Nonharmonic transverse vibration of the H-bonded molecules and the THz spectra in ice and water
ABSTRACT A nonlinear equation of motion is found for the dimer comprising two charged H2O molecules. The THz dielectric response to nonharmonic vibration of a nonrigid dipole, forming the hydrogen bond (HB), is found in the direction transverse to this bond. An explicit expression is derived for the autocorrelator that governs the spectrum generated by transverse vibration (TV) of such a dipole. This expression is obtained by analytical solution of the truncated set of recurrence equations. The far infrared (FIR) spectra of ice at the temperature −7 °C are calculated. The wideband, in the wavenumber (frequency) ν range 0…1000 cm−1, spectra are obtained for liquid water at room temperature and for supercooled water at −5.6 °C. All spectra are represented in terms of the complex permittivity ε(ν) and the absorption coefficient α(ν).The obtained analytical formula for ε comprises the term ε⊥ pertinent to the studied TV mechanism with three additional terms Δεq, Δεμ, and εor arising, respectively, from: elastic harmonic vibration of charged molecules along the H-bond; elastic reorientation of HB permanent dipoles; and rather free libration of permanent dipoles in ‘defects’ of water/ice structure.The suggested TV-dielectric relaxation mechanism allows us: (a) to remove the THz ‘deficit’ of loss ε″ inherent in previous theoretical studies; (b) to explain the THz loss and absorption spectra in supercooled (SC) water; and (c) to describe, in agreement with the experiment, the low- and high-frequency tails of the two bands of ice H2O located in the range 10…300 cm−1. Specific THz dielectric properties of SC water are ascribed to association of water molecules, revealed in our study by transverse vibration of HB charged molecules.
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ABSTRACT: Ionic liquids are defined as salts composed solely of ions with melting points below 100 °C. These remarkable liquids have unique and fascinating properties and offer new opportunities for science and technology. New combinations of ions provide changing physical properties and thus novel potential applications for this class of liquid materials. To a large extent, the structure and properties of ionic liquids are determined by the intermolecular interaction between anions and cations. In this perspective we show that far infrared and terahertz spectroscopy are suitable methods for studying the cation-anion interaction in these Coulomb fluids. The interpretation of the measured low frequency spectra is supported by density functional theory calculations and molecular dynamics simulations. We present results for selected aprotic and protic ionic liquids and their mixtures with molecular solvents. In particular, we focus on the strength and type of intermolecular interaction and how both parameters are influenced by the character of the ions and their combinations. We show that the total interaction between cations and anions is a result of a subtle balance between Coulomb forces, hydrogen bonds and dispersion forces. For protic ionic liquids we could measure distinct vibrational modes in the low frequency spectra indicating clearly the cation-anion interaction characterized by linear and medium to strong hydrogen bonds. Using isotopic substitution we have been able to dissect frequency shifts related to pure interaction strength between cations and anions and to different reduced masses only. In this context we also show how these different types of interaction may influence the physical properties of ionic liquids such as the melting point, viscosity or enthalpy of vaporization. Furthermore we demonstrate that low frequency spectroscopy can also be used for studying ion speciation. Low vibrational features can be assigned to contact ion pairs and solvent separated ion pairs. In conclusion we showed how detailed knowledge of the low frequency spectra can be used to understand the change in interaction strength and structure by variation of temperature, solvent polarity and solvent concentration in ionic liquids and their mixtures with molecular solvents. In principle the used combination of methods is suitable for studying intermolecular interaction in pure molecular liquids and their solutions including additive materials such as nanoparticles.Physical Chemistry Chemical Physics 06/2014; 16(40). DOI:10.1039/c4cp01476f · 4.20 Impact Factor
Angewandte Chemie 04/2009; 121(17):3230-3233. DOI:10.1002/ange.200806224
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ABSTRACT: The dielectric response of elastic vibrations of water molecules along H bonds is analyzed in terms of the model of harmonic oscillators. It is demonstrated that this response corresponds to the submillimeter wavelength range and the translational band in the far-IR spectral range. The spectra of permittivity of light and heavy water are analytically calculated in a wide spectral range (0–1000 cm−1) of the orientational-translational relaxation containing the microwave Debye and far-IR librational bands in addition to the aforementioned bands. For the far-IR librational band, the librational motion of rigid dipoles in a relatively narrow and deep hatpotential well is considered. The molecular configurations that allow such librations are schematically presented. A concept that establishes a relation between the dielectric properties of water and the proton hopping from one H-bonded molecule to another is proposed.Journal of Communications Technology and Electronics 06/2008; 53(6):611-621. DOI:10.1134/S1064226908060016 · 0.36 Impact Factor