bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiempirical calculation of 13C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons. Application to the stereochemical analysis of steroidal side chains
Authors: Gonzalez-Sierra Manuel , Bustos Daniel A. , Ruveda Edmundo A. , Olivieri Alejandro C. , Grasselli Mariano
Published: May 2006

A semiempirical approach for predicting 13C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons has been adapted to a microcomputer program. A series of methyl and dimethyl substituted cholesterols has been studied using this program, and the predicted shifts are in agreement with literature reports. Preferred conformations of the steroidal side chains have been also predicted and agree with previous studies. A simple rule for analyzing the trends in the chemical shift of the carbon C-20, which is sensitive to changes in the configuration at C-22, is also given, not only for hydrocarbon side chains but also for hydroxy substituted compounds.
Crystal and molecular structure of the di-μ-hydroxo-dimolybdenum compound, [Mo(CO)2(OH)(C4H7)(3,5-diMepzH)]2•C6H6
Authors: Rettig Steven J. , Storr Alan , Trotter James
Published: May 2006

The synthesis and X-ray crystal analysis of a hydroxy-bridged molybdenum dimer are described. Crystals of di-μ-hydroxo-bis [dicarbonyl(3,5-dimethylpyrazole)(η3-2-methylallyl)molybdenum(II)] benzene solvate are monoclinic, P21/c, a = 7.508(1), b = 20.747(1), c = 20.717(3) Å, β = 99.536(6)°, Z = 4. The structure was determined by Patterson and Fourier methods and refined by full-matrix least-squares procedures to R = 0.022 for 3197 observed reflections. The dimeric molecule contains two distorted Mo coordination octahedra sharing an OH … OH edge to form a central folded four-membered Mo(μ-OH)2Mo ring. The pyrazole ligands form bent N—H … O intramolecular hydrogen bonds to the bridging OH groups, and the other ligands occupy positions which result in approximate C2 molecular symmetry. Two sets of Mo—OH bonds differ significantly in length, 2.237(2) and 2.140(2) Å; other molecular dimensions are within expected ranges. Molecules are linked into chains along a by O—H … OC hydrogen bonds, and benzene solvate molecules are well defined.
High pressure–temperature aqueous oxidations. III. A kinetic study of the enolization and oxidation of cyclohexanone
Authors: Thomas John W. , Taylor Jay E.
Published: May 2006

The rates of enolization of cyclohexanone have been determined at 145 and 172 °C in D2O and with buffers. The rates of oxidation were evaluated at temperatures of 145, 172, and 193.5 °C with oxygen partial pressures of 20.4 to 131 atm. The rate of enolization was 10–160 times faster than the rate of oxidation thereby supporting the previously proposed concept of enol intermediacy for the oxidation of ketones. The oxidation was first order in cyclohexanone and 1/2 order in oxygen. The rate of oxidation was increased by the addition of traditional phenolic inhibitors. The products isolated were formic, acetic, butanedioic, pentanedioic, hexanedioic, and 5-oxohexanoic acids. The activation parameters were calculated to be ΔH≠, 22 kcal/mol; ΔS≠, −27 eu, log A, 7.6 for the oxidation and ΔH≠, 12 kcal/mol; ΔS≠, −42.3 eu, log A, 4.13 for the enolization. Based on these observations a mechanism has been postulated whereby an oxygen molecule forms a transitory adduct with two enolates of cyclohexanone. The latter may then split by a reversible reaction to form an intermediate which may then isomerize or oxidize to either 2-hydroxycyclohexanone or 1,3-cyclohexanedione. Upon further oxidation the former yields hexanedioic acid. The latter then undergoes a reverse condensation in the aqueous media to 5-oxohexanoic acid which upon further oxidation yields formic plus pentanedioic acid and acetic plus butanedioic acid.
Structure dependent regularities of zero-point plus heat content energies in organic molecules
Authors: Fliszár S. , Poliquin F. , Bǎdilescu I. , Vauthier E.
Published: May 2006

A representative collection of heat content plus zero-point energies deduced in the harmonic oscillator approximation suggests a number of simple additivity rules which allow ZPE + (HT − H0) to be related to structural features. Accurate evaluations are obtained for alkanes, alkenes, alkynes, cycloalkanes, chloroalkanes, and amines, as well as for large and small cycloalkanes in which CH2 is replaced by O.
Electron transfer sensitized photolysis of 'onium salts
Authors: DeVoe R. J. , Sahyun M. R. V. , Schmidt Einhard , Serpone N. , Sharma D. K.
Published: May 2006

We have studied the anthracene-sensitized photolyses of both diphenyliodonium and triphenylsulphonium salts in solution using both steady-state and laser flash photolysis techniques. Photoproducts, namely, phenylated anthracenes along with iodobenzene or diphenylsulphide, respectively, are obtained from both salts with quantum efficiencies of ca. 0.1 at 375 nm. We infer the intermediacy of diphenyliodo and triphenylsulphur radicals formed by single electron transfer from the singlet-excited anthracene. We have developed a quantitative model of this chemistry, and identify the principal sources of inefficiency as back electron transfer, which occurs at nearly the theoretically limiting rate, intersystem crossing from the initially formed sensitizer–'onium salt encounter complex, and in-cage radical recombination.
Thermodynamics of transfer of glycine, diglycine, and triglycine from water to aqueous solutions of urea, glycerol, and sodium nitrate
Authors: Talukdar Himansu , Rudra Sibaprasad , Kundu Kiron K.
Published: May 2006

Standard free energies [Formula: see text] and entropies [Formula: see text] of transfer of glycine (G), diglycine (DG), and triglycine (TG), from water to aqueous mixtures of glycerol (GL) and urea (UH) have been determined from solubility measurements at different temperatures. This was also extended to an ionic cosolvent system like aqueous sodium nitrate solutions for G and DG. The observed [Formula: see text] and [Formula: see text]–composition profiles, as well as those obtained after correcting for the "cavity effect" as estimated by scaled particle theory (SPT), were examined in the light of various interactions. The corrected [Formula: see text]and [Formula: see text] values show a regular function of the peptide chain length of the amino acids and impart useful information regarding the involved relative structural effects of these ionic and non-ionic cosolvents.
Orientational ordering of ethylene on graphite
Authors: Larese J. Z. , Passell L. , Ravel B.
Published: May 2006

A neutron diffraction study of the low temperature structure and loss of orientational order was performed for the low density phase of ethylene adsorbed on graphite. At temperatures below 30 K the ethylene molecules form a centered-rectangular herringbone structure with their C=C bond parallel to the surface. At T > 30 K we observe a gradual loss of orientational order and a simultaneous increase in the near-neighbor spacing. This correlates well with the onset of rotational motions about an axis perpendicular to the C=C bond which remains predominately parallel to the surface plane. We speculate that the transition to this rotator phase is accompanied by a structural change to a triangular lattice.
Calorimetric studies of the phase transition in iodoform
Authors: Boerio-Goates Juliana , Woodfield Brian F.
Published: May 2006

The heat capacity of crystalline iodoform prepared by vacuum sublimination has been measured from 50 to 340 K and that of iodoform prepared by recrystallization from acetone measured from 120 to 300 K. Both samples exhibit a phase transition near 260 K. The heat capacity anomaly in the sublimed material shows several small maxima which appear with variable positions and heights depending upon the thermal pretreatment of the sample. The transition, which was undetected by previously-reported spectroscopic measurements, is ascribed to an incomplete ordering of the molecular dipole.
Excess properties (VE, εE, and HE) of the binary system pyridine (1) + sulfolane (2)
Authors: Jannelli Liliana , Pansini Michele
Published: May 2006

Volumes of mixing and dielectric constants of the binary system pyridine (1) + sulfolane (2) were measured over the entire composition range, 0 ≤ x ≤ 1, at selected temperatures ranging between 293.16 and 323.16 K. Mixing enthalpies were also measured at 303.16 K. Deviations from ideality of the measured properties are consistent with the regular behaviour of sulfolane systems, as evidenced by previous works. Nevertheless the occurrence, in the pyridine-rich region, of a sharp maximum in excess dielectric constants versus composition curves, and of a deep minimum in enthalpies of mixing curves, substantiates the supposition that small dipolar interactions occur between molecules of the components. These steadily decrease in magnitude with increasing temperature and sulfolane content in solution.
The critical temperature, the critical pressure, and the dependence of vapour pressure on temperature for dodecafluorocyclohexane and hexafluorobenzene
Authors: Davies D. R. , Ewing M. B. , Hugill J. A. , McGlashan M. L.
Published: May 2006

The critical temperature Tc, the critical pressure pc, and the dependence of the vapour pressure p on the temperature T, have been measured for dodecafluorocyclohexane and hexafluorobenzene. For the first substance, little had previously been known. For the second substance, previous measurements have been confirmed and extended. The results have been analysed by means of Wagner's equation: In(p/pc) = (Tc/T)(A2τ + A3τ1.5 + A4τ2.5 + A5τ5) where τ = (1 − T/Tc).
Differential effect of tetramethylammonium chloride and sodium chloride on duplex melting temperatures of deoxyoligonucleotides: resolution of a salt effect into specific and non-specific components
Authors: Marky Luis A. , Blumenfeld Kenneth S. , Breslauer Kenneth J.
Published: May 2006

Temperature-dependent uv spectroscopy has been used to determine the differential salt effect of tetramethylammonium chloride (Me4NCl) and sodium chloride (NaCl) on the thermal stabilities of a series of deoxyoligonucleotide duplexes that systematically differ in base composition and sequence. This approach has allowed us to resolve the AT-specific effect of Me4N+ from the more general, non-specific cation effects of Me4N+ versus Na+. Our results reveal that in the absence of AT-specific effects, Me4N+ (relative to Na+) significantly reduces the thermal stability of duplex structures. We offer an interpretation of this differential salt effect in terms of the influence of the condensed cation on water structure at the surface of the duplex. We also observe an AT-specific effect of Me4N+ which results in an increased thermal stability for AT-containing duplexes in Me4NCl relative to NaCl. This specific effect which is governed by mass action is found to be uninfluenced by base sequence and to be a linear function of the percent AT base pairs in the duplex.
Monte Carlo simulation of chlorine adsorbed on graphite
Authors: Suh Soong-Hyuck , O'Shea Seamus F.
Published: May 2006

Monte Carlo calculations are reported for patches of molecular chlorine adsorbed on the basal plane of graphite. A variety of combinations of intermolecular and molecule–surface potentials have been tested, and the resulting structural predictions cover a wide range of behaviour. None of the combinations employed here can explain the recent experimental observations, but it is clear that at least some could be adjusted to do so. However, the lack of thermodynamic data concerning the interaction between the molecules and the surface presents a major obstacle. Further experiments which give information about the energetics of the system are needed before a definitive account of the structure and dynamics can be given.
Retardation of Cd2+and Zn2+ discharge at the dropping mercury electrode by adsorbed phenyl substituted phosphonium cations
Authors: Anastopoulos A. , Christodoulou A. , Moumtzis I.
Published: May 2006

The effect of adsorbed propargyltriphenyl- and vinyltriphenylphosphonium cations on the discharge of Cd2+ and Zn2+ at dropping mercury electrode is studied by direct current polarographic measurements. Adsorption characteristics are determined by capacitance measurements. The size parameters of the activated complex and the surfactant particles are derived and an elucidation of the mechanism of the inhibited reaction is attempted.
Synthesis and metal ion complexation behavior of polycarboxylate 18-crown-6 ethers derived from tartaric acid
Authors: Dutton Philip J. , Fyles Thomas M. , McDermid Susan J.
Published: May 2006

The metal ion complexation behavior of four 18-crown-6 ethers derived from (+)- and meso-tartaric acid is examined. Preparations of a meso-crown ether diacid and of a crown ether hexacid from three units of (+)-tartaric acid are described. Acidity constants and stability constants for complexation of metal cations in aqueous solution were determined by potentiometric titration. The complexes are substantially stabilized by favourable electrostatic interactions and are of similar stability to complexes of cryptands and EDTA. The complexation behavior of the series can be rationalized in terms of electrostatic interactions, direct coordination of the cations by at least one carboxylate from the crown ether periphery, and rigidification of the ligands as the anionic charge increases. Distributions of charge influence the relative stability of isomeric complexes. Highly charged polycarboxylate crown ethers are effective, but relatively unselective, cation complexing agents for a range of cations. The complexes are stable to pH 3 and the ligands can be used as simultaneous pH and metal ion buffers.
Structural studies of organoboron compounds. XXX. 1,9-Dimethyl-3,5,7-triphenyl-2,4,6,8-tetraoxa-1,9-diazonia-5-bora-3,7- diboratatricyclo[5.4.0.03,9]undecane
Authors: Amt Henning , Kliegel Wolfgang , Rettig Steven J. , Trotter James
Published: May 2006

Details of the preparation and physical properties of the title compound are given. Crystals of 1,9-dimethyl-3,5,7- triphenyl-2,4,6,8-tetraoxa-1,9-diazonia-5-bora-3,7-diboratatricyclo[5.4.0.03,9]undecane are orthorhombic, a = 9.4026(3), b = 9.4663(2), c = 24.7462(9) Å, Z = 4, space group P212121. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.036 and Rw = 0.038 for 1536 reflections with I ≥ 3σ(I). The three six-membered and two seven-membered rings comprising the tricyclo[5.4.0.03,9]undecane ring system all have boat or boat-like conformations. Important bond lengths (corrected for libration) are: B(sp3)—N = 1.649(6) and 1.668(6), B(sp3)—O(N) = 1.492(5) and 1.497(6), B(sp3)—O[B(sp2)] = 1.444(6) and 1.438(5), B(sp3)—C = 1.600(6) and 1.594(7), B(sp2)—O = 1.357(6) and 1.370(6), B(sp2)—C = 1.574(6) Å.
Cationic complexes of titanium(III); phosphine substitution reactions
Authors: Seewald Patricia A. , White Graham S. , Stephan Douglas W.
Published: May 2006

The Ti(III) compound [Cp2Ti(CH3CN2)2]2[ZnCl4], 1, has been prepared and crystallizes in the space group Pbca with a = 28.444(11) Å, b = 15.370(4) Å, c = 15.206(5) Å, and Z = 8. The BPh4 salt (i.e., [Cp2Ti(CH3CN)2][BPh4], 2) is a convenient precursor for Ti(III) cationic phosphine complexes. Reaction of 2 with several monodentate phosphines has been monitored by epr spectroscopy. Compound 2 reacts with PMe3 in a stepwise fashion, to replace the coordinated CH3CN molecules, yielding [Cp2Ti(CH3CN)(PMe3)][BPh4], 3, and [Cp2Ti(PMe3)2][BPh4], 4. Complex 4 exhibits a quasi-reversible cyclic voltammogram at −0.40 V vs. SCE, in the presence of excess PMe3. This is indicative of aTi(III)/Ti(II) redox couple thus generating the known Ti(II) species Cp2Ti(PMe3)2. Similar reactions of 2 with PEt3 show that only the monosubstitution product [Cp2Ti(CH3CN)(PEt3)]+,is formed even in the presence of excess phosphine. No substitution was seen with either PCy3 or PPh3. The steric and electronic factors affecting substitution are briefly considered.
Structural and electronic origin of the conformational behavior of biphenyl-like α-diimine ligands. A theoretical study
Authors: Barone Vincenzo , Minichino Camilla , Fliszar Sândor , Russo Nino
Published: May 2006

The conformational behavior of aromatic α-diimines has been analyzed by means of ab initio computations making use of the standard STO-3G minimal basis set. The absolute minimum of the potential energy curve always corresponds to the planar trans conformation and a secondary minimum is also found for a non planar cisoid conformation. An alternate analysis, rooted in bond energy calculations explicitly involving the charges of the bond-forming atoms, indicates that bonded rather than nonbonded interactions are responsible for the observed conformational preferences.
In-situ Fourier transform infrared spectroelectrochemical studies of the oxidation of some tetraphenylporphyrin complexes
Authors: Hinman A. Scott , Pavelich Brad J. , McGarty Karen
Published: May 2006

In-situ FTIR spectroelectrochemistry has been applied to determine difference spectra in the region 1800 to 1000 cm−1 associated with the oxidation of TPPH2, TPPMg, TPPCu, TPPZn, TPPCo, TPPMnCl, and TPPFeCl. In all cases, formation of the π-cation radical results in two characteristic absorbance decreases near 1600 and 1485 cm−1, and five characteristic absorbance increases near 1415, 1350, 1285, 1225, and 1005 cm−1. Formation of the π-dication was observed for TPPZn, TPPMg, and TPPCo, and gave rise to a new absorbance increase near 1600 cm−1, and a shift to higher frequency of bands in the 1200 to 1400 cm−1 region. Perchlorate coordination to oxidized TPPZn, TPPMg, and TPPCo gives rise to bands near 1140 and 1030 cm−1.
Molecular structure of opiate alkaloids. Part II. Crystal structures of 4-methylhomobenzomorphan hydrobromide (I)and 4,12β-dimethylhomobenzomorphan (II)
Authors: Michel André G. , Evrard Guy , Norberg Bernadette , Milchert Eugène
Published: May 2006

The two title compounds belong to the family homobenzomorphans. Their analgesic properties are comparable to morphine but without inducing physical dependence. This work presents the results of X-ray diffraction and NMR studies to elucidate the conformational properties particular to this class of compounds. More specifically, the spatial orientation of the nitrogen lone-pair are compared to those observed for other related molecules. The two molecules adopt the same L-shaped structure and the 16β-methyl substituent present in the second molecule does not modify this structure. Although very different in both structures, the intermolecular interactions do not alter the molecular conformation. The nitrogen lone pair is oriented away from the benzene ring and its orientation is conserved in solution, as proven by nmr studies. The main conclusions are that the seven-membered ring shows very specific conformational properties and that the lone-pair orientation is predictable, according to the position of the nitrogen in the ring. Finally, it is concluded that correlations between the nitrogen lone-pair orientation and the analgesic activity have to be reconsidered.
Reactions between Co(II), 1,2-bis(diphenylphosphino)ethane, and NaBH3CN in the presence and absence of CO. The crystal structure of bis(cyanotrihydroborato)tetrakis(N,N-dimethylformamide)cobalt(II)
Authors: Elliot David J. , Haukilahti Sanna , Holah David G. , Hughes Alan N. , Maciaszek Stanislaw , Barton Richard J. , Luo Yaoguang , Robertson Beverly E.
Published: May 2006

Reactions between Co(II), Diphos, and NaBH3CN lead to Co(BH3CN)2(Diphos)2, 1, or [Co(BH3CN)(Diphos)2]X, 2 (X = ClO4 or BPh4), and, in certain solvents, 2 reacts to produce [Co(CN)(Diphos)2](ClO4). Compound 1 can be reversibly converted to Co(BH3CN)2(DMF)4, 4, via Co(BH3CN)2(Diphos)(DMF). In addition, 1 reacts with CO to form the Co(I) and Co(III) compounds [Co(Diphos)2](CO)]X and [Co(Diphos)2(CN)2]X (X = BH3CN or BPh4). Single crystal X-ray diffraction studies of 4 show that the compound crystallizes in the triclinic space group [Formula: see text], with unit cell parameters a = 7.572(6), b = 9.695(6), c = 9.395(6) Å, α = 81.06(4), β = 68.46(5), γ = 68.19(5)°, V = 595.5 Å3, Z = 1, and dcalcd = 1.202 g cm−3. The structure converged to a conventional R factor of 0.040 for 2841 observations and showed an octahedral arrangement of four O atoms from DMF molecules and two trans N-bound BH3CN groups around the Co(II) center.
Synthesis, spectroscopic and magnetic properties of some pyrazine-bridged iron(II) sulfonate complexes
Authors: Haynes John S. , Sams John R. , Thompson Robert C.
Published: May 2006

Six new pyrazine-bridged complexes of iron(II) have been prepared and studied using magnetic susceptibility, differential scanning calorimetry, and vibrational, electronic, and Mössbauer spectroscopic methods. Fe(pyz)(CF3SO3)2 exhibits a susceptibility maximum at 4.4 K and analysis of the magnetic data according to a two-dimensional S = 2 system in the (isotropic) Heisenberg limit yields best-fit values of J = −0.26 cm−1 and g = 2.20. Variable temperature Mössbauer spectra between 4.2 and 1.6 K are also reported for this compound and show the onset of magnetic hyperfine splitting below ~3.8 K. Analysis of magnetic data for Fe(pyz)2(CH3SO3)2 and Fe(pyz)2(CF3SO3)2•CH3OH according to the two-dimensional Heisenberg model also indicates the presence of weak antiferromagnetic exchange in these complexes (J = −0.18 and −0.20 cm−1, respectively). No clear evidence for magnetic concentration was obtained from our studies on Fe(pyz)2(CIO4)2, Fe(pyz)(p-CH3C6H4SO3)2, and Fe(pyz)(p-CH3C6H4SO3)2•2CH3OH; any antiferromagnetic coupling in these complexes must be considerably weaker than in the other complexes studied.
The thermodynamics of zinc sulphide transport in hydrogen chloride by modified entrainment
Authors: Gardner Peter J. , Pang Peter
Published: May 2006

The equilibrium ZnS(c, sphalerite) + 2HCl(g) = ZnCl2(g) + H2S(g) was studied by modified entrainment in the temperature range 1023 to 1263 K and the free energy equation, ΔG0/J mol−1 = (95060 ± 2700) − (38.7 ± 2.4)T, was obtained. A free energy equation for the corresponding equilibrium, ZnS(c, sphalerite) + 2HCl(g) = ZnCl2(1) + H2S(g), was derived using ancillary thermodynamic data, ΔG0/J mol−1 = (−28340 ± 2800) + (85.6 ± 2.4)T, (638–978 K).
Stepwise reduction of samarium cryptates in propylene carbonate: anions and water concentration effects on the redox behavior of the Sm(III)/Sm(II) couple
Authors: Loufouilou Edouard Laurent , Gisselbrecht Jean Paul
Published: May 2006

The polarographic reduction of samarium(III) cryptates with cryptands 222, 221, and 22 was investigated in propylene carbonate. The samarium(III) cryptates are reduced in two consecutive steps, the first step was reversible and corresponded to the reduction of the Sm(III) to the Sm(II) cryptate. The Sm(III)/Sm(II) redox potential of the cryptates depended on the anion used in the complex and, in the case of the cryptate with chloride anions, a stable mixed complex was observed in propylene carbonate. The Sm(III)/Sm(II) redox potentials of the cryptates were more cathodic than the redox potential of the uncomplexed Sm(III)/Sm(II) couple, which is typical of a lower stability of the reduced cryptate. Propylene carbonate does not stabilize low oxidation states of lanthanides by cryptation. This is at variance with behavior observed previously in other media like water and methanol. Variations of redox potentials as a function of increasing amounts of water were accounted for by solvent shielding of samarium(III) upon encapsulation in cryptands.
Fast atom bombardment (FAB) and chemical ionization (CI) mass spectrometry of γ-carboxyglutamic acid, 5-substituted-4,4-dicarboxyprolines, and their monodecarboxy derivatives glutamic acid and 5-substituted-4-carboxyprolines
Authors: Capasso R. , Pucci P. , Randazzo G. , Malorni A.
Published: May 2006

DL-γ-Carboxyglutamic acid (Gla) reacts easily and quantitatively with aldehydes to give the racemic diastereoisomers of 5-substituted-4,4-dicarboxyprolines. After heating at 100 °C for 30 minutes these amino acids lose a carboxyl group from C-γ of Gla and C-4 of the proline ring to give glutamic acid and the racemic diastereoisomers of cyclic analogs of glutamic acid respectively. This paper reports a study of the above-mentioned compounds by using FAB mass spectrometry in comparison with CI analysis. FAB turned out to be an adequate ionization technique for dealing with such nonvolatile and thermolabile amino acids, and may thus allow the development of a rapid and sensitive method for detecting γ-carboxyglutamic acid in urine and in other biological matrices in pathological cases (i.e., renal stones and blood serum).
Synthesis and X-ray crystal structure of the novel Mo—Cu complex, (Ph2PCH2CH2PPh2)2Cu+[(HBpz″3)Mo(CO)3]2Cu−(where pz″ = 3,5-dimethylpyrazolyl, N2C5H7)
Authors: Rettig Steven J. , Storr Alan , Trotter James
Published: May 2006

The novel complex, (Ph2PCH2CH2PPh2)2Cu+[(HBpz″3)Mo(CO)3]2Cu− (where pz″ = 3,5-dimethylpyrazolyl), has been synthesized by reacting the (HBpz″3)Mo(CO)3− anionic ligand with CuI in the presence of a stoichiometric amount of Ph2PCH2CH2PPh2. Crystals of (Ph2PCH2CH2PPh2)2Cu+[(HBpz″3)Mo(CO)3]2Cu−•3.5C6H6•CH2Cl2 are triclinic, a = 13.421(1), b = 14.931(3), c = 28.523(5) Å, α = 90.41(2), β = 90.19(1), γ = 104.65(1)°, Z = 2, space group [Formula: see text] The structure was solved by heavy atom methods and was refined by full-matrix least-squares procedures to R = 0.047 and Rw = 0.049 for 8779 reflections with I ≥ 3σ(I). The crystal structure consists of (Ph2PCH2CH2PPh2)2Cu+ cations having irregular tetrahedral coordination geometry about the copper atom (Cu—P = 2.263(2)–2.304(2) Å and P—Cu—P = 88.30(7)–134.62(8)°) and novel [(HBpz″3)Mo(CO)3]2Cu− anions having exact Ci and approximate D3d symmetry. The central Cu atom of the anion is bonded to two Mo atoms in a linear fashion and also interacts with six semi-bridging carbonyl ligands, three from each (HBpz″3)Mo(CO)3− ligand. Average bond lengths and angles for the anion are Mo–Cu = 2.565, Mo—C = 1.955, Mo—N = 2.256, Cu—C = 2.309, C—O = 1.174 Å, Mo—C—O = 171.7° and Cu—C—O = 114.7°.
Transformation of resin sandaracopimaric acid into androstane analogs
Authors: Gonzalez Francisco Bermejo , Martin Maravillas Bordell , Mateos Alfonso Fernandez
Published: May 2006

Preparation of the α,β-unsaturated 15-ketosteroid 13 by acid-promoted cyclization of the pimarane diterpene β-hydroxyacid 5 is described.
Open chain nitrogen compounds. Part XIII. 1-Aryl-3-arylthiomethyl-3-methyltriazenes and 3-(arylazo)-1,3-thiazolidines
Authors: Vaughan Keith , Manning Hartford W. , Merrin Marcus P. , Hooper Donald L.
Published: May 2006

Reaction of 3-acetoxymethyl-1-aryl-3-methyltriazenes with sodium thiophenolate or thiocresolate in anhydrous dimethylformamide affords a new series of 3-arylthiomethyltriazenes (2), Ar-S-CH2-NMe-N=N-Ar′. These triazenes are remarkably labile in aqueous buffer and may be good pro-drugs for the active metabolite of the antitumour dimethyltriazenes. The reaction of arenediazonium fluoroborates with 1,3-thiazolidine in aqueous acetone affords a new series of N-arylazo-1,3-thiazolidines (4); the arylazothiazolidines represent a new class of triazene in which the N3 nitrogen is incorporated into a heterocyclic unit, in this case a 1,3-thiazolidine. Nuclear magnetic resonance spectra of the arylazothiazolidines show evidence for rotational isomerism of the exocyclic N2—N3 bond in the triazene moiety.
The dependence of vanadate phenyl ester formation on the acidity of the parent phenols
Authors: Galeffi Bruno , Tracey Alan S.
Published: May 2006

The condensation of vanadate (VO4H21−) with a variety of substituted phenols in aqueous acetone solution has been studied by 51V nuclear magnetic resonance spectroscopy. The formation constants for the various product esters were determined, as were their respective pKa values. Inductive and resonance contributions to vanadate ester formation were investigated using the dual substituent parameter approach. This procedure did not succeed particularly well, although trends were clearly observed. Correlations with Hammett constants were compared with those of the dual parameter approach. The observation of some linear free-energy relationships was interpreted to mean that the inductive and resonance parameters generally ascribed to the various phenols are not directly applicable to vanadate ester formation where d orbitals are probably involved. The results did indicate that resonance effects have a significantly greater influence on ester formation than do inductive effects.
Structural studies of organoboron compounds. XXXII. 5,5-Difluoro-2,2-pentamethylene-1,4,6-trioxa-3a-azonia-5-borata-1,2,3,4,5,6-hexahydrobenzo[h]azulene and 2,2-pentamethylene-5,5-diphenyl-1,4,6-trioxa-3a-azonia-5-borata-1,2,3,4,5,6-hexahydrobenzo[h]azulene
Authors: Kliegel Wolfgang , Tajerbashi Mahmood , Rettig Steven J. , Trotter James
Published: May 2006

Details of the syntheses, physical properties, and crystal structures of the title compounds are reported. Crystals of 5,5-difluoro-2,2-pentamethylene-1,4,6-trioxa-3a-azonia-5-borata-1,2,3,4,5,6-hexahydrobenzo[h]azulene are triclinic, a = 6.1576(3), b = 11.2651(3), c = 10.8118(3) Å, α = 109.747(3), β = 105.807(4), γ = 92.976(4)°, Z = 2, space group [Formula: see text], and those of 2,2-pentamethylene-5,5-diphenyl-1,4,6-trioxa-3a-azonia-5-borata-1,2,3,4,5,6-hexahydrobenzo[h]azulene are monoclinic, a = 9.8549(6), b = 10.9242(6), c = 41.263(2) Å, β = 92.717(6)°, Z = 8, space group P21/n. Both structures were solved by direct methods and were refined by full-matrix least-squares procedures to R = 0.045 and 0.042 for 2344 and 4248 reflections with I ≥ 3σ(I), respectively. The molecules were both found to possess a seven-membered chelate structure, the O,O-chelatїng ligand being strongly bound to the X2B moiety. Two conformational isomers of the X = Ph compound were found in the solid state. The 2-oxazoline N-oxide moieties present in both compounds are the first to be structurally characterized. Important mean libration-corrected bond lengths are: O—B = 1.491 and F—B = 1.387 Å for X = F; O—B = 1.511 and C—B = 1.620 Å for X = Ph.
Stereochemical assignment of erythro and threo isomers of 2-aryloxy-1-(2-piperidyl)ethanol by phase sensitive homocorrelation nuclear magnetic resonance experiments
Authors: Feliz M. , Mauleón D. , Minguillón C.
Published: May 2006

The stereochemical assignment as erythro and threo for two pairs of 2-(aryloxy)-1-(2-piperidyl)ethanols 1 and 2 is described. Their CHO–CHN coupling constants were measured on the phase sensitive 1H–1H homocorrelation spectra (DQF–COSY), optimized by means of the LAOCOON-3 program, and compared with the calculated values from an MM2 conformational analysis. The experimental constants were Jerythro = 3.3/3.8 Hz and Jthreo = 6.4/6.8 Hz, whereas the calculated values were 2.5 and 5.4 Hz, respectively. 13C nuclear magnetic resonance steric shielding of the C(3) carbon atom in the erythro diastereomers confirms the results of the conformational analysis.
Minor metabolites of the cold water soft coral Gersemia rubiformis
Authors: Williams David E. , Andersen Raymond J. , Kingston John F. , Fallis Alex G.
Published: May 2006

The minor metabolites of both Pacific and Atlantic Ocean specimens of the cold water soft coral Gersemia rubiformis have been investigated. Pacific coast animals yielded two new pseudopterane diterpenoids, isogersemolides A (11) and B (12), and four new cembrane diterpenoids, isoepilophodiones A (13), B (14), and C (15) and rubifol (16). Atlantic coast animals yielded 3-acetoxy-β-cubebene (17). The proposed structures of all the new metabolites are based on spectroscopic analysis and chemical interconversions.
A new synthesis of 2,5-anhydro-D-mannose derivatives
Authors: Sastre Juan A. López , Molina José Molina , Olea Dolores Portal , Romero-Avila Cristina
Published: May 2006

Penta-O-acetyl-D-glucose diethyl dithioacetal monoxide 3 was obtained by oxidation of penta-O-acetyl-D-glucose diethyl dithioacetal 2 with 3-chloroperbenzoic acid. By reaction of 3 with sodium methoxide in methanol and further acetylation, 3,4,6-tri-O-acetyl-2,5-anhydro-D-mannose-diethyl dithioacetal monoxide 8 and the reduction product 3,4,6-tri-O-acetyl-2,5-anhydro-1,1-bis(ethylthio)-1-deoxy-D-arabino-hex-1-enitol 4 were obtained. Reduction of 8 yielded the corresponding dithioacetal derivative 9, which was converted to 3,4,6-tri-O-acetyl-2,5-anhydro-D-mannose diethyl acetal 11.
Naphthaloyl group: a new selective amino protecting group for deoxynucleosides in oligonucleotide synthesis
Authors: Dikshit Archana , Chaddha Manjula , Singh R. K. , Misra Krishna
Published: May 2006

The naphthaloyl group has been found to be a selective amino protecting group for deoxycytidine, deoxyadenosine, and deoxyguanosine in oligodeoxyribonucleotide synthesis. All three protected monomers obtained (78–85%), being six-membered cyclic imides, were fairly stable. These protected monomers were used successfully for the preparation of dimers (phosphodiester approach) and tetramers (phosphotriester approach) in solution as well as solid phase, respectively. The group acted as a purification tool due to its high lipophilicity. No adverse effect has been observed either on the glycosidic bond (depurination) or the internucleotidic bond during its removal. The monomeric units were characterized by UV, NMR, and elemental analyses whereas the tetramers were characterized by enzymatic hydrolyses with snake venom phosphodiesterase followed by alkaline phosphatase.
Crystal chemistry of tetraradial species. Part 2. Crystal structures of Et4NI, Ph4PBr•H2O, and Ph4PBr•2H2O
Authors: Vincent Beverly R. , Knop Osvald , Linden Anthony , Cameron T. Stanley , Robertson Katherine N.
Published: May 2006

The crystal structure of Et4NI [Formula: see text], a = 8.860(2) Å, c = 6.933(1) Å, Z = 2) has been redetermined and those of Ph4PBr•H2O ([Formula: see text], a = 10.005(3) Å, b = 10.659(2) Å, c = 10.697(4) Å, α = 102.61(2)°, β = 83.39(3)°, γ = 108.09(3)°, Z = 2) and Ph4PBr•2H2O (Pnma, a = 16.255(4) Å, b = 10.810(4) Å, c = 12.667(9) Å, Z = 4) have been determined for the first time. In the Et4NI structure Et4N+ cations in an extended S4 conformation and I− anions are arranged in a zincblende-type packing. The two hydrate structures are ionic and very similar. The anion in the monohydrate is a centrosymmetric cyclic dimer [Br2(OH2)2]2−; in the dihydrate the anion, which is an almost planar infinite [Br−(OH2)4/2]x chain 11b, appears to be of a novel type. If the H2O molecules are not considered, the ion packing in both hydrates may be regarded as of the anti-NiAs type. The ion packing in R4EX structures and the conformation of the Et4N+ ion in crystals are discussed in some detail.
General description of multiple-modulation effects and sideband production in magnetic resonance
Authors: Sakamoto Hirokazu , Takui Takeji , Itoh Koichi
Published: May 2006

A theory is presented for the sideband structure produced by a general multiple-modulation scheme with arbitrary combination of p-tuple field modulation, q-tuple frequency modulation, and r-tuple amplitude modulation for integral p, q, and r. A general formula describing the multiple-modulation effects is analytically derived using a density matrix formalism under the assumption of negligible saturation; an equivalent result is obtained from the phenomenological Bloch equation. The general formula reveals that field and frequency modulations are equivalent for the sideband production, even in the presence of amplitude modulation. Two particular cases, viz., r-tuple amplitude modulation and double field modulation are dealt with to show the validity and applicability of the general formula.
Ionic conductivity and solid phase transitions in Na2SO4–Cs2SO4 system
Authors: Macdonald Kevin G. , Maclean Charles , Secco E. A.
Published: May 2006

The ac conductivity and DSC calorimetry data reveal interesting solid state interactions and solid phase transition behavior in the Na2SO4–Cs2SO4 binary system. The β → α solid transition of NaCsSO4 is reported for the first time. These data indicate preferential reaction between Cs2SO4 and the high temperature phase I of Na2SO4, exemplifying the Hedvall effect, to form the compound NaCsSO4 which further interacts with excess Na2SO4 or excess Cs2SO4 to form 1:1 association complexes, viz. NaCsSO4:Na2SO4 and NaCsSO4:Cs2SO4. These complexes appear to dissociate at low temperatures reminiscent of spinodal-type decomposition behavior. The reaction enthalpies of NaCsSO4, β → α transition of NaCsSO4 and complex associations are reported.The report contains two unexpected results: (i) a subdued higher ionic conductivity relative to its Rb2SO4 analogue and (ii) a conductivity drop accompanying the high temperature phase transition which is in strong contrast to all the other Na2SO4 – mixed alkali compositions.
Polarimetric and nuclear magnetic resonance studies of the complexation of mercury by thiols
Authors: Shoukry Mohamed M. , Cheesman Bruce V. , Rabenstein Dallas L.
Published: May 2006

The complexation of Hg(II) by glutathione has been studied by polarimetry under conditions of excess ligand with the objective of characterizing formation of the 3:1 complex, Hg(glutathione)3. The optical rotatory power of solutions containing glutathione only and of solutions containing glutathione and Hg(II) at ratios of 2:1, 2.5:1, 3:1, and 4:1 was measured as a function of pH. Acid dissociation constants for the ammonium and thiol groups of glutathione and for the two ammonium groups of Hg(glutathione)2 and the formation constant of the 3:1 complex (Hg(glutathione)2 + glutathione [Formula: see text] Hg(glutathione)3) were determined from the pH dependence of the optical rotatory power. The value obtained for the formation constant, Kf = 1.5 × 103, indicates that binding of the third ligand to form Hg(glutathione)3 is much weaker than binding of the first two glutathione ligands. However, calculations indicate that binding is sufficiently strong that a significant fraction of Hg(II) is present as Hg(glutathione)3 under physiological conditions. Equilibrium constants were also determined by polarimetry and by 13C nuclear magnetic resonance for the displacement of one thiolate ligand by another (RSHgSR + R′SH [Formula: see text] RSHgSR′ + RSH; RSHgSR′ + R′SH [Formula: see text] R′SHgSR′ + RSH). The results indicate that, at pH 5.5 and at physiological pH, the relative stability increases in the order Hg(glutathione)2 < Hg(penicillamine)2 < Hg(mercaptoethylamine) 2. However, when competitive protonation of free ligand is accounted for, it is shown that the intrinsic stability of the complexes increases in the order Hg(penicillamine)2 < Hg(mercaptoethylamine)2 < Hg(glutathione)2, which parallels the order of the Brønsted basicity of the thiolate ligands.
Influence de la complexation sur la réactivité de nitrates d'halogènes
Authors: Gaude Didier , Gellon Gisèle , Goaller Raymond Le , Pierre Jean-Louis
Published: May 2006

Iodine nitrate or bromine nitrate in acetonitrile or in chloroform react with a variety of phenolic substrates to form both halogenated and nitrated products. In the presence of strong complexing agents of halonium ions, no reaction occurs, while in the presence of pyridine or triethylamine, only halogenated phenols exhibiting a strong ortho-directing effect of the phenolic function are produced. Keywords: phenols, iodine nitrate, bromine nitrate, halogenation, nitration.
The preparation and crystal structure of tetrakis(phenylmethoxy)methane
Authors: Latimer Darin R. , Weil John A. , Quail J. Wilson , Robinson Ward T.
Published: May 2006

The new compound tetrakis(phenylmethoxy)methane (also known as benzyl orthocarbonate) has been produced by the action of the sodium salt of benzyl alcohol on trichloronitromethane (chloropicrin), resulting in a white crystalline solid with a melting range of 68–70 °C. The crystal structure has been determined at 153(5) K: triclinic, [Formula: see text], a = 10.161(4) Å, b = 12.664(4) Å, c = 20.472(5) Å, α = 71.78(2)°, β = 82.30(3)°, γ = 71.92(2)°. The results show the symmetry at each central carbon to be approximately S4, with average C—O bond lengths of 1.397(4) Å, O—C—O angles ranging between 113–114° and 107–108°, and C—O—C angles of 115(1)°. Additional evidence for this structure is given by nuclear magnetic resonance, infrared and mass spectra. Keywords: benzyl orthocarbonate, orthocarbonate, tetrakis(phenylmethoxy)methane.
Cocatalysis by an internal carboxyl group and by α-cyclodextrin. The debromination of 4-bromo-4-methyl-2,5-cyclohexadienone-2-carboxylic acid
Authors: Takasaki Bryan K. , Tee Oswald S.
Published: May 2006

Debromination of the title dienone (1), formed during the bromination of 5-methylsalicylic acid (2), is strongly catalyzed by α-cyclodextrin (CD). The bromination of 2 is also catalyzed by CD. The extent of the catalysis of the debromination of 1 is independent of pH in the range pH 3–5 and it tends to saturate at high [Br−] and high [CD]. Analysis of the [Br−] dependence data in the presence of 1 mM CD yields a value of 0.68 M for the dissociation constant of the CD•Br− complex. Also, from the dependence of the rate of debromination on [CD], the anion of the dienone 1 (i.e., 1−) forms a complex with CD with a Kd of 15 mM, which appears to react with Br− at a rate 170 times faster than does free 1−. However, the catalysis is more likely due to the reaction of free dienone, reacting as 1, with the CD•Br− complex, which is 3400 times more reactive than uncomplexed Br−. The combined effect of the internal catalysis by the carboxyl group of 1 and the external catalysis by CD is an overall rate enhancement of 12 million. Keywords: catalysis, cocatalysis, cyclodextrin, debromination, intramolecular.
Crystal, molecular and electronic structure of 1-H-3-methyl-4-amine-5-thione-1,2,4-triazol
Authors: Escobar-Valderrama J. L. , García-Tapia J. H. , Ramírez-Ortíz J. , Rosales M. J. , Toscano R. A. , Valdés-Martínez J.
Published: May 2006

The crystal and molecular structure of the title compound has been determined from three-dimensional X-ray intensity data. The crystals are orthorhombic with space group Pbcm, a = 8.877(4), b = 9.813(2), c = 6.545(2) Å, Z = 4. The structure has been refined to R = 0.029 and Rw = 0.035. The molecules are present in the thione form as planar molecules with only intermolecular interactions. Bond lengths and angles suggest that the electron density is more localized in the N—N—C—S ring fragment. Theoretical calculations (MNDO method) were also carried out on the compound both in the thione and its thiol tautomeric forms. Some comparisons are made between several properties (formation enthalpies, proton affinities, molecular electrostatic potentials, atomic charges) of both tautomers. The results indicate some important differences in reactivity between both forms. Keywords: crystal structure, molecular structure, electronic structure, 1-H-3-methyl-4-amine-5-thione-1,2,4-triazol.
Differences between adsorption properties of two Rhodesian chrysotile samples. Relation with the DTA features introduced by leaching and grinding
Authors: Suquet Hélène
Published: May 2006

Substantial differences are founded between the absorption properties of two Rhodesian chrysotile samples (−1 and −2), which are, in most chemical and structural aspects, highly similar. The phenanthrene and CO2 amounts adsorbed are much less important on chrysotile-2 than on chrysotile-1. This indicates that chrysotile-2 has fewer basic surface sites. DTA and XRD results show that dry grinding and leaching produce a breakdown of the chrysotile structure, yield amorphous silica, and lower the temperature of the dehydroxylation peak. The dehydroxylation peaks of chrysotile-1 and -2 are really doublets. As the low-temperature component of the dehydroxylation peak is greater in chrysotile-2 than in chrysotile-1, the chrysotile-2 contains more ground and (or) leached fibers than chrysotile-1 and consequently shows weaker absorption properties. Keywords: chrysotile, leaching, phenanthrene adsorption.
Esterification of 1,4-dichlorobutane with sodium formate under solid–liquid phase transfer catalysis. A kinetic study
Authors: Zahalka Hayder A. , Sasson Yoel
Published: May 2006

Kinetic data are reported regarding the esterification of 1,4-dichlorobutane with sodium formate catalyzed by quaternary ammonium salts as a model for reactions in series, under solid–liquid phase transfer conditions. The process was found to follow a consecutive first-order mechanism of the general type A → R → S. The reactivity of the quaternary ammonium salts with regard to the counteranion was Cl− > Br− > 1− > HSO4−. The reaction rate was linearly dependent on catalyst concentration up to 12 mol% of catalyst relative to the substrate. Above this concentration the rate was constant and independent of the amount of the catalyst. The activation energy of the two consecutive steps was found to be similar (21 kcal/mol). Therefore, the product distribution (R/S) is not appreciably affected by temperature. A mechanism termed "Thin aqueous boundary layer" is suggested for nucleophilic displacement reactions under solid–liquid phase transfer conditions. Keywords: phase transfer catalysis, series reactions, kinetic study.
The identification of structural fragments of enolic monoterpenes, enolic pheromones, and related natural products, from OH stretching vibration frequencies and relative intensities
Authors: Bacon John F. , Maas John H. van der
Published: May 2006

The OH stretching frequency has proved to be a powerful tool for structural investigations of enolic natural products containing diverse structural elements. The position of unsaturation can in many cases be determined directly from frequency measurements. Primary, secondary, and tertiary α–β and β–γ unsaturation can unambiguously be identified due to the possession of OH bands arising from an interaction of the OH proton with the π system. Frequently, unsaturation further removed than the γ position can be identified, particularly when a double bond is present, intermediate between the distal π system and the OH group. The identification of long-range OH–π system interactions relies on the observation that an intermediate double bond reduces the rotational freedom of the molecule. This reduction in entropy is assumed to be responsible for a significant population of the conformation in which the OH group is associated with the π system. Keywords: infrared, conformation, natural products, alcohols, molecular structure.
A total synthesis of a protected form of the secoxyloganin aglucone
Authors: Cheng Peter T. W. , McLean Stewart
Published: May 2006

A synthesis is described that leads from cyclopentadiene through 5-norbornen-2-one to the monosubstituted cyclopentenone 5; a vinyl substituent is attached by the reaction of a cuprate, and the enolate intermediate is trapped as the silyl enol ether 4, which is converted to 19, a product with the 10-carbon framework and stereochemistry of the secoxyloganin aglucone, and with all functional groups differentially protected and in the correct oxidation state. Explorations of the cuprate reaction required to convert the cyclopentenone 5 to the siloxycyclopentene 4 and of the reactions required for the oxidative cleavage of 4 reveal problems with these reactions when they are applied to compounds that are cyclopentane derivatives. The resolution of these problems leads to reaction sequences that proceed with high yields and excellent diastereoselectivity. Keywords: synthesis, secoxyloganin, vinyl cuprate, oxidative cleavage.