Statistical theory for hydrogen bonding fluid system of AaDd type (I): The geometrical phase transition

The influence of hydrogen bonds on the physical and chemical properties of hydrogen bonding fluid system of A _a D _d type is investigated from two viewpoints by the principle of statistical mechanics. In detail, we proposed two new ways that can be used to obtain the equilibrium size distribution of the hydrogen bonding clusters, and derived the analytical expression of a relationship between the hydrogen bonding free energy and hydrogen bonding degree. For the nonlinear hydrogen bonding systems, it is shown that the sol-gel phase transition can take place under proper conditions, which is further proven to be a kind of geometrical phase transition rather than a thermodynamic one. Moreover, several problems associated with the geometrical phase transition and liquid-solid phase transition in nonlinear hydrogen bonding systems are discussed.     

Statistical theory for hydrogen bonding fluid system of A_aD_d type (I): The geometrical phase transition

The influence of hydrogen bonds on the physical and chemical properties of hydrogen bonding fluid system of AaDd type is investigated from two viewpoints by the principle of statistical mechanics. In detail, we proposed two new ways that can be used to obtain the equilibrium size distribution of the hydrogen bonding clusters, and derived the analytical expression of a relationship between the hydrogen bonding free energy and hydrogen bonding degree. For the nonlinear hydrogen bonding systems, it is shown that the sol-gel phase transition can take place under proper conditions, which is further proven to be a kind of geometrical phase transition rather than a thermodynamic one. Moreover, several problems associated with the geometrical phase transition and liquid-solid phase transition in nonlinear hydrogen bonding systems are discussed.     

Unique evolution of spatial and dynamic heterogeneities on the glass transition behavior of PVPh/PEO blends

Glass transition behavior of hydrogen bonded polymer blends of poly(vinyl phenol)(PVPh) and poly(ethylene oxide)(PEO) is systematically investigated using normal differential scanning calorimetry(DSC) and recently developed multifrequency temperature-modulated DSC(TOPEM),in combination with Fourier transform infrared spectroscopy(FTIR) and nuclear magnetic resonance(NMR) techniques,focusing on the effect of the PEO molecular weight on the spatial and dynamic heterogeneity.It is found,for the first time,that both the glass transition temperature(T_g) and activity energy(E_a) of the blends strongly depend on PEO molecular weight,and a common turning point,which separates the rapid and slow increasing regions,can be found.The interchain hydrogen bonding interactions,both determined by FTIR measurements and obtained from the Kwei equation,decrease with increasing PEO molecular weight,indicating a decrease of the componential miscibility.A series of parameters related to the microscopic spatial and dynamic heterogeneity,such as the activity energy, fragility,nonexponential factor and the size of cooperatively rearranging regions,are calculated from frequency dependency complex heat capacity measured using TOPEM.It is found that each of these parameters monotonically changes with increasing the PEO molecular weight during the glass transition process,demonstrating that hydrogen bonding interaction is the key factor in controlling the spatial and dynamic heterogeneity,thus the glass transition.NMR relaxation results reveal the existence of obvious phase separation large than 5 nm,implying that the cooperatively rearranging regions should be closely related to the interphase region between the two components.The above obtained origin and evolution of spatial and dynamic heterogeneity provide a new insight into the glass transition behavior of polymer blends.     

Employing halogen-halogen interaction to construct high-temperature hybrid perovskite phase transition materials

Organic-inorganic hybrid perovskites (OIHPs) materials with high phase transition temperature (T_p) have been widely studied in the field of molecular switches, solar energy and electric power. At present, the OIHPs with high T_p are generally constructed through molecular design, which can be applied to a wide temperature range. Here, three one-dimensional (1D) OIHPs [R-ClEQ]PbCl₃ (T_p = 442 K), [R-ClEQ]PbBr₃ (T_p= 499 K) and [R-ClEQ]PbI₃ (T_p above m.p.) (R-ClEQ = (R)-N-chloroethyl-3-quinuclidinol) with different T_p are obtained by regulating the halogen-halogen interaction and hydrogen bonding in the system. Especially in [R-ClEQ]PbX₃ (X = Cl, Br and I) crystal system, all the halogen bonds tend to form at approximately 180°angles and the strength of halogen bonding is found to be increased from 1.59 × 10^–3 Hartree to 2.35 × 10^–3 Hartree with increased atom number from Cl to I. The synergistic effect of halogen bonding and hydrogen bonding provide a useful strategy for the design OIHPs phase transition materials with high T_p.     

Direct observation of the anderson transition in hxwo3 bronzes by high-resolution x-ray photoelectron spectroscopy

We report high-resolution X-ray photoemission measurements of semiconducting and metallic hydrogen bronze H_xWO₃ as a function of the hydrogen/tungsten ratio. Complex structures in W 4f core-level spectra can be attributed to two discrete initial states in the semiconducting phase and to three states in the metallic phase. These results, and a comparison of valence and conduction band spectra with band structure calculations, are consistent with an Anderson transition at x ≈ 0.2 and conflict with percolation or Mott-type transition models.     

Crystallographic study of the ammonium/potassium 12-molybdophosphate ion-exchange system

Ammonium and potassium 12-molybdophosphate crystals obtained by gel-diffusion methods, as well as the products obtained by subjecting the parent compounds to ion exchange in solutions containing the alternate cations, have been studied by single-crystal X-ray diffraction studies, using automatic counting techniques. Details of the structures of five compounds of the type (K, NH₄, H₃O)₃(PO₄)Mo₁₂O₃₆, which have all been refined to R < 0.06, show that although the various crystals appear to be isomorphous (space group Pn3m, a ? 11.65Å), the incompleteness of exchange observed with these materials can be rationalized in terms of a phase transition consisting of the reorientation during ion exchange of ammonium ions in the exchanger phase. This is probably caused by a reorganization of the bonding pattern in the clathrated anionic unit to compensate for the rupture of bifurcated hydrogen bonds.     

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability

In this study, we investigate the crystal structures and phase equilibria of butanols+CH₄+H₂O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X‐ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hydrates without any hydrogen‐bonding interactions between the hydroxy group of the guests and the water‐host framework. However, Raman spectra of the binary (1‐butanol+CH₄) hydrate do not show the free OH signal, indicating that there could be possible hydrogen‐bonding interactions between the guests and hosts. We also measure the four‐phase equilibrium conditions of the butanols+CH₄+H₂O systems.     

Computational studies on electron and proton transfer in phenol‐imidazole‐base triads

The electron and proton transfer in phenol‐imidazole‐base systems (base = NH₂^? or OH^?) were investigated by density‐functional theory calculations. In particular, the role of bridge imidazole on the electron and proton transfer was discussed in comparison with the phenol‐base systems (base = imidazole, H₂O, NH₃, OH^?, and NH₂^?). In the gas phase phenol‐imidazole‐base system, the hydrogen bonding between the phenol and the imidazole is classified as short strong hydrogen bonding, whereas that between the imidazole and the base is a conventional hydrogen bonding. The n value in spⁿ hybridization of the oxygen and carbon atoms of the phenolic CO sigma bond was found to be closely related to the CO bond length. From the potential energy surfaces without and with zero point energy correction, it can be concluded that the separated electron and proton transfer mechanism is suitable for the gas‐phase phenol‐imidazole‐base triads, in which the low‐barrier hydrogen bond is found and the delocalized phenolic proton can move freely in the single‐well potential. For the gas‐phase oxidized systems and all of the triads in water solvent, the homogeneous proton‐coupled electron transfer mechanism prevails. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

The analysis of structural and electronic properties for assessment of intramolecular hydrogen bond (IMHB) interaction: a comprehensive study into the effect of substitution on intramolecular hydrogen bond of 4-nitropyridine-3-thiol in ground and electronic excited state

The hydrogen bond strength, molecular geometry, π-electron delocalization, and physical properties such as dipole moment, chemical potential, and chemical hardness of 4-nitropyridine-3-thiol and its 29 derivatives have been studied by means of density functional method with 6-311++G** basis set in gas phase and water solution. Also, the excited-state properties of intramolecular hydrogen bonding in these systems have been investigated theoretically using the time-dependent density functional theory method. The HOMA, NICS, PDI, ATI, FLU, and FLU_π indices as well-established aromaticity indicators have been examined. Natural bond orbital analysis is also performed for better understanding the nature of intramolecular interactions. The electron density and Laplacian (?² ρ) properties, estimated by AIM calculations, indicate that H···O bond possesses low ρ and positive ?² ρ values, which are in agreement with electrostatic character of the HBs, whereas S–H bond has covalent character. Numerous correlations between topological, geometrical, and energetic parameters are also found.     

Effect of carbon chain length of monocarboxylic acids on cloud point temperature of poly(2-ethyl-2-oxazoline)

The temperature-induced phase transition of poly(2-ethyl-2-oxazoline) (PEtOx) aqueous solution under mixing with a series of small carboxylic acids has been studied by turbidity measurements and laser light scattering. It has been found that cloud point temperature (T _cp) of the PEtOx was changed to varying degrees depending upon the pH, ionic strength, molar ratio of acids to 2-ethyl-2-oxazoline unit, and carbon chain length of small carboxylic acids. Significant change in T _cp was observed in the case of hexanoic acid. At acidic pH, an increase in the molar ratio of hexanoic acid to the 2-ethyl-2-oxazoline unit gradually decreased the phase transition temperature of the polymer as compared to the T _cp of pure PEtOx. At original pH 6 (pH?>?pK _a), T _cp shifts to higher value than that of pure PEtOx for lower molar ratios and decreased later on with increasing the molar ratio. The shift in the T _cp is described based on the differences in the driving force of phase transition, including hydrogen bonding between small carboxylic acids and PEtOx polymer and hydrophobic interaction.     

Physicochemical Characterization and Simulation of the Solid–Liquid Equilibrium Phase Diagram of Terpene-Based Eutectic Solvent Systems

The characterization of terpene-based eutectic solvent systems is performed to describe their solid–liquid phase transitions. Physical properties are measured experimentally and compared to computed correlations for deep eutectic solvents (DES) and the percentage relative error e_r for the density, surface tension, and refractive index is obtained. The thermodynamic parameters, including the degradation, glass transition and crystallization temperatures, are measured using DSC and TGA. Based on these data, the solid–liquid equilibrium phase diagrams are calculated for the ideal case and predictions are made using the semi-predictive UNIFAC and the predictive COSMO RS models, the latter with two different parametrization levels. For each system, the ideal, experimental, and predicted eutectic points are obtained. The deviation from ideality is observed experimentally and using the thermodynamic models for Thymol:Borneol and Thymol:Camphor. In contrast, a negative deviation is observed only experimentally for Menthol:Borneol and Menthol:Camphor. Moreover, the chemical interactions are analyzed using FTIR and ¹H-NMR to study the intermolecular hydrogen bonding in the systems.     

The nature of chemical bonding in nitramide

The spatial and electronic structure studies of nitramide NH₂NO₂ suggest that the change in its molecular geometry upon transition from the gas phase to the condensed state is caused by an increase in the contribution of conjugation between functional groups. According to the analysis of the Bader atomic charges, the effects of such conjugation are to a considerable extent governed by intramolecular charge transfer from the amino to the nitro group. From estimation of the contribution of conjugation to the charge transfer it follows that conjugation remains in the isolated molecule. The influence of hydrogen bonding on the increase in the contribution of conjugation and the corresponding charge redistribution in the molecule was considered. Despite the presence of conjugation between functional groups, the planar configuration of the molecule in the crystal is not realized and the crystallographic twofold axis corresponds to superposition of two molecular configurations with C _s symmetry.     

Effects of organophilic montmorillonite on hydrogen bonding, free volume and glass transition temperature of epoxy resin/polyurethane interpenetrating polymer networks

Epoxy resin nanocomposites containing organophilic montmorillonite (oM) and polyurethane were prepared by adding oM to interpenetrating polymer networks (IPNs) of epoxy resin and polyurethane (EP/PU). The dispersion degree of oM in EP/PU matrix was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fourier transform infrared spectrometry (FT-IR) showed that strong interactions existed between oM and EP/PU matrix, and oM had some effect on hydrogen bonding of these EP/PU IPNs nanocomposites. Positron annihilation spectroscopy (PALS) and differential scanning calorimetry (DSC) measurements were used to investigate the effect of oM and PU contents on free volume and glass transition temperature (T_g) of these nanocomposites. The PALS and DSC results clearly showed that the presence of oM led to a decrease in the total fractional free volume, which was consistent with increasing T_g upon addition of oM, ascribed to increasing hydrogen bonding in interfacial regions of oM and EP/PU matrix and enhancing the miscibility between EP phase and PU phase. In addition, with increasing PU content, the total fractional free volume increased, corresponding to decreasing T_g.     

FTIR study on molecular structure of poly(N-isopropylacrylamide) in mixed solvent of methanol and water

The intrachain and interchain hydrogen bonding of poly(N-isopropylacrylamide) (PNIPA) and intermolecular hydrogen bonding between PNIPA chains and the solvent molecules in the mixed solvent of methanol and water have been quantitatively investigated by using Fourier transform infrared (FTIR) spectroscopy at 25 °C. In this spectroscopic system with curve fitting program, we found that in the C-H stretching region, both the N-isopropyl group and the backbone underwent conformational change upon the solvent composition. An analysis of the amide I band suggested that the amide groups of PNIPA were mainly involved in intermolecular hydrogen bonding with water molecules, and the polymer chains were flexible and disordered in the mixed solvent when the methanol volume fraction (χ_v) was lower than 15%. While χ_v was in the range of 15-65%, about 30% of these intermolecular hydrogen bonding between the polymer and water were replaced by intrachain and interchain hydrogen bonding, consequently, PNIPA shrinked as aggregates. If χ_v was above 65%, the interchain hydrogen bonding became predominant due to the solubility characteristics of amphiphilic methanol, and the PNIPA system was homogeneous solution again. We believe that the reentrant transition is related to the weaker interaction between PNIPA molecules and methanol-water complexes, (H₂O)_m(CH₃OH)_n (m/n = 5/1, 5/2, 5/3, 5/4, 5/5) as compared to that between PNIPA and free water or free methanol.     

Extension of the associated perturbed anisotropic chain theory to mixtures with more than one associating component

The Associated Perturbed Anisotropic Chain Theory (APACT), a theory useful for calculation of equilibrium thermodynamic properties for fluid mixtures presented recently by Ikonomou and Donohue has been extended to treat multicomponent mixtures in which two or more components associate. Mixtures containing non-associating components (diluents) also can be treated. Association equilibria for each component hydrogen bonding to itself as well as for cross-association between associating components are considered. Following an approach similar to that used previously in the derivation of APACT, a closed-form equation of state has been derived. Preliminary results show that APACT does well in fitting phase equilibrium data for binary mixtures involving alcohols and water with small values of k_ij. The fits are not as good, however, for systems containing small molecules (water and methanol) and the reasons for this shortcoming are discussed.     

Collisional frequency shift with a primary frequency standard

When a hydrogen atom trapped in finite space formed by two parallel perfectly conducting plates, the transition rates can be modified. Life-time of the hydrogen 2P _1/2-state is estimated to be shorter as much as 56.3ps with a separation distance of b = 1.2μm between two plates. Although the modification is dependent on position of the atom, the life-time can be shorten or lengthen by adjusting the separation distance even for the case that the atom is placed at the center between two plates.     

A time-dependent density functional theory study on the excited state behavior of a novel T2(OH)B molecule

A novel pH-sensitive fluorescent probe T₂(OH)B was selected to theoretically investigate its excited state hydrogen bonding effects and excited state intramolecular proton transfer (ESIPT) process. First, it was verified that one intramolecular hydrogen bond is formed spontaneously in T₂(OH)B itself. Given the geometrical changes, we further confirm that the hydrogen bond should be strengthened in the first excited state. When it comes to the photoexcitation process, we present the charge redistribution around hydrogen bonding moieties facilitate the ESIPT tendency. The increased electronic densities around acceptor promote the attraction of hydrogen protons. The potential energy barrier in the constructed potential energy curves reveals that the ESIPT process of the T₂(OH)B system should be ultrafast. And comparing several nonpolar solvents, we deem solvent polarity plays little role in the ESIPT reaction. Furthermore, we also search the S₁-state transition state structure along with the ESIPT path, based on which we simulate the intrinsic reaction coordinate path. We not only confirm the ESIPT mechanism presented in this work but also clarify the ultrafast excited state process and explain previous experiment. We sincerely hope that our theoretical work could guide novel applications based on the T₂(OH)B system in future.     

A Hydrogen‐Bonded Cyanide‐Bridged [Co2Fe2] Square Complex Exhibiting a Three‐Step Spin Transition

Co‐crystallization of a cyanide‐bridged tetranuclear complex [Co₂Fe₂] with 4‐cyanophenol (CP) gave a hydrogen bonding donor–acceptor system, [Co₂Fe₂(bpy*)₄(CN)₆(tp*)₂](PF₆)₂⋅2 CP⋅8 BN ( 1 ). 1 exhibited a three‐step phase transition between HT, IM1, IM2, and LT phases upon temperature variation. Variable temperature magnetic measurements and structural analyses revealed that the three‐step spin transition is caused by electron‐transfer‐coupled spin transitions (ETCSTs) accompanied with alteration of the hydrogen bonding interactions.     

Epoxy resin/poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL) blends cured with 1,3,5-trihydroxybenzene: miscibility and intermolecular interactions

Epoxy resin (ER)/poly(ethylene oxide) (PEO) and/or poly(e-caprolactone) (PCL) blends cured with 1,3,5-trihydroxybenzene (THB) were prepared via the in situ curing reaction of epoxy monomers in the presence of PEO and/or PCL, which started from the initially homogeneous mixtures of DGEBA, THB and PEO and/or PCL. The miscibility and the intermolecular specific interactions in the thermosetting polymer blends were investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The two systems displayed single and composition-dependant glass transition temperatures (T _gs), indicating the full miscibility of the thermosetting blends. The experimental T _gs of the blends can be well accounted for by Gordon-Taylor and Kwei equations, respectively. The T _g-composition behaviors were compared with those of poly(hydroxyether of bisphnol A) (Phenoxy) blends with PEO and PCL. It is noted that the formation of crosslinked structure has quite different effects on miscibility and intermolecular hydrogen bonding interactions for the thermosetting polymer blends. In ER/PEO blends, the strength of the intermolecular hydrogen bonding interactions is weaker than that of the self-association in the control epoxy resin, which is in marked contrast to the case of Phenoxy/PEO blends. This suggests that the crosslinking reduces the intermolecular hydrogen bonding interactions, whereas the intermolecular hydrogen bonding interactions were not significantly reduced by the formation of the crosslinking structure in ER/PCL blends.