bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evidence for delocalization of the triplet state 3P680 in the D1D2cytb559‐complex of photosystem II
Authors: Kamlowski A. , Frankemöller L. , van der Est A. , Stehlik D. , Holzwart A. R. 
Published: July 2012

AbstractSpin polarized transient ESR spectra of 3P680 in D1D2cytb559‐complexes of Photosystem II are studied as a function of temperature. The spin polarization as well as the rise time of the ESR signals are characteristic of triplet formation via recombination from the primary radical pair P+•680Pheo−•. Below 100K the well‐known spectrum of 3P680 is observed with zero‐field splitting (zfs) parameters associated with the triplet state of monomeric chlorophyll‐a (Chl). Between 100 K and 200 K the spectral pattern changes drastically and above 200 K a considerably narrowed triplet spectrum (i.e. reduced zfs parameters) is observed which suggests a delocalization of the triplet excitation. When the temperature‐dependent spectra are normalized to the same triplet yield, isosbestic points are observed indicating that the spectra are weighted sums of the limiting case spectra at low and high temperature (10 K and 250 K, respectively). This is taken as evidence that a transition between two triplet states of different electronic origin occurs. The coefficients of this superposition follow a sigmoidal temperature dependence which suggests a distribution of activation energies for the triplet delocalization process in accordance with a glass‐like distribution of protein matrix conformations. The narrowing of the spectrum at high temperature can be interpreted as thermally activated delocalization of the triplet excitation over at least two Chl units with different orientations with respect to the PS II reaction center, viz. one oriented with its plane tilted 30° with respect to the membrane plane and the other with its plane perpendicular. The similarities and differences in the cofactor arrangement of PS II and purple bacteria reaction center are discussed.
Photophysical and photochemical properties of porphyrin aggregates
Authors: Siggel U. , Bindig U. , Endisch C , Komatsu T. , Tsuchida E. , Voigt J. , Fuhrhop J.‐H. 
Published: July 2012

AbstractIn aqueous media substituted protoporphyrins and amphiphilic tetraphenylporphyrins aggregate to fibers, tubules, helical ribbons and sheets. Six different types are presented, characterized by the excitonic shifts in the electronic absorption spectra. On excitation, the aggregates from octopusporphyrin show fluorescence and population of the triplet state. In other aggregates fluorescence, triplet state or both are effectively quenched.
X‐and K‐band transient EPR of the light induced radical ion pairs in photosynthetic model systems
Authors: van der Est A. J. , Fuechsle G. , Stehlik D. , Wasielewski M. R. 
Published: July 2012

AbstractThe light induced electron transfer in the complex 2‐tetraalkylphenylenediamine‐Zinc porphyrin‐2‐naphtho‐quinone (TAPD‐ZnP‐NQ) is investigated using transient EPR at X‐band (9 GHz) and K‐band (24 GHz). Spin polarized spectra are presented for the radical pair TAPD+•NQ−• which is generated by light excitation of the complex at 30 K in 2‐methyl‐tetrahydrofuran and the liquid crystal BDH E7. In the liquid crystal, the complex is partially ordered and the spectra depend on the orientation of the sample with respect to the magnetic field. A qualitative interpretation of the spectra on the basis of the coupled, correlated radical pair (CCRP) model is presented. The spectra of the partially oriented samples clearly show that: (i) the exchange interaction, J, makes a substantial contribution to the coupling between the radicals and is of the same order of magnitude as the dipole‐dipole coupling (ii) when this coupling is taken into account, the experimental spectra are consistent with the expected geometry of the complex. This system is one of the few cases in which the two couplings are of similar magnitude and allows their influence on the polarization pattern to be investigated. The value of J≈0.1 mT obtained from the spectra is at least two orders of magnitude larger than that found in the analogous radical pair, P+•Q−•, in photosynthetic reaction centres, although the average distance between the spins in the two systems differ at most by a factor of 1.5. Possible reasons for this difference are discussed.
Novel methodology for topological studies of photosynthetic membrane protein complexes
Authors: Vater J. , Heinze K. , Friedrich B. , Kablitz R , Blokesch A. , Irrgang K.‐D. , Thiede B. , Salnikow J. 
Published: July 2012

AbstractNovel techniques have‐been developed for the investigation of photosynthetic membrane protein complexes from the thylakoid membrane of spinach. The protein components of photosystem II were fractionated by a solid phase extraction procedure using Chromabond cartridges. The cytochrome b6f complex was dissociated with high concentrations of the nonionic detergent n‐dodecyl‐β‐D‐maltoside. Subsequently the subunits of both membrane protein complexes were separated by high resolution reversed phase HPLC. Nearest neighbour relationships were investigated in the cytochrome b6f complex by molecular cross‐linking with o‐phthaldialdehyde. A methodology has been developed to identify the cross‐linking sites on the molecular level by tryptic digestion of the protein conjugates and separation of the fragments by high resolution HPLC. Matrix Assisted Laser Desorption Ionization‐Mass Spectrometry (MALDI‐MS) was used as a sensitive tool to identify the protein components and crosslinked peptide fragments of photosystem II and the cytochrome b6f complex.
GEQUAC, an excess Gibbs energy model for simultaneous description of associating and non‐associating liquid mixtures
Authors: Egner K. , Gaube J. , Pfennig A. 
Published: July 2012

AbstractA physically founded model for the excess Gibbs energy, GEQUAC (Group‐surface Explicit QUAsiChemical theory), presented here, allows the description of associating as well as non‐associating liquid mixtures. This is achieved by explicit accounting for the different poles of the molecular surface between which strong interactions take place in a mixture. The model is tested against results of Monte‐Carlo computer simulations performed in lattice systems. Comparison of GE, HE and TSE determined from GEQUAC, computer simulations, and experiment shows that the thermodynamic behaviour of mixtures containing polar as well as hydrogen‐bonding components can be modeled.
Thermodynamic properties of the molten Tl‐GeTe4 system
Authors: Nakamura Yoshio , Kamiyama Takashi , Itoh Shinji 
Published: July 2012

AbstractFrom the electromotive force method, the chemical potential or activity of Tl in the molten Tl‐GeTe4 system has been measured as a function of composition and temperature. The chemical potential of Tl has then been separated in terms of partial entropy and enthalpy, the both quantities showing an abrupt change from negative to positive values around the stoichiometric composition corresponding to Tl8GeTe4. The integral molar entropy and enthalpy, ΔSm and ΔHm, show sharp minima at the composition Tl8GeTe4 where the electrical conductivity has a sharp minimum and the thermoelectric power changes its sign. From these results it is suggested that the strong interaction between Tl and Te leads to form a chemical entity Tl2Te and the observed thermodynamic properties can be interpreted as a mixture of Tl2Te and unbounded atomic Ge and Te in the GeTe4‐rich side and or atomic Ge and Tl in the Tl‐rich side.
Dielectric constants, refractive indexes and polarizations for 1‐Alcohol +Heptane mixtures at 298.15 and 308.15 K
Authors: Sastry N. V. , Valand M. K. 
Published: July 2012

AbstractNew experimental data on dielectric constants, refractive indexes for the binary mixtures of heptane + 1‐pentanol, + 1‐hexanol, + 1‐heptanol, + 1‐octanol, 1‐decanol and + 1‐dodecanol at 298.15 and 308.15 K are reported. The molar and orientation polarizations were calculated for the pure and mixture components. The excess dielectric functions were estimated. The variation of correlation factor and free energy of mixing has been examined over the whole range of 1‐alcohol mole fractions for all the binary mixtures. The results are interpreted in terms of molecular interactions.
Observation of the nucleophilic displacement (SN2) reaction F− + CH3Br → CH3F + Br− Induced by dissociative electron capture in binary van der waals clusters
Authors: Lehmann Lars , Matejcik Stefan , Illenberger Eugen 
Published: July 2012

AbstractWe report on the SN2 reaction F− + CH3Br → FCH3 + Br− in binary clusters composed of CH3Br and C2F6 in a crossed electron/molecular beam experiment. The reaction is induced by resonance dissociative electron attachment to the C2F6 component where the nucleophile F− is generated at an energy (around 3.8 eV) where CH3Br does not capture electrons to form Br−. The results indicate that the cross section for the SN2 reaction is independent of the total excess energy which can be varied between 1.7 and 5 eV in the present system.
Variable reaction coordinate direct RRKM theory
Authors: Klippenstein Stephen J. , Allen Wesley D. 
Published: July 2012

AbstractVarious aspects of the direct implementation of the variable reaction coordinate formalism for evaluating the kinetics of barrierless reactions are considered. A new discussion of the decoupling of the conserved and transitional modes within this formalism is provided with sample results for NCNO suggesting an optimum simple procedure for implementing this decoupling. The effort involved in replacing analytic potentials with direct ab initio determinations in the Monte Carlo‐based evaluation of the transitional mode partition functions is illustrated through sample calculations for Cl− +CH3Cl, CN + O2, CN + NO, and 1CH2+CO. Some procedures for improving the convergence properties of such direct statistical evaluations are also suggested. A brief summary is given of a procedure for directly evaluating the effects of anharmonicities and certain rovibrational couplings on the density of states for the complex. The results from our previous direct variable reaction coordinate RRKM theory study for the dissociation of singlet ketene are slightly revised on the basis of the improved decoupling scheme. The results for both the dissociation rate constant and vibrational distributions are in quantitative agreement (i.e., within 20% throughout the energy ranges considered) with the corresponding recent experimental results. Finally, a possible procedure for making greater use of the ab initio data, and perhaps also obtaining improved convergence, is summarized.
On nascent angular‐momentum distributions in complex‐forming bimolecular reactions
Authors: Olzmann M. 
Published: July 2012

AbstractA detailed modelling of a complex‐forming bimolecular reaction requires the knowledge of the nascent energy and angular‐momentum distribution of the intermediate complex. A method is presented to calculate the distribution of the orbital angular momenta as a result of reactive collisions in a radial potential. For the case of thermal collisions, analytical solutions are obtained. Reactions of photochemically generated species are also considered. The method is applied to the reaction of CH4 with O+2 and O(1D), respectively.
Picosecond kinetics of trans‐cis‐photoisomerisations: From jet‐cooled molecules to compressed solutions
Authors: Schroeder Jörg 
Published: July 2012

AbstractThe understanding of photoisomerisation reactions involving large amplitude motion is discussed with special emphasis on the merits of systematic studies over wide ranges of physical conditions. It appears that the friction β in such cases is proportional to macroscopic solvent viscosity η, and the original Kramers theory is sufficient to describe the effect of friction in the range from intermediate to strong damping, i.e. from low viscosity fluid to compressed high viscosity liquid solution. This one‐dimensional model may be connected to multidimensional unimolecular rate theory to obtain a representation of the density dependence of the rate coefficient down to the thermal collisionless regime. The apparent failure of this approach in the case of trans‐stilbene can be traced to modifications of the excited state PES by the solvent. This interpretation is consistent over the entire range of physical conditions studied, starting at the jet‐cooled isolated molecule and solvent cluster and ending in compressed liquid solution at very high viscosities. It is shown that detailed information about the PES is urgently needed to provide safer ground for a profound understanding of the photoisomerisation dynamics.
In situ Raman spectroscopy for the study of high‐pressure reactions
Authors: Kessler W. , Luft G. , Zeiß W. 
Published: July 2012

AbstractAn optical high‐pressure cell has been developed for spectroscopic investigations of rapid high‐pressure reactions. It can be coupled with commercial Raman spectrometer. The measuring cell is suitable for use at pressures of up to 2000 bar and temperatures of up to 200°. The advantages of in situ Raman spectroscopy are demonstrated in the course of an investigation of the decomposition of tertiary butyl peroxypivalate in solution. This organic peroxide is a standard initiator for the radical high‐pressure polymerisation of olefins for which it is used at pressures of 1300 to 3000 bar and temperatures of 160 to 190°. The rate of decomposition was measured within a wide pressure and temperature range. From the results the half‐life, the frequency factor, the energy of activation and the activation volume of the peroxide decomposition reaction were determined. The data thus obtained were compared with data in the literature which had been obtained by other methods.
Strain‐birefringence of a polyelectrolyte gel
Authors: Hahn O. , Woermann D. 
Published: July 2012

AbstractResults of measurements of the stress‐optical coefficient and the Young elasticity modulus of ‐SO3− and ‐CH2‐SO3− polyelectrolyte gels are reported. Upon stretching the gels become birefringent. The birefringence is assumed to be caused by a partial orientation of optical anisotropic structures formed by highly cross‐linked regions which are embedded in nonbirefringent regions with a lower concentration of cross links. These results give support to a proposed structural model of the gels which takes into account small‐angle and ultrasmall‐angle X‐ray scattering data as well as information about the transport properties of the gels in form of membranes.
Adsorption and selective oxidation of H2S on alumina powders: In‐situ UV‐VIS studies in a differential reactor
Authors: Melsheimer J. , Lee J.K. , Schlögl R. , Böhm M.C. 
Published: July 2012

AbstractIn‐situ UV‐VIS spectroscopy in transmission and reflection mode as well as on‐line GC and TPD MS were performed on pressed and powdered wafers made from industrial aluminas in different microreactor cells at 400 K. These investigations were carried out on activated samples using a continuous gas flow of He containing either H2S (1%) or SO2 (0.5%) or H2S (1%) with SO2 (0.5%) or O2 (0.5 and 5.0%).The adsorbed reactants lead to an absorption band at 290 nm (H2S and SO2). This absorption band could be attributed to chemisorbed species.Using a differential microreactor for transmission spectroscopy, a conversion of ca. 10% was measured in the steady state and an attribution of the different visible absorption bands to

to

(H2S oxidation by O2) and

to

(modified Claus reaction, 2H2S + SO2  (3/x) Sx + 3H2O) seems plausible. The species S may be responsible for the stationary conversion during the modified Claus reaction.
Intracluster electron and proton transfer in ionic ortho‐xylene/dimethylamine aggregates
Authors: Ring S. , Koch S. , Baumgärtel H. 
Published: July 2012

AbstractHeteroclusters of ortho‐xylene (OX) and dimethylamine (DMA) were studied by resonance‐enhanced two‐photon ionization spectroscopy (R2PI). The obtained mass spectra consist mainly of reaction products from dissociative electron and dissociative proton transfer reactions in the ionic aggregates. In the 1:1 aggregate, a competition between both reaction pathways is observed. Proton transfer, being the favoured reaction pathway in the 1:1 aggregate after resonance enhanced two‐photon ionization, is suppressed when going to higher clusters as a consequence of structural and energetic changes in the aggregates. Due to the high reactivity of the ionic aggregates, intermolecular fragmentation is only observed after intracluster reaction.
A dynamic 15N NMR study of kinetic hydrogen/deuterium isotope and tunnel effects on the triple proton transfer in crystalline 3,5‐dimethylpyrazole
Authors: Aguilar‐Parrilla Francisco , Limbach Hans‐Heinrich , Klein Oliver , Elguero José 
Published: July 2012

AbstractUsing dynamic solid state 15N CPMAS NMR spectroscopy (CP≡cross polarization, MAS≡magic angle spinning) the kinetics of the degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of 15N‐labeled polycrystalline 3,5‐dimethylpyrazole (DMP) have been studied in a wide temperature range. At high temperatures, rate constants of the various isotopic HHH, HHD, HDD, and DDD transfer reactions are obtained in the millisecond timescale by lineshape analysis of partially deuterated doubly 15N‐labeled DMP. At low temperatures, the kinetics were followed by magnetization transfer methods in the laboratory frame. In order to suppress artifacts arising from 15N‐spin diffusion these experiments were performed on singly 15N labeled DMP, partially diluted in the non‐labeled material. As multiple kinetic hydrogen/deuterium isotope effects on triple proton transfer reactions have not yet been studied these effects are modeled theoretically for the single barrier case involving a concerted proton motion and for the triple barrier case where the three protons are transferred stepwise. The experimental kinetic isotope effects obey the rule of the geometric mean, i.e. kHHH/kHHD ≈︁ kHHD/kHDD ≈︁ kHDD/kDDD ≈︁ 3.6 i.e. ≈︁ kHHH/kDDD ≈︁ 47 at 300 K, which is indicative of a single barrier where all hydrons loose zero‐point energy in the transition state. At low temperatures, strong deviations from an Arrhenius behavior are observed for all isotopic reactions, indicating incoherent triple hydron tunneling processes, which are described in terms of a modified Bell tunneling model.
Pressure/composition isotherms of proton conducting SrYb0.05Zr0.95O2.975/H2O by means of nuclear resonance reaction analysis
Authors: Hempelmann R. , Eschenbaum J. , Altmayer M. , Groß B. , Grambole D. , Herrmann F. , Nagengast D. , Krauser J. , Weidinger A. 
Published: July 2012

AbstractFilms of SrYb0.05Zr0.095O2.975 were prepared by sol‐gel processing. Water vapour pressure/hydrogen composition isotherms were recorded ex‐situ comprising pressures between 0 and 84.5 kPa and temperatures between 873 and 1073 K. Nuclear Resonance Reaction Analysis (NRRA) was used to determine the hydrogen concentration and, in addition, depth profiles of hydrogen concentration. The chemical potential of interstitial hydrogen in oxides could be determined; from its temperature dependence thermodynamical constants were calculated. We observe two hydrogen fractions with a site energy difference of 20 kJ/mol. A size effect of the thermodynamic properties is observed below a thickness of 350 nm: nanofilms exhibit higher solubility than bulk material.
NMR studies of catalytic reaction of methanol absorbed in trisodium 12‐molybdophosphate crystal
Authors: Ishimaru Shin'Icbi , Ikeda Ryuichi , Nakamura Nobuo 
Published: July 2012

AbstractThe molecular dynamics and the catalytic reaction of methanol absorbed in 12‐heteropoly compound Na3PMo12O40 were studied by 1H, 2H, 13C, and 31P solid state wideline and/or MAS NMR spectra. Stoichiometric compounds Na3PMO12O40·nCH3OH where n = 6 and 9 were obtained under constant partial pressures. The presence of two kinds of methanol molecules, mobile and almost rigid, was revealed in crystals. Production of dimethylether and reduction of heteropoly anions were observed in the specimen heat‐treated at 200°C. We succeeded to detect an ester‐like species between heteropoly anion and methanol which can be assigned to an intermediate product of the reaction. Based on this finding we proposed a microscopic reaction mechanism of the catalytic reaction.
Colloidal assemblies used as templates to control the size, shape and self organization of nanoparticles
Authors: Pileni M. P. 
Published: July 2012

AbstractIn this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders.Self‐assembly of silver and silver sulfide nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced and, in these aggregates, the particles are highly organized and form pseudo‐crystals with a face‐centered cubic structure for various particle sizes.
Chemistry of nanosized silica‐coated metal particles‐EM‐study
Authors: Giersig Michael , Liz‐Marzán Luis M. , Ung Thearith , Su Dangsheng , Mulvaney Paul 
Published: July 2012

AbstractColloidal gold and silver are difficult to manipulate. The surfaces are normally coated with organic acids or polymers, and conventional methods for thin metal oxide coatings normally result in clumping and heterocoagulation. We have recently developed a method to silica‐coated gold and silver particles which avoids any particle coalescence. The method can be applied to a wide range of nanoparticles [1,2]. In this report we present some results from studies into chemical reactions of nanosized coated metal particles using high resolution transmission electron microscopy. The chemical reactions were carried out on silica‐coated silver and gold nanoparticles in aqueous solution. The reaction of a single Ag@SiO2 nanoparticle with I2 to form AgI was monitored directly by transmission electron microscopy. As well the dissolution of Au cores by cyanide ion led to the creation of monodisperse, hollow, nanosized silica particles.
Structure determination and homogeneous optical properties of CdS/HgS quantum dots
Authors: Mews A. , Banin U. , Kadavanich A. V. , Alivisatos A. P. 
Published: July 2012

AbstractThe structural and optical properties of heterogeneous semiconductor nanoparticles consisting of CdS and HgS are investigated by High Resolution Electron Microscopy (HRTEM) and selective spectroscopy like Hole Burning (HB) and Fluorescence Line Narrowing (FLN). The HRTEM study shows that epitaxy is possible in nanocrystals, provided the crystallites have well defined faceted shapes to begin with. From the HB‐ and FLN experiments homogeneous absorption and fluorescence spectra are calculated. It could be shown that the absorption is coupled to HgS‐like phonons (250 cm−1) whereas the emission frequency is closer to the LO phonon frequency of CdS.