Coupling of orientational and translational motions in viscoelastic fluids. Low frequency depolarized (VH) light scattering

Physical processes in supercooled liquids are discussed and it is explained how elasticity might prevail. Consequently a theory is presented for the VH spectrum of low frequency light scattering from an isotropic solid-like visco-elastic medium containing molecules with orientational degrees of freedom. In contrast with the low frequency VH spectrum from non-elastic fluids the present one contains under specified conditions two central lorentzians instead of only one in addition to two pairs of shifted lorentzians and dispersion curves. The information that can be extracted from comparing this theory to experimental data contains a constant which designates the degree of elasticity and also orientational relaxation times, shear viscosities and coupling constants between the translational and the orientational motions.     

Ion mobility and conductivity in the Li(NH<Subscript>3</Subscript>CH<Subscript>2</Subscript>COO)(NO<Subscript>3</Subscript>) compound

(⁷Li, ¹H) NMR and impedance spectroscopy methods are used to study the ion mobility and conductivity in a complex of the composition Li(NH₃CH₂COO)(NO₃) (I), which has a layered crystal structure. The character of ion motions in lithium and proton sublattices with temperature variation is considered; the types of motions and temperature ranges in which they occur are determined. It is found that above 350 K the dominant process in the lithium sublattice of the compound is Li⁺ ion diffusion. Possible migration paths of lithium ions in the lattice of the compound are analyzed. The specific conductivity of the compound is found to be 2.4×10^–6 S/cm at 393 K.     

Nuclear spin conversion to probe the methyl rotation effect on hydrogen-bond and vibrational dynamics

A noteworthy example of a molecule with coupled large-amplitude motions is provided by acetylacetone (methyl group torsions and intramolecular hydrogen bonds). The molecule was trapped in solid parahydrogen to investigate the complex proton tunneling processes. Nuclear spin conversion in methyl groups is observed and, combined with IR spectra, documents the coupling between high frequency modes and large amplitude motions.     

The magnetic field and temperature dependences of proton spin-lattice relaxation in proteins

The nuclear magnetic relaxation dispersion profiles of lyophilized globular proteins were measured in the frequency range of 10 kHz-30 MHz at temperatures from 156 to 302 K. The existent theory of proton relaxation in immobilized protein systems was critically tested and expended to include contributions of rapid motions of protein side-chain groups. The new theory takes into account the strong coupling between the side-chain protons and the protein backbone, when correlation function cannot be written as a product of the contributions. The measurements showed that while the relaxation rate constant of the protein backbone protons is a linear function of the absolute temperature the side-chain groups exhibit an exponential temperature dependence corresponding to an activated process. Measurements carried out on simple homopolypeptides, polyglycine and polyalanine, provide strong support of the proposed new theory.     

Calculation of the electrical conductivity of some organic semiconductors in strong electric fields

The conductivity is calculated in a semiclassical approximation, which corresponds to the limiting case of strong coupling between the electrons and phonons. The (E) dependence has a minimum and a maximum. The equations directly give the activation energy for the mobility from the voltage-current curves: they can also be used to estimate the constant for the electron-phonon interaction.     

Internal coordinate density of state from molecular dynamics simulation

The vibrational density of states (DoS), calculated from the Fourier transform of the velocity autocorrelation function, provides profound information regarding the structure and dynamic behavior of a system. However, it is often difficult to identify the exact vibrational mode associated with a specific frequency if the DoS is determined based on velocities in Cartesian coordinates. Here, the DoS is determined based on velocities in internal coordinates, calculated from Cartesian atomic velocities using a generalized Wilson's B ‐matrix. The DoS in internal coordinates allows for the correct detection of free dihedral rotations that may be mistaken as hindered rotation in Cartesian DoS. Furthermore, the pronounced enhancement of low frequency modes in Cartesian DoS for macromolecules should be attributed to the coupling of dihedral and angle motions. The internal DoS, thus deconvolutes the internal motions and provides fruitful insights to the dynamic behaviors of a system. © 2015 Wiley Periodicals, Inc.     

Theoretical infrared line shapes of H‐bonds within the strong anharmonic coupling theory and Fermi resonances effects

In this article, we extend a previous work toward presenting a theoretical study of the effects of Fermi resonances and the fundamental anharmonic coupling parameter α between the high‐frequency mode and the H‐bond bridge. The model incorporates (i) both intrinsic anharmonicities of the fast mode (double well potential) and the H‐bond Bridge (Morse potential), (ii) strong anharmonic coupling theory, (iii) Fermi resonances by the aid of an anharmonic coupling between the fast mode and one or several harmonic bending modes, (iv) quadratic modulation of both the angular frequency and the equilibrium position of the X? …Y stretching mode on the intermonomer ? H… motions, and (v) the quantum direct (fast and bending modes) and indirect dampings (slow mode). The IR spectral density is obtained by Fourier transform of the autocorrelation function of the transition dipole moment operator of the X? H bond. The numerical calculation shows that Fermi resonances generate very complicated profiles with multisubstructure and also provide a direct evidence of Fermi resonances which were predicted to be a major feature of H‐bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Synthesis,characterization, and electrochemical and electrical properties of novel mononuclear and binuclear ball-type Zn(II) and Co(II) phthalocyanines substituted with 1a,8b-dihydronaphtho[b]naphthofuro-[3,2-d]furan-7,10-diyl

New mononuclear phthalocyanines [Zn(II) 4 and Co(II) 6] and ball-type bisphthalocyanines [Zn(II) 5 and Co(II) 7] have been synthesized from the corresponding compound 3, which can be obtained from the reaction of 4-nitrophthalonitrile 1 with 1a,8b-dihydronaphtho[b]naphthofuro[3,2-d]furan-7,10-diol 2. The novel compounds have been characterized by elemental analysis, UV/Vis, IR, ¹H NMR and MASS spectroscopy. The electrochemical measurements show the formation of various mixed-valence oxidized and reduced species, due to intramolecular interactions between the two phthalocyanine units in the ball-type binuclear metallophthalocyanines. Detailed studies of the effect of temperature on the electronic properties of the films were investigated by dc conductivity and impedance spectroscopy techniques at temperatures between 290 K and 460 K. Thermally activated conductivity dependence on temperature was observed from the dc measurements. The ac results give a power law behavior in which the frequency exponent decreases with temperature. It was observed that the impedance spectra consist of a curved line at low temperature. These curved lines transform into a full semicircle with increasing temperature.     

Dielectric relaxations of chitosan: The effect of water on the α‐relaxation and the glass transition temperature

In this work thermal relaxations of chitosan are reported by using a novel methodology that includes subtraction of the dc conductivity contribution, the exclusion of contact and interfacial polarization effects, and obtaining a condition of minimum moisture content. When all these aspects are taken into account, two relaxations are clearly revealed in the low frequency side of the impedance data. We focus on the molecular motions in neutralized and non‐neutralized chitosan analyzed by dielectric spectroscopy in the temperature range from 25 to 250 °C. Low and high frequency relaxations were fitted with the Havriliak and Negami model in the 10^?1 to 108 Hz frequency range. For the first time, the low frequency α‐relaxation associated with the glass‐rubber transition has been detected by this technique in both chitosan forms for moisture contents in the range 0.05 to 3 wt % (ca. 18–62 °C). A strong plasticizing effect of water on this primary α‐relaxation is observed by dielectric spectroscopy and is supported by dynamic mechanical analysis measurements. In the absence of water (<0.05 wt %) the α‐relaxation is obscured in the 20–70 °C temperature range by a superposition of two low frequency relaxation processes. The activation energy for the σ‐relaxation is about 80.0–89.0 kJ/mol and for β‐relaxation is about 46.0–48.5 kJ/mol and those values are in agreement with that previously reported by other authors. The non‐neutralized chitosan possess higher ion mobility than the neutralized one as determined by the frequency location of the σ‐relaxation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2259–2271, 2009     

Solute-solvent intermolecular vibronic coupling as manifested by the molecular near-field effect in resonance hyper-Raman scattering

Vibronic coupling within the excited electronic manifold of the solute all-trans-β-carotene through the vibrational motions of the solvent cyclohexane is shown to manifest as the "molecular near-field effect," in which the solvent hyper-Raman bands are subject to marked intensity enhancements under the presence of all-trans-β-carotene. The resonance hyper-Raman excitation profiles of the enhanced solvent bands exhibit similar peaks to those of the solute bands in the wavenumber region of 21,700-25,000 cm(-1) (10,850-12,500 cm(-1) in the hyper-Raman exciting wavenumber), where the solute all-trans-β-carotene shows a strong absorption assigned to the 1A(g) → 1B(u) transition. This fact indicates that the solvent hyper-Raman bands gain their intensities through resonances with the electronic states of the solute. The observed excitation profiles are quantitatively analyzed and are successfully accounted for by an extended vibronic theory of resonance hyper-Raman scattering that incorporates the vibronic coupling within the excited electronic manifold of all-trans-β-carotene through the vibrational motions of cyclohexane. It is shown that the major resonance arises from the B-term (vibronic) coupling between the first excited vibrational level (v = 1) of the 1B(u) state and the ground vibrational level (v = 0) of a nearby A(g) state through ungerade vibrational modes of both the solute and the solvent molecules. The inversion symmetry of the solute all-trans-β-carotene is preserved, suggesting the weak perturbative nature of the solute-solvent interaction in the molecular near-field effect. The present study introduces a new concept, "intermolecular vibronic coupling," which may provide an experimentally accessible∕theoretically tractable model for understanding weak solute-solvent interactions in liquid.     

Solvent effects on environmentally coupled hydrogen tunnelling during catalysis by dihydrofolate reductase from Thermotoga maritima

Protein motions may be perturbed by altering the properties of the reaction medium. Here we show that dielectric constant, but not viscosity, affects the rate of the hydride-transfer reaction catalysed by dihydrofolate reductase from Thermotoga maritima (TmDHFR), in which quantum-mechanical tunnelling has previously been shown to be driven by protein motions. Neither dielectric constant nor viscosity directly alters the kinetic isotope effect of the reaction or the mechanism of coupling of protein motions to tunnelling. Glycerol and sucrose cause a significant increase in the rate of hydride transfer, but lead to a reduction in the magnitude of the kinetic isotope effect as well as an extension of the temperature range over which "passive" protein dynamics (rather than "active" gating motions) dominate the reaction. Our results are in agreement with the proposal that non-equilibrium dynamical processes (promoting motions) drive the hydride-transfer reaction in TmDHFR.     

Deposition of CTAB-terminated nanorods on bacteria to form highly conducting hybrid systems

Gold nanorods and nanospheres capped with positively charged CTAB are deposited on the surface of gram-positive bacterium having negatively charged teichoic acid brushes. The deposition rate is more than an order of magnitude faster than that for peptide and nucleic acid capped nanoparticles. For the nanorods, never been reported before, the strong electrostatic attraction causes the rods to bend conformally over the curved bacterium surface in random orientations. This leads to formation of an efficient percolating cluster (i.e., low contact resistance and high radius of gyration) with 104-fold higher current at 3-fold lower coverage compared to a monolayer from nanospheres. The high monolayer conductivity at only approximately 10% coverage for nanorods has potential for fabricating a novel class of bioelectronic devices by coupling electronic "nanocircuitry" at the surface to the biological function of microorganisms.     

Hydration and temperature interdependence of protein picosecond dynamics

We investigate the nature of the solvent motions giving rise to the rapid temperature dependence of protein picoseconds motions at 220 K, often referred to as the protein dynamical transition. The interdependence of picoseconds dynamics on hydration and temperature is examined using terahertz time domain spectroscopy to measure the complex permittivity in the 0.2-2.0 THz range for myoglobin. Both the real and imaginary parts of the permittivity over the frequency range measured have a strong temperature dependence at >0.27 h (g water per g protein), however the permittivity change is strongest for frequencies <1 THz. The temperature dependence of the real part of the permittivity is not consistent with the relaxational response of the bound water, and may reflect the low frequency protein structural vibrations slaved to the solvent excitations. The hydration necessary to observe the dynamical transition is found to be frequency dependent, with a critical hydration of 0.19 h for frequencies >1 THz, and 0.27 h for frequencies <1 THz. The data are consistent with the dynamical transition solvent fluctuations requiring only clusters of ~5 water molecules, whereas the enhancement of lowest frequency motions requires a fully spanning water network.     

Hydrogen bonding: How much anharmonicity?

In moderately strong hydrogen bonds, hydrogen bond formation increases the anharmonicity constant of the high frequency stretching vibration, significantly but not dramatically. This increase tends to increase with the strength of the hydrogen bond. The main cause of the fine structure and breadth of this band is, however, coupling with both the low frequency stretching and bending vibrations of the bridge, despite the smallness of the coupling constants. Second–order perturbation theory is sufficient to interpret the observed frequencies in the case of moderately strong hydrogen bonds. HCNHF, O–H:O, O–H:N, and N–H:N hydrogen bonds are considered.     

Synthesis of 5-aryl-5′-formyl-2,2′-bithiophenes as new precursors for nonlinear optical (NLO) materials

A series of formyl-substituted 5-aryl-2,2′-bithiophenes 5 were synthesized using two different methods: Vilsmeier-Haack-Arnold reaction (VHA) or through Suzuki coupling. The synthesis of compounds 5 through the Vilsmeier-Haack-Arnold reaction, starting from inexpensive and easily available precursors such as acetophenones, gave the title compounds in low yields after four reaction steps. On the other hand Suzuki coupling of functionalized arylboronic acids 7 and the 5-bromo-5′-formyl-2,2′-bithiophene 6 gave compounds 5 in good yields in only one step.     

Simple model for ac ionic conduction in solids

We present a model for the ac conduction in ionically conducting solids that takes into account, in a simple way, the interaction between carriers. The Coulomb force forms an "ionic atmosphere" that exerts a restoring force on a central ion, whose motion corresponds to an overdamped oscillator. We consider the effect of the relaxation of the ionic atmosphere by introducing an additional equation for the displacement of the potential toward the particle position. The general behavior of the ac conductivity can be understood in terms of two types of motions: motion of the bound ion at high frequencies determined by microscopic friction, and a much slower motion coupled to the surrounding carriers relaxation at low frequencies.     

Explicitly correlated treatment of the Ar-NO+ cation

We present an application of the recently developed explicitly correlated coupled cluster method to the generation of the three-dimensional potential energy surface (PES) of the Ar-NO(+) cationic complex. A good overall agreement is found with the standard coupled clusters techniques employing correlation consistent atomic basis sets (aug-cc-pVnZ, n= D, T, Q) of Wright et al. This PES is then used in quantum close-coupling scattering and variational calculations to treat the nuclear motions. The bound states energies of the Ar-NO(+) complex obtained by both approaches are in good agreement with the available experimental results. The analysis of the vibrational wavefunctions shows strong anharmonic resonances between the low frequency modes (intermonomer bending and stretching modes) and the wavefunctions exhibit large amplitude motions.     

Lateral separation of colloids or cells by dielectrophoresis augmented by AC electroosmosis

Colloidal particles and biological cells are patterned and separated laterally adjacent to a micropatterned electrode array by applying AC electric fields that are principally oriented normally to the electrode array. This is demonstrated for yeast cells, red blood cells, and colloidal polystyrene particles of different sizes and zeta-potentials. The separation mechanism is observed experimentally to depend on the applied field frequency and voltage. At high frequencies, particles position themselves in a manner that is consistent with dielectrophoresis, while at low frequencies, the positioning is explained in terms of a strong coupling between gravity, the vertical component of the dielectrophoretic force, and the Stokes drag on particles induced by AC electroosmotic flow. Compared to high frequency dielectrophoretic separations, the low frequency separations are faster and require lower applied voltages. Furthermore, the AC electroosmosis coupling with dielectrophoresis may enable cell separations that are not feasible based on dielectrophoresis alone.     

Numerical Simulations of the Effect of Microstructure on AC Conductivity of MWCNT/Polymer Nanocomposites

A continuum model is developed to investigate the microstructure‐dependent AC properties of MWCNT/polymer nanocomposites. The AC conductivity of the composite is increased by a higher curl ratio of MWCNTs. At a critical frequency ω₀, the AC conductivity switches to a frequency‐dependent region. For high MWCNT content, the curl curliness of MWCNTs has only a weak influence on the AC conductivity. For medium MWCNT content, the AC conductivity became frequency‐dependent for low frequencies with decreasing curl ratio of MWCNTs, which cannot be explained by correlation length theory. An interpretation based on the linear circuit theory is given. With increasing size of MWCNT clusters, the critical frequency ω₀ increases. It is also affected significantly by the crystallinity the polymer.