bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-resolved ESR study on photochemical formation of radical pair in cyclodextrin cavities
Authors: Murai Hisao , Yamamoto Yoshinori , I'Haya Yasumasa J.
Published: May 2006

The photoreduction of xanthone with diethylaniline in cyclodextrin cavities was studied at 77 K and room temperature by a time-resolved ESR technique. The radical pair observed in β- and γ-cyclodextrins showed inverted spin polarization compared to that of precursor excited triplet xanthone. This result is rationalized by taking account of the fixed orientation of the radical ion pair in the cyclodextrins. Frozen aqueous solutions and dried powder-like samples provided similar results. The spectrum of the radical pair was also detected in an aqueous solution of β-cyclodextrin at room temperature. Key words: cyclodextrins, xanthone, spin polarization, radical ion-pair, time-resolved ESR.
Measurements of far-infrared optical constants of liquids with a synchrotron radiation source
Authors: Fujita Yoshitaka , Ohba Tetsuhiko , Ikawa Shun-Ichi
Published: May 2006

The far-infrared optical constants of liquid acetonitrile, carbon disulfide, and carbon tetrachloride were determined from transmission measurements with a synchrotron radiation source at the Institute for Molecular Science (Okazaki). The small size of the synchrotron radiation source was of benefit to measurements of transmittances that were suited for an analysis based on an optical theory. From the transmittances of liquid acetonitrile at temperatures ranging from 238 to 343 K, both the absorption coefficients and the refractive indices were obtained by using the subtractive Kramers–Kronig relation proposed by Jones and co-workers. The dipole moment of liquid acetonitrile calculated from the optical constants, 4.5 ± 0.1 D, is nearly independent of temperature. The spectra of carbon disulfide and carbon tetrachloride were measured at pressures in the ranges 1–1800 bar and 1–1100 bar, respectively. The absorption coefficients were obtained from the transmittances by neglecting the dispersion of the refractive indices. The zeroth and second moments of the collision-induced absorption bands were obtained as a function of pressure, after resolving the overlap of intramolecular vibrational bands. Key words: far-infrared, optical constants, pressure, liquid, synchrotron radiation source.
A Raman spectral study of solvation and ion association in the systems LiAsF6/CH3CO2CH3 and LiAsF6/HCO2CH3
Authors: Deng Zhongyi , Irish Donald E.
Published: May 2006

The structure of the solvated lithium cation in methyl acetate (MA) solutions has been investigated using Raman spectroscopy. Two bands at 844 and 864 cm−1 have been assigned to two different types of MA: the former is from the bulk solvent and the latter arises from MA molecules solvating the lithium cation. From measurement of changes in intensity of these bands with increasing salt concentration a solvation number of four for Li+ in MA has been inferred. Changes in the Raman bands at ca. 1740 cm−1 suggest that solvation occurs through the carbonyl group. Evidence for contact ion pairing between Li+ and AsF6− is also presented. An equilibrium between solvent-shared ion pairs and contact ion pairs is proposed for which an equilibrium constant is estimated. The system LiAsF6/methyl formate (MF) is similar in structure. Key words: Raman, ion pair formation, lithium and hexafluoroarsenate ions, methyl acetate and formate, lithium ion solvation.
Semiclassical model calculations of weak, strong, and short H-bonds
Authors: Luck Werner A. P. , Wess Thomas
Published: May 2006

The OH potentials of the complexes [Formula: see text] are calculated by an ab initio MO-LCAO-SCF-CISD method with a 6-31G** basis set for 30 different [Formula: see text] distances. The resulting OH vibration levels are calculated by the so-called semiclassical Planck–Sommerfeld phase integral. The experimental H-bond effects on vibration spectra, and their anomalies, could be established by calculations of H-bond energies, OH frequencies, anharmonicities, the Badger–Bauer rule, correlation between OH and [Formula: see text] distances, isotopic effects, and the Ubbelohde effect. For very short H-bonds new properties could be predicted. There seem to exist three classes of H-bond: (1) weak or medium-strong, (2) strong, and (3) very short. The simple phase integral method could demonstrate the anomalous effects of strong H-bonds solely by the transition from two separated minima to one, without necessarily assuming tunnel effects. Key words: strong H-bonds, IR spectra and theory, tunnel effects.
Vibrational spectra and force field studies of [PtCl3(CO)]− anion labelled with 13C16O, 12C18O, and radioactive 14C16O
Authors: Mink J. , Goggin P. L.
Published: May 2006

Detailed infrared and Raman studies are reported for [PtCl3(12C16O)]−, [PtCl3(13C16O)]−, [PtCl3(12C18O)]−, and [PtCl3(14C16O)]− together with assignments. Bands observed for the 14C16O species at 2005, 483, 507, and 466 cm−1 are assigned to CO and PtC stretching, PtCO in-plane and out-of-plane linear bending modes, respectively. Force constants have been calculated using a modified valence force field, with constrained off-diagonal force constants based on [PtCl4]2− and [PtX3(CO)]− (X = Br, I). The effective G-matrix method (EGM) has been used for approximate calculation of CO and PtC stretching force constants for some other square-planar platinum–carbonyl complexes. The results are discussed with the assistance of stretching force constants for some of the simpler complexes. Key words: vibrational spectra, force constant calculation, 13C, 14C, and 18O labelled [PtCl3(CO)]−.
Copper(I)-catalyzed cross-dehydrocoupling reactions of silanes and amines
Authors: Liu H. Q. , Harrod J. F.
Published: May 2006

In the presence of cuprous chloride, hydrosilanes and primary amines react to give silazanes. Methylphenylsilane reacts with (R)-(+)-α-methylbenzylamine to give the two diastereomeric monosilazanes as the only products. Aniline and benzylamine react with methylphenyl- and diphenylsilanes to give primarily the monosilazane, disilazane, or diazasilane as the main product, depending on the reactant ratios. Phenylsilane gives higher oligomers with aniline, mainly the trisilatriazane and trisiladiazanes, but no high molecular weight material. Keywords: silanes, amines, silazanes, cuprous chloride, dehydrocoupling, catalysis.
Orbital reorganization and its energies in the interaction of CO with Sc, Fe, and Cu
Authors: Hirai Kazuo , Kosugi Nobuhiro
Published: May 2006

The interaction of CO with transition metals (M = Cu, Fe, and Sc) is analyzed using a newly developed method within the framework of the Hartree–Fock approximation. This method reveals reorganization of each orbital by the interaction and its energy contribution. To avoid the basis-set dependence in the energy partitioning, the orbital reorganization energy includes both polarization and charge-transfer effects; however, these effects are clearly distinguished by using orbital contours. The interaction in M–CO is discussed not only in terms of widely accepted components, such as metal 4s–4pσ polarization, CO 5σ – metal 3dσ donation, and metal 3dπ – CO 2π* backdonation, but also by using newly found components, metal 3dσ – 4s and 4s–3dσ polarization, CO 5σ – metal 4pσ donation, metal 3dσ – CO 6σ* back-donation, and CO 1π – 2π* polarization. Keywords: metal carbonyl, interaction energy, orbital reorganization.
Relativistic compact effective potentials and efficient, shared-exponent basis sets for the third-, fourth-, and fifth-row atoms
Authors: Stevens Walter J. , Krauss Morris , Basch Harold , Jasien Paul G.
Published: May 2006

Relativistic compact effective potentials (RCEP), which replace the atomic core electrons in molecular calculations, have been derived from numerical Dirac–Fock atomic wavefunctions using shape-consistent valence pseudo-orbitals and an optimizing procedure based on an energy-overlap functional. Potentials are presented for the third-, fourth-, and fifth-row atoms of the Periodic Table (excluding the lanthanide series). The efficiency of molecular calculations is enhanced by using compact Gaussian expansions (no more than three terms) to represent the radial components of the potentials, and energy-optimized, shared-exponent, contracted-Gaussian atomic orbital basis sets. Transferability of the potentials has been tested by comparing calculated atomic excitation energies and ionization potentials with values obtained from numerical relativistic Hartree–Fock calculations. For the alkali and alkaline earth atoms, core polarization potentials (CPP) have been derived which may be added to the RCEP to make possible accurate molecular calculations without explicitly including core-valence correlating configurations in the wavefunction. Keywords: model potentials, effective core potentials, transition metals, relativistic calculations.
Electrosynthesis of thiocyanato and mixed ligand thiocyanato complexes of transition metals by the sacrificial dissolution of metal anodes
Authors: Chakravorti M. C. , Subrahmanyam Gampa V. B.
Published: May 2006

A general method for the electrosynthesis of anionic and mixed ligand nonelectrolytic thiocyanato complexes of transition metals has been developed by the oxidation of sacrificial metal anodes in aqueous, or aqueous ethanolic, medium containing ammonium thiocyanate at room temperature and at an applied potential of 2–7 V. The method is rapid and gives good yield with high purity. Keywords: electrosynthesis, thiocyanato complexes, mixed ligand thiocyanato complexes.
A phosphorus-31 NMR study of solid carbonylhydridotris(triphenylphosphine)rhodium(I). Unusual MAS sideband intensities in second-order NMR spin systems
Authors: Wu Gang , Wasylishen Roderick E. , Curtis Ronald D.
Published: May 2006

The CP/MAS 31P NMR spectrum of carbonylhydridotris(triphenylphosphine)rhodium(I), RhH(CO)(PPh3)3 (1), can be described as a tightly coupled ABMX spin system (X = 103Rh). In contrast to the solution 31P NMR spectrum, the three equatorial phosphorus nuclei are nonequivalent in the solid state and this structural feature allows us to measure the two-bond spin–spin couplings, 2J(31P,31P). A new method is proposed for extracting the principal components of the chemical shift tensor from slow MAS NMR spectra in a tightly J-coupled two-spin system. For compound 1, the principal components of the 31P chemical shift tensors calculated using this method are in good agreement with those obtained from NMR spectra of a static sample. The principal components of the 31P chemical shift tensors determined for 1 are compared with those reported previously for Wilkinson's catalyst, RhCl(PPh3)3. The δ22 component of the 31P chemical shift tensors in 1 shows the largest variation with structure. This is ascribed to differences in the orientation of the P—Cipso bond about the equatorial plane of the trigonal bipyramidal structure. Keywords: rhodium–phosphine compounds, AB spin system, 31P chemical shift tensor, magic-angle spinning, molecular structure.
The thermodynamics of the ionization of 3-hydroxypyridine and pyridoxine in water–dioxane mixtures
Authors: Asensio Salvador B. , Lopez-Cantarero Enrique , Llor Juan
Published: May 2006

A potentiometric method has been used to determine the thermodynamic equilibrium constants for the macroscopic ionization processes of 3-hydroxypyridine and pyridoxine in water–dioxane mixtures (0–70% weight fraction in dioxane) at temperatures from 10 to 50 °C. From these data the reaction standard thermodynamic function changes were obtained for the processes in various water–dioxane mixtures at 25 °C. It was found, in both molecules, that during the first ionization process the contributions of entropy and enthalpy to free energy are similar but that during the second ionization the main contribution comes from entropy, especially in mixtures with a high dioxane content. The effect of the solvent on the tautomeric equilibrium in the net zero charge specie could explain this behaviour.
Intramolecular hydrogen bonding in ribonucleoside sugar hydroxyls. An abinitio study
Authors: Bosch Enric , Moreno Miquel , Lluch José M.
Published: May 2006

Abinitio self-consistent field (SCF) calculations were done for a model of a ribonucleoside where only the ring part of the furanoside form of D-ribose is considered. Full geometry optimization and direct location of stationary points were used in order to characterize the geometry of the minimum energy structure and of the transition state for the proton transfer reaction in the anion. Topological analysis of the charge density and its Laplacian for the minimum energy structure allow us to find clear electronic and geometrical evidence of an intramolecular hydrogen bond. On the other hand, our results show a very low energy barrier for the proton transfer so that the proton jumps easily between the two oxygen atoms.
Integral method for infinite dilution activity coefficients calculation
Authors: Chialvo A. C.
Published: May 2006

An integral method to determine infinite dilution activity coefficients [Formula: see text] from data sets of finite concentration activity coefficients is proposed for binary systems. This method is based upon the integration of the experimental dependence of ln [Formula: see text] over different composition ranges. The limiting behavior of this function is analysed, in order to improve the precision of its integration. The accuracy of this method is demonstrated by the calculation of [Formula: see text] for several systems.
Reactions of lignin model compounds with pulp bleaching chemicals. Reactions of 4-methyltrichlorocatechol
Authors: McKague A. Bruce , Reeve Douglas W. , Rettig Steven J. , Trotter James
Published: May 2006

Aqueous chlorination of 4-methyltrichlorocatechol 2 gave the tetrachlorohydroxycyclohexenone 4, which crystallized as the hydrate 5. Reaction of 4 with aqueous sodium hydroxide resulted in a benzilic acid rearrangement to the tetrachlorodihydroxycyclopentenecarboxylic acid 6. These reactions have important implications with respect to understanding how lignin is removed during pulp bleaching. Crystals of the tetrachlorotrihydroxycyclohexenone 5 are monoclinic, a = 11.4420(6), b = 7.5873(9), c = 12.6443(6) Å, β = 101.266(4)°, Z = 4, space group P21/c, and those of the methyl tetrachlorodihydroxycyclopentenecarboxylate 7 are monoclinic, a = 28.447(1), b = 6.929(1), c = 12.597(1) Å, β = 103.074(5)°, Z = 8, space group C2/c. The structures were solved by direct methods and were refined by full-matrix least-squares procedures to R = 0.028 and 0.034 for 1716 and 2164 reflections with I ≥ 3σ(I), respectively.
Reaction path Hamiltonian for the association reactions CH3 + H and CH3 + D
Authors: Wardlaw David M.
Published: May 2006

Coupling coefficients and normal mode frequencies appearing in the reaction path Hamiltonian formulated by Miller, Handy, and Adams have been calculated for the title reactions as a function of distance along the reaction path. The calculation of the coupling coefficients requires the use of symmetry coordinates which are described herein. It is found that the carbon–hydrogen separation provides an excellent representation of the reaction path coordinate, being linearly related to it. The coupling coefficients for CH3 + H and CH3 + D are approximately the same in the region of variational transition states and do not support the suggestion that an apparent isotope anomaly in the experimentally derived rates of these reactions might be attributable to different dynamics along the reaction path. The relative magnitudes of coupling coefficients for CH3 + H are used to assess some of the usual assumptions in variational transition state theory concerning separability of different types of motion in the transition state region.
Oxidation of L-rhamnose and D-mannose by Cr(VI) in perchloric acid. A comparative study
Authors: Sala Luis F. , Signorella Sandra R. , Rizzotto Marcela , Frascaroli María I. , Gandolfo Fabio
Published: May 2006

The oxidation of L-rhamnose and D-mannose by Cr(VI) in perchloric acid follows the rate law:[Formula: see text]K = 5.40, k0 = 0.00138 M−1 s−1, k1 = 6.71 × 10−4 M−3 s−1 for D-mannose and K = 2.63, k0 = 2.51 × 10−3 M−1 s−1k1 = 2.74 × 10−3 M−3 s−1 for L-rhamnose. This rate law corresponds to the reaction leading to the formation of L-1,4-rhamnonelactone and D-1,4-mannonelactone. No cleavage to carbon dioxide takes place when a 20-fold or higher excess of aldose over Cr(VI) is employed. Relative values of kinetic constants are interpreted in terms of primary hydroxyl group participation in the chromic ester formed in the first reaction step. The free radicals formed during the reaction react with Cr(VI) to yield Cr(V). Two signals (giso = 1.977 and giso = 1.970) ascribed to intermediate Cr(V) complexes were observed by electron paramagnetic resonance (epr) of 0.25–1 N HClO4 solutions saturated with L-rhamnose and Cr2O72−. At higher pH (3–7), only the signal at g = 1.977 was detected. For D-mannose only one signal at g = 1.978 was observed over the whole range of HClO4 concentrations.
Use of lithium (trimethylstannyl)(cyano)cuprate for the conversion of alkyl 2-alkynoates into alkyl (Z)- and (E)-3-trimethylstannyl-2-alkenoates
Authors: Piers Edward , Wong Timothy , Ellis Keith A.
Published: May 2006

Reaction of functionalized alkyl 2-alkynoates (e.g., 13–19) with lithium (trimethylstannyl)(cyano)cuprate (3) under two sets of carefully defined experimental conditions provides, efficiently and stereoselectively, either alkyl (Z)- or (E)-3-trimethylstannyl-2-alkenoates (e.g., 23, 26–31 and 24, 32–37, respectively).
Modeling the effect of Zn(II) on the hydrogen transfer in inhibition processes
Authors: Rozas Isabel , Arteca Gustavo A.
Published: May 2006

We present in this work the results of a rather simplified theoretical approach to some of the processes involved in the inhibition of Zn-containing active sites. Our aim is to calibrate a model in which Zn is replaced by Be. The motivation for this strategy is to provide a qualitatively correct, but fast, approach that can be used in the simulation of many inhibitors interacting with a Zn site. The use of Be allows us to retain some of the essential features of the inhibitor–Zn interaction, while involving much less computation time. It should be noted that we are not modeling a Be-containing active site, but processes occurring in a Zn-containing one. We design a scaling transformation that permits us to relate the electronic properties associated with a given Be–ligand distance to the same electronic properties associated with a desired Zn–ligand distance. The results provided by the Be-containing model are comparable to those corresponding to the actual Zn-containing model. One of the processes we are interested in describing here is the effect that the presence of a Zn(II) cation has on the ability of an inhibitor to donate a proton to an acceptor. This process is of importance in the inhibition of Zn-containing enzymes, such as liver alcohol dehydrogenase (LADH). The inhibitory mechanism of LADH seems to involve first the interaction of a ligand with the Zn cation, followed by the transfer of a proton to an acceptor, and finally the binding of the deprotonated ligand to the nicotinamide adenine dinucleotide (NAD+) coenzyme. In this work, we considered the molecule of formaldehydehydrazone as a model of an inhibitor (e.g., pyrazole). This simplified model contains the strict essentials needed to study proton transfer mediated by a metallic cation. We found a parallelism between the roles of the two metals in the H transfer from this model of inhibitor that allows us to make some qualitative predictions for other processes in Zn from the results obtained exclusively in the presence of Be.
1H nuclear magnetic resonance and molecular orbital estimates of the internal rotational barrier in phenylpropynal in solution and in the vapor
Authors: Schaefer Ted , Sebastian Rudy , Schurko Robert W.
Published: May 2006

The 1H nuclear magnetic resonance spectra of phenylpropynal and 1-phenylpropyne are analyzed for CS2/C6D12 and acetone-d6 solutions. The ensuing spin–spin coupling constants over eight formal bonds are used in assessing the conformational dependence of the one in the propynal derivative, as compared to the one over six bonds in benzaldehyde. The eight-bond coupling constant in phenylpropynal implies, via a hindered rotor model, that the twofold barrier to internal rotation is 5.9 ± 1.6 kJ/mol in both solutions. This number is much smaller than that for the internal barrier in benzaldehyde, reflecting the reduced π electron conjugation in phenylpropynal. Molecular orbital computations, with geometry optimization, confirm the essentially purely twofold internal barrier in the free propynal. The theoretical magnitudes are given for AM1 calculations, as well as for abinitio computations with STO-3G, 3-21G, 6-31G, and 6-31G* bases. To within experimental error, the barrier magnitudes from the split-valence basis sets agree with those obtained in solution.
Identification of the tautomeric structure of H2TMpyP4–DNA complexes from resonance Raman excitation profiles
Authors: Yixian Liu , Koningstein J. A. , Yevdokimov Y.
Published: May 2006

Aspects of the electronic structure of tautomers of tetrakis(N-methyl-4-pyridyl)porphyrin (H2TMpyP4) complexed to DNA have been determined from a study of the resonance Raman excitation profiles of normal modes of H2TMpyP4 in buffer solutions containing monomeric and aggregated molecules and of a H2TMpyP4/DNA complex with r(c) ~ 25. Maxima of the excitation profiles of the complex have been assigned to electronic origins for transitions between the ground state and states in the Soret absorption region of intercalated and externally groove-bound tautomers of H2TMpyP4. The positions of the electronic origins support the assignment of the CD spectrum of the complex. From the shift of the Cβ—H bending mode of the selectively excited resonance Raman spectra of tautomers of the porphyrin molecules it can be concluded that the pyridyl rings of externally bound H2TMpyP4 molecules are twisted to a lesser degree than those rings for intercalated H2TMpyP4. Raman intensity studies also reveal that the resonance-enhanced Raman intensity of modes of aggregated H2TMpyP4 is greater than those of the monomer.
Spectroscopic and reactivity studies of diastereomeric iridium(III) hydride complexes of the chelating tritertiary phosphine ligand Cyttp
Authors: Socol Steven M. , Yang Chihae , Meek Devon W. , Glaser Robert
Published: May 2006

The chelating tritertiary phosphine C6H5P(CH2CH2CH2P(C6, H11)2)2 (Cyttp) in octahedral six-coordinate iridium(III) complexes [IrH2Z(Cyttp](0 or 1+) (Z = Cl, CO, CH3CN, and related ligands) is meridionally bound. The conformation of the Cyttp ligand is such that the phenyl group of the central phosphorus lies out of the meridional plane. This leads to diastereomers in which the Z group is syn or anti to the phenyl group in question. These diastereomers were readily characterized by a general nuclear Overhauser effect difference NMR technique involving an intensity change to the apical-hydrido absorbance syn-to-phenyl upon irradiation of phenyl-ortho protons. A single diastereomer, cis-anti-mer-[IrH2Cl(Cyttp)] (6a), was prepared by (a) refluxing [IrCl(COD)]2 and Cyttp in ethanol or (b) treatment of [IrCl(COD)(Cyttp)] in benzene with excess dihydrogen. The stereochemical course for ligand substitution reactions on 6a was determined using the Cyttp phenyl group as a stereochemical label. Of six chloro-ligand metathesis reactions performed on cis-anti-mer-[IrH2Cl(Cyttp)] (6a), four gave mixtures of the two cis-syn-mer-/cis-anti-mer-[IrH2Z(Cyttp)](0 or 1+) diastereomers (Z = CH3CN, CN, PMe2Ph, and P(OCH2)3CEt), one gave inversion only (the cis-syn-mer compound (Z = CO)), and only one gave retention (cis-anti-mer-[IrH2I(Cyttp)]. A dissociative mechanism is proposed for the substitution reactions. Loss of stereochemical integrity in 6a was also observed in other reactions. Insertions of CS2 into the iridium–meridonal-hydride bond and SnCl2 into the iridium–chlorine bond produced mixtures of cis-syn-mer/cis-anti-mer diastereomers. Electrophiles (C7H7+ in the presence of CH3CN, I2, HCl, and HOAc in the presence of TI+) attack only the meridonal hydride to yield various ratios of geometric and (or) syn/anti isomers. Trends in the relative chemical shifts of the hydride protons and 31P nuclei in syn versus anti isomers are noted and rationalized.
Nature and characterization of the singlet and triplet excited states of the dibenzylideneacetone and di-3-(N-ethylcarbazoylidene)acetone bridging ligands
Authors: Harvey Pierre D. , Daoust Benoit
Published: May 2006

The lowest energy singlet state in di-3-(N-ethylcarbazoylidene)acetone (dNECa), a luminescent model compound at room temperature for dibenzylideneacetone (dba), has been assigned to a charge transfer (CT) state using the medium polarity effect on λmax(F) and the fluorescence polarized spectra of dNECa at 77 K. The dNECa fluorescence quantum yields [Formula: see text] and lifetimes (τF) are solvent sensitive where both [Formula: see text] and τF tend to increase with polarity. The very weak and moderately structured phosphorences have been located for the first time for both dba and dNECa in the 550–800 nm range and are also assigned to a CT state. In this case the CT interactions are less pronounced than λ(0–0)P and τP are much less solvent sensitive. Finally, EHMO type calculations confirm that the CT transition is the lowest energy transition in dba and di-3-(N-methylindoylidene)acetone (a model compound for dNECa), but also suggest that the nπ* state must lie near the CT state in dba.
Excess properties for acetylacetone + pentanols systems at 298.15 K
Authors: Riggio Roque , Martinez Hector E. , Salas Norma Z. de
Published: May 2006

From experimental densities, viscosities, and refractive indexes for the acetylacetone + 1-pentanol, + 2-Pentanol, + isoamyl alcohol, and + tert-amyl alcohol systems at 298.15 K, the excess molar volumes, excess viscosities, excess molar free energies of activation of flow, and excess internal pressures were calculated over the whole concentration range. Conclusions about the molecular interactions in these mixtures were drawn from the variations of the excess functions with the composition.
Total synthesis and establishment of the absolute stereochemistry of (+)-mostueine. Addition of chiral nucleophiles to 3,4-dihydro-2-methyl-9-(p-toluenesulfonyl)-β-carbolinium iodide
Authors: Rey Allan W. , Szarek Walter A. , MacLean David B.
Published: May 2006

A highly convergent synthesis of the pentacyclic indole alkaloid (+)-mostueine (1) is described. The key step involved the coupling of the dianion derived from (1′S)-3-(1′-hydroxyethyl)-4-methylpyridine (4) with the iminium salt 3,4-dihydro-2-methyl-9-(p-toluenesulfonyl)-β-carbolinium iodide (3). Low asymmetric induction (15% de) at the C-1 position of the β-carboline ring system (C-3 of mostueine) was obtained. The nonfermenting baker's yeast-mediated reduction of 3-acetyl-4-methylpyridine provided the hydroxyethylpyridine component in acceptable yield (67%) and high optical purity (99.0% ee). This synthesis of 1 has established that the absolute stereochemistry of mostueine is (3S, 19R).
1992 Merck–Frosst Award Lecture Orbital symmetry and the photochemistry of cyclobutene
Authors: Leigh William J.
Published: May 2006

Direct photolysis of alkyl-substituted cyclobutene derivatives in solution with monochromatic far-UV light sources results in competing ring opening to the corresponding substituted 1,3-butadiene derivatives and fragmentation to the corresponding alkene and alkyne via formal [2 + 2] cycloreversion. The excited states leading to the two types of products have been identified. Cycloreversion occurs stereospecifically and arises as a result of excitation of the π,R(3s) Rydberg state, which is of comparable energy to the π,π* state in alkylcyclobutenes. In spite of its stereospecificity, the reaction is nonconcerted; evidence is presented to suggest that it occurs by a mechanism involving [1,2] migration (ring contraction) to yield a cyclopropylcarbene intermediate, which then fragments to yield the alkyne and alkene. The stereochemistry at C3 and C4 in the starting cyclobutene is retained in the alkene produced in the reaction. Ring opening proceeds nonstereoselectively (from the π,π* singlet state) in well over a dozen systems that have been investigated, and possible mechanisms to account for this are discussed. It is proposed that orbital symmetry plays a role in the reaction, but only in the early stages of ring opening. Bicyclic cyclobutene derivatives in which the cyclobutene double bond is located across the [0] bridge undergo ring opening with a high degree of disrotatory stereoselectivity. Possible reasons for this unusual behavior are discussed in light of recent ab initio theoretical results.
Low-temperature X-ray diffraction studies on [Cu(4-Xpz)2]x linear chain polymers (where X = Cl, Br, CH3, H; pz = pyrazolate)
Authors: Ehlert Martin K. , Storr Alan , Thompson Robert C. , Einstein Frederick W. B. , Batchelor Raymond J.
Published: May 2006

Room temperature and low-temperature (110–140 K) powder diffractograms have been obtained for the polymeric compounds [Cu(4-Xpz)2]x (where X = H, CH3, Cl, and Br), and values of the unit cell parameters (orthorhombic, space group Ibam) a, b, and c have been obtained at both high and low temperatures. A single crystal X-ray diffraction study of the X = H compound at 116 K was completed and the results compared with a published study done at room temperature. The structures of these complexes involve extended chains of pyrazolate-bridged copper ions extending along the c crystallographic axis. The X-ray studies indicate little change in the c parameter with decreasing temperature and small but significant changes in the a or b parameters reflecting changes in interchain packing. This study permits some evaluation of how structural parameters are affected by these variations in interchain packing and how these variations may be affecting the magnitude of magnetic exchange in the compounds.
A deuterium NMR study of molecular dynamics and geometry in two classes of onium salts: (CH3)3E+X− and C6H5M(CH3)3+I−
Authors: Penner Glenn H. , Polson James M. , Daleman Stephen I. , Reid Kara
Published: May 2006

Deuterium NMR measurements are reported for two types of onium salts: (CH3)3E+I−, where E = O (counterion is BF4−), S, Se, or Te, and C6H5M(CH3)3+I−, where M = N, P, or As. Within each class of compounds the activation energy for rotation of the trimethyl groups about the [Formula: see text] axis increases with increasing size of the central atom. In the first class of compounds the C—E—C bond angle decreases with the size of the atom E. In addition the magnitude of the quadrupolar coupling constant, χ, varies with E, ranging from 160 kHz for E = O to 190 kHz for E = Te. This is in qualitative agreement with molecular orbital calculations of the electric field gradients. At low temperatures the 2H NMR spectrum of C6H5N(CH3)3+I− can only be rationalized with a model in which trimethyl rotation is faster than methyl rotation. The 2H NMR of ring (predominantly ortho and para)-deuterated C6H5N(CH3)3+I− is consistent with rapid n-site (n ≥ 3) rotation of the phenyl ring above 390 K. Below 390 K spectra characteristic of two-site, 180°, flips of the phenyl ring are observed. Below 280 K the motion of the phenyl ring is in the rigid lattice limit.
Dynamic behaviour of oxidized glutathione in solution investigated by nuclear magnetic resonance
Authors: Rossi Claudio , Donati Alessandro , Ulgiati Sergio , Sansoni Maria Rosaria
Published: May 2006

In view of the important biological functions of oxidized glutathione (GSSG), we investigated the correlation between dynamic and conformational properties and biological activity. Anisotropic molecular motion characterizes different GSSG molecular districts. This information, obtained by NMR carbon relaxation investigations, suggests that the peptide does not independently assume any stable structure in solution. Analysis of the effects of the addition of a stable spin label to the solution confirmed the absence of conformation of GSSG in D2O. Moreover, the paramagnetic effects observed on proton and carbon nuclei of oxidized glutathione suggest that the dipolar term is the main source of paramagnetic relaxation.
An ab initio study on the reaction of CH(X2π) with CH4
Authors: Yu Zhonghua , Chen Congxiang , Huang Mingbao
Published: May 2006

The mechanism of the reaction CH(X2π) + CH4 has been investigated by ab initio molecular orbital calculations. Addition, insertion, and abstraction–addition reaction paths have been examined by, in total, five methods of approach. The addition reaction path has a rather high energy barrier. Our calculations have implied that the assumed insertion reaction path does not seem to exist for the reaction CH + CH4, and a two-step mechanism (abstraction–addition reaction path) was then proposed. For the abstraction–addition reaction, the reactants, transition state, intermediates, and products were optimized at the HF/3-21G and HF/6-31G* levels, and vibrational frequencies were calculated at the HF/3-21G level. Electronic correlation energy was estimated by means of the Møller–Plesset perturbation theory and configuration interaction method. The excited-state abstraction reaction was also studied in some detail.
An empirically corrected quantum mechanical potential energy curve of internal rotation of acryloyl fluoride, CH2=CH-CF=O
Authors: De Maré George R. , Panchenko Yurii N. , Abramenkov Alexander V. , Bock Charles W.
Published: May 2006

The geometrical parameters of acryloyl fluoride were optimized completely at the MP2/6-31G* computational level for 17 points on the internal rotation potential energy (IRPE) curve for rotation around the formal single carbon–carbon bond. The expansion coefficients of the reduced rotational constant function F(φ) and the four, five, and six-term expansions of the IRPE function,[Formula: see text]were obtained from these data. The theoretical IRPE functions were then refined using only the experimental torsional transition frequencies in both the s-trans and s-cis wells. The IRPE functions obtained are compared with those in the literature, calculated at lower levels of theory in both the rigid and nonrigid rotation approximations. The best representation of the refined IRPE function is given by the six-term expansion with V1 = 71.7, V2 = 1944.8, V3 = 113.0, V4 = −122.8, V5 = −8.7, and V6 = 12.5 cm−1, respectively. From this IRPE function, one correctly predicts the s-trans conformer to be more stable with ΔH0 = 168 cm−1. The barrier to rotation from the s-trans to the s-cis positions, ΔH#, is 2048 cm−1 at 88° from the s-trans well. The advantages of using the nonrigid rotation approximation, based on high-quality quantum mechanical calculations that include correlation effects, to construct the effective IRPE function for molecules are emphasized.
"A-frame" vs. "open-book" geometries in binuclear complexes bridged by diphosphines and mercaptothiazolinate ligands. Unusual examples involving a bridging bis(diphenylphosphino)ethane group
Authors: Xiao Jianliang , Cowie Martin
Published: May 2006

Reaction of the tetracarbonyl species [M(CO)2(µ-mtz)]2 (M = Rh, Ir; mtz = 2-mercaptothiazolinate) with 1 equivalent of the diphosphines (Ph2P(CH2)nPPh2; n = 1 (dppm), 2 (dppe)) yields the compounds [M2(CO)2(µ-L)(µ-mtz)2] (M = Rh, L = dppm (1), dppe (3); M = Ir, L = dppm (2)), which readily undergo oxidative addition of iodine to give [M2I2(CO)2(µ-L)(µ-mtz)2] (M = Rh, L = dppm (4), dppe (6); M = Ir, L = dppm (5)). When 2 equivalents of dppm are used, the A-frame compounds [M2(CO)2(η1-mtz)(µ-mtz)(dppm)2] (M = Rh (7), Ir (8)) are afforded. In solution the dangling mtz group of 7 undergoes exchange with both the free mtz anion and the bridging mtz ligand. Compounds 7 and 8 are also produced by treatment of trans-[MCl(CO)(dppm)]2 (M = Rh, Ir) with 2 equivalents of the mtz anion. Reaction of these dppm-bridged dichloro species with 1 equivalent of the mtz anion yields [M2Cl(CO)2(µ-mtz)(dppm)2] (M = Rh (9a), Ir (10)). Compound 9a undergoes reversible Cl− dissociation to give [Rh2(CO)2(µ-mtz)(dppm)2][Cl] (9b), which is also the stable form in the solid. Reaction of 9 with CO gives the carbonyl-bridged species [Rh2(CO)2(µ-CO)(µ-mtz)(dppm)2][Cl]. The structures of 6 and 9b have been determined by X-ray crystallography. Compound 6 crystallizes in the triclinic space group [Formula: see text] with one-half equivalent of THF per asymmetric unit in a cell having a = 9.856(3) Å, b = 14.078(6) Å, c = 16.245(5) Å, α = 103.66(3)°, β = 93.21(3)°, γ = 92.91(3)°, V = 2182(1) Å3, and Z = 2, and has refined to R = 0.045 and Rw = 0.057 on the basis of 433 parameters varied. Compound 9b crystallizes with one equivalent of CH2Cl2 in the monoclinic space group P21/n with a = 11.400(1) Å, b = 21.944(2) Å, c = 22.134(1) Å, β = 92.494(7)°, V = 5532(1) Å3, and Z = 4, and has refined to R = 0.062 and Rw = 0.082 on the basis of 613 parameters varied.
Excess molar volumes and viscosities for the n-decane + 1-chlorodecane system at different temperatures
Authors: Holgado Mercedes E. F. De Ruiz , Schaefer Cecilia R. De , Davolio Franco , Katz Miguel
Published: May 2006

Excess molar volumes, excess viscosities, and excess energies of activation for viscous flow have been determined for the n-decane + 1-chlorodecane system at different temperatures, over the whole concentration range. The Prigogine–Flory–Patterson model for solution thermodynamics has been used to calculate the excess molar volumes. Grunberg and Nissan, McAllister, Teja and Rice, and Schrodt and Akel models have been used to calculate viscosity coefficients and these were compared with experimental data for the mixtures.
Electroréduction de dérivés dinitrés aromatiques. III. Rôle du sulfate de titanyle comme médiateur redox
Authors: Martre Anne-Marie , Danciu Virginia , Mousset Guy
Published: May 2006

The electrochemical behavior of titanium oxysulfate, used as a redox mediator in the reduction of 4,4′-dinitrodibenzyl, is studied in a 5 N H2SO4/EtOH 50:50 medium. The role played by electrogenerated Ti3+ ions in the chemical reduction of nitro groups is also defined. The results obtained allow the determination of the most suitable conditions leading to selective syntheses. Macroscale electrolyses at a constant potential in the presence of TiOSO4 give 4,4′-diaminodibenzyl with a great purity and a higher yield (93%) than without mediator (75%). The homogeneous chemical reduction of 4,4′-dinitrodibenzyl by first electrogenerating the Ti3+ ions, then adding the organic substrate, is particularly selective. The nature of the reduction products depends on the [Ti3+]/[dinitro derivative] ratio. The 4-amino-4′-nitrodibenzyl is synthesized in a 70% yield for a 6:1 ratio whereas the amount of diamino derivative increases and can constitute the sole reduction product for higher ratios. A comparison is made with 1,5- and 1,8-dinitronaphthalenes and 2,2′-dinitrodiphenyl. Using a 6:1 ratio, 1-amino-8-nitronaphthalene is not obtained while 45% of 1-amino-5-nitronaphthalene and 25% of 2-amino-2′-nitrodiphenyl are formed by reduction of the corresponding dinitro derivatives.
Reversible electrode potentials for formation of solid and liquid chlorozirconate and chlorohafnate compounds
Authors: Kipouros G.J. , Flengas S.N.
Published: May 2006

The standard electrode potentials for the formation of the pure solid and molten compounds Li2ZrCl6, Li2HfCl6, Na2ZrCl6, Na2HfCl6, K2ZrCl6, K2HfCl6, Cs2ZrCl6, and Cs2HfCl6 have been calculated from measured vapour pressures corresponding to their thermal decomposition at equilibrium and from available thermochemical data. Reversible potentials for the formation of Na2ZrCl6 and of K2ZrCl6 in solution according to the reaction[Formula: see text]where A is Na or K, have been calculated from available equilibrium vapour pressures as functions of the mole fractions of the alkali hexachlorocompounds. Standard potentials for the above reaction and "formal" potentials are also given. The latter are useful in predicting the electrochemical behaviour of dilute solutions of the hexachlorozirconates in alkali metal chlorides.
Switchover of reactions of solvated electrons with nitrate ions and ammonium ions in propanol–water solvents
Authors: Kang Tae Bum , Freeman Gordon R.
Published: May 2006

The reaction rate constants of [Formula: see text] with ammonium nitrate (~ 0.1 mol m−3) in 1-propanol-water and 2-propanol–water binary solvents correspond to [Formula: see text] reaction in the water-rich solvents, and to [Formula: see text] reaction in alcohol-rich solvents. The overall rate constant is smaller in solvents with 40–99 mol% water, with a minimum at 70 mol% water. The Arrhenius temperature coefficient is 26 kJ mol−1 in each pure propanol solvent, increases to 29 kJ mol−1 at 40 mol% water, then decreases to 17 kJ mol−1 in pure water solvent. The high reaction rates in the single component solvents, alcohol or water, are limited mainly by solvent processes related to shear viscosity (diffusion) and dielectric relaxation (dipole reorientation). Rate constants reported for concentrated solutions (50–1000 mol m−3) of ammonium and nitrate salts in methanol (Duplâtre and Jonah. J. Phys. Chem. 95, 897 (1991)) have been quantitatively reinterpreted in terms of the ion atmosphere model.
Kinetics and mechanism of the oxidation of alcohols by ferrate ion
Authors: Lee Donald G. , Gai Huifa
Published: May 2006

A kinetic study of the reduction of ferrate ion under basic conditions has been completed. The observation that a typical aliphatic ether, tetrahydrofuran, is oxidized at a rate comparable to that of aliphatic alcohols, such as cyclopentanol, indicates that the reaction between ferrate and alcohols is likely initiated by attack of the oxidant at an α-C—H bond, a conclusion that is consistent with the occurrence of primary deuterium kinetic isotope effects (2.8–4.3 at 25 °C) when α-hydrogens are replaced by deuterium. Because only acyclic products are obtained from the oxidation of cyclobutanol by ferrate, it may be concluded that free radical intermediates are involved in the reaction. The insensitivity of the reaction rates to substituent effects during the oxidation of substituted mandelic acids indicates that substantial charges are not built up in the transition state. All of these observations are most readily accommodated by a mechanism in which the reaction is initiated by a 2 + 2 addition of an Fe=O bond to the α-C—H of an alcohol to give an organometallic intermediate that subsequently decomposes by homolytic cleavage of the resulting C—Fe bond. Comparisons are made with the reactions between alcohols and other high-valent transition metal oxides.
Ion-pair state spectroscopy of HgCl
Authors: Bascal H.A. , Jordan K.J. , Lipson R.H.
Published: May 2006

Fluorescence excitation spectra of HgCl produced and cooled in a corona-excited supersonic jet discharge are presented. The dominant feature between ≈ 368 and 267 nm is an extensive vibrationally and isotopically resolved band system which is assigned to the B2Σ+(ν′) ← X2Σ+(ν″ = 0, 1, and 2) valence-to-ion-pair state transition of the radical. Transition wavenumbers for vibrational bands with 75 ≥ ν′ ≥ 21 and 2 ≥ ν″ ≥ 0 were measured, and B-state Dunham parameters were derived which are valid between 75 ≥ ν′ ≥ 0. The equilibrium bondlength for the B-state was determined to be 2.960(2) Å by Franck–Condon calculations. B–X Franck–Condon factors are tabulated which are expected to help in the interpretation of HgCl2 photodissociation and HgCl laser spectra.
Raman studies of cyanate: Fermi resonance, hydration and hydrolysis to urea
Authors: Brooker Murray H. , Wen Nanping
Published: May 2006

Raman spectra were measured for potassium cyanate in the solid phase and as aqueous solutions in H2O and D2O for freshly prepared and for aged solutions. The results indicated that the assignment of the Fermi doublet, ν1 and 2ν2, for solid potassium cyanate was reversed from the assignment for the aqueous solution. The Fermi doublet has an associated pair of hot bands at 1191 and 1315 cm−1 which originate from the 638 cm−1 ν2 state, 010. Assignment of the hot bands was confirmed by studies of solid potassium cyanate at liquid-N2 temperature, room temperature, and at 473 K. Raman spectra of aged aqueous potassium cyanate revealed that the cyanate ion hydrolyzed slowly and spontaneously at room temperature (even without added ammonium) to produce urea and a carbamate, carbonate equilibrium mixture in parallel reactions. Hydrolysis of cyanate in aqueous ammonium chloride solution resulted in almost total conversion of cyanate to urea. The reaction was not reversible under ambient conditions. Differences in peak frequencies and half-widths were observed for the cyanate dissolved in H2O compared to solutions in D2O. The results provide evidence for strong hydrogen bonding of cyanate to water and are consistent with greater structure in the D2O solution. Theoretical ab initio calculations indicated that the water molecules hydrogen bond well at both the oxygen and nitrogen atoms of cyanate although the hydrogen bond to nitrogen was found to be slightly stronger.
Indole alkaloids from "hairy roots" of Rauwolfia serpentina
Authors: Falkenhagen Heike , Stöckigt Joachim , Kuzovkina INNA N. , Alterman Irina E. , Kolshorn Heinz
Published: May 2006

Three minor indole alkaloids have been isolated from "hairy roots" of Rauwolfia serpentina Benth.: 3-epi-α-yohimbine (1), 18β-hydroxy-3-epi-α-yohimbine (2), and 12-hydroxyajmaline (3). The latter compound (3) is a novel alkaloid. The alkaloid structures have been determined by spectroscopic data.
Thermodynamic properties of the Na–Bi–Pb system
Authors: Tumidajski Peter J.
Published: May 2006

Thermodynamic properties of Na in an equimolar Bi–Pb alloy were determined at 723 K for compositions of 0.0075 ≤ XNa ≤ 0.1000 from electromotive force measurements on Na metal concentration cells with a solid Na–β-Al2O3 electrolyte. A coulometric titration technique was used to electrochemically generate the Na metal in the Bi–Pb alloy. The results indicate that, as Na is added to Bi–Pb, the liquid becomes more associated and suggests complex species formation with an ionic character intermediate between those observed for the constituent binaries.