bluuebunny/crossref_metadata_2025 huggingface dataset:
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

The 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

The 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.
Photochemically Induced Oxidative and Reductive Regeneration of NAD(P)+/NAD(P)H Cofactors: Applications in Biotransformations
Authors: Willner Itamar , Maidan Ruben , Willner Bilha
Published: November 2013

Photosensitized regeneration of NAD(P)H cofactors is accomplished by biocatalyzed and artificially catalyzed transformations in photochemical assemblies. Photogenerated N,N'‐dimethyl‐4,4′‐bipyridinium radical cation, MV+., acts as electron carrier for the reduction of NADPH in the presence of the enzyme ferredoxin reductase and for the reduction of NADH in the presence of lipoamide dehydrogenase. For the photogeneration of MV+. and subsequent NADPH formation, three different photosensitizers are applied: Ru(bpz)23+, Ru(bpy)23+, and Zn—TMPyP4+. The highest quantum yield for NADPH formation is observed with Ru(bpz)32+ and is ϕ = 1.7 × 10−1. For NADH regeneration only Zn‐TMPyP4+ can be applied. Ru(bpy)32+ and Ru(bpz)2+3 interact with NADH in their excited or oxidized forms and therefore cannot be used as light‐active compounds in the system. The NADPH regeneration cycle has been coupled to the biocatalyzed synthesis of glutamic acid. Although Ru(bpz)32+ is 42.5‐fold more efficient than Ru(bpy)32+ in the regeneration of NADPH, the synthesis of glutamic acid is improved only by a factor of 2 in the presence of Ru(bpz)32+, implying that the coupled process is rate limiting. Oxidative regeneration of the NAD+ cofactor is accomplished in a photosystem that includes Ru(bpy)32+ as photosensitizer. The photoprocess is coupled to dehydrogenation of ethanol, propanol, lactic acid, and alanine with concomitant H2 evolution. A photosystem that includes Ru(bpy)32+ as photosensitizer, ascorbate as electron donor, and chloro‐tris‐(3‐diphenylphosphinobenzene sulfonate)Rh(I), RhCl(dpm)33‐, is catalytically active in the photoinduced regeneration of NAD(P)H cofactors. Mechanistic investigations show that photogenerated Ru(bpy)3+ mediates the generation of a hydrido‐rhodium complex that acts as a charge relay for the production of NAD(P)H.
Concerning the Stereochemistry of SE'‐Type Reactions
Authors: Matassa Victor G. , Jenkins Paul R. , Kümin Albin , Damm Lorenz , Schreiber Jakob , Felix Dorothee , Zass Engelbert , Eschenmoser Albert
Published: November 2013

The steric course of an azidolytic SE' desilylation of a chiral allyl silane and experiments on electrophilic alkylations of ketene‐O,N‐acetals are described. In this context, we point to some merits of combining qualitative concepts of σ‐conjugation with the ẗ‐bond model of double bonds.
On the Absence of Anomalous Singularities in the S‐Matrix Version of the Kohn Variational Principle for Quantum Scattering
Authors: Gaucher Lionel F.X. , Miller William H.
Published: November 2013

Recent calculations by Apagyi, B.; Lévay, P.; Ladányi, K. (Phys. Rev. A, 1988, 37: 4577) have shown that anomalous (i.e., spurious, unphysical) singularities arise in e− + H(1s) scattering calculations using the Schwinger variational principle. It has been known for a long time that such singularities can exist in the K‐ matrix version of the Kohn variational principle, but this is the first time (to our knowledge) that they have been seen in the Schwinger variational principle. This paper reports the results of calculations for this same problem using the S‐ matrix version of the Kohn variational principle (Zang, J.Z.H.; Chu, S.I.; Miller, W.H. J. Chem. Phys., 1988, 88: 6233), showing that there are no such anomalous singularities in this approach.
Time‐Dependent Quantum‐Mechanical Approaches to the Continuous Spectrum: Scattering Resonances in a Finite Box
Authors: Hammerich Audrey Dell , Muga J. Gonzalo , Kosloff Ronnie
Published: November 2013

Several novel aspects of scattering resonances are studied. An expression, valid for a finite box, relating the continuum phase shift with the energy shift and unperturbed level separation is proposed and applied to obtain the resonance parameters. The effect of the resonance on propagating a wavepacket in imaginary time is studied. It is observed that the resonance strongly affects the cumulants of the energy distribution. In particular, a local minimum of the first derivative of the energy with respect to time (proportional to the second cumulant) serves to estimate the resonance energy and wavefunction. Once the estimate is known, the autocorrelation function is used to evaluate the resonance width. Alternatively, a new iterative approach is developed that is capable of selectively yielding an arbitrary band of energy eigenvalues and eigenfunctions on a grid. This method is applied to give those energy levels that are of interest for the discrete computation of the resonant phase shift, i.e., those close to resonance. Exact (analytical) and approximate results are in good agreement for a particular separable potential model in one dimension. These methods can be extended to realistic potentials in higher dimensions.
Theory of Diatomic Photodissociation to Atomic Hyperfine Structure States
Authors: Lee Sungyul , Williams Carl J. , Freed Karl F.
Published: November 2013

We present a general theory of the photodissociation of diatomic molecules to the hyperfine sublevels of open‐shell atomic fragments. The hyperfine interactions affect the fragment hyperfine sublevel populations, polarizations, etc., either when the fragment atomic hyperfine sublevels are optically resolved, when resonances possess hyperfine splittings, or both. The case of hyperfine splittings in resonances is of particular interest for states whose principle parentage is singlet, but which exhibit hyperfine splitting due to nonadiabatic mixing with close‐lying triplet states. Such resonances have been observed in the photodissociation of SiH+ (Sarre, P.J. et al., Phil. Trans. R. Soc. London, 1988, A324: 233). We also show how our nonadiabatic theory in the presence of hyperfine interactions reduces to the general theory of diatomic photodissociation to open‐shell atoms of Singer et al. (J. Chem. Phys., 1983, 79: 6060) when resonance hyperfine splittings or hyperfine structure states of the fragments are not resolved in particular experiments.
Unimolecular Reactions Revisited
Authors: Rice Stuart A. , Gaspard Pierre
Published: November 2013

A brief review of recent developments in the theory of unimolecular reactions is presented, with emphasis on the work carried out at The University of Chicago. The subjects considered are: the development of a paradigm for the control of selectivity of a photoinduced reaction; the development of an alternative form for the classical mechanical theory of the rate of a unimolecular reaction; the beginnings of an understanding of the influence of classical mechanical chaos on the corresponding quantum‐mechanical description of an unbounded system; and the possible decoupling of a part of an N‐degree‐of‐freedom system from the rest of the system on a time scale of interest with respect to subsystem internal energy transfer and chemical reaction. The intellectual linkages between these subjects are very briefly sketched.
Electron Transfer in a Dynamically Disordered Polar Medium
Authors: Nadler Walter , Marcus Rudolph A.
Published: November 2013

Intramolecular electron transfer (ET) processes with the main energetic contributions coming from the solvent reorganization are investigated for a polar medium that exhibits dynamic disorder. Dynamic disorder provides a description of the anomalous relaxational behavior of correlation functions in complex glass‐like systems, alternative to static disorder. In particular, the questions addressed are whether time‐resolved observation of nonexponential ET in such a medium can readily distinguish experimentally between static and dynamic disorder and whether a contribution of intramolecular degrees of freedom to the ET can be identified by it.
Structure and Photochemistry of C4 as Studied by the Coulomb Explosion Imaging Method
Authors: Algranati M. , Feldman H. , Kella D. , Malkin E. , Miklazky E. , Naaman R. , Vager Z. , Zajfman J.
Published: November 2013

The Coulomb Explosion Imaging (CEI) method was used to determine the structure and photochemistry of carbon clusters. The cyclic structure of C4 was found using a combination of laser photodetachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 ± 0.1 eV, more than 1 eV lower than the electron affinity of the linear isomer.
Resonance Positions and Widths for Time‐Periodic Hamiltonians by the Complex Coordinate Method
Authors: Moiseyev Nimrod , Korsch H. Jürgen
Published: November 2013

The complex coordinate method combined with the Floquet‐Bloch theory has the advantage of treating the metastable quasi‐energy states (i.e., resonances) of time‐periodic Hamiltonians in terms of bound state formalism. The formalism derived here is a general one. It enables one to use complex basis functions when needed and to calculate the resonance positions and widths for any time‐periodic Hamiltonians. A numerical example is given to illustrate the potential use of the proposed method in the study of the dynamics of laser‐driven systems and of time‐periodic classical chaotic Hamiltonians.
Photoinitiated Reactions of H Atoms with CO2: OH(v = 0) Rotational Excitation from the 239‐nm Photolysis of CO2HI Complexes
Authors: Hoffmann G. , Oh D. , Chen Y. , Engel Y.M. , Wit‐Tig C.
Published: November 2013

Photoinitiated H + CO2 → OH + CO reactions are discussed, with emphasis on reactions in CO2HI complexes. Under single‐collision bulk conditions, reaction probability rises with collision energy by two orders of magnitude throughout the range 10300–19000 cm−1 (ΔH = 8960 cm−1). Modest probabilities at collision energies well above threshold are interpreted as due to the inability of the heavier nuclei to move fast enough to trap the H atom on the HOCO potential surface. The pronounced increase in reaction probability with collision energy can be due to impact‐induced distortion of the CO2 frame, localizing the H atom in a shallow region of the HOCO potential surface long enough for the heavier nuclei to move toward the HOCO equilibrium geometry, thus capturing the H atom. Measurements of nascent OH(v = 0) R, T excitations indicate a significant bias toward product translation and away from OH rotation at the highest collision energies. OH LIF spectra taken at different collision energies provide a map of nascent OH(v = 0) rotational excitation for different values of E†, the HOCO† energy in excess of the OH + CO product channel. With CO2HI complexes, pairwise I‐H and H‐CO2 repulsions before HOCO† is formed increase the I‐HOCO† speed at the expense of HOCO† internal excitation. It is pointed out that with CO2HBr, the Br atom is 3.6 Å from the C atom along a line perpendicular to the CO2 axis, with the H atom localized near one of the O atoms. CO2HI is expected to be qualitatively similar. The OH(v = 0) rotational distribution obtained using 239‐nm photolysis of CO2HI complexes differs markedly from that obtained under single‐collision conditions at the same photolysis wavelength, the former being colder and qualitatively distinct from any of the OH(v = 0) distributions obtained at a single collision energy. The OH(v = 0) rotational distribution obtained using CO2HI complexes can be reconciled with a bimodal P(E†) distribution (e.g., ∼ 30% at E† ∼ 800 cm−1 and ∼ 70% at E† ∼ 6000 cm−1). The 6000‐cm−1 component is attributed to the squeezed‐atom effect (E† = 7880 cm−1 for single‐collision conditions at the same photolysis wavelength), while the origin of the other component is uncertain. It may derive from (i) mechanisms that produce HOCO† with low E† values, (ii) mechanisms that relax HOCO† and/or OH such as interactions with the nearby I atom and (iii) higher‐than‐binary complexes.
High Performance Liquid Chromatographic Studies of Some Cyclodextrin Complexes
Authors: Landau Sharon , Grushka Eli
Published: November 2013

The complex formation between β‐cyclodextrin and nitrophenol isomers was studied with the aid of reversed phase liquid chromatography. The effects of temperature, pH, and methanol content on the formation constants of the nitrophenol/β‐cyclodextrin complexes were investigated. The complex formation constants were found to decrease with increasing temperatures. This decrease in formation constant was independent of the pH of the mobile phase. From the temperature dependence of the formation constants we were able to extract enthalpies and entropies of the complex formation. The values calculated in this work agree favorably with published values. Three pHs were examined: 3.57, 4.65, and 5.6. The formation constants of the ortho and meta isomers were highest at the lowest pH and leveled off at the two higher pHs. The para isomer showed a minimum in the formation constant at pH 4.65. This behavior was independent of the pH of the mobile phase.
Graph‐Theory Derived Models for the Skeletal Chemical Bonding in Organometallic Metal Carbonyl Clusters
Authors: King R.B.
Published: November 2013

Ideas derived from topology and graph theory can be used to model the skeletal chemical bonding in metal carbonyl clusters as well as boranes and carboranes. In such molecules delocalized skeletal bonding occurs when there is a mismatch between the number of internal orbitals provided by a vertex atom and the number of polyhedral edges meeting at that vertex. Such global delocalization in deltahedral metal clusters may be regarded as a three‐dimensional analogue of the aromaticity in planar polygonal molecules such as benzene. Elementary graph‐theoretical considerations provide a basis for the 2n+2 skeletal electrons normally observed in n vertex delocalized deltahedral metal clusters lacking tetrahedral chambers. Electron‐rich n‐vertex metal clusters have more than 2n + 2 skeletal electrons and form polyhedra having at least one face with at least 4 edges. Electron‐poor n‐vertex metal clusters have less than 2n + 2 skeletal electrons and form deltahedra having at least one tetrahedral chamber. Ideas derived from group theory and graph theory can be used to analyze molecular orbital energy parameters for the most symmetrical deltahedral borane anions BnHn2− (particularly n = 6 and 12) computed using either semiempirical extended Hückel methods or ab initio methods based on Gaussian orbitals. Some of the fundamental aspects of the graph‐theory derived models for skeletal bonding in delocalized deltahedral metal clusters are closely related to the tensor surface harmonic theory of Stone.
Multinuclear Metal Carbonyl Alkoxide and Aryloxide Derivatives as Models for Metal Carbonyls Adsorbed on Metal Oxide Supports
Authors: Darensbourg Donald J. , Mueller Brian L. , Bischoff Christopher J. , Johnson Cathy C. , Sanchez Kathryn M. , Reibenspies Joseph H.
Published: November 2013

The synthesis and characterization of two tungsten carbonyl dimers containing bridging alkoxide or aryloxide ligands are described. The crystal and molecular structures of [PPN]2[W2(CO)8(OCH2CF3)2], 1, and [Et4N]3[W2(CO)6‐(OPh)3]‐CH3CN, 2, are reported and compared with the structures of tetranuclear tungsten derivatives previously described. The dimer 1 crystalizes in the triclinic space group P 1 with unit cell parameters a = 13.460(11) Å, b = 12.318(5) Å, c = 13.842(10) Å, α = 82.73(5)°, β = 59.11(5)°, γ= 80.09(5)°, V = 1938(2) Å3, and Z = 1. The complex 2 crystalizes in the monoclinic space group P21/n with unit cell parameters a = 11.954(2) Å, b = 19.359(4) Å, c = 26.462(5) Å, β = 102.50(16)°, V = 5979(2) Å3, Z = 4. Molecular modeling software was utilized to construct a tetranuclear derivative from 1 similar to the structurally characterized [W(CO)3OH]4−4 tetramer. The two tetramers were found to possess similar molecular parameters. This supports the contention that dimers of type 1 are the precursors of the tetramers. Comparisons of the tungsten alkoxides and aryloxides with the behavior of W(CO)6 on γ‐alumina are provided.
Fluid Flow in Restricted Geometries
Authors: Sheng Ping , Zhou Minyao
Published: November 2013

This paper addresses two topics concerning fluid flow in restricted geometries. The first topic is on the relationship between the dynamic permeability κ(ω), where ω denotes frequency, and the microstructure of a porous medium. It is shown that when κ is normalized by its static value κ0, and the frequency ω by a characteristic ω0 particular to the sample, the resulting dimensionless function magnified image, with magnified image and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \tilde \omega = }\omega {\rm /}\omega _{\rm 0} $\end{document}, is dominated by the geometry of the throat regions in a porous medium. If the pore cross‐sectional area S varies slowly near the throats, i.e., dS/dz ≅ 0 where z is the distance normal to the cross section, then \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \kappa ({\rm \tilde \omega)} $\end{document} is an approximate universal function independent of other features of the microstructure. Both theoretical and experimental evidences are presented for such scaling behavior. When scaling holds, the dynamic permeability κ(ω) is found to contain only two pieces of geometric information, and the knowledge of either the low‐frequency or the high‐frequency asymptotic constants of κ(ω) would enable one to deduce the other missing parameters. In particular, since the high‐frequency asymptotic constants of κ(ω) can be related to the electrical formation factor and a weighted volume‐to‐surface ratio, the static permeability value κ0 may be directly deduced from the knowledge of these parameters. Whereas the first topic concerns single‐phase flow, the second topic of this paper involves the displacement of a fluid by another immiscible fluid in a capillary tube. Here the basic physics lies in the dynamics of the moving contact line, defined as the intersection of the moving fluid‐fluid interface with the tube wall. Through first‐principle hydrodynamic calculations and comparison with experiments, the macroscopic behavior of fluid displacement is linked to the microscopic parameters governing the region around the contact line. It is shown that there are two frictional forces associated with the motion of the contact line. One is the viscous stress, which is primarily responsible for the deformation of the fluid‐fluid interface as the velocity of the moving interface increases. The other frictional force, found experimentally to vary as the square root of velocity, is explained on the basis of capillary‐wave excitation at the fluid‐fluid interface due to contact line motion over wall roughness. Excellent agreement between theory and experiment is obtained.
Fluorescence Quenching in Three to Zero Dimensions: Effect of Dimensionality on a Diffusion‐Limited Reaction
Authors: Almgren Mats , Alsins Jan
Published: November 2013

The diffusion‐limited fluorescence quenching of pyrene by a hydrophobic benzophenone derivative, 1‐(4‐benzoyl)‐phenylhexane, was investigated in the different phases of the binary system hexaethyleneglycol‐mono‐n‐dodecylether (C12E6)‐water, from the normal homogeneous solution of the neat surfactant, via a sequence of liquid crystalline phases, to the aqueous micellar solution. These phases provide nonpolar domains of various dimensionality for the reactants: three dimensions in the neat surfactant, two in the lamellar phase, one in the hexagonal phase, and zero in the small micelles. The results were evaluated using models with appropriate dimension for the diffusion‐controlled reaction. The resulting diffusion coefficients at 20°C were obtained as 3 × 10−11 m2 s−1 in the neat surfactant, around 6 × 10−11 m2 s−1 in the lamellar phase, and 10 × 10−11 m2 s−1 in the hexagonal phase. The activation energy for the diffusion process increased from 32 kJ mol−1 in the neat surfactant to 42 kJ mol−1 in the hexagonal phase. The differences between the phases may be due to the uncertainty in the values of the size parameters for the structures; in any case, they are not large, indicating that it is mainly the geometry that is important for the deactivation kinetics, whereas the mobilities of the reactants are less affected by changes in packing.
Sequential Annulation in Molecular Construction. A Short, Stereocontrolled Synthesis of (±)‐Hirsutene
Authors: Paquette Leo A. , Moriarty Kevin J. , Shen Chien‐Chang
Published: November 2013

A total synthesis of (±)‐hirsutene has been achieved in 9 steps and in 14% overall yield via a twofold cycloadditive scheme that utilizes 4,4‐dimethylcyclopentene, 1‐(trimethylsilyloxy)butadiene, and ethylene as the three key building blocks.
Regioselective Oxidation of Polyalkoxy Naphthalenes: Formation of Naphthoquinones by Ammonium Cerium(IV) Nitrate Oxidation of Methoxymethyl Ethers
Authors: Clive Derrick L. J. , Middleton Donald S.
Published: November 2013

A proper choice of methoxy and methoxymethyl groups for protection of the hydroxyls of polyoxygenated naphthalenes allows selective oxidation of either ring of the naphthalene structure with ammonium cerium(IV) nitrate; for example, 9 can be converted into 10, and 11 into 12.
Biocatalysis as a Rational Approach to Enantiodivergent Synthesis of Highly Oxygenated Compounds: (+)‐ and (—)‐Pinitol and Other Cyclitols
Authors: Hudlicky Tomas , Rulin Fan , Tsunoda Toshiya , Luna Hector , Andersen Catherine , Price John D.
Published: November 2013

Bacterial oxygenation of halogenated aromatic compounds yields arene cis‐diols of type 2 of high enantiomeric purity. Further oxygenation of these extremely versatile synthetic intermediates provides for stereo‐ and regioselective introduction of additional functionalities such as hydroxyl groups. Several rational methods of oxygenation (epoxidation, osmylation, singlet oxygen addition, and ozonolysis) are used to produce, in short synthetic routes, cyclitol derivatives of medicinal importance. Proper symmetry considerations lead to the development of enantiodivergent synthetic design of target compounds from a single enantiomer of a starting diol. These principles are illustrated on the short syntheses of (+)‐ and (‐)‐pinitol. Stereocontrolled oxidative transformation of diols of type 2 are exemplified in the synthesis of conduritol C and dihydroconduritol C. Full experimental details are provided for all compounds. A guide to the stereorational design of any stereoisomer of cyclohexane (poly)ols or carbohydrates is provided at the end of the paper.
Stereoselective Total Synthesis of (±)‐Ptilocaulin and its 7‐Epimer. A Strategy Based On the Use of an Intramolecular Nitrile Oxide Olefin Cycloaddition (INOC) Reaction
Authors: Keshava Murthy K.S. , Hassner Alfred
Published: November 2013

A stereoselective total synthesis of (±)‐ptilocaulin 1, an antimicrobial anticancer agent as well as of its 7‐epimer 23 is described. The strategy involves an aldol condensation of the dilithio dianion of the Z‐aldoxime (13) with a ketone 6 to produce 11 and the stereospecific intramolecular cycloaddition of the nitrile oxide 5, generated in situ from 11. The resulting isoxazoline 4 was converted to hydroxy ketone 3 which serves as a starting point for introduction of the fourth stereocenter at C‐7. Lithium in ethylamine reduction of 3 produces stereospecifically the 7β‐methyl compound which leads to ptilocaulin 1, while catalytic hydrogenation of 3 introduces the 7α‐methyl substituent and finally 7‐epiptilocaulin 23. H‐ and C NMR analysis permits differentiation between isomers of intermediates. When, in the last step of the synthesis of 1 or 23 from 19 or 22 and guanidine, a higher temperature (>130 °C) was applied, an interesting disproportionation to aminopyrimidine 25 or 28 and cyclic guanidine 24 or 27 was observed.
Radiationless Transitions in CdSe Quantum Crystallites
Authors: Brus Louis
Published: November 2013

This 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

We 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

Several 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

Although 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

The 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

This 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

A 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

The 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

Intermolecular 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

The 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

NMR 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

Results 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

Using 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

The 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

Nitrate 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

Ozone 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

Vertical 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

Kinetic 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.