bluuebunny/crossref_metadata_2025_split huggingface dataset:
Bixon‐Jortner Revisited
Authors: Kommandeur Jan , Meerts W. Leo 
Published: November 2013

AbstractAfter twenty years, the theory of radiationless transitions in molecules is reviewed. Its qualities and its strengths are discussed. It is pointed out, however, that this theory does not explain properly the quantum yield for nonresonant excitation, which should approach unity for large values of detuning. Insertion of an intermediate state remedies this situation and leaves the theory largely unaffected. The requirement of an intermediate state may explain the rather arbitrary variation of quantum yields among organic molecules and it may also be responsible for the rather too large number of interacting states observed in a high‐resolution molecular eigenstate spectrum.
Kinetically Controlled Aggregation in Reactive Adsorbate Overlayers
Authors: Becker Oren M. , Silverberg Marvin , Ben‐Shaul Avinoam 
Published: November 2013

AbstractThe kinetics of surface chemical reactions depend critically on the lateral distribution of the adsorbed reactants. In some cases, the adsorbates are aggregated into islands and the rate of reaction is strongly influenced by the structural (sizes and shapes) and energetic (lateral interaction potentials) characteristics of the aggregated overlayer. Adsorbate aggregation or segregation on surfaces may be induced by a variety of mechanisms, three important types of which are briefly discussed here. The first process is driven by strong lateral interactions between the adsorbed particles, which favor their organization in an ordered two‐dimensional phase, but because of kinetic constraints the system only reaches a long‐lived metastable state characterized by finite islands with highly ramified edges. A somewhat more complex aggregation‐segregation process, in a system comprised of two interacting adsorbate species, is also described, and the consequences with respect to surface reaction kinetics are demonstrated. The second aggregation process reflects (Eden‐type) island formation during adsorption of molecules from the gas phase. More specifically, these islands result from the so‐called (extrinsic) precursor‐mediated chemisorption mechanism. Finally, we discuss the segregation of adsorbed reactants induced by the bimolecular annihilation reaction A + B → 0, under steady‐state conditions. Unlike in the first example, this segregation process takes place even in the absence of adsorbate interactions (except for the reactive A‐B interaction) and is most pronounced in the absence of particle diffusion.
Coherent Vibrational Wave Packet Dynamics in Femtosecond Laser Excitation of Diatomic Molecules
Authors: Letokhov V.S. , Tyakht V.V. 
Published: November 2013

AbstractA scheme is discussed for real‐time observation of a finite vibrational motion of diatomic molecules. Observation schemes based on the Franck‐Condon principle, i.e., the “persistence” of a coordinate wave packet in an ultrashort excitation, are proposed.
Dynamic Behavior of Individual Rovibronic States in S1 Benzene
Authors: Schubert Ulrich , Riedle Eberhard , Neusser Hans J. , Schlag Edward W. 
Published: November 2013

AbstractPulsed Doppler‐free two‐photon excitation of single rotational states of the 141 vibronic state of benzene with a resolution of 70 MHz is employed to investigate the influence of molecular rotation on the decay behavior of these states. The analysis of the completely resolved (Δv = 15 MHz) spectrum of the 1410 band, measured with a CW laser, makes it possible to specify whether individual states are perturbed by other (“dark”) S1 states and allows the precise determination of the degree of admixture of these background states. The vibrational identity of the background states is known from emission spectra of single rovibronic states. Unperturbed rovibronic states were investigated up to J'K' = 6761, and a pure exponential decay was found which does not depend on molecular rotation in accord with a nonradiative decay of S1 benzene in the statistical limit. The decay of perturbed states, which was also found to be purely exponential, depends strongly on the degree of admixture of the background state and on its vibrational identity. Values of the decay time of two identified “dark” zero‐order S1 vibrational states, which cannot be directly excited by optical transitions, are determined by evaluation of the decay behavior of the perturbed states.
Homogeneous Desulfurization of Thiols and Sulfides by Yb[N(SiMe3)2]2.2 THF. Relevance to Heterogeneous and Biological Desulfurizations and the Synthesis of Lanthanide(II) Sulfides
Authors: Gretz Efi , Vetter William M. , Stecher Hilmar A. , Sen Ayusman 
Published: November 2013

AbstractYb[N(SiMe3)2]2·2THF was found to desulfurize thiols (RSH) and dibenzyl sulfide. With thiols, the extent of desulfurization was a function of the relative stability of the corresponding radical, R·. Radicals were also involved in the desulfurization of dibenzyl sulfide. A unified mechanism involving a series of one‐electron redox steps leading to the generation of organic radical intermediates has been proposed.
Molecular Adsorption on Porous Silica Gels from Binary Solutions
Authors: Drake J. M. , Levitz Pierre , Klafter Joseph 
Published: November 2013

AbstractIn this paper we present a detailed experimental study of molecular adsorption on interfaces of silica gels from binary solutions of alcohol/toluene. The adsorption measurements are corroborated with infrared spectra of the surface hydroxyl groups and with NMR results. The adsorption isotherms are analyzed within the framework of the Everett approach and possible scaling relations between alcohol monolayer capacities and adsorbate cross sections are discussed. We conclude that reliable morphological information on the interfaces cannot be obtained from these adsorption experiments.
Synthesis of (4S, 10R, 11S)‐4‐Methyl Juvenile Hormone I by Using Chiral Building Blocks of Biochemical Origin
Authors: Mori Kenji , Fujiwhara Mitsumko 
Published: November 2013

AbstractThe title compound 1 was synthesized starting from (2S, 3S)‐2‐ethyl‐3‐hydroxy‐2‐methylcyclohexanone (B) and (R)‐3‐methyl‐5‐pentanolide (3). The key step for the preparation of B was the reduction of the corresponding diketone with a yeast, Pichia terricola KI0117, and that for the preparation of 3 was pig liver esterase (PLE)‐catalyzed hydrolysis of C to give D.
Synthetic Receptors Possessing Non‐Covalent, Macrocyclic Rings
Authors: Libman J. , Tor Y. , Dayan I. , Shanzer A. , Lifson S. 
Published: November 2013

AbstractIn an attempt to provide access to biomimetic ion binders we aimed at molecules that would adopt conformationally restricted arrangements by virtue of non‐covalent, intramolecular interactions, and allow extension by suitable ion binding sites. This article introduces four Types of C3‐symmetric trispeptides, which satisfy both of these requirements. These trispeptides have been assembled from 1,3,5‐tris(aminomethyl)benzene (Type 1), tris(2‐aminoethyl)amine (Type 2), and two homologous triscarboxylates (Types 3 and 4) as anchors by symmetric extension with amino acid residues through peptide linkages. Spectral examination, supported by EFF calculations, demonstrates that all these compounds adopt chiral, propellerlike conformations which are stabilized by interstrand H‐bonds between amide linkages of different kinds. The stability of these conformations at equilibrium depends on the nature of the anchor, the amino acid residue and the solvent. Their H‐bond patterns share common features by consistently generating 15‐membered H‐bond rings. The differences between the four Types of trispeptides are interpreted in terms of a subtle balance between (i) stabilization through H‐bond formation, and (ii) imposed strain and entropy loss through the formation of non‐covalent, polycyclic arrangements. The success of this approach in providing biomimetic analogs of siderophores (iron carriers of microbial origin), by extension with catecholate or hydroxamate binding sites, is also indicated.
Activation of Glycerophosphocholine Phosphodiesterase in Friend Leukemia Cells Upon In‐Vitro Induced Erythroid Differentiation. 31P and 1H NMR Studies
Authors: Podo Franca , Carpinelli Giulia , Ferretti Amalia , Borghi Paola , Proietti Enrico , Belardelli Filippo 
Published: November 2013

AbstractThis paper reports the results of 31P and 1H NMR studies on alterations of glycerophosphocholine phosphodiesterase (EC 3.1.4.2) activity in Friend leukemia cells, upon in vitro induced erythroid differentiation, in which cells undergo a transition from a highly proliferative (proerythroblastic) to a more mature (normoblastic) state. Experiments on cell homogenates incubated in the presence of added glycerophosphocholine (GroPCho) demonstrated that in differentiated cells the activity of GroPCho phosphodiesterase was at least three times higher than in undifferentiated control cells. Activation of the same enzyme was also observed in previous studies on FLC tumors implanted in syngeneic mice and treated in vivo with antitumor cytokines. These results, together with the decrease of GroPCho reported also in other cell systems undergoing cell transitions to more mature forms, suggest the hypothesis that an activation of GroPCho phosphodiesterase might play a role in cell maturation and, perhaps, also in tumor cell proliferation. An interpretation is then proposed of the alterations observed in the levels of phospholipid precursors and/or catabolites in FLC in vitro and in vivo, in light of possible correlations between phospholipid turnover, cell stimulation, and signal transduction.
The Concept of FID‐detected Hole‐burning in Pulsed EPR Spectroscopy
Authors: Wacker Thomas , Sierra Gustavo A. , Schweiger Arthur 
Published: November 2013

AbstractA new class of pulsed EPR experiments, based on FID‐detected hole‐burning, is introduced. A transient spectral hole burnt into an inhomogeneously broadened EPR line by means of a selective microwave pulse is shifted or broadened by different types of perturbations and is subsequently recorded in a single experiment via an FID following a nonselective microwave pulse. The proposed approach often exceeds corresponding electron spin‐echo experiments with respect to sensitivity and resolution. A number of applications of the FID‐detected hole‐burning technique are discussed and the predicted features of the new experiments are verified by model calculations and experiments.
Pulsed ESR at 140 GHz
Authors: Prisner T.F. , Un S. , Griffin R.G. 
Published: November 2013

AbstractWe report pulsed electron spin resonance (ESR) experiments at a static magnetic field (B0) of 5.0T, corresponding to a microwave frequency of ˜140 GHz for g = 2 electrons. The experiments were performed with a transmission spectrometer, utilizing a 300 watt extended interaction klystron (EIK) operating in pulsed mode, together with a crossed waveguide Fabry‐Perot resonator which provided 30–40 dB of receiver isolation. Signal averaging was accomplished by mixing the pulse from the EIK with a 139.95 GHz reference oscillator, which was subsequently mixed with a local oscillator to generate a trigger signal for a digital oscilloscope. Concurrently, the ESR signal was mixed to an intermediate frequency using a second low frequency oscillator which was phase‐locked to the same source as the trigger oscillator and detected with a transient digitizer. In this configuration we achieved π/2 pulses of −14 ns, and the isolation of the Fabry‐Perot provided short spectrometer dead times (20–50 ns, including the pulse) for subsequent detection of the free induction decays. The performance of the spectrometer is demonstrated with spectra obtained from crystals of fluoranthenyl hexafluorophosphate.
Synthesis and Photochemistry of Quantum‐Size Semiconductor Particles in Solution and in Modified Layers
Authors: Weller H. , Eychmüller A. , Vogel R. , Katsikas L. , Hässelbarth A. , Giersig M. 
Published: November 2013

AbstractWe describe the preparation of highly monodisperse, quantum‐size CdS particles. In these samples a band splitting into discrete exciton‐like states is observed. A mechanism for the excitonic fluorescence is presented involving shallow traps as a reservoir for electrons which finally recombine with free positive holes across the bandgap. Quantum‐size CdS and PbS particles prepared in situ on highly porous, polycrystalline TiO2 electrodes work as highly efficient sensitizers. Photocurrent yields of more than 70% are reported. In the case of PbS, it is shown that an efficient electron transfer is possible only with particles in the size of the quantization regime.
Ultrasmall Metal Oxide Particles: Preparation, Photophysical Characterization, and Photocatalytic Properties
Authors: Bahnemann Detlef W. 
Published: November 2013

AbstractThis paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2‐propanol is described. Quantum (Q)‐size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower‐lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue‐shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pHzpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface‐bound molecules and free radical intermediates is presented to explain these differences in reactivity.
Modelling Photochemical Reactivity of Organic Systems — A New Challenge to Quantum Computational Chemistry
Authors: Bernardi Fernando , Olivucci Massimo , Robb Michael 
Published: November 2013

AbstractThe applicability of modern methods of quantum computational chemistry to the problem of modelling photochemical mechanisms is discussed. It is demonstrated that one of the central mechanistic features is the surface‐crossing region which takes the form of a conical intersection. It is shown that this mechanistic feature can yield a new rationalization of common experimental observations such as the simultaneous production of different photoproducts, the occurrence of rigid stereochemistry, and the occurrence of very low fluorescence quantum yields. In addition, a simple VB model can often lead to accurate predictions about the nature of the surface‐crossing region and the nature of the recoupling pathways on the ground‐state surface.
A GAUSSIAN‐1 Type Ab Initio MO Study of the [CH2NO]+ Potential Energy Surface. Theoretical Support for the Existence of Three Experimentally Distinguishable Isomers in the Gas Phase
Authors: Hrušák Jan , Holthausen Max C. , Goldberg Norman , Iraqi Muhammad , Koch Wolfram , Schwarz Helmut 
Published: November 2013

AbstractThe potential energy surface (PES) of [CH2NO]+ ions with an NCO connectivity has been explored by GAUSSIAN‐1 type calculations. Six isomers having an NCO backbone are found and characterized as minima on the PES; three of them are separated by isomerization barriers reasonably large enough to allow an experimental verification. The favored protonation site of neutral isocyanic acid (HNCO) is the nitrogen atom (proton affinity, calculated = 171.4 kcal/mol; experimental = 173 ± 1 kcal/mol) leading to the most stable isomer H2NCO+ (1). For the protonation of NCOH at the nitrogen atom we predict a proton affinity for the cyanic acid of 179.7 kcal/mol. The resulting ion HNCOH+ (2) is 15.9 kcal/mol higher in energy than 1. The third stable isomer 5 having the NCO connectivity can be formally viewed as a complex between CN and the water radical cation; 5 is 72.9 kcal/mol less stable than H2NCO+ (1). In addition, two high‐energy isomers HN‐C(H)O+(3) and HO‐C(H)N+ (4) have been found, but according to the shape of the PES, they are predicted not to be accessible experimentally. The calculated thermochemical data are in excelled agreement with experimentally available values. The calculations are extended to the unimolecular fragmentation channels, which should be observed in mass‐spectrometric experiments. The analysis of the results suggests that the hydrogen‐atom elimination process is favored over all other dissociation channels, followed by loss of NH2‐· and NH.
Electronic Spectral Shifts and Linebroadening of Heteroclusters
Authors: Heidenreich Andreas , Jortner Joshua 
Published: November 2013

AbstractIn this paper we report on molecular dynamics simulations of the lineshapes of the absorption spectra of perylene Arn heteroclusters (n = 2–22), which rest on the spectral density method in conjunction with the excited‐state potential modelling scheme. Inhomogeneous semiclassical absorption lineshapes were calculated by averaging of microcanonical spectra over the accessible phase space region. The size dependence and isomer specificity of the spectral shifts and spectral linewidths were elucidated, with the spectral shifts providing a powerful tool for the identification of structural isomers. Some information on isomerization dynamics was inferred from the temperature dependence of the linewidths, which mark the onset of correlated surface motion for (5|0) and (5|5) heteroclusters, as well as the onset of 2D ⇄ 3D isomerization and side crossing for (22|0) heteroclusters.
Photoinduced Charge‐Transfer Dissociation in van der Waals Complexes. III. Na···FCH3
Authors: Polanyi J.C. , Wang J.‐X. , Yang S.H. 
Published: November 2013

AbstractComplexes Na···FCH3)n, n = 1–6, in beams were identified by photoionization time‐of‐flight (TOF) mass spectrometry. Dissociation of Na···FCH3 was studied by photodepletion. The depletion spectrum (450–750 nm) suggested excitation of Na in the complex followed by charge‐transfer to yield Na+···FCH3− → NaF + CH3. The depletion cross section was substantial, peaking at 1.1 Å2.
The Reactions of O(1D) with Propane and Water Monomers and Clusters
Authors: Hurwitz Y. , Rudich Y. , Naaman R. 
Published: November 2013

AbstractReactions of O(1D) with water and propane monomers and clusters were investigated via a crossed molecular beam experiment and by dissociation of ozone in a water‐ozone complex, applying laser‐induced fluorescence for the detection of the OH product. The rotational and spin‐orbit state populations were analyzed. A strong preference for the 2Π3/2 spin‐orbit state of the OH product is observed. The new results presented here demonstrate the effect of the initial rotational temperature of propane on the spin‐orbit states distribution in the product OH. The preference for the low 2Π3/2 state is attributed to conservation of electronic angular momentum. It is conserved through the curve crossing which occurs in the entrance channel and during the lifetime of the long‐lived collision complexes. A quantitative model is presented which rationalizes all the experimental observations.
Preparing Reagents: Time Dependence of HCl(v =1, J) Alignment Following Pulsed Infrared Excitation
Authors: Orr‐Ewing A.J. , Simpson W.R. , Rakitzis T.P. , Zare R.N. 
Published: November 2013

AbstractWe discuss the use of photon absorption as a means of aligning reagents for studies of dynamical stereochemistry. A linearly polarized infrared beam from an optical parametric oscillator prepares HCl(v = 1, J = 1), and the alignment is monitored at subsequent times by (2+1) resonance‐enhanced multiphoton ionization. The degree of alignment oscillates in time caused by hyperfine quantum beats. Experimental observations are well matched by calculations that account for the presence of unresolved hyperfine structure. Following reagent preparation by optical pumping, a cumulative hyperfine‐depolarization coefficient can be used to describe the effective reagent alignment in a subsequent chemical reaction.
Relative Laser Phase in the Coherent Control and Interference Control of Photodissociation Branching Ratios
Authors: Brumer Paul , Chen Zhidang , Shapiro Moshe 
Published: November 2013

AbstractProduct properties in molecular photodissociation can be controlled by simultaneous excitation through a number of coherent excitation routes. Examples of simplified methods for maintaining laser phase coherence of the multiple routes, required for control, are described. Of particular interest is a new high field approach where multiple paths are generated in such a manner as to make relatively coherent lasers unnecessary.
Acetylcholine Recognition by an Aromatic Host: the Role of an All‐Hydrogen Topology in Simulations of the Cation‐π Interaction
Authors: Axelsen Paul H. 
Published: November 2013

AbstractThe contribution of cation‐π interactions to the affinity of a synthetic aromatic host for acetylcholine in a molecular mechanics simulation was estimated using a thermodynamic cycle‐free energy perturbation approach. Compared to a host in which aromatic dipole ring charges were set to zero, the affinity of the host for acetylcholine is greater by 1.8 kcal/mole when using a set of typical simulation parameters. The role of four ether oxygens in the host is probably negligible. Although acetylcholine binding may be driven in large part by hydrophobic effects, cation‐π interactions most likely make a significant contribution to the selectivity of this host for quaternary ammonium cations.
The Kinetic Inversion Problem in Ligand‐Binding Heme Proteins
Authors: Lerner Daniel M. , Agmon Noam 
Published: November 2013

AbstractFor a two‐dimensional potential of intersecting parabolic bowls, we obtain an analytical approximation for the survival probability in the limit that the reactive mode is diffusive and the perpendicular protein mode frozen. The approximation is used in an automated computer algorithm for determining the parameters of the potential. Using this program, we are able to obtain a reasonably good fit to the low‐ temperature data of CO binding to horse myoglobin. We find, however, that features of the potential such as the saddle‐point energy and the effective barrier, expected to dictate the high‐temperature kinetics, are not uniquely determined for this potential from fitting the low‐temperature data.
Conformational Sampling of Loop Structures Using Genetic Algorithms
Authors: Ring Christine S. , Cohen Fred E. 
Published: November 2013

AbstractBLoop, a method for generating plausible loop conformations for protein models, is described. Conformations that are compatible with the local sequence are generated by using the search mechanisms of genetics algorithms to explore the possible combinations of tetrapeptide conformers. Because the amino acid propensities that are used to judge the quality of the structures are derived from a data set of known protein structures, BLoop reproduces many features of database loop searches. But unlike these strategies that are limited by their contents, BLoop is capable of generating novel conformations. The expected accuracy of the database and BLoop methods for loop generation are compared on a set of loops as a function of residue length.