bluuebunny/crossref_metadata_2025_split huggingface dataset:
Multi‐band Adiabatic Inversion Pulses for Use with the 8th‐order Hadamard Spectroscopic Imaging Technique
Authors: Goelman Gadi , Leigh John S. 
Published: November 2013

AbstractRf pulses with frequency responses which correspond to the rows of the 8th‐order Hadamard matrix are developed. These pulses can be used in the B1 insensitive Hadamard Spectroscopic Imaging multi‐volume localization technique. It is shown how any adiabatic rf pulse, which inverts spin population at a well defined frequency band, can be modified to invert populations at several frequency bands simultaneously. Conditions on the frequency and amplitude functions which allow such modifications are defined. For increasing amplitude the interaction between the bands produces nonlinear irradiation at several frequencies. By using an integrated form of the Floquet formalism and perturbation theory, the interaction between the bands is described analytically by an effective Hamiltonian. From this description the conditions which guarantee minimization of the unwanted irradiation are defined. The experimental results agree very well with the theoretical predications.
Multiple Quantum Nuclear Coherences in Pulsed ENDOR Spectroscopy
Authors: Thomann Hans , Bernardo Marcelino 
Published: November 2013

AbstractExperimental and theoretical aspects of multiple quantum (MQ) NMR coherence phenomena for paramagnetically coupled nuclei are presented. The spin interactions responsible for MQ NMR coherences observed in pulsed EPR and pulsed ENDOR (Electron Nuclear Double Resonance) are contrasted with those responsible for MQ coherences in pulsed NMR spectroscopy. MQ NMR coherences in pulsed ENDOR spectroscopy are demonstrated experimentally and described using an operator formalism approach. Detailed discussions of the phase incrementation technique for detecting MQ NMR coherences in ENDOR spectroscopy, different microwave excitation schemes for the preparation of nuclear spin order, off‐resonance excitation, and the evolution of MQ coherences under free precession are presented. Experimental results on a single crystal of irradiated malonic acid and on the conjugated polyene polymer poly acetylene are presented.
Surface Modification of CdS Colloids with Mercaptoethylamine
Authors: Kamat Prashant V. , de Lind van Wijngaarden Michael , Hotchandani Surat 
Published: November 2013

AbstractSurface modification as well as size control of CdS colloids in acetonitrile has been achieved with mercaptoethylamine (MEA). The CdS emission yield increases as it interacts with the surface modifier. A decrease in the hole‐trapping yield was observed in optically excited CdS colloids which were modified with MEA. Use of such surface modifiers in bridging a semiconductor colloid with a gold colloid has also been demonstrated.
Comparative Semiconductor Photophysics of II‐VI (CdS) and I‐VII (AgBr) Quantum Dots
Authors: Johansson Katy , Cowdery Robin , O'Neil Michael , Rehm Julie , Mclendon George , Marchetti Alfred , Whitten David G. 
Published: November 2013

AbstractWhen CdS and related II‐VI direct bandgap materials form semiconductor clusters in which the absorbance properties are dominated by quantum size effects, the emission properties are particularly sensitive to surface defects, which are associated with the unfilled valence of surface Cd2+ ions. These mid‐bandgap states result in large emission Stokes shifts. This shift can be overcome by binding ligand‐like amines to the surface. By contrast, the more ionic I‐VII materials (e.g., AgBr) show free exciton emission rather than surface defect emission. Since AgBr is an indirect bandgap material, the most obvious effect of size restriction is an increase in the free exciton probability which results from a breakdown in the momentum selection rules at small cluster size.
Size‐Quantized Nanocrystalline Semiconductor Films
Authors: Hodes Gary 
Published: November 2013

AbstractThis paper is a review of our work on nanocrystalline semiconductor films which exhibit pronounced size quantization effects in three dimensions, manifested by large blue‐shifts in their optical absorption spectra. The films are prepared by either chemical solution deposition (CdSe and PbSe) or by electrodeposition from nonaqueous electrolytes (CdSe and CdS). Except for PbSe, where the nanocrystals are surrounded by a matrix, the films are comprised of aggregated nanocrystals. Crystal size (typically from <4 nm to >6 nm), and therefore absorption spectra, can be controlled by deposition temperature, illumination during deposition (for chemically deposited films), solution composition, and post annealing. The crystal size dependence of chemically‐deposited films on experimental parameters (temperature, illumination, reactant concentrations) is discussed. CdSe nanocrystals were epitaxially electrodeposited on single‐crystal Au. The nanocrystal distribution (isolated or aggregated) could be controlled by deposition current and temperature. All these films exhibit photoelectrochemical behavior but no corresponding solid state photovoltaic behavior. A model is proposed based on electron/hole separation by kinetic differences in charge injection into an electrolyte rather than by a built‐in space charge layer in the semiconductor. The films can behave as both ‘n’ ‐ or ‘p’‐type, with respect to direction of photocurrent flow, by changes in the semiconductor surface properties and/or the electrolyte, in agreement with this model.
Charge Transfer at High Excitation Power of Solids
Authors: Mikhelashvili M.S. , Kapinus E.I. 
Published: November 2013

AbstractElectron transfer between excited donor molecules followed by the back reaction is treated theoretically for a solid solution at high excitation power. An expression for the average donor cation probability P+(t) is derived. Calculations of the time dependence for P+(t) are presented. The results indicate that the maximum value of P+(t) in the nonlinear quenching process is less than the corresponding values of P+(t) for the linear quenching at a low excitation power due to the depletion of the acceptors with time t. The restriction of the reaction volume slows down the charge separation if the back charge transfer takes place.
Analytic Second Derivatives of the Potential Energy Surface
Authors: Handy Nicholas C. , Tozer David J. , Laming Gregory J. , Murray Christopher W. , Amos Roger D. 
Published: November 2013

AbstractOne of J. A. Pople's leading contributions in ab initio quantum chemistry was his 1979 paper in which he and his colleagues presented calculations for the analytic second derivatives of the Self Consistent Field Potential Energy surface. This led to the automatic characterization of stationary points as minima or transition states. Recently there has been an upsurge of interest in computational density functional theory (DFT). Here we describe our implementation of analytic gradients and second derivatives for the Kohn‐Sham potential energy surface. This parallels similar developments by Pople and his coworkers. We examine Pople's idea of differentiating the weights in the quadrature scheme to ensure that the energy gradient is exactly zero at the energy minimum. We present some calculations on small molecules which are as near ‘exact’ as possible for the LDA and BLYP functional which we use. In other words we use large basis sets and a large number of quadrature points to evaluate the extra (nonanalytical) integrals. We do not use any fitting procedures and no other approximations are introduced.
Scaling Factors for Obtaining Fundamental Vibrational Frequencies and Zero‐Point Energies from HF/6–31G* and MP2/6–31G* Harmonic Frequencies
Authors: Pople John A. , Scott Anthony P. , Wong Ming Wah , Radom Leo 
Published: November 2013

AbstractNew scaling factors have been determined for obtaining fundamental vibrational frequencies and zero‐point vibrational energies from harmonic frequencies calculated at the HF/6–31G* and MP2/6–31G* levels. The scaling factors for the fundamental frequencies have been derived from a comparison of a total of 1066 calculated frequencies for 122 molecules with corresponding experimental values, while the zero‐point energy scaling factors were determined from a comparison of the computed values with the experimental zero‐point energies for a set of 24 molecules. The scaling factors recommended are, respectively, 0.8929 and 0.9427 for HF/6–31G* and MP2/6–31G* fundamental frequencies, and 0.9135 and 0.9646 for HF/6–31G* and MP2/6–31G* zero‐point energies. RMS errors were determined to be around 50 cm−1 for the HF and MP2 fundamental frequencies, and around 0.4 kJ mol−1 for the HF and MP2 zero‐point energies.
Combining Synchronous Transit and Quasi‐Newton Methods to Find Transition States
Authors: Peng Chunyang , Bernhard Schlegel H. 
Published: November 2013

AbstractA linear synchronous transit or quadratic synchronous transit approach is used to get closer to the quadratic region of the transition state and then quasi‐newton or eigenvector following methods are used to complete the optimization. With an empirical estimate of the hessian, these methods converge efficiently for a variety of transition states from a range of starting structures.
Reaction of Sr with CH2Cl2, CHCl3, and CCl4; Laser‐Induced Fluorescence Studies on Relaxed and Unrelaxed SrCl
Authors: Keuzer Freek , Bulthuis Jaap , Stolte Steven 
Published: November 2013

AbstractA combined spectroscopic and chemical study is performed on thermal and nonthermal SrCl. The Δv = 0 sequence of the B2Σ ← X2Σ electronic transition is used for absorption spectroscopy up to high vibrational quantum numbers v with the technique of laser‐induced fluorescence. Effective spectroscopic constants are derived that are valid up to high v. SrCl is investigated both as a product of oven reactions and of the reaction of Sr beam atoms with the chlorinated methane molecules in the observation region (henceforth referred to as the gas‐phase reaction), leading to thermal and nonthermal spectra, respectively. One of the possible oven reactions, the metal‐salt reaction Sr + SrCl2 → 2SrCl, yields a thermal spectrum with a vibrational temperature equal to the oven temperature. The reaction Sr + CH2Cl2 → SrCl + CH2Cl apparently has a small cross section for the gas phase, since only a thermal spectrum is observed. The reaction Sr + CHCl3 → SrCl + CHCl2 yields a combined thermal and nonthermal spectrum. The gas‐phase reaction leads to SrCl product molecules with a vibrational population distribution peaked around v = 17. The reaction Sr + CCl4 → SrCl + CCl3 has a large cross section for the gas phase with a distribution peaked around v = 27. In this reaction, nearly all available energy is disposed into internal energy of the SrCl product.
Raman Studies of Hydrogen Bonding in Polyamides
Authors: Triggs Nancy E. , Valentine James J. 
Published: November 2013

AbstractWe report Raman and pre‐resonant Raman spectra of the polyamides N,N‐ tetramethylene bisbutamide, N,N‐hexamethylene bishexamide, and 7,14‐diazacyclotetradecane‐1,6‐dione, as well as of a low molecular weight prepolymer of nylon 6,6 aimed at characterizing the nature of hydrogen bonding in these compounds. The relative intensities of the C=O stretch and C‐N stretch bands, the frequency width of the C=O band, the frequency of the N‐H stretch band, and the frequency width of the N‐H band indicate that these compounds do not show the Raman signatures of hydrogen bonding that we have established in our previous studies of many model amides. In this regard, these polyamides behave like the lactams 2‐azacyclononanone and 2‐azacyclotridecanone, for which all the Raman spectral indicators of hydrogen bonding are similarly negative. These results indicate that whatever the nature of the intermolecular interactions in these polyamides, they do not result in the changes in vibrational and electronic structure that we believe are associated with hydrogen bonding.
Derivation of Class II Force Fields. III. Characterization of a Quantum Force Field for Alkanes
Authors: Maple J.R. , Hwang M.‐J. , Stockfisch T.P. , Hagler A.T. 
Published: November 2013

AbstractRecently, a quantum mechanical Class II force field (QMFF) was derived from a fit of HF/6–31G* ab initio energy and energy derivative data for alkanes, and a comparison of this quantum force field and the ab initio energy and energy derivatives was presented. In this work, the quantum force field is further evaluated with regard to its accuracy, and, more importantly, transferability. A detailed comparison between structures, frequencies, and energies calculated from quantum mechanics and from the classical analytical form is given for a set of molecules selected from both those used in the original training set and molecules selected from outside the training set. None of these properties were used directly in the original derivation of the force field. In order to assess the importance of anharmonic and coupling interactions that occur in and contribute to molecular energy surfaces, the results are compared to a diagonal quadratic force field. It is demonstrated that the QMFF functional form is capable of calculating the ab initio bond lengths, bond angles, torsion angles, and conformational energy differences to an rms accuracy of 0.003 Å, 0.4°, 1.2°, and 1.0 kcal/mol, respectively. This compares quite well to corresponding deviations of 0.006 Å, 0.8 Å, 2.3°, and 3.3 kcal/mol for a harmonic diagonal force field. Excluding three‐ or four‐membered rings, the QMFF rms frequency deviations were 24 cm−1, which again is much better than the ˜100 cm−1 deviations for the harmonic diagonal force field. Larger average rms frequency deviations of 36 and 71 cm−1 were found with QMFF for molecules with three‐ and four‐membered rings. An in‐depth analysis of C‐H and C‐C bond length, H‐C‐H, H‐C‐C, and C‐C‐C bond angle, and C‐C‐C‐C torsion angle deviations is also presented, along with a similar characterization of frequency deviations in C‐H stretching, C‐C stretching, C‐C‐C bending, and torsion modes. It is concluded from these results that the use of quantum energy surfaces allows us to derive a (Class II) functional form which is not only more accurate, but also more transferable than previous generation force fields.
Dynamics of tRNA: Experimental Neutron Spectra Compared with a Normal Mode Analysis
Authors: Nardi F. , Doster W. , Tidor B. , Karplus M. , Cusack S. , Smith J.C. 
Published: November 2013

AbstractA comparison is made of experimental inelastic neutron scattering spectra from tRNA with spectra calculated from a normal mode analysis of the molecule. The experimental data indicate that a dynamic transition occurs with temperature, as is seen in proteins. At low temperatures a broad peak is seen in the dynamic structure factor, due to the lowest frequency collective modes. This peak is centered at ˜40 cm−1, somewhat higher in frequency than that observed in small globular proteins. The vibrational frequency distribution calculated from the normal mode analysis rises to a broad maximum at 50 cm−1, in general accord with the experiment. However, the lowest frequency vibrations in the harmonic model (<40 cm−1) are not present in the experimental sample. Possible reasons for this are discussed.
Effect of Solvent Quality on Pure and Mixed Brushes
Authors: Chen C. , Dan N. , Dhoot S. , Tirrell M. , Mays J. , Watanabe H. 
Published: November 2013

AbstractWe investigate, theoretically and experimentally, the effect of solvent quality on pure, one‐component, and two‐component brushes. Both mean‐field and self‐consistent field models are developed, and their predictions compared to surface forces experiments on polystyrene and mixed polystyrene–polybutadiene brushes. We find that in pure brushes the structure and interactions between surfaces are highly sensitive to solvent quality. In a good solvent, the brush is extended and the interactions are repulsive, while in a poor solvent the brush is “collapsed” and the interactions are strongly attractive. Mixing two incompatible chains significantly reduces the sensitivity of the brush to solvent properties. While one of the chains collapses in a selective solvent, the other chain remains solvated, thus providing long‐range repulsive interactions between surfaces. Our experiments show that the interactions between surfaces carrying a 1:1 mixed brush of styrene–butadiene are repulsive even in hexane, a poor solvent for polystyrene. Theoretically, we calculate that incorporating only 10% of an equi‐molecular‐weight chain into a collapsed brush can eliminate attractive interactions between surfaces.
Structure and Shear Response in Nanometer‐Thick Films
Authors: Thompson Peter A. , Robbins Mark O. , Grest Gary S. 
Published: November 2013

AbstractSimulations of the structure and dynamics of fluid films confined to a thickness of a few molecular diameters are described. Confining walls introduce layering and in‐plane order in the adjacent fluid. The latter is essential to transfer of shear stress. As the film thickness is decreased, by increasing pressure or decreasing the number of molecular layers, the entire film may undergo a phase transition. Spherical molecules tend to crystallize, while short‐chain molecules enter a glassy state with strong local orientational and translational order. These phase transitions lead to dramatic changes in the response of the film to imposed shear velocities v. Spherical molecules show an abrupt transition from Newtonian response to a yield stress as they crystallize. Chain molecules exhibit a continuously growing regime of non‐Newtonian behavior where the shear viscosity drops as v−2/3 at constant normal load. The same power law is found for a wide range of parameters, and extends to lower and lower velocities as a glass transition is approached. Once in the glassy state, chain molecules exhibit a finite yield stress. Shear may occur either within the film or at the film/wall interface. Interfacial shear dominates when films become glassy and when the film viscosity is increased by increasing the chain length.
Metals in Ancient Israel: Archaeological Interperetation of Chemical Analysis
Authors: Shalev Sariel 
Published: November 2013

AbstractOver a thousand metal finds are now known from Chalcolithic and Early Bronze habitation sites, burial caves, and hoards in Israel and Jordan. To answer the question as to how these artifacts were made, more than 200 objects were sampled for metallographic analysis and for chemical analysis by electron‐probe X‐ray microanalysis and by atomic absorption spectrophotometry.In the Chalcolithic period (4th millennium BC), within a confined zone of southern Israel and Jordan, three entirely different groups of metal objects were found together. Various materials (Cu‐As‐Sb; Au; Cu) from totally separate sources were used in diverse production methods to produce specific classes of objects, of a specific shape and color, totally different from one another.In contrast, the components of Early Bronze Age (3rd millennium BC) metal production indicate a fundamental change in the structure and level of craft specialization. This change may be recognized mainly by the unity of the repertory of objects and the use of the same source‐metal (Ag; Au; Cu) for a wider range of products, as well as by the total technological and geographical separation between extraction and production.The transformation from proto‐urban to urban society documents an important stage of social, economic, and political development in Early Bronze Age Israel and Jordan. Many explanations have been offered for this transformation, most of them based on external intervention or stimulus. This paper adds to the existing arguments for the beginning of urbanization, the missing local socioeconomic factor, based on changes in social complexity as reflected in the first two thousand years of metallurgy.
Application of Artificial Pigments to Structure Determination and Study of Photoinduced Transformations of Retinal Proteins
Authors: Nakanishi Koji , Crouch Rosalie 
Published: November 2013

AbstractThe protonated Schiff bases of all‐trans‐retinal and its double bond isomers, 11‐cis‐ and 13‐cis‐retinals, comprise the chromophores of bacteriorhodopsin, sensory rhodopsin, rhodopsin, and the Chlamydomonas photoreceptor pigment. Absorption of photons by these chromophores is directly responsible for the functioning of the various photopigments. Clarification of their extremely complex structures and mechanisms requires multidisciplinary collaboration. The study of synthetic retinal analogs and pigments reconstituted from analogs provides a powerful and indispensable tool for such clarification. This article summarizes the application of retinal analogs in the bioorganic, biophysical, and biochemical investigations of the various retinal pigments.
Application of NMR‐Spectroscopy to Retinal Proteins
Authors: Engelhard Martin , Bechinger Burkhard 
Published: November 2013

AbstractThe application of NMR spectroscopy to the retinal proteins bacteriorhodopsin (BR) and rhodopsin is reviewed. 2H‐, 15N‐, and 13C‐solid‐state NMR spectroscopy have contributed considerably to understanding the conformation and chemical environment of the protonated retinylidene Schiff base in BR as well as in rhodospin. The data from both pigments clarified the mechanism of the opsin shift which is quite different for rhodopsin and BR. An analysis of the chemical shifts of isotopically labeled aspartic acid, tyrosine, and proline incorporated into BR‐provided evidence for the protonation state of Asp and Tyr, and the isomerization state of the Xaa—Pro peptide bond, respectively. Solid‐state NMR spectroscopy was also applied to the investigation of the photocycle intermediates of BR, as well as bathorhodopsin and metarhodopsin II, which are formed after light‐activation of rhodopsin. Solution NMR spectroscopy of BR solubilized in detergents or organic solvents, as well as of opsin‐derived peptide segments, was also applied to the investigation of the two‐ and three‐dimensional structure of BR.