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Register a new userRevealing excess protons in the infrared spectrum of liquid water
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The most common species in liquid water, next to neutral H$_2$O molecules, are the H$_3$O$^+$ and OH$^-$ ions. In a dynamic picture, their exact concentrations depend on the time scale at which these are probed. Here, using a spectral-weight analysis, we experimentally resolve the fingerprints of the elusive fluctuations-born short-living H$_3$O$^+$, DH$_2$O$^+$, HD$_2$O$^+$, and D$_3$O$^+$ ions in the IR spectra of light (H$_2$O), heavy (D$_2$O), and semi-heavy (HDO) water. We find that short-living ions, with concentrations reaching $sim 2%$ of the content of water molecules, coexist with long-living pH-active ions on the picosecond timescale, thus making liquid water an effective ionic liquid in femtochemistry.
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The starting point to understanding cluster properties is the putative global minimum and all the nearby local energy minima; however, locating them is computationally expensive and challenging due to a combinatorial explosion problem. The relative populations and spectroscopic properties of a molecule that are a function of temperature can be approximately computed by employing statistical thermodynamics. Here, we investigate temperature-entropy-driven isomers distribution on Be$_6$B$_{11}^{-}$ fluxional cluster and the effect of temperature on their infrared spectroscopy and relative populations. We identify the vibration modes possessed by the cluster that significantly contribute to the zero-point energy. A couple of steps are considered for computing the temperature-dependent relative population: First, using a genetic algorithm coupled to density functional theory, we performed an extensive and systematic exploration of the potential/free energy surface of Be$_6$B$_{11}^{-}$ cluster to locate the putative global minimum and elucidate the low-energy structures. Second, the relative populations temperature effects are determined by considering the thermodynamic properties and Boltzmann factors. The temperature-dependent relative populations show that the entropies and temperature are essential for determining the global minimum. We compute the temperature-dependent total infrared spectra employing the Boltzmann factor weighted sums of each isomers infrared spectrum and find that at finite temperature, the total infrared spectrum is composed of an admixture of infrared spectra that corresponds to the spectrum of the lowest energy structure and its isomers located at higher energies. The methodology and results describe the thermal effects in the relative population and the infrared spectra.
The paper describes the investigation of the properties of silver besilicate salt colloids in water medium. Ag6Si2O7 was obtained in a soft conditions, in water medium, at temperatures and pressure close to room ones. The morphology and crystallinity of matter under investigations appear to depend on the growth conditions. Slow growth resulted in quasi-crystalline yellowish whiskers, while high concentration of precursors led to fast formation of the quasi-amorphous aggregates. The elemental constitution of object under study was determined with the help of Comebax-INCA set-up, and was then proved by chemical analysis. The ratio of elements was close to Ag6Si2O7, and depended a little whether the sample was taken from the sediments of reaction or from dried residue of a liquid medium. The experiments in Raman technique revile several well-resolvable bands in whisker-like samples, but only few of them were able to detect in quasi-amorphous material. In the last case the bands were broader, but the positions of weak broad bands were close to the positions of corresponding bands in whiskers. We can suggest, that the bands, common in two former cases, are due to oscillation in bepyramidal molecules of Ag6Si2O7, and the bands, seen only at whiskers, are due to long-rang order. The very special peculiarity was observed under chemical test with hydrochloric acid. The white flakes sedimentation appeared after addition of NaCl but do not appeared when HCl was added. This result can be explained if to assume the existence of silicon acid shell around the colloidal particles of silver besilicate. The model of silicon acid shell around colloids was proved by high resolution electron microscopy.
When analyzing the broadband absorption spectrum of liquid water (10^10 - 10^13 Hz), we find its relaxation-resonance features to be an indication of Frenkels translation-oscillation motion of particles, which is fundamentally inherent to liquids. We have developed a model of water structure, of which the dynamics is due to diffusion of particles, neutral H2O molecules and H3O+ and OH- ions - with their periodic localizations and mutual transformations. This model establishes for the first time a link between the dc conductivity, the Debye and the high frequency sub-Debye relaxations and the infrared absorption peak at 180 cm-1. The model reveals the characteristic times of the relaxations, 50 ps and 3 ps, as the lifetimes of water molecules and water ions, respectively. The model sheds light on the anomalous mobility of a proton and casts doubt on the long lifetime of a water molecule, 10 hours, commonly associated with autoionization.
Among the many existing molecular models of water, the MB-pol many-body potential has emerged as a remarkably accurate model, capable of reproducing thermodynamic, structural, and dynamic properties across waters solid, liquid, and vapor phases. In this work, we assessed the performance of MB-pol with respect to an important set of properties related to vapor-liquid coexistence and interfacial behavior. Through direct coexistence classical molecular dynamics simulations at temperatures 400 K < T < 600 K, we calculated properties such as equilibrium coexistence densities, vapor-liquid interfacial tension, vapor pressure, and enthalpy of vaporization, and compared the MB-pol results to experimental data. We also compared rigid vs. fully flexible variants of the MB-pol model and evaluated system size effects for the properties studied. We found that the MB-pol model predictions are in good agreement with experimental data, even for temperatures approaching the vapor-liquid critical point; this agreement was largely insensitive to system size or the rigid vs. flexible treatment of the intramolecular degrees of freedom. These results attest to the chemical accuracy of MB-pol and its high degree of transferability, thus enabling MB-pols application across a large swath of waters phase diagram.
An accurate ab initio theory of the H-bond structure of liquid water requires a high-level exchange correlation approximation from density functional theory. Based on the liquid structures modeled by ab initio molecular dynamics by using maximally localized Wannier functions as a basis, we study the infrared spectrum of water within the canonical ensemble. In particular, we employ both the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) and the state-of-the-art meta-GGA level approximation provided by the strongly constrained and appropriately normed (SCAN) functional. We demonstrate that the SCAN functional improves not only the water structure but also the theoretical infrared spectrum of water. Our analyses show that the improvement in the stretching and bending bands can be mainly attributed to better descriptions of directional H bonding and the covalency at the inter- and intramolecular levels, respectively. On the other hand, better agreements in libration and hindered translation bands are due to the improved dynamics of the H-bond network enabled by a less structured liquid in the experimental direction. The spectrum predicted by SCAN shows much better agreement with experimental data than the conventionally widely adopted PBE functional at the GGA level.
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