bluuebunny/crossref_metadata_2025_split huggingface dataset:
High‐Resolution Laser Techniques Applied To Reaction Dynamics
Authors: Vetter Raymond J. 
Published: November 2013

AbstractIn this paper, using the particular case of the Cs(7P) + H2 → CsH + H reaction, it is shown how the application of high‐resolution continuous‐wave (CW) laser techniques to crossed‐beam experiments can yield information about the mechanism of reactive collisions. Firstly, CW laser techniques allow the preparation of Cs atoms in given fine and hyperfine structure levels; for this reaction, measurements of hyperfine cross‐sections vs. different orientations of the atomic orbital show how the molecular symmetry of the reagents controls the efficiency of the harpooning process involved. Secondly, the laser‐induced fluorescence (LIF) technique of detection allows the characterization of CsH products in given rotational levels. The high‐precision scanning of the CW laser frequency over absorption profiles of CsH molecules results in the determination of the Doppler shifts, which are associated with the recoil velocity of the products; two laser beam arrangements are used to provide the product velocity and angular scattering probability. A “forward” peaking is discovered, which depends on collision energy but not on product rotation. Finally, the technique of saturated absorption is applied for the first time to reactive experiments; it could lead to methods for selecting products which scatter in a given plane or in given directions.
Vibrational Breakup of Triatomic Collision Complexes: A Model Study for H+‐H2
Authors: Drolshagen Gerhard , Gianturco Francesco Antonio , Toennies J. Peter 
Published: November 2013

AbstractThe predissociative behavior of the H+3 system is studied from the point of view of purely vibrational coupling between the rotationally‐averaged motions of the neutral diatomic and the bound proton. By decoupling the effects of the usual rotational barriers from those induced by the potential strength as a function of relative separations, one is able to see that the vibrational interchannel coupling shows strong propensity and highly nonstatistical propagation among bound levels and that the inner chaotic motion often assumed for this system is mostly controlled by the presence of multiple angular momentum barriers. Tunneling through these multiple rotational momentum barriers appears to be responsible for the slow dissociation seen in spectroscopic experiments.
Quasi‐Classical Trajectory Studies of H + HX → H2 + X (X = Cl, Br, I) Reaction at High Collision Energy
Authors: Aker Pamela M. , Valentini James J. 
Published: November 2013

AbstractQuasi‐classical trajectory calculations have been performed to investigate the dynamics of the H + HX → H2 + X reactions, where X = Cl, Br, and I. Calculations were performed for two collision energies, 16 and 36 kcal mol−1. The results show that the dynamics of the abstraction reactions at these high collision energies are rather unusual. The product energy and angular distributions can be accounted for by a mechanism that involves an isolated interaction between the two H atoms in the system to form H2 followed by hard‐sphere scattering of this newly formed H2 off the X atom.
Quantitating the Effect of Solvent Type on Neutral Macrocycle‐Mediated Cation Transport in Liquid Membranes
Authors: Izatt R.M. , Bruening R.L. , Bruening M.L. , Lamb J.D. 
Published: November 2013

AbstractThe recently confirmed diffusion‐limited transport model for neutral macrocyle‐mediated cation transport in membranes has been used to quantify the membrane solvent effects on the fundamental parameters in the model and their relative importance. The fundamental parameters involved are the extraction equilibrium constant, macrocycle partition coefficient, and diffusion coefficient. Experimental measurements and predictive correlation calculations have been made to quantify these fundamental parameters. Membrane stability has also been quantified by looking at solvent solubilities and solvent compatibility with various membrane types. It was found that membrane stability vs. rapid transport is the major choice to be made in choosing a membrane solvent. Retention of the macrocycle in the membrane and diffusion coefficients are greatest with short carbon chain solvents containing few chlorine atoms. On the other hand, high boiling points and low water solubility are obtained with long carbon chain solvents containing increasing numbers of chlorine atoms. Extraction equilibrium constants also decrease with increasing carbon chain lengths and increasing chlorination, but the decrease is not uniform like the changes in the other parameters. Little effect of solvent type on membrane selectivity was observed. The transport model and quantification of these fundamental parameters should allow for accurate predictions in the choice of solvent in designing stable membranes with optimal transport properties.
Field‐Flow Fractionation (FFF) and Related Techniques for the Separation of Particles, Colloids and Macromolecules
Authors: Levin Shulamit 
Published: November 2013

AbstractThin‐channel flow techniques can be used for the separation of samples ranging over 15 orders of magnitude in molecular weight, namely, macromolecules, colloids, and particles. The first part describes the group of field‐flow fractionation techniques. Flow system components are illustrated and described, as well as mode of operation and theory behind retention in the thin channel. An update of current topics under research is given, namely, normal retention mode of Brownian particles (under 1 μm in diameter) as compared to the steric retention mode of non‐Brownian particles (above 1 μm in diameter). Optimization of the separation by field programming is briefly discussed.Another group of thin‐channel techniques for small‐scale processing, the split‐flow thin (SPLITT) cells, in which fractionation is based on mass transport across the channel thickness, is presented in the second part. Principles of separation in this technology as well as possible applications are illustrated and discussed.
The Natural Bite Angle of Chelating Diphosphines
Authors: Casey Charles P. , Whiteker Gregory T. 
Published: November 2013

AbstractThe natural bite angles of chelating diphosphine ligands have been determined by molecular mechanics calculations using the MACROMODEL program with a modified AMBER force field. The natural bite angle (βn) is defined as the preferred chelation angle determined only by ligand backbone constraints and not by metal valence angles. Potential energy diagrams for diphosphine chelates were constructed to estimate chelate flexibility. Molecular mechanics calculations have been used to select diphosphine ligands with natural bite angles of 120° for diequatorial chelation in trigonal bipyramidal metal complexes.
Stereochemical Control in Palladium‐catalyzed Allylic Etherification
Authors: Keinan Ehud , Roth Zeev 
Published: November 2013

AbstractTwo alternative approaches for increasing the stereospecificity of Pd(0)‐catalyzed allylic etherification are presented, both of which also suppress product isomerization and elimination. One of them involves addition of chlorotrimethylsilane to a benzene reaction mixture. In the model reaction studied, it increases the ratio of the two stereoisomeric products, arising from either retention or inversion of configuration at the allylic carbon, from 3:1 to 9:1, respectively. The second approach involves the employment of both donor‐ and acceptor‐chelating ligands. Essentially 100% stereospecificity (retention of configuration) was achieved when the reaction was carried out in benzene at 60°C in the presence of Pd2(dba)3 and bis‐1,2‐diphenylphosphinoethane.
Reversible Carbon‐Carbon Bond Cleavage of a 3‐Vinyl‐1‐Cyclopropene by Rh(I). Molecular Structures of Two Sterically Crowded 1,2,3,5‐η‐Pentadienediyl Complexes of Rh(III)
Authors: Donovan Bernadette T. , Egan James W. , Hughes Russell P. , Spara Paul P. , Trujillo Hernando A. , Rheingold Arnold L. 
Published: November 2013

Abstract1,2,3‐Tri‐tert‐butyl‐3‐vinyl‐1‐cyclopropene 6a reacts with [RhCl(C2H4)]2 to give the dimeric 1,2,3,5‐η‐pentadienediyl complex 7a. The cyclopentadienyl derivative of this complex, 8a, is obtained by reaction of 7a with T1(C5H5). Crystal structures of both 7a and 8a were determined: 7a; orthorhombic, Pbcn, a = 28.136 (4) Å, b = 10.637 (2) Å, c = 12.154 (2) Å, V = 3637.6 (1.2) Å3, and Z = 4: 8a; triclinic, P 1 ‐bar, a = 9.906 (2) Å, b = 9.736 (2) Å, c = 12.126 (2) Å, α = 76.07 (2)°, β = 78.21 (2)°, γ = 65.34 (2)°, V = 1024.6 (3) Å3, and Z = 2. Treatment of 7a with one equivalent of PMe3 per Rh center results in regeneration of the vinylcyclopropene, demonstrating the reversibility of the ring opening reaction. Deuterium labelling studies show that both the ring opening and closing reactions proceed with retention of configuration at the vinyl olefin. Thus the trans‐deuterated vinylcyclopropene 6b reacts with [RhCl(C2H4)2]2 to give 7b in which deuterium is located exclusively in the syn position. Treatment of 7b with T1(C5H5) yields 8b. Treatment of 7b with one equivalent of PMe3 per Rh center results in formation of 6b as the sole organic product. Unlike their triphenyl relative 2, the tri‐Bu complexes 7a, 7b, 8a, and 8b are stereochemically rigid in solution at room temperature, and do not undergo an η3 → η1 → η3 isomerization on the NMR time scale. However, after prolonged heating, syn‐anti site exchange of H and D slowly takes place with 8b.
Stereoselective Alkylations of Enolates Derived from Ligands Attached to the Indenyl Iron Chiral Auxiliary [(η5‐C9H7)Fe(CO)(PPh3)]: X‐ray Crystal Structure and Conformational Analysis of [(η5‐C9H7)Fe(CO)(PPh3)COCH3]
Authors: Davies Stephen G. , Sarah Holland K. , Sutton Kevin H. , Mcnally John P. 
Published: November 2013

AbstractThe preparation, X‐ray crystal structure, and conformational analysis of the acetyl complex [(η5 ‐ C9H7)Fe(CO)(PPh3)COCH3] (5) are described. Deprotonation and alkylation of complex 5 generates the corresponding propanoyl (6) and butanoyl (7) complexes. Deprotonation and alkylation of complexes 6 and 7 to generate complementary diastereoisomers of the corresponding 2‐methylbutanoyl complex occur with high stereoselectivities. The aldol reaction of the diethylaluminium enolate derived from 5 with acetaldehyde also shows a high stereoselectivity (95:5). The reactions of the indenyl complex 5 clearly parallel those for the cyclopentadienyl analogue and this is rationalised in terms of the conformational analysis.
Unsaturated Bis(phosphido)‐bridged Heterobimetallic Polyhydrides via Dihydrogen Activation
Authors: Thomas Baker R. , Glassman Timothy E. , Ovenall Derick W. , Calabrese Joseph C. 
Published: November 2013

AbstractHydrogenation (1 atm) of (PCy2)2Re(μ‐PCy2)2M(1,5‐COD) (M = Rh, Ir; Cy = cyclohexyl; COD = cyclooctadiene), (PCy2)2Re(μ‐PCy2)2Rh(DMPE) (DMPE = [Me2PCH2]2), [(PCy2)2ReH(μ‐PCy2)2Rh(DMPE)]BF4 and (PCy2)2ReH(μ‐PCy2)2Pd‐(PPh3) proceeds stepwise with initial additions of H2 across the Re = P multiple bonds to give ReH(PCy2H) moieties. Further addition of H2 occurs only for M = Rh. The DMPE complex gives the tetrahydride (PCy2H)2ReH3(μ‐PCy2)2RhH(DMPE), which loses PCy2H at 80 °C to give (PCy2H)ReH4(μ‐PCy2)2Rh(DMPE). Hydrogenation of the COD complex affords the stable hexahydride (PCy2H)ReH5(μ‐PCy2)2RhH(PCy2H) via the cyclooctenyl complex. (PCy2)2ReH(μ‐PCy2)2Rh(η3‐C8H13). While the hexahydride is an active homogeneous catalyst precursor for the hydrogenation of alkenes, dienes, and alkynes, attempts to activate the Re center for C‐H bond activation have not yet been successful. 31P and 1H DNMR spectroscopy have been used to investigate the solution dynamics of the heterobimetallic polyhydrides, and several X‐ray diffraction studies serve to define the solid state structures.
Percolation Model of Nuclear Magnetic Relaxation in Porous Media: Slow Diffusion
Authors: Mendelson Kenneth S. 
Published: November 2013

AbstractNuclear magnetic relaxation in a porous medium is simulated by a random walk on the open sites of a percolation lattice. This model is used to study relaxation in the case that diffusion to the pore surface is slow compared to relaxation at the surface. The computed decay curves can be fit by sums of exponentials. For slow diffusion the principal relaxation time is independent of the surface relaxation rate. It is inversely proportional to the diffusion coefficient and directly proportional to the mean square displacement of the walkers. It appears that a measurement of nuclear magnetic relaxation in the slow diffusion range will yield only an average pore size. This pore size is given by the root mean square displacement to the surface for a random walker starting with equal probability from any point in the pore space.
Reaction Kinetics in Restricted Spaces
Authors: Kopelman Raoul , Koo Yong‐Eun 
Published: November 2013

AbstractReactions in restricted spaces rarely get stirred vigorously by convection and are thus controlled by diffusion. Furthermore, the compactness of the Brownian motion leads to both anomalous diffusion and anomalous reaction kinetics. Elementary binary reactions of the type A + A → Products, A + B → Products, and A + C → C + Products are discussed theoretically for both batch and steady‐state conditions. The anomalous reaction orders and time exponents (for the rate coefficients) are discussed for various situations. Global and local rate laws are related to particle distribution functions. Only Poissonian distributions guarantee the classical rate laws. Reactant self‐organization leads to interesting new phenomena. These are demonstrated by theory, simulations, and experiments. The correlation length of reactant production affects the self‐ordering length scale. These effects are demonstrated experimentally, including the stability of reactant segregation observed in chemical reactions in one‐dimensional spaces, e.g., capillaries and microcapillaries. The gap between the reactant A (cation) and B (anion) actually increases in time and extends over millimeters. Excellent agreement is found among theory, simulation, and experiment for the various scaling exponents.
Influence of the Ionization Potential and of the Interfacial Electric Field on Photoinduced Hole Transfer Processes in Langmuir‐Blodgett Films
Authors: Itaya Akira , van der Auweraer Mark , Verschuere Bart , De Schryver Frans C. 
Published: November 2013

AbstractThe presence of a monolayer of ω‐(2‐anthryl)‐alkanoic acid on an anthracene or perylene single crystal modifies the kinetics of photosensitized hole injection. An increase in the chain length of the ω‐(2‐anthryl)‐alkanoic acid leads to a decrease in the quantum yield of the sensitized hole current (SHC). This effect becomes more important when the ionization potential of the dye is decreased. This argument supports the hypothesis that electron transfer is assisted by an interaction between the HOMO of the dye and that of the alkyl chain. The permanent electric field existing at the interface between the monolayer of ω‐(2‐anthryl)‐alkanoic acid and the monolayer containing the dye is also expected to influence the free energy gain associated with this electron transfer process. When the dye is diluted by tripalmitine instead of arachidic acid, the change in the electrical potential difference at the interface with the monolayer of ω‐(2‐anthryl)‐alkanoic acid leads to a tenfold decrease in the quantum yield of the SHC.
Fractal Nature of Fluorescence Quenching and its Relation to Percolative Conduction in Water‐in‐Oil Microemulsions
Authors: Lianos Panagiotis , Malliaris Angelos 
Published: November 2013

AbstractResults of fluorescence quenching and electrical conductivity measurements in water‐in‐oil microemulsions were analyzed in terms of fractal models. It was found that for SDS/water/pentanol/alkane microemulsions there exists a connection between the fractal nature of the reaction domain and the magnitude of electrical conductivity. Conductivity values increase as the domain becomes less fractal.
Asymmetric Intramolecular Ene Reaction Catalyzed by a Chiral Titanium Reagent and Synthesis of (—)‐ε‐Cadinene
Authors: Narasaka Koichi , Hayashi Yujiro , Shimada Satoru , Yamada Jun 
Published: November 2013

AbstractA highly diastereo‐ and enantio‐selective intramolecular ene reaction was developed by employing a chiral titanium reagent generated in situ from TiCl2(OPri)2 and a tartrate‐derived chiral 1,4‐diol. This reaction afforded masked cyclohexanone and cyclopentanone derivatives with high optical purities which were synthetically useful chiral building blocks. From the intramolecular ene product (—)‐ε‐cadinene was synthesized by the use of the newly developed intramolecular Michael reaction.
Mixed Crystals as Host‐Guest Systems for Probing Molecular Interactions
Authors: Vaida M. , Weissbuch I. , Lahav M. , Leiserowttz L. 
Published: November 2013

AbstractA method is described for probing interactions between molecules at the surfaces of growing crystals. For this purpose, mixed crystals have been treated as unique host‐guest systems where the guest molecules are occluded into the host crystal through stereospecific interactions occurring at the different growing surfaces of the host. Owing to the process of molecular recognition at such surfaces, the adsorption and occlusion of the guest molecules takes place at specific sites, resulting in a reduction in symmetry of the mixed crystal compared to the pure host. The concept of reduction in symmetry is general, and has been instrumental for probing subtle molecular interactions at the surfaces of growing crystals, for the transformation of centrosymmetric single crystals into crystals with polar arrangement, and for “absolute” asymmetric synthesis inside centrosymmetric crystals.
Bifunctional Binding and Catalysis in Host‐Guest Chemistry
Authors: Breslow Ronald 
Published: November 2013

AbstractSome dimers of cyclodextrin have been prepared that can bind appropriate substrates in water with binding constants as strong as those of strong antigen/antibody complexes. Doubly linking two cyclodextrins with connections at neighboring glucose units produces a hinged molecule that resembles a clamshell. Another isomer is also produced, in the so‐called loveseat configuration, in which the two cyclodextrin units cannot cooperate in binding a single substrate. When the two links are of different length, the clamshell dimer preferentially binds a non‐linear substrate with an affinity exceeding 1011 M−1, at least 103 better than the binding of a linear substrate isomer. Binding of substrates occurs by induced fit, in which the host closes around and engulfs the substrate. Mechanistic studies on models for the enzyme ribonuclease have led to a proposed mechanism different from that normally suggested for the enzyme. This mechanism has guided the synthesis of a new enzyme mimic with improved properties. Isotope studies show that this enzyme mimic uses a simultaneous bifunctional mechanism, as does the enzyme. The double binding of the transition state by the two imidazole units of the enzyme mimic occurs because of their linkage. The geometric preference for this binding reflects anon‐linear transition state.
Enantioselective Hosts for Neutral Derivatives of Amino Acids
Authors: Clark Still W. , Kilburn Jeremy D. , Sanderson Philip E.J. , Liu Ruiping , Wiley Michael R. , Hollinger Frank P. , Hawley Ronald C. , Nakajima Makoto , Bernardi Anna , Hong Jong‐In , Namgoong Sung Keon 
Published: November 2013

AbstractA series of macropolycyclic receptors for hydrogen bond donor/acceptor guests were prepared and studied. Molecular modeling and X‐ray crystallography provided structural information, while NMR titration studies allowed determination of free energies of association. Some of die receptors prepared showed enantioselectivity as high as 3 kcal/mol in binding derivatives of simple amino acids in organic solvents. These studies show that conformational homogeneity is one of the most important characteristics of a selective host and that molecular modeling can play an important role in the design of such substances.
Supramolecular Photochemistry and Photophysics. Adducts of Metal Complexes with the Natural Ionophore Lasalocid A Anion
Authors: Ballardini Roberto , Gandolfi Maria Teresa , Moya Maria Luisa , Prodi Luca , Balzani Vincenzo 
Published: November 2013

AbstractThe formation of adducts of Cr(bpy)33+, Cr (phen)33+, Ru(bpy)32+, and Pt(bpy)(NH3)22+ (bpy = 2, 2′‐bipyridine; phen = 1, 10‐phenanthroline) with the lasalocid A anion (LAS‐) in CH2Cl2 solution has been investigated by means of electronic absorption and emission spectroscopy. Cr(bpy)33+ forms a 1:1 adduct with LAS−. In this adduct, the fluorescence of LAS− is quenched, whereas the phosphorescence intensity and lifetime of Cr(bpy)33+ increase. In the case of Cr(phen)33+, only a dynamic quenching of the Cr(phen)33+ phosphorescence by LAS− is observed. For Ru(bpy)32+, either no adduct is formed or the interaction is too weak to cause changes in the absorption and emission properties. For Pt(bpy)(NH3)22+, evidence has been obtained for formation of a 1:2 Pt(bpy)(NH3)22+ / LAS− adduct, where the LAS− fluorescence is statically quenched.
Effects of Electrostatic and π—π Interactions on the Stabilities of Xanthene Dye—4,4′‐Bipyridinium Complexes: Structural Design of a Geared Supramolecular Machine
Authors: Willner Itamar , Eichen Yoav , Doron Amihood , Marx Sharon 
Published: November 2013

AbstractXanthene dyes and substituted 4,4′‐bipyridinium salts form well defined 1:1 complexes in solution and in the crystalline state. Electrostatic, charge‐transfer, and π‐π interactions are the driving forces for the formation of these complexes. Electrostatic interactions contribute 1.1–1.3 kcal/mol per ion pair, and interring interactions, i.e., charge transfer and π‐π interactions, contribute 6.2–6.3 kcal mol−1 to the complexes' stabilization. Application of a bis‐bipyridinium host results in enhanced binding of eosin Y, 1, due to increased π‐π and donor‐acceptor interactions and complementary electrostatic attractions. The binding properties of the bipyridinium moiety to eosin Y is controlled by its functionalization with a photochromic azobenzene unit. The association constants of 8‐trans and 8‐cis to eosin Y, 1, are 3000 M−1 and 38000 M−1, respectively. This difference in the association constants of the diad isomers to 1 allows the photo‐controlled reversibly‐geared association and dissociation of the diad to and from eosin Y in the supramolecular assembly.
The Mechanisms of Making Molecules to Order
Authors: Brown Christopher L. , Philp Douglas , Spencer Neil , Fraser Stoddart J. 
Published: November 2013

AbstractThe mechanisms of formation of a [2]catenane and one of its molecular components have been investigated. While the synthesis of the tetracationic cyclophane, [BBIPYBIXYCY]4+ from bipyridine (BP) and 1,4‐bis(bromomethyl)benzene (BBB), directed by the template, 1,5–bis[2(2–hydroxyethoxy)ethoxy]naphthalene(1/5BHEEN) becomes less efficient (23 to 5%) under ultra‐high pressure reaction conditions (12 kbars), the self‐assembly of {[2]‐[BPP34C10]‐[BBIPYBIXYCY] catenane}4+ from BP and BBB in the presence of BPP34C10 can be achieved with increased efficiency (18 to 42%) at 12 kbars. This difference in the trends of the yields can be ascribed to the enhanced templating action of BPP34C10 relative to that of 1/5BHEEN when two moles of BP and two moles of BBB are being employed to construct the tetracationic cyclophane. The self‐assembly of the [2]catenane from BP, BBB, and BPP34C10 has been followed by 1H NMR spectroscopy in D7‐DMF solution. On the basis of this spectroscopic evidence and supporting chemical data, the formation of {[2]‐[BPP34C10]‐[BBIPYBIXYCY] catenane}4+ from two moles of BP, two moles of BBB, and one mole of BPP34C10 is believed to proceed via the monoquatemary intermediate, [MBXYBIPY]+, which has not been isolated, and the dicationic species, [BBIPYXY]2+, which has been isolated and shown to be an intermediate in the self‐assembly process leading to the [2]catenane, presumably via the [BXYBBIPYXY]3+ trication — the final intermediate which again has not been isolated.