Hydrogen bond mediated self-assembly of two diblock copolymers

Two chemically dissimilar diblock copolymers, polybutadiene-b-poly(acrylic acid), PBd-b-PAA (M_w = 5.8–4 kg mol⁻¹) and poly(styrene)-b-poly(ethylene oxide), PS-b-PEO (M_w = 9–5 kg mol⁻¹) were blended in an effort to achieve morphologies typical of triblock copolymers. Blend compatibility was achieved by the hydrogen bond driven association of the PAA block of one diblock with the PEO block of the other. Small angle X-ray scattering was used to determine the morphologies of the compositions, which were further investigated using transmission electron microscopy and selective staining techniques. The crystallinity of the PEO block was determined by differential scanning calorimetry. The hydrogen bond interactions between PEO and PAA yielded a complex triblock lamellar morphology of the form PS-b-(PEO/PAA)-b-PBd-b-(PEO/PAA). This morphology was stable when crystallization of PEO was suppressed by sufficient interaction with PAA.     

Measuring acetic acid dimer modes by ultrafast time-domain Raman spectroscopy

Acetic acid is capable of forming strong multiple hydrogen bonds and therefore different dimeric H-bonded structures in neat liquid phase and in solutions. The low frequency Raman spectra of acetic acid (neat, in aqueous solution and as a function of temperature) were obtained by ultrafast time and polarization resolved optical Kerr effect (OKE) measurements. Isotropic OKE measurements clearly reveal a specific totally symmetric mode related to the dimeric structure H-bond stretching mode. The effects of isotope substitution, water dilution and temperature on this mode were investigated. These results together with anisotropic OKE measurements and density functional theory calculations for a number of possible dimers provide strong evidence for the cyclic dimer structure being the main structure in liquid phase persisting down to acetic acid concentrations of 10 M. Some information about the dimer structure and concentration dependence was inferred.     

Polarized IR spectra of the hydrogen bond in acetic acid crystals

The paper presents the results of our investigations of the polarized IR spectra of the hydrogen bond in crystals of acetic acid, CH₃COOH, as well as in crystals of three deuterium isotopomers of the compound: CH₃COOD, CD₃COOH and CD₃COOD. The spectra were measured at 283 K and at 77 K by a transmission method using polarized light. Theoretical analysis of the results concerned the linear dichroic effects, together with the H/D isotopic and temperature effects observed in the solid-state IR spectra of the hydrogen and of the deuterium bond at the frequency ranges of the ν_O–H and the ν_O–D bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of one of the quantitative theories of the IR spectra of the hydrogen bond, i.e. the “strong-coupling” theory or the “relaxation” theory when a hydrogen bond dimer model is used. From the spectra obtained it resulted that the strongest exciton coupling involved the closely spaced hydrogen bonds, belonging to different chains of associated acetic acid molecules. These results contradict the former explanation of the spectra within a model, which assumed a strong vibrational exciton coupling between four hydrogen bonds in a unit cell. On analyzing the spectra of isotopically diluted crystalline samples of acetic acid it has been proved that a non-random distribution of the protons and deuterons takes place in the hydrogen bond lattices. This non-conventional isotopic effect is a result of dynamical co-operative interactions involving hydrogen bonds in the system. Simultaneously it has been also found that in an individual hydrogen bonded chain in the crystals, distribution of the hydrogen isotope atoms H and D was fully random. The H/D isotopic “self-organization” mechanism most probably involves a pair of hydrogen bonds from each unit cell where each hydrogen bond belongs to a different chain.     

Hydrogen bond mediated supramolecular micellization of diblock copolymer mixture in common solvents

We report a new approach toward preparing self-assembled hydrogen-bonded complexes having vesicle and patched spherical structures from two species of block copolymers in nonselective solvents. Two diblock copolymers, poly(styrene-b-vinyl phenol) (PS-b-PVPh) and poly(methyl methacrylate-b-4-vinylpyridine) (PMMA-b-P4VP), were synthesized through anionic polymerization. The assembly of vesicles from the intermolecular complex formed after mixing PS-b-PVPH with PMMA-b-P4VP in THF was driven by strong hydrogen bonding between the complementary binding sites on the PVPH and P4VP blocks. In contrast, well-defined patched spherical micelles formed after blending PS-b-PVPh with PMMA-b-P4VP in DMF: the weaker hydrogen bonds formed between the PVPh and P4VP blocks in DMF, relative to those in THF, resulted in the formation of spherical micelles having compartmentalized coronas consisting of PS and PMMA blocks.     

The stability of the acetic acid dimer in microhydrated environments and in aqueous solution

The thermodynamic stability of the acetic acid dimer conformers in microhydrated environments and in aqueous solution was studied by means of molecular dynamics simulations using the density functional based tight binding (DFTB) method. To confirm the reliability of this method for the system studied, density functional theory (DFT) and second order M?ller-Plesset perturbation theory (MP2) calculations were performed for comparison. Classical optimized potentials for liquid simulations (OPLS) force field dynamics was used as well. One focus of this work was laid on the study of the capabilities of water molecules to break the hydrogen bonds of the acetic acid dimer. The barrier for insertion of one water molecule into the most stable cyclic dimer is found to lie between 3.25 and 4.8 kcal mol(-1) for the quantum mechanical methods, but only at 1.2 kcal mol(-1) for OPLS. Starting from different acetic acid dimer structures optimized in gas phase, DFTB dynamics simulations give a different picture of the stability in the microhydrated environment (4 to 12 water molecules) as compared to aqueous solution. In the former case all conformers are converted to the hydrated cyclic dimer, which remains stable over the entire simulation time of 1 ns. These results demonstrate that the considered microhydrated environment is not sufficient to dissociate the acetic acid dimer. In aqueous solution, however, the DFTB dynamics shows dissociation of all dimer structures (or processes leading thereto) starting after about 50 ps, demonstrating the capability of the water environment to break up the relatively strong hydrogen bridges. The OPLS dynamics in the aqueous environment shows--in contrast to the DFTB results--immediate dissociation, but a similar long-term behavior.     

Theoretical study of hydrogen and deuterium bond in glutaric acid crystal dimer

In the spirit of the work of Blaise et al. [J Chem Phys, 2005, 122, 64306], we have extended their quantum theoretical approach by accounting for the intrinsic anharmonicity of the slow frequency mode, which is described by a Morse potential to reproduce the polarized infrared line shapes of glutaric acid dimer and its deuterium derivative at different temperatures. In this approach, the adiabatic approximation is performed for each separate H‐bond bridge of the dimer, and a strong nonadiabatic correction is introduced into the model via the resonant exchange between the fast mode excited states of the two moieties. Working within the strong anharmonic coupling theory, according to which the high‐frequency mode is anharmonically coupled to the H‐bond bridge, this approach incorporated the Davydov coupling between the excited states of the two moieties, the quantum direct and indirect dampings and the intrinsic anharmonicity of the H‐bond bridge. The spectral density was obtained within the linear response theory by Fourier transform of the damped autocorrelation functions. The numerical results show that the theoretical line shapes of the glutaric acid dimer are in fairly good agreement with the experimental ones. Using a minimum number of independent parameters, this theoretical approach fits correctly the experimental line shapes of the glutaric acid dimer. The effects of deuteration and temperature have been successfully reproduced by our calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Hydrogen bond energies of 2-aminopyridine dimer and 2-aminopyridine-2-pyridone complex formation

The formation energies of 2-aminopyridine dimer and 2-aminopyridine-2-pyridone complex were measured in C₆D₆ solution by ¹H-NMR spectroscopy and values of 25.1 and 44.6 kJ/mol were obtained. The former value is about twice the magnitude of hydrogen bond energies which are generally observed for the N---HN system, and latter value is about half the sum of the formation energies of the 2-aminopyridine and 2-pyridone dimers. The formation energies of 2-aminopyridine dimer and 2-aminopyridine-2-pyridone complex were calculated by the CNDO/2 method, and their formation energies (−ΔH) were estimated to be 28.5 and 42.8 kJ/mol, respectively.     

Theoretical interpretation of the line shape of the gaseous acetic acid cyclic dimer

A general quantum theoretical approach of the nu(X-H) IR line shape of cyclic dimers of weakly H-bonded species in the gas phase is proposed. In this model, the adiabatic approximation (allowing to separate the high frequency motion from the slow one of the H-bond bridge), is performed for each separate H-bond bridge of the dimer and a strong nonadiabatic correction is introduced into the model via the resonant exchange between the fast mode excited states of the two moieties. The present model reduces satisfactorily to many models in the literature dealing with more special situations. It has been applied to the cyclic dimers (CD(3)CO(2)H)(2) and (CD(3)CO(2)D)(2) in the gas phase. It correctly fits the experimental line shape of the hydrogenated compound and predict satisfactorily the evolution in the line shapes, to the deuterated one by reducing simply the angular frequency of the H-bond bridge and the anharmonic coupling parameter by the factor 1/ square root of 2.     

Ab initio and DFT studies of hydrogen bond interactions in difluoroacetic acid dimer

Density functional theory (DFT) and ab initio calculations were performed for difluoroacetic acid (DFA). Eight theoretically possible conformers were considered and by using conformational analysis only three stable conformers were found. The hydrogen bonding interaction of DFA complex has been investigated using DFT and ab initio methods for cis conformers. Stabilization energies of dimers including basis set superposition error and ZPE were found in the range 8.89–13.08 kcal mol⁻¹. It was found that EFC dimer is slightly more stable. Red shift of O–H bond in the range −226.3 to 505.7 cm⁻¹ predicted for dimers. The natural bond orbital analysis was applied to characterize nature of the interaction.     

Hydrogen bonding of single acetic acid with water molecules in dilute aqueous solutions

In separation processes, hydrogen bonding has a very significant effect on the efficiency of isolation of acetic acid (HOAc) from HOAc/H₂O mixtures. This intermolecular interaction on aggregates composed of a single HOAc molecule and varying numbers of H₂O molecules has been examined by using ab initio molecular dynamics simulations (AIMD) and quantum chemical calculations (QCC). Thermodynamic data in aqueous solution were obtained through the self-consistent reaction field calculations and the polarizable continuum model. The aggregation free energy of the aggregates in gas phase as well as in aqueous system shows that the 6-membered ring is the most favorable structure in both states. The relative stability of the ring structures inferred from the thermodynamic properties of the QCC is consistent with the ring distributions of the AIMD simulation. The study shows that in dilute aqueous solution of HOAc the more favorable molecular interaction is the hydrogen bonding between HOAc and H₂O molecules, resulting in the separation of acetic acid from the HOAc/H₂O mixtures with more difficulty than usual.     

稀醋酸/水溶液中单个醋酸与水分子的氢键结构

在醋酸/水体系的工业分离中,溶液中的氢键对分离效率有很大影响.本文采用两种第一性原理方法,即从头算分子动力学模拟(AIMD)和量子化学计算(QCC),对由单个醋酸和不同水分子所组成聚合体的氢键相互作用进行了研究,采用极化统一模型和自洽反应场模型计算得到了聚合体在水溶液中的热力学数据.从QCC计算的气相和水溶液中的聚合自由能表明六元环在两种状态下都为最优结构,热力学数据反映出的各种结构的相对稳定性与AIMD模拟的环分布符合得相当一致.研究表明,由于存在醋酸和水分子间的氢键作用,稀醋酸/水溶液中的醋酸分离要比在浓醋酸溶液中困难得多.     

Hydrogen bond studies

The oxonium ion, H₃O⁺, has been studied with the MO-LCAO method in order to determine its equilibrium geometry. The main purpose has been to study effects of the external electrostatic forces exerted on the ion situated in crystals whose structures are experimentally known. Calculations have also been performed on the free ion, where the energy minimum is found for a non-planar conformation with H-O-H angles of 116.6° and O-H distances of 0.96 Å. The effect of an external field is essentially to lengthen the O-H distances and decrease the H-O-H angles in order to form approximately linear hydrogen bonds.

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Zusammenfassung Die Gleichgewichtsgeometrie des Oxoniumions H₃O⁺ wurde mit Hilfe der MO-LCAO Methode bestimmt. In erster Linie sind die Effekte, die von externen elektrostatischen Kräften auf das Ion in Kristallen mit bekannter Struktur ausgeübt werden, untersucht worden. Es wurden aber auch Rechnungen für das freie Ion durchgeführt, wobei die minimale Energie im Falle der nicht planaren Konformation mit H-O-H-Winkeln von 116,6° und einem O-H-Abstand von 0,96 Å erhalten wurde. Der Effekt eines externen Feldes besteht hauptsächlich in einer Verlängerung der O-H-Bindung und einer Verringerung des H-O-H-Winkels, so daß näherungsweise lineare Wasserstoffbindungen gebildet werden.

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Part 65 will appear in Acta Cryst. in the near future.     

Anion-exchange behaviour of various metals on DEAE-cellulose in mixed acetic acid-nitric acid media

A number of nitrato complexes of metals have been found to be adsorbed on DEAE-cellulose from mixed acetic acid-nitric acid media, although none can be adsorbed from aqueous nitric acid solutions. The distribution coefficients of Sc, Mo, La, Sm, W, Re, Bi, Th and U are given as functions of acetic acid and nitric acid concentrations (sometimes in the presence of hydrogen peroxide to prevent hydrolytic precipitation). For 25 other metals column adsorption behaviour is described for a 90% acetic acid-10% 7.6M nitric acid mixture. Favourable differences in the distribution coefficients allow useful separations such as FeMoW and USmMoBiTh, to be achieved.     

Hydrogen bond-induced vibronic mode mixing in benzoic acid dimer: a laser-induced fluorescence study

Laser-induced dispersed fluorescence spectra of benzoic acid dimer in the cold environment of supersonic jet expansion have been reinvestigated with improved spectral resolution of measurements. The spectra are analyzed with the aid of the normal mode vibrations of the dimer calculated by the ab initio quantum chemistry method at the DFT/B3LYP/6-311+G(*) (*) level of theory. The analysis reveals that the low-frequency intermolecular hydrogen bond modes are mixed extensively with the carboxyl as well as aromatic ring vibrations upon electronic excitation. The mode mixing is manifested as the complete loss of mirror symmetry relation between the fluorescence excitation and dispersed fluorescence spectra of the S(1) origin, and appearance of large number of cross-sequence transitions when the DF spectra are measured by exciting the low-energy vibrations near the S(1) origin. The cross-sequence bands are found in all the cases to be the combinations of two nontotally symmetric fundamentals consisting of one of the intermolecular hydrogen bond modes and the other from the aromatic ring and carboxyl group vibrations. The implications of this mode mixing on the excited state dynamics of the dimer are discussed.     

Hydrogen bond formations between pyrazine and formic acid and between pyrazine and trichloroacetic acid

The hydrogen bond formations between pyrazine and formic acids and between pyrazine and trichloroacetic acids were studied through observation of the Raman and infrared spectra for mixture of pyrazine and formic acid and also mixture of pyrazine and trichloroacetic acid at 77 K. It was observed that the mutual exclusion principle held for the Raman and infrared spectra of both mixtures, even for the spectra of the samples whose mixing mole ratio of acids was very low. This fact clearly indicates that the hydrogen bonded molecule does not exist in the form of formic acid-pyrazine or trichloroacetic acid-pyrazine whose geometry belongs to the Cs point group, but exists in the form of formic acid-pyrazine-formic acid or trichloroacetic acid-pyrazine-trichloroacetic acid belonging to the C(i) point group.     

Hydrogen bond network formation

The signs of the dynamic dichroism of the NH stretch and the amide I bands in nylons were found to be counterintuitive. Experiments show that polymer chains tend to align in the direction of an applied tensile strain. The CH stretching bands in nylons exhibit the expected negative dynamic dichroism indicating chain alignment in the strain direction. The ΔA′ peaks for the NH and amide I bands are positive. The ΔA′ peak for the NH band is also unusual in that it has a derivative shape. This can be explained by band shifts brought about by anisotropic changes in the intermolecular spacing in the glassy polymer. Above T_g the derivative shape disappears but the ΔA′ peak for both the NH and amide I absorption remain positive. We postulate that the positive ΔA′ peaks of the NH and amide I bands result from a hydrogen bonding network where stress is transmitted through a network consisting of covalent chains connected by hydrogen bonds.     

Peptide bond vibrational coupling

Neutral trialanine (Ala3), which is geometrically constrained to have its peptide bond at Phi and Psi angles of alpha-helix and PPII-like conformers, are studied at the B3LYP/6-31+G(d,p) level of theory to examine vibrational interactions between adjacent peptide units. Delocalization of the amide I, amide II, and amide III3 vibrations are analyzed by calculating their potential energy distributions (PED). The vibrational coupling strengths are estimated from the frequency shifts between the amide vibrations of Ala3 and the local amide bond vibrations of isotopically substituted Ala3 derivatives. Our calculations show the absence of vibrational coupling of the amide I and amide II bands in the PPII conformations. In contrast, the alpha-helical conformation shows strong coupling between the amide I vibrations due to the favorable orientation of the C=O bonds and the strong transitional dipole coupling. The amide III3 vibration shows weak coupling in both the alpha-helix and PPII conformations; this band can be treated as a local independent vibration. Our calculated results in general agree with our previous experimental UV Raman studies of a 21-residue mainly alanine-based peptide (AP).     

Selective catalytic oxidation of ethanol to acetic acid on dispersed Mo-V-Nb mixed oxides

The direct oxidation of ethanol to acetic acid is catalyzed by multicomponent metal oxides (Mo-V-NbO(x)) prepared by precipitation in the presence of colloidal TiO(2) (Mo(0.61)V(0.31)Nb(0.08)O(x)/TiO(2)). Acetic acid synthesis rates and selectivities (~95 % even at 100 % ethanol conversion) were much higher than in previous reports. The presence of TiO(2) during synthesis led to more highly active surface areas without detectable changes in the reactivity or selectivity of exposed active oxide surfaces. Ethanol oxidation proceeds via acetaldehyde intermediates that are converted to acetic acid. Water increases acetic acid selectivity by inhibiting acetaldehyde synthesis more strongly than its oxidation to acetic acid, thus minimizing prevalent acetaldehyde concentrations and its intervening conversion to CO(x). Kinetic and isotopic effects indicate that C-H bond activation in chemisorbed ethoxide species limits acetaldehyde synthesis rates and overall rates of ethanol conversion to acetic acid. The VO(x) component in Mo-V-Nb is responsible for the high reactivity of these materials. Mo and Nb oxide components increase the accessibility and reducibility of VO(x) domains, while concurrently decreasing the number of unselective V-O-Ti linkages in VO(x) domains dispersed on TiO(2).     

Hydrogen bond lifetime in supercritical water

Molecular dynamics simulations of water were performed in a wide range of supercritical temperatures and pressures with using TIP4P-HB model potential. The effect of state parameters on microdynamics of hydrogen bonds was studied. Isobaric (P = 30, 50, 80, 100 MPa) and isothermal (T = 673 K) dependencies of average hydrogen bond lifetime were calculated.