bluuebunny/crossref_metadata_2025 huggingface dataset:
Proton dynamics and the tautomerization potential in benzoic acid crystals
Authors: Neumann M. A. , Craciun S. , Corval A. , Johnson M. R. , Horsewill A. J. , Benderskii V. A. , Trommsdorff H. P.
Published: July 2012

Two‐proton exchange along the two hydrogen bonds mediates the tautomerization in benzoic acid (BA) dimers. Optical spectroscopy and quasi‐elastic neutron scattering (QENS) have been employed to characterize the proton dynamics in doped and pure BA crystals. The proton motion in BA is governed by a multidimensional potential energy surface (PES), and recent theoretical methods, based on a perturbative instanton approach, to describe tunneling in such PES are presented. This PES is also modulated by the interaction with the solid state environment as manifest by the energy difference between the otherwise equivalent tautomers. The value of this energy difference in pure crystals is an important parameter in the data analysis of NMR and QENS. Both methods give mutually consistent values that differ significantly from earlier determinations via infrared and 13C NMR as well as a recent evaluation using neutron diffraction data. The energy difference between tautomers is altered for dimers in the vicinity of impurity molecules. This is the basis for the optical spectroscopic methods, which enable a direct and accurate determination of the level structure and tautomerization dynamics of these coupled dimers in the limit of very low temperatures, where coherent tunneling is also observed in some cases. Measurements with new impurity molecules make it possible to monitor simultaneously at least 10 different tautomer configurations and prove that the influence of the probe molecules on the proton dynamics is small. The transition to thermally activated barrier crossing at higher temperatures is accessed via the width of the QENS line that is determined by the inverse of the proton correlation time. The quantitative data analysis of the scattered intensity as a function of temperature and scattering angle yields the energy difference, A, between the two wells (A/kB=90+20 K), the length (0.686 Å), and direction of the proton jump vector. These measurements complement NMR investigations presented in the preceding paper.
Observation of thermal tautomerism of thermochromic salicylideneaniline derivatives in the solid state by 15N CPMAS NMR down to cryogenic temperatures
Authors: Benedict Claudia , Langer Uwe , Limbach Hans‐Heinrich , Ogata Hironori , Takeda Sadamu
Published: July 2012

The temperature dependent solid state 15N CPMAS NMR spectra of 15N enriched salicylideneaniline derivatives, N‐salicylideneaniline (SA), N,Nî‐bissalicylidene‐p‐phenylenediamine (BSP) and N,Nî‐di(2‐hydroxy‐1‐naphthylidene)‐p‐phenylenediamine (DNP) were measured in order to study possible proton transfer processes in the NHO hydrogen bonds. For DNP a remarkable change of the 15N chemical shifts of more than 50 ppm was observed between 26 and 388 K by using the CPMAS NMR technique down to cryogenic temperatures. The result clearly indicates very small enthalpy differences among the four tautormers associated with the combination of two possible forms, OH and NH form, of the two NHO hydrogen bonds in a DNP molecule.
Coherent and incoherent dihydrogen dynamics in a ruthenium trihydride complex with the tris(pyrollyl)phosphine ligand
Authors: Gründemann Stephan , Limbach Hans‐Heinrich , Rodriguez Venancio , Donnadieu Bruno , Sabo‐Etienne Sylviane , Chaudret Bruno
Published: July 2012

The ruthenium IV trihydride complex Cp*RuH3(Ppy3) (1) accommodating the π‐accepting tris(pyrollyl)phosphine ligand has been synthesized by standard methods. This complex exhibits average exchange couplings J of the coherent dihydrogen exchange and average rate constants k of the incoherent dihydrogen exchange which are significantly larger than the corresponding triphenylphosphine and tricyclohexylphosphine analogs. No kinetic HH/HD isotope effects on the incoherent exchange are observed within the margin of error. Using a simple kinetic model involving incoherent vibrational excitation it is shown that in principle both the coherent and the incoherent processes should be related. However, whereas the energy of activation of k corresponds to the barrier of rotation, the energy of activation of the coherent dihydrogen exchange J is much smaller i.e. determined by states well below the barrier. Nevertheless, it is found that chemical modification which reduces the energy of activation of J also reduces the energy of activation of k which demonstrates the relation between both kinds of processes.
On the unexpected isotope effect on the exchange coupling constant in partially deuterated transition‐metal trihydride complexes
Authors: Gelabert Ricard , Moreno Miquel , Lluch José M.
Published: July 2012

In this paper we attempt to explain the unusual isotope effect displayed by the 1H‐NMR scalar H‐H coupling of the hydrides in complexes [CpIrL(H)3]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc. 112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three‐coupled well system, but when a deuterium is introduced the system is better described as a double‐coupled well. Finally, it is shown that the coupling in the case of a double‐well system is greater than in an equivalent triple‐well system, in this way giving a plausible explanation to the experimental facts reported.
Indirect characterisation of hydrogenation intermediates using PASADENA NMR spectroscopy — evolution of zero‐quantum coherence in AB spin systems
Authors: Hübler P. , Natterer J. , Bargon J.
Published: July 2012

An indirect method is presented for the detection and characterisation of intermediates that occur during homogeneously catalysed hydrogenations. In combination with parahydrogen as a reactand, this method takes advantage of NMR spectroscopy and zero‐quantum coherence that becomes modified under the Hamiltonian of an intermediate. The method allows the investigation of highly reactive intermediates that are inaccessible for conventional analytical methods. A theoretical framework is presented based on a density matrix description. It is shown that PASADENA (Parahydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment) NMR spectra of strongly coupled spin systems (i.e., AB systems) contain structural information of intermediates, even if they cannot be detected directly.
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

The 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

The 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

Is 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

A 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

The 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

2‐(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

Encapsulation 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

We 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

Picosecond 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

The 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

The 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

The 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

This 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

From 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

The 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

Kossel 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

Measurements 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

Zirconium 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

The 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.
Dielectric constants, refractive indexes and polarizations for acrylic esters +1‐alcohol binary mixtures at 298.15 and 308.15 K
Authors: Sastry Nandhibatla V. , Valand Mahendra K.
Published: July 2012

Dielectric constants and refractive indexes of binary mixtures of four 1‐alcohols (1‐heptanol, 1‐octanol, 1‐decanol and 1‐dodecanol) with three acrylic esters (methyl, ethyl and butyl acrylates) were measured at 298.15 and 308.15 K. The molar and orientation polarizations were calculated. The excess dielectric functions viz. excess dielectric constants, excess molar and orientation polarizations were evaluated. For a given ester the excess dielectric functions become less negative with the increase in the carbon chain length of 1‐alcohols. The variation of Kirkwood correlation factor, gK, with the ester mole fractions at 298.15 K for all the mixtures was examined. The results show that the disordering interactions are more predominant in the ester low regions followed by ester‐ester like interactions in the ester‐rich regions.
Adsorption of amide‐containing alkanethiols on gold
Authors: Zhang J. , Zhang H.L. , Chen M. , Zhao J. , Liu Z.F. , Li H.L.
Published: July 2012

Monolayers of a new series of amide‐containing alkanethiol compounds, CH3(CH2)n‐2C(O)NHC2H4SH (referred as CnC2SH, n=4, 5, 5, 8), were self‐assembled from ethanol solutions onto gold and their structures have been characterized using the reflectance absorption infrared spectroscopy (RA‐FTIR) and wettability measurement. It has been proved that the CO and N‐H bonds are parallel to the gold surface and the orientations of alkyl chain in these Self‐Assembled Monolayers (SAMs) are perpendicular to the surface. The special orientation of the amide moiety and molecular axis are suggested to be attributed to the hydrogen‐bonding formed between the amide moieties of neighbor molecules. Meanwhile, wettability measurement reveals that the packing degree of the SAMs depends on the length of the alkyl chains in the thiol molecules. These results demonstrate that both the hydrogen‐bonding and the length of the alkyl chain exerts the tremendous influnece on the orientation of amide‐containing alkanethiol molecules. And also, it give us a way to control the molecular orientation in SAMs in molecular level.
LIF detection of the IO‐radical and kinetics of the reactions I+O3 → IO+O2, O(3P)+I2 → IO+I, O(3P)+CH3I → IO+CH3 and O(3P)+CF3I → IO+CF3 in the temperature range 230 to 310 K
Authors: Hölscher D. , Fockenberg Chr. , Zellner R.
Published: July 2012

Laser induced fluorescence (LIF) from the (2,0) and (3,0) band of the A2∏3/2 ← X2∏3/2 transition of IO was detected. Using this detection, the rate constants for the reactions (1) I+O3 → IO+O2, (2) O(3P)+I2 → IO+I, (3) O(3P)+CH3I → IO+CH3 and (4) O(3P)+CF3I → IO+CF3 were determined. Reactions (1), (3) and (4) were investigated over me temperature range 230 to 310 K, whereas reaction (2) was measured only at 295 K. The rate constants for these reactions can be described by the following Arrhenius expressions:
Excited state electron transfer in 1,1′‐binaphthyl‐2,2′‐diyl‐phosphato‐pentaamminecobalt(III)
Authors: Kunkely Horst , Vogler Arnd
Published: July 2012

The phosphate ester 1,1′‐binaphthyl‐2,2′‐diyl‐phosphate shows an intense fluorescence of the binaphthyl moiety. In the complex 1,1′‐binaphthyl‐2,2′‐diyl‐phosphato‐pentaammine‐cobalt(III) this emission is quenched by excited state electron transfer to Co(III). As a consequence the phosphate ester undergoes an oxidative cleavage while Co(III) is reduced to Co(II) with ϕ = 0.023±0.002 at λirr=313 nm.
Picosecond IR study of the UV photo‐induced decomposition of Bis(naphthoyl)peroxide and of a naphthyl peroxycarbonate
Authors: Aschenbrücker J. , Buback M. , Ernsting N.P. , Schroeder J. , Steegmüller U.
Published: July 2012

Picosecond UV pump/IR probe experiments were carried out on bis(1‐naphthoyl)peroxide (1) and on tert‐butyl‐peroxy‐2‐naphthylcarbonate (2) dissolved in CCl4. After excitation by a 308 nm pulse the formation of CO2 is measured with a time resolution of 1.8 ps via the IR‐absorption of the v3‐mode at around 2340 cm−1 for delay times up to 550 ps. A pronounced temporal and spectral evolution of the v3‐absorption band of CO2 is found which is assigned to the initial formation of vibrationally excited CO2 and subsequent vibrational cooling by energy transfer to the surrounding solvent. CO2 concentration versus time profiles are obtained from the time dependence of the integrated v3‐band absorbance. The data suggest that 2 decomposes in a consecutive stepwise process whereas 1 undergoes concerted bond scission leading to CO2 formation within the time resolution of the experiment.
Elementary reactions in molecular ions — from thermal towards state‐selective experiments
Authors: Weitzel Karl‐Michael
Published: July 2012

Two important classes of experiments aimed at the energetics, kinetics and the mechanism of unimolecular reactions of molecular ions are discussed. The first kind of experiment is based on a thermal sample of molecules. As an example the energetically lowest reaction channel in the ethane ion, i.e. the H2 loss reaction, is presented. The second kind of experiment deals with state‐selected ions. Here as an example the photodissociation of HBr* ions is discussed.
The [4+2]‐cycloaddition of singlet oxygen (1Δg) to aromatic hydrocarbons: An Easy Method for Determination of Oxygen Concentrations in Organic Solvents
Authors: Neumann Marcus , Hild Markus , Brauer Hans‐Dieter
Published: July 2012

By determination of the photooxidized amount of a singlet oxygen (1Δg) acceptor one can calculate the oxygen concentration in solution. Rubrene was used as acceptor due to its photochemical properties. Acceptor concentrations were determined both gravimetrically and photometrically: Using this new method, the oxygen solubility was examined in nine aromatic solvents. The solubility parameters have been verified by an indirect method based on the diffusion‐controlled fluorescence quenching of pyrene by ground state oxygen. The rate constants are compared with the prediction of the Stokes Einstein equation for a diffusion‐controlled process.
Interaction between Cl− and linear and branched polyethyleneimine in aqueous solution studied by 35Cl‐ and 37Cl‐NMR
Authors: Meier Urs , Schläpfer Carl Wilhelm
Published: July 2012

Counter anion condensation in solutions of LPEI (linear polyethylene imine) and BPEI (branched polyethylene imine) was studied by 35Cl‐ and 37Cl‐NMR relaxation as a function of the charge density of the polycation, which is controlled by the degree of protonation of the polybase determined by the pH of the solution. The condensation in BPEI solutions is proportional to the degree of protonation. In LPEI solutions the counterion condensation is controlled by the charge and conformation of the polyion in solution, which changes upon protonation.
Electrochemically induced surface changes on microstructured Pt films on a Solid YSZ electrolyte
Authors: Poppe J. , Schaak A. , Janek J. , Imbihl R.
Published: July 2012

Thin (500 Å) microstructured Pt films on single crystalline yttrium stabilized zirconia (YSZ) prepared by photolithography have been used to study the processes at the interface between the solid electrolyte and the Pt film when electrical potentials are applied. The experiments are performed under UHV conditions with a standard three‐electrode set‐up employing photoemission electron microscopy (PEEM) as spatially resolving method. Upon electrochemical “pumping” of oxygen by applying a positive electric potential to the Pt film we observe in PEEM a darkening of the Pt film reflecting a work function (WF) increase; a negative potential leads to a WF decrease. No time‐dependent spatial concentration profiles which would have been indicative of the slow diffusional spreading of a spillover species were observed.
Investigation of the crystallization of lead titanate and lead zirconate titanate with X‐ray diffraction and differential thermal analysis
Authors: Merkle R. , Bertagnolli H.
Published: July 2012

Lead titanate (PT) and lead zirconate titanate (PZT) were prepared by the sol‐gel process from lead acetate and zirconium and titanium n‐propoxide dissolved in 2‐methoxyethanol. After pyrolysis of the organic residues, the crystallization kinetics were investigated. In situ X‐ray diffraction of the isothermal crystallization yields Avrami‐exponents of ≈︁ 3 for the perovskite formation of titanium‐rich samples. Both the pyrochlore and the perovskite formation of zirconium rich samples exhibit Avrami exponents of ≈︁ 1. Investigation of the non‐isothermal crystallization with differential thermal analysis (DTA) yields Avrami exponents that are similar to the isothermally determined values. The activation energy is 500 kJ/mol for the formation of the pyrochlore phase, and 300–350 kJ/mol for the formation of the perovskite phase. Possible crystallization mechanisms are discussed.
Novel general linear concentration rules and their theoretical aspects for multicomponent systems at constant chemical potentials — Ideal‐Like Solution Model and Dilute‐Like Solution Model
Authors: Wang Zhi‐Chang
Published: July 2012

General simple rules have been established both for binary and higher‐order systems related to their pure components and for ternary and higher‐order systems related to their binary subsystems. In this paper we present two new general linear concentration rules, together with their strict thermodynamic aspects and main statistical explanations, for the real systems {A1+A2+…+Aq+B+C+…+Z} related to their subsystems {A1+A2+…+Aq+i} (i∈{B,C,…,Z}) at constant chemical potentials of A1, A2,…,Aq. One of the linear rules holds good for the real system having zero interchange energy among B, C,…,Z, while the other linear rule, for the real system, having nonzero interchange energies among them. In the simplest case of q=1, they are identical with Zdanovskii's rule for mixed electrolyte aqueous solutions, semi‐ideal hydration model for mixed nonelectrolyte aqueous solutions, linear solubility equation for carbon in ternary alloys, and Wang's general linear concentration rules and corresponding solution models for the real systems {A+B+C+…+Z} related to their binary subsystems {A+i} at constant chemical potential of A, respectively, which quantitatively fit the literature experiments for organic and organic‐inorganic mixtures, aqueous and nonaqueous solutions, alloys, molten salt mixtures, slags and nonstoichiometric solid compounds under normal and supercritical conditions. In order to verify their applicabilities at q≥2, isopiestic measurements for fifteen quaternary and quinary aqueous solutions, each containing two unsaturated solutes and one or two saturated solutes, and joint solubility measurements for four quaternary systems, each containing two saturated solutes and two organic solvents, are reported at T=298.15 K. The experimental results are in excellent agreement with the new linear rules.
Reconciling ab initio and semiempirical approaches to lattice stabilities
Authors: Grimvall Göran
Published: July 2012

Starting around 1970, two methods have been developed that may give information about the lattice stability Hα/βHα–Hβ, i.e. the enthalpy difference between two phases, α and β, of a pure element or compound. The ab initio method is purely theoretical and relies on electron structure calculations. The CALPHAD method derives a Gibbs energy function GHTS from experimental thermodynamic data, including phase diagrams. A long‐standing problem has been that the two methods gave very different lattice stabilities for some transition metals, e.g., W. The reason for this discrepancy has recently been clarified, when the role of dynamical lattice instabilities was realised. This paper gives an account of how the discrepancy emerged and is now resolved. The interpretation and practical applicability of the Gibbs energy function in the CALPHAD approach to systems containing dynamical instabilities is clarified.
Thermodynamic description of the stable Pb‐Sn‐Zn and the metastable Al‐Cu‐Mg miscibility gaps
Authors: Hurtado I. , Jantzen T. , Spencer P.J.
Published: July 2012

The ternary system Pb‐Sn‐Zn displays a large miscibility gap in the liquid phase which extends from the binary system Pb‐Zn. The calculated isothermal sections at 714, 757, 805, 842 and 877 K, in which two liquid phases are shown to exist in equilibrium with each other, will be presented and compared with experimental data. The calculated activities of Zn in the liquid phase for different isopleth sections are in good agreement with experimental data.

The appearance of GP Zones in Al‐Cu‐Mg alloys may be described by a Gibbs energy function which predicts a miscibility gap in the fcc (Al) phase. The new function was first obtained for the binary Al‐Cu system and then extended into the ternary to describe the precipitation of GPZB zones. The Thermo‐Calc calculation package was used for this purpose.
Contribution of the electron subsystem state variations to the melting entropy of covalent crystals
Authors: Glazov V. M. , Koltsov V.B.
Published: July 2012

Analysis of the whole complex of the electrophysical and thermodynamic properties of semiconductors allowed us to conclude that melting entropy can be presented by three basic components, i.e. by the position, vibration and electron contributions. Possible methods of calculating the above contributions have been discussed, e.g. it has been shown that the electron contribution to the melting entropy of semiconductors that are metallized makes up a considerable part of the total melting entropy, which accounts for the anomalous values of the melting entropy of the substances discussed.
The enthalpy of formation and the heat capacity of liquid and undercooled liquid Al‐Cu‐La alloys
Authors: Zhou S. H. , Sommer F.
Published: July 2012

The integral enthalpy of mixing δH of liquid lanthanum‐rich and aluminium‐rich Al‐Cu‐La alloys was determined by solution calorimetry at 1078 K. An adiabatic calorimeter was used to measure the heat capacity Cp of liquid Al25Cu20La55 and Cu67La33 alloys. The association model was applied to calculate the thermodynamic functions of liquid Al‐Cu‐La alloys using the model parameter of the three limiting binary alloys. The results show systematic positive deviations from the experimental data. An additional ternary association reaction was assumed with a stoichiometry near to the composition of maximum deviation between calculated and experimental ΔH values. The new description reveals a good agreement with the experimental ΔH(x, 1078) and the Cp(x, T) data. The calculated Cp(T) curve of the undercooled liquid Al25La55Cu20 alloy exhibits a maximum.
Phase equilibria in aluminum‐carbon system at high temperatures
Authors: Gokcen N. A. , Oden L. L.
Published: July 2012

The partial excess Gibbs energies of carbon and aluminum in liquid Al‐C were used with the equilibrium vapor pressures of pure Al and pure C to obtain the phase diagrams (1) at 1 bar of total pressure, and (2) at pressures sufficient to suppress the gas phase. The first diagram contains [gas+graphite → liquid] tectoid‐type reaction at 2900±20K with the tectoid composition of 0.681 mol fraction of aluminum. The [gas/graphite] boundary is subject to large errors due to the large errors in the vapor pressure of pure graphite, but its general form is acceptable. The second diagram consists entirely of the condensed phases with peritectic reaction [liquid+graphite → Al4C3] at 2429 K. The [liquid/graphite] phase boundary was obtained from the graphite solubility in liquid solutions at 2429‐2800 K, yielding the partial excess Gibbs energy of carbon in solution.

The resulting phase boundary is as reliable as the melting point of graphite, taken to be 4130 K.
Experimental and thermodynamic investigations in the Ti‐Si‐C system
Authors: Du Yong , Schuster J. C.
Published: July 2012

The Si‐rich part of the 1100°C isothermal section and the isopleths at 5, 10 and 15 at.% C in the Ti‐Si‐C system were determined by means of DTA and XRD techniques. Two ternary eutectic reaction temperatures (1330°C for LSi+SiC+TiSi2 and 1485°C for LTiSi+TiSi2+Ti5Si3Cx) and three transition reaction temperatures (1485°C for L+Ti5Si3CxTi3SiC → 2+TiSi2, 1473°C for L+Ti3SiC2TiSi2+SiC, and 1341°C for L+TiCbcc‐(Ti)+Ti5Si3Cx) were observed. The decomposition temperature of the ternary compound Ti3SiC2 was determined by using a Pirani technique. By considering the experimental results from the present work and literature data, a consistent thermodynamic data set for the Ti‐Si‐C system was obtained.
Gibbs energy of formation of sic: A contribution to the thermodynamic stability of the modifications
Authors: Kleykamp Heiko
Published: July 2012

The Gibbs energy of formation ΔfGo of hexagonal α‐SiC was determined by electromotive force (emf) measurements between 1200 and 1300 K using the galvanic cell Si, SiO2|Th(Y)O2|SiO2, α‐SiC, C which gives (T in K): ΔfGo (α‐SiC) =‐94770+24.24·T J/mol. The third‐law enthalpy of formation was calculated as ΔfHo298 (α‐SiC)=‐74.4 kJ/mol at 298 K. In order to examine more closely the relative stability of α‐SiC and cubic β‐SiC, Gibbs energy of transformation ΔtrG measurements were made by the same method between 1100 and 1300 K using the cell β‐SiC, C, SiO2|Th(Y)O2|SiO2, C, α‐SiC. An emf of about 20 mV of the meta‐stable cell was measured up to 5 h cell operation. This observation implies that α‐SiC is the more stable modification in the investigated temperature range yielding ΔtrG (βα‐SiC) ≈︁ −8 kJ/mol at 1200 K.
The nickel‐rich part of the Ga‐Ni phase diagram and the corresponding phase relations in the ternary Ga‐Ni‐Sb system
Authors: Micke K. , Markovski S. L. , Ipser H. , van Loo F.J.J.
Published: July 2012

In the past years, metal/III‐V compound semiconductor interfaces have been investigated intensively in order to find a suitable metallization scheme meeting the required performance, reproducibility and stability criteria. Equilibrium phase diagrams provide a base for understanding the interfacial reactions, hence solid state equilibria at 600 and 900°C in the Ga‐Ni‐Sb system have been experimentally determined and are presented in this article. In the course of the work the nickel‐rich part of the Ga‐Ni phase diagram has been revised by means of DTA and the diffusion couple technique. This study was undertaken to clarify the true temperature range of stability for the compounds Ga2Ni3 and Ga9Ni13 that is of significant importance for establishing the Ga‐Ni‐Sb phase diagram.