Vibrational phase relaxation of O–H stretch in bulk water: Role of large amplitude angular jumps and negative cross-correlations among the forces on the O–H bond

The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function (Cω(t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of Cω(t) is found to be responsible for the ultrashort T2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of Cω(t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (FO(t)) and hydrogen (FH(t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate.

Subquadratic wavenumber dependence of the structural relaxation of supercooled liquid in the crossover regime

As a liquid is progressively supercooled toward its glass transition temperature, an intriguing weakening of the wavenumber (q) dependence of the structural relaxation time tau(q) in the intermediate-to-large q limit is observed both in experiments and simulation studies. Neither continuous Brownian diffusive dynamics nor discontinuous activated events can alone explain the anomalous wavenumber dependence. Here we use our recently developed theory that unifies the mode coupling theory for continuous dynamics, with the random first order transition theory treatment of activated discontinuous motion as a nucleationlike instanton process to understand the wavenumber dependence of density relaxation. The predicted smooth change in mechanism of relaxation from diffusive to activated, in the crossover regime, is wavevector dependent and appears to be responsible for the observed subquadratic,nalmost linear, q dependence of the relaxation time.

Stress responses in mycobacteria

Mycobacterium tuberculosis is a successful pathogen that overcomes numerous challenges presented by the immune system of the host. This bacterium usually establishes a chronic infection in the host where it may silently persist inside a granuloma until, a failure in host defenses, leads to manifestation of the disease. None of the conventional anti-tuberculosis drugs are able to target these persisting bacilli. Development of drugs against such persisting bacilli is a constant challenge since the physiology of these dormant bacteria is still not understood at the molecular level. Some evidence suggests that the in vivo environment encountered by the persisting bacteria is anoxic and nutritionally starved. Based on these assumptions, anaerobic and starved cultures are used as models to study the molecular basis of dormancy. This review outlines the problem of persistence of M. tuberculosis and the various in vitro models used to study mycobacterial latency. The basis of selecting the nutritional starvation model has been outlined here. Also, the choice of M. smegmatis as a model suitable for studying mycobacterial latency is discussed. Lastly, general issues related to oxidative stress and bacterial responses to it have been elaborated. We have also discussed general control of OxyR-mediated regulation and emphasized the processes which manifest in the absence of functional OxyR in the bacteria. Lastly, a new class of protein called Dps has been reviewed for its important role in protecting DNA under stress.

Stress concentration and load diffusion in rectangular panels with constant stress flanges

The stress concentration that occurs when load is diffused from a constant stress member into thin sheet is an important problem in the design of light weight structures. By using solutions in biharmonic polar-trigonometric series, the stress concentration can be effectively isolated so that highly accurate information necessary for design can be obtained. A method of analysis yielding high accuracy with limited effort is presented for rectangular panels with transverse edges free or supported by inextensional end ribs. Numerical data are given for panels with length twice the width.

A model to similate the strength and deformation of concrete in compression

A simple model is developed to represent the strength and deformational characteristics of concrete when subjected to a rate of strain or rate of stress or creep or relaxation testing under uniaxial compression.

Steady flow of a maxwell fluid in a porous cylindrical annulus with circular cross-section

When a fluid with memory is injected into any flow region some assumptions regarding the initial state of stress have to be made in order to determine the state of stress at any subsequent instant. For a Maxwell fluid, it is assumed that the fluid near the surface of injection is suddenly stressed and responds by starting flow in accordance with the mechanical model chosen. The flow of a Maxwell fluid with a single relaxation time has been determined under the above assumption in the following two cases: (i) annulus between two porous concentric circular cylinders, and (ii) space between two porous and infinitely extending parallel plates. The nature of flow in the present case is similar to that of the Reiner-Rivlin fluids obtained by Narasimhan2).

Elastic stress analysis of jointed half-planes

Using a Fourier-integral approach, the problem of stress analysis in a composite plane consisting of two half-planes of different elastic properties rigidly joined along their boundaries has been solved. The analysis is done for a force acting in one of the half-planes for both cases when the force acts parallel and perpendicular to the interface. As a particular case, the interface stresses are evaluated when the interface is smooth. Some properties of the normal stress at the interface are discussed both for plane stress and plane strain conditions.

Dispersion of the stress-optic coefficient in glasses

The stress-optic coefficient (n3/2)(q11-q12) has been determined for a series of 18 optical glasses of different compositions in the wavelength range 5700-3200 Å. The coefficients are negative for all the glasses except for a high-lead-content glass of density 6·7 and refractive index 1·89. The numerical value of the coefficient decreases as one proceeds to the ultraviolet. This behaviour is just the opposite of what is observed in fused silica. By applying Mueller's theory, the strain polarizability constant and its dispersion have been evaluated.

Dielectric relaxation and electrical conductivity in Bi5NbO10 oxygen ion conductors prepared by a modified sol–gel process

Crystalline Bi5NbO10 nanoparticles have been achieved through a modified sol–gel process using a mixture of ethylenediamine and ethanolamine as a solvent. The Bi5NbO10 nanoparticles were characterized by X-ray diffraction (XRD), differential scanning calorimetry/thermogravimetry (DSC/TG), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and Raman spectroscopy. The results showed that well-dispersed 5–60 nm Bi5NbO10 nanoparticles were prepared through heat-treating the precursor at 650 °C and the high density pellets were obtained at temperatures lower than those commonly employed. The frequency and temperature dependence of the dielectric constant and the electrical conductivity of the Bi5NbO10 solid solutions were investigated in the 0.1 Hz to 1 MHz frequency range. Two distinct relaxation mechanisms were observed in the plots of dielectric loss and the imaginary part of impedance (Z″) versus frequency in the temperature range of 200–350 °C. The dielectric constant and the loss in the low frequency regime were electrode dependent. The ionic conductivity of Bi5NbO10 solid solutions at 700 °C is 2.86 Ω−1 m−1 which is in same order of magnitude for Y2O3-stabilized ZrO2 ceramics at same temperature. These results suggest that Bi5NbO10 is a promising material for an oxygen ion conductor.

Vibrational force constant and electron relaxation in H2 and Li2

The force constants of H2 and Li2 are evaluated employing their extended Hartree-Fock wavefunctions by a polynomial fit of their force curves. It is suggested that, based on incomplete multiconfiguration Hartree-Fock wavefunctions, force constants calculated from the energy derivatives are numerically more accurate than those obtained from the derivatives of the Hellmann-Feynman forces. It is observed that electrons relax during the nuclear vibrations in such a fashion as to facilitate the nuclear motions.