bluuebunny/crossref_metadata_2025_split huggingface dataset:
Studies of Liquid Crystalline Solutions Using NMR Spectroscopy
Authors: Emsley J.W. 
Published: November 2013

AbstractThe form of the NMR spectra obtained from liquid crystalline samples is discussed. It is shown how orientational order parameters can be derived from quadrupolar splittings and dipolar couplings, and their use in investigating possible forms for potentials of mean torque is illustrated for rigid solute molecules. For non‐rigid molecules, the observed partially‐averaged quadrupolar and dipolar couplings depend upon the nature of the internal rotations about bonds, as well as on the orientational order; it is shown how both of these factors can be investigated by NMR for both mesogenic molecules and for non‐rigid solutes. Finally, it is shown how deuterium NMR is being used to study field‐induced orientational order in the isotropic phase of mesogenic molecules.
Electron Spin Echo Modulation Studies of Dipolar Hyperfine Interactions of Catalytic Reaction Intermediates Involving Transition Metal Ions on Oxide Surfaces
Authors: Kevan Larry 
Published: November 2013

AbstractThe application of electron spin echo modulation (ESEM) pulsed electron spin resonance methods to detect dipolar hyperfine interactions of catalytic reaction intermediates involving transition metal ions on oxide surfaces is described. This provides a method to determine critical aspects of the geometrical structure of such intermediates. One example involves catalysis of ethylene dimerization by paramagnetic Ni+ on silica. ESEM results show direct coordination of two and three ethylene molecules to Ni+ as reaction intermediates in ethylene dimerization, depending on the activation temperature of the catalyst pretreatment. A second example involves catalysis of ethylene dimerization by paramagnetic Pd+ in X—zeolite. ESEM results show direct coordination of successively one and two molecules of ethylene to Pd+ prior to dimerization. Thus ESEM methods provide a powerful tool to develop a molecular picture of the course of catalytic reactions on surfaces.
EPR Study of Cation Radicals of Short Chain Polyenes
Authors: Prasad L. Sreeramachandra , Ding Ruisong , Bradford Eric G. , Kispert Lowell D. , Wang Hanqing 
Published: November 2013

AbstractShort chain polyene cation diene and triene radicals generated by X‐ray irradiation of different polyenes diluted in a CFCl3 matrix have been detected by EPR and the magnitudes of the g and isotropic hyperfine coupling constants have been obtained. The structure of the resulting cation radicals were deduced by comparing the experimental coupling constants to those derived from an INDO calculation. An estimate was obtained of the unpaired spin density as a function of chain length, suggesting that resolved hyperfine structure in the EPR spectra of long chain carotenoid cation radicals should not be observed.
1H and 2H ENDOR Investigations of Flavin Radicals Bound to Riboflavin Binding Protein from Egg White
Authors: Bretz Norbert H. , Henzel Norbert , Kurreck Harry , Müller Franz 
Published: November 2013

AbstractRiboflavin‐binding apoprotein isolated from egg white has been used as a protein matrix for a variety of substituted and/or deuterated flavins. The neutral radical states of the reconstituted flavoproteins were studied by ENDOR (electron nuclear double resonance) spectroscopy in disordered solids. For the first time all major hyperfine coupling constants of a flavin cofactor embedded within a protein could be measured and unambiguously assigned to molecular positions. Some conclusions about the geometrical arrangement of the flavin within the protein pocket and about molecular motions could be drawn.
Medium‐Ring Compounds 30. cis‐1,6‐Cyclodecanediol: Molecular Compounds with the trans Isomer and with Itself
Authors: Ermer Otto , Vincent Beverly R. , Dunitz Jack D. 
Published: November 2013

AbstractThe crystal structures of cis‐1,6‐cyclodecanediol (1) and of the molecular compound formed between cis and trans‐1,6‐cyclodecanediol in the ratio 2:1 (2) have been established by X‐ray diffraction. The two substances are closely isostructural. (Crystal data: both monoclinic, P21/c, Z = 6; (1) a = 6.665(6), b = 18.074(3), c = 14.351(8) Å, β = 119.77(6)°, V = 1500(2) Å3; (2) a = 6.679(8), b = 18.074(4), c = 14.314(10) Å, β = 119.64(8)°, V = 1502(3) Å3). The asymmetric unit of the molecular compound contains one cis molecule in a general position and one trans molecule at a center of symmetry. In the crystal of the pure cis diol a set of disordered cis molecules mimics the corresponding centrosymmetric trans molecules in the molecular compound. The molecules have the stable BCB cyclodecane ring conformation, except for the disordered cis molecules, which have a conformation about 2 kJ mol−1 higher in energy (molecular mechanics calculations). The intricate three‐dimensional hydrogen‐bond network found in both crystals is presumably especially favorable from an energetic viewpoint.
Electronic Interactions of Cyclopropyl Groups: Synthesis and Photoelectron Spectra of Model Compounds Derived from Triasterane, Nortricyclane and Norbornane
Authors: De Meijere Armin , Michelsen Klaus , Gleiter Rolf , Spanget‐Larsen Jens 
Published: November 2013

AbstractThe syntheses of the new hydrocarbons trimethylenetriasterane12, trispirocyclopropanetriasterane3, trispirocyclopropanenortricyclane4, and of the trispirocyclopropanenorbornanes5, 6, and7are reported.12contains six vinylcyclopropane units rigidly held in an antiperiplanar (ap) conformation in aD3hsymmetric framework.3and4are the first model compounds with bicyclopropyl units rigidly fixed in ans‐trans(ap) conformation (Θ = 180°). Their PE spectra reveal that the interaction parameter βww= −1.73 eV for this arrangement is the same as that for Θ = 0°.
Can Electron Transmission from Aza‐Crown Ethers Contribute to Diels‐Alder π‐Facial Selectivity?
Authors: Gugelchuk Melinda , Paquette Leo A. 
Published: November 2013

AbstractA pair of isodicyclopentafulvenes that carries an exocyclic phenyl group para‐substituted with an aza‐15‐crown‐5 or aza‐18‐crown‐6 ring has been prepared. These molecules (5a and 5b) and their respective NaClO4 (11) and KSCN (12) complexes were examined for the extent of π‐facial stereoselectivity operational during their Diels‐Alder cycloaddition to (Z)‐1,2‐bis(phenylsulfonyl)ethylene (2) under conditions of high pressure. Whereas 5a and 5b engage in [4 + 2] condensation with a 2.5–3.1 preference for dienophile capture from the above‐plane direction, their complexes show a substantially reduced tendency for doing so (14/13 = 1.2–1.4). However, this response goes contrary to those para‐substituent effects previously observed. The linear correlation noted earlier conformed to an increased preference for above‐plane cycloaddition to 2 as the para‐substituent became progressively more electron‐withdrawing. Probable reasons for this seemingly divergent behavior are presented.
Azo Bridges from Azine 9. [2 + 2 + 1] Cycloadditions of Parallel C=C and N=N Bonds with Dihalocarbenes
Authors: Hünig Siegfried , Schmitt Michael 
Published: November 2013

AbstractSchemes 1 and 2 are examples of type A compounds in which the groups ‐N=N‐ and ‐CH=CH‐ are held in a close parallel position. They react with dichloro‐ and fluorochlorocarbene to form a unique pyrazolidine moiety (3–5) in a formal [2 + 2 + 1] cycloaddition which, despite the substitution pattern, is stable to ionization and hydrolysis according to Bredt's rule. The reaction is considered to start with the addition of the carbenes to the azo group. The azomethine imine thus formed subsequently undergoes a rapid [3 + 2] dipolar cycloaddition with the bridging C=C bond. The different isomers from 3–5 are identified from their NMR spectra.
Benzyl Ethers as Chromophores in the Exciton Chirality Method for Absolute Configurational Studies of Compounds with Hindered Hydroxyls
Authors: Takeda Reiji , Park Myung Hwan , Nakanishi Koji 
Published: November 2013

Abstractp‐Phenylbenzyl ethers and, in general, other benzyl ethers, can be substituted for benzoates in the exciton chirality method for determining absolute configurations of polyols. This is useful when dealing with hydroxyl groups that resist acylation. If necessary, the phenylbenzylates can be oxidized to the corresponding phenylbenzoates with concomitant enhancement in sensitivity.
Hyperthermal Surface Ionization
Authors: Danon Albert , Amirav Aviv 
Published: November 2013

AbstractSurface ionization of molecules with hyperthermal kinetic energy (1–20 eV) was found to be very efficient, demonstrating several unique features. We have used the technique of aerodynamic acceleration in supersonic seeded beams in order to obtain molecular kinetic energies in the range 1–20 eV. Three types of hyperthermal surface ionization (HSI) processes were observed: (a) surface‐molecule electron transfer was demonstrated in the I2/diamond system where negative molecular iodine (I−2) ions were produced; (b) molecule‐surface electron transfer was found for the anthracene molecule where positive molecular anthracene ions were generated; (c) hyperthermal surface‐induced dissociative ionization (HSIDI) was observed in 1‐iodopropane, 1‐chloro‐3‐iodopropane, benzyl bromide, and many other molecules. In these processes, we have observed a large current of negative halogen ions and positive molecular residue ions (ion‐pair formation). The mechanism of HSI is described in terms of electron transfer processes. It occurs due to a curve crossing between the neutral scattering interaction potential surface and the ionic interaction potential surface and nonunity reneutralization second curve crossing of the scattered molecules or fragment ions.
Excitation Transfer in Porous Silicas — A Fractal Approach
Authors: Pines Dina , Huppert Dan 
Published: November 2013

AbstractIn this paper, we present both experimental data and a theoretical study of the excited state energy transfer in irregular systems. Picosecond time‐resolved luminescence techniques were employed to resolve the fluorescence curves of both donor and acceptor molecules. By analyzing the kinetics of direct electronic energy transfer, the effective dimensions describing the distributions of adsorbed dye molecules on the surfaces of silica gels and controlled‐pore glasses were obtained. The same dimensions for silica gels were obtained in fluorescence depolarization experiments in one‐component (only donors) systems and in migration‐accelerated trapping (indirect energy transfer) measurements.
The Isolated Binary Collision Picture of Vibrational Energy Transfer Processes in Condensed Phases
Authors: Krueger Herman , Weitz Eric 
Published: November 2013

AbstractA selective review of experimental studies of vibrational energy transfer in cryogenic solutions is presented, with a focus on work carried out within the last five years. The applicability of isolated binary collision (IBC) models to a number of experimentally studied systems is considered. Also discussed are possible circumstances under which current IBC treatments may break down and possible experimental studies that might test the range of applicability of this model.
Two‐Color Charge Transfer Transitions as a Probe of Electronic Relaxation and Photodissociation Dynamics at High Densities: Molecular Halogens in Xenon
Authors: Lawrence William G. , Apkarian V. Ara 
Published: November 2013

AbstractTwo‐color charge transfer transitions are developed as a probe of intermolecular potentials, electronic relaxation, and photodissociation dynamics of halogens in condensed rare gases. Two illustrative examples are given: electronic relaxation of Br2 in liquid Xe and recombination dynamics of Cl atoms in high pressures of Xe. The Br2 A(3π1+u) and A'(3π2+u) states, which are in fast equilibrium, are not quenched by the solvent in liquid Xe. In dilute solutions, they relax radiatively via the A' state with a lifetime of ∼ 100 μs. In concentrated solutions, diffusion‐controlled self‐quenching of Br2(A/A') proceeds with a probability of 0.07 per encounter. An encounter is found to be equivalent to four gas kinetic collisions. The photodissociation of Cl2 and recombination of Cl atoms in gaseous Xe, 1–55 atm, are studied. The recombination kinetics is well reproduced by incorporation of diffusion‐controlled encounter frequencies in the rate constants. An equilibrium constant for formation of XeCl(X) of 2.3 × 10−22 cm3 is derived from the kinetic analysis.
On the Mechanism of Palladium(0)‐Catalyzed Reactions of Allylic Substrates with Nucleophiles. Origin of the Loss of Stereospecificity
Authors: Bäckvall Jan‐E. , Granberg Kenneth L. , Heumann Andreas 
Published: November 2013

AbstractThe mechanism of the loss of stereospecificity in palladium‐catalyzed nucleophilic substitution of allylic substrates has been investigated. Eight substrates (cis and trans isomers of 1a‐d) and two nucleophiles (Et2NH and NaCH(SO2Ph)2) were studied. In the animation reactions two pathways are responsible for the formation of anomalous inversion product, viz., isomerization of the starting material (path B, Scheme 2) and isomerization of the π‐allyl intermediate via displacement of palladium by Pd(0) (path C, Scheme 2), the latter of which predominates. In the alkylation the results indicate that loss of stereospecificity is caused only by path C. The use of a more reactive substrate increased the stereospecificity of the reaction and suppressed the isomerization pathway. An analysis of the kinetics is consistent with the hypothesis that path C is the major pathway for the stereochemical loss.
Structure and Dynamics in the Interfacial Zone Between Partly Compatible Polymers
Authors: Klein Jacob 
Published: November 2013

AbstractThe interface between two partly miscible polymers is characterized by a finite interfacial region of width w comparable to the polymer size. The composition variation and in particular the time development of this restricted zone have recently been studied experimentally using a new depth‐profiling approach based on nuclear reaction analysis. The results show w to vary quantitatively in a way consistent with mean‐field models, while its time development appears to follow a power law w ∝ tα, with the mean value of the exponent α = 0.34 ± 0.06 being significantly lower than its free interdiffusion value of 0.5.
Synthesis of Cembranolides by SE′ Cyclization of Enantioenriched α‐Alkoxy Allylstannanes
Authors: Marshall James A. , Gung Benjamin W. 
Published: November 2013

AbstractA total synthesis of the unnamed cembranolide 35 is described starting from enal XV, an intermediate derived from geraniol. Key transformations include asymmetric reduction of stannyl ketone 1 with (R)‐(+)‐BINAL‐H to give the hydroxy stannane 2 of high ee, and cyclization of the derived MOM ether 6 to the cis‐(Z) hydroxy enol ether 7 in high yield. Further elaboration of 7 to the natural product was achieved by OH‐directed hydroalanation‐iodination of the derived diol 17, methylation of iodide 17 with methyl cuprate and selective protection, oxidation, lactonization, and α‐methylenation. The resulting product was identified by comparison of 1H NMR, 13C NMR, IR, and optical rotation with an authentic sample. The synthesis defines the absolute configuration of the natural cembranolide.
A Discriminating Protecting Group Strategy for Alcohols and Amines
Authors: Liotta Louis J. , Ganem Bruce 
Published: November 2013

AbstractThe protection of alcohols and amines as O‐ and N‐benzyl derivatives has long played a central role in organic synthesis. For cases where the conventional method of benzylation under strongly basic conditions is not applicable, we now describe an alternate method using phenyldiazomethane that proceeds under near‐neutral conditions with HBF4 as a catalyst. O‐Benzylation occurs at −40 °C and works well on primary, secondary, and tertiary alcohols. The technique is compatible with ester, acetal, and reactive halide functionalities. Under the same conditions PhCHN2 also N‐benzylates primary and secondary amines, albeit more slowly. However S‐benzylation is only marginally successful. As a consequence aminoalcohols like 2‐hydroxymethylpiperidine can be O‐benzylated cleanly and in good yield. Thus HBF4‐mediated benzylations using PhCHN2 achieve a kinetic selectivity not customarily possible in the standard Williamson ether synthesis. Preliminary experiments with other, substituted phenyldiazomethanes indicate the method can be extended to the production of substituted benzyl ethers and amines.
MyristoylCoA:protein N‐Myristoyltransferase: Probing Host‐Guest Interactions Using Synthetic Substrates
Authors: Gokel George W. , Lu Tianbao , Rudnick David A. , Jackson‐Machelski Emily , Gordon Jeffrey I. 
Published: November 2013

AbstractMyristoylCoA:protein N‐myristoyl transferase (NMT) catalyzes the cotranslational covalent attachment of myristate (C14:0) to the ammo‐terminal glycine residues of a variety of biologically important proteins. An extensive survey of myristic acid analogs has previously allowed us to infer the conformational constraints imposed upon the fatty acid when bound to the enzyme's acylCoA binding site. A series of myristic acid analogs has now been synthesized and studied in an in vitro assay to probe further the conformation of NMT's bound ligand and to assess the importance of the distance between the carboxyl and the proposed bend in the fatty acid.
Topological Editing of Cross‐Relaxation Networks
Authors: Macura Slobodan , Fejzo Jasna , Hoogstraten Charles G. , Westler William M. , Markley John L. 
Published: November 2013

AbstractWe have used the elements of graph theory to describe NMR cross‐relaxation networks in macromolecules and to analyze different experiments used for topological editing. We propose a new experiment, block decoupled NOESY (B.D. NOESY), that splits the cross‐relaxation network of a macromolecular system into two noninteracting subdomains. Splitting is achieved by arranging for the effective z‐components of the magnetization in one subdomain, as viewed from the other subdomain, to be zero. The B.D. NOESY experiment, in favorable cases, can usefully simplify the analysis of cross‐relaxation spectra by removing certain ambiguities. We demonstrate the method by separating the aromatic resonances from the rest of the cross‐relaxation network of a small globular protein: turkey ovomucoid third domain, Mr 6000. The resulting spectrum provides a clearer picture of cross‐relaxation pathways that involve only aromatic or only aliphatic spins. By comparing the B.D. NOESY spectrum with the normal NOESY spectrum, we were able to identify cross peaks that contain contributions from indirect magnetization transfer (spin diffusion) mediated by the aromatic side chains. In the terminology of graph theory, these experiments decompose topological networks of cross‐relaxation into two subgraphs, the join of which generates the original graph.
Orientation Dependence of Electron Spin Phase Memory Relaxation Times in Copper(II) and Vanadyl Complexes in Frozen Solution
Authors: Du Jing‐Long , More Kundalika M. , Eaton Sandra S. , Eaton Gareth R. 
Published: November 2013

AbstractThe orientation dependence of the electron spin phase memory relaxation time (T) was measured for vanadyl 5‐(4‐carboxyphenyl)‐10, 15, 20‐tritolylporphyrin at 22, 50, and 100 K, copper(II) bis(diethyldithiocarbamate) at 50 and 100 K, and copper(II) 5, 10, 15, 20‐tetratolylporphyrin at 50, 70, and 100 K in frozen solution. Tm was determined by fitting a single exponential to two‐pulse electron spin echo data. The values of Tm were strongly dependent on the orientation of the molecule in the magnetic field. Longer values were obtained when the magnetic field was along a principal axis or along a non‐canonical turning point in the spectrum. Shorter values of Tm were observed at intermediate orientations. The orientation dependence of Tm is attributed to molecular motion. The EPR spectra for the three systems examined are approximately axial, so the relevant motion is motion of the molecular z axis with respect to the external magnetic field. Longer values of Tm (slower relaxation) occur for orientations at which the resonant condition is less sensitive to a change in orientation of the molecular z axis. Shorter values of Tm (faster relaxation) occur at orientations for which the resonant condition is more sensitive to a change in orientation of the molecular z axis.
GaAs Quantum Dots
Authors: Nozik A.J. , Uchida H. , Kamat P.V. , Curtis C. 
Published: November 2013

AbstractGaAs quantum dots have been prepared by reacting gallium acetylacetonate with trimethylsilylarsine in triethylene glycol ether at 216 K. The colloidal dispersion of GaAs nanocrystals has been characterized by transmission electron microscopy, electron diffraction, ultrafiltration, optical absorption and photoluminescence spectroscopy, and transient pump‐probe spectroscopy; the spectral data were also fitted to a model that accounts for the size‐dependent optical properties of a Gaussian distribution of particle sizes. Our results indicate that the spectral region above 470 nm can be assigned solely to GaAs nanocrystals having a mean diameter of 40.0 Å with a standard deviation of 9.7 Å. The origin of spectral features below 470 nm has not yet been determined conclusively.
Femtosecond Chemical Dynamics in Solution: Photodissociation of I3‐
Authors: Banin Uri , Kosloff Ronnie , Ruhman Sanford 
Published: November 2013

AbstractWe report a study of the UV photolysis of I3− in alcohol solutions, using femtosecond time‐resolved transient transmission experiments. We interpret our results to indicate that photoexcitation leads to direct formation, within 300 fs, of diiodide ions which are vibrating coherently. The time scales for vibrational dephasing, vibrational relaxation, and reorientation of the fragment ions are 400 fs, 4 ps, and 5 ps, respectively. UV transient transmission experiments were conducted in order to characterize the time scales for recombination. Recombination takes place on several time scales. A fast component is assigned to primary recombination, followed by vibrational relaxation on the ground state of I3−. The prominence of this recombination route is found to be strongly dependent on the solvent. Finally, time domain quantum simulation techniques are employed in order to gain insight into the nature of the nascent diiodide vibrational distribution, and to introduce a semiquantitative model for the stage of bond fission for isolated ions.
Femtosecond‐Picosecond Laser Photolysis Studies on Proton Transfer Process of Excited 1‐Pyrenol‐Triethylamine Hydrogen Bonding Complex in Solutions
Authors: Miyasaka Hiroshi , Wada Keisuke , Ojima Seishi , Mataga Noboru 
Published: November 2013

AbstractThe proton transfer (PT) process in the excited hydrogen bonding (HB) complex of 1‐pyrenol and triethylamine, leading to the formation of a hydrogen‐bonded ion pair (HBIP*) of excited pyrenolate anion and triethylammonium cation, PyO−* ··· H‐N+(C2H5)3, and subsequent reaction processes of HBIP* in various solvents were investigated by picosecond and femtosecond laser photolysis, as well as by steady‐state absorption and fluorescence measurements. Time constants for the PT process taking place in the time region of a few picoseconds were almost independent of the polarity of the solvent and it was suggested that the intermolecular slow motion in the HB complex, such as the intermolecular vibration and/or the torsional motion of the two moieties, might regulate the PT process. Moreover, the ionic dissociation process following the photoinduced PT process in polar solvent was directly measured. The dynamic behaviors of the excited intermolecular HB complex and underlying mechanisms are discussed by comparing these results with those on the intramolecular photoinduced PT process and with results of investigations of electron transfer systems.