bluuebunny/crossref_metadata_2025 huggingface dataset:
Photosynthetic oxygen evolution: H/D isotope effects and the coupling between electron and proton transfer during transitions S2⟹3 and S3⟹S4→S0
Authors: Bögershausen Oliver , Haumann Michael , Junge Wolfgang
Published: July 2012

The oxygen evolving complex (OEC) of photosystem II (PS II) incorporates a tetra Mn‐cluster, tyrosine (Yz) and probably one histidine residue (X) as redox cofactors. Four quanta of light drive the OEC through the increasingly oxidized states S0⟹S1⟹S2⟹S3⟹S4 to yield O2 during S4→S0. It has been speculated that some oxidized cofactor abstracts hydrogen from bound water. This implies that its oxidoreduction is electroneutral and linked to its deprotonation. To identify such steps we investigated the rates of electron transfer and proton release as function of the D2O/H2O ratio, the pH, and the temperature in thylakoids and PS II core particles. Upon oxidation of X on S2⟹S3, a rise of the pH from 5 to 8 increased the rate of the electron transfer to Yz by a factor of 2.5 and substitution of D2O for H2O gave an isotopic ratio of 2.1. Contrastingly, during all other transitions, including the O2‐evolving step S4→S0, the electron transfer rate was much less sensitive to these parameters (factors of ≤ 1.4). These results suggest a kinetical steering role of proton transfer only during S2⟹S3. We propose that X* (His*?) serves as a hydrogen acceptor for bound water during S4⟹S0.
On the influence of multiple scattering contributions to the extended X‐ray absorption fine structure (EXAFS) spectra of the photosystem II manganese complex
Authors: Dittmer Jens , Dan Holger
Published: July 2012

The catalytic center of photosynthetic water oxidation is a tetra‐nuclear manganese complex which presumably is covalently bound to the polypeptides of the Photosystem II (PS II) protein complex. Extended X‐ray Absorption Fine Structure (EXAFS) spectra were simulated for various likely structural elements of the PS II manganese complex. The influence of multiple scattering contributions to the EXFAS spectra is assessed. It is found that for distances below 3.5 Å multiple scattering contributions to the EXAFS spectrum of the PS II manganese complex are presumably negligible. However, for distances above 3.5 Å multiple scattering contributions to the EXAFS spectrum of the PS II manganese complex are likely to be of relevance. We conclude that the amino acid residues (glutamate, aspartate, histidine) which are ligands of the PS II manganese complex could give rise to the EXAFS Fourier peak at about 4 Å.
Pulsed EPR experiments on radical pairs in photosynthesis: Comparison of the donor‐acceptor distances in photosystem I and bacterial reaction centers
Authors: Zech Stephan G. , Lubitz Wolfgang , Bittl Robert
Published: July 2012

Pulsed EPR spectroscopy is applied to the radical pairs PQ in bacterial reaction centers of Rhodobacter sphaeroides R‐26 and PA in photosystem I from Synechococcus elongatus. For both radical pairs an out‐of‐phase echo was observed showing a deep modulation as a function of the spacing between the two microwave pulses. From the modulation frequency the dipolar and isotropic couplings between the radical pair spins were derived. For the bacterial reaction centers a dipolar coupling of D≈︁ −121 μT was obtained, resulting in a distance of about 28.4 Å between the electron spins of the radical pair. This value is in good agreement with the distance derived from the x‐ray structure of Rhodobacter sphaeroides R‐26. From the faster modulation in the PS I sample we obtained a dipolar coupling of D≈︁ −170μT, indicating a distance of 25.4 Å between the primary donor P700 and the acceptor quinone A1. For both radical pairs, only a small isotropic coupling of J ≈︁ 1 μT, consistent with transient EPR spectra, was found.
Biomimetic constructs. De‐novo design of redox proteins
Authors: Rau Harald K. , Haehnel Wolfgang
Published: July 2012

A modular strategy for the de‐novo synthesis of four‐helix bundle proteins has been established. The individual helices and a cyclic decapeptide are purified to homogeneity after deprotection before they are coupled chemo‐ and regioselectively in predetermined orientations. The orientation of the helices relative to each other as well as of the N‐terminus is controlled by different side chain protecting groups. The new protein adapts the predicted structure of the heme binding core of the transmembrane cytochrome b to a soluble form. This protein with a molecular mass of 14,293 with antiparallel orientation of the four helices binds two heme groups in its hydrophobic interior via four histidines. Circular dichroism studies indicate a high helical content of the complex and a free energy of folding of −30.4kJ/mol. The two heme groups have midpoint potentials of −106 and −170 mV, respectively. The modular design of the de‐novo protein can easily be modified and provides the basis for detailed physical studies of molecular parameters.
Ivestigation of energy‐ and charge‐transfer processes using self‐organized porphyrin aggregates
Authors: Chernook Andrei V. , Shulga Alexander M. , Zenkevich Eduard I. , Rempel Ulrich , von Borczyskowski Christian
Published: July 2012

Self‐assembled triads and pentads are formed from Zn‐porphyrin or Zn‐chlorin dimers and pyridyl‐substituted free base of porphyrins, chlorins or tetrahydroporphyrins. A two‐point co‐ordination of the central Zn ions in the dimer with the nitrogen atoms of the pyridyl substituents of the free bases is essential for the complex formation. The complexation process has an equilibrium constant of the order of 107 M−1 at room temperature. An activation energy of the thermal dissociation of the complexes of 0.8 eV is found, which does not differ very much for different kinds of aggregates. Competing energy and charge transfer processes taking place in the aggregates have been identified by temperature dependent steady state fluorescence measurements.
Miscellanea. Femtosecond infrared spectroscopy on bacteriorhodopsin using a broad band carbon monoxide laser
Authors: Dziewior Reiner , Oilier Rolf
Published: July 2012

A pump‐probe mid infrared transient absorption apparatus was built, employing a broad band continuous wave carbon monoxide laser and an optical gating technique resulting in femtosecond time resolution.

It was used to monitor the all‐trans to 13‐cis isomerization of the retinal chromophore in Bacteriorhodopsin (BR). The C=C in‐phase stretch vibration, located at 1529cm−1 in the light adapted, unphotolyzed state BR570 served as marker band. Photoexcitation leads to a bleach centered at this wavenumber and to a partial recovery corresponding to the non unity quantum yield of the BR photocycle. On the picosecond time scale the high energy shoulder of this bleach is superimposed by newly created IR absorption strength, due to the corresponding vibrational modes of the J‐ and the K‐state.
Relativistic Effects in Heavy‐Element Chemistry
Authors: Heß Bernd Artur
Published: July 2012

The nature of relativistic effects in the electronic structure of atoms and molecules is explained and an account of theoretical methods for relativistic calculations in a quantum‐mechanical framework is given. The focus is on the requirement of reducing the number of degrees of freedom in the theory in order to obtain computationally efficient and still highly accurate approximations. Some applications illustrating the relativistic effects in coinage metal atoms and molecules are discussed.
Charge Transfer Like Complexes of Organic Dyes Adsorbed at Colloidal Silver studied by Cyclic Voltammetry, UV‐vis and SERS Spectroscopy
Authors: Nickel U. , Halbig P. , Gliemann H. , Schneider S.
Published: July 2012

After adding cationic dyes like oxazine 170, malachite green, and crystal violet to a solution of colloidal silver sometimes an absorption band appears at wavelengths > 750 nm which is neither caused by the monomeric dye nor by a coagulation of the colloid. Based on electrochemical measurements this new absorption band is assigned to a chinhydron‐like adsorption geometry of the dye. Usually this band is only observed at low concentration of the dye, whereas at higher concentration the formation of H‐aggregates occurs. There seems to be a relationship between the intensity of this band and the SERS activity.
Non‐Cottrell Behaviour of Potentiostatic Processes in Mixed Conductors with Comparable Ionic and Electronic Conductivities
Authors: Preis Wolfgang
Published: July 2012

A theoretical investigation of potentiostatic polarization processes in mixed conductors with arbitrary electronic transport numbers has been performed. As the potentiostatic polarization is accompanied by a chemical diffusion process inside the mixed conductor located between blocking electrodes, Fick's second law of diffusion has been solved for asymmetric electrochemical cells in order to obtain concentration profiles of the mobile species, current‐time relationships and the impedance of the asymmetric cell. Ohmic interfacial resistances as well as the ohmic bulk resistance owing to the total conductivity of the mixed conductor are considered. The theoretical results reveal that chemical diffusion processes in mixed conductors with comparable ionic and electronic conductivities show non‐Cottrell behaviour resulting in non‐Cottrell type current‐time relationships. These relations allow the exact description of the short‐time behaviour of potentiostatic diffusion processes occurring during chronoamperometric measurements. In addition, the non‐Cottrell behaviour of potentiostatic processes in mixed conductors with arbitrary electronic transport numbers employed in symmetric electrochemical cells is discussed.
Solvatochromism in Alcohol‐Water Mixtures: Effects of the Molecular Structure of the Probe
Authors: Novaki Luzia P. , El Seoud Omar A.
Published: July 2012

Solvatochromic behavior of 2,6‐diphenyl‐4‐(2,4,6‐triphenyl‐1‐pyridinio)‐1‐phenolate (RB), 1‐methyl‐8‐oxyquinolinium betaine (QB), sodium 1‐methyl‐8‐oxyquinolinium betaine‐5‐sulphonate (QBS), and 1‐methyl‐3‐oxypyridinium betaine (PB) was studied UV/Vis spectrophotometrically in binary mixtures of water with methanol, ethanol, 1‐propanol, 2‐methyl‐2‐propanol, ethane‐1,2‐diol, 2‐methoxyethanol, and 2‐n‐butoxyethanoI. The solvent polarity parameter, ET (in kcal/mole), was calculated from the position of the longest‐wavelength absorption band of the solvatochromic probe, and was then correlated with χw, the mole fraction of water in the mixture. Except for PB in aqueous methanol, aqueous ethanol and aqueous 1‐propanol, plots of ET versus χw are not linear because of preferential solvation of the probe by one component of the mixed solvent and, when applicable, because of solvent microheterogeneity. In the same aqueous alcohol mixtures, deviation from linearity in the ET versus χw plots increases as a function of the increasing hydrophobic character of the probe. For the same solvatochromic probe in different aqueous mixtures, the above mentioned deviation increases as a function of increasing the chain length of the alcohol. Preferential solvation of the probe is described in terms of the so called “solvent fractionation factor, ϕs”, whose value indicates the relationship between composition of the solvation shell of the probe, and composition of the bulk solvent. In the absence of solvent microheterogeneity, all values of ϕs are less than unity, which indicates preferential solvation of the probe molecule by the alcohol. An attempt to evaluate contributions from preferential solvation and solvent microheterogeneity to solvatochromism in aqueous methanol, and aqueous THF showed that the approach used by Park et al. [J. Phys. Org. Chem. 7, 757 (1994)] is not applicable.
Density Measurements for Expanded Fluid Selenium up to the Liquid‐vapour Supercritical Region by the X‐Ray Absorption Method
Authors: Hosokawa Shinya , Kuboi Takeshi , Tamura Kozaburo
Published: July 2012

Density of expanded fluid selenium was measured in the temperature and pressure ranges up to 1650°C and 700 bar including the liquid‐vapour critical point by the x‐ray absorption method. For the measurements, a new type of cell made of polycrystalline sapphire and a high‐pressure vessel of authors' own design were developed. The critical temperature, pressure and density of 1615 ± 5°C, 385 ± 5bar, 1.85 ± 0.03g cm−3, respectively, were obtained. We found that isochores bend in the semiconductor‐metal transition region. The shape of the isochore seems an elongated inverse‐S letter. The diameter of the liquid‐vapour coexistence curve is deviated from the law of the rectilinear diameter to the liquid side slightly around the SC‐M transition region and strongly near the critical point. A distinct crossover was also found in the critical exponent β; the value of β drastically changes from 0.30 to 0.10 when the critical point is approached.
The nature of valence band holes in pure and Fe‐doped NiO: An ab initio Hartree‐Fock study
Authors: Mackrodt W. C.
Published: July 2012

First principles periodic Hartree‐Fock calculations of the first ionised state of Fe0.125Ni0.875 are reported from which the Mulliken populations, charge and spin distributions and atom projected densities of valence band states provide direct evidence of Fe(3d) holes in complete agreement with recent conductivity measurements on materials with approximately the same composition. This is in marked contrast to the self‐trapped hole in NiO, also reported here, and the bound hole in Li‐doped NiO where both experiment and a recently reported calculation find holes of predominantly O(2p) character.
Defect structure and solubility range of gallium‐doped cobalt oxide)
Authors: Schönfeld D. , Laqua W.
Published: July 2012

The electrical conductivity σ of polycrystalline cobalt oxide doped with gallium oxide, Co1‐xGa2x/3O, was measured as a function of temperature (800– 1400°), oxygen activity (10−4–0.21) and composition x (0–0.125). The phase boundary of the solid solution as a function of temperature and oxygen activity was ascertained by a well‐defined change of the slope in the σ‐T‐Arrhenius plot. As a function of Ga2O3‐content, the electrical conductivity runs through more or less distinct minima at all temperatures and oxygen activities. To explain these dependencies, a defect model was developed, which contains different defect associates and a 4:1‐cluster. It emerges from a model in which associated holes contribute to the total conductivity as well as free holes. Due to this fact different mobilities result for the respective holes.
Influence of the preparation conditions on the properties of electropolymerised polyaniline
Authors: Brandl Volker , Holze Rudolf
Published: July 2012

Polymerisation conditions have a remarkable effect on the electrochemical properties of polyaniline. Care has to be taken to ensure identical conditions for electrochemical experiments when films electropolymerised differently are examined in comparison, especially when films are prepared potentiodynamically. UV‐vis spectra of PANI films show a hypsochromic shift with increasing potential, and a theory of this phenomenon is presented. It is based on the assumption of a distribution of chain segments of different effective conjugation length.
Conformational fluctuation and ion permeation of lipid bilayers doped with azobenzene derivative: Infrared spectroscopic study
Authors: Tanaka Motomu , Kohno Yoshiumi , Yonezawa Yoshiro , Shimidzu Takeo
Published: July 2012

We have fabricated the liposome dispersions of L‐α‐dimyristoylphosphatidylcholine (DMPC) doped with an amphiphilic azobenzene derivative, 4‐octyl‐4'‐(5‐carboxypentamethyleneoxy)azobenzene (8A5). The conformation of the DMPC liposome membrane was changed reversibly according as photoisomerization of 8A5. Fourier transform infrared spectroscopy (FTIR) was applied to study interactions between dopants and host lipids. Dependence of FTIR spectra of the CH2 stretching bands on the molar ratio of 8A5 and on temperature indicates that 8A5 molecules induce a large fluctuation on the molecular packing of the membrane around the phase transition temperature. We have carried out K+ ion permeation measurements across the liposome membrane in varying molar ratios at T = 283–303 K. A large overshoot and the following relaxation of the ion permeability were observed with UV irradiation at T = 293 K. We have proposed that the specific photoresponse is operated by two factors: the expansion of pores (‘quasi‐channels’) due to trans‐ti‐cis isomerization of 8A5 and the increase of the boundaries between gel phase and liquid‐crystalline phase around the phase transition temperature. FTIR spectroscopy yields useful information about the conformational fluctuations of the DMPC bilayer induced by photoisomerization of 8A5.
Competitive counterion binding to poly(alkylene phosphate)s related to natural teichoic acids. The Poisson‐Boltzmann approach
Authors: Ostrowska‐Czubenko Jadwiga , Czubenko Marek
Published: July 2012

The problem of competitive metal ion binding to poly(alkylene phosphate)s, considered as synthetic models of natural teichoic acids, was analysed theoretically applying the Poisson‐Boltzmann (PB) equation to a cylindrical cell model. There were calculated the radial distribution functions as well as the degree of ion binding of mono‐and divalent metal ions to polyphosphate ions in solutions of different concentrations ranging from 10−5 M to 10−2 M and containing different amounts of M⊕ and M2+ counterions. For comparison, analogous calculations were performed also for poly(styrenesulfonate)s. The results indicate the preferential binding of divalent metal ions to polyphosphate ions over monovalent ones. The selectivity for M2+ ions is concentration dependent and increases with decreasing the polymer solution concentration. It results from PB calculations, that condensation of divalent metal ions at the surface of polyion chains seems to be responsible for preferential binding of biologically important metal ions, for example magnesium, to natural teichoic acids and their synthetic analogues.
Erratum: “Primary reaction of oxygenic photosynthesis”
Authors: Witt H. T.
Published: July 2012

The orientation of Fig. 19 is wrong. It has been turned upside down. The correct orientation is depicted below. (In the reprints of the article Fig. 19 is correctly orientated.)
State‐to‐state studies of intramolecular dynamics and unimolecular reactions
Authors: Kuhn B. , Boyarkin O. V. , Rizzo T. R.
Published: July 2012

Double‐resonance vibrational overtone excitation spectra of small molecules, in combination with laser‐induced fluorescence product detection, provide information on both their energetics and intramolecular energy redistribution dynamics. Application of this approach to measure OH stretch overtone spectra of HOOH and NH2OH near their dissociation threshold provides estimates of the O‐O and N‐O bond energies of 17051.8±4 cm−1 and 21620±20 cm−1 respectively.

In symmetrical molecules, such as HOOH, the double‐resonance excitation technique allows us to prepare molecules in either pure OH stretch overtone levels or combination levels in which the excitation energy is divided between two identical OH stretch oscillators. Comparison of the resulting vibrational overtone linewidths indicates no clear difference between the pure OH stretch and the local‐local combination levels. However, comparison of the energy transfer rates from pure OH stretch vibrations and those in which one quantum of energy is put in a low frequency mode reveals faster rates from the latter. This observation clearly indicates that coupling strengths depend markedly on the vibrational character of the coupling partner and not simply on the total energy.

Comparison of the OH stretch overtone spectra of the structurally similar molecules HOOH, NH2OH, and CH3OH reveal similar rates of vibrational energy redistribution, despite the difference in the density of states in these molecules. This observation reinforces the notion that energy flow between the OH stretch and dark background states is not statistical but rather is controlled by coupling to a small subset of background states.
Intramolecular energy transfer from isotope selective overtone spectroscopy by vibrationally assisted dissociation and photofragment ionization
Authors: Hippler Michael , Quack Martin
Published: July 2012

Overtone spectroscopy by vibrationally assisted dissociation and photofragment ionization (OSVADPI) has been coupled with the detection of fragments in a mass spectrometer allowing isotope selective overtone spectroscopy (ISOS) on naturally occurring isotopomer mixtures. The room temperature CH‐chromophore spectra of CF2HCl (polyad component Nj = 41 near 11430 cm−1) and of chloroform (Nj = 21 near 5942 cm−1 and Nj = 31 near 8726 cm−1) have been investigated in this way and discussed in relation to intramolecular energy transfer. The features of the central Q‐branch regions are assigned with resolution of the Cl‐isotopomers, and Q‐branch maxima and Cl‐isotopomer shifts are determined. The effect of the strong Fermi resonance, mixing CH‐stretching and bending modes, on the position of vibrational hot band transitions is demonstrated. We describe in detail the extension of our technique into the near and mid infrared range with a high resolution optical parametric oscillator system which has a wide range of potential applications as a general infrared spectroscopy with isotope and mass selection (IRSIMS).
The influence of hindered rotations on recombination/dissociation kinetics
Authors: Wagner Albert F. , Harding Lawrence B. , Robertson Struan H. , Wardlaw David M.
Published: July 2012

The variational canonical flexible transition state theory expressions for the thermal reaction rate constant are reviewed and then applied to the barrier‐less recombination reactions H + CCH, H + CF3, and CH + H2. The rate constant expressions can classically accommodate any description of the hindering potential governing relative orientation changes between the reactants at a given distance. The distance held fixed during the relative orientation changes can be fully optimized. The three applications display comparable and large variations between the harmonic oscillator and free rotor limits of the hindering potential. The actual hindering potentials are all derived from similar ab initio electronic structure calculations. With these potentials, the rate constant for H + CF3 is somewhat free‐rotor like, the rate constant for CH + H2 is mostly harmonic‐oscillator like, and the rate constant for H+CCH is largely in between these two limits.
The calculation of vibrational eigenstates by MINRES filter diagonalization
Authors: Yu Hua‐Gen , Smith Sean C.
Published: July 2012

A spectral filtering method reported recently [Faraday Disc. 102, 17 (1996)], which utilizes the optimal expansion of the Green operator in a finite Lanczos subspace, is used to construct a new scheme for the calculation of bound vibrational states by filter diagonalization. The method requires storage of only two real vectors in the primary representation, i.e., those required to generate the Lanczos subspace. Calculations for the HO2 molecule show that the new scheme efficiently generates converged eigenvalues within a nominated energy window which maybe scanned through the bound spectrum, and utilizes just a single Lanczos subspace (generated once only). It not only is more efficient than the regular Lanczos algorithm for computing high‐lying eigenstates, but also has the interesting property of eliminating duplicated and ghost eigenvalues, which can cause problems in interpretation of the regular Lanczos spectrum.
Rigidity factors in unimolecular reactions
Authors: Troe Jürgen
Published: July 2012

Unimolecular reactions with loose activated complexes, i.e. simple bond fission and the reverse bond formation processes, are conveniently characterized by phase space theory (PSD, which accounts for the maximum attraction potential between the fragments, and by additional rigidity factors which are the result of the anisotropy of the potential. This article summarizes recent calculations of specific rigidity factors frigid (E, J) and of thermal rigidity factors frigid (T) for a series of different potentials such as, e.g., ion‐dipole, dipole‐dipole, or standard valence potentials. The calculations have been made by rigorous statistical adiabatic channel (SACM) in combination with classical trajectory (CT) calculations.
Quantum and classical calculations of adiabatic and nonadiabatic capture rates for anisotropic interactions
Authors: Nikitin E. E. , Troe J.
Published: July 2012

A uniform and systematic approach to the calculation of the capture rate constant is presented in the case of most typical long‐range intermolecular interactions. We consider ion‐linear dipole rotor, ion‐axially symmetric quadrupole rotor and two dipole linear rotors with additional isotropic ion‐induced dipole and induced dipole‐induced dipole interactions. The results for the capture rate constants are given in analytical form for nonadiabatic and adiabatic capture of classical as well as quantum rotors in very wide range of dynamic and kinematic parameters of the capture events.
Measurement of the rate constant k(E) of the unimolecular H2 elimination from ethane radical cations in a reflectron mass spectrometer
Authors: Güthe Felix , Weitzel Karl‐Michael
Published: July 2012

A brief review of the literature concerning the H2 loss reaction from ethane radical cations is given. The reaction most likely proceeds by tunneling of H atoms through a transfer barrier. In this contribution a new experimental approach is presented, which combines the high energy resolution of a zero kinetic energy (ZEKE) electron spectrometer with the high kinetic resolution of a reflecting ion time of flight spectrometer in a coincidence experiment. With this technique it is possible to measure the rate constant k(E) of the title reaction an ion energy resolution of 10 meV directly over three orders of magnitude. The implementation of this technique will ultimately enable the study of D2 loss from isotopically labeled ethane‐d6 cations.
Unimolecular decomposition of large organic radicals with low reaction thresholds: Decomposition and reversible isomerization of n‐pentyl radicals
Authors: Tsang Wing , Bedanov Vladimir , Zachariah Michael R.
Published: July 2012

The decomposition kinetics of n‐pentyl radicals in the high temperature regime where steady state distributions are not achieved and reversible isomerization to decomposing 2‐pentyl radicals is of importance has been analyzed through the solution of the time‐dependent master equation. The reactions are characterized by low activation thresholds and lead to large rate constants that vary with time. Particular attention is paid to branching ratios for direct decomposition of the n‐pentyl radical and the decomposition that follows reversible isomerization to the 2‐pentyl species. The behavior of the system under a variety of conditions is described and the use of branching ratios as a possible means of characterizing the reactions in a manner that is compatible with present methods for employing kinetic data for simulating complex chemical phenomenon is considered. At higher temperatures, rate constants and branching ratios collapse to limiting values. The combination of limiting and the onset of steady state behavior complicates the situation at lower temperatures. The overall behavior of such systems is highly dependent on the magnitude of the barrier to isomerization. The extent of departures from the high pressure‐branching ratios are defined.
Kinetics and energy partitioning in complex‐forming reactions between neutral species
Authors: Smith Ian W. M.
Published: July 2012

A short survey is presented of some recent experiments on the kinetics and dynamics of complex‐forming reactions. Differences and similarities between bimolecular reactions in which a pair of products are formed and collisionally assisted association reactions are considered. Some emphasis is placed on those reactions whose rates are determined by ‘capture’ on the long‐ and medium‐range attractive potential acting between the reagents. Such processes are discussed in relation to specific reactions, such as those of CN with O2 and C2H2, OH with butenes, and CH with O2, C2H2, N2, CO and H2, for which rate constants have been determined over most of the temperature range from 10 to 1000 K. The CH + H2 system is considered in some detail as it serves as a prototype of complex‐forming reactions in which there is competition between collisionally assisted association (to CH3 radicals) and bimolecular reaction (to CH2 + H).
Collisional energy transfer probabilities in the deactivation of highly vibrationally excited aromatics
Authors: Hold U. , Lenzer T. , Luther K. , Reihs K. , Symonds A.
Published: July 2012

Full sets of experimentally determined “collisional energy transfer probability distributions”, P(E', E), have become available first time for the deactivation of highly vibrationally excited large molecules. These data from KCSI measurements (kinetically controlled selective ionization) on the vibrational relaxation of toluene ( = 50000 cm−1) and azulene ( = 30000 and 20000 cm−1) are discussed with emphasis on systematic aspects. Under proper conditions the KCSI measurements become self‐calibrating. This provides i.a. very high quality results on <ΔE>, free from possible calibration errors. Detailed second moments are given and consequences on energy distributions during relaxation are discussed. The P(E', E) distributions show extended wings of finite probabilities for transfer of large ΔE. Their relative role is characterized. A double‐exponential representation of P(E' E) is useful, mainly for atomic and shall molecular collider gases. However, a more universal and better analytical representation of P(E', E), using one parameter less, is recommended. The fraction of “super‐collisions” (independent from details of definition) is very small.
Progress on the modeling of the collisional energy transfer mechanism in unimolecular reactions
Authors: Nordholm Sture , Börjesson L. E. B. , Ming Liu , Svedung Harald
Published: July 2012

The RRKM theory of unimolecular reaction rates is a statistical mechanical theory based on an assumption of microcanonical equilibrium in the reactant phase space. The energy transfer in reactant medium collisions was originally described by a canonical strong collision assumption, i.e., an assumption of full thermal equilibration in each collision. In our work we first introduce a microcanonical strong collision assumption which gives the RRKM theory a consistent form. We then introduce parameterizations of the degree of weakness (nonergodicity) of the collisions. A concept of collision efficiency is defined. The weakness of the collision is expressed in terms of reduced subsets of active reactant and medium degrees of freedom. The corresponding partially ergodic collision theory (PECT) yields physical functional forms of the collisional energy transfer kernelP(E', E). In order to resolve the energy and temperature dependence and the dependence on interaction strength a multiple encounter theory is introduced (PEMET). Initially each encounter may be described by a semiempirical PECT model. Eventually the encounters may be resolved by quantum dynamical calculations of the semiclassical or CAQE (classical approach/quantum encounter) type. Simple statistical collision models only distinguish between “hits and misses”. In reality the energy transfer efficiency exhibits characteristic fall off with increasing impact parameterb.Thisb‐dependence can be explicitly accounted for in the master equation for the reaction rate coefficient.
Photodissociation and size analysis of (HBr)n clusters
Authors: Baumfalk R. , Buck U. , Frischkorn C. , Gandhi S. R. , Lauenstein C.
Published: July 2012

Hydrogen bromide clusters that are generated in an adiabatic expansion with Ar are size‐selected by scattering from a He or a Ne beam and a subsequent separation by a velocity selector. The complete fragmentation pattern for electron impact ionization of clusters n = 3,5,7,10, and 15 is obtained for electron energies of 70 eV. Up to cluster sizes of n = 15 strong fragmentation is observed. Based on these results the cluster sizes are measured as a function of different source conditions. For the same experimental conditions the (HBr)n clusters are photoionized at 243.1 nm. A time‐of‐flight mass spectrometer with low extraction fields is used to obtain the translational energy distributions of hydrogen atoms. Several distinct features are observed which sensitively depend on the cluster size. At small cluster sizes a tail appears towards lower energies in the fast hydrogen kinetic energy distribution which is caused by few collisions with the cluster molecules. Peaks at higher energies are attributed to internally excited HBr molecules which gain their energy in collisions with photofragmented H atoms. Small clusters exhibit pure rotational excitation with averaged J = 7, while the larger ones show vibrational excitations to v = 1 with J = 8 and v = 2. Finally, we observe a narrow H component peaking near zero velocity and exhibiting an isotropic angular distribution. This effect appears only at larger cluster sizes and is attributed to a complete caging of the photofragments.
Intramolecular charge transfer, photoisomerisation and rotational reorientation of trans‐4‐dimethylamino‐4′‐cyanostilbene in liquid solution
Authors: Il'Ichev Yurii V. , Zachariasse Klaas A.
Published: July 2012

In the excited singlet state of trans‐4‐dimethylamino‐4′‐cyanostilbene (DCS) ultrafast intramolecular charge transfer (ICT) takes place followed by trans‐cis photoisomerisation from a highly polar charge transfer (CT) state. The photoisomerisation of DCS is studied as a function of temperature in three homologous solvent classes, nalkanes, n‐alkylnitriles and n‐alcohols. The photoisomerisation activation barrier Etc increases with viscosity for DCS in n‐alkane solvents, whereas in the polar n‐alkylnitriles and n‐alcohols the separate influence of viscosity and polarity on the isomerisation reaction cannot be disentangled. The activation barrier for the trans‐cis photoisomerisation is considerably larger in acetonitrile and methanol, highly polar solvents of low viscosity, than in the nonpolar n‐heptane, due to the strongly polar character of the CT state as compared to the transition state of the isomerisation reaction. This polarity trend is reversed in solvents with the lowest polarity and the highest viscosity of the homologous series of the n‐alkylnitriles and the n‐alcohols. Simultaneous operation of polarity, viscosity and temperature as factors changing the potential energy surfaces, leads to isoviscosity plots that appear to indicate that the trans‐cis photoisomerisation of DCS does not depend on solvent viscosity. From measurements of fluorescence decay times as a function of pressure (to 4000 bar) it follows, however, that the photoisomerisation rate constant ktc decreases by a factor of 2.0 for a 3.3 fold viscosity increase, resulting in a fractional viscosity dependence, ktc = η−α with α = 0.61. When the increase of the permittivity with pressure is taken into account, α becomes 0.53. From the observation of a linear relationship between the rotational relaxation time τrot and the viscosity, 1/τrot=η−1.0 it is concluded that the Stokes‐Einstein‐Debye equation holds for the overall rotation of the DCS molecule, showing that viscosity is a good measure of solvent friction and that the fractional viscosity dependence with α < 1 is not due to a nonlinear viscosity behaviour.
Scavenging of hydrocarbon radicals from flames with dimethyl‐disulfide II. Hydrocarbon radicals in fuel‐rich low‐pressure flames of acetylene, ethylene, 1,3‐butadiene and methane with oxygen
Authors: Hausmann M. , Homann K.‐H.
Published: July 2012

The new technique of free‐jet condensation/scavenging with dimethyl‐disulfide has been applied for the quantitative analysis of hydrocarbon radicals and carbenes in flat premixed hydrocarbon/oxygen flames burning with fuel‐rich mixtures at 27 mbar. The results are reported as profiles of mole fractions of the radicals. The limit of detectability was about 1·10−7 mole fraction. Qualitatively the radicals are very similar with the four fuels, but there are large differences in their quantities. While in the methane flame only C1 and C2 radicals were present in detectable concentration, higher radicals up to naphthyl could be detected with the unsaturated fuels. Although methyl could not be determined quantitatively, it was found to be the major hydrocarbon radical in all flames. Methoxy radical was only found in the methane flame. Whereas, for example, C2, C3H2 and C3H are typical high‐temperature species, C2H is surprisingly formed at the beginning of the oxidation zone at relatively low temperature. Vinyl and the vinyl‐type radicals C2nH3 (n = 1 to 4) are in equilibrium with acetylene and the polyynes (C2nH2) at maximum flame temperature and in the burned gas. Since phenyl peaks always after benzene, it is concluded that it is mainly a degradation product from benzene and other lower aromatics. The mechanism of formation of C2H and C2, the equilibria of the C2nH3 and the relation between aliphatic and aromatic radicals are discussed.
Preferential solvation of some silver(I) salts in iso‐dielectric mixtures of methanol and N‐Methyl‐2‐pyrrolidinone at 30°
Authors: Varadarajan T.K. , Ramakrishna T.V. , Kalidas C.
Published: July 2012

The solvation behavior of various silver(I) salts, viz. silver (I) iodate, bromate and sulfate in iso‐dielectric solvent mixtures of methanol and N‐methyl‐2‐pyrrolidinone (NMP) has been investigated using Gibbs transfer energies and solvent transport number measurements. The Gibbs transfer energies of the salts were split into their individual ionic contributions using both negligible liquid junction potential method (nLJP) and the reference electrolyte method using tetraphenylarsonium tetraphenylborate (TATB). The results show a significant hetero‐selective solvation of all salts beyond XNMP = 0.5 with silver ion being selectively solvated by NMP and the anions by methanol.
Measurements of thermal conductivity of aqueous LiCl and LiBr solutions from 293 to 473 K at pressures up to 100 MPa
Authors: Abdulagatov I.M. , Magomedov U.B.
Published: July 2012

High‐pressure thermal conductivities of H2O+LiCl and H2O + LiBr have been measured with a parallel‐plate apparatus. Data cover the pressure range from 0.1 to 100 MPa at temperatures from 293 to 473 K. The concentrations studied were 0.025, 0.05, 0.10. 0.15, 0.20, and 0.25 mass fraction of salts. The estimated accuracy of the method is about ± 1.6%. The pressure, temperature, and concentration dependence of the thermal conductivity has been studied. Measurements were made on six isobars, namely: 0.1, 20, 40, 60, 80, 100 MPa. The thermal conductivity shows a linear dependence on pressure and concentration at all isotherms. Along each isobar a given concentration shows the thermal‐conductivity maximum at temperatures about 413 K. The measured values of thermal conductivity are compared with the results of other investigators.
NMR study of liquid and supercritical benzene
Authors: Asahi Nobuyoshi , Nakamura Yoshio
Published: July 2012

Density, spin‐lattice relaxation time and self‐diffusion coefficient of benzene have been measured under vapor‐liquid coexistence from room temperature up to above the critical temperature. Measurements were made for four samples with the density in the supercritical region being respectively 0.302, 0.250, 0.154 and 0.101 g cm−3. The density has been determined by the NMR one‐dimensional projection technique, and the Carr‐Purcell‐Meiboom‐Gill method has been adopted to the measurement of the self‐diffusion coefficient. The results obtained have been discussed in terms of hard sphere models and various contributions to the relaxation rates have been separated and the correlation times for each molecular motion have been estimated at 470 K. It has been shown that the spin‐rotational relaxation is dominant near and above the critical temperature.
Raman spectral study of WCl6 in alkali chloride melts
Authors: Carountzos George , Kontoyannis Christos G. , Østvold Terje
Published: July 2012

The chemistry of tungsten chlorides in the LiCl/KCl eutectic has been investigated by Raman spectroscopy between 400 and 500°. It has been found that W(VI) is reduced to W(V) directly by the free Cl− ion in the melt and the octahedral complex ion WCL−6 is formed. Addition of metals (Al, Zn, Pb, W) causes further reduction of W(V). Depending on the metal added WCl2‐6 or W metal can be obtained.
The heterogeneous reaction of HONO and HBr on ice and on sulfuric acid
Authors: Seisel Sabine , Rossi Michel J.
Published: July 2012

The heterogeneous reactions of HONO with HBr on ice, solid and supercooled liquid H2SO4 solution (40–95 wt%) were studied in the temperature range 180 to 200 K for the solid substrates (ice, H2SO4) and 210 to 270 K for the liquid solutions of H2SO4 using a Knudsen flow reactor.

The uptake coefficient of HONO onto frozen aqueous HBr solutions at 190 K was determined and resulted in a mean value of γ = (2.3 ± 1.2)·10−2. On ice the HBr uptake rate is high with a mean value of γ = 0.32±0.12 between 180 and 200 K and is unaffected by the presence of HONO. The uptake of HONO onto ice is a function of the HBr concentration in the condensed phase and for the highest HBr flows used an uptake coefficient of γ = 2.2·10−2 was found.

On frozen H2SO4 solutions the uptake coefficient of HBr varies with the concentration of the solution from about 0.25 at 10 wt% to less than 1·10−4 in the absence and 5·10−4 in the presence of HONO at 95 wt%. The uptake of HONO in the presence of HBr was found to vary with the HBr concentration and to be approximately a factor of two lower than on the ice surface.

On liquid H2SO4 the uptake coefficients of HBr and HONO both strongly depended on the concentration of the solution. HONO shows the greatest interaction at 95 wt% with γ = 2·10−2 and decreases to about 5·10−3 at 40 wt%. In comparison HBr only shows a weak interaction of γ of approximately 5·10−4 at 95 wt% and a strong interaction of γ = 4·10−2 at 40 wt%.

On all three substrates the concurrent uptake of HONO and HBr was found to be reactive forming the only identified bromine containing reaction product BrNO. The variation of the substrates from strong acid liquid to solid neutral provides information about the nature of the heterogeneous interaction.
Singlet and triplet state quenching of aromatic cations by molecular oxygen
Authors: Wirp Christoph , Brauer Hans‐Dieter , Bendig J.
Published: July 2012

Rate constants of fluorescence quenching by molecular oxygen, kSq, of 4 acridizinium ions and of 7 protonated heterocyclic compounds as well as protonated 6‐aminochrysene have been determined in acetonitrile at room temperature. They adopt values in the range between 3.0×108 M−1 s−1 and 2.9×1010 M−1 s−1. Singlet oxygen (1O2) efficiencies for the oxygen quenching of the singlet state, ϕSΔ, and the triplet state, ϕTΔ, could be determined for most of the compounds. The results indicate that fluorescence quenching can be described here as competition between energy transfer and a process involving the second excited triplet state, kSq shows at first approximation a correlation with the energy gap between the singlet state and the second excited triplet state. Quenching of the first excited triplet state by oxygen is ruled mainly by energy transfer. Rate constants, kTq, could be determined for 5 compounds and vary between 6.0×108 M−1 s−1 and 1.1 × 109 M−1 s−1.
Magnetic field effects on the luminescence of p‐phenylenediamine derivatives
Authors: Sacher M. , Grampp G.
Published: July 2012

A positive magnetic field effect (MFE) on the luminescence of three different N,N,N',N'‐tetraalkyl‐p‐phenylenediamines (TAPD) has been detected by field‐modulated MARY (MAgnetic Field Effects on Reaction Yields) spectroscopy in a binary solvent mixture of toluene/dimethylsulfoxide containing more than 20% vol. dimethylsulfoxide and in neat dimethylsulfoxide. The p‐phenylenediamine derivatives' luminescence in these solvents could be increased by application of small external magnetic fields in the order of less than 30 mT (1T = 104G) and the B1/2 values could be measured. In a comparative experiment the N,N‐dimethylaniline (DMA) fluorescence did not show such a behaviour. In conclusion a reaction scheme involving an electron transfer process between the p‐phenylenediamines and the solvent dimethylsulfoxide is suggested.
Experimental verification of gas phase bromine enrichment in reactions of HOBr with sea salt doped ice surfaces
Authors: Kirchner U. , Benter T. H. , Schindler R. N.
Published: July 2012

Significant gas phase bromine enrichment is experimentally verified on flowing gaseous HOBr over ice surfaces doped with sodium halides of sea salt composition. It is argued that formation of Br2Cl− in the condensed phase followed by its dissociation with release of Br2 into the gas phase accounts for this phenomenon.
Consideration of the thermodynamic stability of iodine species in the Bray‐Liebhafsky reaction
Authors: Stanisavljev Dragomir
Published: July 2012

The standard free energy of formation of IO* and HIO2 in water solution at 298 K are estimated to be ΔG0IO· = 120 kJ/mol and ΔGHIO2 = −75 kJ/mol. Values of standard reduction potentials for half‐reactions HIO2 + 2H++2e− → HIO + H2O, IO* + H+ + e− → HIO, IO−3 + 3H+ + 2e → HIO2 + H2O and HIO2 + H+ + e → IO* + H2O are calculated to be E0 = 1.35 V, E0 = 2.3 V, 0.92 V and E0 = 0.4 V. Energetic requirements during the process of oxidation of iodine in acidic media are introduced as an important criteria in examination of the models of the Bray‐Liebhafsky reaction mechanism.
Formation of Ca‐O clusters and their protonation and hydration in direct laser vaporization
Authors: Liu Xiang‐Hong , Zhang Xiao‐Guang , Wang Xiu‐Yan , Lou Nan‐Quan
Published: July 2012

Calcium oxide clusters produced in the direct laser vaporization of different precursor solid samples CaO, Ca(OH)2 and CaCO3 have been investigated by a time‐of‐flight mass spectrometer. Besides [Ca(CaO)n]+ and [(CaO)n]+ clusters, considerable protonated clusters [H(CaO)n]+ and hydrated clusters [H(CaO)n(H2O)m]+ have also been found in the TOF mass spectra. The characteristic of the abundance distribution of the clusters shows that the formation process of calcium oxide clusters should proceed along the growth course of the rock salt structure, which is further supported by the reactivity of [H(CaO)n]+ clusters with H2O produced in the direct laser vaporization.
Complexing ability of monensin anion for alkali metal cations in binary mixtures of dipolar aprotic solvents
Authors: Wagner‐Czauderna E. , Rzeszotarska J. , Orłowska E. , Kalinowski M. K.
Published: July 2012

Cyclic voltammetric studies have been carried out on monensin anion complexes with the alkali metal cations in binary mixtures of dimethylsulfoxide with acetonitrile, acetone and hexamethylphosphoric triamide. respectively. The log Ks values obtained for the complexity constants can be well described by empirical relation log Ks = αDN + β, where DN stands for the effective donor number of a given solvent mixture and α and β mark the regression coefficients. This result is in agreement with the predominant role of cation solvation. The complexes show a sharp stability maximum for the Na+ complex in all media under investigation.
Size dependence of bond length and binding energy in palladium and gold clusters
Authors: Krüger Sven , Vent Stefan , Rösch Notker
Published: July 2012

All‐electron density functional calculations on a series of highly symmetric palladium clusters up to nuclearity 147 yield an essentially linear correlation between the average bond length and the average coordination. The average binding energy per atom is found to be proportional to the inverse cluster radius and thus mainly determined by the decreasing surface to volume ratio. Comparison to recently obtained results for the corresponding series of gold clusters confirms the wider validity of these size dependence relationships.