Statistical theory for hydrogen bonding fluid system of AaDd type (Ⅱ): Properties of hydrogen bonding networks

Making use of the invariant property of the equilibrium size distribution of the hydrogen bonding clusters formed in hydrogen bonding system of AaDd type, the analytical expressions of the free energy in pregel and postgel regimes are obtained. Then the gel free energy and the scaling behavior of the number of hydrogen bonds in gel phase near the critical point are investigated to give the corresponding scaling exponents and scaling law. Meanwhile, some properties of intermolecular and intramolecular hydrogen bonds in the system, sol and gel phases are discussed. As a result, the explicit relationship between the number of intramolecular hydrogen bonds and hydrogen bonding degree is obtained.     

Statistical theory for hydrogen bonding fluid system of AaDd type (Ⅱ): Properties of hydrogen bonding networks

Making use of the invariant property of the equilibrium size distribution of the hydrogen bonding clusters formed in hydrogen bonding system of A_aD_d type, the analytical expressions of the free energy in pregel and postgel regimes are obtained. Then the gel free energy and the scaling behavior of the number of hydrogen bonds in gel phase near the critical point are investigated to give the corresponding scaling exponents and scaling law. Meanwhile, some properties of intermolecular and intramolecular hydrogen bonds in the system, sol and gel phases are discussed. As a result, the explicit relationship between the number of intramolecular hydrogen bonds and hydrogen bonding degree is obtained. Supported by the National Natural Science Foundation of China (Grant Nos. 20303006 and 20574016), the Natural Science Foundation of Hebei Province (Grant Nos. B2006000959 and B2004000093) and the Natural Science Foundation of Education Committee of Hebei Province (Grant No. 2003101)     

Reaction dynamics and statistical theory for the growth of hydrogen bonding clusters

The similarities between the formation of hydrogen bonds and polycondensation reactions are stated from the statistical viewpoint, and then taking the hydrogen bonding system of AaDd type as an example, the growing process of hydrogen bonding clusters is investigated in terms of the theory of reaction dynamics and statistical theory for polymeric reactions. The two methods lead to the same conclusions, stating that the statistical theory for polymerization is applicable to the hydrogen bonding systems. Based on this consideration, the explicit relationship between the conversions of proton-donors and proton-acceptors and the Gibbs free energy of the system under study is given. Furthermore, the sol-gel phase transition is predicted to take place in some hydrogen bonding systems, and the corresponding generalized scaling laws describing this kind of phase transition are obtained.     

Statistical theory for hydrogen bonding fluid system of AaDd type (Ⅲ): Equation of state and fluctuations

The equation of the state of the hydrogen bonding fluid system of AaDd type is studied by the principle of statistical mechanics. The influences of hydrogen bonds on the equation of state of the system are obtained based on the change in volume due to hydrogen bonds. Moreover, the number density fluctuations of both molecules and hydrogen bonds as well as their spatial correlation property are investigated. Furthermore, an equation describing relation between the number density correlation function of "molecules-hydrogen bonds" and that of molecules and hydrogen bonds is derived. As application,taking the van der Waals hydrogen bonding fluid as an example, we considered the effect of hydrogen bonds on its relevant statistical properties.     

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.     

Cooperative effect in hydrogen bonding of N-methylacetamide in carbon tetrachloride solution confirmed by NMR and IR spectroscopies

Recently we carried out ab initio molecular orbital calculations of the hydrogen bond energies in the dimers and trimers of N-methylacetamide (NMA), and found the existence of a cooperative effect in the hydrogen bonding, by which formation of one hydrogen bond in a hydrogen-bonded chain enhances the strength of another hydrogen bond. In order to confirm the existence of such a cooperative effect, we have determined experimentally the enthalpy changes occurring upon hydrogen bonding of NMA in CCl4 solution. First, following the method proposed by us, the population fractions of free (non-bonded) NH protons are obtained from the observed amide proton NMR chemical shifts and the IR intensities of the free NH stretching bands. Next, the enthalpy changes are evaluated by analyzing the equilibrium between the free and bonded states of an NH proton. In this analysis, the existence of the CCl4 solvent is taken into account. The stabilization energy of hydrogen bonds in a trimer, as compared with twice the hydrogen bond energy in a dimer, is 5.4 kJ mol(-1), in good agreement with the calculated value (5.9 kJ mol(-1)). This result provides experimental confirmation of the existence of a cooperative effect in hydrogen bonding.     

Statistical theory for hydrogen bonding fluid system of A a D d 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.     

Evaluation of the individual hydrogen bonding energies in N-methylacetamide chains

The individual hydrogen bonding energies in N-methylacetamide chains were evaluated at the MP2/6-31+G** level including BSSE correction and at the B3LYP/6-311++G(3df,2pd) level including BSSE and van der Waals correction. The calculation results indicate that compared with MP2 results, B3LYP calculations without van der Waals correction underestimate the individual hydrogen bonding energies about 5.4 kJ mol^?1 for both the terminal and central hydrogen bonds, whereas B3LYP calculations with van der Waals correction produce almost the same individual hydrogen bonding energies as MP2 does for those terminal hydrogen bonds, but still underestimate the individual hydrogen bonding energies about 2.5 kJ mol^?1 for the hydrogen bonds near the center. Our calculation results show that the individual hydrogen bonding energy becomes more negative (more attractive) as the chain becomes longer and that the hydrogen bonds close to the interior of the chain are stronger than those near the ends. The weakest individual hydrogen bonding energy is about ?29.0 kJ mol^?1 found in the dimer, whereas with the growth of the N-methylacetamide chain the individual hydrogen bonding energy was estimated to be as large as ?62.5 kJ mol^?1 found in the N-methylacetamide decamer, showing that there is a significant hydrogen bond cooperative effect in N-methylacetamide chains. The natural bond orbital analysis indicates that a stronger hydrogen bond corresponds to a larger positive charge for the H atom and a larger negative charge for the O atom in the N-H?O=C bond, corresponds to a stronger second-order stabilization energy between the oxygen lone pair and the N-H antibonding orbital, and corresponds to more charge transfer between the hydrogen bonded donor and acceptor molecules.     

Pressure and temperature dependence of the hydrogen bonding in supercritical ethanol: a computer simulation study

As a step toward deeper insight on the "hydrogen bonding" in supercritical ethanol (scEtOH), we carried out NVT molecular dynamics simulations of the fluid over a wide range of temperatures and pressures. The fluid was studied at SC conditions for which thermodynamic and spectroscopic (NMR, infrared, Raman, dielectric) data are available. The various site-site pair distribution functions (pdf's) were calculated, and their temperature and pressure dependence was obtained. It was found that over the thermodynamic conditions investigated here, scEtOH remains highly structured. Moreover, the characteristic behavior of the first peaks in H-H, O-O, and H-O pdf's reveals that hydrogen bonds still exist in scEtOH. The analysis focuses also on the reorientational dynamics of the bond unit vectors O-H, C-O, and of the permanent dipole moment of the molecules as well as the total dipole moment of the sample. The corresponding Legendre time correlation functions were discussed in connection to the "hydrogen bonding" in the fluid and in the context of experimental results. Specifically, the behavior of the O-H dynamics exhibits the well-known associative nature of the molecules in the system. A further analysis of the hydrogen bonds was carried out, and the degree of aggregation (average number of H-bonds per molecule) was obtained and compared with results from NMR chemical shift studies. Also the estimated monomer and free O-H groups in the fluid were compared with results from IR and Raman vibrational spectroscopy. The percentage analysis fi of the liquid and scEtOH molecules, with i = 0, 1, 2, 3, ... hydrogen bonds per molecule, has been obtained. The results show the existence of small, linear-chain oligomers formed mainly by two molecules, whereas the number of the three body oligomers, and specifically that of four body oligomers in the sample, is relatively small.     

基于双嘧啶和双巴比妥酸的氢键组装体的理论研究

用AM1方法对双嘧啶和双巴比妥酸以及二者以1:1的摩尔比通过分子间多重氢键自组织形成的二体进行了几何构型优化,得到氢键键能。并在此基础上用INDO-CI方法讨论了三者的UV-Vis光谱,结果与实验值符合得很好,证实了多重氢键导致新的超分子聚集体的生成,并讨论了氢键在超分子聚集体形成过程中的作用本质。     

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.     

The role of hydrogen bonding in supercooled methanol

The role of hydrogen bonding on the microscopic properties of supercooled methanol has been analyzed by means of molecular dynamics simulations. Thermodynamic, structural, and dynamical properties have been investigated in supercooled methanol. The results have been compared with those of an ideal methanol-like system whose molecules have the same dipole moment as the methanol but lack sites for hydrogen bonding. Upon cooling the methanol samples, translational relaxation times increase more rapidly than reorientational ones. This effect is much more important when hydrogen bonds are suppressed. Suppression of hydrogen bonds also results in lower critical temperatures for diffusion and for several characteristic relaxation time constants. The anisotropy of individual dynamics and the existence of dynamical heterogeneities have also been investigated.     

Effect of hydrogen bonding on properties of hexafluorosilicates with heterocyclic cations

The known data regarding the effects of interionic hydrogen bonding on properties of onium hexafluorosilicates with heterocyclic cations are summarized. Thermal stability parameters and water solubilities for this type of salts are shown to correlate with the number of strong and medium-strength hydrogen bonds or the number of hydrogen donors in the salt structure.     

Aromaticity-controlled tautomerism and resonance-assisted hydrogen bonding in heterocyclic enaminone-iminoenol systems

The contribution of aromaticity and intramolecular hydrogen bonding to relative stability, for a set of (1H-azahetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs, has been investigated by means of quantum chemical DFT and ab initio methods up to the MP4(SDTQ)/AUG-cc-pVDZ and MP2/AUG-cc-pVTZ levels of theory. It is found that the relative energy of the tautomers is governed by the change in the degree of heterocycle aromaticity upon intramolecular hydrogen transfer. An analysis of geometrical parameters of a hydrogen-bonded system reveals a clear relationship between the aromaticity of the heterocycle, the conjugation in a resonant spacer, and the strengths of the intramolecular hydrogen bonds. This allows the conclusion to be drawn that intramolecular N-H...O and O-H...N hydrogen bonds formed are found to be resonance-assisted and their strength is dependent on the pi-donating/accepting properties of the heterocycle. On the basis of the results of the calculations, a simple model describing the mechanism of resonance assistance of hydrogen bonding has been suggested.     

Statistical theory for hydrogen bonding fluid system of AaDd type (Ⅲ): Equation of state and fluctuations

The equation of the state of the hydrogen bonding fluid system of A_aD_d type is studied by the principle of statistical mechanics. The influences of hydrogen bonds on the equation of state of the system are obtained based on the change in volume due to hydrogen bonds. Moreover, the number density fluctuations of both molecules and hydrogen bonds as well as their spatial correlation property are investigated. Furthermore, an equation describing relation between the number density correlation function of “molecules-hydrogen bonds” and that of molecules and hydrogen bonds is derived. As application, taking the van der Waals hydrogen bonding fluid as an example, we considered the effect of hydrogen bonds on its relevant statistical properties. Supported by the National Natural Science Foundation of China (Grant Nos. 20303006 and 20574016), the Natural Science Foundation of Hebei Province (Grant Nos. B2006000959 and B2004000093) and the Natural Science Foundation of Education Committee of Hebei Province (Grant No. 2003101)     

How resonance assists hydrogen bonding interactions: an energy decomposition analysis

Block-localized wave function (BLW) method, which is a variant of the ab initio valence bond (VB) theory, was employed to explore the nature of resonance-assisted hydrogen bonds (RAHBs) and to investigate the mechanism of synergistic interplay between pi delocalization and hydrogen-bonding interactions. We examined the dimers of formic acid, formamide, 4-pyrimidinone, 2-pyridinone, 2-hydroxpyridine, and 2-hydroxycyclopenta-2,4-dien-1-one. In addition, we studied the interactions in beta-diketone enols with a simplified model, namely the hydrogen bonds of 3-hydroxypropenal with both ethenol and formaldehyde. The intermolecular interaction energies, either with or without the involvement of pi resonance, were decomposed into the Hitler-London energy (DeltaEHL), polarization energy (DeltaEpol), charge transfer energy (DeltaECT), and electron correlation energy (DeltaEcor) terms. This allows for the examination of the character of hydrogen bonds and the impact of pi conjugation on hydrogen bonding interactions. Although it has been proposed that resonance-assisted hydrogen bonds are accompanied with an increasing of covalency character, our analyses showed that the enhanced interactions mostly originate from the classical dipole-dipole (i.e., electrostatic) attraction, as resonance redistributes the electron density and increases the dipole moments in monomers. The covalency of hydrogen bonds, however, changes very little. This disputes the belief that RAHB is primarily covalent in nature. Accordingly, we recommend the term "resonance-assisted binding (RAB)" instead of "resonance-assisted hydrogen bonding (RHAB)" to highlight the electrostatic, which is a long-range effect, rather than the electron transfer nature of the enhanced stabilization in RAHBs.     

Amide-ligand hydrogen bonding in reverse micelles

One approach to modeling the second coordination shell of metalloproteins is to pair amide-containing counterions with metal complexes to form hydrogen bonds in the solid state. In a more general approach, we have designed a surfactant counterion that can sustain hydrogen bonding interactions with metal complexes in solution. The surfactant is cationic and incorporates an amide as part of its headgroup to form hydrogen. The surfactant forms hydrogen bonding reverse micelles that accommodate anionic metal complexes in their polar core. In reverse micelles containing an iron(III) hexacyanide complex, spectroscopic evidence suggests that the anion is confined to the polar core region in solution. Single-crystal X-ray diffraction data on the surfactant ferricyanide system reveals a layered structure with interdigitated alkyl chains and an extensive network of hydrogen bonds that link amide groups to the cyanide ligands and to neighboring headgroups.     

Hydrogen bonding of water, hydrogen sulphide and related acceptors with electron donors

Properties of hydrogen bonds formed by 1:1 interaction of H₂O with oxygen, nitrogen, sulphur and other electron donors have been evaluated by extended Hückel and CNDO methods and the results are discussed in relation to the experimental data. A detailed analysis of the variation of the dissociation energies and charge densities with bond distances is presented for the amine-water system. 1:2 complexes of water with donors are found to contain weaker hydrogen bonds than 1:1 complexes. Results of molecular orbital calculations on the hydrogen bonding of H₂S and CH₃SH with some donors are presented. The theoretical value of hydrogen bond dissociation energy varies linearly with the overlap population, and stretching force constant of the hydrogen bond as well as with the experimental O—H frequency shift.     

Carbohydrate intramolecular hydrogen bonding cooperativity and its effect on water structure

Molecular dynamics (MD) simulations combined with water-water H-bond angle analysis and calculation of solvent accessible surface area and approximate free energy of solvation were used to determine the influence of hydroxyl orientation on solute hydration and surrounding water structure for a group of chemically identical solutes-the aldohexopyranose sugars. Intramolecular hydrogen bond cooperativity was closely associated with changes in water structure surrounding the aldohexopyranose stereoisomers. The OH-4 group played a pivotal role in hydration as it was able to participate in a number of hydrogen bond networks utilizing the OH-6 group. Networks that terminated within the molecule (OH-4 --> OH-6 --> O-5) had relatively more nonpolar-like hydration than those that ended in a free hydroxyl group (OH-6 --> OH-4 --> OH-3). The OH-2 group modulated the strength of OH-4 networks through syndiaxial OH-2/4 intramolecular hydrogen bonding, which stabilized and induced directionality in the network. Other syndiaxial interactions, such as the one between OH-1 and OH-3, only indirectly affected water structure. Water structure surrounding hydrogen bond networks is discussed in terms of water-water hydrogen bond populations. The impact of syndiaxial versus vicinal hydrogen bonds is also reviewed. The results suggest that biological events such as protein-carbohydrate recognition and cryoprotection by carbohydrates may be driven by intramolecular hydrogen bond cooperativity.     

Role of hydrogen bonding in cellulose deformation: the leverage effect analyzed by molecular modeling

The axial modulus of the cellulose Iβ crystal is as high as 120–160 GPa. The importance of hydrogen bonds is often emphasized in this context, although intrinsic stiffness of the hydrogen bonds is relatively low. Here, hydrogen bond–covalent bond synergies are investigated quantitatively using molecular mechanics and molecular dynamics simulations for the so-called leverage effect, a model introduced recently in which strains for intra-molecular hydrogen bonds are higher than for the cellulose chain as a whole, thereby amplifying their contribution to the total stiffness. The present work also includes simulation of the hydrogen bonding band shifts in vibrational spectra during cellulose deformation, which are compared with FT-IR data. The leverage effect hypothesis was supported by the results, although the total contribution to cellulose stiffness is only 12 %. Hydrogen bonding is still critically important and would lower the modulus much more than 12 %, if “artificially” removed in the model. The reason is that intra-molecular hydrogen bonding preserves the crystal structure and directs axial deformation mechanisms towards higher energy deformation and high stiffness.