bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantum cluster equilibrium theory of liquids part I: Molecular clusters and thermodynamics of liquid ammonia
Authors: Ludwig R. , Weinhold F. , Farrar T. C. 
Published: July 2012

AbstractThe quantum cluster equilibrium (QCE) theory is presented for liquid ammonia. The cluster equilibria that dictate phase composition are determined by the rigorous techniques of quantum statistical thermodynamics in the canonical ensemble, based on the ab initio partition function. The characteristic features of the supra‐molecular clusters which comprise the QCE model are discussed in terms of binding energies, geometries, cooperativity and charge transfer. Many possible equilibrium structures of ammonia are strongly disfavored for enthalpic and/or entropic reasons and do not show up in the QCE cluster populations. The numerical accuracy of the resulting QCE model is demonstrated by comparison with experimental data for thermodynamic properties of ammonia such as Clausius‐Clapeyron pressure dependence and specific heat.
Closed‐Loops of liquid‐liquid immiscibility in binary mixtures of equal sized molecules predicted with a simple theoretical equation of state
Authors: Yelash L. V. , Kraska T. 
Published: July 2012

AbstractSeveral types of closed‐loop liquid‐liquid immiscibility behavior in binary mixtures have been found with a simple isotropic model fluid. The calculations are based on an equation of state composed by the Carnahan‐Starling hard sphere repulsion and the van der Waals‐attraction. The extension to binary mixtures is accomplished with the usual quadratic one‐fluid mixing rules. The results are presented in a global phase diagram as introduced by van Konynenburg and Scott. Several closed‐loop phase diagrams similar to those discovered by Boshkov ten years ago with a Lennard‐Jones equation of state have been found. The results are discussed in the context of the topological and physical origin of closed‐loop behavior in systems of isotropic mixtures.
Ambient temperature removal rate constants for singlet methylene with some nitrogen and sulphur compounds
Authors: Staker Warren S. , King Keith D. , Gutsche Greg J. , Lawrance Warren D. 
Published: July 2012

AbstractThe technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, 1CH2 (ā A1), with some nitrogen and sulphur compounds at ambient temperature (298±2 K). A measurement of the removal rate constant for 1CH2 with NH3 yielded a value of (3.33±0.19)xl0−10 cm3 molecule−1 s−1 in good agreement with the only previously reported measurement. Removal rate constants for the reactants CH3NO2, CH3CH2NO2, CH3CH2CH2NO2, CH3CN, CH3CH2CH2CN, CH2=CHCN, H2S, CH3CH2SH, CH3SCH3, CH3SSCH3, and CH3COSH were measured for the first time and found to be (3.23±0.28), (3.46±0.69), (3.55±1.28), (4.51±0.21), (3.64±0.41), (4.10±0.26), (3.87±0.22), (4.77±0.25), (4.53±0.23), (5.37±0.53), and (4.59±0.32)x10−10 cm3 molecule−1 s−1 respectively. The calculated removal efficiencies (removal rate constant relative to the Lennard‐Jones collision rate constant) are indicative of reaction being the dominant process leading to loss of 1CH2. Comparisons are drawn between the reactivities of the nitrogen and sulphur functional groups and their hydrocarbon analogues, and between the sulphur compounds and their isoelectronic oxygen analogues.
Fine‐Structure analysis of ClO fragments in the mode‐specific predissociation of chlorine dioxide
Authors: Delmdahl R. F. , Welcker S. , Gericke K. ‐H. 
Published: July 2012

AbstractThe photodissociation of OClO (2A2) ClO (X2πΩ,ν,J,Λ)+O (3P) in the 424‐450 nm region and in the 356‐371 nm region has been investigated by monitoring the ClO (X2πΩ,ν = 0,J,ν) fragments using two‐photon laser‐induced fluorescence. In the long‐wavelength regime the product vibrational excitation is clearly a function of the particular initial type of motion of OClO (2A2v1,v2,V3). This behavior is significantly reduced at shorter photolysis wavelengths. The population of the fragments rotational levels is observed to resemble Boltzmann distributions of around 1000 K. A population ratio of about P(2π3/2): P(2π1/2) = 3.5 is obtained for the two spin‐orbit states of ClO. The ν‐doublet states (A′) and (A″) of the ClO fragments are equally preferred in the dissociation process.
Hydrogen transfer in the porphin anion: A quantum dynamical study of vibrational effects
Authors: Brackhagen O. , Scheurer Ch. , Meyer R. , Limbach H.‐H. 
Published: July 2012

AbstractThe observed temperature‐dependent hydrogen transfer rate constants for the H, D, and T isotopic species of the porphin anion have been reproduced by two different quantum dynamical models. The approach is based on the master equation for the system‐state populations. The models involve adapted localization properties and a simplified interaction with a quantum mechanical heat bath. A one‐dimensional model describes a hindered circular transfer motion of the hydrogen atom between its four stable sites in the plane of the porphin ring framework. With five parameters that are common to the three isotopic species it yields agreement with the observed data. One of the parameters, the height of the fourfold potential energy barrier, predicts an acceptable estimate for the in‐plane N‐H wagging frequency. Interactions of the transfer motion with individual vibrations have been studied for a series of two‐dimensional model systems. There the circular transfer motion is combined with ring framework modes having properties similar to those obtained for porphin by Li and Zgierski [J. Phys. Chem. 95, 4268 (1991)]. For vibrational modes near or above 960 cm−1, strong interactions are found to be unlikely, as they would yield kinetic isotope effects and/or apparent activation energies in disagreement with the experimental data. For low frequency modes of the ring framework, however, sizeable couplings with the transfer cannot be ruled out. An interaction of the type suggested by quantum chemical results reported by Vangberg and Ghosh [J. Phys. Chem. B. 101, 1496 (1997)] has been included in a two‐dimensional model involving a rectangular displacement of the pyrrole rings. With low vibrational frequency, a very large coupling strength, and a barrier close to the predicted one, this model has also been found to be compatible with experiment.
NQR Studies of charge distribution in hydrogen bonded chloroacetates
Authors: Zdanowska‐Fraczek Maria 
Published: July 2012

Abstract35Cl spectroscopy was applied to study the hydrogen bond symmetry as well as the isotope and pressure effects on it in a wide range of (2:1) salts of chloroacetic trichloroacetic and difluorochloroacetic acids. The NQR results were discussed qualitatively on the base of a variational correlated ground state wave function theory of the single hydrogen bond
A new view of the quantum dynamics for proton transfer along hydrogen bonds: Vibrational spectroscopy with neutrons
Authors: Fillaux F. , Nicolaï B. , Baron M. H. , Lautié A. , Tomkinson J. , Kearley G. J. 
Published: July 2012

AbstractInelastic neutron scattering is an ideal tool to observe proton transfer dynamics along hydrogen bonds in the quantum regime. “Tunnelling” transitions associated with the quantum transfer of a single proton have been observed for potassium hydrogen carbonate, KHCO3, and similar hydrogen bonded complexes in the solid state. More recently, the tunnelling transition was observed for a polypeptide: polyglycine II. The “tunnelling” bands are rather sharp. Therefore, the proton transfer is largely decoupled from the heavy atom dynamics. Force‐field calculations including INS intensities confirm that the proton dynamics are almost totally decoupled from those of the surrounding heavy atoms. This is in contrast to the “vibration assisted tunnelling” model that supposes a large modulation of the double minimum potential by the low frequency modes of the hydrogen bond. The double minimum potential functions governing the proton transfer are discussed.
Localization of hydrogen bond deuterons in proton sponges by dipolar solid state 15N NMR spectroscopy
Authors: López Concepción , Lorente Philippe , Claramunt Rosa María , Marín Javier , Foces‐Foces Concepcion , Llamas‐Saiz Antonio L. , Elguero Jose , Limbach Hans‐Heinrich 
Published: July 2012

AbstractIt is shown that dipolar solid state 15N NMR spectroscopy can be employed in order to estimate the positions of hydrogen bond deuterons in polycrystalline 15N labeled proton sponges containing an intramolecular N…H…N hydrogen bond. For singly 15N labeled 1, 8‐bis(dimethylamino)naphtalene. LPF6 (1‐h: LH, 1‐d: LD), an asymmetric hydrogen bond with cubic average distances between the two nitrogen atoms and the hydrogen bond deuteron of 1.19 Å and 1.47 Å are obtained. These values are in accordance with the N…H distances of 1.19(7) Å and 1.43(6) Å, obtained previously by x‐ray diffraction at 150 K.
Nuclear magnetic resonance of hydrogen bonded clusters between F− and (HF)n: Experiment and theory
Authors: Shenderovich Ilja G. , Smirnov Sergei N. , Denisov Gleb S. , Gindin Vladimir A. , Golubev Nikolai S. , Dunger Anita , Reibke Rebecca , Kirpekar Sheela , Malkina Olga L. , Limbach Hans‐Heinrich 
Published: July 2012

AbstractLiquid state 1H and 19F NMR experiments in the temperature range between 110 and 150 K have been performed on mixtures of tetrabutylammonium fluoride with HF dissolved in a 1:2 mixture of CDF3 and CDF2Cl. Under these conditions hydrogen bonded complexes between F− and a varying number of HF molecules were observed in the slow proton and hydrogen bond exchange regime. At low HF concentrations the well known hydrogen bifluoride ion [FHF]− is observed, exhibiting a strong symmetric H‐bond. At higher HF concentrations the species [F(HF)2]−, [F(HF)3]− are formed and a species to which we assign the structure [F(HF)4]−. The spectra indicate a central fluoride anion which forms multiple hydrogen bonds to HF. With increasing number of HF units the hydrogen bond protons shift towards the terminal fluorine's. The optimized gas‐phase geometries of [F(HF)n]−, n = 1 to 4, calculated using ab initio methods confirm the D∞h, C2v, D3h and Td symmetries of these ions. For the first time, both one‐bond couplings between a hydrogen bond proton and the two heavy atoms of a hydrogen bridge, here 1JHF and 1JHF where |1JHF|≥|1JHF'|, as well as a two‐bond coupling between the heavy atoms, here 2JFF, have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. 1JHF and 2JSF>0, and 1JHF|. <0. Ab initio calculations of NMR chemical shifts and the scalar coupling constants using the Density Functional formalism and the Multi‐configuration Complete Active Space method show a reasonable agreement with the experimental parameters and confirm the covalent character of the hydrogen bonds studied.
Formation, stability and fragmentation of biomolecular clusters in a supersonic jet investigated with nano‐ and femtosecond laser pulses
Authors: Meffert Anja , Grotemeyer Jürgen 
Published: July 2012

AbstractGas phase reactions have often been discussed to play a role in the MALDI ionisation mechanism. Therefore the cluster formation of several typical MALDI matrix substances like ferulic acid (= 4‐hydroxy‐3‐methoxy‐cinnamic acid), vanillic acid (= 4‐hydroxy‐3‐methoxy‐benzoic acid), sinapic acid (= 3,5‐dimethoxy‐4‐hydroxy‐cinnamic acid) and 2,5‐dihydroxy‐benzoic acid was investigated after laser desorption into a supersonic beam of argon by means of time‐of‐flight mass spectrometry. Not only the formation of homogeneous matrix clusters but also that of heterogeneous clusters with some small amino acids was studied. The different neutral clusters formed in the supersonic expansion were ionised by a multiphoton process employing either nano‐ or femtosecond laser pulses. Ionisation conditions were chosen to avoid direct ionisation of the small, non aromatic amino acids. Consequently the ionisation process of the neutral clusters is determined by the MALDI matrix compounds provided with an aromatic chromophore.Strong differences in the detection of cluster ions due to the laser pulse length employed for multiphoton ionisation were observed. Only femtosecond activation led to mass spectra with intense signals of the cluster ions. In addition in the case of femtosecond ionisation protonated amino acids were detected in the mass spectra. As direct ionisation of the free amino acids is not possible under the chosen ionisation conditions because they lack an adequate chromophore these protonated amino acids are assumed to be formed via an intracluster proton transfer in the heterogeneous dimer and subsequent decay of the ionised cluster (dissociative proton transfer). Such well‐known processes for heterogeneous clusters consisting of a substituted aromatic molecule and small polar solvent molecules may be involved in the MALDI ionisation process.
Pathways for the photochemical hydrogen abstraction by n, π*‐excited states
Authors: Nau Werner M. 
Published: July 2012

AbstractAs a result of recent experimental and theoretical investigations, new aspects of the photochemical hydrogen abstraction by n, π*‐excited states have emerged. The absolute reactivity of singlet‐excited n, π* states exceeds that of the corresponding triplet states, but the singlet reaction turns out to be chemically much less efficient than the triplet reaction. Hence, radiationless deactivation of the singlet states represents the dominant pathway for interaction with hydrogen donors. The fraction of singlet encounters undergoing deactivation appears to increase with decreasing reaction exothermicity. Hence, when relatively inert solvents like methanol or chloroform act as hydrogen donors towards singlet‐excited azoalkanes, the reaction becomes entirely inefficient. This constitutes an example of a novel fluorescence quenching mechanism, which is referred to as an “aborted” hydrogen abstraction. The inefficiency of the singlet reaction can be rationalized, making use of the most recent theoretical findings, in terms of the occurrence of a conical intersection, i.e., a real surface crossing, rather than the previously implicated avoided crossing. The conical intersection provides an efficient channel for nonadiabatic return to the ground‐state reactants and, thus, serves to account for the observed inefficiency of product formation from the singlet state. In general, the conical intersection appears to follow the transition state for hydrogen abstraction, but the consequences of a situation, where the conical intersection precedes the transition state are also discussed. The latter becomes reasonable for endothermic hydrogen abstractions, for which one would then expect a unit efficiency of singlet deactivation. The absolute reactivities in hydrogen abstractions, the involvement of CT interactions and tunneling, and the behavior of different n, π* chromophores (azoalkanes versus ketones) are also discussed in view of the most recent experimental data. General rules for photochemical hydrogen abstractions by n, π*‐excited states are expressed.
Vibration‐assisted intermolecular hydrogen tunneling in photoreactive doped molecular crystals: Effect of temperature and pressure
Authors: Prass B. , Stehlik D. , Chan I. Y. , Trakhtenberg L. I. , Klochikhin V. L. 
Published: July 2012

AbstractThe pressure and temperature dependence of the reaction rate of the photochemical intermolecular H‐transfer reaction in acridine‐doped fluorene single crystals exhibit all typical features of solid state H‐tunneling. Independent Raman data including the pressure dependence have identified three low energy inter‐ and intramolecular vibrational fluctuations as reaction promoting modes. The mode identification allows a more subtle analysis of the H‐transfer reaction rate behavior on the basis of recent theoretical concepts. Surprisingly, the complete data set k (P, T) can be satisfactorily reproduced by an average one frequency mode model with parameters consistent with an average of the parameters calculated for the individual reaction promoting modes.
D. Proton transfer in complex systems, liquids and biological systems: Proton transfer reactions at the surface of Ice. Heterogeneous reactions involved in stratospheric ozone depletion
Authors: Bianco Roberto , Gertner Bradley J. , Hynes James T. 
Published: July 2012

AbstractTwo heterogeneous reactions on an ice surface that are relevant in connection with stratospheric ozone depletion are discussed theoretically: the hydrolysis of chlorine nitrate, ClONO2, to produce HOC1 and HNO3, and the acid ionization of hydrochloric acid HC1. For the former, a recently proposed proton transfer mechanism actively involving the ice lattice is supported by examination of the ClONO2 hydrolysis energetics when proton transfer is prevented. For the latter, a previously proposed picture of facile HCl ionization at, but not on, the ice surface is supported by the results of a quantum treatment of the proton and of a model investigation of the free energetics of ionization atop the ice surface.
Quantum dynamical entanglement of protons in liquid water: New Raman light scattering and inelastic neutron scattering experiments
Authors: Chatzidimitriou‐Dreismann C. A. , Abdul‐Redah T. , Streffer R. M. F. 
Published: July 2012

AbstractRecent theoretical work indicated that the short‐time quantum entanglement between closely lying protons (or other particles) in condensed matter may be experimentally accessible, even at room temperature. This effect has been predicted to cause the appearance of certain “anomalous” components in the scattering intensity, the “anomaly” being caused by the fact that an external field cannot interact properly with domains containing quantum correlated protons (or H‐bonds). First experimental evidence for this novel effect has been provided with the aid of Raman light scattering from liquid H2O/D2O mixtures. Very recently, associated deep inelastic (Compton) neutron scattering experiments on liquid H2O/D2O mixtures gave an independent confirmation of the considered effect. The results of both experiments, which have found so far no conventional interpretation, provide direct evidence for short‐lived quantum entanglement of adjacent protons (and deuterons) in condensed systems atT≈︁ 293 K. In addition, we present first results of two new experiments concerning: (1) Raman light scattering on solutions of NaCl in H2O/D2O; (2) deep inelastic neutron scattering on solutions of urea (H2N‐CO‐NH2) in H2O/D2O.
Proton self‐exchange of para‐substituted N,N‐dimethylanilines and their conjugate acids in aprotic solvents
Authors: Goez Martin , Heun Rainer 
Published: July 2012

AbstractThe intermolecular proton transfer between the title compounds was investigated by dynamic NMR spectroscopy. The mechanism was found to be direct proton transfer, other transfer pathways being excluded by the experimental conditions (aprotic solvents with an extremely low water content of about 5×10−4M). It was shown that the observed exchange rates are identical with the rates of the proton transfer proper; association or dissociation pre‐ or postequilibria are not rate determining. Magnitude and solvent dependence of the kinetic isotope effects, as well as the influence of the solvent polarity on the intrinsic activation barrier ΔG‡int indicate considerable solvent reorganization during the transfer. These observations, as well as correlations between the activation parameters of proton self‐exchange and electron self‐exchange of the substrates studied provide some evidence that inner‐sphere and outer‐sphere contributions to ΔG‡int might be separable.
The contributions of secondary proton bridges to the catalytic power of the serine proteases
Authors: Bibbs Jeffrey A. , Garoutte Michael P. , Wang Bing , Tittel Paul D. , Schowen K. Barbara , Schowen Richard L. 
Published: July 2012

AbstractThe curvature of proton‐inventory functions (rate constants in mixtures of protium and deuterium oxides) for hydrolysis of substrates of varying structure by trypsins from four different sources has been used as a signal of transition‐state stabilization by a short, strong hydrogen bonds (“low‐barrier hydrogen bonds”) at the histidine‐aspartate site of the catalytic triad. Previous estimates suggested contributions as large as 85 kJ mol−1 but the current findings are consistent with contributions of no more than a few kJ mol−1.
Quantum dynamics of proton transfer processes in enzymatic reactions. Simulations of phospholipase A2
Authors: Bala P. , Grochowski P. , Lesyng B. , Mccammon J. A. 
Published: July 2012

AbstractFor the past few years we have been developing a theoretical model capable of describing quantum dynamical processes which occur in molecular systems and in enzymatic active sites. This, in particular, includes proton transfer processes.Our most advanced studies have been performed for phospholipase A2, an enzyme which hydrolyzes phospholipids. The potential energy function for the active site is computed using an Approximate Valence Bond (AVB) method. The dynamics of the key proton in the enzyme's active site is described either by the classical molecular dynamics (MD/AVB model) or by the time‐dependent Schroedinger equation. The dynamics of the remaining atoms of the enzyme are described using classical MD. The coupling between the quantum proton and the classical atoms is accomplished via Hellmann‐Feynman forces, as well as the time‐dependence of the potential energy function in the Schroedinger equation (QCMD/AVB model).The quantum proton transfer from the water molecule to histidine is followed by a nucleophilic attack of the created OH− group. The processes are simulated using the parallelized QCMD code.
Structural investigations of fluid carbon tetrafluoride by statistical mechanical calculations and Monte Carlo simulation
Authors: Bassen A. , Waldner I. , Bertagnolli H. 
Published: July 2012

AbstractStatistical mechanical calculations with the extended reference interaction site model (XRISM) and Monte Carlo simulations are performed on fluid CF4. The parameters of a five‐center (12‐6) Lennard Jones potential with added point charges are deduced. The results are compared with total atom pair correlation functions recently determined by neutron diffraction experiments at three supercritical densities. Apart from the good agreement between experimental and theoretical results, we found that depending on the method used, different potentials are able to describe the investigated system. The atom pair correlation functions and angular distribution functions indicate a completely disordered arrangement of the molecules in fluid CF4 with no significant short‐range orientational order except for very close distances. The comparison of the results with former Reverse Monte Carlo (RMC) simulations allows the conclusion that the RMC method is an effective tool, equivalent to the well established Monte Carlo and XRISM methods, in determining the microscopic structure of fluid CF4, but without the exact knowledge of intermolecular potentials.
Diffusive motion of water molecules in harmotome studied by neutron scattering
Authors: Stockmeyer Rolf 
Published: July 2012

AbstractThe quasielastic neutron scattering from H2O and D2O in harmotome, a natural zeolite with the unit cell content Ba2(AlO2)4 (SiO2)12nH2O, has been measured at temperatures between 294 and 423 K with loadings of n = 2, 4, and 12 water molecules per unit cell. From the observed coherent scattering relaxation times for D‐D correlations from 2 to 11 ps were deduced. Fitting the incoherent quasielastic peaks from H2O to a two‐site jump model yielded residence times of the protons in the range from 4 to 6 ps.