Dielectric behaviour and hydrogen bond molecular interaction study of formamide-dipolar solvents binary mixtures

The dielectric constant of binary mixtures of formamide with some common dipolar aprotic and protic solvents has been investigated at sixteen molar concentrations over the entire mixing range at 30 °C. The solvents used for binary mixtures with formamide are water, dimethylsulphoxide, N,N-dimethylformamide, acetone, 1,4-dioxane, mono-, di- and trihydric alcohols, and homologous series of 2-alkoxyethanol, 2-(2-alkoxyethoxy)ethanol, and ethylene glycol oligomers. The concentration dependent plots of excess dielectric constant and Kirkwood correlation factor were used to explore the complexes formed between unlike molecules, the molar ratio of a stable adduct, dipolar ordering, hydrogen-bond molecular connectivities, and their strength in the binary mixtures. Results confirmed that the complexation strength of dipolar aprotic solvents with formamide strongly depends on the value of solvent dielectric constant. The dependence of the hydrogen-bond complexation on number of hydroxyl groups and molecular size of the homologous series of the solvents is recognized from the comparative excess dielectric constant values of the mixed solvents.     

A theoretical perspective of the nature of hydrogen-bond types – the atoms in molecules approach

Hydrogen bonds and their strength were analysed based on their X–H proton–donor bond properties and the parameters of the H–Y distance (Y proton acceptor). Strong, moderate and weak interactions in hydrogen-bond types were verified through the proton affinities of bases (PA), deprotanation enthalpies of acids (DPE) and the chemical shift (σ). The aromaticity and anti-aromaticity were analysed by means of the NICS (0) (nucleus-independent chemical shift), NICS (1) and ΔNICS (0), ΔNICS (1) of hydrogen-bonded molecules. The strength of a hydrogen bond depends on the capacity of hydrogen atom engrossing into the electronegative acceptor atom. The correlation between the above parameters and their relations were discussed through curve fitting. Bader's theory of atoms in molecules has been applied to estimate the occurrence of hydrogen bonds through eight criteria reported by Popelier et al. The lengths and potential energy shifts have been found to have a strong negative linear correlation, whereas the lengths and Laplacian shifts have a strong positive linear correlation. This study illustrates the common factors responsible for strong, moderate and weak interactions in hydrogen-bond types.     

Intermolecular dynamical charge fluctuations in water: a signature of the H-bond network

We report a simulation of deuterated water using a Car-Parrinello approach based on maximally localized Wannier functions. This provides local information on the dynamics of the hydrogen-bond network and on the origin of the low-frequency infrared activity. The oscillator strength of the translational modes, peaked around approximately 200 cm-1, is anisotropic and originates from intermolecular--not intramolecular--charge fluctuations. These fluctuations are a signature of a tetrahedral hydrogen-bonding environment.     

Temperature-dependent hydrogen-bond geometry in liquid water

We have determined the hydrogen-bond geometry in liquid water from 0 to 80 degrees C by combining measurements of the proton magnetic shielding tensor with ab initio density functional calculations. The resulting moments of the distributions of hydrogen-bond length and angle are direct measures of thermal disorder in the hydrogen-bond network. These moments, and the distribution functions that can be reconstructed from them, impose quantitative constraints on structural models of liquid water.     

Hydrogen-bond dynamics of interior and surface molecules in a water nanocluster: temperature and size effects

Molecular dynamics simulations are employed to investigate the effects of temperature and size on the hydrogen-bond dynamics of interior molecules and surface molecules in a water nanocluster. The flexible three-centred (F3C) water model is invoked in the simulations. To inspect the dynamics of the interior hydrogen bonds and the surface hydrogen bonds, a spherical water nanocluster is modelled and then divided into interior molecules and surface molecules according to the density profile of the water nanocluster. It is observed that at higher temperatures the average number of hydrogen bonds decreases and yields faster hydrogen-bond relaxation for both interior molecules and surface molecules of the water nanocluster. Furthermore, the surface molecules have a lower average number of hydrogen bonds than the interior molecules. The lifetime of the surface hydrogen bonds is slightly longer than that of the interior hydrogen bonds, whereas the hydrogen-bond structural relaxation time of the surface molecules is more obviously slower than that of the interior molecules. Regarding the size effect, a larger water nanocluster is seen to have a larger average number of hydrogen bonds and a longer hydrogen-bond structural relaxation time.     

Is the hydrophobic effect unique to water? The relation between solvation properties and network structure in water and modified water models

The properties of water as a solvent are related to the structure of its liquid phase which in turn depends on the intermolecular potential. In order to explore this relationship we investigate the properties of liquids formed by a number of modified water models. Changing the molecular geometry changes short-range molecular correlations and the network of hydrogen bonds. The solubility and anomalously low entropy of non-polar solutes is only slightly reduced. Reducing the hydrogen-bond strength increases the solubility and removes the low entropy of solution of uncharged spheres. We conclude that the hydrophobic effect depends on the existence of hydrogen bonds and other strong intermolecular interactions but not on the presence of a three-dimensional network.     

缔合流体在高温高压下的自扩散系数研究

在统计力学理论基础上,本文提出了一个考虑氢键对自扩散系数影响的方程．这个方程为非氢键贡献部分与氢键贡献部分之积,其中自扩散系数的非氢键部分由Ｌｅｎｎａｒｄ－Ｊｏｎｅｓ链模型求得,而一个分子中的平均氢键数随温度和密度的变化关系使用统计缔合流体理论得到。链节之间的相互作用能量参数由粘度的关联式获得,其它四个参数则由扩散系数的实验数据获得。对７个典型的缔合流体在相当宽的温度压力范围内计算的平均相对百分误差为５．９８％。     

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.     

Phase transitions of interfacial water at 165 and 240 K. Connections to bulk water physics and protein dynamics

We are considering water adsorbed as a monolayer on Vycor, a porous silica glass. The interfacial water molecules interact with the substrate through hydrogen bonding with the numerous silanol (Si-OH) groups present all over the surface. This special form of water exhibits peculiar dynamical properties. A combined calorimetric, diffraction, high resolution quasi-elastic and inelastic neutron scattering study shows that interfacial water experiences a glass transition at 165 K and a liquid-liquid transition at 240 K from a low-density to a high density-liquid. We show that this unusual behaviour, compared to the bulk, is due to a strong weakening of the hydrogen-bond strength, possibly due to the reduced number of hydrogen-bonds engaged by water molecules when they are in an interfacial two dimensional situation. The connections of these findings to the physics of bulk water and protein dynamics are discussed.     

盐离子对双链DNA分子弹性影响的研究P

在DNA延伸过程中,考虑到氢键相互作用以及盐浓度(Na+)对氢键和堆积作用的影响,给出了修正的ZZO模型,得到了力-延伸曲线随盐浓度的变化规律.结果表明:随盐浓度的增加,延伸相变力非线性增加,并最终趋于稳定,理论计算结果与实验测量一致.     

Direct observation of hydrogen-bond exchange within a single water dimer

The dynamics of water dimers was investigated at the single-molecule level by using a scanning tunneling microscope. The two molecules in a water dimer, bound on a Cu(110) surface at 6 K, were observed to exchange their roles as hydrogen-bond donor and acceptor via hydrogen-bond rearrangement. The interchange rate is approximately 60 times higher for (H2O)2 than for (D2O)2, suggesting that quantum tunneling is involved in the process. The interchange rate is enhanced upon excitation of the intermolecular mode that correlates with the reaction coordinate.     

Hydrogen-bond equilibria and lifetimes in a monohydroxy alcohol

Dielectric loss spectra covering 13 decades in frequency were collected for 2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like relaxation, typical for several classes of hydrogen-bonded liquids. The thermal variation of the dielectric absorption amplitude agrees well with that of the hydrogen-bond equilibrium population, experimentally mapped out using near infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond switching rate does not define the frequency position of the prominent absorption peak. This contrasts with widespread notions and models based thereon, but is consistent with a recent approach.     

Structural Parameters and Thermodynamics of the Formation of Molecular Water Clusters

The formation of molecular water clusters is simulated using the theoretical density functional theory/ B3LYP/6-311+G(d,p) method. The spatial configurations of 29 clusters with 2 to 28 water molecules are calculated. The dipole moments, the complete complex-formation enthalpy, and the enthalpy of the successive joining of water molecules are determined with the basis-set superposition error taken into account. The features of the geometric structure and the hydrogen-bond strength of water clusters are analyzed on the basis of the obtained theoretical data. The complex-formation enthalpy is revealed to depend periodically on the number of water molecules in a cluster. It is found that clusters with molecules whose number is a multiple of four are energetically most advantageous. When a molecular cluster is built starting with 17 molecules, the cluster structure is changed, resulting in that one end of the complex rolls up into a prismatic configuration.     

Dipolar response and hydrogen-bond kinetics in liquid water in square-wave time-varying electric fields

Non-equilibrium molecular dynamics simulations of liquid water have been performed at 298 K in the presence of external time-varying electric fields, approximating a square wave, of varying peak intensity (0.005–0.1 V/Å) in the microwave to far-infrared frequency range (20–500 GHz). Significant non-thermal field effects were noted in terms of dipolar response and acceleration of hydrogen-bond kinetics. The coupling between the total dipole moment and the external field has been investigated and autocorrelation functions (ACFs) of both the total dipole moment and the average of the individual molecular dipole moment along the laboratory axis of the applied fields exhibited coupling, with the former showing a stronger coupling and the latter showing coupling to lower magnitude fields. The maximum alignment achieved has been computed as a function of field intensities and frequencies: the lower frequencies show a greater maximum alignment as the system had more time within each field cycle to respond. The normalised probability distribution and the hydrogen-bond ACFs have been computed: the ACF showed a clear effect over shortening the hydrogen-bond relaxation time. The field effects over the molecules’ transitions from four to five hydrogen bonds have been computed. There was an enhancement of fewer molecules undergoing transitions and a dampening for a larger proportion of molecules, depending on the external fields’ periods.     

Langmuir-Blodgett膜摩擦分子动力学模拟和机理研究

采用分子动力学方法,模拟了在两块石英基板上由脂肪酸(C₁₅H₃₁COOH)组成的单层Langmuir-Blodgett (LB)膜间的摩擦特性,探究了超薄膜在滑动过程中的摩擦和结构机理.得出对于单层LB膜在滑动过程中,在速度小于60m/s时,随着速度的增大,其剪切压增大;在速度大于60 m/s时,剪切压随速度的增加而减小.其链的倾斜角随着滑动速度的增加而减小.单层膜内的分子之间以氢键方式形成了较大的分子簇,由此导致了剪切压呈现较长的周期性,但在单层膜之间无氢键形 关键词： 分子动力学模拟 朗缪尔布洛杰特膜 纳米摩擦 氢键     

1-氨基萘印迹聚合物分子识别特性的光谱学研究

采用分子自组装印迹技术合成了一种对1-氨基萘有高度选择性的分子印迹聚合物新材料。应用紫外光谱、红外光谱、X射线光电子能谱和1H核磁共振波谱等研究了印迹聚合物的结合位点和识别机理。结果表明模板分子与功能单体通过氢键作用形成1:1型配合物,配合物的稳定常数K=5.537×104L/mol。1-氨基萘分子氨基上的氮原子是质子接受体,功能单体甲基丙烯酸分子羧基上的氢原子是氢键的质子给予体,是与1-氨基萘形成氢键作用的选择性识别位点。利用平衡吸附试验研究了印迹聚合物的结合特性,表明印迹聚合物对1-氨基萘分子具有特异的识别性能。     

绿色木霉纤维素酶分子内氢键特征的研究

运用能够揭示蛋白质分子基团振动特征的激光拉曼光谱分析技术,对绿色木霉纤维素酶中的CBHⅡ在固态以及两种pH值的液态中酶分子内的氢键状态进行了分析。结果表明相对于纤维素酶固体干粉,液态中酶分子酰胺Ⅰ羰基氧原子作为氢键质子受体的能力为上升;酰胺Ⅱ与酰胺Ⅰ的β结构特征峰变化倾向相似。在3种样品中,酶蛋白中Trp均为强氢键供体,从成键能力方面证实了以往同类酶空间分析的结果。固体与pH 6.0水溶液酶蛋白中酪氨酸酚羟基成键能力也是强的。根据游离巯基分析可以判断绿色木霉CBHⅡ的成熟肽CBD与瑞氏木霉CBHⅡ的CBD有相似的二硫键构成。     

A comparative study of DRL-lift and lift on integrated polyisobutylene polymer matrices

This paper presents a comparative study of polymer pixel on sensors obtained by Laser Induced Forward Transfer (LIFT) assisted by a triazene polymer as Dynamic Release Layer (DRL). Polyisobutylene (PIB) was selected as model for chemoselective polymers which could be used as hydrogen-bond acidic polymer for vapor sensors.     

Vibrational multilevel quantum coherence due to anharmonic couplings in intermolecular hydrogen bonds

Femtosecond three-pulse photon-echo studies of acetic acid dimers in solution reveal multilevel coherence of O-H stretching excitations caused by the anharmonic coupling between the high-frequency stretching and low-frequency hydrogen-bond motions. We demonstrate for the first time that such multilevel coherence determines the nonexponential decay of the macroscopic O-H stretching polarization, whereas spectral diffusion processes play a minor role. The dephasing time of individual vibrational transitions contributing to the overall polarization is approximately 200 fs.