Neat H2O Research Articles

Intermolecular/Interionic Vibrations of 1-Methyl-3-n-octylimidazolium Tetrafluoroborate Ionic Liquid and H2O Mixtures

We report here the low-frequency spectra, resulting from the intermolecular/interionic vibrational dynamics, of aqueous mixtures of an ionic liquid, 1-methyl-3-n-octylimidazolium tetrafluoroborate, with the H(2)O mole fractions of 0.2, 0.4, and 0.6 and the neat ionic liquid and H(2)O within the frequency range of 0.1-700 cm(-1) by means of femtosecond Raman-induced Kerr effect spectroscopy. Addition of H(2)O induces tiny effects on the line shape of the low-frequency Kerr spectrum of the ionic liquid: ca. a 2 cm(-1) red shift in the first moment of the low-frequency spectrum has been observed for a transition from the neat ionic liquid to the binary mixture containing 0.6 mol fraction of H(2)O. Surface tension and liquid density of the mixture also accompany minimal changes upon addition of H(2)O. These results suggest that H(2)O molecules localize at the interface between the ionic and nonpolar regions, and the interionic interaction in the ionic region is weakly perturbed by the existence of H(2)O. On the other hand, successive addition of H(2)O in the mixture slows down the picosecond overdamped relaxation process measured in the 3-300 ps range even though the shear viscosity of the mixture decreases substantially.

Dissociative adsorption of hydrogen fluoride onto amorphous solid water

Adsorption of hydrogen fluoride (HF) onto amorphous solid water films at 50 K is reported to yield a strong absorbance continuum in their reflection-absorption infrared spectra (RAIRS). This and other complex features observed in the RAIRS spectra of stratified binary composite HF:H(2)O nanoscopic films deposited onto Pt(111) are interpreted quantitatively using a classical optics model. Comparison with experimental data allows us to determine that the absorbance continuum is due to absorption within the film (as opposed to trivial optical effects) and that the extent of intermixing and uptake is mostly limited to the first few molecular layers. Furthermore, extensive isotope scrambling is demonstrated by the observation of similar Zundel continua upon codeposition of neat HF, or DF, and H(2)O vapors onto Pt(111) at 50 K. These observations are consistent with those expected from extensive ionic dissociation of HF upon dissolution within, and adsorption onto, ASW at 50 K.

Measurements of spectral parameters of water-vapour transitions near 1388 and 1345 nm for accurate simulation of high-pressure absorption spectra

Quantitative near-infrared absorption spectroscopy of water-vapour overtone and combination bands at high pressures is complicated by pressure broadening and shifting of individual lines and the blending of neighbouring transitions. An experimental and computational methodology is developed to determine accurate high-pressure absorption spectra. This case study investigates two water-vapour transitions, one near 1388 nm (7203.9 cm−1) and the other near 1345 nm (7435.6 cm−1), for potential two-line absorption measurements of temperature in the range of 400–1050 K with a pressure varying from 5–25 atm. The required quantitative spectroscopy data (line strength, collisional broadening, and pressure-induced frequency shift) of the target transitions and their neighbours (a total of four H2O vapour transitions near 1388 nm and six transitions near 1345 nm) are measured in neat H2O vapour, H2O–air and H2O–CO2 mixtures as a function of temperature (296–1000 K) at low pressures (<800 Torr). Precise values of the line strength S(T), pressure-broadening coefficients γair(T) and , and pressure-shift coefficients δair(T) and for the ten transitions were inferred from the measured spectra and compared with data from HITRAN 2004. A hybrid spectroscopic database was constructed by modifying HITRAN 2004 to incorporate these values for simulation of water-vapour-absorption spectra at high pressures. Simulations using this hybrid database are in good agreement with high pressure experiments and demonstrate that data collected at modest pressures can be used to simulate high-pressure absorption spectra.

Time-Domain Calculations of the Polarized Raman Spectra, the Transient Infrared Absorption Anisotropy, and the Extent of Delocalization of the OH Stretching Mode of Liquid Water

The polarized Raman spectrum and the time dependence of the transient infrared (TRIR) absorption anisotropy are calculated for the OH stretching mode of liquid water (neat liquid H2O) by using time-domain formulations, which include the effects of both the diagonal frequency modulations (of individual oscillators) induced by the interactions between the dipole derivatives and the intermolecular electric field, and the off-diagonal (intermolecular) vibrational coupling described by the transition dipole coupling (TDC) mechanism. The IR spectrum of neat liquid H2O and the TRIR anisotropy of a liquid mixture of H2O/HDO/D2O are also calculated. It is shown that the calculated features of these optical signals, including the temperature dependence of the polarized Raman and IR spectra, are in reasonable agreement with the experimental results, indicating that the frequency separation between the isotropic and anisotropic components of the polarized Raman spectrum and the rapid decay (approximately 0.1 ps) of the TRIR anisotropy of the OH stretching mode of neat liquid H2O are mainly controlled by the resonant intermolecular vibrational coupling described by the TDC mechanism. Comparing with the time evolution of vibrational excitations, it is suggested that the TRIR anisotropy decays in the time needed for the initially localized vibrational excitations to delocalize over a few oscillators. It is also shown that the enhancement of the dipole derivatives by the interactions with surrounding molecules is an important factor in generating the spectral profiles of the OH stretching Raman band. The time-domain behavior of the molecular motions that affect the spectroscopic features is discussed.

Polymorphic Behavior in Protein−Surfactant Mixtures: The Water−Bovine Serum Albumin−Sodium Taurodeoxycholate System

Mixtures containing water, bovine serum albumin (BSA), and sodium taurodeoxycholate (NaTDC), a component of the bile in mammals, have been investigated in a wide range of composition and pH. Depending on the concentration of both solutes and the pH, solutions, precipitates, and gels are formed. Under spontaneous pH conditions, the transport properties in dilute solutions indicate the occurrence of significant interactions between BSA and the surfactant. Conversely, acidic media favor the formation of nonsoluble protein-surfactant complexes, with subsequent precipitation. The nucleation kinetics of the protein-surfactant complexes in solid form and the related precipitation processes can be slow or fast, depending on the overall solute content and the mole ratio. At high concentrations, a gel, extending on both sides of the charge neutralization line, and two-phase regions are observed. Gels shrink in open air and swell in the presence of excess water. Depending on concentration and temperature, the gels transform from an essentially liquidlike behavior to that peculiar to true gels (when G' > or = G''). The thermal gelation threshold, the temperature above which G' > or = G'', depends on BSA and NaTDC content and is concomitant to moderate heat effects, inferred by differential scanning calorimetry (DSC). The above data also indicate that the protein thermal denaturation in the gel is shifted to higher temperatures compared to water. Such a stabilizing effect is presumably related to the occurrence of both electrostatic and hydrophobic interactions with NaTDC. Water self-diffusion in the gels is slightly slower than that in the bulk and poorly sensitive to composition: it is about 65% the value of neat H2O in a wide concentration range, irrespective of the BSA, or NaTDC, concentration. A peculiar behavior is also observed in 23Na longitudinal and transverse relaxation rates. The T1 and T2 values, measured at 105.75 MHz on BSA-NaTDC gels, indicate that the motions determining the NMR relaxation of the sodium ions in the hydration layer of the protein-surfactant aggregates are not slow, having frequencies comparable with the Larmor one. The above properties, especially the rheological and the spectroscopic ones, are important for understanding the behavior of gels based on protein-surfactant mixtures.

‘Green’ Synthesis of Nitro Alcohols Catalyzed by Solid-Supported Tributylammonium Chloride in Aqueous Medium

A series of 1-aryl-2-nitroalkan-1-ols (5a-m; Table 2) was prepared in yields of 64-94% by Henry reaction between aromatic aldehydes and nitroalkanes in the presence of a dual catalytic system of KOH and polystyrene-supported tributylammonium chloride (PsTBAC; 2). The reactions were performed either in H2O containing 10% of THF or, in the case of liquid reagents (aldehydes and nitroalkanes), in neat H2O at room temperature. The immobilized phase-transfer catalyst, designed to be used for large-scale industrial processes, could be easily separated from the products, and effectively re-used several times, basically without loss in activity.

Hydrogen isotope effects and mechanism of aqueous ozone and peroxone decompositions.

Hydrogen peroxide exalts the reactivity of aqueous ozone by reasons that remain obscure. Should H2O2 enhance free radical production, as it is generally believed, a chain mechanism propagated by (.OH/.O2-) species would account for O3 decomposition rates in neat H2O, HR-O3, and in peroxone (O3 + H2O2) solutions, HPR-O3. We found, however, that: (1) the radical mechanism correctly predicts HR-O3 but vastly overestimates HPR-O3, (2) solvent deuteration experiments preclude radical products from the (O3 + HO2-) reaction. The modest kinetic isotope effect (KIE) we measure in H2O/D2O: HR-O3/DR-O3 = 1.5 +/- 0.3, is compatible with a chain process driven by electron- and/or O-atom transfer processes. But the large KIE found in peroxone: HPR-O3/DPR-O3 = 19.6 +/- 4.0, is due to an elementary (O3 + HO2-) reaction involving H-O2- bond cleavage. Since the KIE for the hypothetical H-atom transfer: O3 + HO2- HO3. +.O2-, would emerge as a KIE1/2 factor in the rates of the ensuing radical chain, the magnitude of the observed KIE must be associated with the hydride transfer reaction that yields a diamagnetic species: O3 + HO2- HO3- + O2. HO3-/H2O3 may be the bactericidal trioxide recently identified in the antibody-catalyzed addition of O2(1Deltag) to H2O.

A physical adsorption model of the dependence of ClONO2 heterogeneous reactions on relative humidity

We present a model of heterogeneous reactivity based on physical adsorption that describes the observed relative humidity dependence of the ClONO2 reaction probability with H2O and HCl on sulfuric acid tetrahydrate ice surfaces (SAT) and with H2O on nitric acid trihydrate (NAT). The laboratory data are modeled using only two parameters for a given system, the measured reaction probability on a neat H2O ice surface, and a constant from the BET theory which describes the fraction of an acid hydrate surface covered by H2O as a function of relative humidity. The model indicates that ClONO2 reactivity with both HCl and H2O on SAT and with H2 O on NAT is controlled by the surface coverage of H2O. In contrast, the reaction of ClONO2 +HCl on NAT is better described by an alternative model based on reactivity in solutions formed within a porous ice by capillary liquid absorption.