A calorimetric study of the glass transition kinetics in polyisoprene

The connection between the dielectric and calorimetric relaxation behaviours of synthetic polyisoprene Cariflex IR 305 is studied. A similar comparison of dielectric and dilatometric results was described in [1]. The heat capacity was measured during heating of samples prepared with different thermal history. Experimental results were compared with the heat capacity curves calculated for a model based on the multiparameter theory of Kovacs et al. [4]. The model considers the relaxation system as being composed of a set of subsystems characterized by different relaxation times. The distribution of relaxation times and their temperature dependence were taken from the diclectric measurement. The relaxation time of a subsystem from posed to depend, not only on the actual, temperature of the sample, but also on the deviation of this subsystem from equilibrium, or alternatively, on the deviation of the system as a whole. The comparison between the measured and modeled curves shows that both influences must be taken into account in order to explain the experimental results.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday.     

Synthesis,characterisation and thermal properties of hydrazinium metal oxalate hydrates. crystal and molecular structure of hydrazinium copper oxalate monohydrate

Complexes of the formula (N₂H₅)₂M(C₂O₄)₂·nH₂O where M = Co, Ni, Cu and n = 3, 2, 1 respectively have been prepared and their spectral, magnetic and thermal properties investigated. The copper complex, (N₂H₅)₂Cu(C₂O₄)₂·H₂O has a square planar geometry with no coordination of hydrazinium cations as confirmed by a single crystal X-ray study. Nickel and cobalt form octahedral complexes and X-ray powder patterns reveal that the complexes are not isomorphous.     

Phase transition in the phenanthrene crystal. A spectroscopic study

The Raman spectrum of the phenanthrene crystal was investigated between ?190 and +90°C. The results indicate that intermolecular forces play an important role in the solid—solid phase transition at 70°C. The assumption of a molecular conformational change was not confirmed.     

Electron and nuclear positions in the short hydrogen bond in urotropine-N-oxide.formic acid

The crystal structure of urotropine-N-oxide.formic acid, as determined from multiple temperature single-crystal X-ray diffraction experiments in the range 123-295 K and from neutron diffraction at 123 K, is reported. There is a strong hydrogen bonding interaction between the OH of formic acid and the N-oxide of urotropine, with the oxygen-oxygen distance ranging from 2.4300(10) to 2.4469(10) A. The electron density of the hydrogen atom associated with this interaction was located in the Fourier difference maps of the spherical atom refinement after all heavy atom positions were determined. The maximum of the electron density associated with the hydrogen bond is located approximately 1.16 A from the formate segment, though the distribution of electron density is very broad. The electron density associated with the H atom is thus shown by these accurate X-ray diffraction experiments to be approximately centered at all temperatures studied. This was conclusively confirmed by single-crystal neutron diffraction data obtained at 123 K, from which statistically equivalent O-H distances of 1.221(7) and 1.211(7) A were obtained.     

A reexamination of the ice III/IX hydrogen bond ordering phase transition

Ice III is a hydrogen bond disordered crystal which when cooled 1 K / min or faster transforms to an antiferroelectric hydrogen bond ordered structure, ice IX. Throughout its region of stability, experiments indicate that the H bonds in ice III are, in fact, partially ordered, i.e., some proton arrangements are preferred. In addition, there has been evidence that the structure of ice IX retains some residual disorder after the transition. Diffraction experiments and calorimetry apparently conflict with regard to the degree of ordering at the ice III/IX transition. Mean field statistical mechanical theories have been used to link partial occupations from diffraction data with thermodynamics. In this work, we investigate the ice III/IX proton ordering phase transition using electronic density functional theory calculations for small unit cells, extended to simulate the phase transition in a large unit cell using graph invariants. In agreement with experiment, we observe partial ordering over a wide range of temperatures as ice III transforms to partially disordered ice IX, near 126 K, which becomes fully ordered at lower temperatures. We compare our results from full statistical mechanical simulations with mean field models, finding small errors for the low-temperature ice IX phase and much larger errors for the high-temperature ice III phase. The failure of mean field theories may explain the apparent conflict between diffraction experiments and calorimetry.     

A model study of the interaction of a hydrogen bond with a π-electron system

A semi-empirical CNDO/2 calculation of the structure of the diimide-hydrogen fluoride hydrogen bonded complex is reported. The results are compared to calculations on a similar saturated system in an attempt to elucidate the effect of a neighboring -system on a hydrogen bond, and to uncomplexed diimide to study the effect of the hydrogen bond on the -system.     

A theoretical study of the hydrogen bond donor capability and co-operative effects in the hydrogen bond complexes of the diaza-aromatic betacarbolines

In this work, we present a quantum mechanical investigation on the hydrogen bond interactions of N(9)-methyl-9H-pyrido[3,4-b]indole, MBC, and N(2)-methyl-9H-pyrido[3,4-b]indole, BCA, with different hydrogen bond donors. Thus, it has been analysed the influence that the hydrogen bond donor strength and the co-operative effect of the increasing number of donor molecules have on the shape of the potential energy surfaces versus the N···H distances, r(N–H). To rationalize the nature of the interactions, the Bader theory has been applied and the characteristics of the bond critical points analysed. The results show that two different hydrogen bond complexes can be formed depending on the donor capabilities or the number of donor molecules included in the calculations. The topological parameters from the Bader theory are used to justify the statement that the analysed interactions can be classified as weak or partially covalent hydrogen bond interactions, respectively. As experimentally observed, weak hydrogen bond donors form weak hydrogen bond complexes, called HBC. Upon the increase of the donor strength the N···H proton is shifted nearest to the nitrogen atom giving rise to the observation of a stronger hydrogen bond complex, the proton transfer complex, PTC. The most outstanding result of these studies is the fact that the formation of the PTC can also be managed just by changing the number of donor molecules, that is, by a co-operative effect of the hydrogen bonds.     

A non-empirical study of the hydrogen bond between peptide units

A detailed study of hydrogen bonding in the linear dimers of formamide has been performed in a non-empirical SCF framework. The stabilization energy is studied as a function of the distance between the monomers and a decomposition of the energy into the coulomb, exchange and polarization and charge transfer contributions is given as well as a study of the underlying changes in the electron distribution. The angular displacements around the carbonyl oxygen are studied in the antiparallel dimer and show a very shallow minimum. The effects of hydrogen bonding on the NH stretching are satisfactorily accounted for.

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Zusammenfassung Die Wasserstoffbindung in den linearen Dimeren des Formamids wird im Rahmen der nichtempirischen SCF-Theorie behandelt. Die Stabilisierungsenergie wird als Funktion des Abstands der Monomeren untersucht. Dabei wird auch die Zerlegung der Energie in Coulomb-, Austausch-, Polarisations- Ladungsübertragungs-Anteile gegeben. Die dabei stattfindenden Änderungen in der Elektronenverteilung werden untersucht. Die winkelabhängigen Verschiebungen um das Sauerstoffatom der Carbonyl-Gruppe werden für das antiparallele Dimere untersucht und zeigen ein schwaches Minimum. Der Einfluß der Wasserstoffbindung auf die NH-Streckung wird befriedigend wiedergegeben.

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Résumé Une étude détaillée de la liaison hydrogène dans les dimères linéaires de la formamide est faite par une méthode SCF non-empirique. L'énergie de stabilisation est étudiée en fonction de la distance entre les monomères et de l'angle des liaisons C=O et HN. Une décomposition de l'énergie en ses composantes coulombienne, d'échange et de polarisation plus transfert de charge a été faite ainsi qu'une étude concomittante des déplacements électroniques correspondants.

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After this article was completed, we received in communication a preprint by P. A. Kollman and L. C. Allen where they make a partition of the hydrogen-bond energy into electrostatic and delocalization contributions. These corresponds to our E ¹ and E _P+CT.     

A calorimetric study on the low temperature dynamics of doped ice V and its reversible phase transition to hydrogen ordered ice XIII

Doped ice V samples made from solutions containing 0.01 M HCl (DCl), HF (DF), or KOH (KOD) in H(2)O (D(2)O) were slow-cooled from 250 to 77 K at 0.5 GPa. The effect of the dopant on the hydrogen disorder --> order transition and formation of hydrogen ordered ice XIII was studied by differential scanning calorimetry (DSC) with samples recovered at 77 K. DSC scans of acid-doped samples are consistent with a reversible ice XIII <--> ice V phase transition at ambient pressure, showing an endothermic peak on heating due to the hydrogen ordered ice XIII --> disordered ice V phase transition, and an exothermic peak on subsequent cooling due to the ice V --> ice XIII phase transition. The equilibrium temperature (T(o)) for the ice V <--> ice XIII phase transition is 112 K for both HCl doped H(2)O and DCl doped D(2)O. From the maximal enthalpy change of 250 J mol(-1) on the ice XIII --> ice V phase transition and T(o) of 112 K, the change in configurational entropy for the ice XIII --> ice V transition is calculated as 2.23 J mol(-1) K(-1) which is 66% of the Pauling entropy. For HCl, the most effective dopant, the influence of HCl concentration on the formation of ice XIII was determined: on decreasing the concentration of HCl from 0.01 to 0.001 M, its effectiveness is only slightly lowered. However, further HCl decrease to 0.0001 M drastically lowered its effectiveness. HF (DF) doping is less effective in inducing formation of ice XIII than HCl (DCl) doping. On heating at a rate of 5 K min(-1), kinetic unfreezing starts in pure ice V at approximately 132 K, whereas in acid doped ice XIII it starts at about 105 K due to acceleration of reorientation of water molecules. KOH doping does not lead to formation of hydrogen ordered ice XIII, a result which is consistent with our powder neutron diffraction study (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758). We further conjecture whether or not ice XIII has a stable region in the water/ice phase diagram, and on a metastable triple point where ice XIII, ice V and ice II are in equilibrium.     

Subangstrom crystallography reveals that short ionic hydrogen bonds, and not a His-Asp low-barrier hydrogen bond, stabilize the transition state in serine protease catalysis

To address questions regarding the mechanism of serine protease catalysis, we have solved two X-ray crystal structures of alpha-lytic protease (alphaLP) that mimic aspects of the transition states: alphaLP at pH 5 (0.82 A resolution) and alphaLP bound to the peptidyl boronic acid inhibitor, MeOSuc-Ala-Ala-Pro-boroVal (0.90 A resolution). Based on these structures, there is no evidence of, or requirement for, histidine-flipping during the acylation step of the reaction. Rather, our data suggests that upon protonation of His57, Ser195 undergoes a conformational change that destabilizes the His57-Ser195 hydrogen bond, preventing the back-reaction. In both structures the His57-Asp102 hydrogen bond in the catalytic triad is a normal ionic hydrogen bond, and not a low-barrier hydrogen bond (LBHB) as previously hypothesized. We propose that the enzyme has evolved a network of relatively short hydrogen bonds that collectively stabilize the transition states. In particular, a short ionic hydrogen bond (SIHB) between His57 Nepsilon2 and the substrate's leaving group may promote forward progression of the TI1-to-acylenzyme reaction. We provide experimental evidence that refutes use of either a short donor-acceptor distance or a downfield 1H chemical shift as sole indicators of a LBHB.     

A study of the hydrogen bond by means of comparative calculations

The nature of the hydrogen bond is investigated by means of a comparative analysis of some hydrogen bonded and hydrogen-like bonded systems. It is concluded that the hydrogen bond is determined by electrostatic interaction between the proton and the region of high electron density in the neighbour molecule.     

Nature of one-dimensional short hydrogen bonding: bond distances, bond energies, and solvent effects

On the basis of recently synthesized calix[4]hydroquinone (CHQ) nanotubes which were self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds (SHB), we have investigated the nature of 1-D SHB using first-principles calculations for all the systems including the solvent water. The H-bonds relay (i.e., contiguous H-bonds) effect in CHQs shortens the H...O bond distances significantly (by more than 0.2 A) and increases the bond dissociation energy to a large extent (by more than approximately 4 kcal/mol) due to the highly enhanced polarization effect along the H-bond relay chain. The H-bonds relay effect shows a large increase in the chemical shift associated with the SHB. The average binding energies for the infinite 1-D H-bond arrays of dioles and dions increase by approximately 4 and approximately 9 kcal/mol per H-bond, respectively. The solvent effect (due to nonbridging water molecules) has been studied by explicitly adding water molecules in the CHQ tube crystals. This effect is found to be small with slight weakening of the SHB strength; the H...O bond distance increases only by 0.02 A, and the average binding energy decreases by approximately 1 kcal/mol per H-bond. All these results based on the first-principles calculations are the first detailed analysis of energy gain by SHB and energy loss by solvent effect, based on a partitioning scheme of the interaction energy components. These reliable results elucidate not only the self-assembly phenomena based on the H-bond relay but also the solvent effect on the SHB strength.     

D and Cs NMR study of the hydrogen bond network and antiferroelectric phase transition of cesium trihydrogen selenite

The ²D and ¹³³Cs NMR spectra of deuterated and protiated single crystals of antiferroelectric cesium trihydrogen selenite have been studied in the high- and low-temperature phases. The number of chemically nonequivalent hydrogen bonds, their lengths, and directions in the unit cell were determined from deuteron electric field gradient tensors. The deuterons of centered hydrogen bonds have been found disordered in the paraelectric phase over two equivalent sites on either side of a center of symmetry. The antiferroelectric phase transition is accompanied by order-disorder phenomena of the H system and displacive behavior of the heavy-ion system.     

A theoretical study of the sulfenate–sulfoxide rearrangement. Effect of the hydrogen bond complexation

A DFT study of the thermal and radical sulfenate–sulfoxide rearrangement of derivatives of 3‐propenyl sulfoxide has been carried out. The effect of the substitution and hydrogen bond complexation has been analyzed. The results show that without external factors the radical breakdown path is the one preferred by the alkyl and aromatic derivatives while the unsubstituted system proceeds preferentially through a two‐step series of [1,3]‐ and [2,3]‐sigmatropic shifts. The inclusion of a hydrogen bond donor interacting with the oxygen atom increases the stability of all the species except the radical and the final products. Thus, in the dimethyl derivative the radical and two‐step processes present similar limiting steps. The analysis of the electron density of the systems provides some relationships between the properties at the bond critical point and the interatomic distances for the S···C and H···O cases. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2391–2397, 2010     

Forty years of progress in the study of the hydrogen bond

Structural Chemistry - The author looks back at developments over the last few decades concerning the H–bond. The list of atoms involved as proton donor and acceptor has broadened...     

Thermal properties of hydrazinium fluorometallates of the first row transition elements

Thermal analyses of hydrazinium fluorometallates of the first row transition elements and zinc are described. The intermediate compounds of the thermal decompositions are either ammonium fluorometallates or adducts of metal fluorides with hydrazine. The end products of the thermal analyses are in all cases metal fluorides, except in the case of the copper compound where elemental copper is obtained.     

Car-Parrinello and path integral molecular dynamics study of the hydrogen bond in the chloroacetic acid dimer system

We have studied the double proton transfer (DPT) reaction in the cyclic dimer of chloroacetic acid using both classical and path integral Car-Parrinello molecular dynamics. We also attempt to quantify the errors in the potential energy surface that arise from the use of a pure density functional. In the classical dynamics a clear reaction mechanism can be identified, where asynchronized DPT arises due to coupling between the O-H stretching oscillator and several low energy intermolecular vibrational modes. This mechanism is considerably altered when quantum tunneling is permitted in the simulation. The introduction of path integrals leads to considerable changes in the thermally averaged molecular geometry, leading to shorter and more centered hydrogen bond linkages.     

An adsorption and calorimetric study of the interaction of hydrogen with chromium oxide

The chemisorption of hydrogen on porous chromium oxide was studied at temperatures up to 723 K under the conditions of controlled oxygen, hydrogen, and water contents in the samples. The molar heats of chemisorption were measured at temperatures up to 473 K. Hydrogen was found to be chemisorbed (310 kJ/mol) in the form of water and absorbed into oxide volume (165 kJ/mol) in the form of coordination bound atoms. Changes in the molar heat of the processes during chemisorption were caused by changes in the ratio between chemisorption and sorption. The phenomena observed and quantitative results could be explained by simple Langmuir concepts without assumptions of surface heterogeneity.     

A theoretical study of the effect of a tetraalkylammonium counterion on the hydrogen bond strength in Z-hydrogen maleate.

High-level ab initio calculations (B3LYP/6-31+G and QCISD(T)/6-311+G**) were carried out to resolve the disagreement between recent experimental and computational estimates of the relative strength of the intramolecular hydrogen bond in Z-hydrogen maleate anion with respect to the normal hydrogen bond in maleic acid. The computational estimates for the strength of the intramolecular hydrogen bond in the gas-phase maleate anion are in a range of 14-28 kcal/mol depending on the choice of the reference structure. Computational data suggest that the electrostatic influence of a counterion such as a tetraalkylammonium cation can considerably weaken the hydrogen bonding interaction (by 1.5-2 times) in the complexed hydrogen maleate anion relative to that in the naked anion. The estimated internal H-bonding energies for a series of Z-maleate/R4N+ salts (R = CH3, C2H5, CH3CH2CH2CH2) range from 8 to 13 kcal/mol. The calculated energy differences between the E- and Z-hydrogen maleates complexed to Me4N+, Et4N+, and Bu4N+ cation are 4.9 (B3LYP/6-31+G(d,p)) and 5.7 and 5.8 kcal/mol (B3LYP/6-31G(d)). It is also demonstrated that the sodium cation exerts a similar electrostatic influence on the hydrogen bond strength in bifluoride anion (FHF-). The present study shows that while low-barrier short hydrogen bonds can exist in the gas phase (the barrier for the hydrogen transfer in maleate anion is only 0.2 kcal/mol at the QCISD(T)/6-311+G//QCISD/6-31+G level), whether they can also be strong in condensed media or not depends on how their interactions with their immediate environment affect their strength.