The combined effect of cations and anions on the dynamics of water

We studied the orientational relaxation of the OD-stretch vibration of HDO molecules in concentrated solutions of alkali-halide salts (NaCl, NaI, CsCl and KI) in isotopically diluted water (4% D(2)O in H(2)O), using polarization-resolved femtosecond infrared pump-probe spectroscopy (fs-IR). We were able to distinguish the orientational dynamics of the water molecules solvating the halide ions from the dynamics of the bulk water and the water solvating the cations. We found that the reorientation of the halide-bound molecules shows two strongly different components (2.0 ± 0.3 ps and 9 ± 1 ps), related to a wiggling motion of the OD group hydrogen-bonded to the anion, and rotational diffusion of the molecule over the charged anion surface, respectively. The relative amplitudes of the two components are dependent on the nature of both the anion and cation, and on the concentration. These results show that cations can have a profound effect on the solvation shell dynamics of their counter-ions.     

Structural inhomogeneity of interfacial water at lipid monolayers revealed by surface-specific vibrational pump-probe spectroscopy

We report vibrational lifetime measurements of the OH stretch vibration of interfacial water in contact with lipid monolayers, using time-resolved vibrational sum frequency (VSF) spectroscopy. The dynamics of water in contact with four different lipids are reported and are characterized by vibrational relaxation rates measured at 3200, 3300, 3400, and 3500 cm(-1). We observe that the water molecules with an OH frequency ranging from 3300 to 3500 cm(-1) all show vibrational relaxation with a time constant of T(1) = 180 ± 35 fs, similar to what is found for bulk water. Water molecules with OH groups near 3200 cm(-1) show distinctly faster relaxation dynamics, with T(1) < 80 fs. We successfully model the data by describing the interfacial water containing two distinct subensembles in which spectral diffusion is, respectively, rapid (3300-3500 cm(-1)) and absent (3200 cm(-1)). We discuss the potential biological implications of the presence of the strongly hydrogen-bonded, rapidly relaxing water molecules at 3200 cm(-1) that are decoupled from the bulk water system.     

Structure of the aqueous solvated electron from resonance Raman spectroscopy: lessons from isotopic mixtures

The structure and thermodynamics of the hydrated electron are probed with resonance Raman spectroscopy of isotopic mixtures of H(2)O and D(2)O. The strongly enhanced intramolecular bends of e(-)(H(2)O) and e(-)(D(2)O) produce single downshifted bands, whereas the e(-)(HOD) bend consists of two components: one slightly upshifted from the 1,446 cm(-1) bulk frequency to 1,457 cm(-1) and the other strongly downshifted to approximately 1,396 cm(-1). This 60 cm(-1) split and the 200 (120) cm(-1) downshifts of the OH (OD) stretch frequencies relative to bulk water reveal that the water molecules that are Franck-Condon coupled to the electron are in an asymmetric environment, with one proton forming a strong hydrogen bond to the electron. The downshifted bend and librational frequencies also indicate significantly weakened torsional restoring forces on the water molecules of e(-)(aq), which suggests that the outlying proton is a poor hydrogen bond donor to the surrounding solvent. A 1.6-fold thermodynamic preference of the electron for H(2)O is observed based on the relative intensities of the e(-)(H(2)O) and e(-)(D(2)O) bands in a 50:50 isotopic mixture. This equilibrium isotope effect is consistent with the downshifted vibrational frequencies and a relative reduction of the zero-point energy of H(2)O bound to the electron. Our results enhance the cavity model of the solvated electron and support only those models that contain water monomers as opposed to other molecular species.     

A new structure of water layer on Cu(111) electrode surface during hydrogen evolution

A layered structure of water molecules formed on a Cu(111) electrode surface during hydrogen evolution in sulfuric acid solution was studied by surface X-ray diffraction and infrared reflection absorption methods. Water molecules in the surface layers take a closest pack-like stacking structure with nearest-neighbor oxygen-oxygen distances in intra-(0.322(5) nm) and inter-(0.275(15) nm) layers of multi-domains; the infrared spectra of the layered water on the Cu electrode surface showed the existence of free OH(OD) and hydrogen-bonded OH(OD) of water molecule.     

Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two-Dimensional Heterodyne-Detected VSFG Spectroscopy

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular-level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate-ion (TPB⁻)/water and tetraphenylarsonium-ion (TPA⁺)/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of the anionic TPB⁻ interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen-bonded OH groups, which are hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of the cationic TPA⁺ interface only show the presence of usual hydrogen-bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.     

Ab initio and DFT studies of the vibrational spectra of hydrogen-bonded PhOH...(H2O)4 complexes

The vibrational characteristics (vibrational frequencies and infrared intensities) for the hydrogen-bonded complex of phenol with four water molecules PhOH...(H2O)4 (structure 4A) have been predicted using ab initio and DFT (B3LYP) calculations with 6-31G(d,p) basis set. The changes in the vibrational characteristics from free monomers to a complex have been calculated. The ab initio and B3LYP calculations show that the observed four intense bands at 3299, 3341, 3386 and 3430 cm(-1) can be assigned to the hydrogen-bonded OH stretching vibrations in the complex PhOH...(H2O)4 (4A). The complexation leads to very large red shifts of these vibrations and very strong increase in their IR intensity. The predicted red shifts for these vibrations with B3LYP/6-31G(d,p) calculations are in very good agreement with the experimentally observed. It was established that the phenolic OH stretching vibration is the most sensitive to the hydrogen bonding. The predicted red-shift with the B3LYP/6-31G(d,p) calculations for the most stable ring structure 4A (-590 cm(-1)) is in better agreement with the experimentally observed than the red-shift, predicted with SCF/6-31G(d,p) calculations. The magnitude of the wavenumber shift is indicative of relatively strong OH...H hydrogen-bonded interaction. The complexation between phenol and four water molecules leads to strong increase of the IR intensity of the phenolic OH stretching vibration (up to 38 times).     

Vibrational and orientational dynamics of water in aqueous hydroxide solutions

We report the vibrational and orientational dynamics of water molecules in isotopically diluted NaOH and NaOD solutions using polarization-resolved femtosecond vibrational spectroscopy and terahertz time-domain dielectric relaxation measurements. We observe a speed-up of the vibrational relaxation of the O-D stretching vibration of HDO molecules outside the first hydration shell of OH(-) from 1.7 ± 0.2 ps for neat water to 1.0 ± 0.2 ps for a solution of 5 M NaOH in HDO:H(2)O. For the O-H vibration of HDO molecules outside the first hydration shell of OD(-), we observe a similar speed-up from 750 ± 50 fs to 600 ± 50 fs for a solution of 6 M NaOD in HDO:D(2)O. The acceleration of the decay is assigned to fluctuations in the energy levels of the HDO molecules due to charge transfer events and charge fluctuations. The reorientation dynamics of water molecules outside the first hydration shell are observed to show the same time constant of 2.5 ± 0.2 ps as in bulk liquid water, indicating that there is no long range effect of the hydroxide ion on the hydrogen-bond structure of liquid water. The terahertz dielectric relaxation experiments show that the transfer of the hydroxide ion through liquid water involves the simultaneous motion of ~7 surrounding water molecules, considerably less than previously reported for the proton.     

Preferential solvation within hydrophilic nanocavities and its effect on the folding of cholate foldamers

The conformations of three cholate foldamers and one molecular basket were studied by fluorescence and NMR spectroscopy. In nonpolar solvents (e.g., hexane/ethyl acetate or ethyl acetate) mixed with a small amount of a polar solvent (e.g., alcohol or DMSO), the cholate oligomer folded into a helix with the hydrophilic faces of the cholates turned inward. Folding created a hydrophilic nanocavity preferentially solvated by the entrapped polar solvent concentrated from the bulk. This microphase separation of the polar solvent was critical to the folding process. Folding was favored by larger-sized polar solvent molecules, as fewer such molecules could occupy and solvate the nanocavity, thus requiring a smaller extent of phase separation during folding. Folding was also favored by smaller/acyclic nonpolar solvent molecules, probably because they could avoid contact with the OH/NH groups within the nanocavity better than larger/cyclic nonpolar solvent molecules.     

Adsorption of organosulfur species at aqueous surfaces: molecular bonding and orientation

Four sulfur-containing compounds important to tropospheric chemistry have been examined at the vapor/H2O and vapor/D2O interfaces. These adsorbates, DMS, DMSO, DMSO2, and DMSO3, were studied by surface tension and vibrational sum-frequency spectroscopy (VSFS). Each adsorbate is surface active and each orients with the hydrophobic methyl groups pointed out of the plane of the interface. Their influence on the interfacial water structure is adsorbate dependent. Strong and weak interactions with surface water are observed as well as reorientation of subsurface water molecules, resulting in an increase in interfacial thickness.     

Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D‐HD‐SFG Spectroscopy

Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface—whereas in bulk water the coupling is homogeneous. For strongly hydrogen‐bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near‐surface region. For weakly hydrogen‐bonded OH groups that absorb around 3500 cm^?1, which are assigned to the outermost, yet hydrogen‐bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen‐bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.     

Solvation dynamics in Ni+ (H2O)n clusters probed with infrared spectroscopy

Infrared photodissociation spectroscopy is reported for mass-selected Ni+ (H2O)n complexes in the O-H stretching region up to cluster sizes of n = 25. These clusters fragment by the loss of one or more intact water molecules, and their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water. The first evidence for hydrogen bonding, indicated by a broad band strongly red-shifted from the free OH region, appears at the cluster size of n = 4. At larger cluster sizes, additional red-shifted structure evolves over a broader wavelength range in the hydrogen-bonding region. In the free OH region, the symmetric stretch gradually diminishes in intensity, while the asymmetric stretch develops into a closely spaced doublet near 3700 cm(-1). The data indicate that essentially all of the water molecules are in a hydrogen-bonded network by the size of n = 10. However, there is no evidence for the formation of clathrate structures seen recently via IR spectroscopy of protonated water clusters.     

Infrared spectroscopy of hydrogen-bonded 2-fluoropyridine-water clusters in supersonic jets

The hydrogen-bonded clusters of 2-fluoropyridine with water were studied experimentally in a supersonic free jet and analyzed with molecular orbital calculations. The IR spectra of 2-fluoropyridine-(H2O)(n) (n = 1 to 3) clusters were observed with a fluorescence detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The frequencies of OH stretching vibrations show that water molecules bond to the nitrogen atom of 2-fluoropyridine in the clusters. The hydrogen-bond formation between aromatic CH and O was evidenced in the 1:2 and 1:3 clusters from the experimental and calculated results. The overtone vibrations of the OH bending mode in hydrogen-bonded water molecules appear in the IR spectra, and these frequencies become higher with the increase of the number of water molecules in the clusters. The band structure of the IR spectra in the CH stretching region changes depending on the number of coordinating water molecules.     

Cooperative interaction of n-butylammonium ion with 1,3-alternate tetrapropoxycalix [4]arene: NMR and theoretical study

The interaction of 1,3-alternate tetrapropoxycalix[4]arene (1) with n-butylammonium ion (2) in CD(2)Cl(2) was examined using (1)H, (13)C and (14)N NMR spectroscopy and DFT (density functional theory) calculations. NMR shows that 1 forms with 2 an equimolecular hydrogen-bonded complex with the equilibrium constant 5.91 x 10(3) l/mol at 296 K. The structure of the complex can be shown to be asymmetric at 203 K, with 2 interacting by hydrogen bonds with the two ethereal oxygen atoms of one half of 1 and with the pi system of the other half, but is rapidly averaged to an apparent C(4h) symmetry by chemical exchange at higher temperatures. Using two related but independent techniques based on transverse and rotating-frame proton relaxation, it is shown that only an intermolecular exchange of 2 between the bound and free states takes place, in contrast to previously studied interaction of 1 with H(3)O(+). Its correlation time is 0.169 ms. It is shown by DFT calculations that such swift exchange is not possible without a cooperative interaction of both 2 and 1 with several molecules of water present. Similarities and contrasts between the exchange processes of 2 and H(3)O(+) bound to 1 are discussed, in particular with respect to the apparent quantum tunneling of the latter inside the molecule of the complex.     

Solvent-induced polymorphism of three-dimensional hydrogen-bonded networks of hexakis(4-carbamoylphenyl)benzene

The crystal structures for three types of three-dimensional (3-D) hydrogen-bonded networks of hexakis(4-carbamoylphenyl)benzene (1), the network morphologies of which depend greatly on crystallization conditions, have been determined. When this compound is crystallized from hot DMSO, the resulting crystals, 1.12DMSO (orthorhombic, Pca2(1)), showed a 3-D hydrogen-bonded porous network (type A) via 1-D catemer chains as a hydrogen-bonding motif of six primary amide groups. The type A network creates chambers surrounded by six molecules of 1 and channels along the c axis to give the highest porosity among the network polymorphs of 1 investigated here. Crystallization from a boiling mixture of n-PrOH and water gave 1.6n-PrOH (monoclinic, P2(1)/c), which exhibits another type of 3-D hydrogen-bonded porous network (type B) via cyclic dimers as another hydrogen-bonding motif of six primary amide groups. The type B network leads to triangle-like channels along the a axis having a cross section of ca. 9.2 x 9.7 x 9.7 A (including van der Waals radii). The crystal structure of 1.H(2)O (monoclinic, P2(1)/c), which was produced under hydrothermal conditions, showed a nonporous 3-D hydrogen-bonded network chain of amide groups (type C) composed of a mixed hydrogen bonding motif of helical catemer chains/cyclic dimer/catemer. Solvent-induced topological isomerism of these 3-D hydrogen-bonded networks of 1 arises from (i) the guest inclusion ability based on a radially functionalized hexagonal structure of 1, (ii) the correlation between the hydrogen bond donor ability of the syn and anti protons of the primary amide group in host 1 and the hydrogen bond acceptor ability of the oxygen atoms of 1 and guest solvents, and (iii) the polarity of the bulk crystallization solvents.     

Effect of temperature and concentration on the structure of tert-butyl alcohol/water mixtures: near-infrared spectroscopic study

The effect of temperature and concentration on the structure of tert-butyl alcohol/water binary mixtures in the alcohol-rich region (X(H2O) < 0.3) has been studied by using Fourier transform near-infrared (FT-NIR) spectroscopy. The obtained results demonstrate that the addition of a small amount of water to tert-butyl alcohol (2-methyl-2-propanol, abbreviated as TBA) leads to minor changes in the structure of neat TBA and suggest that molecules of TBA in the mixture are in the same environment as those in pure TBA. The bands of water are red-shifted in the mixture relative to bulk water, implying that the molecules of water in TBA are involved in stronger hydrogen bonding. The present experimental data give no evidence for the existence of nonbonded water in the mixture. Even at a very low content of water, the main NIR bands of water (nu(2) + nu(3) and nu(1) + nu(3)) have two components showing markedly different behavior upon an increase in temperature. From the power spectra, it is seen that the extent of intensity changes due to the free OH groups of TBA is smaller in the mixture relative to pure TBA. All of these results support the model of chain-end bonding of water molecules to TBA associates. An increase in X(H2O) reduces the population of nonbonded OH groups of TBA, yet both processes do not appear at the same rate. The amount of bonded OH groups of water increases faster than that of the nonbonded ones. It seems that the water-water interaction becomes more important as X(H2O) increases. At high alcohol content, the position of the CH alkyl stretching bands is constant, evidencing a negligible role of the hydrophobic hydration in the mixture.     

Ultrafast vibrational energy relaxation of the water bridge

We report the energy relaxation of the OH stretch vibration of HDO molecules contained in an HDO:D(2)O water bridge using femtosecond mid-infrared pump-probe spectroscopy. We found that the vibrational lifetime is shorter (~630 ± 50 fs) than for HDO molecules in bulk HDO:D(2)O (~740 ± 40 fs). In contrast, the thermalization dynamics following the vibrational relaxation are much slower (~1.5 ± 0.4 ps) than in bulk HDO:D(2)O (~250 ± 90 fs). These differences in energy relaxation dynamics strongly indicate that the water bridge and bulk water differ on a molecular scale.     

Combined electronic structure/molecular dynamics approach for ultrafast infrared spectroscopy of dilute HOD in liquid H2O and D2O

We present a new approach that combines electronic structure methods and molecular dynamics simulations to investigate the infrared spectroscopy of condensed phase systems. This approach is applied to the OH stretch band of dilute HOD in liquid D2O and the OD stretch band of dilute HOD in liquid H2O for two commonly employed models of water, TIP4P and SPC/E. Ab initio OH and OD anharmonic transition frequencies are calculated for 100 HOD x (D2O)n and HOD x(H2O)n (n = 4-9) clusters randomly selected from liquid water simulations. A linear empirical relationship between the ab initio frequencies and the component of the electric field from the solvent along the bond of interest is developed. This relationship is used in a molecular dynamics simulation to compute frequency fluctuation time-correlation functions and infrared absorption line shapes. The normalized frequency fluctuation time-correlation functions are in good agreement with the results of previous theoretical approaches. Their long-time decay times are 0.5 ps for the TIP4P model and 0.9 ps for the SPC/E model, both of which appear to be somewhat too fast compared to recent experiments. The calculated line shapes are in good agreement with experiment, and improve upon the results of previous theoretical approaches. The methods presented are simple, and transferable to more complicated systems.     

Adsorption structures of water in NaX studied by DRIFT spectroscopy and neutron powder diffraction

Diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurements (4000-1500 cm(-1)) and the results of neutron powder diffraction have been combined to study the structure of adsorption complexes of water in a NaX zeolite at different water loadings (25, 48, 72, and 120 water molecules per unit cell, respectively). Sharp bands corresponding to non-hydrogen-bonded OH groups of water molecules and broad associate bands due to hydrogen-bonded molecules are observed in the DRIFT spectra. We observe a remarkable downshift of the high-frequency associate band in a narrow temperature interval when the water amount decreases from 120 to 72 molecules per unit cell, which could signify some kind of "phase transition" for the water inside the zeolite cavities. Neutron powder diffraction results show that water molecules are predominantly localized in or near the 12-ring windows. Water molecules with hydrogen-bonded and non-hydrogen-bonded OH groups were found, in agreement with the observation of sharp and broad bands in the DRIFT spectra. We find strong evidence for the formation of cyclic hexamers of water molecules localized in the 12-ring windows, which are further stabilized by hydrogen bonds to framework oxygen atoms.     

近红外光谱用于低温水结构的分析

采用近红外光谱分析并结合拉曼光谱和分子动力学模拟研究了二甲基亚砜(DMSO)-水混合物中水在低温时的结构,对DMSO降低水冰点的机理进行了讨论.通过对不同DMSO含量混合物的近红外和拉曼光谱分析,发现了DMSO与水相互作用的光谱信息,表明DMSO与水在混合物中主要以摩尔比1∶2和2∶1的氢键结构(DW2和D2W)的形式存在,结构形式主要取决于DMSO和水在混合物中的比例.通过对水和30%(摩尔分数)DMSO-水混合物的温控近红外光谱分析,发现DW2结构抑制四面体水结构的形成是混合物冰点降低的主要原因.采用分子动力学模拟对DMSO-水混合物体系进行的模拟进一步证明了结论的可靠性.