Vibrational spectroscopy of single methanol molecules attached to liquid water clusters

Study on hydrogen bonding network in aqueous methanol solution by Raman spectroscopy

The Raman spectra of aqueous methanol solution with various concentrations were measured at room temperature and atmospheric pressure. We found that the CO stretching vibration mode of methanol showed a significant blue shift at V_m (V_m represents the volume fraction of methanol) >0.4, while the CH symmetric and asymmetric stretching vibration modes exhibited red shift under the same conditions. These results illustrate that the variation of hydrogen bond between methanol and water molecules lead to a phase transition of the methanol-water complex at V_m = 0.4. Furthermore, the red shift of the CH vibration mode indicates that there is no hydrogen bond formed between the CH₃ group of methanol and water molecules. In addition, we found that the frequency shift of C-H is affected by the hydrogen bond C-O…H-O formed between methanol and water molecules, and the corresponding theoretical discussion is given. Finally, the phase transition process of methanol-water complex in methanol-water binary solution was given by theoretical analysis.

Coexistence of ice clusters and liquid-like water clusters on the Ru(0001) surface

Detailed RAIRS spectra reveal rich and varied local hydrogen bonding structures inside the two types of water clusters found on the Ru(0001) surface.

The water-hydrophobic interface: neutral and charged solute adsorption at fluorocarbon and hydrocarbon self-assembled monolayers (SAMs)

Adsorption of small molecular solutes in an aqueous solution to a soft hydrophobic surface is a topic relevant to many fields. In biological and industrial systems, the interfacial environment is often complex, containing an array of salts and organic compounds in the solution phase. Additionally, the surface itself can have a complex structure that can interact in unpredictable ways with small solutes in its vicinity. In this work, we studied model adsorption processes on hydrocarbon and fluorocarbon self-assembled monolayers by using vibrational sum frequency spectroscopy, with methanol and butylammonium chloride as adsorbates. The results indicate that differences in surface functionality have a significant impact on the organization of adsorbed organic species at hydrophobic surfaces.

Dynamics and mechanism of structural diffusion in linear hydrogen bond

Dynamics and mechanism of proton transfer in a protonated hydrogen bond (H-bond) chain were studied, using the CH(3)OH(2)(+)(CH(3)OH)(n) complexes, n = 1-4, as model systems. The present investigations used B3LYP/TZVP calculations and Born-Oppenheimer MD (BOMD) simulations at 350 K to obtain characteristic H-bond structures, energetic and IR spectra of the transferring protons in the gas phase and continuum liquid. The static and dynamic results were compared with the H(3)O(+)(H(2)O)(n) and CH(3)OH(2)(+)(H(2)O)(n) complexes, n = 1-4. It was found that the H-bond chains with n = 1 and 3 represent the most active intermediate states and the CH(3)OH(2)(+)(CH(3)OH)(n) complexes possess the lowest threshold frequency of proton transfer. The IR spectra obtained from BOMD simulations revealed that the thermal energy fluctuation and dynamics help promote proton transfer in the shared-proton structure with n = 3 by lowering the vibrational energy for the interconversion between the oscillatory shuttling and structural diffusion motions, leading to a higher population of the structural diffusion motion than in the shared-proton structure with n = 1. Additional explanation on the previously proposed mechanisms was introduced, with the emphases on the energetic of the transferring proton, the fluctuation of the number of the CH(3)OH molecules in the H-bond chain, and the quasi-dynamic equilibriums between the shared-proton structure (n = 3) and the close-contact structures (n ≥ 4). The latter prohibits proton transfer reaction in the H-bond chain from being concerted, since the rate of the structural diffusion depends upon the lifetime of the shared-proton intermediate state.

Determination of the Adsorption Isotherm of Methanol on the Surface of Ice. An Experimental and Grand Canonical Monte Carlo Simulation Study

The adsorption isotherm of methanol on ice at 200 K has been determined both experimentally and by using the Grand Canonical Monte Carlo computer simulation method. The experimental and simulated isotherms agree well with each other; their deviations can be explained by a small (about 5 K) temperature shift in the simulation data and, possibly, by the non-ideality of the ice surface in the experimental situation. The analysis of the results has revealed that the saturated adsorption layer is monomolecular. At low surface coverage, the adsorption is driven by the methanol-ice interaction; however, at full coverage, methanol-methanol interactions become equally important. Under these conditions, about half of the adsorbed methanol molecules have one hydrogen-bonded water neighbor, and the other half have two hydrogen-bonded water neighbors. The vast majority of the methanols have a hydrogen-bonded methanol neighbor, as well.

Size-selected methyl lactate clusters: fragmentation and spectroscopic fingerprints of chiral recognition

A comprehensive experimental study of the OH stretching vibrations of size-selected clusters of enantiopure and racemic methyl lactate is presented. For the size selection, we measured angular dependent mass spectra and time-of-flight distributions at the different fragment masses. In this way the fragmentation of these clusters upon electron impact ionization is obtained. The largest fragment masses of the neutral (MLac)n clusters are the protonated (MLac)n-1H+ ions. The results of a pressure dependent study in an FTIR jet experiment are compared with completely size-selected experiments based on atomic beam deflection and depletion spectroscopy. The size assignments and spectra agree for dimers and trimers. Structures and spectral information for the trimer and the tetramer at density functional and MP2 level are provided. Selective self-aggregation and chiral recognition was observed for homochiral trimers. They exhibit a ring structure bound by OH...OH hydrogen bonds. A spectacular switch in the hydrogen bonding topology was observed for the tetramer. The homochiral enantiomer exhibits cooperative OH...OH bonding, while the heterochiral version shows isolated OH...O=C bonding in a symmetric SRSR arrangement. The crucial ingredients for this identification are the size-selective IR spectra with their different shifts and line patterns which are reproduced by the calculations.

Effect of pressure on hydrogen bonding in liquid methanol

We have investigated the effect of pressure on the hydrogen bonding in liquid methanol using Raman spectroscopy. Specifically, we have measured the OH and CO stretching modes and assigned the bands, in agreement with recent IR and crossed molecular beam experiments on methanol clusters. At about 7 to 8 kbar, we note indications that the intrinsic nature of the methanol clusters in our samples has changed. Our results provide support for and extend conclusions derived from Monte Carlo simulations, explain anomalies observed by previous researchers, and provide new insights into general hydrogen-bonding phenomena.

H-densities: a new concept for hydrated molecules

It is common to represent molecules by "ball-and-stick" models that represent static positions of atoms. However, the vibrational states of water molecules involved in hydrogen bonding have wide amplitudes, even in their ground states. Here we introduce a new representation of this wide-amplitude vibrational motion: H-density plots. These plots represent the delocalized zero-point vibrational motion of terminal hydrogen atoms of water molecules weakly bound to other molecules. They are a vibrational analogy to electron densities. Calculations of the H-densities for complexes of water with water, benzene, phenol, and DNA bases are presented. These are obtained using the quantum diffusion Monte Carlo method. Comparisons of measured and calculated rotational constants provide experimental evidence of the new concept.