Real-time measurement of the orientational dynamics of aqueous solvation shells in bulk liquid water

We present a study of the orientational dynamics of aqueous solvation shells of halogenic anions in bulk water solution with femtosecond two-color midinfrared spectroscopy. These orientational dynamics have time constants between 2.5 and 12 ps, depending on the type of anion and the temperature. We find that the solvation shell of the larger ion I- shows faster dynamics than that of the smaller ions Cl- and Br-.     

First-principles molecular dynamics simulations of uranyl ion interaction at the water/rutile TiO2(110) interface

The effects of temperature and solvation on uranyl ion adsorption at the water/rutile TiO₂(110) interface are investigated by Density Functional Theory (DFT) in both static and Born–Oppenheimer molecular dynamics approaches. According to experimental observations, uranyl ion can form two surface complexes in a pH range from 1.5 to 4.5. Based on these observations, the structures of the complexes at 293 K are first calculated in agreement with vacuum static calculations. Then, an increase in temperature (293 to 425 K) induces the reinforcement of uranyl ion adsorption due to the release of water molecules from the solvation shell of uranyl ion. Finally, temperature can modify the nature of the surface species.     

离子配对动力学不一定遵循Eigen-Tamm机理

Eigen-Tamm模型认为水溶液中的离子对从单水分子桥链离子对到紧密离子对是离子对溶剂化平衡的动力学控制性步骤,两态之间自由能垒最高,相互转化速率最慢. 设计了两类带有单位电荷的离子对模型(2.0:x和x:2.0,固定离子对中的阳离子或阴离子的半径为2 ?,另一个为不同半径的阴阳离子),通过计算离子对不同距离的平均力势来获取自由能曲线,发现2.0:x系列离子对由于溶剂水的作用,从单水分子桥链离子对到紧密离子对转化的自由能垒有明显减低,并不是离子成对动力学平衡中的最慢步骤,与Eigen-Tamm 模型描述存在偏差.     

Interaction of Li ions with ethylene carbonate (EC): Density functional theory calculations

Electronic structures of Li⁺ ion-ethylene carbonate (EC) complexes were studied by density functional theory. The structural, electronic and dynamical properties of Li⁺-EC complexes were studied for both an isolated EC molecule and clusters including Li⁺ ion. Our structural analysis showed only one type of Li⁺ coordination with EC through Li⁺?OC which was supported by the vibration spectral analysis for interaction between Li⁺ ion and a solvent (EC) molecule. It was analyzed that the solvation energy and Mulliken charge of Li+ ion solvated by EC molecule decrease with increase in number of EC molecule. However, electron affinity shows the opposite change. This analysis with solvation energy, electron affinity and Mulliken charge supported the stabilization of 4-coordinated solvation shell among [Li⁺(EC)_n]_n=1-5 complexes.     

Kinetic sonication effects in aqueous acetonitrile solutions. Reaction rate levelling by ultrasound

The kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated with and without ultrasonic irradiation in acetonitrile–water binary mixtures containing 0.008 to 35 wt.% of acetonitrile and the kinetic sonication effects (k_son/k_non) were calculated. Molecular dynamics (MD) simulations of the structure of the solutions were performed with ethyl acetate as the model ester. The ester is preferentially solvated by acetonitrile. The excess of acetonitrile over water in the solvation shell grows fast with an increase in the co-solvent content in the bulk solution. In parallel, the formation of a second solvation shell rich in acetonitrile takes place. Significant kinetic sonication effects for the hydrolysis were explained with facile destruction of the diffuse second solvation shell followed by a rearrangement of the remaining solvent layer under sonication. The rate levelling effect of ultrasound was discussed. In an aqueous-organic binary solvent, independent of the solvent composition, the ultrasonic irradiation evokes changes in the reaction medium which result in an almost identical solvation state of the reagent thus leading to the reaction rate levelling.     

Kinetic sonication effects in light of molecular dynamics simulation of the reaction medium

Molecular dynamics (MD) simulation of the structure of ethyl acetate solutions in two water–ethanol mixtures was performed at 280 and 330 K. The MD simulations revealed that ethyl acetate was preferentially solvated by ethanol, water being mainly located in the next solvation layer. With increasing temperature ethanol was gradually replaced by water in the first solvation shell. These findings explain the decrease in the rate of ester hydrolysis with increasing molar ratio of ethanol in the solution as the reaction rate was linearly dependent on the relative ethanol content in the first solvation shell of the ester. Predominance of ethanol results in decreased polarity and water activity in the shell and accordingly in a decreased reaction rate. Based on the results of the MD simulations, the principal conclusion of this work is that ultrasound enhances the kinetic energy (the effective temperature) of species in the solution and, in this way, evokes shifts in the solvation equilibria thus affecting the reaction rate. It appears that ultrasound does not completely break down the solvent shells or clusters in the solution as previously believed. Phenomena of thermo-solvatochromism and reaction rate levelling by ultrasound in binary solvents are described.     

Combined multi-level quantum mechanics theories and molecular mechanics study of water-induced transition state of OH~- + CO_2 reaction in aqueous solution

The presence of a solvent interacting with a system brings about qualitative changes from the corresponding gasphase reactions. A solvent can not only change the energetics along the reaction pathway, but also radically alter the reaction mechanism. Here, we investigated the water-induced transition state of the OH~- + CO_2→ HCO_3~- reaction using a multi-level quantum mechanics and molecular mechanics method with an explicit water model. The solvent energy contribution along the reaction pathway has a maximum value which induces the highest energy point on the potential of mean force. The charge transfer from OH~- to CO_2 results in the breaking of the OH~- solvation shell and the forming of the CO_2 solvation shell. The loss of hydrogen bonds in the OH~-solvation shell without being compensated by the formation of hydrogen bonds in the CO_2 solvation shell induces the transition state in the aqueous solution. The calculated free energy reaction barrier at the CCSD(T)/MM level of theory, 11.8 kcal/mol, agrees very well with the experimental value, 12.1 kcal/mol.     

Water ordering under laser radiation

New results of experiments on nonlinear laser photoacoustics, nonlinear infrared spectroscopy, and Raman scattering of water are presented. It has been found that the liquid water should be considered as a sufficiently inhomogeneous liquid. The solvation water shells around ions can play the role of inhomogeneities. It has been experimentally established that there is a positive feedback between electrolytic dissociation and solvation shell formation. A model considering the kinetics of two water structures (H-network structure and solvation shell structure) is proposed. It was shown that many features of this model are similar to that of the well-known “chemical clock” model. It has been shown that this model explains some water “anomalies” as well as new experimental facts.     

Ultrafast dynamics of electron localization and solvation in ice layers on Cu(111)

The femtosecond dynamics of localization and solvation of photoinjected electrons in ultrathin layers of amorphous solid H2O and D2O have been studied by time- and angle-resolved two-photon-photoelectron spectroscopy. After electron transfer from the metal substrate into the conduction band of ice, the excess electron localizes within the first 100 fs in a state at 2.9 eV above E(F), which is further stabilized by 300 meV on a time scale of 0.5-1 ps due to molecular rearrangements in the adlayer. A pronounced change of the solvation dynamics at a coverage of approximately 2 bilayers is attributed to different rigidity of the solvation shell in the bulk and near the surface of ice.     

First-principles molecular-dynamics simulations of a hydrated electron in normal and supercritical water

A first principles study of a hydrated electron in water at ordinary and supercritical conditions is presented. In the first case, the electron cleaves a cavity in the hydrogen bond network in which six H2O molecules form the solvation shell. The electron distribution assumes an ellipsoidal shape, and the agreement of the computed and the experimental optical absorption seems to support this picture. At supercritical conditions, instead, the H-bond network is not continuous and allows us to predict that the electron localizes in preexisting cavities in a more isotropic way. Four water molecules form the solvation shell but the localization time shortens significantly.     

Ab initio molecular dynamics study of aqueous formaldehyde and methanediol

The solvation shell of aqueous formaldehyde has been studied by ab initio molecular dynamics. Two different DFT approaches using BLYP and PBE functionals were explored. The results show only a slightly different mobility in the solvation shells and allow characterization of the hydrogen bonded structure with a H₂C?=?O··HOH hydrogen bond lifetime of ca. 3 ps. Formaldehyde hydrolysis was studied by following the reverse process, methanediol decomposition, by Blue Moon constrained MD showing that four water molecules are directly involved in the reaction and assisted by the whole hydration shell. The total energy of the aqueous methanediol to formaldehyde inter-conversion process is calculated with a barrier height of ca. 95?kJ?mol^?1 while the corresponding free energy barrier is only ΔG^??=?46?kJ?mol^?1 at 300?K.     

Experimental and computational studies on the solvation of lithium tetrafluorobrate in dimethyl sulfoxide

Solvation structures of the lithium cation and tetrafluorobrate anion in dimethyl sulfoxide (DMSO) were investigated by Raman spectroscopy and ab initio calculations at various salt concentrations. The SO and C S stretching bands were used to monitor the structural change of the solvation shell. It has been shown that the solvation number of Li⁺, calculated by the changes in intensities of the C S asymmetric and symmetric stretching bands, is consistent with the value predicted by ab initio calculations. The wavenumber shift of the C H stretching band is suggested to be the result of the anion solvation and the dissociation of the associated DMSO molecules. Copyright © 2007 John Wiley & Sons, Ltd.     

Ionic solvation and association in LiCl aqueous solution: a density functional theory,polarised continuum model and molecular dynamics investigation

In this work, the ionic solvation and association behaviours in the LiCl aqueous solution were investigated using density functional theory (DFT), a polarised continuum model and classical molecular dynamics simulations. DFT calculations of LiCl(H₂O)_1–6,8 clusters show that contact ion pair (CIP) and solvent-shared ion pair (SSIP) conformers of LiCl(H₂O)_n (n ≥ 4) clusters are generally energetic both in the gas phase and in the aqueous solution. Some SSIP conformers may be slightly more stable than their CIP isomers when at least eight water molecules are incorporated in the inner hydration shells of LiCl hydrates. The transformation between CIP and SSIP conformers is easy by overcoming a small energy barrier, which mainly results from the hydration shell reorganisation of Li⁺. Molecular dynamics simulations show that ion pairs or ion clusters can be found in the LiCl aqueous solution, and the probability of CIP conformers or ion clusters presented in the LiCl solution generally increases with rise in temperature. However, the presentation of ion pairs or ion clusters in the LiCl aqueous solution does not inevitably lead to the nucleation of LiCl crystallisation.     

Nanoscale molecular superfluidity of hydrogen

We present a microscopic quantum theoretical analysis of the nanoscale superfluid properties of solvating clusters of para-H2 around the linear OCS molecule. Path-integral calculations with N=17 para-H2 molecules, constituting a full solvation shell, show the appearance of a significant superfluid response to rotation around the molecular axis at T=0.15 K. This low-temperature superfluid response is highly anisotropic and drops sharply as the temperature increases to T approximately 0.3 K. These calculations provide definitive theoretical evidence that an anisotropic superfluid state exists for molecular hydrogen in this microscopic solvation layer.     

Solvation structure of magnesium,zinc, and alkaline earth metal ions in N,N‐dimethylformamide,N,N‐dimethylacetamide,and their mixtures studied by means of Raman spectroscopy and DFT calculations—Ionic size and electronic effects on steric congestion

The solvation structure of magnesium, zinc(II), and alkaline earth metal ions in N,N‐dimethylformamide (DMF) and N,N‐dimethylacetamide (DMA), and their mixtures has been studied by means of Raman spectroscopy and DFT calculations. The solvation number is revealed to be 6, 7, 8, and 8 for Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, respectively, in both DMF and DMA. The δ (O C N) vibration of DMF shifts to a higher wavenumber upon binding to the metal ions and the shift Δν(= ν_bound − ν_free) becomes larger, when the ionic radius of the metal ion becomes smaller. The ν (N CH₃) vibration of DMA also shifts to a higher wavenumber upon binding to the metal ions. However, the shift Δν saturates for small ions, as well as the transition‐metal (II) ions, implying that steric congestion among solvent molecules takes place in the coordination sphere. It is also indicated that, despite the magnesium ion having practically the same ionic radius as the zinc(II) ion of six‐coordination, their solvation numbers in DMA are significantly different. DFT calculations for these metalsolvate clusters of varying solvation numbers revealed that not only solvent–solvent interaction through space but also the bonding nature of the metal ion plays an essential role in the steric congestion. The individual solvation number and the Raman shift Δν in DMF–DMA mixtures indicate that steric congestion is significant for the magnesium ion, but not appreciable for calcium, strontium, and barium ions, despite the solvation number of these metal ions being large. Copyright © 2006 John Wiley & Sons, Ltd.     

Hydration of the chloride ion in concentrated aqueous solutions using neutron scattering and molecular dynamics

Neutron scattering experiments were performed on 6 m LiCl solutions in order to obtain the solvation structure around the chloride ion. Molecular dynamics simulations on systems mirroring the concentrated electrolyte conditions of the experiment were carried out with a variety of chloride force-fields. In each case the simulations were run with both full ionic charges and employing the electronic continuum correction (implemented through charge scaling) to account effectively for electronic polarisation. The experimental data were then used to assess the successes and shortcomings of the investigated force-fields. We found that due to the very good signal-to-noise ratio in the experimental data, they provide a very narrow window for the position of the first hydration shell of the chloride ion. This allowed us to establish the importance of effectively accounting for electronic polarisation, as well as adjusting the ionic size, for obtaining a force-field which compares quantitatively to the experimental data. The present results emphasise the utility of performing neutron diffraction with isotopic substitution as a powerful tool in gaining insight and examining the validity of force-fields in concentrated electrolyte solutions.     

Ion solvation dynamics in supercritical fluids

We present a theoretical study of ion solvation dynamics in a supercritical solvent. Molecular dynamics simulations show a significant difference between equilibrium and nonequilibrium solvent response functions, especially pronounced at medium and low solvent densities. We propose a simple analytical theory for the nonequilibrium solvation function based on the generalized nonlinear Smoluchowski-Vlasov equation. The theory is shown to be in excellent agreement with simulation over a wide range of supercritical solvent densities.     

高浓度NaBF4/DMF溶液中的离子溶剂化和离子缔合

利用振动光谱技术和量子化学方法研究了NaBF4／DMF溶液中的离子缔合和离子溶剂化现象。DMF分子的谱带变化表明,Na^ 与DMF分子的相互作用是通过DMF分子羰基上的氧原子进行的。Na^ 的溶剂化层内含有4个DMF分子,呈近似的四面体结构。而BF4^-谱带的变化表明,溶液中存在着离子缔合,有直接接触离子对生成。直接接触离子对的含量随溶液浓度增加而增大。     

Anomalous power law decay in solvation dynamics of DNA: a mode coupling theory analysis of ion contribution

Several time dependent fluorescence Stokes shift (TDFSS) experiments have reported a slow power law decay in the hydration dynamics of a DNA molecule. Such a power law has neither been observed in computer simulations nor in some other TDFSS experiments. Here we observe that a slow decay may originate from collective ion contribution because in experiments DNA is immersed in a buffer solution, and also from groove bound water and lastly from DNA dynamics itself. In this work we first express the solvation time correlation function in terms of dynamic structure factors of the solution. We use mode coupling theory to calculate analytically the time dependence of collective ionic contribution. A power law decay in seen to originate from an interplay between long-range probe–ion direct correlation function and ion–ion dynamic structure factor. Although the power law decay is reminiscent of Debye–Falkenhagen effect, yet solvation dynamics is dominated by ion atmosphere relaxation times at longer length scales (small wave number) than in electrolyte friction. We further discuss why this power law may not originate from water motions which have been computed by molecular dynamics simulations. Finally, we propose several experiments to check the prediction of the present theoretical work.