as such, is the first isolated and fully characterized open-chain non-chelate boron complex of a cycloimidate N-oxide (cyclohydroximate). Bond lengths (corrected for libration) involving the tetrahedrally coordinated boron atom are OB = 1.530(3) and FB = 1.370(2)1.391(2) Å.Key words: crystal structure, organoboron compound, boron compound.
Étude de la conformation d'imines thiophéniques
Authors: Oussaid Boualem , Ayad Mohamed , Garrigues Bernard , Fayet Jean-Pierre , Tran Le Trang
Published: May 2006

Conformation of 2-iminomethylthiophenes and of the 2,5-bis(methyliminomethyl)thiophene has been elucidated from theoretical investigations, dipole moment measurements, use of stereospecific coupling through five bonds, as well as Overhauser effect experiments. Only one conformation has been found: (2ctrans) for the 2-iminomethyl thiophenes and (2c 5ctrans) for the 2,5-bis(methyliminomethyl)thiophene. Keywords: thiophene, imine, conformation.
Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system
Authors: Milne Paul H. , Wayner Danial D. M. , DeCosta Dayal P. , Pincock James A.
Published: May 2006

The electrochemical oxidation and reduction potentials of a number of substituted 1-methylnaphthalenes (1a-l) and 1-naphthylmethyl radicals (2a-l•) as well as 2-methylnaphthalene (3) and the 2-naphthylmethyl radical (4•) have been measured by cyclic voltammetry and photomodulation voltammetry. The oxidation potentials correlate with σ+ (ρ+ = −7.1 and −8.4 for 1 and 2• respectively) while the reduction potentials correlate with σ− (ρ− = 10.1 and 13.0 for 1 and 2• respectively). The relative magnitude of the ρ values can be rationalized when the charge density distribution in these systems is considered. This leads to the interesting conclusion that even though a full charge is placed in the π-system of 1 when it is oxidized or reduced, the fraction of the charge that accumulates at C4 is actually less than in 2+ or 2− where only 50–70% of the charge is delocalized into the ring. A correlation between ρ for the redox reactions of 1, 2•, benzyl, diphenylmethyl, and cumyl and the calculated (AM1) charge density at C4 is established, implying that the sensitivity of the corresponding ions to substituent effects increases as the fraction of charge at that site increases. The redox data have been used in thermochemical cycles in order to estimate the substituent effect on the homolytic, mesolytic, and heterolytic cleavage reactions of 1 and its corresponding radical ions. The implication of these results on the C—C cleavage versus deprotonation of radical cations and on the photochemical homolysis versus heterolysis of naphthylmethyl halides and acetates is discussed. Keywords: electrochemistry, homolysis, heterolysis, naphthylmethyl, substituent effect.
Synthetic applications of glycidic thiolesters. Regioselective reduction to 1,3-diols and 2,3-epoxy alcohols
Authors: Liu Hsing-Jang , Luo Weide
Published: May 2006

Glycidic thiolesters were shown to undergo regioselective reduction with Raney nickel to give 1,3-diols. With sodium borohydride at room temperature and lithium aluminum hydride at −78 °C, the reduction of glycidic thiolesters was found to proceed chemoselectively to furnish 2,3-epoxy alcohols. Keywords: glycidic thiolesters, reduction, 1,3-diols, 2,3-epoxy alcohols.
The reduction of α-bromocamphor by tertiary amines
Authors: Chen Jian Jeffrey , Tanner Dennis D.
Published: May 2006

The reduction of α-bromocamphor to camphor by N,N-dimethylaniline (DMA) at high temperatures (> 200 °C) proceeds via a free radical chain sequence. The reduction can be effected with DMA or triethylamine (TEA) in acetonitrile at much lower temperatures in the presence of di-tert-butylperoxide. The chain termination step is the dimerization of the camphor radical. These reductions presumably constitute an example of an electron transfer chain mechanism involving a tertiary amine as the chain propagating species. Keywords: reduction, tertiary amines, α-bromocamphor, chain reaction.
On the accuracy of the algebraic approximation in electronic structure calculations for the ground states of H32+ and H43+ using basis sets of atom-centred Gaussian-type functions
Authors: Wilson S
Published: May 2006

Sequences of even-tempered basis sets of atom-centered Gaussian-type functions are employed to explore the accuracy that can be achieved for the ground state energies of the H32+ and H43+ species in calculations using finite basis sets. These systems are considered as prototypes for the treatment of polyatomic molecules of arbitrary symmetry within the algebraic approximation. Keywords: algebraic approximation, basis sets, even-tempered, universal.
Abinitio study on radical anions of nitro compounds
Authors: Ramondo Fabio
Published: May 2006

The results of abinitio molecular orbital calculations on some nitro compounds, RNO2 (R = CH3, CF3, CH2F, BH2, BF2, BHF, C6H5) and on the corresponding radical anions are reported. The geometries of the neutral and charged species were optimized at the SCF and MP2 levels of theory employing the 6-31G* and the 6-31 + G* basis sets. The rotation about the CN and BN bonds reveals distinct conformations for each molecule and vibrational frequencies were determined for such structures. All the employed levels of theory predict structural differences between neutral and charged molecules. The major geometrical changes occurring by electron adding to RNO2 consist in the lengthening of the NO bonds and in the shortening of the CN and BN bonds. The radical anions are calculated to be pyramidal at nitrogen in the stable conformers of CH3NO2−, CF3NO2−, CH2FNO2−, while, for the π-electron-accepting substituents (BH2, BHF, BF2, C6H5), the anion is planar. Electron correlation energy corrections in the framework of Møller–Plesset perturbation theory were included to determine relative stabilities between different conformations. At the MP4/6-31G*//MP2/6-31G* level, an easy rotation of the NO2 group is predicted for all the radical anions and neutral molecules with the exception of BH2NO2−, BF2NO2−, and BHFNO2−, which show high torsional barriers about the BN bond. Keywords: nitro compounds, radical anions, abinitio calculations, molecular structure.
Optimization of quantum Monte Carlo wavefunctions using analytical derivatives
Authors: Bueckert Hartmut , Rothstein Stuart M. , Vrbik Jan
Published: May 2006

We show how to optimize many-parameter wavefunctions for use in quantum Monte Carlo by deriving and utilizing formulas for analytical rather than numerical evaluation of the required derivatives. We present these in a form which is easily vectorizable for use on a supercomputer. We also discuss several technical issues of variational Monte Carlo to ensure both an unbiased and efficient optimization. Finally, we illustrate our optimization scheme's numerical performance by optimizing ground-state wavefunctions for LiH and H2O, each with more than 100 variational parameters. Keywords: quantum Monte Carlo, optimization, electron correlation, LiH, H2O.
Universality among topological properties of electron density associated with the hydrogen–hydrogen nonbonding interactions
Authors: Cioslowski Jerzy , Mixon Stacey T.
Published: May 2006

Nonbonding repulsive interactions between hydrogen atoms, separated by less than ca. 2.18 Å and connected to two carbon atoms in the 1,4-positions, are associated with additional pairs of bond and ring critical points in the electron density, ρ(r), and the corresponding attractor interaction lines. Such topological features of ρ(r) are present in some planar benzenoid hydrocarbons, including chrysene, benzanthracene, and phenanthrene. They also appear in conformations of the biphenyl molecule with the torsional angles between the benzene rings lying within the range of 0°–27°. Properties such as the bond point – ring point distance, the difference in ρ(r) at the bond point and the ring point, and the bond ellipticity are found to follow universal functional dependencies with the hydrogen–hydrogen distance as the controlling variable. The same is true about the corresponding difference in the second derivatives of ρ(r) in the direction of the vector connecting the bond and ring points. Keywords: atoms in molecules, electron density, attractor interaction lines, steric repulsions.
A GTO basis set calculation of properties of the continuum. Photoionization and resonances of He
Authors: Moccia Roberto , Spizzo Pietro
Published: May 2006

By using the K-matrix technique for the continuum states that was previously employed with particularly diffuse L2 basis sets, it is shown that GTO bases are also capable of yielding accurate values for the properties belonging to the electronic continuum. The method has been tested for helium and proved of satisfactory accuracy also for the analysis of the autoionizing states. The results include the phase shifts of the continuum states of the 1Seand 1P° manifolds, the properties of the lowest resonances of these symmetries, the ground state photoionization cross section, and the S contribution to the 1s2p1P° photoionization cross section. The results obtained suggest that the proposed technique should be useful for computing molecular differential photoionization cross sections by exploiting the widely used codes that employ GTO bases. Keywords: photoionization, Gaussian basis sets, helium, autoionizing states.
Computational studies on hydrates of alkaline-earth dications
Authors: Klobukowski Mariusz
Published: May 2006

SCF and Møller–Plesset calculations were done for the hydrated alkaline-earth ions in the gas phase. The systems studied were M2+(H2O)n, where M = Mg, Ca, and Sr, and n = 1,2,3,4, and 6. The successive ion–water bond energies were in the range anticipated on the basis of extrapolation of results from experimental studies on larger hydrates. Keywords: hydrates, alkaline-earth cations, quantum-chemical calculations.
The electronic structure of small transition metal clusters
Authors: Tatewaki Hiroshi , Tomonari Mutsumi
Published: May 2006

In this paper we report the energy level distributions for transition metal clusters obtained by symmetry-broken and symmetry-constrained SCF methods. Investigated clusters were Fe4, Ni4, Cu4, Zn4, Fe6, Ni6, Cu6, and Zn6. Contracted Gaussian-type basis sets of the same size generated by the method of Tatewaki and Huzinaga were used throughout the paper. It will be shown that the d-hole localization model as well as the model with a two-electron-like ionization process is essential to discuss the electronic structure of the positive ion states, and the models bring the energy level distribution of the clusters rather close to the density of the state of the corresponding metals. The calculated first ionization potentials agree quite well with those of experiment. For example, the experimental ionization potential for Fe4 is between 6.3 and 6.5 eV, while calculated values are 5.9 and 6.3 eV for the 3d electron ionization (d−1) and 4s electron ionization (s−1), respectively. If we define the Fermi level of the clusters as the first ionization potential, the Fermi level for Fe4 is composed of the d−1or d−1 + s−1 state. The Fermi level for Ni4 is found to be s−1or s−1 + d−1. Those of Cu4 and Zn4 are definitely of the s−1 state. Further discussion is also presented for larger clusters of M6 (M = Fe, Ni, Cu, and Zn). Keywords: transition metal clusters, ionization potential, abinitio SCF calculations, energy level distributions.
77Se NMR spectroscopic study of the molecular composition of sulfur–selenium melts
Authors: Chivers Tristram , Laitinen Risto S. , Schmidt Kenneth J.
Published: May 2006

A series of six sulfur–selenium mixtures containing 0.4, 5, 15, 25, 35, and 45 mol% of selenium were heated in evacuated ampoules at 430 °C for 4 h and, keeping the material molten at 135 °C, their natural-abundance 77Se NMR spectra were recorded. The equilibrium melt in each case was found to contain both heterocyclic SenS8−n species and polymeric material. The most abundant eight-membered selenium sulfide rings were SeS7, which was the main cyclic component in all melts studied in this work, and 1,2-Se2S6, the quantity of which increased with increasing selenium content of the melt. In addition the melts contained smaller amounts of 1,3, 1,4, and 1,5 isomers of Se2S6, 1,2,3, 1,2,4, and 1,2,5 isomers of Se3S5, 1,2,3,4, 1,2,3,5, and 1,2,5,6 isomers of Se4S4, 1,2,3,4,5-Se5S3, 1,2,3,4,5,6-Se6S2, and Se8. The relative distribution of the cyclic products as a function of the selenium content in the melt was virtually identical with that obtained earlier in the CS2 solutions of the quenched melts, implying that the extracted quenched melts actually represent the original melt composition as far as the SenS8−n species are concerned. The resonances due to the polymeric material in the melts could best be interpreted by the statistical random distribution of selenium and sulfur in the polymeric chains, the probability of occurrence of the different fragments being governed by the overall composition of the melt. As the selenium content increases, the relative amount of selenium bound in the polymer seems to increase at the expense of heterocyclic selenium sulfides. Keywords: selenium sulfides, sulfur–selenium melt, 77Se NMR spectroscopy.
Preparation and characterization of Tl(1,2-C2H4Cl2)B(OTeF5)4: a metal complex of a least coordinating solvent and a least coordinating anion
Authors: Hurlburt Paul K. , Anderson Oren P. , Strauss Steven H.
Published: May 2006

Addition of B(OTeF5)3 to TIOTeF5 in the weakly coordinating solvents dichloromethane, 1,2-dichloroethane, and 1,1,2-trichlorotrifluoroethane produces solutions of M(solv)x+B(OTeF5)4−. When the solvent was 1,2-dichloroethane, the crystalline compound Tl(1,2-C2H4Cl2)B(OTeF5)4 was isolated and studied by X-ray crystallography: triclinic, space group [Formula: see text], a = 9.221 (4), b = 11.396(5), c = 12.538 (4) Å, α = 110.75 (3)°, β = 101.72(3)°, γ = 99.74 (3)°, Z = 2, T = −116 °C. The Tl(1,2-C2H4Cl2)+ cation contains a five-membered chelate ring with Tl—Cl distances of 3.138 (4) and 3.179 (3) Å. The metal ion is weakly bonded to four B(OTeF5)4− counterions, with nine Tl—F interactions that range from 2.950 (5) to 3.981 (8) Å. When the solvent is dichloromethane or 1,1,2-trichlorotrifluoroethane, only the unsolvated solid salt TlB(OTeF5)4 can be isolated by crystallization. This salt is thermally unstable, slowly forming TlOTeF5 and volatile B(OTeF5)3. Keywords: noncoordinating anion, noncoordinating solvent, metal ion solvation.
Tris-triphenylsiloxy compounds of aluminium
Authors: Apblett Allen W. , Warren Alison C. , Barron Andrew R.
Published: May 2006

The reaction of AlMe3 with three equivalents of HOSiPh3 in THF results in the formation of the solvent complex Al(OSiPh3)3(THF) (1). Hydrolysis of 1 yields the stable water complex Al(OSiPh3)3(H2O)(THF)2 (2) in which the THF molecules are hydrogen bonded to the coordinated water ligand. Compounds 1 and 2 have been fully characterized by 1H, 13C, 17O, 27Al, and 29Si NMR and IR spectroscopy. In addition, variable temperature 1H NMR of 2 has been employed to investigate the steric interactions between the phenyl rings of adjacent siloxide ligands. The molecular structures of the solvates 1•(THF) and 2•(THF)1.25 have been determined by X-ray crystallography. 1•(THF): monoclinic P21/c, a = 10.03 (1), b = 23.758 (6), c = 23.294 (7) Å, β = 101.13 (6)°, Z = 4, R = 0.084, Rw = 0.094. 2•(THF)1.25: cubic [Formula: see text], a = 23.034 (3) Å, Z = 4, R = 0.093, Rw = 0.099. Keywords: aluminium, siloxide, hydrolysis, complex, NMR spectroscopy.
Metal-selenolate chemistry: stereochemistry of adamantane-type clusters of formula [(μ-SePh)6(MSePh)4]2− (M = Zn and Cd)
Authors: Vittal Jagadese J. , Dean Philip A. W. , Payne Nicholas C.
Published: May 2006

The structures of three tetramethylammonium salts containing anions of formula [(μ-SePh)6(MSePh)4]2− (M = Zn and Cd) were determined by single crystal X-ray diffraction techniques. The Zn salt crystallizes in different space groups depending upon the solvent combination used in the synthesis. Thus crystals of (Me4N)2[Zn4(SePh)10], 1, grown from a mixture of methanol, acetonitrile, and acetone are triclinic, space group [Formula: see text] with cell dimensions a = 13.214(2), b = 23.859(2), c = 13.072(1) Å, α = 91.134(8), β = 113.350(8), γ = 79.865(9)°, and Z = 2. In the absence of acetone, a solvated crystal (Me4N)2[Zn4(SePh)10]•CH3CN, 2, is formed, which belongs to the monoclinic space group P21/n with a = 14.248(1), b = 39.722(2), c = 13.408(1) Å, β = 97.132(5)°, and Z = 4. The Cd salt (Me4N)2[Cd4(SePh)10], 3, crystallizes in the monoclinic space group P21/c, with a = 20.830(2), b = 14.282(1), c = 25.872(1) Å, β = 99.626(6)°, and Z = 4. These three salts are the first examples of homoleptic, tetranuclear selenolatometal(II) anions with (μ-Se)6M4 cages of adamantane-type stereochemistry. In each case the phenyl substituents of the bridging ligands adopt the configuration [aae, aae, aee, aee], which has the minimum number of two 1,3-axial–axial non-bonding substituent interactions. Keywords: selenolate complexes, synthesis, X-ray crystallography, isomerism, adamantane stereochemistry.
Polyether carboxylic acids incorporating a benzodioxinic subunit: synthesis and complexing ability of sodium and potassium
Authors: Bosseray Pascal , Coudert Gérard , Guillaumet Gérald , Jeminet Georges , Tissier Madeleine , Juillard Jean
Published: May 2006

We report here the synthesis of new polyoxygenated macrocyclic derivatives with a benzodioxinic subunit and a carboxylic side chain (2, 3), along with some of their corresponding acyclic polyethers (1). Formation constants of sodium and potassium complexes with both the acid and anion forms of these compounds were obtained from potentiometric measurements. The enhancement of both complexing ability and selectivity afforded by the presence of a carboxylic side chain is discussed. Keywords: polyether carboxylic acids, macrocycles, benzo-1,4-dioxin, sodium and potassium complexes, solvent methanol.
Studies of the effect of hydrogen bonding on the absorption and fluorescence spectra of all-trans-retinal at room temperature
Authors: Alex S. , Thanh H. Le , Vocelle D.
Published: May 2006

Ultraviolet (UV)–visible and fluorescence spectra were obtained for complexes of ATR and TFA at different ratios and in four different solvents: hexane, chloroform, dichloromethane, and methanol. In the first three solvents, a large excess of TFA generates retinylic cations that absorb from 459 to 600 nm. Also, in CHCl3, Raman spectroscopy and fluorescence indicate that some aggregated species like ATR:(TFA)n, with λmax of ca. 470 nm, are present. In methanol, TFA protonates the solvent and it is CH3O+H2 which interacts with ATR so that only blue-shifted H-bonded ATR is present. From these results, it is shown that in the tautomeric equilibrium [Formula: see text], form (1) is always favored in the ground state whatever the solvent. In the excited state in hexane and in methanol, (1) is rapidly transformed into (2). In CH2Cl2 and CHCl3, this transformation is absent so that there is no energy dissipation, with the result that the retinal complexes become more unstable. Keywords: all-trans-retinal, fluorescence, H-bonds, trifluoroacetic acid, UV–vis spectroscopy.
The aldol condensation of acetone with acetophenone
Authors: Guthrie J. Peter , Wang Xiao-Ping
Published: May 2006

The kinetics and equilibria involved in the aldol condensation of acetone, acting as carbon acid, and acetophenone have been studied in aqueous alkaline solution. The enone isolated is the E isomer. The reactions are all first order in hydroxide, with rate and equilibrium constants (defined for E-enone as initial compound) of: k12 = (5.55 ± 0.17) × 10−6 M−1 s−1, k21 = (8.00 ± 0.40) × 10−6 M−1 s−1, K21 = (1.44 ± 0.55) (ketol to E-enone), K24 = 0.160 ± 0.033 (ketol to Z-enone), K32 = (1.89 ± 0.26) × 10−3 M−1 (acetone plus acetophenone to ketol), k23 = 0.180 ± 0.005 M−1 s−1, k32 = (3.41 ± 0.49) × 10−4 M−2 s−1. There is an equilibration of the two enones in base that is faster than hydration to the ketol: k14 = (3.14 ± 0.84) × 10−5 M−1 s−1; k41 = (2.81 ± 0.61) × 10−4 M−1 s−1; K14 = 0.112 ± 0.019. To analyze the behavior of the enone:ketol equililbrium system in acid we simultaneously fitted analytical data for all three species (E-enone, Z-enone, and ketol) to a kinetic model, so that the rate constants were determined by the best fit to all of the data for an experiment.
Organometallic oxides and sulfides: preparation of [(η-C5H5)Ti]5(μ3-S)6, [(η-C5H5)Ti]5(μ3-S)5(μ3-S2)(μ4-O), and(η-C5H5)2Ti(SH)(μ-O)Ti(η-C5H5)(μ-S)2- Ti(η-C5H5)2 by reductive aggregation of (η-C5H5)2Ti(SH)2 with zinc powder or by oxidation of (η-C5H5)2Ti(CO)2 with H2S in the presence of water
Authors: Bottomley Frank , Day Robert W.
Published: May 2006

Reduction of (η-C5H5)2Ti(SH)2 with zinc powder gave a mixture of products, of which [(η-C5H5)Ti]5(μ3-S)6 (1), [(η-C5H5)Ti]5(μ3-S)5(μ3-S2)(μ4-O)(2),(η-C5H5)2Ti(SH)(μ-O)Ti(η-C5H5)(μ-S)2Ti(η-C5H5)2 (3),[(η-C5H5)Ti]4(μ2-S2)4(μ2-O)(μ4-O) (4), and [(η-C5H5)Ti(μ3-S)]4 (5) were identified by a combination of fractional crystallization and X-ray diffraction, electron spin resonance spectroscopy, or mass spectrometry. The source of the oxygen in 2, 3, and 4 is believed to be the zinc oxide coating on the zinc powder. A similar mixture of products was obtained from the reaction between (η-C5H5)2Ti(CO)2 and H2S in the presence of small quantities of water. The structures of 2 and 3 are discussed in detail. 2 has a trigonal bipyramidal arrangement of Ti atoms (average (Ti(ax)—Ti(eq) 3.22 Å, average Ti(eq)—Ti(eq) distance 3.24 Å) except for one long equatorial Ti—Ti edge of 3.60 Å, which is spanned by the μ3-S2 unit. 2 is monoclinic, P21/n, a = 10.585(1), b = 28.133(4), c = 31.408(2) Å, β = 91.20(1)°, Z = 4 (for three independent molecules of 2 and one C6H5CH3), R = 0.071 for 6911 observed reflections and 1141 refined parameters. 3 is triclinic, [Formula: see text] a = 9.146(1), b = 12.290(1), c = 13.027(1) Å, α = 100.72(1), β = 104.39(1), γ = 110.55(1)°, Z = 2, R = 0.037 for 3046 observed reflections and 304 parameters. The Ti—O—Ti unit in 3 has an angle of 175.6(2)°.
Ecuadorin, a novel tetranortriterpenoid of Guareakunthiana: structure elucidation by 2-D NMR spectroscopy
Authors: Mootoo Baldwin S. , Játiva Cumandá , Tinto Winston F. , Reynolds William F. , McLean Stewart
Published: May 2006

Ecuadorin (1), C29H34O10, a novel tetranortriterpenoid, has been isolated from Guareakunthiana (Meliaceae) collected in Ecuador. Its structure has been established by spectroscopic methods, mainly 2-D NMR spectroscopy; the value of the FLOCK pulse sequence in the determination of 2- and 3-bond 13C–1H connectivities is illustrated. Ecuadorin has been reduced to the dihydro and tetrahydro derivatives (2 and 3), and these have provided information regarding the stereochemistry of ecuadorin. Stereochemical arguments have been based on observed vicinal 1H–1H coupling constants and NOE measurements, as well as on biosynthetic hypothesis.
Isolation and characterization of enniatins from Fusariumavenaceum DAOM 196490
Authors: Blais L. A. , ApSimon J. W. , Blackwell B. A. , Greenhalgh R. , Miller J. D.
Published: May 2006

Dried mycelia from a 15 L stirred-jar fermentation culture of Fusariumavenaceum (DAOM 196490) were extracted with dichloromethane and partitioned between hexane and methanol/water. Recrystallization of the hexane fraction yielded 6.750 g of a crude mixture of enniatins. Fast atom bombardment mass spectrometry showed, in accord with previous studies, that enniatin A was the predominant enniatin present, with smaller amounts of enniatins A1 and B1. HPLC purification of 500 mg of the enniatin mixture resulted in the isolation of 300 mg of enniatin A, 114 mg of enniatin A1, 9 mg of enniatin B1, and 46 mg of a new, N-methylleucine-containing enniatin (A2). This is the first reported isolation of a pure N-methylleucine-containing enniatin from Fusarium, though the existence of such compounds had previously been inferred from hydrolysis and NMR data. The purification and complete characterization of enniatins A1, B1, and A2 are reported.
Formation, reactions, and NMR spectra of 1,20-cycloatidanes
Authors: Edwards Oliver E. , Dvornik Dusan , Kolt Ralph J. , Blackwell Barbara A.
Published: May 2006

Imines derived from the alkaloid atisine gave N-acetyl 1,20-cycloatidane derivatives when heated with acetic anhydride. Vigorous alkaline hydrolysis cleaved the cyclopropane ring, regenerating the parent imine. The 1H and 13C NMR spectra of several 1,20-cyclo derivatives have been assigned and compared to those of the parent imines 2. All of the N-acetyl compounds showed doubling of the majority of the NMR resonances, due to amide rotamers. The effects of the cyclopropane ring current are noted.
Atrazine in mineral soil: the analytical chemistry of speciation
Authors: Gamble Donald S. , Ismaily L. Ali
Published: May 2006

The HPLC–microfiltration method previously described for determining atrazine and hydroxyatrazine species in organic soil has been adapted to a mineral soil. The method was able to monitor atrazine concentrations of less than 1.0 × 10−6 M, which is over 40 times lower than the atrazine concentrations measured in the earlier organic soil experiments. During kinetics.experiments, it also monitored four free and sorbed chemical species instead of only one. Measurements were obtained with much better accuracy and productivity than can be obtained by conventional methods. The atrazine sorption capacity of the mineral soil was also measured. The analytical chemical data produced can support heterogeneous chemical kinetics calculations.
Atrazine in mineral soil: chemical species and catalysed hydrolysis
Authors: Gamble Donald S. , Khan Shahamat U.
Published: May 2006

Equilibrium and kinetics parameters have been evaluated at 25.0 °C for the heterogeneous catalysis of atrazine hydrolysis in slurries of a chemically characterized mineral soil. The fraction of acidic sites that accounts for sorption capacity, and the sorption equilibrium function resemble those for humic acid and organic soil. Sorption and desorption half-lives increased with increasing coverage of sorption sites. The sorption half-lives ranged from 3.6 to 735 days. The desorption half-lives ranged from 1 to 11 days. The hydrolysis half-lives ranged from 9.6 to 168 days and are consistent with Brönsted acid catalysis theory. The relationship of independent variables to data scatter has been analyzed. The information obtained should be useful for water and solute transport models.
Single-ion enthalpies and entropies of transfer from water to aqueous urea solutions at 298.15 K
Authors: Hakin Andrew W. , Beswick Colin L.
Published: May 2006

In this paper we report enthalpies of solution at infinite dilution [Formula: see text] at 298.15 K for tetraphenylarsonium chloride (Ph4AsCl), sodium tetraphenylborate (NaBPh4), sodium chloride (NaCl), sodium bromide (NaBr), and sodium iodide (NaI) in water and aqueous solutions containing 5, 10, 20, and 30% urea by weight. Enthalpies of transfer from water to aqueous urea solutions are reported. Single-ion enthalpies of transfer [Formula: see text] have been calculated using the tetraphenylarsonium tetraphenylborate, (TATB) reference electrolyte assumption. These single-ion enthalpy data have been combined with single-ion Gibbs functions of transfer [Formula: see text] reported in the literature to obtain single-ion entropies of transfer [Formula: see text] for the urea + water mixed solvent system. The results of this single-ion analysis are discussed in terms of the impact of electrolytes on the structure of aqueous urea solutions.
Synthesis of β-O-4 lignin model dimers and their chlorinated derivatives
Authors: Shen Xinhua , Heiningen Adriaan Van
Published: May 2006

The unknown lignin model dimers with hydroxymethyl group at the para position of the phenoxy moiety and their chlorinated derivatives (compounds 1–13, except 8) have been synthesized. During the preparation of 6 from 5, a byproduct was isolated and identified as 9. The compounds 4, 7, and 8 were obtained as a mixture of erythro and threo isomers in the ratio of about 5:4, estimated from their 13C NMR spectra, and the erythro isomer 8a was crystallized from the mixture. Chlorinated derivatives 12a (erythro) and 12b (threo) were also isolated. Their relative configuration was assigned on the basis of both 13C and 1H NMR spectroscopy.
The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor
Authors: Schaefer Ted , Cox Kerry J. , Sebastian Rudy
Published: May 2006

not available
Electronic configuration and structure of paramagnetic iron dinitrosyl complexes
Authors: Bryar Traci R. , Eaton Donald R.
Published: May 2006

The electronic and geometric structures of paramagnetic iron dinitrosyl complexes have been investigated using electron spin resonance, infrared spectroscopy, and X-ray crystallography. It is concluded that these compounds are best described as 17 electron complexes with a d9 configuration rather than the d7 configuration assumed by most previous investigators. The anisotropy of the g values, determined from the electron spin resonance spectra of frozen solutions, varies considerably from complex to complex. The results are consistent with the supposition that all of the complexes have a distorted tetrahedral geometry, but the nature of the distortion changes as the ligands are varied. As a result of this variation there are changes in the nature of the spin-containing d orbital. Ligands containing hard, nonpolarizable donor atoms such as oxygen or fluorine produce a distortion towards a planar geometry, placing the odd electron in a predominantly [Formula: see text] orbital, while those containing softer donor atoms such as phosphorus or sulfur give complexes with a different type of distortion, leading to placement of the odd electron in a predominantly [Formula: see text] orbital. Nitrogen and halide donor ligands produce smaller distortions, leading to spin-containing molecular orbitals with contributions from a mixture of d orbitals. In accordance with this model, the crystal structure of [Fe(NO)2I2]− has been found to be only slightly distorted from regular tetrahedral coordination about the iron atom.
Thermally stable coordination compounds intercalated in montmorillonite clay mineral
Authors: Labhsetwar N. K. , Shrivastava O. P.
Published: May 2006

Montmorillonite is a natural layered clay mineral having myriad applications due mainly to its remarkable ion exchange, intercalation, and swelling properties. It can act as a good host for several compounds. Two intercalated coordination compounds of Cu(II) and Ni(II) with 2,2′-bipyridyl have been prepared in the interlayer spaces of montmorillonite structure. They are characterized on the basis of elemental analysis, infrared spectroscopy, magnetic studies, and X-ray diffraction. Both compounds show similarity in their structure and properties with the respective normal complexes of similar composition. These compounds show good thermal stability. The present intercalated compounds might be useful for their nitrogen-immobilizing properties and high-temperature applications.
Synthèses du β-L-rhamnopyranoside d'hordénine et d'acylgluco-L-rhamnopyranoses peracétylés
Authors: Boughandjioua Nadia , Chao Lin Rui , Skaltsounis Alexios-Leandros , Seguin Elisabeth , Tillequin François , Koch Michel
Published: May 2006

The stereoselective synthesis of hordenine β-L-rhamnopyranoside (3) was achieved using 4-O-acetyl-2,3-O-carbonyl-α-L-rhamnopyranosyl bromide (14) as key intermediate. The regiospecific syntheses of the peracetyl derivatives of (E)-O-(6-O-cinnamoyl-β-D-glucopyranosyl)-(1 → 2)-, (1 → 3)-, and (1 → 4)-α-L-rhamnopyranoses and of their 4-acetoxycinnamoyl counterparts were achieved through the use of various types of rhamnose-derived intermediates, i.e., 1,2-O-orthoester for the (1 → 2) series, 2,3-O-orthoester for the (1 → 3) series, and 2,3-O-isopropylidene for the (1 → 4) series.
A CAD study of isobaric (CH3)3SiArH+ ions (ArH = benzene, toluene) formed by association and protonation
Authors: Li Xiaoping , Stone John A.
Published: May 2006

Collision-assisted dissociation has been used to compare the ions formed by (a) the addition of trimethylsilylium ((CH3)3Si+) to benzene and toluene, and (b) the protonation of trimethylsilyl-substituted aromatics. The spectra of the isobaric ions are essentially identical at both high (8 keV) and low (<15 eV) collision energies. Proton transfer to trimethylamine is observed for all ions at centre of mass collision energies below 1.5 eV. The results show that the decomposing and reacting structures are the same for all these isobaric ions, whether formed by protonation or by addition of trimethylsilylium to aromatic, but they do not definitively resolve the question of the structure of their ground state, i.e., whether (CH3)3Si is π or σ bonded.
A study of computed rotational barriers of methyl groups and 1H and 13C spin-lattice relaxation rates in some steroids
Authors: Colebrook Lawrence D. , Qing Ning
Published: May 2006

1H and 13C spin-lattice relaxation rates of the methyl groups of 15 steroids have been measured, and rotational barriers of the methyl groups have been calculated, using force field methods. Correlations between relaxation rates and computed rotational barriers have been investigated. 1H and, 13C relaxation rates are consistent, and correlate with rotational barriers in simpler steroids. Steroids that are highly substituted with polar groups, and that must be investigated in polar solvents, tend to show poor correlations, an observation that is attributed to limitations in the computational model employed, which does not account for the effects of specific solvation or molecular association.
Polyphenol interactions. The copigmentation case: thermodynamic data from temperature variation and relaxation kinetics. Medium effect
Authors: Dangles Olivier , Brouillard Raymond
Published: May 2006

Copigmentation of malvin (a common anthocyanic pigment) with a series of naturally occurring colourless organic molecules (copigments) has been investigated by UV–visible absorption. The temperature-dependence of the visible spectrum of the copigmented solutions yields the copigmentation enthalpy and entropy changes. The experiments are carried out at pH 3.5 (malvin flavylium cation copigmentation) and pH 5.5 (copigmentation of the malvin quinonoidal bases). Copigmentation appears to be clearly enthalpically driven (exothermic process with unfavourable entropy change) in the case of chlorogenic acid and (+)-catechin. Concerning caffeine and tryptophan, favourable entropy changes, probably arising from solvation effects, have been recorded. Similar experiments in water–ethanol binary solvents have shown that the dramatic weakening of copigmentation on cosolvent addition is essentially entropic in origin. Salt effects on copigmentation have been investigated as well. Most of the results are discussed with a view to pointing out the major contributions to the magnitude of the copigmentation. Finally, relaxation kinetic measurements on the malvin–rutin system give striking evidence of the strong slowing down that a good copigment causes to the anthocyanin fading process (hydration). From the decrease in the apparent first-order rate constant of hydration on copigment addition, a general method for a quick determination of the copigmentation stability constants is proposed.
Synthesis of a series of 3-aryl-1-methyltriazene 1-oxides with substituents in the ortho or para position in the aryl group
Authors: Cameron Lynn M. , Vaughan Keith , Hooper Donald L.
Published: May 2006

A series of 3-aryl-1-methyltriazene 1-oxides, with a variety of substituents in the ortho or para position, have been prepared by coupling of the appropriate diazonium salt with TV-methylhydroxylamine. The substituents range from electron withdrawing, i.e., CO2Et, NO2, CN, COCH3, Br, and CF3, to electron releasing, i.e., OCH3. In all cases studied, the triazenes exist as the 1-oxide tautomer; there is no evidence for the presence of a 3-hydroxy tautomer in any of these cases. The 1-oxide structures have been confirmed by IR and NMR spectroscopy and by elemental analysis of new compounds. In the case of the -CO2Et and -CN substituents, a significant change to lower frequency is seen in the N—H vibration frequency in the IR spectra when the substituent is moved from the para to the ortho position. This change can be readily explained by the presence of intramolecular H-bonding, which is confirmed by the NMR spectra. The presence of an ortho substituent with H-bonding potential (i.e., CO2Et, -COCH3, or NO2) causes a downfield shift of the N—H proton resonance.
The reaction of boron trichloride – tertiary amine adducts with pseudohalide salts under phase transfer catalysis conditions
Authors: Atchekzaï Jean , Bonnetot Bernard , Mongeot Henri , Boufi Sami , Frange Bernard
Published: May 2006

The reaction of boron trichloride – tertiary amine adducts D•BCl3 with pseudohalide salts MX was investigated in a homogeneous medium or in two-phase systems under liquid/liquid or solid/liquid phase transfer catalysis conditions (PTC) as a potential route to the adducts D•BX3 (X = NCS, NCO). Best results were obtained with solid/liquid PTC with tetraglyme as catalyst. By suitable choice of the tertiary amine, solvent, and stoichiometry, the symmetric compounds D•BX3 were thus prepared with good yields, the same reaction leading either to mixed species D•BX3-nCln (n = 1, 2; X = NCS, NCO) or to the tetrapseudohaloborate anions BX4− + (X = NCS, NCO). The mixed tetrahaloborates BX4-n Yn− could be obtained by exchange reaction from BX4− + BY4− (X = NCS, Y = F, Cl, Br) or from BY3 + Bu4NX (X = NCS; Y = F, Cl, Br; X = NCO; Y = F). All these compounds were characterized by 11B and, in one case, by 1H and 13C NMR. Pairwise additivity is observed for the 11B NMR chemical shifts.
Syntheses of model oligosaccharides of biological significance. XII. Synthesis, NMR, and conformational analysis of trideuteriomethyl 4-O-(β-D-mannopyranosyl)- 2-acetamido-2-deoxy-β-D-glucopyranoside: the use of DEPT 1H to 13C transfer for T1 measurements and NOE assignments of tightly coupled 1H nuclei
Authors: Lee Ho Huat , Congson Ligaya N. , Whitfield Dennis M. , Radics Lajos R. , Krepinsky Jiri J.
Published: May 2006

The title disaccharide Manp(β1–4)GlcpNAcβ1-OCD3 has been prepared by a short synthetic sequence through the inversion of configuration from gluco to manno involving benzyl Glcp(β1–4)GlcpNAc. The disaccharide was subjected to detailed high-field 1H and 13C NMR study. First, conventional and 2D spectra were run to afford a complete set of assigned spectral parameters. Next, steady-state 1H–{1H} NOE and 1H, 13C spin lattice relaxation experiments were performed to infer dynamic spectral data related to molecular conformation. Owing to tight couplings and signal overlaps in the 1H spectrum, proton relaxation and selected NOE data were obtained via 13C NMR after transfer of the actual, non-equilibrium, proton magnetization to the 13C frequency domain. Using this experimental approach it has been found that T1's for Manp H-4 and GlcpNAc H-2 were substantially longer than those of the other sugar ring protons, and that the principal interresidue dipolar contact in Manp(β1-4)GlcpNAcβ1-OCD3 takes place between Manp H-1 and GlcpNAc H-4. Subsequently, a conformational analysis of the disaccharide was executed by means of a semiempirical method comparing Boltzman-averaged computed observables with experimental T1 and NOE values. The results suggest that the disaccharide has substantial conformational flexibility. Its exo-anomeric-stabilized conformational minimum at approximately [Formula: see text] of the glycosidic rotational angle is significantly populated but this global minimum does not represent the only rotational form available to the molecule.
The anomeric effect in thio derivatives of benzocycloheptene
Authors: Désilets S. , St-Jacques M.
Published: May 2006

The conformational properties of six thio derivatives of benzocycloheptene were studied by 1H and 13C high-field NMR spectroscopy in solution at low temperature. Both parent compounds 2 and 6, with sulfur at positions 3 and 1, respectively, were found to exist in the chair conformation. The 2-substituted derivatives of 2 (3: Y = OCH3 and 4: Y = Cl) revealed the presence of the Ca form only. In contrast, the 2-substituted derivatives of 6 (7: Y = OCH3 and 8: Y = Cl), with sulfur at position 1, showed mixtures of three conformers, namely Ca:Ce:TB (Ca = axial chair, Ce = equatorial chair, and TB = twist-boat), whose populations in CHF2Cl were, respectively, 93:5:2 and 85:12:3. The predominance of the Ca form in both series argues in favour of a strong anomeric effect in spite of the large departure from coplanarity (~35°) in the:—S—C—Y fragment of both 7 and 8. This noncoplanarity appears to weaken only slightly the n–σ* overlap characteristic of the endo-anomeric effect, contrary to earlier observations for the:—O—C—Y fragment in 5. The differences between the effects of O and S are discussed.
Dielectric enrichment in binary solvent mixtures. The intramolecular hydrogen bond in N-alkyl-substituted o-nitroanilines. Substituent effects
Authors: Cattana Rosa , Silber Juana J. , Anunziata Jorge
Published: May 2006

In this work we report solvatochromism studies on o-nitroaniline and several N-alkyl-o-nitroaniline derivatives used as solutes in solvent mixtures of an "inert" nonpolar cosolvent, cyclohexane, and THF as a solvent with hydrogen bond acceptor ability. These studies allowed us to establish the competition between inter- and intramolecular hydrogen bond in solutes. This was done by comparing the magnitude of the local inhomogeneity induced by the solute-in-solvent mixture, that is, "preferential solvation," using Suppan's dielectric enrichment model as modified by us to be applied to electronic transitions. Since preferential solvation accounts for dielectric as well as specific interactions while dielectric enrichment only for the former, it was shown by comparison that it is possible to separate both effects and even quantify them. It was deduced that N-alkyl-o-nitroanilines form a strong intramolecular hydrogen bond, which remains unbroken even in polar solvents and hydrogen bond acceptors such as dimethyl sulfoxide. This was also confirmed by solvatochromic studies in pure solvents. On the other hand, a symmetrical dependence of the effects of alkyl substituents and solvents on the shifts of the absorption spectra was observed.
Lambertellin, a metabolite of Ciboriagordonii Funk
Authors: Poulton Gerald Arthur , Bushnell Gordon William , Li Yun-Long
Published: May 2006

Lambertellin, 9-hydroxy-3-methyl-2H-naphtho[2,3-b]pyran-2,5,10-trione, previously identified from species of Lambertella and Pseudospiropes, has been isolated from the discomycete Ciboriagordonii Funk, identified on needles of the western hemlock Tsugaheterophylla. Spectral data and a crystal structure are given for lambertellin, confirming the structure in the literature.
Kinetics and mechanism of oxidation of nickel(II) tetraazamacrocycles by the peroxydisulphate anion in aqueous and binary aqueous mixtures
Authors: Haines Robert I. , Northcott Sandra J.
Published: May 2006

The kinetics of oxidation of several nickel(II) tetraazamacrocycles by the peroxydisulphate anion have been studied in water and in binary aqueous mixtures. The reactions proceed via an ion-pairing pre-equilibrium, followed by metal ion-assisted peroxy-bond fissure within the ion-pair solvent shell. The derived rate law is[Formula: see text]Ion-pairing constants have been determined and have been found to be little influenced by steric factors, but do depend on solvent composition. Rate constants have been extracted using the rate expression and activation energies have been estimated from temperature dependences.
Azetinone formation is not competitive with intermolecular reactions of a β-lactam-4-ylidene
Authors: Zoghbi Michel , Warkentin John
Published: May 2006

3-Phenoxy-1-phenyl-2-azetidinon-4-ylidene (β-lactam-4-ylidene) 2a, was generated by thermolysis of a spiro-fused β-lactam oxadiazoline precursor (1). Fast 1,2-H migration, a characteristic reaction of singlet carbenes that would convert 2a to 3-phenoxy-1-phenyl-3-azetin-2-one (4a) could not be demonstrated. Added 1, 3-diphenylisobenzofuran (6) did not afford the [4 + 2] cycloadduct (7) expected from 4a but, instead, the isomeric E- and Z-4-[1-(2-benzoylphenyl)-1-phenyl]-methylene-3-phenoxy-1-phenylazetidin-2-ones (9). Those compounds can be rationalized as the products of rearrangement of first-formed [2 + 1] adducts of 6 and the ylidene. The structure of the Z isomer of 9 was established by means of single crystal X-ray diffraction. Generation of 2a in methanol-d4, either neat or 3.3 M in benzene, afforded the isomeric products of carbene insertion into the OD bond of methanol-d4. Structural isomers expected from addition of methanol-d4 to 4a could not be detected. The results suggest that 1,2-H migration in 2a is relatively slow, with kH ≤ 1.4 × 106 s−1 at 100 °C, which is slower than analogous rearrangement of benzylchlorocarbene by 200-fold or more.
Computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes. Evidence that n–π orbital mixing in the HOMO is not the source of the syn π-facial selectivity in Diels–Alder reactions
Authors: Werstiuk Nick H. , Ma Jiangong , Macaulay John B. , Fallis Alex G.
Published: May 2006

A series of C5 monomethyl (1a–1f) and pentamethylcyclopentadienes (2a–2f) bearing stereogenic C5 heteroatom substituents (-NH2, -OH, -OCH3, and -SCH3) have been studied computationally (abinitio and AM1) and with ultraviolet photoelectron (pe) spectroscopy. In the case of compounds 1d–1f and 2d–2f, the conformations 3 and 4 of Cs symmetry with the lone paris anti to the cyclopentadiene ring are computed to be the most stable geometries. On the other hand, the twisted-anti conformers 5a and 5b are the most stable geometries of 1c and 2c. Analysis of the computed MO eigenvalues (orbital energies), MO eigenvectors (orbital coefficients), and the pe spectra of cyclopentadienes 2a–2f established that n–π orbital mixing is not important in the HOMO's of 1c–1d or 2c–2d. That the ionization energy of the HOMO is found to be virtually independent of the substituent at C5 in the series 2a–2f, provides support for the computational results. Because 2c–2d undergo Diels–Alder reactions selectively, syn to the heteroatom substituent, n–π orbital mixing in the HOMO cannot be the source of the π-facial selectivity observed for these compounds.