bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ionic motions and phase transitions in solid trimethylethylammonium tetrafluoroborate studied by 1H, 19F, and 14N NMR, powder X‐ray diffraction, and differential scanning calorimetry
Authors: Ono Hiroshi , Ikeda Ryuichi , Ishida Hiroyuki 
Published: July 2012

Abstract1H, 19F, and 14N NMR, powder X‐ray diffraction, and differential scanning calorimetry (DSC) were studied for trimethylethylammonium tetrafluoroborate, (CH3)3NC2H5BF4. Five solid phases, named I, II, III, IV, and V in the order of decreasing temperature, were obtained in the temperature range 77‐630 K. The transition temperatures and the corresponding enthalpy changes determined by DSC were 222 K (0.16 kJ mol−1), 239 K (1.8 kJ mol−1), 293 K (2.1 kJ mol−1), and 362 K (4.5 kJ mol−1). X‐ray powder patterns showed that Phase I and II form an NaCl‐type cubic lattice (a = 10.14 Å, Z = 4) and a tetragonal lattice (a = 8.59, c = 6.24 Å, Z = 2), respectively. The NMR study revealed that the cationic orientation in Phase I and II is dynamically disordered. Moreover, since the self‐diffusion of anions and the isotropic rotation of cations were observed in Phase I, this phase can be considered as a mesophase very close to the plastic phase. In Phases II, III, IV, and V we have detected the isotropic reorientation of anions, the anisotropic tumbling of cations, the C'3 reorientation of the whole (CH3)3N group about C‐N bond axis, and the C3 reorientation of the CH3 groups. The motional parameters were evaluated for these cationic and anionic motions.
Fermi resonance structure and femtosecond quantum dynamics of a chiral molecule from the analysis of vibrational overtone spectra of CHBrClF
Authors: Beil Andreas , Luckhaus David , Quack Martin 
Published: July 2012

AbstractThe rovibrational spectra of bromochlorofluoromethane (CHBrClF) were measured at intermediate (0.1 cm−1) and high resolution (0.001 cm−1) between the far infrared (20 cm−1) and the visible range of the spectrum (14000 cm−1) by interferometric Fourier transform techniques and diode laser techniques in stationary samples and supersonic jets. All fundamentals and numerous combinations and overtone transitions could be assigned and their band positions and integrated strengths were determined. A quantitative interpretation of the complex multiple resonance structure of the CH chromophore spectra (about 50 assigned bands) is given in terms of an effective anharmonic CH‐stretching‐bending Hamiltonian and in terms of vibrational variational calculations on a 3‐dimensional grid using ab initio (MP2 and B3LYP) and empirically adjusted potential hypersurfaces for the CH chromophore normal coordinate subspace. The systematic analysis of anharmonic resonances of the alkyl CH chromophore in symmetric top (C3v) and asymmetric top (Cs) molecules is herewith extended to chiral molecules. We provide a detailed analysis of the chiral Cs symmetry breaking coupling in the effective Hamiltonian (coupling constant |k′sab| =(20 ± 10) cm−1) and in the potential (Csab and further terms) and their relationship. The implications for the femtosecond intramolecular dynamics for CHBrClF are discussed in terms of quantum dynamical results for time‐dependent populations, survival probabilities, wavepackets and also time‐dependent symmetries and their violations (formal a′ and a′′ species for vibrational Cs and l quantum number for vibrational C∞v symmetry). We have also developed an improved synthesis of CHBrClF, which is suitable for the preparation of large quantities (several mol) of the pure (racemic) compound, and have demonstrated enantiomer separation by gas chromatography.
Intramolecular dynamics of small molecules ‐ empirical evaluation of adiabatic and nonadiabatic corrections
Authors: Ogilvie J. F. 
Published: July 2012

AbstractFrom published analyses of vibration‐rotational spectra of diatomic and linear triatomic molecules, we evaluated the term coefficients Ykl, and various Zkl in terms of mechanical (related to vibration and rotation of the nuclei about the centre of mass), adiabatic, and nonadiabatic vibrational and rotational effects (as the associated electrons follow imperfectly the nuclei). We propose generalisations about relative orders of magnitudes of mechanical, adiabatic and nonadiabatic effects that pertain to intramolecular dynamics of free small molecules, and discuss the importance of taking into account all these effects in either comparing results deduced from experiment with those drawn from computations of molecular electronic structure or deducing structural information from spectral parameters.
Homogeneous ice nucleation observed in single levitated micro droplets
Authors: Krämer B. , Schwell M. , Hübner O. , Vortisch H. , Leisner T. , Rühl E. , Baumgärtel H. , Wöste L. 
Published: July 2012

AbstractHomogeneous ice nucleation rates in single isolated micro droplets of water are determined under various atmospheric conditions inside an electrodynamic Paul‐trap. The droplet size is measured by angle‐resolved detection of the scattered light from a Helium‐Neon laser. The freezing process is detected by a sudden change of the depolarization ratio which is an indication for the formation of nonspherical particles.The experimentally determined homogeneous ice nucleation rates JLS are JLS = (2.2±0.4)·108 cm−3 s−1 at T = (−37.2±0.15)°C and JLS = (5.6±1.0)·107 cm−3 s−1 at T = (−37.7±0.15)°C.These values are much more precise as determined by measurements inside a cloud expansion chamber. The method of single particle observation eliminates complications arising from these nucleation rate measurements. The results underline the theory that one single ice germ of certain size is sufficient to induce the freezing of the whole droplet. The depolarization ratio of the scattered intensity in the parallel and perpendicular polarization plane Is/Ip from the frozen particles exhibits a broad distribution with a mean value of Is/Ip = 0.3.
Trapping and charge stabilization in chlorosome containing bacteria: Comparative study on Chloroflexus aurantiacus and Chlorobium limicola
Authors: Schmidt Kristiane , Trissl Hans‐Wilhelm 
Published: July 2012

AbstractTrapping and charge stabilization in intact cells of Chloroflexus aurantiacus and Chlorobium limicola were studied by means of photovoltage kinetics and fluorescence emission spectra upon excitation in the chlorosome. In Chloroflexus two electrogenic phases with time constants of 100 ps for trapping (P+ H− formation) from the core‐complexes and 530 ps for charge stabilization (QA reduction) were found. In Chlorobium there were also two electrogenic phases of 105 ps and 650 ps. The later phase most likely represents the charge stabilization on the iron‐sulfur center, Fx. A comparison of stationary fluorescence spectra of both species indicates that the trapping time from the core pigments in Chlorobium is significantly faster than in Chloroflexus. This proves the existence of at least two intermediary acceptors between the primary donor (P 840) and Fx in the Chlorobium reaction center. In both species the photovoltage signal in the low excitation limit appeared after a lag phase of 55 to 65 ps, which is ascribed to the energy transfer from the chlorosome to the core pigments.
Primary charge separation. The primary processes of bacterial photosynthesis — ultrafast reactions for the optimum use of light energy
Authors: Zinth W. , Arlt T. , Wachtveitl J. 
Published: July 2012

AbstractFemtosecond spectroscopy is used to study the molecular mechanisms of the primary electron transfer. Data on native and mutated reaction centers of Rhodopseudomonas viridis show, that primary electron transfer is an ultrafast stepwise reaction. The electron is transferred via a chain of pigments: In a first reaction step the electron is transported from the special pair to the accessory bacteriochlorophyll with a time constant of ≈ 3 ps. A second, faster reaction carries the electron with 0.65 ps to a bacteriopheophytin. Experiments on mutated reaction centers with strongly modified reaction times yield additional information on energetics, reorganisation energies and electronic coupling of the reaction center. A consistent theoretical treatment of the data shows that standard non‐adiabatic theory describes well the primary electron transfer process. It also reveals that the realised reaction parameters optimise the reaction centers for highest possible quantum yield.
Effects of zwitterionic detergents on the primary donor of bacterial reaction centers
Authors: Müh Frank , Rautter Joachim , Lubitz Wolfgang 
Published: July 2012

AbstractThe effects of zwitterionic detergents on the spectroscopic properties of the primary electron donor P of photosynthetic reaction centers (RCs) from the purple nonsulfur bacterium Rhodobacter sphaeroides are investigated. When RCs are solubilized with N‐(n‐alkyl)‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonates (sulfobetaines) the maximum of the characteristic near‐infrared absorption of P, λmax, is shifted to lower wavelengths at room temperature as compared with nonionic detergents. While the range of the blueshift (λmax = 850 – 862 nm) is identical for different sulfobetaines, the dependence of the shift on the detergent/RC ratio varies systematically with the number of carbon atoms in the n‐alkyl‐chain. N‐lauryl‐N,N‐dimethyl‐amine‐N‐oxide (LDAO), a zwitterionic detergent with a less polar head group, is only able to induce blueshifts of λmax down to 859 nm at much higher detergent/RC ratios. ENDOR experiments performed on the primary donor radical cation P+• reveal that two distinct states of P exist in such samples. Thus the optical bandshifts are caused by transitions between two spectroscopic species, called P866 and P850. We interpret the detergent effect as an electrostatic influence of the detergent head groups on the equilibrium between two protein conformations which in turn affect the structure of the primary donor.
ENDOR and pulsed EPR studies of photosynthetic reaction centers: Protein‐cofactor interactions
Authors: Lendzian F. , Rautter J. , Käß H. , Gardine A. , Lubitz W. 
Published: July 2012

AbstractThe influence of specific protein‐cofactor interactions on the electronic structure of the primary donor cation radical P+• and the acceptor anion radicals Q−•A and QB−•B in wild type and mutant reaction centers of photosynthetic bacteria is investigated by ENDOR and Pulsed EPR techniques. The results show that hydrogen bonds to the primary donor have a strong effect on the distribution of the unpaired electron over the two BChl halves of the special pair, P+•. A correlation between the rate of reduction of P+• by cytochrome C2 and the spin density distribution within the dimer is found. ESEEM made it possible to detect 14N nuclear quadrupole resonances for Q−•A and Q−•B that are assigned to nitrogen atoms of ligating amino acids (histidines and peptide backbone). The deduced hyperfine couplings indicate significant delocalization of spin density onto these residues. This indicates an active role of these ligands in the electron transfer from QA to QB.
Magnetic investigations on a valence‐delocalized dinuclear Fe(II)‐Fe(III) complex
Authors: Saal C , Mohanta S. , Nag K. , Dutta S. K. , Werner R. , Haase W. , Duin E. , Johnson M. K. 
Published: July 2012

AbstractThe magnetic properties of a valence delocalized mixed valent Fe(II)‐Fe(III) complex have been investigated. On the basis of magnetic susceptibility measurements and NIR absorption spectroscopy the double exchange parameter was determined to B = 940cm−1 while for the isotropic exchange interaction an antiferromagnetic contribution of, J = −100 cm−1 was obtained. The results were further discussed with regard to vibronic interaction. For the vibronic‐interaction parameter A only an upper bound of 2500 cm−1 could be given.
Self‐organized electron transfer as a possible element in the photosynthetic process
Authors: Pohlmann L. , Tributsch H. 
Published: July 2012

AbstractAs energy fluxes in several systems far from thermodynamic equilibrium can be largely enhanced under nonlinear conditions permitting local entropy export, this should also be possible on a more microscopic scale for biological electron transfer processes. As a precondition a molecular feedback loop must exist, which transfers a small fraction of the energy, which drives the reaction, through the surrounding medium to assist the electron transfer in a non‐linear way. This can lead to microscopic self‐organization, i. e. to a temporary dissipative structure which may significantly improve electron transfer through a non‐equilibrium electron distribution and a decrease of the activation barrier. The problem is theoretically modelled as a Kramers approach in which the friction term may become active. A formula for “self‐organized” electron transfer is obtained which yields — for finite activation barriers — significantly higher rates than the Marcus formula. The presented model may help to explain how biological catalysts have evolved the ability of generating favourable energy barriers for multi‐electron transfer near the thermodynamic overall potential.
On the Chemical Interpretation of the One‐Electron Density Matrix of Some Ionic Solids: LiH, LiF, and LiFHF
Authors: Asthalter Tanja , Weyrich Wolf 
Published: July 2012

AbstractTwo‐dimensional cuts through one‐electron density matrices of three ionic solids in the position representation were calculated from restricted Hartree–Fock wavefunctions. The basis‐set quality was assessed by comparison of calculated Compton profiles/reciprocal form factors with experimental ones. The off‐diagonal regions of the density matrix contain the main information about chemical bonding in the crystal. The decomposition of the total density matrix into band contributions shows that the valence band always dominates in those regions. Furthermore, one‐electron density matrices of small subunits of the crystal and of symmetrically orthogonalised crystal wavefunctions were calculated. Their similarity and differences with respect to the solid are discussed.
Intermetallic Phases at the Cathodically Polarized Interface Platinum/Polycrystalline Yttria Stabilized Zirconia
Authors: Barbi G. B. , Chiodelli G. , Magistris A. , Manara A. 
Published: July 2012

AbstractA deep cathodic polarization has been applied to a Platinum/Polycrystalline Yttria Stabilized Zirconia interface under low oxygen pressure. In the temperature range 973–1273 K the Electromotive Force recorded after the current cut off did not show any rest potential at a level corresponding to nucleation of a zirconium metal phase, likely because such process is kinetically unfavoured with respect to that of formation of intermetallic phases, whose stability introduces a limitation of the local oxygen chemical potential depression.Two rest potentials were found at less negative potentials which levels correspond to intermetallic phases which very high stability can be accounted for d–d orbitals bonding, as described by the Engel‐Brewer correlation.In some EMF decays it was observed the occurrence of a third rest potential, located at ∼ −450 mV (reference pure oxygen) that could be attributed to a transitory presence of a Pt–Zr–O phase.
In Situ Determination of Acidity Level in Concentrated Phosphoric Acid Solutions by Potentiometric Method at Imposed Current Intensity
Authors: Hammouti B. , Oudda H. , Elmaslout A. , Benayada A. , Bessiere J. 
Published: July 2012

AbstractThe device constituted by two past electrodes: ferrocene and orthochloranil at fixed current potentiometry, can be used as a good sensor for the in‐situ determination of phosphoric acid concentration (2.0–14.7 M). Good agreement was observed regarding the values obtained with our method and the spectrophotometric values in synthetic phosphoric media. Different factors affecting the variation of potential at imposed current such as current intensity, temperature, stirring rate of solution and H+ ion concentrations are studied. The potential variations measured by our device between ferrocene and orthochloranil electrodes have the same values as those obtained by the voltamperometric curves. A new acidity function Ri(H), which represents the determination of the H+ activity has been introduced and compared to Strehlow Ro(H) and Hammet Ho functions. A good agreement has been observed between the Ro(H) and Ri(H) functions.
Self‐Diffusion Coefficient and Apparent Molar Volume Studies of Crown Ethers in Aqueous (D2O) and CDCl3 Solutions
Authors: Patil K. J. , Heil S. R. , Holz M. , Zeidler M. D. 
Published: July 2012

AbstractWe have measured the self‐diffusion coefficient and density properties of 15‐crown‐5 and 18‐crown‐6 ethers in aqueous (D2O) and CDCl3 solutions in the range of 0 to 6 m concentration of ethers. Some measurements for 1 m crown in D2O with various concentrations of alkali‐metal bromides are also reported. It has been observed that self‐diffusion coefficients of the crowns are small and decrease with increase in concentration of the crown both in D2O and CDCl3 solutions. The density data in D2O were used to obtain the apparent molar volumes of the crown φv as a function of concentration. It has been observed that φv goes through a minimum at about 2 m of crown concentration. Similar measurements and calculations of 1 m crowns in D2O in various salt solutions indicate that D varies peculiarly as a function of salt concentration. In case of KBr solutions, D increases initially and goes through a flat maximum at about 1–2 m of KBr concentration. The apparent molar volumes of the salts calculated in 1 m 18‐crown‐6 and 1 m 15‐crown‐5 solutions exhibit anomalous dependence on salt concentration. These results are explained on the basis of solute‐solute, solute‐solvent, ion‐crown and ion‐ion interactions in solution phase.
Kinetics of the Reactions of NCO Radicals with NO and NH3
Authors: Becker K. H. , Kurtenbach R. , Schmidt F. , Wiesen P. 
Published: July 2012

AbstractThe rate constants for the reaction of NCO (X2Π) radicals with NO and NH3 were measured at 20 Torr total pressure in temperature ranges of 290–1098 K and 295–882 K, respectively, using ClNCO excimer laser photolysis for NCO radical formation in combination with laser‐induced fluorescence detection of NCO. In the temperature range investigated the NCO + NO reaction exhibits a negative temperature dependence which is described by the following three parameter Arrhenius equation:

, with E0 in units of kJ/mol.For the NCO + NH3 reaction the measurements exhibit a positive temperature dependence over the temperature range investigated with a slight upwards curvature. A modified three parameters Arrhenius fit provides a good description of the experimental data:

, with E0 in units of kJ/mol. In addition, the rate constant of the NCO + NH3 reaction was found to be pressure independent in the range 10–193 Torr at 295 K.
Investigation of sorption and intracrystalline diffusion of benzene and toluene on silicalite‐1
Authors: Cartarius Ralph , Vogel Herbert , Dembowski Jürgen 
Published: July 2012

AbstractSorption and diffusion measurements were carried out with a novel, fully automatic gravimetric adsorption apparatus, which makes it possible to measure the adsorption (thermodynamic data) and diffusion (kinetic data) of adsorbates on adsorbents simultaneously in a short time. For that reason this device is best suited for catalyst improvement and development in the industrial laboratory. The well characterized system benzene/silicalite‐1 was first used as a reference system to check the performance and applicability of the new apparatus. A comparison of our experimental results with literature data in the temperature range of 280–353 K shows the expected agreement. We found a striking lack of diffusion data of toluene in silicalite‐1. Consequently our next aim was the examination of the toluene/silicalite‐1 system. The adsorption isotherms and intracrystalline diffusivities of toluene on monocrystals of silicalite‐1 were investigated in the temperature range of 283–333 K. The comparison of the systems benzene, toluene and xylol‐isomers on silicalite‐1 shows the influence of one additional methyl group on the adsorptive and diffusive behaviour. The most remarkable features of the comparison is that the obtained toluene diffusivities are greater than benzene diffusities, as it was already recognized in a comparison of benzene and p‐xylol diffusivities, and that the activation energy of the toluene diffusion shows an abrupt increase at the phase transition concentration.
Sensitivity analysis of complex chemical equilibria in heterogeneous systems
Authors: Fishtik Ilie , Geană Dan 
Published: July 2012

AbstractSensitivity analysis of an equilibrium multiple chemical reaction system with several pure condensed phases and one ideal gas phase is considered. It is shown that, in this case, response equilibria also play an important role. The particularity of such systems is that the system's response may be presented as a sum of contributions coming only from those response equilibria defined by gas phase species. The effect of temperature, pressure and initial amounts of species is analyzed for gasification of coal (graphite).