Vibrational Spectrum Of Water Research Articles

Twist and Stretch: Assignment and Surface Charge Sensitivity of a Water Combination Band and Its Implications for Vibrational Sum Frequency Spectra Interpretations.

The vibrational spectrum of water at hydrophobic/hydrophilic interfaces is crucial to understanding complex surface chemistry phenomena. Vibrational sum frequency (VSF) spectroscopy is a valuable nonlinear spectroscopic technique for exploring the details of vibrational spectra of molecules at surfaces. However, spectral assignments and analysis of VSF spectra are often more nuanced than in linear spectroscopy. This study is aimed at understanding the source of the broad VSF signal in the oil-water surface water spectrum at energies slightly higher than traditionally examined, beyond the unbound free OH oscillator of surface water molecules. Analyzing isotopic dilutions of the aqueous solvent with VSF spectroscopy, we demonstrate that this signal is due to a combination band of water stretch and libration motions. The spectral characteristics of this band are found to be highly sensitive to the sign and magnitude of the surface charge induced by adsorption of both anionic and cationic surfactants. The results have implications for VSF measurements of the C-H stretching vibrations of various adsorbates when studied with D2O as the aqueous solvent. Because the vibrational signal from the water combination band is dependent on surface charge, it is imperative to include the presence of the combination band when fitting surface spectra.

Understanding the Origins of Dipolar Couplings and Correlated Motion in the Vibrational Spectrum of Water.

The combination of vibrational spectroscopy and molecular dynamics simulations provides a powerful tool to obtain insights into the molecular details of water structure and dynamics in the bulk and in aqueous solutions. Applying newly developed approaches to analyze correlations of charge currents, molecular dipole fluctuations, and vibrational motion in real and k-space, we compare results from nonpolarizable water models, widely used in biomolecular modeling, to ab initio molecular dynamics. For the first time, we unfold the infrared response of bulk water into contributions from correlated fluctuations in the three-dimensional, anisotropic environment of an average water molecule, from the OH-stretching region down to the THz regime. Our findings show that the absence of electronic polarizability in the force field model not only results in differences in dipolar couplings and infrared absorption but also induces artifacts into the correlated vibrational motion between hydrogen-bonded water molecules, specifically at the intramolecular bending frequency. Consequently, vibrational motion is partially ill-described with implications for the accuracy of non-self-consistent, a posteriori methods to add polarizability.

Vibrational Spectra of Proximal Water in a Thermo-Sensitive Polymer Undergoing Conformational Transition Across the Lower Critical Solution Temperature

The vibrational spectrum of water near a thermo-sensitive polymer poly(N-isopropylacrylamide) (PNIPAM) undergoing conformational transition through the lower critical solution temperature (LCST) is calculated using molecular dynamics simulations. The characteristic structural features observed at the atomic scale for these proximal water molecules in a solvated polymer chain while undergoing the conformational transition are strongly correlated to their vibrational densities of states. Comparison of the vibrational spectrum below LCST for the proximal water with the vibrational spectrum obtained for bulk water reveals a significant fraction of the hydrogen bonding between the proximal water molecules and the polymer side groups. Hydrogen-bonded bridges of water molecules are formed between two adjacent and alternate monomers. This network of hydrogen bonding results in formation of locally ordered water molecules at temperatures below the LCST. Analysis of the simulation trajectories confirms the presence of a quasi-stable solvation structure near the PNIPAM. The calculated vibrational spectra for proximal water above the LCST suggest significantly reduced hydrogen bonding with the polymer and indicate a reduction in the structural stability of proximal water around a collapsed polymer chain. Systematic trends in the observed peak intensities and frequency shifts at the low- and high-frequency ends of the spectrum can be correlated with the structural and dynamical changes of water molecules below and above the LCST transition, respectively, for various polymer chain lengths. The simulations reveal that, compared to bulk water, the libration bands are blue shifted and OH stretch bands red shifted for water in proximity to PNIPAM with 30 monomer units below the LCST. The simulations suggest that vibrational spectra can be used as a predictive tool for quantifying atomic-scale structural transitions in solvation of thermo-sensitive polymers such as PNIPAM.

Vibrational manifestations of strong non-Condon effects in the H3O+·X3 (X = Ar, N2, CH4, H2O) complexes: A possible explanation for the intensity in the “association band” in the vibrational spectrum of water

The harmonic approximation provides a powerful approach for interpreting vibrational spectra. In this treatment, the energy and intensity of the 3N- 6 normal modes are calculated using a quadratic expansion of the potential energy and a linear expansion of the dipole moment surfaces, respectively. In reality, transitions are often observed that are not accounted for by this approach (e.g. combination bands, overtones, etc.), and these transitions arise from inherent anharmonicities present in the system. One interesting example occurs in the vibrational spectrum of H(2)O((l)), where a band is observed near 2000 cm(-1) that is commonly referred to as the "association band". This band lies far from the expected bend and stretching modes of the water molecule, and is not recovered at the harmonic level. In a recent study, we identified a band in this spectral region in gas-phase clusters involving atomic and molecular adducts to the H(3)O(+) ion. In the current study we probe the origins of this band through a systematic analysis of the argon-predissociation spectra of H(3)O(+)·X(3) where X = Ar, CH(4), N(2) or H(2)O, with particular attention to the contributions from the non-linearities in the dipole surfaces, often referred to as non-Condon effects. The spectra of the H(3)O(+) clusters all display strong transitions between 1900-2100 cm(-1), and theoretical modeling indicates that they can be assigned to a combination band involving the HOH bend and frustrated rotation of H(3)O(+) in the solvent cage. This transition derives its oscillator strength entirely from strong non-Condon effects, and we discuss its possible relationship to the association band in the spectrum of liquid water.

On the low-frequency vibrational spectrum of water

A new assignment is proposed for the low-frequency region of the vibrational spectrum of water, based on a comparison of the times of relevant occurring events. The time-matching is made between the dynamics of the rupture and formation time of the hydrogen bond, and the so-called bending and stretching oscillation periods of the O–H ··· O tetrahedral network. The same assignment also emerges by comparing thermal and solute concentration effects. Furthermore, a critical re-examination of the literature already existing on the subject corroborates the now proposed assignments for the modes at 60 and 170 cm−1. These are considered to arise first from a transverse acoustic phonon with a low-frequency contribution sensitive to temperature and concentration effects, and second from external motions such as translations or restricted rotations of small water clusters inside a cage, and subjected to its potential field.

Hydrogen bond structure and vibrational spectrum of water at a passivated metal nanoparticle

Molecular Dynamics computer simulations of gold passivated nanoparticles in water are presented. We show that butanethiol passivated gold nanoparticles of ∼3 nm diameter exhibit weak de-wetting, similar to that observed in extended hydrophobic surfaces. The nanoparticle–water interface is characterised by a water density depletion region, where the water molecules show a dramatic reduction in the number of hydrogen bonds with respect to bulk water. This weakening of the hydrogen bond structure is accompanied by the orientation of the dipole water vector towards the nanoparticle surface. The investigation of the vibrational spectrum of interfacial water at the nanoparticle surface supports the existence of free OH bonds. Overall our results indicate that small passivated metal nanoparticles (∼3 nm diameter) behave like hydrophobic surfaces.

Study of Vibrational Spectra of Interlayer Water in Sodium Beidellite by Molecular Dynamics Simulations

To elucidate the molecular structure and water-clay interaction of interlayer water in sodium type smectite (hydrated layered alminosilicate), vibrational spectra of water were investigated using molecular dynamics (MD) simulations based on the free flexible force field model. Vibrational spectra of water were obtained by Fourier transformation of the velocity autocorrelation function of the hydrogen atom. Two distinct bands were found at 3365 and 3500 cm - 1 in the stretching vibrational spectrum of interlayer water. The former band was assigned to an O-H bond unbound to the clay surface, while the latter was attributed to O-H vibrations bound to the clay surface through hydrogen bonding. The hydrogen bond distance (the H...O distance) between water and the clay surface (H w a t e r ...O c l a y = 0.22 nm) was larger than that between water molecules (H w a t e r ...O w a t e r = 0.19 nm). Detailed comparison of simulation results with IR spectroscopic observations indicated good agreement. The hydrogen bond structure and the vibrational spectrum of interlayer water suggest no rigid network structure of water molecules (icelike water) near the smectite surface.

Vibrational spectrum of water at the liquid/vapor interface.

The infrared spectrum of water molecules at the liquid/vapor interface of pure water and the liquid/vapor interface of a water-methanol mixture is calculated using molecular dynamics. A comparison with the IR spectrum calculated for bulk water shows a significant population of free OH bonds, in qualitative agreement with a recent experiment by Du et al. Phys. Rev. Lett. 70 2313 1993. It is also demonstrated that the free OH IR signal is quenched by a moderate increase in the concentration of methanol in bulk water, which has been recently observed by Du et al. Science 264 826 1994.

The vibrational spectrum of water in liquid alkanes

The water wire hypothesis of hydrogen-ion transport in lipid bilayers has prompted a search for water aggregates in bulk hydrocarbons. The asymmetric stretching vibration of the water dissolved in n-decane and in a number of other alkanes and alkenes has been observed. The water band in the alkanes is very wide and fits to the results of a J-diffusion calculation for the water rotation. This implies that the water is freely rotating between collisions with the solvent and certainly not hydrogen bonded to anything. The existence of water aggregates is thus most unlikely. In contrast, water in an alkene is hydrogen bonded to the solvent molecules (although not to other water molecules) and shows an entirely different spectrum.

Analysis of the OH bend and stretch region in the vibrational spectrum of water

A method for calculating the infrared absorption spectrum of water in a range of different physical environments is described. Calculations are based on Monte Carlo simulations together with local-mode analyses of the H 2O vibrations. Results for liquid water and amorphous ice show clearly that the major features of the OH stretching motions are reproduced with good accuracy.

Fermi-Resonance Minima in the Vibrational Spectrum of Water in Crystalline Hydrates

The infrared absorption and Raman spectra of polycrystalline tetrachlorocuprate(II) dihydrates M2CuCl4.2H2O (M = K, Rb, Cs, NH4) contain minima in the OH stretching region which sharpen at low temperature. These minima are shown to be 'Evans holes', or negative absorption features, caused by Fermi resonance between the broad and intense H2O stretching fundamental V1 and a relatively narrow band due to the overtone of the H2O bending vibration 2V2. Our findings confirm that Fermi resonance can lead to unusual spectral features, which must be taken into account in the analysis of the stretching region of the spectra of H2O and D2O in condensed phases, as has been done by Scherer etal. for the spectrum of liquid water.

Comment on possible motional effects on the vibrational spectrum of water in the fundamental stretching region

The i.r. spectrum of water in four organic solvents has been carefully recorded in the v(OH) region. Dipole correlation functions obtained through Fourier transform of the high-frequency side of the v 3 band confirm essentially free rotation of water in the two non-polar solvents chloroform and carbontetrachloride, on a time scale of the order 0.1 psec. In dioxane solution, a shoulder observed on the high-frequency side of the v 3 band, similar to that observed in the pure liquid, is assigned to non-H-bonded molecules. It is suggested that exchange motions between free and H-bonded species in dioxane solution and in the pure liquid, affect the position and width of this shoulder component, and also the overall band shapes. The effect occurs on a time-scale of the order 0.1 psec.