bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photochemically Induced Oxidative and Reductive Regeneration of NAD(P)+/NAD(P)H Cofactors: Applications in Biotransformations
Authors: Willner Itamar , Maidan Ruben , Willner Bilha 
Published: November 2013

AbstractPhotosensitized regeneration of NAD(P)H cofactors is accomplished by biocatalyzed and artificially catalyzed transformations in photochemical assemblies. Photogenerated N,N'‐dimethyl‐4,4′‐bipyridinium radical cation, MV+., acts as electron carrier for the reduction of NADPH in the presence of the enzyme ferredoxin reductase and for the reduction of NADH in the presence of lipoamide dehydrogenase. For the photogeneration of MV+. and subsequent NADPH formation, three different photosensitizers are applied: Ru(bpz)23+, Ru(bpy)23+, and Zn—TMPyP4+. The highest quantum yield for NADPH formation is observed with Ru(bpz)32+ and is ϕ = 1.7 × 10−1. For NADH regeneration only Zn‐TMPyP4+ can be applied. Ru(bpy)32+ and Ru(bpz)2+3 interact with NADH in their excited or oxidized forms and therefore cannot be used as light‐active compounds in the system. The NADPH regeneration cycle has been coupled to the biocatalyzed synthesis of glutamic acid. Although Ru(bpz)32+ is 42.5‐fold more efficient than Ru(bpy)32+ in the regeneration of NADPH, the synthesis of glutamic acid is improved only by a factor of 2 in the presence of Ru(bpz)32+, implying that the coupled process is rate limiting. Oxidative regeneration of the NAD+ cofactor is accomplished in a photosystem that includes Ru(bpy)32+ as photosensitizer. The photoprocess is coupled to dehydrogenation of ethanol, propanol, lactic acid, and alanine with concomitant H2 evolution. A photosystem that includes Ru(bpy)32+ as photosensitizer, ascorbate as electron donor, and chloro‐tris‐(3‐diphenylphosphinobenzene sulfonate)Rh(I), RhCl(dpm)33‐, is catalytically active in the photoinduced regeneration of NAD(P)H cofactors. Mechanistic investigations show that photogenerated Ru(bpy)3+ mediates the generation of a hydrido‐rhodium complex that acts as a charge relay for the production of NAD(P)H.
Concerning the Stereochemistry of SE'‐Type Reactions
Authors: Matassa Victor G. , Jenkins Paul R. , Kümin Albin , Damm Lorenz , Schreiber Jakob , Felix Dorothee , Zass Engelbert , Eschenmoser Albert 
Published: November 2013

AbstractThe steric course of an azidolytic SE' desilylation of a chiral allyl silane and experiments on electrophilic alkylations of ketene‐O,N‐acetals are described. In this context, we point to some merits of combining qualitative concepts of σ‐conjugation with the ẗ‐bond model of double bonds.
On the Absence of Anomalous Singularities in the S‐Matrix Version of the Kohn Variational Principle for Quantum Scattering
Authors: Gaucher Lionel F.X. , Miller William H. 
Published: November 2013

AbstractRecent calculations by Apagyi, B.; Lévay, P.; Ladányi, K. (Phys. Rev. A, 1988, 37: 4577) have shown that anomalous (i.e., spurious, unphysical) singularities arise in e− + H(1s) scattering calculations using the Schwinger variational principle. It has been known for a long time that such singularities can exist in the K‐ matrix version of the Kohn variational principle, but this is the first time (to our knowledge) that they have been seen in the Schwinger variational principle. This paper reports the results of calculations for this same problem using the S‐ matrix version of the Kohn variational principle (Zang, J.Z.H.; Chu, S.I.; Miller, W.H. J. Chem. Phys., 1988, 88: 6233), showing that there are no such anomalous singularities in this approach.
Time‐Dependent Quantum‐Mechanical Approaches to the Continuous Spectrum: Scattering Resonances in a Finite Box
Authors: Hammerich Audrey Dell , Muga J. Gonzalo , Kosloff Ronnie 
Published: November 2013

AbstractSeveral novel aspects of scattering resonances are studied. An expression, valid for a finite box, relating the continuum phase shift with the energy shift and unperturbed level separation is proposed and applied to obtain the resonance parameters. The effect of the resonance on propagating a wavepacket in imaginary time is studied. It is observed that the resonance strongly affects the cumulants of the energy distribution. In particular, a local minimum of the first derivative of the energy with respect to time (proportional to the second cumulant) serves to estimate the resonance energy and wavefunction. Once the estimate is known, the autocorrelation function is used to evaluate the resonance width. Alternatively, a new iterative approach is developed that is capable of selectively yielding an arbitrary band of energy eigenvalues and eigenfunctions on a grid. This method is applied to give those energy levels that are of interest for the discrete computation of the resonant phase shift, i.e., those close to resonance. Exact (analytical) and approximate results are in good agreement for a particular separable potential model in one dimension. These methods can be extended to realistic potentials in higher dimensions.
Theory of Diatomic Photodissociation to Atomic Hyperfine Structure States
Authors: Lee Sungyul , Williams Carl J. , Freed Karl F. 
Published: November 2013

AbstractWe present a general theory of the photodissociation of diatomic molecules to the hyperfine sublevels of open‐shell atomic fragments. The hyperfine interactions affect the fragment hyperfine sublevel populations, polarizations, etc., either when the fragment atomic hyperfine sublevels are optically resolved, when resonances possess hyperfine splittings, or both. The case of hyperfine splittings in resonances is of particular interest for states whose principle parentage is singlet, but which exhibit hyperfine splitting due to nonadiabatic mixing with close‐lying triplet states. Such resonances have been observed in the photodissociation of SiH+ (Sarre, P.J. et al., Phil. Trans. R. Soc. London, 1988, A324: 233). We also show how our nonadiabatic theory in the presence of hyperfine interactions reduces to the general theory of diatomic photodissociation to open‐shell atoms of Singer et al. (J. Chem. Phys., 1983, 79: 6060) when resonance hyperfine splittings or hyperfine structure states of the fragments are not resolved in particular experiments.
Unimolecular Reactions Revisited
Authors: Rice Stuart A. , Gaspard Pierre 
Published: November 2013

AbstractA brief review of recent developments in the theory of unimolecular reactions is presented, with emphasis on the work carried out at The University of Chicago. The subjects considered are: the development of a paradigm for the control of selectivity of a photoinduced reaction; the development of an alternative form for the classical mechanical theory of the rate of a unimolecular reaction; the beginnings of an understanding of the influence of classical mechanical chaos on the corresponding quantum‐mechanical description of an unbounded system; and the possible decoupling of a part of an N‐degree‐of‐freedom system from the rest of the system on a time scale of interest with respect to subsystem internal energy transfer and chemical reaction. The intellectual linkages between these subjects are very briefly sketched.
Electron Transfer in a Dynamically Disordered Polar Medium
Authors: Nadler Walter , Marcus Rudolph A. 
Published: November 2013

AbstractIntramolecular electron transfer (ET) processes with the main energetic contributions coming from the solvent reorganization are investigated for a polar medium that exhibits dynamic disorder. Dynamic disorder provides a description of the anomalous relaxational behavior of correlation functions in complex glass‐like systems, alternative to static disorder. In particular, the questions addressed are whether time‐resolved observation of nonexponential ET in such a medium can readily distinguish experimentally between static and dynamic disorder and whether a contribution of intramolecular degrees of freedom to the ET can be identified by it.
Structure and Photochemistry of C4 as Studied by the Coulomb Explosion Imaging Method
Authors: Algranati M. , Feldman H. , Kella D. , Malkin E. , Miklazky E. , Naaman R. , Vager Z. , Zajfman J. 
Published: November 2013

AbstractThe Coulomb Explosion Imaging (CEI) method was used to determine the structure and photochemistry of carbon clusters. The cyclic structure of C4 was found using a combination of laser photodetachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 ± 0.1 eV, more than 1 eV lower than the electron affinity of the linear isomer.
Resonance Positions and Widths for Time‐Periodic Hamiltonians by the Complex Coordinate Method
Authors: Moiseyev Nimrod , Korsch H. Jürgen 
Published: November 2013

AbstractThe complex coordinate method combined with the Floquet‐Bloch theory has the advantage of treating the metastable quasi‐energy states (i.e., resonances) of time‐periodic Hamiltonians in terms of bound state formalism. The formalism derived here is a general one. It enables one to use complex basis functions when needed and to calculate the resonance positions and widths for any time‐periodic Hamiltonians. A numerical example is given to illustrate the potential use of the proposed method in the study of the dynamics of laser‐driven systems and of time‐periodic classical chaotic Hamiltonians.
Photoinitiated Reactions of H Atoms with CO2: OH(v = 0) Rotational Excitation from the 239‐nm Photolysis of CO2HI Complexes
Authors: Hoffmann G. , Oh D. , Chen Y. , Engel Y.M. , Wit‐Tig C. 
Published: November 2013

AbstractPhotoinitiated H + CO2 → OH + CO reactions are discussed, with emphasis on reactions in CO2HI complexes. Under single‐collision bulk conditions, reaction probability rises with collision energy by two orders of magnitude throughout the range 10300–19000 cm−1 (ΔH = 8960 cm−1). Modest probabilities at collision energies well above threshold are interpreted as due to the inability of the heavier nuclei to move fast enough to trap the H atom on the HOCO potential surface. The pronounced increase in reaction probability with collision energy can be due to impact‐induced distortion of the CO2 frame, localizing the H atom in a shallow region of the HOCO potential surface long enough for the heavier nuclei to move toward the HOCO equilibrium geometry, thus capturing the H atom. Measurements of nascent OH(v = 0) R, T excitations indicate a significant bias toward product translation and away from OH rotation at the highest collision energies. OH LIF spectra taken at different collision energies provide a map of nascent OH(v = 0) rotational excitation for different values of E†, the HOCO† energy in excess of the OH + CO product channel. With CO2HI complexes, pairwise I‐H and H‐CO2 repulsions before HOCO† is formed increase the I‐HOCO† speed at the expense of HOCO† internal excitation. It is pointed out that with CO2HBr, the Br atom is 3.6 Å from the C atom along a line perpendicular to the CO2 axis, with the H atom localized near one of the O atoms. CO2HI is expected to be qualitatively similar. The OH(v = 0) rotational distribution obtained using 239‐nm photolysis of CO2HI complexes differs markedly from that obtained under single‐collision conditions at the same photolysis wavelength, the former being colder and qualitatively distinct from any of the OH(v = 0) distributions obtained at a single collision energy. The OH(v = 0) rotational distribution obtained using CO2HI complexes can be reconciled with a bimodal P(E†) distribution (e.g., ∼ 30% at E† ∼ 800 cm−1 and ∼ 70% at E† ∼ 6000 cm−1). The 6000‐cm−1 component is attributed to the squeezed‐atom effect (E† = 7880 cm−1 for single‐collision conditions at the same photolysis wavelength), while the origin of the other component is uncertain. It may derive from (i) mechanisms that produce HOCO† with low E† values, (ii) mechanisms that relax HOCO† and/or OH such as interactions with the nearby I atom and (iii) higher‐than‐binary complexes.
High Performance Liquid Chromatographic Studies of Some Cyclodextrin Complexes
Authors: Landau Sharon , Grushka Eli 
Published: November 2013

AbstractThe complex formation between β‐cyclodextrin and nitrophenol isomers was studied with the aid of reversed phase liquid chromatography. The effects of temperature, pH, and methanol content on the formation constants of the nitrophenol/β‐cyclodextrin complexes were investigated. The complex formation constants were found to decrease with increasing temperatures. This decrease in formation constant was independent of the pH of the mobile phase. From the temperature dependence of the formation constants we were able to extract enthalpies and entropies of the complex formation. The values calculated in this work agree favorably with published values. Three pHs were examined: 3.57, 4.65, and 5.6. The formation constants of the ortho and meta isomers were highest at the lowest pH and leveled off at the two higher pHs. The para isomer showed a minimum in the formation constant at pH 4.65. This behavior was independent of the pH of the mobile phase.
Graph‐Theory Derived Models for the Skeletal Chemical Bonding in Organometallic Metal Carbonyl Clusters
Authors: King R.B. 
Published: November 2013

AbstractIdeas derived from topology and graph theory can be used to model the skeletal chemical bonding in metal carbonyl clusters as well as boranes and carboranes. In such molecules delocalized skeletal bonding occurs when there is a mismatch between the number of internal orbitals provided by a vertex atom and the number of polyhedral edges meeting at that vertex. Such global delocalization in deltahedral metal clusters may be regarded as a three‐dimensional analogue of the aromaticity in planar polygonal molecules such as benzene. Elementary graph‐theoretical considerations provide a basis for the 2n+2 skeletal electrons normally observed in n vertex delocalized deltahedral metal clusters lacking tetrahedral chambers. Electron‐rich n‐vertex metal clusters have more than 2n + 2 skeletal electrons and form polyhedra having at least one face with at least 4 edges. Electron‐poor n‐vertex metal clusters have less than 2n + 2 skeletal electrons and form deltahedra having at least one tetrahedral chamber. Ideas derived from group theory and graph theory can be used to analyze molecular orbital energy parameters for the most symmetrical deltahedral borane anions BnHn2− (particularly n = 6 and 12) computed using either semiempirical extended Hückel methods or ab initio methods based on Gaussian orbitals. Some of the fundamental aspects of the graph‐theory derived models for skeletal bonding in delocalized deltahedral metal clusters are closely related to the tensor surface harmonic theory of Stone.
Multinuclear Metal Carbonyl Alkoxide and Aryloxide Derivatives as Models for Metal Carbonyls Adsorbed on Metal Oxide Supports
Authors: Darensbourg Donald J. , Mueller Brian L. , Bischoff Christopher J. , Johnson Cathy C. , Sanchez Kathryn M. , Reibenspies Joseph H. 
Published: November 2013

AbstractThe synthesis and characterization of two tungsten carbonyl dimers containing bridging alkoxide or aryloxide ligands are described. The crystal and molecular structures of [PPN]2[W2(CO)8(OCH2CF3)2], 1, and [Et4N]3[W2(CO)6‐(OPh)3]‐CH3CN, 2, are reported and compared with the structures of tetranuclear tungsten derivatives previously described. The dimer 1 crystalizes in the triclinic space group P 1 with unit cell parameters a = 13.460(11) Å, b = 12.318(5) Å, c = 13.842(10) Å, α = 82.73(5)°, β = 59.11(5)°, γ= 80.09(5)°, V = 1938(2) Å3, and Z = 1. The complex 2 crystalizes in the monoclinic space group P21/n with unit cell parameters a = 11.954(2) Å, b = 19.359(4) Å, c = 26.462(5) Å, β = 102.50(16)°, V = 5979(2) Å3, Z = 4. Molecular modeling software was utilized to construct a tetranuclear derivative from 1 similar to the structurally characterized [W(CO)3OH]4−4 tetramer. The two tetramers were found to possess similar molecular parameters. This supports the contention that dimers of type 1 are the precursors of the tetramers. Comparisons of the tungsten alkoxides and aryloxides with the behavior of W(CO)6 on γ‐alumina are provided.
Fluid Flow in Restricted Geometries
Authors: Sheng Ping , Zhou Minyao 
Published: November 2013

AbstractThis paper addresses two topics concerning fluid flow in restricted geometries. The first topic is on the relationship between the dynamic permeability κ(ω), where ω denotes frequency, and the microstructure of a porous medium. It is shown that when κ is normalized by its static value κ0, and the frequency ω by a characteristic ω0 particular to the sample, the resulting dimensionless function magnified image, with magnified image and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \tilde \omega = }\omega {\rm /}\omega _{\rm 0} $\end{document}, is dominated by the geometry of the throat regions in a porous medium. If the pore cross‐sectional area S varies slowly near the throats, i.e., dS/dz ≅ 0 where z is the distance normal to the cross section, then \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \kappa ({\rm \tilde \omega)} $\end{document} is an approximate universal function independent of other features of the microstructure. Both theoretical and experimental evidences are presented for such scaling behavior. When scaling holds, the dynamic permeability κ(ω) is found to contain only two pieces of geometric information, and the knowledge of either the low‐frequency or the high‐frequency asymptotic constants of κ(ω) would enable one to deduce the other missing parameters. In particular, since the high‐frequency asymptotic constants of κ(ω) can be related to the electrical formation factor and a weighted volume‐to‐surface ratio, the static permeability value κ0 may be directly deduced from the knowledge of these parameters. Whereas the first topic concerns single‐phase flow, the second topic of this paper involves the displacement of a fluid by another immiscible fluid in a capillary tube. Here the basic physics lies in the dynamics of the moving contact line, defined as the intersection of the moving fluid‐fluid interface with the tube wall. Through first‐principle hydrodynamic calculations and comparison with experiments, the macroscopic behavior of fluid displacement is linked to the microscopic parameters governing the region around the contact line. It is shown that there are two frictional forces associated with the motion of the contact line. One is the viscous stress, which is primarily responsible for the deformation of the fluid‐fluid interface as the velocity of the moving interface increases. The other frictional force, found experimentally to vary as the square root of velocity, is explained on the basis of capillary‐wave excitation at the fluid‐fluid interface due to contact line motion over wall roughness. Excellent agreement between theory and experiment is obtained.
Fluorescence Quenching in Three to Zero Dimensions: Effect of Dimensionality on a Diffusion‐Limited Reaction
Authors: Almgren Mats , Alsins Jan 
Published: November 2013

AbstractThe diffusion‐limited fluorescence quenching of pyrene by a hydrophobic benzophenone derivative, 1‐(4‐benzoyl)‐phenylhexane, was investigated in the different phases of the binary system hexaethyleneglycol‐mono‐n‐dodecylether (C12E6)‐water, from the normal homogeneous solution of the neat surfactant, via a sequence of liquid crystalline phases, to the aqueous micellar solution. These phases provide nonpolar domains of various dimensionality for the reactants: three dimensions in the neat surfactant, two in the lamellar phase, one in the hexagonal phase, and zero in the small micelles. The results were evaluated using models with appropriate dimension for the diffusion‐controlled reaction. The resulting diffusion coefficients at 20°C were obtained as 3 × 10−11 m2 s−1 in the neat surfactant, around 6 × 10−11 m2 s−1 in the lamellar phase, and 10 × 10−11 m2 s−1 in the hexagonal phase. The activation energy for the diffusion process increased from 32 kJ mol−1 in the neat surfactant to 42 kJ mol−1 in the hexagonal phase. The differences between the phases may be due to the uncertainty in the values of the size parameters for the structures; in any case, they are not large, indicating that it is mainly the geometry that is important for the deactivation kinetics, whereas the mobilities of the reactants are less affected by changes in packing.
Sequential Annulation in Molecular Construction. A Short, Stereocontrolled Synthesis of (±)‐Hirsutene
Authors: Paquette Leo A. , Moriarty Kevin J. , Shen Chien‐Chang 
Published: November 2013

AbstractA total synthesis of (±)‐hirsutene has been achieved in 9 steps and in 14% overall yield via a twofold cycloadditive scheme that utilizes 4,4‐dimethylcyclopentene, 1‐(trimethylsilyloxy)butadiene, and ethylene as the three key building blocks.
Regioselective Oxidation of Polyalkoxy Naphthalenes: Formation of Naphthoquinones by Ammonium Cerium(IV) Nitrate Oxidation of Methoxymethyl Ethers
Authors: Clive Derrick L. J. , Middleton Donald S. 
Published: November 2013

AbstractA proper choice of methoxy and methoxymethyl groups for protection of the hydroxyls of polyoxygenated naphthalenes allows selective oxidation of either ring of the naphthalene structure with ammonium cerium(IV) nitrate; for example, 9 can be converted into 10, and 11 into 12.
Biocatalysis as a Rational Approach to Enantiodivergent Synthesis of Highly Oxygenated Compounds: (+)‐ and (—)‐Pinitol and Other Cyclitols
Authors: Hudlicky Tomas , Rulin Fan , Tsunoda Toshiya , Luna Hector , Andersen Catherine , Price John D. 
Published: November 2013

AbstractBacterial oxygenation of halogenated aromatic compounds yields arene cis‐diols of type 2 of high enantiomeric purity. Further oxygenation of these extremely versatile synthetic intermediates provides for stereo‐ and regioselective introduction of additional functionalities such as hydroxyl groups. Several rational methods of oxygenation (epoxidation, osmylation, singlet oxygen addition, and ozonolysis) are used to produce, in short synthetic routes, cyclitol derivatives of medicinal importance. Proper symmetry considerations lead to the development of enantiodivergent synthetic design of target compounds from a single enantiomer of a starting diol. These principles are illustrated on the short syntheses of (+)‐ and (‐)‐pinitol. Stereocontrolled oxidative transformation of diols of type 2 are exemplified in the synthesis of conduritol C and dihydroconduritol C. Full experimental details are provided for all compounds. A guide to the stereorational design of any stereoisomer of cyclohexane (poly)ols or carbohydrates is provided at the end of the paper.
Stereoselective Total Synthesis of (±)‐Ptilocaulin and its 7‐Epimer. A Strategy Based On the Use of an Intramolecular Nitrile Oxide Olefin Cycloaddition (INOC) Reaction
Authors: Keshava Murthy K.S. , Hassner Alfred 
Published: November 2013

AbstractA stereoselective total synthesis of (±)‐ptilocaulin 1, an antimicrobial anticancer agent as well as of its 7‐epimer 23 is described. The strategy involves an aldol condensation of the dilithio dianion of the Z‐aldoxime (13) with a ketone 6 to produce 11 and the stereospecific intramolecular cycloaddition of the nitrile oxide 5, generated in situ from 11. The resulting isoxazoline 4 was converted to hydroxy ketone 3 which serves as a starting point for introduction of the fourth stereocenter at C‐7. Lithium in ethylamine reduction of 3 produces stereospecifically the 7β‐methyl compound which leads to ptilocaulin 1, while catalytic hydrogenation of 3 introduces the 7α‐methyl substituent and finally 7‐epiptilocaulin 23. H‐ and C NMR analysis permits differentiation between isomers of intermediates. When, in the last step of the synthesis of 1 or 23 from 19 or 22 and guanidine, a higher temperature (>130 °C) was applied, an interesting disproportionation to aminopyrimidine 25 or 28 and cyclic guanidine 24 or 27 was observed.