bluuebunny/crossref_metadata_2025_split huggingface dataset:
B. Proton localization in hydrogen bonds: The proton potential function and dynamics in sodium hydrogen bis(4‐nitrophenoxide) dihydrate
Authors: Kreevoy Maurice M. , Marimanikkuppam Sudha , Young Victor G. , Baran Jan , Szafran Miroslaw , Schultz Arthur J. , Trouw Frans 
Published: July 2012

AbstractThe structure of crystalline sodium hydrogen bis(4‐nitrophenoxide) dihydrate (1) has been studied by X‐ray diffraction at 105, 173, and 297 K; and by neutron diffraction at 105 K. Vibrational spectra of 1 have been obtained by infrared absorption at temperatures from 14 K to room temperature, and also by inelastic neutron scattering at 15 K. 1 is a Speakman‐Hadži compound, with an O,O distance of 243 pm (neutron diffraction) or 246 pm (X‐ray diffraction) at 105 K. This distance increases to 248 pm (X‐ray diffraction) at 297 K. The neutron structure suggests that the bridging hydrogen is distributed between two positions, separated by 35 pm, at the lowest temperature.The asymmetric O,H stretching frequency was found by inelastic neutron scattering, using isotopic substitution, at 605 cm−1. A number of IR fingerprint bands which are separate below 200 K merge between 200 and 250 K, simplifying the appearance of the spectra. Prominent examples occur around 1600 cm−1 (a) and around 1450 cm−1 (b). The (a) pair is separated by about 28 cm−1 and the (b) pair about 20 cm−1. At low temperature 1 appears to be a mixture of degenerate tautomers. At higher temperature either the proton shift accelerates, adn band averaging occurs, or else the proton is centralized. If the acceleration model is adopted the tautomer lifetime is about 0.5 ps at the convergence temperature, 225 K. Partial or complete centralization may also be responsible for the changes in the IR spectrum. Thermal excitation of one or more low frequency vibrations causes the acceleration and/or centralization. The O,H stretching frequency is too high to be substantially excited at the coalescence temperature, so this cannot be the relevant vibration. The regular increase in the O,O distance with temperature makes it seem unlikely that centralization is the sole cause of the spectral changes.
X‐Ray diffraction, IR, UV and NMR studies on proton transfer equilibrating phenol‐N‐base systems
Authors: Sobczyk Lucjan 
Published: July 2012

AbstractThe analysis of X‐ray diffraction data on 1:1 phenol‐tertiary amine complexes shows that shortest bridges OHN of ca. 2,52 Å length are realized when the H‐atom is located close to the centre of a bridge. This is well reflected in the correlation curves dO…N vs dC…O and dN…H and dO…H vs dC‐O. The value of dC‐O can be treated as a measure of the proton transfer (PT) degree. Several phenol‐amine systems exhibit very broad (continuous) absorption in infra‐red that can be interpreted in terms of Zundel theory based on extreme hydrogen bond polarizability. However in many cases this broad absorption is limited to high frequency range (above 650 or 1000 cm−1 showing an asymmetric potential energy curves for adiabatic fast proton motion. This is particularly well seen for analogues of phenol‐amine complexes with intramolecular O–H…N hydrogen bonds, i.e. Mannich bases at low temperatures when the charge separation in the PT species is enhanced due to the aggregation processes and ordering of solvent molecules. The studies of deuterium isotope effect on 13C NMR spectra and behaviour of 15N chemical shift and 1H‐15N coupling constant provide a convincing evidence of the influence of temperature and dimerization on the PT equilibrium and localisation of the proton in a bridge.
Symmetry of hydrogen bonds in solution, an overview
Authors: Perrin Charles L. , Nielson Jennifer B. , Kim Yeong‐Joon 
Published: July 2012

AbstractIs a hydrogen bond symmetric (single‐well potential, unique structure with a centered hydrogen) or asymmetric (double‐well potential, pair of tautomers with hydrogen jumping between donor atoms)? This question has received renewed interest because short, strong hydrogen bonds have been proposed to provide stabilization in some enzyme‐catalyzed reactions. Among the methods that have been applied are high‐level calculations and the primary isotope shift. One of the most direct is the NMR method of isotopic perturbation, which will be illustrated with 3‐hydroxypropenal‐d and with monoanions of mono‐18O‐labeled dicarboxylic acids. Both of these are found to be asymmetric. The results on hydrogen maleate and phthalate were unanticipated, since these are symmetric in crystals. The difference had been attributed to the disorder of aqueous environment, and it was thought that these are symmetric in nonpolar solvents, as had been indicated by the primary isotope shifts. Yet further studies of 18O‐induced isotope shifts, especially at ipso carbons, show that a wide variety of these monoanions exist as two equilibrating tautomers in all solvents.
One‐bond deuterium isotope effects on 15N chemical shifts in Schiff bases
Authors: Hansen Poul Erik , Sitkowski Jerzy , Stefaniak Lech , Rozwadowski Zbigniew , Dziembowska Teresa 
Published: July 2012

AbstractA series of 15N enriched Schiff bases have been investigated by means of 15N NMR at variable temperature (300‐200 K). Most reliable results are obtained below 250 K, as the exchange of the chelate proton is sufficiently slow on the NMR time scale at this temperature. The 15N chemical shifts are found to vary linearly with the one bond 15N‐H coupling constants. Deuterium isotope effects on 15N chemical shifts, 1Δ15N(D), show a characteristic S‐shape, when plotted vs. the one‐bond 15N‐H coupling constants. From this graph the intrinsic isotope effects of the two forms can be estimated as 5Δ15N(D)int˜‐1.5 ppm and 1Δ15N(D)int˜3 ppm. The intrinsic isotope effects for the OH‐form is shown to vary only little for the various substituted compounds investigated. The difference in 15N chemical shifts of the OH‐form minus the NH‐form is estimated to 100‐140 ppm.The dependence of 1Δ15N(D) on the mole fraction is similar to that found for 2ΔC(OD) for other intramolecularly hydrogen bonded cases, β‐diketones and Mannich bases and looks very similar to that determined for 1Δ15N(D) of the organic acid‐pyridine complex.
Radicals as hydrogen bond acceptors
Authors: Alkorta I. , Rozas I. , Elguero J. 
Published: July 2012

AbstractThe ability of carbon radicals to act as hydrogen bond acceptors has been evaluated using ab initio theoretical methods. A hybrid Hartree‐Fock Density Functional Theory based method (B3LYP), Møller‐Plesset at the second perturbation order (MP2), and Quadratic CI methods that include single and double excitation (QCISD), have been used. The complexes formed by four radicals CH3(d), C2H4(t), (CH3)2C(t) and (CH3)3C(d)] with four standard hydrogen bond donors [FH, H2O, HCN and H3N] have been studied and their geometry, interaction energy, and electronic properties, within the Atoms in Molecules (AIM) methodology framework have been analyzed. The energy and geometry results show that the studied radicals are poor hydrogen bond (HB) acceptors and the strength of the HBs qualitatively correlates with the molecular electrostatic potential (MEP) minimum of the isolated radicals. The atomic partition of different properties using the AIM methodology indicates that HBs complexes involving radicals behave differently to other HBs formed between neutral molecules.
C. Excited state proton transfer: Keto‐enol tautomerism in the triplet state of hydroxyphenylbenzoxazoles in an alkane glass: Hydrogen tunneling and isotope effects down to 2 K
Authors: Nickel Bernhard , Grellmann Karl H. , Stephan Jens S. , Walla Peter J. 
Published: July 2012

Abstract2‐(2î‐Hydroxyphenyl)benzoxazole (HBO) and its meta‐methyl derivative (m‐MeHBO) can exist in two tautomeric forms, an enol tautomer (E) and a keto tautomer (K). The stable ground state is an enol state, 1E, and the lowest excited singlet state is a keto state, 1K*. Intersystem crossing 1K*→3K* is followed by triplet‐state tautomerization (TST) 3K*→3E* if the energy relation E(3K* — E(3E* = hex > 0 holds. In an alkane glass, the wavenumber x is > 0 for ≈︁ 90% of all HBO molecules and for virtually all m‐MeHBO molecules. The kinetics of TST of HBO, m‐MeHBO and of the deuteriooxy analogues DBO and m‐MeDBO in an alkane glass was studied between ≈︁ 2 and ≈︁ 80 K. TST can be observed by measuring the transient absorption after pulsed excitation, due to the different absorption spectra of 3K* and 3E*. The basic results are: (1) TST is a tunnel reaction and its kinetics is independent of temperature between 2 and 25 K. (2) The rate constant kKE of TST depends on the energy difference hex. Due to the wide distribution of x, the kinetics of TST is multiexponential; an empirical biexponential description of TST yields the ratios k1k2 ≈︁ 5 for the rate constants and A1/A2 ≈︁ 2 for the preexponential factors. (3) The wide distribution of the rate constant kKE can be rationalized by a kinetic model, in which the generation of phonons is an important rate‐limiting factor. (4) The TST of DBO is ≈︁ 1700 times slower than that of HBO, and the TST of m‐MeDBO is ≈︁ 700 times slower than that of m‐MeHBO. ‐ Clear glassy solutions without cracks were obtained by using absorption cells made of polymethylmethacrylate (PMMA), whose thermal expansion coefficient is similar to that of hydrocarbon glasses.
The involvement of rotational processes in the intramolecular proton‐transfer cycle
Authors: Douhal Abderrazzak 
Published: July 2012

AbstractEncapsulation processes and picosecond time‐resolved transient absorption spectra of 2‐(2î‐hydroxyphenyl) imidazo[1,2‐a]pyridine have been used to elucidate the fluorescence band shift and the transient absorption dynamics (in the picosecond time scale) of the proton transfer phototautomer in cyclodextrins and in cyclohexane, respectively. Restriction of the internal motion of the phototautomer by cyclodextrins leads to a 70 nm blue shift of the proton transfer fluorescence band. A shaping time (14 ps) of the transient absorption spectrum has been observed and assigned to a combination of this internal rotational motion and a vibrational‐cooling process of the phototautomers in the excited state. Both studies suggest the involvement of rotational processes in the intramolecular proton‐transfer cycle.
A mechanistic study of competing dissociative reactions in small ionic clusters
Authors: Ring S. , Baumgärtel H. 
Published: July 2012

AbstractWe have studied dissociative electron and dissociative proton transfer processes in cationic dimer aggregates of ortho‐xylene/dimethylamine. Appearance potentials for the products DMA+ and DMAH+ are measured. Mass spectra as a function of total photon energy are evaluated for protonated and deuterated dimers to study the branching ratio and the internal reaction process for the two competing reaction channels. H/D‐isotope effects are studied in order to determine the internal reaction process. The dissociation process following the internal reaction was studied by observing asymmetric broadenings in time‐of‐flight signals.
Solvent effects upon the picosecond dynamics of proton transfer in 2‐(N‐methylanilino)acetophenone
Authors: Dreyer Jens , Peters Kevin S. 
Published: July 2012

AbstractPicosecond pump‐probe spectroscopy is used to examine the dynamics of singlet excited state proton transfer in 2‐(N‐methylanilino)acetophenone in a series of solvent mixtures of benzene and acetonitrile. A factors and energies of activation are derived from temperature‐dependent studies. The rate constants show a moderate solvent dependence, but Arrhenius parameters differ considerably between polar and unpolar solvents. It is concluded that proton transfer occurs via nonadiabatic tunneling into different vibrational channels of the product state.
Competitive geminate quenching and geminate recombination reactions of 1‐naphthol
Authors: Pines Ehud , Tepper Dimitry , Magnes Ben‐Zion , Pines Dina , Barak Tamar 
Published: July 2012

AbstractThe back proton recombination following the photo‐dissociation of 1‐naphthol was studied by a time‐correlated single‐photon counting apparatus. The measured picosecond time‐resolved fluorescence decay profiles were analyzed using the numerical solution of the Debye‐Smoluchowski equation with coupled reversible and irreversible proton back‐recombination reactions at the source.A full kinetic analysis shows that both the reversible geminate recombination reaction, which assumes at long times a t−3/2 dependence, and the irreversible geminate quenching reaction, which assumes at long times a t−1/2 dependence, are significant in determining the excited‐state kinetics of 1‐naphthol.
Isotope and temperature effects in ultrafast proton‐transfer from a strong excited‐state acid
Authors: Pines E. , Pines D. , Barak T. , Magnes Ben‐Zion , Tolbert L. M. , Haubrich J. E. 
Published: July 2012

AbstractThe introduction of naphthol derivatives that exhibit excited‐state acidities comparable to strong mineral acids, has opened the way for direct investigation of ultrafast proton‐transfer in aqueous solutions and alcohols. We report on the direct measurement of the isotope and temperature effects on 5‐cyano‐1‐naphthol excited‐state proton dissociation in the full composition range of water‐methanol mixtures. The results indicate that in pure water the proton‐transfer rate is almost activationless in the proton coordinate and is mainly controlled by the solvent. In pure methanol the reaction shows down considerably and is mainly controlled by activation along the proton coordinate.
Quantum and classical simulations of an excess proton in water
Authors: Pavese Marc , Voth Gregory A. 
Published: July 2012

AbstractThe dynamics of excess proton motion in bulk water is investigated through both quantum and classical molecular dynamics simulations. Results are given both for the dynamics of the excess proton itself, as well as for the solvent dynamics which influence it. The shuttling of the excess proton between two solvating water molecules has been modeled using the empirical valence bond EVB model of Lobaugh and Voth. A critical analysis of the methodology used to model the proton exchange barrier is also given.
Proton transport along water chains and NADH hydride transfer in solution
Authors: Decornez Hélène , Drukker Karen , Hurley M. M. , Hammes‐Schiffer Sharon 
Published: July 2012

AbstractThis paper presents simulations of two biologically important processes involving proton or hydride transfer reactions. The first process is proton transport along a protonated chain of three water molecules. This process is induced with a linearly increasing external electric field. The multiconfigurational molecular dynamics with quantum transitions (MC‐MDQT) method is utilized to calculate the real‐time nonequilibrium quantum dynamics of this process. The results illustrate that nonadiabatic effects play an important role in the proton dynamics for water chains under certain nonequilibrium conditions. The second process discussed in this paper is the hydride transfer reaction from ethanol to an NAD+ analog (1‐methyl‐nicotinamide) in water with a divalent zinc ion catalyst. The structures, relative energies, and charge distributions are calculated for the reactant, product, and transition state of a complex including the NAD+ analog, ethanol, zinc, and three water molecules at the Hartree‐Fock level with a polarized valence double zeta basis set. This information is utilized to parametrize an augmented molecular mechanical potential, which is implemented to calculate the classical free energy curve for this reaction in water. The results indicate that solvation effects significantly alter the energetics of this reaction.
On proton transport in perovskite‐type oxides and plastic hydroxides
Authors: Kreuer K. D. , Münch W. , Traub U. , Maier J. 
Published: July 2012

AbstractFrom a critical review of the experimental data on proton conducting oxides and new results of quantum MD‐simulations on proton transport in the model perovskite‐type oxide BaCeO3 further evidence is given for a rapid rotational diffusion of protonic defects (hydroxide ions on oxygen sites) with a comparatively slow proton exchange between neighbouring oxygens being the rate limiting step of long range proton transport. Proton transfer is suggested to occur almost barrierless in contracted transition state configurations of the type O‐H‐O which are virtually insensitive towards solvent effects. The energy of this configuration depends strongly on the softness of the oxide lattice, which is not only determined by the cation/oxygen bonding but also by the formation of hydrogen bonds of the type O‐H…O. Hydrogen bonding is expected to promote proton transport in oxides and the application of pressure, which generally increases the hydrogen bond strength, is anticipated to lower the activation energy for proton mobility.While protons in oxides may be approximately considered to be random walkers, proton transport in hydroxides is suggested to be highly correlated. For the cubic high temperature phases of alkaline metal hydroxides this is reflected by a high proton diffusion coefficients, while virtually no proton conductivity is observed. This effect is also expected to become relevant for proton transport in oxides with very high concentrations of protonic defects.
Unimolecular dissociation: A state‐specific quantum mechanical perspective
Authors: Schinke Reinhard , Beck Christian , Grebenshchikov S. Yu. , Keller Hans‐Martin 
Published: July 2012

AbstractThe dynamics of unimolecular dissociation of small polyatomic molecules is discussed in the light of exact quantum mechanical calculations. The central quantities are vibrational resonances, that is quasi‐bound states embedded in the continuum. The main concepts are illustrated with two prototypes, HCO and HNO, both in their ground electronic states. While HCO is a system with an extremely low density of states, a mostly regular internal vibrational dynamics and a ‘tight’ transition state, HNO has a higher density of states, behaves more chaotically and has a ‘loose’ transition state. In both cases the resonance widths and hence the dissociation rates strongly fluctuate, as functions of energy, about the average. In addition to the rates, the final state distributions of the products and their relationship to the dynamics beyond the transition state are also discussed in detail. These systems are well suited to elucidate the contrast between state‐ and mode‐specificity in molecular breakup. An essential aspect is the test of approximations like classical mechanics and statistical models.
Kossel rings from colloidal dispersions at rest and under sheared conditions
Authors: Dux Ch. , Reus V. , Versmold H. 
Published: July 2012

AbstractKossel rings of colloidal dispersions obtained with colloids at rest and under flowing conditions are described. When flowing only Kossel rings of planes with a component in the direction of the flow do not vanish. Compared with the dispersion at rest the number of Kossel rings of the dispersions under flow is reduced. Experimental examples are presented.
Semiconducting behavior in the liquid Ag‐As2X3 System (X: Te, Se)
Authors: Usuki Takeshi , Itoh Keiji , Uemura Osamu , Kameda Yasuo 
Published: July 2012

AbstractMeasurements of the electrical conductivity, magnetic susceptibility and thermoelectric power have been carried out on liquid Ag‐As2Te3 and Ag‐As2Se3 systems. The liquid Ag‐As2Te3 system exhibits a deep minimum of the electrical conductivity, diamagnetic peak of the magnetic susceptibility and rapid drop of the positive thermoelectric power at the stoichiometric composition of Ag6As2Te3, reflecting the occurrence of the metal‐semiconductor transition. The liquid Ag‐As2Se3 system also has the diamagnetic peak of magnetic susceptibility and sign reversion of thermoelectric power near the composition of Ag6As2Se3. However, the electrical conductivity in the selenide system indicates a negative temperature dependence like metals and takes a maximum value at this composition. The observed results have been discussed with the help of the pseudogap model developed by Enderby and Barnes. In this result, the unusual behavior of electrical conductivity in the selenide system has been deduced to be associated with the conduction and valence band edge‐parameters rather than the pseudogap width.
Influences of nuclear size and shape and nuclear spin on chemical isotope effect of zirconium‐crown complex
Authors: Fujii Toshiyuki , Yamamoto Tadashi , Inagawa Jun , Watanabe Kazuo , Nishizawa Kazushige 
Published: July 2012

AbstractZirconium isotopes were fractionated by liquid‐liquid extraction system using dicyclohexano‐18‐crown‐6. The experimental enrichment factors showed failure from the Bigeleisen‐Mayer approximation, which had been believed since their article of 1947. The nuclear size and shape and the nuclear spin result in the additional isotope effects on the Bigeleisen‐Mayer approximation. The contributions of the nuclear mass and of the field shifts of s‐orbital and d‐orbital to the chemical isotopic enrichment factors were calculated by adding the correction terms onto the Bigeleisen‐Mayer approximation. The scaling factor ratios which represent weight of contributions of the mass effect and the field shift effect in the separation factors were independent of the initial condition of aqueous phase and indicated constant values throughout every condition, while the scaling factors themselves had individual values. The scaling factor of the nuclear spin was also calculated by means of the hyperfine splitting spectra of 91Zr; w'=−0.118±0.003. The constant value of w' showed that the nuclear magnetic field was not disturbed in the chemical exchange reaction.
Kinetics of the electroporative deformation of lipid vesicles and biological cells in an electric field
Authors: Kakorin S. , Neumann E. 
Published: July 2012

AbstractThe electric conductivity of a suspension of unilamellar lipid vesicles of phosphatidylcholine and phosphatidylglycerol, filled with electrolyte, increases after exposure to external field pulses. The electrolyte efflux through the electropores is described in terms of volume decrease under Maxwell stress at constant membrane surface area. Applying the Hagen‐Poiseuille Ansatz, the exact analytical solution for the kinetics of the electromechanical vesicle deformation is obtained in terms of a LambertW function. The initial and final phases of the volume decrease can be approximated by exponential functions with the time constants τ and 2τ, respectively. It is found that τ is proportional to E2·a9, describing the extreme sensitivity of the deformation to the vesicle radius a in electric field of strength E.The kinetic analysis yields the membrane bending rigidity κ = 3.0±0.3·10−20 J. At the field strength E = 1.0 MV m−1 and in the range of pulse duration 5 ≤ tE/ms ≤ 60, the number of water‐permeable electropores is found to be N=35±5 per vesicle of radius rp = 50 nm with mean pore radius rp=0.9±0.1 nm.The kinetic analysis developed here for vesicles is readily applied to cell membranes, aiming at physicalchemical guidelines to optimize the membrane electroporation techniques for the direct transfer of drugs and genes into tissue cells.