bluuebunny/crossref_metadata_2025_split huggingface dataset:
Radiationless Transitions in CdSe Quantum Crystallites
Authors: Brus Louis 
Published: November 2013

AbstractThis article reviews recent experiments on the band edge luminescence of 32‐Å‐diameter, near‐wurtzite CdSe particles. Time, wavelength, and polarization resolution of the emission reveal a complex kinetics where the hole is mobile on the surface. A model is outlined involving strong resonance between three‐dimensional quantized internal states and the intrinsic surface states due to reconstruction. The hole appears to localize on the surface within about 150 fs.
Towards Monodisperse Semiconductor Clusters: Preparation and Characterization of ˜ 13‐Å Thiophenolate‐capped CdS Clusters
Authors: Wang Ying , Harmer Mark , Herron Norman 
Published: November 2013

AbstractWe report that two 10‐Å [Cd20S13(SPh)22]−8 clusters may be fused together to form a 13‐Å CdS cluster, which shows a sharp absorption peak at 370 nm and a luminescence peak at 390 nm. This is supported by the concentration dependence of the absorption spectra, the luminescence‐excitation spectra, the capillary zone electrophoresis separation, the electrochemical measurement, and the powder X‐ray data. This cluster fusion process occurs naturally in solution at high concentrations of [Cd20S13(SPh)22]−8. It can also be induced electrochemically by a two‐electron oxidation process. We also show that capillary zone electrophoresis and electrochemical synthesis are two useful new techniques for obtaining monodisperse CdS clusters. Their potentials and limitations are discussed.
Size Control and Surface Modification of Colloidal Semiconductor Particles
Authors: Luangdilok Carmela , Meisel Dan 
Published: November 2013

AbstractSeveral methods for the control of semiconductor particle sizes, in particular in the quantum size regime, are surveyed. Rational choice of solvent polarity, reaction temperature, concentration of reactants, solution pH, and counterions will determine the size and size distribution of the particles. Sizes of colloidal particles formed in microheterogeneous environments will often be predetermined by the dimensions of the micropores and cavities within the network of the medium. This is the situation in zeolite and clay cavities, and in polymer networks. Self‐aggregation of amphiphillic molecules often generates microdomains which may be utilized to direct the particle precursors or the particles themselves into these domains and thus limit their growth. Finally, complexation at the surface of the particles may limit and control the growth processes of the particles. The latter approach also allows functional modification of the particles' surface.
Chemical and Optical Properties of Small Metal Particles in Aqueous Solution
Authors: Henglein Arnim 
Published: November 2013

AbstractAlthough colloidal metals are systems with a huge amount of interface, relatively few surface chemistry studies of these systems have been made. A number of surface reactions on metallic silver particles (4‐nm diameter) are described in the present article and the accompanying changes in the optical properties are discussed. The reactions include the deposition of excess electrons and positive holes, changes in the Fermi level position and reactivity upon adsorption of various molecules, and photochemical reactions. The preparation and properties of oligomeric silver clusters in aqueous solution are also described. The clusters have strong absorption bands, undergo electron transfer with many acceptors, photoemit electrons with large quantum yields, and react eagerly with nucleophiles. The cluster Ag42+, a special form of dimeric silver, is stable towards oxygen, whereas larger clusters are readily oxidized. The chemical changes associated with the colloid → atom transition are demonstrated by discussing various properties such as the electrochemical potential of silver aggregates as a function of size. Cluster formation in the reduction of other metals (Pd, Cu, Au, Pb) in aqueous solution is also discussed briefly.
Redox Reactions of Thallium Clusters in Aqueous Solution
Authors: Mulvaney Paul 
Published: November 2013

AbstractThe redox chemistry of thallium clusters in aqueous solution has been investigated by pulse radiolysis and γ‐radiolysis. Using Ru(bpy)32+ as a redox indicator, it is shown that the redox potential of the Tl42+/+ couple is more positive than −1.27 V NHE, and that the Tl+/Tl2+ couple's redox potential is more negative than −1.27 V NHE. Ru(bpy)3+ reduces Tl42+ with a second‐order rate constant of 6±2 × 108 M−1s−1. Metallic particles of thallium are predicted to have a surface plasmon absorption band at 205 nm in water. The reduced Tl content in a sol can be measured by air‐free titration with CCl4 or methylviologen.
Elucidating the Molecular Origins of Solute/Solvent Friction
Authors: Hartman R.S. , Alavi D.S. , Waldeck D.H. 
Published: November 2013

AbstractThis paper describes experimental studies of the friction a solute molecule experiences in a solvent. More specifically, this work focuses on the dielectric friction needed to describe the rotational diffusion of medium‐sized molecules (displaced volume of a few hundred cubic angstroms) in polar solvents and electrolyte solutions.
“Condon” and “non‐Condon” Nonlinear Spectroscopy of Polyatomic Molecules in Solutions: Solvation Dynamics Study
Authors: Fainberg Boris 
Published: November 2013

AbstractA theoretical basis for the solvation dynamics study of complex molecules in solutions by resonance four‐photon spectroscopy has been developed. A non‐Markovian theory of four‐photon spectroscopy of electronic transitions in complex molecules has been generalized for the “non‐Condon” tensor case. It has been shown that for definite conditions the cubic susceptibility χ(3), describing four‐photon interaction, can be expressed as a product of “Condon” and non‐Condon parts. The latter describes the mixing effect of different electronic molecular states by nuclear motions (Herzberg‐Teller effects). The Condon part depends on the excitation frequency, and the non‐Condon one depends on the polarization state of the pump and the probe beams. It has been shown that the Condon resonance transient grating spectroscopy is a good method for the investigation of ultrafast solvation dynamics.The dependence of the non‐Condon part of χ(3) on the polarizations of the interacting beams permits one to formulate the principles of a new method for the investigation of Herzberg‐Teller effects on electronic molecular transitions by transient polarization four‐photon spectroscopy.
Barrier Widths and Tunneling in the Four‐Centered Syn Elimination of H‐X from Ethyl‐X. The Role of Transition State Asymmetry
Authors: Wolfe Saul , Kim Chan‐Kyung 
Published: November 2013

AbstractThe syn elimination of HX from CH3CH2X produces a C2H4‐HX complex, which is stabilized relative to ethylene and HX. The primary kinetic H/D isotope effects in the complex‐forming steps of such reactions have been examined ab initio for X = H, BH2, CH2, NH2, NH3+, OH, OH2+, F, Cl, and Br, using 3–21G, 3–21G(d), 6–31G(d), and MP2/6–31G(d)optimized structures and vibration frequencies. Four‐centered transition structures are found for all but X = H, BH2, and CH3; with BH2, no transition structure exists at MP2/6–31G(d); with H and CH3, the transition structures are three‐centered. In addition, although the syn mechanism has lower energy for X=H than the four‐centered Woodward‐Hoffman allowed anti mechanism, C‐H, C‐C, and H‐H bond breaking would be favored over either pathway. The semiclassical primary kinetic isotope effect increases systematically as the central atom of a neutral X is varied from left to right along a row, or down a column, of the periodic table. Concurrently, the X‐H‐C angle in the transition state increases. The increase in kH/kD as the X‐H‐C angle becomes more linear is the direction predicted by E.S. Lewis for hydride (deuteride) transfer, and by More O'Ferrall for hydron transfer. One‐dimensional corrections to these isotope effects predict significant quantum mechanical tunneling only in the cases of F and OH, for which the Bell and Eckart barriers are narrower than those obtained from intrinsic reaction coordinate (IRC) calculations. In contrast, for NH2, where tunneling seems unimportant, the Eckart barrier is wider than the IRC. A quantitative measure of barrier width is the imaginary frequency of the transition state. Tunneling occurs when this is large, when the barrier is large, and, most importantly, when the transition structure is symmetrical.
Urea: Potential Functions, log P, and Free Energy of Hydration
Authors: Duffy Erin M. , Severance Daniel L. , Jorgensen William L. 
Published: November 2013

AbstractIntermolecular potential functions have been developed for urea. Ab initio 6–31G(d) calculations were performed on urea‐water complexes to obtain interaction energies and hydrogen‐bond lengths that were used in developing the partial charges for the OPLS model. An important test was made by computing through Monte Carlo simulations the difference in chloroform/water partition coefficients for urea and acetamide, Δ log P. The accord between the computed result from statistical perturbation theory for TIP4P water and OPLS chloroform (2.0 ± 0.1 ) and the experimental value (1.9) is excellent. The computed absolute free energy of hydration of acetamide (‐9.5 ± 0.4 kcal/mol) also matches the experimental data at 25 °C (‐9.68 kcal/mol). These facts provide confidence in the computed value of −13.6 ± 0.4 kcal/mol for the absolute free energy of hydration of urea, an experimentally unavailable result. The water structure around urea and acetamide was also characterized; the average numbers of solute‐water hydrogen bonds are 5 for urea and 3–4 for acetamide.
The History and Evolution of Gaussian Basis Sets
Authors: Shavitt Isaiah 
Published: November 2013

AbstractThe history and evolution of Gaussian basis sets in molecular electronic structure calculations are reviewed, from the original proposals by McWeeny and Boys to the current high‐quality generally‐contracted basis sets of the atomic natural orbital and correlation‐consistent types. Various explorations of modified forms, generalizations, and alternatives are mentioned, but the focus is on the mainstream developments whose origin can largely be traced to the work of Reeves and his students. The introduction of various standard sequences of basis sets, such as those of Pople and coworkers, with well‐documented and extensive test results, and the wide distribution of easy‐to‐use computer codes, such as the GAUSSIAN series, have contributed greatly to the explosion in the use of ab initio calculations of molecular structure in the chemistry research community.
Calculation of NMR Chemical Shifts — The Third Dimension of Quantum Chemistry
Authors: Cremer Dieter , Olsson Lars , Reichel Felix , Kraka Elfi 
Published: November 2013

AbstractNMR chemical shift calculations provide the basis for an intensive collaboration between quantum chemists and experimentalists. Calculated shift data can be used to describe the magnetic properties of a molecule, to identify unknown compounds by comparison of experimental and theoretical shift values, to determine equilibrium geometries, to investigate conformational changes, to elucidate the mechanism of molecular rearrangements, to determine solvent effects on NMR data, to identify complexation or coordination of soluted molecules by solvent molecules, to detect electronic structure changes caused by the medium, and to describe chemical bonding. This is demonstrated by three examples, namely the determination of the equilibrium structure of the homotropylium cation, the description of BH3NH3 in solution or condensed phases, and the investigation of stannyl cation complexes in solution. IGLO calculations of 13C, 11B, 15N, and 119Sn chemical shifts with DZ+P or TZ+P basis sets lead to the following results: (1) The homotropylium cation possesses an equilibrium 1,7 distance of 2 Å that is indicative of strong through‐space interactions and, as a consequence, homoaromatic character. (2) In solution, the charge transfer from NH3 to BH3 is increased, which leads to a decrease of the BN bond length, an increase of the dipole moment, and a shielding of both the B and the N nucleus. The experimental δ(11B) and δ(15N) values can be reproduced when the geometry effect and the direct solvent effect are included in the shift calculations. (3) Stannyl cations form strongly‐bounded coordination complexes with solvent molecules (binding energy: ≥ 50 kcal/mol) that make the cation properties, in particular δ(119Sn) values, similar to those of covalently‐bounded stannyl compounds. An experimental detection of stannyl cations in solution by NMR spectroscopy should only be possible by extensive solvent variations.
Laser Applications in Chemistry and Biology: Stimulation, Observation, and Manipulation
Authors: Seeger Stefan , Sick Volker , Volpp Hans‐Robert , Wolfrum Jürgen 
Published: November 2013

AbstractResults from three different types of laser‐based experiments are presented. Using translationally‐excited H atoms, the reaction dynamics of H(2S) + O2(3Σg−) → OH(2Π) + O(3P) was investigated. In order to study the influence of selective reagent translational excitation on the reactivity, absolute reaction cross sections and nascent OH product quantum‐state distributions were measured at different collision energies by means of the laser photolysis/laser‐induced fluorescence (LP/LIF) “pump‐probe” technique. The measured absolute reaction cross sections: σR(1.0eV) = (0.20 ± 0.08) Å2, σR(1.6 eV) = (0.58 ± 0.17) Å2, and σR(1.9 eV) = (0.35 ± 0.14) Å2 clearly confirm the presence of a pronounced maximum in the excitation function (Keßler, K.; Kleinermanns, K. J. Chem. Phys. 1992, 97: 374). The experimental results are compared with results from quasiclassical and recent 3D quantum scattering calculations on ab initio potential energy surfaces.In addition, results from experiments are reported in which two‐dimensional laser light sheet techniques were applied to investigate both the NO formation in a domestic natural gas burner and fuel mixing processes in combustion engines. Images of temperature and NO concentration distributions, as well as series of images of the fuel distribution, are presented.Finally, recent results of laser applications in biology are reported. A new concept in biodiagnostics using multiplex dyes, which have different characteristic fluorescence lifetimes but identical excitation and emission spectra, to distinguish different biomolecules has been developed. First experiments are described in which multiplex dyes in capillaries were identified by steady‐state fluorescence spectroscopy using a diode laser system. In combination with a pattern recognition technique, a detection limit of about 100 molecules per 25 picoliters detection volume was determined. The possible application in DNA sequencing is discussed. Results from laser‐induced cell fusion experiments, in which the laser nanoscalpel and photonic tweezers techniques were combined, are also presented.
The Photodissociation of Pyridine at 193 nm
Authors: Prather Kimberly A. , Lee Yuan T. 
Published: November 2013

AbstractUsing photofragmentation translational spectroscopy, the primary processes involved in the photodissociation of pyridine are investigated at 193 nm. Upon absorption of a single photon, HCN + C4H4, CH3 + C,H2N, C3H3 + C2H2N, and H + C5H4N are the observed primary pathways, with relative yields of 59%, 21%, 11%, and 9%, respectively. This paper reports the experimental details and results of this study. Comparison of the results of this study with a previous study in our laboratory investigating the photolysis of benzene serves as a direct measure of the strong influence substitution of a heteroatom in an aromatic ring can have on a molecule's photochemistry.
Fundamental Interactions of Molecules (Na2, Na3) with Intense Femtosecond Laser Pulses
Authors: Baumert Thomas , Gerber Gustav 
Published: November 2013

AbstractThe real‐time dynamics of multiphoton ionization and fragmentation of molecules Na2 and Na3 has been studied in molecular beam experiments employing ion and electron spectroscopy together with femtosecond pump‐probe techniques. Experiments with Na2 and Na3 reveal unexpected features of the dynamics of the absorption of several photons as seen in the one‐ and three‐dimensional vibrational wave packet motion in different potential surfaces and in high laser fields:In Na2 a second major resonance‐enhanced multiphoton ionization (REMPI) process is observed, involving the excitation of two electrons and subsequent electronic autoionization. The possibility of controlling a reaction by controlling the duration of propagation of a wave packet on an electronically‐excited surface is demonstrated. In high laser fields, the contributions from direct photoionization and from the second REMPI process to the total ion yield change, due to different populations in the electronic states participating in the multiphoton ionization (MPI) processes. In addition, a vibrational wave packet motion in the electronic ground state is induced through stimulated emission pumping by the pump laser. The 41Σ+g shelf state of Na2 is given as an example for performing frequency spectroscopy of high‐lying electronic states in the time domain. Pure wave packet effects, such as the spreading and the revival of a vibrational wave packet, are investigated.The three‐dimensional wave packet motion in the Na3 reflects the normal modes in the X and B states, and shows in addition the pseudorotational motion in the B state in real time.
Nitrate Radicals in Tropospheric Chemistry
Authors: Platt Ulrich , Heintz Frank 
Published: November 2013

AbstractNitrate radicals are being recognized as key intermediates in a growing list of important chemical processes in the atmosphere. Here, the role of nitrate radicals (NO3) in tropospheric chemistry is discussed, with special emphasis on results from field measurements, most of which have been made by differential optical absorption spectroscopy (DOAS), with matrix‐isolation electron spin resonance being an alternative technique. Nitrate radicals were observed in the atmosphere at peak mixing ratios of 350 ppt. Long‐term observation of NO3 shows that 24‐h averages in rural air masses are closer to a few ppt. Nevertheless, the NO3 radical plays an important role in the non‐photochemical conversion of NOx to HNO3. Also, NO3 is a strong oxidizing agent and initiates the night‐time removal of atmospheric trace species such as olefins, aromatic hydrocarbons, and organic sulfur compounds. Finally, night‐time peroxy radical production and release of reactive halogen species from sea salt aerosol might be initiated by NO3 reactions.
On Vehicle VOC and NOx Emissions from the Standpoint of the Tropospheric Ozone Problem
Authors: Pierson William R. 
Published: November 2013

AbstractOzone is formed in and downwind of urban areas from urban emissions of NOx and volatile organic compounds (VOCs) in the presence of sunlight. The main sources of VOCs in polluted air are motor vehicles, industrial solvents, processes in the petroleum and chemical industries, and vegetation. The main NOx sources are stationary‐source fuel combustion (mainly electric utilities) and motor vehicles. Recent studies have demonstrated that VOC emissions from motor vehicles have been seriously underestimated, and this may well explain why ambient O3 has not responded well to control efforts. This review presents an overview of the sources, formation, and potential abatement strategies for O3 pollution in the troposphere, with particular emphasis on the mobile source contribution to O3 formation.
Vertical Sulfur Dioxide, Ozone, and Heavy Metal Concentration Profiles Above the Southern Bight of the North Sea
Authors: Otten Philippe , Injuk Jasna , van Grieken René 
Published: November 2013

AbstractVertical profiles of SO2 and O3 concentrations were measured continuously within the lower atmosphere during 18 flights over the Southern Bight of the North Sea. The average SO2 concentration below the temperature inversion layer ranged from 1.9 to 19.5 ppb. The highest levels were observed when the air masses came from the southeast‐east and under variable wind sector conditions: 15.2 and 12.0 ppb, respectively. The highest SO2 concentrations were found at altitudes between 100 and 200 m. The overall average O3 concentration was 46 ppb, with values of 30–40 ppb at sea level to 60–70 ppb at altitudes above 100 m. Low O3 concentrations reflect O3 depletion through reaction with freshly emitted pollutants, while high O3 concentrations are an indication of photochemical activity. O3 concentrations are primarily dependent on seasonal influences, while SO2 levels are dictated more by the history of the sampled air mass. The results are discussed in relation to the heavy metal concentrations, measured within the same experiment. In spite of some differences, SO2 and trace metals showed similar trends. The individual SO2 and O3 profiles clearly illustrated the high variability in vertical concentration distributions.
New Heavy Isotope Studies in Archaeology
Authors: Budd Paul , Haggerty Randolph , Mark Pollard A. , Scaife Brett , Thomas Richard G. 
Published: November 2013

AbstractKinetic theory relating to the separation of isotopes in nonequilibrium evaporation, originally developed in the 1920s, is discussed in the context of the possibility of anthropogenic lead isotope fractionation in ancient metallurgical processes. From a consideration of the chemistry of lead and lead compounds and the results of a small trial experiment, it is concluded that any isotopic fractionation of lead is unlikely to be archaeologically significant given current measurement precision. However, application of the same theory to the tin isotope system shows that substantial and measurable isotopic fractionation is likely to result from melting and remelting bronze. Furthermore, recent studies show that tin isotopes do not vary significantly in terrestrial ores. This raises the possibility that tin isotope ratios will be systematically altered by melting, casting, and recycling bronze. This paper discusses the principles of tin isotope fractionation in ancient metallurgy and the potential for tin isotope studies to yield quantitative data on prehistoric metal recycling.
Stable Isotopes of Carbon and Oxygen as a Possible New Tool for Estimating Firing Temperatures of Ancient Pottery
Authors: Nissenbaum Arie , Killebrew Ann 
Published: November 2013

AbstractThe ratios of the stable isotopes of carbon (13C/12C) and oxygen (18O/16O) in calcite in clays that contain carbonate and that were fired at temperatures between 500 and 700 °C displayed a good correlation with the firing temperature for both isotopes. The isotopic composition indicated that the original carbonate was either completely decomposed or that it exchanged with environmental CO2 to obliterate the original isotopic signature. The isotopic effects seem to be kinetically controlled. The isotope thermometer is used to compare refiring temperatures of pottery which was fired under controlled conditions. The differences between calculated and actual temperatures did not exceed 132 °C and in most cases was about 50 °C or less. Isotopic analysis of pottery from Tel Miqne‐Ekron and Deir el Balah, dated to the 13th–12th centuries BC, gave an average ancient firing temperature of about 600 °C. If shown to be of general validity, then perhaps this technique can be used also for estimating preparation temperatures of ancient mortar, especially when preserved in dry climates where later changes would be minimized.