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Proteins at interfaces play important roles in cell biology, immunology, bioengineering, and biomimetic material design. Many biological processes are based on interfacial protein action, ranging from cellular communication to immune responses and the protein-driven mineralization of bone. Despite the importance of interfacial proteins, comparatively little is known about their structure. The standard methods for studying crystalline or solution-phase proteins (X-ray diffraction and NMR spectroscopy) are not well-suited for studying proteins at interfaces, and for these proteins we still lack a corresponding technique that can provide the same level of structural resolution. This is not surprising in view of the challenges involved in probing the structure of proteins within monomolecular films assembled at a very thin interface in situ. Vibrational sum-frequency generation (SFG) spectroscopy has the potential to overcome this challenge and investigate the structure and dynamics of proteins at interfaces at the molecular level with subpicosecond time resolution. While SFG studies were initially limited to simple model peptides, the past decade has seen a dramatic advancement of experimental techniques and data analysis methods that has made it possible to also study interfacial proteins and their folding, binding, orientation, hydration, and dynamics. In this review, we first explain the principles of SFG spectroscopy and the experimental and theoretical methods to measure and analyze protein SFG spectra. Then we give an extensive overview of the interfacial proteins studied to date with SFG. We highlight representative examples to demonstrate recent advances in probing the structure of proteins at the interfaces of liquids, membranes, minerals, and synthetic materials.

We report the real-time measurement of the ultrafast reorientational motion of water molecules at the water-air interface, using femtosecond time- and polarization-resolved vibrational sum-frequency spectroscopy. Vibrational excitation of dangling OH bonds along a specific polarization axis induces a transient anisotropy that decays due to the reorientation of vibrationally excited OH groups. The reorientation of interfacial water is shown to occur on subpicosecond time scales, several times faster than in the bulk, which can be attributed to the lower degree of hydrogen bond coordination at the interface. Molecular dynamics simulations of interfacial water dynamics are in quantitative agreement with experimental observations and show that, unlike in bulk, the interfacial reorientation occurs in a largely diffusive manner.

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.

Duramycin is a small tetracyclic peptide which binds specifically to ethanolamine phospholipids (PE). In this study, we used lipid monolayers consisting of 1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) and various phosphatidylcholines (PC) to investigate the effect of duramycin on the organization of lipids and its influence on surrounding water molecules, using vibrational sum-frequency generation spectroscopy in conjunction with surface pressure measurements and fluorescence microscopy. The results show that while duramycin has no effect on the PC lipid monolayers, it induces significant disorder of PE molecules and causes an increase of the PE monolayer surface pressure. Duramycin adopts a β-sheet conformation and is well-ordered at the air-water interface as well as after binding to PE. Our results are consistent with duramycin inserting into the PE monolayer via its hydrophobic end, exposing phenylalanine residues to the lipid. Binding of duramycin to PE broadens the hydrogen-bond distribution of lipid-bound water molecules, notably increasing the fraction of the less strongly hydrogen-bonded, possibly undercoordinated, water molecules. Fluorescence microscopy reveals that the interaction of duramycin with PE causes a change in the shape of the liquid-condensed domains of the PE monolayer from circular to horseshoe-like, indicating a reduction of line tension at the boundary of the two lipid phases. These results reveal that the first steps in the disruption of membrane integrity by duramycin consist of a reduction of the line tension, a decrease in the lipid order, and a weakening of the hydrogen bonding network of water around PE.

A theoretical framework is presented for the design and analysis of ultrafast time- and polarization-resolved surface vibrational spectroscopy, aimed at elucidating surface molecular reorientational motion in real time. Vibrational excitation with linearly polarized light lifts the azimuthal symmetry of the surface transition-dipole distribution, causing marked, time-dependent changes in the surface sum-frequency generation (SFG) intensity. The subsequent recovery of the SFG signal generally reflects both vibrational relaxation and reorientational motion of surface molecules. We present experimental schemes that allow direct quantification of the time scale of surface molecular reorientational diffusive motion.

Surfaces and interfaces play important roles in many processes and reactions and are therefore intensively studied, often with the aim of obtaining molecular-level information from just the interfacial layer. Generally, only the first few molecular layers next to the interface are relevant for the surface processes. In the past decades, 2nd-order nonlinear spectroscopies including sum-frequency generation and second harmonic generation have developed into powerful tools for obtaining molecularly specific insights into the interfacial region. These approaches have contributed substantially to our understanding of a wide range of physical phenomena. However, along with their wide-ranging applications, it has been realized that the implied surface-specificity of these approaches may not always be warranted. Specifically, the bulk quadrupole contribution beyond the electric dipole-approximation for a system with a weak nonlinear interface signal, as well as the diffuse layer contribution at charged interfaces, could mask the surface information. In this perspective paper, we discuss the surface-specificity of 2nd-order nonlinear spectroscopy, especially considering these two contributions. [ABSTRACT FROM AUTHOR]

Interfacial water structures have been studied intensively by probing the O-H stretch mode of water molecules using sum-frequency generation (SFG) spectroscopy. This surface-specific technique is finding increasingly widespread use, and accordingly, computational approaches to calculate SFG spectra using molecular dynamics (MD) trajectories of interfacial water molecules have been developed and employed to correlate specific spectral signatures with distinct interfacial water structures. Such simulations typically require relatively long (several nanoseconds) MD trajectories to allow reliable calculation of the SFG response functions through the dipole moment-polarizability time correlation function. These long trajectories limit the use of computationally expensive MD techniques such as ab initio MD and centroid MD simulations. Here, we present an efficient algorithm determining the SFG response from the surface-specific velocity-velocity correlation function (ssVVCF). This ssVVCF formalism allows us to calculate SFG spectra using a MD trajectory of only ~100 ps, resulting in the substantial reduction of the computational costs, by almost an order of magnitude. We demonstrate that the O-H stretch SFG spectra at the water-air interface calculated by using the ssVVCF formalism well reproduce those calculated by using the dipole moment-polarizability time correlation function. Furthermore, we applied this ssVVCF technique for computing the SFG spectra from the ab initio MD trajectories with various density functionals. We report that the SFG responses computed from both ab initio MD simulations and MD simulations with an ab initio based force field model do not show a positive feature in its imaginary component at 3100 cm-1. [ABSTRACT FROM AUTHOR]

The isothermal crystallization process of thin amorphous solid water (ASW) films on Ru(0001) has been investigated in real time by simultaneously employing helium atom scattering, infrared reflection absorption spectroscopy, and isothermal temperature-programmed desorption. The measurements reveal that the crystallization mechanism consists of random nucleation events in the bulk of the ASW films, followed by homogeneous growth. Morphological changes of the solid water film during crystallization expose the water monolayer just above the substrate to the vacuum during the crystallization process. [ABSTRACT FROM AUTHOR]

Despite extensive study the phase behavior of phospholipid monolayers at an air–water interface is still not fully understood. In particular recent vibrational sum-frequency generation (VSFG) spectra of DPPC monolayers as a function of area density show a sharp transition in the order of the lipid chains at 1.10 nm2/molecule. This is in a region where the lateral pressure as a function of area is effectively constant. We have investigated the nature of this transition by studying the phase behavior of DPPC monolayers as a function of area density using molecular-dynamics simulations. The changes in order within the monolayer as a function of area density correlate well with the experimental signal. At 0.58 nm2/molecule we observe the onset of lateral separation of highly ordered and disordered lipids, indicating the coexistence of a gel-like liquid condensed and a fluidlike liquid expanded phase. At 0.97 nm2/molecule the monolayer ruptures, marking the onset of the liquid–gas (G) coexistence region. This is much earlier than suggested by fluorescence microscopy results and implies that at the point of rupture, the initial pores have an equilibrium size smaller than ∼500 nm in diameter. The rupture of the monolayer leads to a sharp increase in the overall lipid order that explains the sharp transition observed in the VSFG measurements. VSFG measurements thus may represent a sensitive means to determine the onset of the liquid–gas (G) coexistence region for such systems. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical investigation of the adsorption, desorption, and dissociation of NO on the stepped Pt (533) surface. By combining temperature programmed desorption and reflection absorption infrared spectroscopy, information about the adsorption sites at different temperatures is obtained. Surprisingly, metastable adsorption structures of NO can be produced through variation of the dosing temperature. We also show that part of the NO molecules adsorbed on the step sites dissociates around 450 K. After dissociation the N atoms can desorb either by combining with an O fragment, or with another N atom, resulting in NO and N2. The N2 production can be enhanced by coadsorbing CO on the surface: CO scavenges the oxygen atom, thereby suppressing associative recombinative desorption of N and O atoms. Density functional theory calculations are used to reveal the adsorption energies and vibrational frequencies of adsorbed NO as well as barriers for dissociation of NO and for diffusion of N atoms. The combined experimental results and theoretical calculations reveal that dissociation of NO is the rate limiting step in the formation of N2.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Interfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atomtoward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle. [ABSTRACT FROM AUTHOR]

With infrared transient hole-burning spectroscopy we have investigated the influence of OD···X hydrogen bonds on the vibrational line shape of O–D stretch vibrations in acid zeolites. The effect of hydrogen bonding on the line shape depends critically on the type of hydrogen bond. For hydrogen bonding in a rigid structure, the hydrogen bond determines the inhomogeneous linewidth, but the homogeneous linewidth is determined by coupling to a ∼200 cm-1 lattice mode as concluded from the temperature dependence of the dephasing rate. When the hydrogen bond is formed with an adsorbing molecule, the coupling between the high-frequency O–D stretch vibration and the low-frequency OD···X hydrogen-bond stretching mode does determine the homogeneous linewidth. The difference between the two systems can be explained by the different hydrogen-bond potentials. Variation of the adsorbate provides a means of obtaining conclusive information on the coupling mechanism between the high-frequency O–D stretching mode and the low-frequency OD···X hydrogen-bond stretching mode. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The interplay of electronic and nuclear degrees of freedom in semiconductor hybrid organic–inorganic perovskites determines many of their fundamental photophysical properties. For instance, charge carriers are dressed with phonons, that is, form polarons, and combination modes composed of strongly mixed localized vibrations and delocalized phonons can provide pathways for electronic energy relaxation and dissipation. Mixing of the different types of nuclear motion in vibrational combination modes requires their strong coupling. The direct measurement of coupling between the high‐frequency N−H stretch modes of the organic methylammonium and formamidinium ions and low‐frequency Pb−I phonon modes of the inorganic sub‐lattice in hybrid organic–inorganic perovskites is presented. The results reveal direct and substantial coupling between the non‐covalently interacting organic and inorganic sub‐lattices. [ABSTRACT FROM AUTHOR]

Water is very different from liquids of similar molecular weight, and one of its unique properties is the very efficient transfer of vibrational energy between molecules, which arises as a result of strong dipole-dipole interactions between the O-H oscillators. Although we have a sound understanding of such energy transfer in bulk water, we know less about how, and how quickly, transfer occurs at its interface with a hydrophobic phase, because specifically addressing the outermost monolayer is difficult. Here, we use ultrafast two-dimensional surface-specific vibrational spectroscopy to probe the interfacial energy dynamics of heavy water (D2O) at the water/air interface. The measurements reveal the presence of surprisingly rapid energy transfer, both between hydrogen-bonded interfacial water molecules (intermolecular), and between O-D groups sticking out from the water surface and those located on the same molecule and pointing towards the water bulk (intramolecular). Vibrational energy transfer occurs on sub-picosecond timescales, and its rates and pathways can be quantified directly. [ABSTRACT FROM AUTHOR]

Understanding water-membrane interactions is a fundamental issue in biophysics since these interactions are at the basis of many key molecular processes occurring in membranes. The hydrogen-bond network of water molecules is highly dynamic and its dynamical structure influences membrane fluidity and proton transport across membranes. We investigate the dynamics of water hydrogen-bonds network in model membranes using dielectric relaxation spectroscopy in the TeraHertz range. This frequency interval is suitable for obtaining information on the collective low-energy modes of the hydrogen-bond network of water molecules. In this paper we review the technique and present some preliminary experimental results. [ABSTRACT FROM AUTHOR]

We present a novel setup to elucidate the dynamics of interfacial molecules specifically, using surface-selective femtosecond vibrational spectroscopy. The approach relies on a fourth-order nonlinear optical interaction at the interface. In the experiments, interfacial molecules are vibrationally excited by an intense, tunable femtosecond midinfrared (2500–3800 cm-1) pump pulse, resonant with the molecular vibrations. The effect of the excitation and the subsequent relaxation to the equilibrium state are probed using broadband infrared+visible sum frequency generation (SFG) light, which provides the transient vibrational spectrum of interfacial molecules specifically. This IR pump-SFG probe setup has the ability to measure both vibrational population lifetimes as well as the vibrational coupling between different chemical moieties at interfaces. Vibrational lifetimes of interfacial molecules are determined in one-dimensional pump-SFG probe experiments, in which the response is monitored as a function of the delay between the pump and probe pulses. Vibrational coupling between molecular groups is determined in two-dimensional pump-SFG probe experiments, which monitor the response as a function of pump and probe frequencies at a fixed delay time. To allow for one setup to perform these multifaceted experiments, we have implemented several instrumentation techniques described here. The detection of the spectrally resolved differential SFG signal using a combination of a charge-coupled device camera and a piezocontrolled optical scanner, computer-controlled Fabry–Pérot etalons to shape and scan the IR pump pulse and the automated sample dispenser and sample trough height corrector are some of the novelties in this setup. [ABSTRACT FROM AUTHOR]

THz time-domain spectroscopy is used to measure the frequency dependent (0.2-2.0 THz) complex refractive index of a pure liquid crystal (LC), 4'-n-pentyl-4-cyanobiphenyl (5CB), and its LC colloids with SiO2 particles. While the refractive index of the pure LC is found to vary markedly due to distinct orientated domains within the sample, the LC colloids provide us with very stable and reproducible spectra, from which we can reliably deduce the optical constants of pure 5CB using effective medium theory. While the absorption coefficient is found to be very small, the refractive index of 5CB decreases considerably over our probe frequency range. [ABSTRACT FROM AUTHOR]

We report on the energy flow dynamics in model membranes, investigated by surface-specific time-resolved (femtosecond) sum frequency generation spectroscopy. This recently developed technique allows us to probe energy dynamics selectively at the water/lipid interface. We report vibrational relaxation dynamics of C-H stretch modes in the lipid alkyl chains, and reveal that incoherent energy transfer occurs from the excited CH2 groups to the terminal CH3 groups. We also find evidence for strong anharmonic coupling between different CH2 and CH3 modes. Relaxation and the energy transfer processes within the lipid alkyl chain occur on (sub-)picosecond timescales. Studies of the dynamics on different lipid phases (gel or liquid crystalline phase) reveal a marked independence of the dynamics on the precise molecular conformation of the lipids. In addition, we report the energy transfer dynamics between membrane-bound water and lipids, and find that the transfer of heat between water and lipids occurs remarkably fast: heat is transferred across the monolayer, from the polar head group region of the lipid to the end of the alkyl chain, within 1 ps. These results demonstrate the potential of using ultrafast surface-specific spectroscopies to elucidate biomolecular dynamics at membrane surfaces. [ABSTRACT FROM AUTHOR]

We investigate the phase behavior of surface-functionalized silica colloids at both the molecular and macroscopic levels. This investigation allows us to relate collective properties such as aggregation, gelation, and aging directly to molecular interfacial behavior. By using surface-specific vibrational spectroscopy, we reveal dramatic changes in the conformation of alkyl chains terminating submicrometer silica particles. In fluid suspension at high temperatures, the interfacial molecules are in a liquid-like state of conformational disorder. As the temperature is lowered, the onset of gelation is identified by macroscopic phenomena, including changes in turbidity, heat release, and diverging viscosity. At the molecular level, the onset of this transition coincides with straightening of the carbon-carbon backbones of the interfacial molecules. In later stages, their intermolecular crystalline packing improves. It is the increased density of this ordered boundary layer that increases the van der Waals attraction between particles, causing the colloidal gas to aggregate. The approach presented here can provide insights into phase transitions that occur through surface modifications in a variety of colloidal systems. [ABSTRACT FROM AUTHOR]

Abstract: Experimental and theoretical investigations of the C–O stretch vibration of CO on a stepped platinum surface (Pt(533)) reveal distinct differences between spectra obtained with reflection absorption infrared spectroscopy (RAIRS) and sum-frequency generation (SFG). The SFG data reveal large variations in the C–O stretch intensity with increasing coverage, whereas the RAIRS data show a continuous increase. The data can be quantitatively reproduced using a model that accounts for the different dependence of SFG and RAIRS on the dipole–dipole coupling. [Copyright &y& Elsevier]

Vibrational sum frequency generation (SFG) spectroscopy is a versatile technique in surface science, which enables the study of physical and chemical properties of interfaces at the molecular scale. This technique reliably measures molecular vibrational resonances. However, utilization of the SFG spectroscopy in its conventional implementation to reveal correlations between vibrational and electronic resonances at interfaces meets substantial practical limitations. The limitations can, in principle, be overcome by using Fourier-transform SFG (FT SFG) spectroscopy and resolving both frequency axes for the second-order nonlinear optical susceptibility. Here, we experimentally demonstrate this approach using two model systems, quartz and a decanoic acid monolayer at the water/air interface. Implementation of the FT SFG spectroscopy requires heterodyne detection and, thus, the technique can provide the phase-resolved response of molecular resonances. To this end, we develop the phasing procedure for this spectroscopy. [ABSTRACT FROM AUTHOR]

The surfactant sodium dodecyl sulfate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. We report a microscopic model for the interaction between the surfactant and water and between water molecules at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct subensembles of water in the presence of this negatively charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O-H groups, i.e., localized O-H stretch vibrations, and those whose O-H stretch vibrations are delocalized, i.e., shared between multiple O-H bonds. The two subensembles are coupled, with subpicosecond energy transfer occurring between them. This is markedly different from O-H bonds at the air-water interface, which are less heterogeneous, and indicates that the water molecules that interact with the surfactant headgroups have hydrogen-bonding properties different from those of water molecules interacting with the other water molecules. [ABSTRACT FROM AUTHOR]

Time-resolved and two-dimensional sum frequency generation (TR-SFG and 2D-SFG) spectroscopies are promising tools in the experimental study of molecular dynamics, specifically at interfaces. Most implementations of TR/2D-SFG spectroscopy rely on a pump-probe scheme, where an excitation pulse excites a fraction of interfacial molecules into the first excited state of a specific vibrational mode, and a subsequent SFG probe pair detects the time-dependent changes of the surface vibrational response. In steady state SFG spectroscopy, phase-resolved detection (also known as heterodyne-detection), as opposed to SFG intensity measurements, has been shown to be useful in unraveling the steady-state response of interfacial vibrations. Here, we explore the merits of phase-resolved TR/2D-SFG spectroscopy. This purely theoretical and numerical study reveals that, for a typical response from aqueous interfaces, the intensity 2D-SFG measurements contain the same information content as phase-resolved 2D-SFG measurements. We specifically analyze the frequency-dependence of the bleach lifetime (reflecting vibrational relaxation), and the time-dependent slope of the on-diagonal features observed in a 2D spectra. We show that for different systems, the intensity-based and phase-resolved 2D-SFG measurements provide the same information and are quantitatively very similar. We investigate the effect of different lineshapes, anharmonicity, and nonresonant signal contributions, and show that none of these effects substantially change the conclusion that intensity-based and phase-resolved 2D-SFG measurements provide equivalent information. [ABSTRACT FROM AUTHOR]

A simple refluxing wet-chemical approach is employed for fabricating In 2 S 3 /Pt-TiO 2 heterogeneous catalysts for hydrogen generation under visible light irradiation. When the mass ratio between Pt-TiO 2 and cubic-phased In 2 S 3 (denoted as In 2 S 3 /Pt-TiO 2 ) is two, the composite catalyst shows the highest hydrogen production, which exhibits an 82-fold enhancement over in-situ deposited Pt-In 2 S 3 . UV–vis diffuse reflectance and valence band X-ray photoelectron spectra elucidate that the conduction band of In 2 S 3 is 0.3 eV more negative compared to that of TiO 2 , favouring charge separation in the nanocomposites. Photoelectrochemical transient photo-current measurements and optical pump – terahertz probe spectroscopic studies further corroborate the charge separation in In 2 S 3 /Pt-TiO 2 . The migration of photo-induced electrons from the In 2 S 3 conduction band to the TiO 2 conduction band and subsequently into the Pt nanoparticles is found to occur within 5 ps. Based on the experimental evidence, a charge separation process is proposed which accounts for the enhanced activity exhibited by the In 2 S 3 /Pt-TiO 2 composite catalysts. [ABSTRACT FROM AUTHOR]

Using combined theoretical and experimental approaches, we demonstrate that the bond orientation of water at the water-vapor interface depends markedly on the water isotope (H-D) composition. While the interfacial water structures of H2O and D2O are indistinguishable, the intramolecular symmetry breaking in HDO is directly reflected at the surface: the OD bonds preferably orient down towards the bulk water, whereas the OH bond tends to orient up into the vapor phase. Path integral molecular dynamics simulations show good agreement with surface-specific sum-frequency generation (SFG) spectroscopy results, revealing that the distinct interfacial bond orientations originate from nuclear quantum effects. The enhanced localization of the heavier D atom leads to stronger hydrogen bonds, giving rise to OD bonds pointing down into the bulk. [ABSTRACT FROM AUTHOR]

The energetically unfavorable termination of the hydrogen-bonded network of water molecules at the air/water interface causes molecular rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theoretical simulation provides new insights. The complex χ(2) spectra of isotopically diluted water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and molecular dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface-in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk-but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ(2) spectra is the intramolecular anharmonic coupling, i.e., Fermi resonance. [ABSTRACT FROM AUTHOR]

We study the influence of the amphipilic compound tetramethylurea (TMU) on the dynamical properties of water, using dielectric relaxation spectroscopy in the regime between 0.2 GHz and 2 THz. This technique is capable of resolving different water species, their relative fractions, and their corresponding reorientation dynamics. We find that the reorientation dynamics of water molecules in the hydration shell of the hydrophobic groups of TMU is between 3 (at low concentrations) and 10 (at higher concentrations) times slower than the dynamics of bulk water. The data indicate that the effect of hydrophobic groups on water is strong but relatively short-ranged. With increasing temperature, the fraction of water contained in the hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water becomes smaller. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate that marked variations exist in hydrogen bonding interactions of interfacial water at different aqueous interfaces. The average hydrogen bond strength and its distribution are inferred from surface sum-frequency generation (SFG) spectra through the center frequency and width, respectively, of the O–D stretch vibration of isotopically diluted HDO in H2O. The use of partially deuterated water prevents complications due to intramolecular vibrational coupling, which we show gives rise to features in the SFG spectra that are unrelated to hydrogen bonding interactions. At the water–air interface, the SFG spectrum in the hydrogen-bonded region strongly resembles the bulk Raman spectrum, indicating that, at this interface, the interfacial hydrogen bonding properties are very similar to those in bulk water. In contrast, for silica–water and lipid–water interfaces, interfacial hydrogen bonding is substantially stronger, with a larger degree of heterogeneity. [Copyright &y& Elsevier]

Abstract: We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems. [Copyright &y& Elsevier]

Abstract: Using terahertz time-domain spectroscopy (THz-TDS) the complex dielectric response of mixtures of the prototypical ionic liquid [1-butyl-3-methylimidazolium]+:[BF4]− ([bmin]BF4) with water is investigated. The dielectric response can be described well using a two-term Debye model, and these two relaxation processes in the THz region are followed as a function of volume fraction of the constituent liquids. For the mixture, slow and fast Debye relaxation times are larger than the values expected from interpolation of pure water and pure ionic liquid values, pointing to increased structural coherence within the mixture. [Copyright &y& Elsevier]

The laser-induced movement of CO molecules over a platinum surface was followed in real time by means of ultrafast vibrational spectroscopy. Because the CO molecules bound on different surface sites exhibit different C-O stretch vibrational frequencies, the site-to-site hopping, triggered by excitation with a laser pulse, can be determined from subpicosecond changes in the vibrational spectra. The unexpectedly fast motion — characterized by a 500-femtosecond time constant — reveals that a rotational motion of the CO molecules, rather than pure translation, is required for this diffusion process. This conclusion is corroborated by density functional theory calculations. [ABSTRACT FROM AUTHOR]

The article discusses the implementation of femtosecond two dimensional-infrared (2D-IR) spectroscopy. It is a sequence of coherent interactions between the sample and the IR laser fields designed to determine molecular structures and dynamics. It mentions that it contains information about vibrational mode coupling. Moreover, it is useful in studying the structure and reactivity of molecular adsorbate layers in catalytic systems and the interactions of membranes and membrane proteins.