bluuebunny/crossref_metadata_2025 huggingface dataset:
Donors for Organic Metals: Molecular Aspects
Authors: Sandman Daniel J. , Ceasar Gerald P.
Published: November 2013

Physicochemical issues are discussed which are relevant to chemists interested in the synthesis of unknown compounds to serve as precursors to organic metals. Molecular topics are emphasized, physical and structural concepts which motivate synthetic activity in chalogen chemistry are presented, and new results in selenium and tellurium systems are summarized. Two model representations for donor molecular structures based on polyenes and polyacenes are presented and known examples are discussed. Quantitative correlations between gas phase vertical ionization potential and solution electrochemical redox potential are examined using donors of interest for organic metals as data. Limitations of such correlations and consequences of these shortcomings are given. Finally, possible future directions for several of these topics are posed.
Electron Donation in Photosystem II
Authors: Thompson Lynmarie K. , Miller Anne‐Frances , De Paula Julio C. , Brudvig Gary W.
Published: November 2013

The pathways of electron donation in Photosystem II (PS II) as studied by electron paramagnetic resonance (EPR) and time‐resolved optical spectroscopy are discussed. The EPR studies have defined two competing pathways of electron donation in PS II, from cytochrome b559 and from the Mn site of the oxygen‐evolving center. The kinetics of re‐reduction of the primary electron donor of PS II (P680). as measured by optical spectroscopy, are re‐evaluated in light of the EPR results. We propose that the 35‐μs kinetic component is due to the reduction of chlorophyll, an alternate electron donor on the cytochrome b559 pathway, rather than to the reduction of P680. The chlorophyll/cytochrome b559 pathway has been proposed to be part of a cyclic electron transfer pathway around PS II; we suggest that photooxidation of chlorophyll is the first step leading to photoinhibition and that cytochrome b559 serves to protect PS II from photoinhibition by rapidly re‐reducing the oxidized chlorophyll (Thompson, L.K.; Brudvig, G.W. Biochemistry, 1988, 27: 6653). These results and proposals are summarized in an overall scheme of electron transfer pathways and rates in PS II.
Interpretation of the Electric Field Sensitivity of the Primary Charge Separation in Photosynthetic Reaction Centers
Authors: Moser Christopher C. , Alegria Guillermo , Gunner M.R. , Leslie Dutton P.
Published: November 2013

Popovic et al. (Biochim. Biophys. Acta, 1986, 851: 38–48) have shown that the efficiency of light‐induced charge separation in a monolayer of oriented photosynthetic reaction center (RC) is sensitive to the electric field. With the data provisionally interpreted as indicating a sensitivity of the picosecond electron transfer to field‐induced free‐energy change of the charge‐separated states, current models for the light‐induced picosecond electron transfer in the RC from bacteriochlorophyll dimer (BChl2) to bacteriopheophytin, assisted by an interposed bacteriochlorophyll monomer (BChl), are challenged to fit the results. A model presuming a simple field‐sensitive Franck‐Condon overlap between BChl2 and BChl, based on single classical vibration coupled to electron transfer, can only fit the data if the free‐energy change does not match the reorganization energy. An even simpler model presuming a rapid field‐sensitive thermal equilibrium between BChl2 and the nearby BChl fits equally well. However, a model involving a superexchange electronic coupling between the excited BChl2 and BChl, with or without similar field‐dependent Franck‐ Condon overlap factors, provides the best fits to the data and suggests that the relevant dielectric constant of the RC may be about 1 with a BChl2‐BChl energy gap of about 100 meV.
Experimental Evidence for the Anisotropic Nature of the Transient EPR Spectrum from Photosystem I Observed in Cyanobacteria
Authors: Bock Christian H. , Stehlik Dietmar , Thurnauer Marion C.
Published: November 2013

Early transient EPR spectra at room temperature, with high spectral (K‐band, 24 GHz) and time (50 ns) resolution, are presented for whole cells of deuterated cyanobacteria (Synechococcus lividus). A full range, between high and low microwave power, is covered. The true low‐power spectrum of the charge‐separated state in Photosystem I prior to electron transfer to the iron‐sulfur center acceptors could be established. The anisotropic nature of the signal has been confirmed with the observation of transient nutations at high microwave power, which indicate possible motional effects or spin dynamics.
Homonuclear Magnetization Transfer Experiments Using Isotropic and Nonisotropic Mixing Schemes
Authors: Bax Ad
Published: November 2013

The principles of homonuclear magnetization transfer via isotropic and non‐isotropic mixing schemes are discussed. Practical aspects of such experiments concern the minimization of the generation of multiple quantum coherence, minimization of rf power requirements, suppression of NOEs during mixing and minimization of relaxation losses. The analogy between these experiments and heteronuclear decoupling is briefly discussed. The efficiencies of several mixing schemes are compared and it is demonstrated experimentally that the resolution and sensitivity of the homonuclear cross polarization methods can be superior to the conventional COSY experiment.
The Structure of Pb+ Laser Centres in KMgF3 from Optically Detected Electron Spin Resonance and Electron Nuclear Double Resonance
Authors: Fockele M. , Lohse F. , Spaeth J.‐M.
Published: November 2013

Laser action of X‐rayed lead doped KMgF3 crystals from an unknown defect was recently reported by Hörsch and Paus.1 By applying optical detection of ESR and ENDOR, the structure of two very similar laser active Pb centres could be determined from the selectively measured hyperfine and superhyperfine interactions. They consist of a Pb+ substituting for a K+ next to an F− vacancy along <110> with a next nearest K+ vacancy for charge compensation in two different positions (Pb+(1) centres). The absorption band for the excitation of the laser emission at 1.4eV could be identified from the optical excitation spectra and from a special magneto‐optical ESR technique. The structural properties, as well as the explanation of their laser properties within a crystal field model, are analogous to those of Tl0(1) centres in alkali halides and Pb+(1) centres in alkaline earth fluorides. A Pb++ perturbed FA centre was also observed.
The Hydration of Vanadyl Doped into Magnesium Acetate: Single Crystal Proton ENDOR at 20 K
Authors: Atherton Neil M. , Oliva Cesare
Published: November 2013

The single crystal ESR and proton ENDOR of VO2+ doped into magnesium acetate tetrahydrate have been studied at 20 K. The VO2+ ion occupies Mg2+ sites, replacing Mg2+ together with one water molecule. Hyperfine tensors for the six protons in the three coordinating water molecules have been determined and their dipolar components used to construct a model of the proton geometry. The relation of the structure to that of {VO(H2O)5}2+ in a Tutton salt and in frozen aqueous solutions of vanadyl sulphate is discussed.
Spin‐Polarised (by CIDEP) Silicon‐Containing Radicals
Authors: Buglass John C. , McLauchlan Keith A.
Published: November 2013

Polarised ESR signals, observed from silicon‐centred radicals produced by irradiation of either di‐tert butyl peroxide or acetone in the presence of a silicon hydride, are presented and analysed. The novel feature that radical pair mechanism CIDEP can be used to accentuate weak sidebands due to radicals containing rare isotopes (here 29Si) is demonstrated. Adduct radicals formed by reaction with maleic anhydride and vinyltrimethylsilane were also studied. With the former, the adduct produced and observed in early‐time experiments remains the same at room temperature and above; in steady‐state studies this has been reported not to be so.
ESR and ENDOR Probes of Skeletal Conformations Implications for Conformations and Orientations of Chlorophylls In Vivo
Authors: Forman A. , Renner M.W. , Fujita E. , Barkigia K.M. , Evans M.C.W. , Smith K.M. , Fajer J.
Published: November 2013

ESR and ENDOR studies of sterically hindered porphyrins and chlorins provide clear evidence of the flexibility of the porphyrin skeleton, in agreement with crystallographic results. Examples considered are the cation radicals of zinc tetraphenyl‐octaethylporphyrin and of the zinc and cobaltous nitrosyl complexes of octaethyl‐ and methyl octaethyl‐chlorins. Extrapolation of the above results suggests that, in vivo, the protein pocket into which the chromophores fit and neighboring residues provide ready means of altering the conformations and properties of the pigments. These considerations can be applied to the primary acceptors of photosystems I and II: a combination of theoretical calculations, model studies and ENDOR results for the chlorophyll and pheophytin acceptors of photosystems I and II, respectively, suggests specific orientation of the 2‐vinyl substituents of the chromophores and, in addition, supports the existence of hydrogen bonding at the 9‐keto group. The implications of these results are that the protein environment helps control the orientations of the macrocycles and substituents, and thereby optimizes the relative orientations of donors and acceptors to facilitate electron transfer.
Intramolecular, Reductive Coupling of Bis‐α,β‐unsaturated Ketones
Authors: Wenkert Ernest , Moeller Peter D. R.
Published: November 2013

The reduction‐rearrangement chemistry of two rigidly held doubly vinylogous β‐diketones has been investigated. One hydrophenanthrene dienedione was a tricyclic intermediate in the Wilds total steroid synthesis and the other was prepared from the Wieland‐Miescher ketone. Lithium‐ammonia reduction yielded a cyclopropanedione and a cyclopropanoketol, respectively. Treatment of these compounds with acid furnished cyclopropane‐cleaved ketoenones.
Crystal and Molecular Structures of Triasterane Derivatives
Authors: Irngartinger Hermann , Hauck Jürgen , De Meijere Armin , Michelsen Klaus , Machinek Reinhard
Published: November 2013

The geometries of trimethylenetriasterane 1, triasteranetrione 2, and trispirocyclopropanetriasterane 3 have been determined by X‐ray crystal structure analyses. All three compounds have D3h symmetry and contain two skeletal cyclopropane rings which are symmetrically cis‐trisubstituted. The vinyl and the carbonyl substituents in 1 and 2 apparently exert about the same acceptor effect on the central cyclopropane rings and cause a small but significant lengthening of the endocyclic C‐C bonds (1.517 Å in 1 and in 2) in comparison to an average normal bond length in cyclopropanes of 1.508 Å, while in 3 no effect of the spirocyclopropane groups on the bond length in the central rings (1.506 Å) is evident The vinylcyclopropane and bicyclopropyl subunits in 1 and 3 respectively are ideally antiperiplanar and almost undistorted with respect to the parent hydrocarbons as evidenced by the bond angles and 13C‐13C coupling constants (1JCC).
Thiocyanate Ion Promoted E ⇌ Z Isomerization of a‐Bromo‐β,β‐Diactivated Electrophilic Olefins: An Apparent High Nucleofugality of NCS−
Authors: Rappoport Zvi , Avramovitch Bianca , Karni Miriam , Apeloig Yitzhak
Published: November 2013

(E)‐β‐Chloro‐α‐phenylcinnamaldehyde and (E)‐ and (Z)‐methyl α‐tert‐butoxycarbonyl‐p‐nitro‐β‐bromocinnamates in the presence of NCS− ion in acetonitrile undergo an (E) ⇌ (Z) isomerization which is faster than the vinylic substitution. (E)‐ and (Z)‐methyl α‐cyano‐β‐mesyloxy‐ (and β‐chloro‐) p‐nitrocinnamates undergo substitution under similar conditions without preceding isomerization. MO calculations gave the energy differences between the eclipsed and the perpendicular conformers of carbanions −CH2CH2X and (NC)2ČH2X (X = Cl, NCS). These rotational barriers denote the hyperconjugation stabilization energy resulting from interaction of the C‐X bond and the 2p(C−) orbital. Both HCA values are high, those for X = Cl being 15–20% higher than for X = NCS. The limited literature data on the nucleofugality of NCS− suggests that it can have a nucleofugality which is not much lower than that of Cl− or Br−. Consequently, NCS−‐catalyzed isomerizations of electrophilic vinyl chlorides and bromides, which involve a 180° internal rotation followed by a faster expulsion of NCS− than of Cl− from the initially formed intermediate carbanion Y'Y−C‐C(R)(Cl)SCN, are not unreasonable, especially if solvation, which was not included in the calculations, plays an important role in the nucleofuge expulsion process.
Propellanes 93. Diels‐Alder Reactions of Thioimido‐ and Lactamo‐Propellanes with 4‐Methyl‐1,2,4‐Triazoline‐3,5‐dione
Authors: Ashkenazi Pnina , Ginsburg David
Published: November 2013

A convenient method is described for the synthesis of γ‐lactams from various imido‐propellanes via the thio derivatives by desulfurization with tribulyltin hydride. Diels‐Alder reactions between thioimido‐, lactamo‐, and imido‐propellanes with methyl triazolinedione are compared.
Reactivity of CO2 on Clean and Hydrogen‐Covered Rhenium Single Crystal Surfaces
Authors: Chacham Ilan , Asscher Micha
Published: November 2013

The adsorption and dissociation of CO2 were studied on clean and H2‐precovered Re(0001) and Re(1120) surfaces. Molecular CO2 lies linear and parallel to the surface at crystal temperatures below 115 K, as indicated by high‐resolution electron energy loss spectroscopy. Upon heating the surface during temperature‐programmed desorption experiments, a fraction of the molecules dissociates. It is found that dissociation is more effective on the smooth Re(0001) surface than on the rough Re(1120) one. A model for a possible active site which may account for the observed structure sensitivity is discussed. At adsorption temperatures above 150 K, a sharp decrease in the dissociation probability is observed, which is explained in terms of a faster competing desorption process. H2 preadsorbed on Re(0001) was found mostly to block the adsorption of molecular CO2 and to destabilize the adsorbate. As a result, the dissociation of CO2 decreases with increasing H2 coverage.
Triplet State Interrogation in Supersonic Beams by Surface Electron Ejection
Authors: Sneh Ofer , Cheshnovsky Ori
Published: November 2013

Recently, we have developed an experimental method for the detection of triplet states generated by laser excitation in supersonic beams. It is based on electron ejection from low work‐function surfaces by metastable triplet states. We have detected both directly laser‐excited triplets and triplets generated via intersystem crossing from laser‐excited singlet states. Here, we review the applications of this method and discuss its mechanism. By comparing the laser‐induced fluorescence (LIF) spectrum and Surface Electron Ejection by Laser‐Excited Metastables (SEELEM), we have measured relative triplet formation quantum yields for several aromatic compounds. By utilizing a detector mounted on a translational stage, we could vary the distance between the pulsed laser excitation and the detector and measure the decay rates of triplets in molecular beams. The major advantage of the method is in extending the measurement of triplet lifetimes to the ∼ 1‐ms range. The combination of LIF, SEELEM, and fluorescence quantum yields enabled us to discriminate between intersystem crossing and internal conversion in isoquinoline. SEELEM is now being utilized in studying the spectroscopy and the dynamics of directly laser‐excited triplet states. Although the oscillator strength of the lowest triplet state of pyrazine is about 10−8, we have measured the spectrum and the decay rates of its various vibronic levels. Our results support the notion that surface Penning ionization is the mechanism of SEELEM. The detector is insensitive to vibrational energy (thus enabling the distinction between intersystem crossing and internal conversion). The detection sensitivity of triplets rises with the excess electronic energy and with the lowering of the surface work‐function.
Metal‐Metal Coordination Chemistry: Free Clusters of Group 12 Elements with Sodium
Authors: Heiz Ueli , Röthlisberger Ursula , Vayloyan Arthur , Schumacher Ernst
Published: November 2013

A systematic study of the mass spectra of particles formed in a supersonic nozzle expansion of Zn, Cd, and Hg vapors together with Na was performed. The particles discussed in this paper contain only one heavy atom and less than 12 Na ligands. Mass spectra from the neutral particles were generated by photoionization at five wavelengths (250, 260, 280, 300, and 320 nm) with a slit width corresponding to 16.5 and 3.3 nm, respectively. The abundances were corrected to relative differential ionization cross‐sections. Photoionization potentials for Nan M with 5 < n < 10 and M = Zn, Cd, Hg were measured. Below n = 6 no mixed clusters were found. The first abundance maximum occurred for all M at n = 8. A second maximum was with n = 16 or n = 18 (not covered in this paper). This selectivity can be rationalized by recent ab initio calculations by Fantucci et al. (J. Chem. Phys.; in press) and by jellium calculations, which give a closed shell at n = 8 with the occupations 1s21p62s2. Ionization potentials clearly show a level reversal to 1p‐2s‐1d in contrast to the homonuclear jellium with 1p‐1d‐2s. This is similar to previous results with KnMg (Kappes, M. et al., Chem. Phys. Lett., 1985, 119: 11). Molecular dynamics simulations with empirical potentials show that the heavy metal migrates to the center of a cube in the frozen asymptote of NagM. At finite temperatures and for larger central atoms, a square antiprismal arrangement seems to be more stable.
Designing Large‐Scale Adsorptive Separations
Authors: Lightfoot E.N. , Gibbs S.J. , Athalye A.M. , Scholten T.H.
Published: November 2013

Scale‐up of adsorptive separations is discussed in terms of separation economics and underlying transport theory. Examination of economic data suggests that recovery of valuable solutes, such as proteins, from dilute solution is dominated by the costs of processing large masses of unwanted materials. Separation processes are classified into concentration, fractionation, and purification, and the corresponding adsorptive operations are discussed. It appears desirable to make substantial changes in both equipment configuration and operating mode upon scale‐up, and hence careful design with a firm basis in transport theory is warranted. Existing difficulties in modelling such operations, especially at the production scale, are reviewed.
The Influence of Magnetic Pretreatment on Membrane Performance
Authors: Lin Israel J.
Published: November 2013

The paper presents some data from an experimental investigation into the use of magnetic pretreatment, with the objective of maintaining high permeate flux and high selectivity.

It was conclusively demonstrated that magnetic pretreatment inhibits and delays formation of clogging deposits on the membrane. Solid suspensoids present in the feed stock or forming in the system are removed continuously and effectively by means of a closed‐bottom hydrocyclone.
Polar Interfacial Interactions in RPLC
Authors: van Oss C.J.
Published: November 2013

The three principal forces that govern the interaction between dissolved proteins and solid (flat or particulate) surfaces immersed in a polar liquid are:

A. Lifshitz‐van der Waals forces (LW interactions)

B. Polar (hydrogen bond, or Lewis acid‐base) forces (AB interactions, which include “hydrophobic” interactions)

C. Electrostatic forces (EL interactions)

EL interaction energies can be determined via electrokinetic measurements. LW and AB interaction energies are derived from the LW component and the electron‐acceptor (γ⊕) and electron‐donor (γ⊖) parameters of the AB component of the surface tension, all three of which can be determined by contact angle (Θ) measurements on (a) hydrated layers of protein and (b) flat layers of particulate surface, using at least 3 polar liquids in each case. By this approach, the optimal conditions for attachment as well as for detachment of a given protein to a well‐characterized solid surface or particle can be determined quantitatively. Reversed phase liquid chromatography of immunoglobulin G (1) on phenyl Sepharose (2) shows that the elution of the protein, by an increasing concentration of ethylene glycol (EG) in the aqueous medium (3), occurs at the EG concentration where the value of ΔG132TOT changes from negative to positive.
Mathematical Modeling of In Situ Vapor Stripping of Contaminated Soils
Authors: Wilson David J. , Clarke Ann N. , Mutch Robert D.
Published: November 2013

Mathematical models are developed for evaluating the feasibility of in situ vapor stripping for the removal of volatile organics from contaminated soils. The models simulate both laboratory soil stripping columns and field‐scale vacuum extraction wells. Local equilibrium between the condensed and vapor phases is assumed for the volatile compounds; Henry's law or more complex isotherms may be used to describe this equilibrium. Methods for determining Henry's constants from lab data and for determining soil permeability from field data are presented. The models are used to predict the effects of well depth, Henry's constant, screened radius of well, and zone of influence radius. The use of impervious caps and passive vent wells to direct gas flow is discussed, and the effects of buried obstacles and evaporative cooling are examined. The possibility of recontamination from non‐aqueous phase liquid (NAPL) underlying the vadose zone is investigated, as is the ability of the vapor stripping technique to remove pools of NAPL floating on the water table below the vadose zone. An estimate of the maximum time required for cleanup at a site is made.
Synthesis, Characterization, and Reactivity of trans‐Ru(X)(Cl)(nbd)(dppb) (X = H, Cl; nbd = 2,5‐norbornadiene; dppb = 1,4‐bis(diphenylphosphino)butane), Including the X‐ray Crystal Structure of the Dichloride
Authors: Dekleva Thomas W. , Joshi Ajey M. , Thorburn Ian S. , James Brian R. , Evans Stephen V. , Trotter James
Published: November 2013

The ruthenium (II) diene complexes [Ru(X)(Cl)(nbd)(dppb)] (X = Cl, H; nbd = 2,5‐norbornadiene; dppb = PPh2(CH2)4PPh2) have been prepared and characterized spectroscopically. The X‐ray crystal structure of RuCl2(nbd)(dppb) (crystal data at 22°C: space group P1, a = 10.896 (1) Å, b = 15.168(2) Å, c = 10.829 (1) Å, α = 103.02(1)°, β = 107.08(1)°, γ = 81.65(1)°, Z = 2, R = 0.054 for 6420 reflections) shows an octahedral geometry at Ru, with the chloro ligands slightly distorted from a trans configuration (Cl)(1)‐Ru‐C1(2) = 168.4°); the unit cell contains two molecules of the complex and one molecule of benzene. Reaction of this complex with H2, in presence of Proton Sponge (PS, 1,8‐bis(dimethylamino)naphthalene) as base, is complicated by initial dissociation of nbd, and [Ru2Cl5(dppb)2]−‐PSH+ is the major product. A minor product, the hydrido(diene) complex trans‐RuCl(nbd)(dppb) 5, characterized spectroscopically, is more effectively synthesized from (a) trans‐Ru(H)Cl(nbd)(PPh3)2, 1, and dppb, or (b) reaction of RuCl2(dppb)‐(PPh3) with H2 in presence of nbd and PS. Complex 5 is unreactive toward H2 or CO while 1 has been shown previously to give η2‐H2 and norbornenoyl derivatives, respectively; the differences in reactivity are discussed.
Pericyclic Organometallic Reactions. Substituent Effects on Cycloadditions and Rearrangements of (η4‐cycloheptatriene)Fe(CO)3 Derivatives
Authors: Goldschmidt Zeev , Hezroni‐Langerman Dafna , Gottlieb Hugo E. , Bakal Yizhak
Published: November 2013

The mechanism of the reactions of (η4‐cycloheptatrien‐1‐al)Fe(CO)3 and its ethyleneglycol acetal with tetracyanoethylene (TCNE) was studied. The aldehyde undergoes a fast reversible 3+2 cycloaddition involving the free aldehyde‐substituted double bond, and a slow irreversible 3+2 cycloaddition at the coordinated site. The latter, σ,π‐allylic complex undergoes a slow interconversion with the corresponding 5 +2 isomer via the pericyclic [3,3]‐sigmahaptotropic rearrangement. In contrast, the acetal reacts with TCNE at the free acetal‐substituted double bond to give a single kinetic 3+2 adduct, which, under thermodynamic conditions, undergoes a [4,4]‐sigmahaptotropic rearrangement to the 6 + 2 adduct. The reaction kinetics was followed by 1H NMR. The effect of substituents on the reaction course is discussed, and the detailed mechanism of both the cycloadditions and rearrangements is described.
On Intergranular Contacts in Sedimentary Rocks
Authors: Cohen Morrel H.
Published: November 2013

In the present paper, we review the evidence that intergranular contacts in sedimentary rocks are normally much smaller than the grain size. We show that an analysis by Ghosh and Fuchs of the frequency‐dependent dielectric constant of a brine‐filled whitestone and a sandstone implies a ratio of contact radius to grain size in the 10−3 to 10−2 range. We argue that sintering is prevented in situ by strong repulsive forces arising from charging and hydration of the grain surfaces in contact with the brine within the pore space. We show, in fact, that considering only these repulsive forces consolidation is prevented, thereby reducing the question of the origin of small contacts to the question of the nature of the consolidation process in the presence of strong repulsions. We outline briefly a mechanism of consolidation, but defer detailed study of consolidation for future work.
Capillary Condensation and Surface Flow in Microporous Vycor Glass
Authors: Abeles Benjamin , Chen Liang Fan , Johnson Jack W. , Drake J.M.
Published: November 2013

Depending on the size of the pores and on the temperature and pressure of the fluid, flow in microporous materials can be through gas diffusion, surface diffusion, and viscous liquid flow driven by capillary forces or by hydraulic pressure. We have studied these flow regimes with toluene in untreated Vycor glass with a pore radius of 31 Å and Vycor glass with a pore radius of 21 Å whose pores were derivatized with C18H38. The observed increase in viscosity with decreasing pore size is ascribed to the attractive interaction of the monolayer adsorbed on the pore walls. The surface diffusion coefficient at 22°C is 5 × 10−5 cm2 s−1 for both the derivatized and underivatized Vycor, with an activation energy of 3 kcal in the temperature range 22–100°C.
Polymers in Slabs, Slits, and Pores
Authors: Whittington Stuart G. , Soteros Christine E.
Published: November 2013

When a polymer is subject to a geometrical constraint, such as being confined to a pore, there may be a loss in configurational entropy. If a linear polymer is in dilute solution in a good solvent, then it can be modelled as a self‐avoiding walk on a lattice and, in this case, it is possible to make rigorous statements about this entropy loss. We shall discuss some of these results for self‐avoiding walks in a half‐space, a slab, a prism, etc. We then examine the dependence of this entropy loss on the architecture of the polymer and ask if ring or star or comb polymers all lose the same amount of entropy when confined in the same way. It turns out that this depends on the nature of the confinement. We also look at the case where a polymer interacts with a surface and investigate how the adsorption behavior depends on the architecture of the polymer. The behavior is strikingly different in two and in three dimensions.
An Enantioselective Total Synthesis of (+)‐Elenoic Acid and the Non‐Natural Enantiomers of Tetrahydroalstonine, Aricine, and Reserpinine
Authors: Hölscher Peter , Knölker Hans‐Joachim , Winterfeldt Ekkehard
Published: November 2013

A simple enantioselective synthesis of (+)epi‐elenoic acid is reported and its transformation into (+)‐elenoic acid is described. Well‐established procedures transform these compounds into the antipodes of tetrahydroalstonine, aricine, and reserpinine.
Polarizing Nuclear Spins Via Photo‐Excited Triplet States
Authors: Schmidt J. , van den Heuvel D.J. , Henstra A. , Lin T.‐S. , Wenckebach W. Th.
Published: November 2013

In this paper we discuss two methods to achieve high nuclear polarizations at room temperature using pulsed EPR techniques and triplet spins which are optically excited by a laser pulse. The results of our experiments, which are performed on a single crystal of napthalene doped with pentacene, show that an unusually high proton spin polarization can be achieved and that the transfer occurs in a time determined by the dipolar interaction between the electron spins of the pentacene guests and the proton spins on the surrounding napthalene molecules.
On Multiple Quantum NMR of Coupled Spins in Solids
Authors: Ba Yong , Veeman Wiebren S.
Published: November 2013

It is shown that odd‐order multiple quantum coherences can be generated and detected in a system of dipolar coupled spins in a solid. The odd‐order multiple quantum coherences can be detected in various ways. In one particular experiment positive‐ and negative‐order quantum coherences can be distinguished. The theory is demonstrated by experiments on adamantane.
Macromolecule Structure Refinement by Relaxation Matrix Analysis of 2D NOE Spectra: Effect of Differential Internal Motion on Internuclear Distance Determination in Proteins
Authors: Kumar Anil , James Thomas L. , Levy George C.
Published: November 2013

Interproton distances obtained from NMR experiments are used to determine the high‐resolution structure of biopolymers. The accuracy of internuclear distances extracted from proton homonuclear 2D NOE spectra can be improved by using a complete relaxation matrix analysis, taking into account all proton dipole‐dipole interactions as well as internal motions. The effect of internal motion on interproton distances, determined from the 2D NOE cross‐peak intensities, has been examined using a new version of the mardigras program, which iteratively determines interproton distances from experimental 2D NOE peak intensities using the complete relaxation matrix. The internal motion has been specified by the ‘model‐free’ approach in terms of an overall correlation time, local correlation times, and order parameters. Using a simulation for the 58‐residue protein bovine pancreatic trypsin inhibitor (BPTI), it is shown that if the local order parameter and correlation time for a given residue differ substantially (but within the experimentally determined range of values) from the average order parameter and correlation time, the error in the interproton distance may be as large as 0.6 Å.
A Picosecond Study of the Photophysics of CdS Clusters Grown in situ in Reversed Micelles
Authors: Barzykin Alexander V. , Fox Marye Anne
Published: November 2013

A study of the dynamics of the electronic states of CdS semiconductor clusters grown in situ in reversed micelles via picosecond pump‐probe technique reveals efficient transient bleaching of the excitonic absorption. This effect is attributable mainly to hole localization near the trapped electron, with no evidence of electron ejection (forming a solvated electron) detectable despite the apparent proximity of the cluster to the included water pool. Bleaching efficiency is cluster‐size dependent, and recovery kinetics are governed by the nature of the particle surface. Similar decay behavior is observed in cluster fluorescence. Generally, the decay is characterized by two components, the faster attributed to recombination of a mobile, detrapped electron with a trapped hole, and the slower to an exchange interaction between trapped carriers. The observed strong multiexponentionality of the decay results from a distribution in trap energy levels, as well as from a distribution of distances between traps on the cluster surface. By varying preparation conditions, particles of different sizes with different surface defects can be grown in reversed micelles in a reproducible way so that their electronic properties can be controlled.
Size‐Quantized Semiconductor Particulate Films Generated in situ at Monolayers
Authors: Fendler Janos H.
Published: November 2013

Methodologies for the in situ generation of size‐quantized semiconductor particulate films (SQSPFs) are described. Cadmium, zinc, lead, and copper sulfides have been generated from their divalent metal ion precursors electrostatically attached to well‐compressed, negatively charged monolayers by the infusion of hydrogen sulfide. Cadmium selenide has been generated at positively charged dioctadecyldimethylammonium bromide monolayers floating on an aqueous subphase which contained appropriate concentrations of sodium selenosulfate and cadmium nitrilotriacetate. SQSPFs have been characterized by absorption spectra, electron microscopy, scanning tunneling microscopy, X‐ray diffraction, and electrochemical measurements. Evidence is presented for the oriented growth and preparation‐dependent rectification behavior of SQSPFs.
Formation and Photoreaction of PbI2, HgI2, and BiI3 Layered Ultrasmall Particles
Authors: Nosaka Yoshio , Fukuyama Toshiaki , Horiuchi Masato , Fujii Nobuyuki
Published: November 2013

Formation of ultrasmall particles of PbI2 in methanol was observed by means of stroboscopic stopped‐flow spectroscopy. Pulse‐laser induced electron transfer from PbI2, HgI2, and BiI3 to viologens was measured for various laser intensities and for various viologen concentrations. The electron transfer yields were 0.24, 0.08, and 0.03 for HgI2, PbI2, and BiI3, respectively. The laser‐intensity dependence differed from the previous observation for CdS and showed that the low yield is not attributable to the direct electron‐hole recombination in the particle.
Ultrafast Torsional Relaxation from the Barrier Region for an Excited‐State Isomerization in Solution: 9‐Carbonylanthracenes
Authors: Johnson Alan E. , Jarzęba WŁOdztmierz , Walker Gilbert C. , Barbara Paul F.
Published: November 2013

The class of compounds containing a carbonyl group substituted at the 9 position of anthracene have been studied as new model systems for torsional isomerization in solution. Franck‐Condon excitation yields an excited‐state distribution near the barrier for isomerization in S1. Using time‐resolved fluorescence spectroscopy, we have determined that the relaxation from the barrier region is subpicosecond for three different compounds in all solvents studied. In fact, the relaxation seems to be almost independent of solvent at room temperature, indicating that most of the local friction felt by the twisting carbonyl is internal friction rather than solvent friction. Apparently the small carbonyl is effectively shielded from the solvent by the anthracene ring so the solvent friction is not strong. In contrast to the weak solvent coupling, previously reported gas‐phase spectra indicate that the torsion may be strongly coupled to the other vibrational modes of the molecule. Energy flow out of the torsion into these other vibrational modes appears to occur on the subpicosecond time scale.
The Lowest Singlet and Triplet States of o‐Benzyne: Spin‐Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries
Authors: Karadakov Peter B. , Gerratt Joseph , Raos Guido , Cooper David L. , Raimondi Mario
Published: November 2013

The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state of o‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐plane sp2‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns corresponding to an arynic structure.
Ethynylvinylidene
Authors: Collins Charlene L. , Hu Ching‐Han , Yamaguchi Yukio , Schaefer Henry F.
Published: November 2013

Ethynylvinylidene, an energetically low‐lying isomer of diacetylene, has been studied using ab initio molecular electronic structure theory. Geometries of diacetylene, ethynylvinylidene, and the transition state between them have been optimized using the double‐ζ plus polarization (DZP) and triple‐ζ plus double polarization function (TZ2P) basis sets with the self‐consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double substitutions (CCSD), and CCSD with perturbatively included connected triple substitutions [CCSD(T)] methods. At the TZ2P CCSD(T) level of theory only single point energies have been determined at the TZ2P CCSD geometries. Harmonic vibrational frequencies and infrared (IR) intensities are reported for the three stationary points at the DZP SCF, TZ2P SCF, and DZP CISD levels of theory. For ethynylvinylidene, rotational constants, dipole moments, and dipole moment components along the principal axes are also given. This isomer is found to be an extremely shallow minimum on the C4H2 potential energy surface, and the classical barrier to rearrangement (to diacetylene) is vanishingly small at the highest level of theory.
Addition of Methyl Radical to Substituted Alkenes: A Theoretical Study of the Reaction Mechanism
Authors: Wong Ming Wah , Pross Addy , Radom Leo
Published: November 2013

Ab initio calculations at the QCISD(T)/6–311G**level have been carried out to study the addition of the methyl radical to a series of substituted alkenes, and the results analyzed with the aid of the curve‐crossing model. It is found that (a) reaction exothermicity is the main factor that dominates reactivity, (b) polar contributions to the transition states are generally small and of minor energetic consequences, and (c) the general observation that π‐electron‐accepting substituents in the alkene enhance reactivity is a secondary correlation that is a consequence of the effect of these substituents on reaction exothermicity. There is no evidence for the prevalent view that the methyl radical is generally nucleophilic towards alkenes.
NDDO‐Based CI Methods for the Prediction of Electronic Spectra and Sum‐Over‐States Molecular Hyperpolarization
Authors: Clark Timothy , Chandrasekhar Jayaraman
Published: November 2013

NDDO‐based (AM1) configuration interaction (CI) calculations have been used to calculate the wavelength and oscillator strengths of electronic absorptions in organic molecules and the results used in a sum‐over‐states treatment to calculate second‐order hyperpolarizabilities. The results for both spectra and hyperpolarizabilities are of acceptable quality as long as a suitable CI‐expansion is used. We have found that using an active space of eight electrons in eight orbitals and including all single and pair‐double excitations in the CI leads to results that agree well with experiment and that do not change significantly with increasing active space for most organic molecules. Calculated second‐order hyperpolarizabilities using this type of CI within a sum‐over‐states calculation appear to be of useful accuracy.
Radiative and Nonradiative Decay of Electronically Excited HNF and DNF
Authors: Chen Jing , Dagdigian Paul J.
Published: November 2013

Decay lifetimes for a range of HNF and DNF levels in their excited Ã 2A' electronic states, in specific K' stacks and in some cases in individual N' rotational levels, are reported. Above a threshold energy, these lifetimes are found to drop, in a K'‐ and N'‐dependent way, below the extrapolated purely radiative lifetimes. This reduction in lifetime is ascribed to the onset of nonradiative, predissociative decay. Lifetimes for individual N' rotational levels in the HNF(Ã2A') (0,4,0) and DNF(Ã2A') (0,6,0) vibrational manifolds are also reported. These are found to fluctuate in a seemingly random manner vs. N', reflecting the variation of the coupling to the dissociation continuum. The implications of these results and, from other experiments, the direct observation of NH(X3Σ−) and ND(X3Σ−) photofragments for the dynamics of the nonradiative decay of electronically excited HNF and DNF are discussed.
Photofragment Translational Spectroscopy of Fluorinated Halomethanes
Authors: Felder Peter , Yang Xuefeng , Baum Georg , Huber Robert
Published: November 2013

The primary and secondary dissociation processes of CF2Br2 and CF2BrI have been studied by photofragment translational spectroscopy in a pulsed molecular beam. The photolysis of CF2Br2 at 193 nm produces mainly the fragment pair CF2Br + Br and, in a competing primary process, CF2 + Br2. The CF2Br radicals are formed with sufficient internal energy to overcome the barrier of the unimolecular secondary dissociation to CF2+ Br. The photodissociation of CF2BrI at 248 nm also proceeds along two decay pathways involving the fission of the weak C‐I bond and the stronger C‐Br bond, respectively. The former reaction produces spin‐orbit excited iodine atoms I(2P1/2) and stable CF2Br radicals, whereas the latter reaction produces unstable CF2I radicals. The results of this study are compared with those of other workers, and an account of the ultraviolet photochemistry of fluorinated halomethanes is given.
Effective Methods for Estimation of Binding Energies in Computer‐Aided Drug Design
Authors: Warshel A. , Tao H. , Fothergill M. , Chu Z.‐T.
Published: November 2013

The development of an efficient and accurate method for computer‐aided drug design presents a major challenge for computational chemistry. The present work addresses this challenge by examining the accuracy and speed of several alternative strategies. Previous studies of the binding of ligands to the MC603 antibody and new studies of ligand binding to endothiapepsin are considered. It is found that the semimacroscopic version of the Protein Dipoles Langevin Dipoles method (the PDLD/S method) approach can provide qualitative results with reasonable execution times. Much more time is required for the linear response approximation, although it yields accurate results in some cases. Overall, it is still not clear what generation of computers will be needed for reliable and fast drug screening. Nevertheless, computer simulation is clearly useful for correlation X‐ray structures of enzyme‐inhibitor complexes with the corresponding binding energies.
Measurement of NOx, SO2, SPM, and O3 at a High Altitude Station in Northwestern Turkey
Authors: Tuncel Semra G. , Baykal A. Hadi , Tuncel Gürdal , Özer Ulviye
Published: November 2013

Temporal variation of NO, NO2, SO2, SPM (suspended particulate matter), and O3 were measured at a high altitude station in the northwestern part of Turkey. The long‐ and short‐term trends in the concentrations of measured parameters were investigated to understand sources contributing to the observed concentrations of these pollutants. Higher concentrations of O3 during summer months were consistent with higher photochemical production with increased solar flux. Seasonal patterns observed in other parameters were consistent with their source strengths during different periods of the year. O3 showed a peak at approximately 3 pm, together with SO2 and NO2. SPM, on the other hand, showed a different diurnal pattern with a maximum concentration occurring during night hours. The main sources of O3, SO2, and NO2 on Mt. Uludag were found to be transported from the town of Bursa and from a nearby ski resort. The vehicular activities on the road which connects the town of Bursa to the ski resort are also an observable source of O3 and its precursors, but not of SO2 and SPM. High concentrations of O3 were observed during night hours. Unlike daytime peaks, these peaks were sporadic and did not correlate with the SO2 and NO2. These night‐time O3 concentrations were suspected to be due to injection from the stratosphere.
Solubilization of Proteins in Membranes
Authors: Dan N. , Safran S.A.
Published: November 2013

The solubility of proteins in membranes depends both on the protein properties and membrane characteristics. In this paper, we calculate the concentration of membrane inclusions as a function of their concentration in solution and the physical properties of the membrane. We find that the membrane deformation plays a key role in determining the adsorption. When there is no thickness mismatch between the inclusions and the membrane, the effective adsorption energy decreases with the lipid spontaneous curvature and hence strongly favors protein adsorption into the membrane.
Trace Element Characteristics of Central Anatolian Obsidian Flows and Their Relevence to Pre‐History
Authors: Yellin Joseph
Published: November 2013

This report concerns the chemical characterization of obsidian sources from Central Anatolia by neutron activation analysis. The sources covered in this study were Göllü Dağ, Hotamiş Dağ, Köru Dağ, Nenezi Dağ, and Çatköy. The study reported here was undertaken with the object of obtaining an accurate “fingerprint” of the compositions of obsidian sources that would permit tracing the origin of archaeological obsidian artifacts to their sources, and in particular, archaeological obsidian recovered from prehistoric sites in Israel. The extent of the sampling within the sources was more thorough than hitherto reported and the results are of high precision and accuracy.
Properties and Photoactivity of Rhodopsin Mutants
Authors: Sakmar Thomas P. , Fahmy Karim
Published: November 2013

A variety of spectroscopic and biochemical studies of recombinant site‐directed mutants of rhodopsin and related visual pigments have been carried out. These studies have elucidated key structural elements common to visual pigments, such as a conserved disulfide bond. In addition, systematic analysis of the chromophore‐binding pocket in rhodopsin and cone pigments has led to an improved understanding of the mechanism of the opsin‐shift, and of particular molecular determinants underlying color vision in humans. Identification of conformational changes which occur upon rhodopsin photoactivation has been of particular recent concern. Assignment of these molecular alterations to specific regions in the receptor has been attempted by studying native opsin regenerated with synthetic retinal analogues or recombinant opsins regenerated with 11‐cis‐retinal. Individual molecular groups that undergo structural alterations during photoactivation have been identified. Analysis of particular mutant pigments in which specific groups are locked into their respective “on” or “off” states has provided a framework to identify determinants of the active conformation as well as the minimal number of intramolecular transitions required to switch between inactive and active conformations. A simple model for the active state of rhodopsin can be compared to structural models of its ground state to localize chromophore‐protein interactions that may be important in the photoactivation mechanism.