bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractPopovic 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

AbstractEarly 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

AbstractThe 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

AbstractLaser 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

AbstractThe 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

AbstractPolarised 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

AbstractESR 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

AbstractThe 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

AbstractThe 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

Abstract(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

AbstractA 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

AbstractThe 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

AbstractRecently, 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

AbstractA 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

AbstractScale‐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

AbstractThe 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

AbstractThe 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

AbstractMathematical 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

AbstractThe 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

AbstractThe 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

AbstractIn 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.