bluuebunny/crossref_metadata_2025_split huggingface dataset:
Infrared Multiphoton Excitation of Small Molecules
Authors: Bloembergen N. , Burak I. , Mazur E. , Simpson T. B. 
Published: November 2013

AbstractThe collisionless infrared multiphoton excitation of small molecules, OCS, SO2, NO2 and DN3 is discussed. Despite the small absorbance of IR photons by these molecules, evidence for the population of highly excited vibronic levels below the dissociation threshold is obtained. Multiphoton dissociation is observed, as well. The average number of photons absorbed and dissociation yields depend predominately on the laser intensity. High order multiphoton processes couple the ground vibrational level with the quasicontinuum of excited levels below and above the dissociation threshold.
Stark Shifting and Double Resonance Decomposition in IR MPD Pump‐and‐Probe Experiments
Authors: Rayner D. M. , Hackett P. A. 
Published: November 2013

AbstractWe report upon the dynamics of the IR MPD of perfluoroalkyl iodides using visible laser induced multiphoton ionization of free iodine atoms in their ground state (2P3/2) as the probe technique. Stark shifting of an atomic MPI line by an intense infrared field was observed for the first time. The importance of an IR/visible double resonance dissociation through an excited electronic state was demonstrated and monitored through MPI of the excited spin‐orbit state (2P1/2) which is produced uniquely through this pathway. The consequences of the complications caused by these two effects for pump‐and‐probe experiments are discussed and methods to minimise their effect are suggested.
Conformation, Coordination and Ion Selectivity of the Ionophore X206
Authors: van Roey Patrick , Duax William L. , Strong Phyllis D. , David Smith G. 
Published: November 2013

AbstractThe conformation and the cation coordination of the ionophore X206, which selectively transports potassium over sodium, are evaluated in an effort to correlate the structural features of the ionophore with its ion selectivity. For this purpose, the crystal structure of the Na+‐complex of X206 has been determined by X‐ray diffraction and is compared with those of the free acid form of X206, its Ag+‐complex and the K+‐complex of the closely related ionophore alborixin. The ionophore conformation is very similar in all four structures. The carboxylic acid end is essentially invariant, but small changes in torsion angles of up to 26° at the other end of the molecule, combined with differences in the positions of the cations, result in very different coordination geometries for the cations. The conformations of the Ag+‐ and K+‐complexes resemble the free acid conformation more closely than does that of the Na+‐complex. The K+ ion is 8 coordinate while the Ag+ and Na+ ions are 6 and 5 coordinate, respectively. Empirical calculations of the bond strength on the basis of the coordination bond lengths show a stronger binding of K+ than of Na+ which is consistent with the greater selectivity for binding of the former.
Heavy Metal Ionophores: Correlations Among Structural Parameters of Complexed Nonpeptide Polyamino Acids
Authors: Stezowski J. J. , Hoard J. L. 
Published: November 2013

AbstractNonpeptide polyamine acid ligands such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) are members of a family of nonspecific heavy metal ionophores. Drawing upon the Cambridge Crystallographic Data Files and the results of a crystal structure determination for BaNd(III)DTPA·3H2O, we have looked for correlations between structural parameters and the physical and chemical properties of the metal ions complexed. Good correlations were found for primary metal‐ligand (PMtL) interaction parameters (parameters involving only atoms interacting directly with the metal). Meaningful correlations were also found for secondary metal–ligand (SMtL) interaction parameters and for parameters involving only ligand atoms (L parameters). The correlations observed are interpreted as indicating that (a) contact distances between oxygen atoms in the metal ion coordination sphere are important to the determination of average Mt–O bond distances, (b) the Mt–ethylenediamine rings display considerably less sensitivity to contact distances in the coordination polyhedron than do Mt–amino acid rings and (c) there appears to be a meaningful correlation between the Mt–O bond distance and the amount of single bond character in the associated CO (carboxylate) bond.
Biomimetic Systems for the “Visible” Copper‐Site CuA in Cytochrome c Oxidase
Authors: Toftlund Hans , Becher Jan , Olesen Preben H. , Pedersen Jens Z. 
Published: November 2013

AbstractRecent work on the structure of the CuA site of cytochrome c oxidase is described. Synthetic model systems containing two thiolate sulfur and two nitrogen donors are reviewed. The electronic model of CuA is discussed and a new dynamic model, which accounts for the atypical ESR properties of this enzyme, is suggested.
Stereoselective Hydrolysis of Amino Acid Esters Catalyzed by β‐Cyclodextrin‐L(or D)‐histidine Compounds
Authors: Ohkubo Katsutoshi , Nagamura Hiroo 
Published: November 2013

AbstractThe catalytic activity of β‐cyclodextrin‐L(or D)‐histidine compounds, β‐CD‐D‐His(e) (2a), β‐CD‐L‐His(e) (2b), and β‐CD‐D‐D‐His(a) (2c), for the stereoselective hydrolysis of N‐acyl‐amino acid esters (3a‐f) was examined at 25° C (pH 7.90). The ability of 2a‐c to bind the ester substrate into the cyclodextrin cavity and to attack the bound substrate by the nucleophilic imidazolyl group of the histidine moiety resulted in extremely high activity for those catalysts. However, the stereoselection of the enantiomeric esters during the inclusion of the substrate by 2a‐c was found to be opposite, in terms of chiral specificity, to that during the esterolysis of 2a‐c in the molecular cavity: that is, the stereoselective activity of 2a‐c is very small. The catalytic activities of 2a‐c are also influenced by the structural differences in 3a‐f, which result in the different modes of substrate inclusion by the catalyst through hydrophobic forces.
Preparation and Electrochemical Properties of bis‐2.2′‐(4.5‐Dimethyl‐1.3‐dithiolium)‐1.4‐phenylene‐bis‐perchlorate
Authors: Fabre J.M. , Giral L. , Harms R.H. , Heinze J. , Mortensen J. 
Published: November 2013

AbstractThe title compound C16H16S4(ClO4)2, hereafter called (TMCHDT) (ClO4)2, has been synthesized by a simple two‐step reaction sequence. Its electrochemical behaviour was studied by cyclic volammetry under superdry conditions. Different results were obtained depending on the nature of the solvent. In propionitrile, the voltammograms showed an irreversible two‐electron transfer wave at Ep = +0.050 V vs. Ag/AgCl. In methylenechloride, however, two separate, reversible one‐electron transfer waves were observed at half wave potentials of EI1/2 = −0.107 V and EII1/2 = −0.321 vs. Ag/AgCl, respectively.
Micellar‐Promoted Stereospecific Hydrolysis of Amino Acid Esters Catalyzed by Peptide‐Like Nucleophiles
Authors: Ohkubo Katsutoshi , Hirayama Kazuyuki , Yamaki Kazuhiro 
Published: November 2013

AbstractThe efficiency of peptide‐type nucleophiles (Z‐L‐Leu‐L‐His (2a), Z‐D‐Leu‐L‐His (2b, Z‐L‐His‐L‐Leu (2c), Z‐L‐Ala‐L‐His‐OMe (2d), Z‐L‐Leu‐L‐His‐OMe (2e), Z‐L‐Phe‐L‐His‐OMe (2f), Z‐L‐Leu‐L‐His‐L‐Leu (3a), Z‐D‐Leu‐L‐His‐L‐Leu (3b), and Z‐L‐Leu‐L‐Leu‐L‐His (3c)) in the stereospecific hydrolysis of p‐nitrophenyl N‐acyl‐phenylalanates (4a‐b) was examined at 10–30°C (pH 7.68) in an solution of hexadecyltrimethylammonium bromide (5). The hydrophobic and chiral amino acid group which is adjacent to the nucleophilic L‐histidine part in 2a‐f (or 3a‐c) was found to play an important role in determining the closeness of approach between the reactive nucelophile and substrate. The nucleophiles 2a and 3a gave high reactivity and stereospecificity in the hydrolysis of 4b possessing a long N‐acyl chain, and the maximum enantiomer rate ratio (L/D), 16.1 (25°C) or 26.2 (10°C), was obtained with mixed micelles of 3a and 5. The structural effect of the nucleophiles on the stereospecific esterolysis of 4a‐b is also discussed in detail on the basis of kinetic parameters (Kb/N, km, ΔH‡, and ΔS‡).
X‐Ray Analysis of trans‐9, 10‐Dihydroxy‐9,10‐diphenyl‐9,10‐dihydroanthracene and Its 1:2 Molecular Complex with Methanol, and Structural Comparison with the Related 1:1 Ethanol and 1:1 1,4‐Butanediol Adducts
Authors: Toda Fumio , Tanaka Koichi , Nagamatsu Shinji , Mak Thomas C.W. 
Published: November 2013

Abstracttrans‐9,10‐Dihydroxy‐9,10‐diphenyl‐9,10‐dihydroanthracene (2) and its 1:2 molecular complex (3) with methanol have been subjected to X‐ray analysis. The crystal structure of 2 is built of discrete molecules held by π interactions between neighboring parallel phenyl rings and van der Waals forces, whereas that of 3 features virtually rhombic hydrogen‐bonded (OH)4 rings, each comprising two methanol fragments and two hydroxy groups, linking the molecular moieties into a chain‐and‐layer arrangement. The molecular stereochemistry, hydrogen bonding, and packing modes in these two compounds are compared with those in the 1:1 adducts of 2 with ethanol and 1,4‐butanediol. Crystal data: for 2, space group P1, a = 6.814(1), b = 8.119(1), c = 9.053(1)Å, α = 103.66(1), β = 96.30(1), γ = 106.55(1)°, and Z = 1 : for 3, space group C2/c, a = 12.165(3), b = 12.484(3), c = 16.349(4)Å, β = 110.33(2)° and Z = 4. Based on observed MoKα data, the structures of 2 and 3 have been refined to RF = 0.069 for 1139 reflections and RF = 0.070 for 1412 reflections, respectively.
Anion Exchange Kinetics in Resins of High Selectivity. Part I. Analysis of Theoretical Models
Authors: Helfferich F.G. , Liberti L. , Petruzzelli D. , Passino R. 
Published: November 2013

AbstractThis paper reviews five kinetic models potentially applicable to ion exchange with resins of high selectivity: liquid‐phase mass transfer control with linear driving force; solid‐phase diffusion control; chemical reaction control with SN2 mechanism; chemical reaction control with shrinking core mechanism; and solid‐phase diffusion with irreversible equilibrium reaction. The parameters affecting the rate are analyzed.
Synthesis and Thermal Rearrangement of 4‐Heterohepta‐1,2,5,6‐tetraenes
Authors: Cheng Y.S. Patrick , Garratt Peter J. , Neoh Soon Bin , Rumjanek Victor M. 
Published: November 2013

Abstract4‐Heterohepta‐1,6‐diynes can be rearranged by base catalysis under carefully controlled conditions to the corresponding 4‐heterohepta‐1,2,5,6‐tetraenes. Thermal rearrangment of these tetraenes gives the corresponding dimers by a process in which the rate‐determining step is first order. The rate constants for these reactions have been determined and the relative rates accounted for in terms of an aromatic contribution afforded to the presumed diradical intermediates. In the two cases investigated it was found that when the thermolyses were conducted in the presence of 3O2 then cyclic peroxides were formed.
The SN2 Transition State. 6: Breakdown of the Reactivity–Selectivity Principle in the SN2 Reaction of α‐Halocarbonyl Compounds. A Molecular Orbital Analysis [1]
Authors: Kost Daniel , Aviram Kalman 
Published: November 2013

AbstractA molecular orbital (MO) analysis of the SN2 transition state (TS) of α‐carbonyl derivatives is described with various nucleophile leaving group pairs. On the basis of this analysis it is shown that the enhanced selectivity toward nucleophiles observed in α‐halocarbonyl compounds is directly associated with SN2 rate enhancement. Both observations result from a stabilizing frontier orbital interaction between a high lying π‐type three center orbital along the reaction coordinate axis and the carbonyl π‐system. The magnitude of interaction is greater for more powerful nucleophiles, resulting in both greater reactivity as well as additional selectivity relative to simple alkyl halides.
Using Macromolecular Dynamics Simulations to Interpret Experiments
Authors: Levy Ronald M. 
Published: November 2013

AbstractThe use of molecular dynamics simulations to study the internal motions and structural flexibility of proteins and nucleic acids has received a great deal of attention in recent years. The attraction of the method is, in part, its potential for studying detailed properties of biological macromolecules that are only indirectly accessible to experiment. Equally important is the use of the method to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques are emphasized: (1) nuclear magnetic resonance relaxation spectroscopy; (2) refinement of macromolecular X‐ray structures; and (3) vibrational spectroscopy.
Restrained Molecular Dynamics Procedure for Protein Tertiary Structure Determination from NMR Data: A Lac Repressor Headpiece Structure Based on Information on J‐coupling and from Presence and Absence of NOE's
Authors: De Vlieg J. , Boelens R. , Scheek R. M. , Kaptein R. , van Gunsteren W. F. 
Published: November 2013

AbstractIn recent years a procedure has been developed by which the three‐dimensional (3D) structure of biomolecules can be derived from 2D NMR data. This procedure combines model building with restrained energy minimization (EM) and molecular dynamics (MD) techniques. Distance information from NOE's is incorporated in the form of an upper limit distance restraining term that is added to the interatomic potential function.Here, two improvements of the refinement procedure are introduced. First, the information that is contained in empty parts of 2D‐NOE spectra is transformed into, so‐called, non‐NOE's which are modelled by adding a lower limit distance restraining term to the potential function for each non‐NOE proton pair. Secondly, the information that is contained in the occurrence of large J‐coupling constants for specific dihedrals is modelled by adding a sinusoidal dihedral angle restraining term to the potential function for each dihedral angle with a large J‐value.The improved refinement procedure is tested by application of the lac repressor headpiece. Both the inclusion of non‐NOE data and the inclusion of J‐coupling information markedly improve the result of the restrained MD refinement of headpiece. In the refinement procedure MD simulation is used for searching configuration space. Energy barriers which are too high to be crossed by MD are surmounted by manually changing the model structures on a picture system. The resulting close non‐bonded contacts are relaxed by EM.The final structure of headpiece satisfies essentially all 169 NOE and 9529 non‐NOE distance constraints as well as the 6Cα – Cβ dihedral angle values corresponding to the measured J‐coupling values.
Chemistry of Organic Polyheterocyclic Conductors
Authors: Wudl Fred 
Published: November 2013

AbstractThe synthesis and some properties of polymeric heterocycles as well as the latest results in the area of soluble conducting polymers are reviewed. Polyheterocycles with long chain substituents exhibit unusual properties in solution while retaining a relatively high conductivity in the solid state. All known methods for the preparation of conducting polymers are presented and their merits discussed. Among the insoluble polymers, the electrochemical methods are still the best because they afford films on a large variety of substrates.
Chemical and Physical Properties of Capped and Uncapped Alkylthio Substituted Tetrathiafulvalenes and Their Charge Transfer Complexes
Authors: Saito Gunzi , Kumagai Hiroaki , Katayama Chuji , Tanaka Chizuko , Tanaka Jiro , Wu Peiji , Mori Takehiko , Imaeda Kenichi , Enoki Toshiaki , Inokuchi Hiroo , Higuchi Yoshiki , Yasuoka Noritake 
Published: November 2013

AbstractA series of tetrakis(n‐alkylthio)tetrathiafulvalene (alkyl‐methyl to octadecyl; TTCn‐TTF (n = 1‐18) has been synthesized. Synthesis, structure, and some physical properties of these uncapped analogues of bis(ethylenedithiolo)tetrathiafulvalene (BEDT‐TTF) are described and compared with those of BEDT‐TTF. Several charge transfer complexes of TTC1‐TTF are presented in terms of their electrical conductivities, ionicity, structures, and intermolecular interactions and compared with those of BEDT‐TTF compounds. Molecular conformation, mode of the sulfur‐to‐sulfur atomic contacts, and requirements for an organic metal are discussed from the viewpoint of the molecular design.
[FeII(MeCN)4]2+(ClO4−)2 and [FeIIICl33] as Mimics for the Catalytic Centers of Peroxidase, Catalase and Cvtochrome P‐450
Authors: Sawyer Donald T. , Spencer Lee , Sugimoto Hiroshi 
Published: November 2013

AbstractAddition of [FeII(MeCN)24+(ClO−4)2 to solutions of hydrogen peroxide in dry acetonitrile (MeCN) catalyzes a rapid disproportionation of H2O2 via the initial formation of an adduct, [FeII(HOOH)↔Fe(O)(OH2)]2+, which oxidizes a second H2O2 to dioxygen. This intermediate also cleanly oxidizes substituted hydrazines, alcohols, aldehydes, and thioethers by a two‐electron process. The products for these H2O2 oxidations are consistent with those that result from catalase‐ and some peroxidase‐catalyzed processes. In the same aprotic medium (MeCN) anhydrous FeIIICl3 catalyzes the demethylation of N,N‐dimethylaniline, the epoxidation of olefins, and the oxidative cleavage of 1‐phenyl‐1,2‐ethanediol (and other 1,2‐diols) by hydrogen peroxide. A mechanism is proposed in which an initial Lewis acid‐base interaction of FeIIICl3 with H2O2 generates a highly electrophilic FeIII‐oxene species as the reactive intermediate. For each class of substrate the products closely parallel those that result from their enzymatic oxidation by cytochrome P‐450. Because of (a) the close congruence of products, (b) the catalytic nature of the FeIIICl3/H2O2 reaction mimic, and (c) the similarity of the dipolar aprotic solvent (acetonitrile) to the proteinaceous lipid matrix of the biomembrane, the form of the reactive intermediate may be the same in each case. This is in contrast to the prevailing view that cytochrome P‐450 acts as a redox catalyst to generate an Fe(V)‐oxo species or an Fe(IV)‐oxo cation radical as the reactive intermediate.
Optical Nuclear Spin Polarization (ONP) Studies of Short‐Lived Excited Triplet States in Anthracene Crystals Doped with Phenazine. Structure and Mechanism of Reversible Photochemical H‐Transfer
Authors: Buntkowsky G. , Nack M. , Stehlik D. , Vieth H.‐M. 
Published: November 2013

AbstractIn anthracene crystals doped with phenazine two types of ONP active triplet states are identified: delocalized triplet excitons of anthracene and localized triplet states with small zero‐field splitting and characteristic hyperfine interaction (hfi). All three basic ONP mechanisms can be demonstrated in one system with transfer of the photoinduced electron polarization (OEP) to the nuclear spins by (a) static hfi in the level crossing region (LAC‐ONP), (b) time modulated hfi inducing electron‐nuclear cross relaxation (Overhauser effect) and (c) rf saturation of forbidden transitions (RF‐ONP, solid effect). The localized triplet states are identified as a radical pair product of a H‐transfer photoreaction, in which an H atom is abstracted from an anthracene X‐trap to the neighboring phenazine guest molecule, between two translationally equivalent lattice sites along the crystalline b‐axis. Kinetic rates determined with RF‐ONP techniques indicate a cyclic reaction with slow radical pair product formation and faster, spin selective, triplet product decay. Analogies are discussed with similar H‐transfer reactions in doped fluorene. Whereas anthracene acts as H‐acceptor when doped in fluorene, it becomes H‐donor in the anthracene crystal doped with phenazine.
The Ambident Nucleophilic Center. Stereochemical Consequences of HOMO‐LUMO and HOMO‐HOMO Dominant Processes
Authors: Wolfe Saul , Livneh Mordechai , Cohen Drora , Hoz Shmaryahu 
Published: November 2013

AbstractThe nitrogen centers of pyridine, quinoline, pyrrole anion, carbazole anion, and acetonitrile possess, in each case, two energetically non‐equivalent electron pairs: one, a non‐bonding orbital, lies in the molecular plane or on the bond axis; the other is part of a π‐type orbital, and is perpendicular to the molecular plane or the bond axis. Simple molecular orbital considerations predict that the π‐type orbital will always be the higher lying and, therefore, the preferred donor orbital in a HOMO‐LUMO controlled frontier molecular orbital interaction with the HOMO and LUMO of an electrophilic reagent. This stereoelectronic effect seems to have been observed experimentally in the case of carbazole anion, but not in the cases of quinoline or acetonitrile (present work). The reasons for the different behavior of these nucleophiles have been analyzed using the semi‐empirical procedure AMI. It is pointed out that the lower lying non‐bonding orbital can control the stereochemical course of the reaction when HOMO‐HOMO interactions dominate. This will occur when the non‐equivalent electron pairs of the nucleophile lie close to the HOMO of the electrophilic reagent, as is the case in the reactions of pyridine, quinoline, and acetonitrile with methyl choride or trimethyloxonium cation. Transition structures for N‐methylation of acetonitrile and pyrrole anion have been located and are, respectively, linear and planar. However, a 45° bend of the pyrrole transition structure leads to only a 29‐fold rate retardation at 25°C, compared to the 300‐fold retardation calculated for acetonitrile.
Efficient Regio‐ and Stereocontrolled Synthesis of Highly Functionalized Polycyclic Systems Using the Diels‐Alder Addition of Stannyl‐Substituted Vinylketeneacetals
Authors: Stork Gilbert , Kahne Daniel 
Published: November 2013

AbstractThe vinylketeneacetal obtained by deprotonation‐silylation of methyl 3‐trimethylstannyl‐2‐butenoate undergoes Diels‐Alder addition regiospecifically with 5‐hydroxy‐1,4‐naphthoquinone to lead to organometallic derivatives in which the organic portion is a differentially protected polyketonic hydroanthracene system.