The influence of water molecule coordination to a metal ion on water hydrogen bonds

The geometry of hydrogen bonds in the crystal structures from the Cambridge Structural Database and calculated data show that water coordination to a metal ion has a remarkable influence on hydrogen bonds. The calculated energies of hydrogen bonds of coordinated water are much stronger, even if the aqua complex is neutral.     

Water's hydrogen bonds in the hydrophobic effect: a simple model

We propose a simple analytical model to account for water's hydrogen bonds in the hydrophobic effect. It is based on computing a mean-field partition function for a water molecule in the first solvation shell around a solute molecule. The model treats the orientational restrictions from hydrogen bonding, and utilizes quantities that can be obtained from bulk water simulations. We illustrate the principles in a 2-dimensional Mercedes-Benz-like model. Our model gives good predictions for the heat capacity of hydrophobic solvation, reproduces the solvation energies and entropies at different temperatures with only one fitting parameter, and accounts for the solute size dependence of the hydrophobic effect. Our model supports the view that water's hydrogen bonding propensity determines the temperature dependence of the hydrophobic effect. It explains the puzzling experimental observation that dissolving a nonpolar solute in hot water has positive entropy.     

The contribution of hydrogen bonds to the surface tension of water

The surface energy, the surface free energy and the surface entropy of liquid water are calculated from the decrease in the number of hydrogen bonds in the surface layer, estimated on the bash of a simplified aater structure scheme. In the calculations of the free energy density function only the hydrogen-bond interactions between molecules are taken into consideration. The resulting surface free energy of water is ≈43 mN/m at 25°C. The calculated temperature dependence is consistent with that observed.     

Intramolecular hydrogen bonds and conformations of the 1,4-butanediol molecule

IR and Raman spectra of 1,4-butanediol (BD) versus variations in the medium (CCl₄, CH₃CN), concentration, temperature, and phase state were obtained. The observed changes attest to the conformational variety of BD molecules under the experimental conditions. On the basis of the analysis of the v(OH) region it is concluded that both in the gas phase and in CCl₄ solution conformers of BD with free OH groups coexist with conformers with O-H...O intramolecular hydrogen bonds. The difference in enthalpies, H, for the groups of conformers with and without intramolecular hydrogen bonds was found from the temperature dependence of the v(OH)_free and v(OH)_intra band intensities. The structures and energies for 70 possible spectrally and energetically distinguishable conformers of BD that do not take into account intramolecular hydrogen bonds were calculated by molecular mechanics with account of electron lone pairs of oxygen atoms. Using the experimental values of H and the calculated relative conformer energies,E, the intramolecular hydrogen bond energyE _intra= 3.7 kcal mol^–1 was found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1572–1577, September, 1993.     

Theoretical model for a tetrad of hydrogen bonds and its application to interpretation of infrared spectra of salicylic acid

Theoretical model of vibrational interactions in hydrogen-bonded salicylic acid dimer is presented which takes into account the adiabatic couplings between high- and low-frequency O-H and O...O stretching vibrations, resonance interactions between both intermolecular hydrogen bonds and between inter- and intramolecular hydrogen bonds, and Fermi resonance between the O-H stretching fundamental and the first overtone of the O-H in-plane bending vibrations. The model is used for theoretical simulation of the nu(s) stretching bands of salicylic acid and its OD derivative at 300 K. The effect of deuteration is successfully reproduced by our model. Infrared, far infrared, Raman, and low-frequency Raman spectra of the polycrystalline salicylic acid and its deuterated derivative have been measured. The geometry and experimental frequencies are compared with the results of density-functional theory calculations performed at the B3LYP6-31 ++ G**, B3LYP/cc-pVTZ, B3PW916-31 ++ G**, and B3PW91/cc-pVTZ levels. O-H, O-D, and O...O stretching frequencies are used in theoretical simulation of the nu(s) stretching bands.     

Adsorption of water soluble ionic/hydrophobic diblock copolymer on a hydrophobic surface

Summary We consider the adsorption of aA-B diblock copolymer on a planar hydrophobic surface in aqueous solution. The hydrophobic anchor (A) block is envisioned to avoid water and adsorbs on the solid-liquid interface in a collapsed state. The buoy block (B) is a polyelectrolyte which expands in solution and forms a brush whose structure depends strongly on the ionic strength of the solution. The minimization of the grand canonical free energy of the system gives access to the surface density (), the thickness of the collapsed layer (L _A ) and the thickness of the external polyelectrolyte layer (L _B ). These three parametersL _B ,L _A and are functions of the molecular weight of the anchored block (N _A ), the molecular weight of the buoy block (N _B ), the charge of the polymer (Z) and the ionic strength of the aqueous solution ( _s ).     

Effect of alcohol-water exchange and surface scanning on nanobubbles and the attraction between hydrophobic surfaces

4-(4′-Butoxyphenyl)phenyl-β-O-d-glucoside was synthesized as a low-molecular-mass gelator. It was able to immobilize a variety of aqueous and organic solvents in large amounts through the formation of three-dimensional self-assembled fibrillar networks. The morphologies of the aggregates depended on the nature of solvent where they were formed. Planar ribbons were observed in water, while helical ribbons were dominant in toluene and sheets in CHCl₃. The xerogel from water exhibited a lamellar structure with a d-spacing of 2.45 nm as demonstrated by 1D-WAXD, indicating a bilayer structure interdigitated with butoxy tails, whereas the xerogel from CHCl₃ or toluene yielded a lamellar structure with a d-spacing of 3.05 nm, implying a bilayer structure interdigitated with glycosyl heads. Increasing the content of 1,4-dioxane in water caused a gradual transformation from planar to twisted ribbons and then tubes.     

The effect of the hydrogen bonds on the molecule conformation in 2-biphenylmethanol

Infrared spectra of hydrogen-bonded organic compound 2-biphenylmethanol has been studied in the wide temperature range from 320 to 12 K. It has been found that IR spectra drastically changed with temperature in the 700–850, 1290–1500, and 3100–3600 cm⁻¹ spectral regions, where deformational vibrations of phenyl rings and methanol group, as well as ν(OH) stretching vibration manifest themselves. The analysis of the computer simulation of the IR spectra for various 2-biphenylmethanol conformers, which are characterized by different combinations of angles of the mutual orientation of the phenyl rings relative to each other and a methanol group relative to the phenyl ring allowed us to conclude that the hydrogen bonding results not only in the shortened of the intermolecular bonds, but also in the increasing of the angle between phenyl ring and methanol group. In other words, hydrogen bonding leads to the changes in the molecule conformation.     

Interstitial water and the formation of low barrier hydrogen bonds: A computational model study

The B3LYP/D95+(d,p) analysis of the uncharged low barrier hydrogen bond (LBHB) between 4‐methyl‐1H‐imidazole (Mim) and acetic acid (HAc) shows that uncharged LBHBs can be formed either by adding three water molecules around the cluster or by placing the Mim–HAc pair in a dielectric environment created by a polarizable continuum model with a permittivity larger than 20.7. The permittivity of environment around uncharged LBHB can be lowered significantly by including water molecules into the system. A Mim–HAc LBHB stabilized with one water molecule observed in diethyl ether (ε = 4.34), with two water molecules in toluene (ε = 2.38), and with three water molecules in vacuo (ε = 1). Solvation models with different numbers of water molecules predict average differences in the proton affinities of the hydrogen bonded bases (ΔPA) for stable uncharged LBHB systems in vacuo to be 91.5 kcal/mol being different from the ΔPA values close to zero in charge‐assisted LBHB systems. The results clearly indicate that small amounts of interstitial water molecules at the active site of enzymes do not preclude the existence of LBHBs in biological catalysis. Our results also show that interstitial water molecules provide a useful clue in the search for uncharged LBHBs in an enzymatic environment and the number of water molecules can be used as a relative measure for the polarity around the direct environment of LBHBs. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Lifetimes and the dynamics of hydrogen bonds in liquid water

Based on Raman spectra of light, heavy, and half-heavy water in the region of O-H and O-D stretching vibrations and on the independent-oscillator model, it is shown that a peculiarity of the liquid state of water is the nonequivalence of O-H groups of water molecules in hydrogen bonding. The structure of liquid water and the mechanism of its molecular mobility are considered in this context.     

Theory of electron localization and its application to blue-shifting hydrogen bonds

We propose a theory of electron localization or stabilization by electron localization through the interactions between occupied (i) and vacant (j*) orbitals under certain conditions, which have been believed so far to cause only electron delocalization. Electrons localize when the electrons redistributed by the interaction are more stable in the i-th occupied orbital than in the overlap region: h(ij*) > s(ij*)h(ii) for s(ij*) > 0. Electron delocalization occurs when h(ij*) < s(ij*)h(ii) for s(ij*) > 0. The h(ij*) and s(ij*)h(ii) terms represent the energy of the electrons in the overlap region and the energy of the redistributed electrons in the occupied orbital, respectively. The theory of electron localization is substantiated by the correlation of the C-H bond lengths of fluorinated methanes H(4-n)CF(n) (n = 1, 2, 3) to the electron population of the σ(CH) bonding orbital, and successfully applied to understanding blue-shifting hydrogen bonds in F(3)CH···X (X = CO, N(2), OC, Ne, OC(CH(3))(2)) and designing some proton donors, HCO(2)CH(3) and hypervalent molecules HPF(4) and HSF(5), for blue-shifting hydrogen bonds.     

Growth dynamics of water drops on a square-pattern rough hydrophobic surface

Condensation on rough or superhydrophobic substrates can induce wetting behavior that is quite different from that of deposited or impinging drops. We investigate the growth dynamics of water drops in a well-controlled condensation chamber on a model rough hydrophobic surface made of square pillars. After having followed growth laws similar to those observed on flat surfaces, a transition to an air-pocket-like state occurred because of the bridging of the drops between the pillars. Another transition to the more stable Wenzel state is later ensured by a noticeable pillar self-drying process. Condensation ends up in a few large drops in a mixed Wenzel penetration regime. The drops are fed by neighboring channels and the adjacent pillars stay almost dry, a remarkable and seemingly general property of rough hydrophobic substrates.     

From a spin-crossover molecule to the bulk electronic behaviour of a material: the paramount role of hydrogen bonds in molecular electronics

The exact role of the hydrogen bond network in a spin-crossover material is unambiguously shown: changing the −N-H…O(nitro)salicylidene− ‘communication wire’ between adjacent Fe ions in the unidi-mensional chains of a two-step spin-transition material to the poorer −N-C-H…O(nitro)salicylidene− pathway results in a gradual and partial spin conversion of Fe^II in otherwise similar unidimensional chains of complex molecules. Cooperativity in the spin-crossover phenomenon indeed results from the effectiveness of the inter-molecular communication.     

Transition from moderate to strong hydrogen bonds: its identification and physical bases in the case of O-H...O intramolecular hydrogen bonds

A systematic investigation aimed at identifying the transition from moderate (M) to strong (S) hydrogen bonds (HBs) and the physical bases of the main geometry-based HB strength classifications reported in the literature has been undertaken using the quantum theory of atoms in molecules (QTAIM). Correlations between the Laplacian of the electron density (rho) at the O...H hydrogen bond critical points (HBCPs), nabla2rhohb, specifically between the more intuitive parameter Lhb = -nabla2rhohb and other QTAIM parameters, have also been explored. The transition from MHBs to SHBs has been identified as the minimum (maximum) in the geometric dependence of Lhb (nabla2rhohb). For O-H...O intramolecular (IM) HBs (including resonance-assisted HBs), the transition is obtained, in a truly remarkable agreement with the existing geometry-based HB strength classifications, when the O...O (O...H) distance is approximately 2.51 ( approximately 1.55) A and when the ratio of the potential energy density (|Vhb|) to the kinetic energy density (Ghb) approximately 1.3. Accordingly, the ranges of the |Vhb|/Ghb ratios are >2-1.3 and 1.3-1 for, respectively, SHBs and MHBs. When the O...O distance is not a genuine indicator of HB strength, the |Vhb|/Ghb ratio and other parameters should be considered to characterize the strength of the HBs. Rationalizations have been provided by way of decoding the physical bases of the transition in terms of the properties of rho and the mechanical characteristics of the interactions that created the HBCPs. Lhb was found to correlate, with a very high degree of fidelity, with at least three parameters (in addition to O...O and O...H distances and the IMHB energy), Vhb/Ghb, Hhb/rhohb (the ratio of the total energy density, Hhb, to the electron density, rhohb (the so-called bond degree parameter)), and deltahb(O,H) (the delocalization index), demonstrating the importance and utility of Lhb (nabla2rhohb) for the study of HB interactions. A new refined energetics-based classification of O-H...O IMHB strengths has been advanced. The approach taken in this investigation can be extended to other HB systems.     

Influence of water on the surface of hydrophilic and hydrophobic room-temperature ionic liquids

Vibrational spectroscopy, sum frequency generation, has been used to study the gas-liquid interface of imidazolium-based, room-temperature ionic liquids. The ionic liquids are divided into two categories on the basis of their miscibility with water, as hydrophobic (immiscible) and hydrophilic (miscible). The spectroscopy results indicate that water will reorient the cations of the hydrophobic ionic liquid, while in the hydrophilic ones they remain unaffected. Thus, under low water conditions the plane of the imidazolium ring remains parallel to the surface for both types of ionic liquid. However, at high water content the ring will reorient to along the surface normal for the hydrophobic ionic liquid but remain parallel to the surface for the hydrophilic one. This is a reversible process.     

Theoretical study on the interlay of hydrogen bonds in the trimers involving HCN and water

In this study, the hydration of hydrogen cyanide (HCN) has been investigated by means of quantum chemical ab initio calculations at the MP2/6‐311++G(3df,2p) level. Various HCN· · ·( H₂O)₂ and (HCN)₂· · ·H₂O complexes were optimized. Geometrics and energetics in these complexes have been analyzed. The hydration of the H atom leads to an elongation of the N?C and C? H bonds, whereas the hydration of the N atom results in a contraction of the N?C bond and a little elongation of the C? H bond. The interaction energy between each molecule pair in the trimers (except in HCN? H₂O? H₂O trimer) is increased relative to that in the respective dimer. The cooperativity of hydrogen bond in HCN? H₂O? H₂O trimer plays a negative contribution to the total interaction energy of the complex, whereas that in the other trimers is a positive contribution. Geometry and energy in H₂O? H₂OO? HCN? H₂O tetramer have also been analyzed. The binding energies in the trimers and tetramer have been studied by means of many‐body interaction analysis. The mechanism of HCN hydration was suggested. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Dependence of the conduction of a single biphenyl dithiol molecule on the dihedral angle between the phenyl rings and its application to a nanorectifier

The transport properties of a biphenyl dithiol (BPD) molecule sandwiched between two gold electrodes are studied using the nonequilibrium Green's function method based on the density functional theory. In particular, their dependence on the dihedral angle (phi=90 degrees -180 degrees ) between two phenyl rings is investigated. While the dihedral-angle dependence of the density of states projected on the BPD molecular orbitals is small, the transport properties change dramatically with phi. The transmission at the Fermi energy exhibits a minimum at phi=90.0 degrees and greatly increases with phi. The ratio of the maximum obtained at phi=180 degrees to the minimum exceeds 100. As an application of this characteristic transport behavior, a BPD molecule functionalized with NH(2) and NO(2) groups is considered. It is found that this molecule works as a nanorectifier.     

Measurement of the charge number per adsorbed molecule and packing densities of self-assembled long-chain monolayers of thiols

We have applied a recently developed method (Langmuir 2006, 22, 5509-5519) to determine charge numbers per adsorbed molecule and packing densities in self-assembled monolayers (SAMs) of octadecanethiol (C18SH), a representative long-chain thiol. Our method yields values of area per molecule that are physically reasonable, in contrast to the popular reductive desorption method, which gives molecular areas that are smaller than those determined by the van der Waals radii. In a nonadsorbing electrolyte, we were able to model the dependence of the charge number per adsorbed molecule on the electrode potential, taking into account that the desorption process is a substitution reaction between the solvent and the adsorbate. We have also shown that the charge number per adsorbed thiol is affected by the specific adsorption of the anion of the electrolyte. In the latter case, the thiol competes for adsorption sites at the surface not only with water but also with the anion of the electrolyte, and this competition has an effect on the measured charge number.     

The apparent contact angle of water droplet on the micro-structured hydrophobic surface

The apparent contact angle of Cassie-Baxter state water droplets can be calculated by the existing theoretical formula, but due to the defects of the micro-structured hydrophobic surface and some inevitable tiny disturbances in the experiment, Cassie-Baxter state water droplets will appear partly in Wenzel state, that is, the mixed state water droplets. In this paper, apparent contact angles of Cassie-Baxter state and mixed state water droplets on micro-structured hydrophobic surfaces are compared. The rese...     

微结构疏水表面上液滴的表观接触角

虽然微结构疏水表面上Cassie-Baxter状态液滴的表观接触角已有理论预测公式,但实验研究发现,由于微结构疏水表面上的瑕疵,以及液滴受到的各种轻微扰动等原因,很容易造成Cassie-Baxter状态液滴局部区域出现Wenzel状态的情况(即混合状态),而有关混合状态液滴表观接触角的研究还比较少.本文通过对比相同微结构疏水表面上Cassie-Baxter状态和混合状态液滴表观接触角的大小,发现将Cassie-Baxter预测公式中的固液接触面积分数F换成最外缘三相接触线处的局部固液接触面积分数F′,则能同时较好地预测上述两种情况下液滴的表观接触角;通过进一步的研究发现,表观接触角的大小仅与最外缘三相接触线处的固液接触状态有关,而与其他处的固液接触状态无关.该结果对于进一步认识微结构疏水表面上液滴的表观接触角以及润湿性质具有重要意义.