Search Results - (Author, Cooperation:R. Viswanathan)

2013-10-05

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Adenosine Triphosphatases/chemistry/metabolism ; Chromatin/chemistry/*metabolism ; Chromatin Immunoprecipitation/*methods ; Cross-Linking Reagents/chemistry ; DNA-Binding Proteins/chemistry/metabolism ; Formaldehyde/chemistry ; Gene Dosage ; *Gene Expression Regulation ; Kinetics ; Promoter Regions, Genetic ; Saccharomyces cerevisiae Proteins/chemistry/metabolism ; TATA-Binding Protein Associated Factors/chemistry/metabolism ; TATA-Box Binding Protein/chemistry/*metabolism ; Transcription Factors/chemistry/metabolism

2011-07-08

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Binding Sites ; Crystallization ; Crystallography, X-Ray ; DNA/chemistry/genetics/metabolism/ultrastructure ; Encephalitozoon cuniculi/*chemistry ; Fungal Proteins/*chemistry/*metabolism/ultrastructure ; Microscopy, Electron ; Models, Biological ; Models, Molecular ; Promoter Regions, Genetic/genetics ; Protein Conformation ; Protein Structure, Tertiary ; Structure-Activity Relationship ; Substrate Specificity ; TATA-Box Binding Protein/*chemistry/*metabolism/ultrastructure ; Transcription Factor TFIIB/chemistry/metabolism

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Functional sensitivity analysis is used to study the effect of potential structure upon the elastic scattering of He atoms from a one-dimensional surface. The calculations are implemented by computing the total scattering wave functions from a wave packet calculation by a Møller wave operator method. The functional sensitivities of the various diffraction probabilities for several angles of incidence and surface corrugation are studied. The method is extended to examine the role of potential structure for a surface with adsorbed impurities. It was observed that the various diffraction processes draw from local regions of the potential in very different ways. At high angles of incidence for back scattering and particularly for strong surface corrugation, the large protruding portions of the surface cast a "shadow'' of lower dynamical sensitivity. Results of this type should ultimately be insightful for the inversion of experimental data to obtain the interaction potential.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

In situ infrared vibrational spectra of azide ions specifically adsorbed on polycrystalline silver electrodes immersed in aqueous electrolytes of composition 1 to 30 mM azide and 0.1 M perchlorate are reported for the potential range 0.25 to 0 V vs Ag/AgCl (3 M KCl). Adsorbed azide was not detected spectroscopically between 0 and −0.9 V, a range known from differential capacitance measurements to allow adsorption onto the silver electrodes. These experimental observations have been interpreted with the help of ab initio calculations of the vibrational frequencies of [AgN3]0 and [AgN3]− for two configurations with C∞v and C2v symmetries representing simple models for azide adsorbed with its configuration axis perpendicular and parallel to the electrode surface, respectively. Besides the charge states specified, calculations are also reported for static applied fields along the symmetry axis. Taken together, the experiments and calculations support a model in which azide adsorbs oriented perpendicular to the surface at the higher surface concentrations induced by potentials more positive than OV, and is adsorbed with long axis parallel to the surface at negative potentials.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

An electronic controller for an electromechanical shutter is described. The synchronization pulse output from a thyratron-triggered pulsed laser is used to keep the shutter open and to unblock the optical path followed by the laser beam for specific selected laser pulses. Effectively, ultralow repetition rates down to 0.001 Hz are obtained when the pulsed laser is operated at normal repetition rates between 0.5 and 2 Hz.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Highly c-axis oriented good-quality (Tc of 88 K and Jc of 105 A/cm2 at 20 K) thin films of Y1Ba2Cu3O7−δ have been deposited on stainless-steel substrates by an in situ all-laser process involving use of laser-deposited Y-ZrO2-Ag composite film as a barrier layer. These results are compared with those obtained for the case of the use of a bilayer configuration of Ag and Y-ZrO2 to emphasize the importance of employing a composite film as a barrier layer.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The density expansion method is used to determine the pair distribution function of a nonuniform fluid in one dimension in which a two-phase interface is induced by an external field. When the fluid is modeled by the lattice gas with nearest-neighbor forces and the external field by the step function that changes sign at the middle of the chain, terms of arbitrary order in the density expansion can be determined exactly. The kernels of the first- and second-order terms are evaluated in closed form and the pair distribution function is evaluated at each order. These results are tested against the exact expression of the pair distribution function of the same model interface, which was recently obtained by the transfer matrix method. It is found, especially for strong intermolecular forces, that the first- and second-order approximations are not only unable to improve the severe disagreement between the zeroth-order approximation and the exact result, but further aggravate this disagreement, suggesting that the density expansion of the pair distribution function is at best asymptotically convergent.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Theoretical computations were carried out to determine the structure and molecular parameters of the BNSi molecule. The most stable isomer is found to have a BNSi linear geometry. Thermal functions as derived from the theoretical computed molecular parameters were used in the evaluation of the thermodynamic properties of BNSi from high-temperature Knudsen effusion mass spectrometric equilibrium data. From the reactions analyzed by the second-law and third-law methods, the enthalpy of formation, ΔfH0o, and of atomization, ΔaH0o, in kJ mol−1, for BNSi, were obtained as 398±16 and 1078±17, respectively. © 1999 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Condensed phase mixtures (Ge+Si+Si3N4) and (Si+Si3N4) were evaporated from boron nitride Knudsen cells and the partial pressures of Si3N(g),Si2NSi2(g), and Si(g) measured with a mass spectrometer at temperatures between 1770 and 2000 K. The thermal functions for Si3N(g) were calculated from theoretical molecular constants, and the enthalpy change for the dissociation reaction: Si3N(g)=Si2N(g)+Si(g) determined. With the revised atomization enthalpy of Si2N(g), ΔatomHmo=1011±12 kJ mol−1 at T=0 K and 1020±12 kJ mol−1 at T=298.15 K, the atomization enthalpy of Si3N(g) was derived as 1298±19 kJ mol−1 at T=0 and 1312±19 kJ mol−1 at T=298.15 K. These values in combination with the enthalpies of formation of Si(g) and N(g) yielded the enthalpies of formation ΔfHmo at T=298.15 K: 352±15 kJ mol−1 for Si2N(g), and 511±22 kJ mol−1 for Si3N(g). Experimental and theoretical bond dissociation energies have been compared and discussed, indicating a very strong bonding of nitrogen to Si3. © 1997 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The influence of 60-keV N+2 ion implantation on the aqueous degradation of the Y1Ba2Cu3O7−x high-Tc superconductor is studied by the small-angle x-ray diffraction technique. Structural depth profiling is performed by registering the x-ray data at several small angles of incidence between 0.3° and 10.0° within Seeman–Bohlin geometry. Examination of samples implanted at a dose value of 3×1017 ions/cm2 and subjected to different aqueous treatments reveals that implantation arrests the degradation process and imparts structural stability to the superconductor. Resistivity temperature data are also provided for specific cases of interest.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We calculate the exact expression of the pair distribution function in the interface of the one-dimensional lattice gas, in which the separation of phases is achieved by imposing an external field which changes sign at the middle of the lattice. The exact distribution functions of arbitrary order can then be obtained from an earlier theorem of Percus. The pair distribution function is shown to decay exponentially as a function of interparticle distance, with a decay length equal to the correlation length of the spontaneous density fluctuations in either bulk liquid or gas phases. This exact result enables us to test approximate theories of the pair distribution function of nonuniform fluids. The exact expression of the direct correlation function is obtained from recent studies of Percus, and of Robert and Widom, and it also is used to test approximate theories of the direct correlation function of nonuniform fluids. Extremely severe disagreement is found between exact results and all commonly used approximations based on local thermodynamics à la van der Waals, while excellent, nearly perfect agreement is found for the dynamic capillary wave-like approximation of Kalos, Percus, and Rao.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The semiempirical valence-bond surface formulated by Viswanathan et al. [J. Phys. Chem. 89, 1428 (1985)] for the unimolecular dissociation of SiH2 has been fitted to an analytical function of the type suggested by Murrell and co-workers [J. Phys. Chem. 88, 4887 (1984)]. The fitted surface accurately represents most of the experimental and CI results. The dynamics of the unimolecular dissociation of SiH2 to form Si and H2 have been investigated by classical trajectory methods on this fitted surface. The effect of describing the initial state of the molecule using normal and local mode approximations has been studied. In spite of the presence of the heavier atom, no bond or mode specificity is observed. The product energy distribution is found to be statistical. Using the RRK model, the high-pressure limiting rate coefficient is found to be k(T,∞)=3.38×1012 exp[−61.6 kcal mol−1/RT] s−1, which is less than the dissociation rate for SiH4. This has been attributed to the higher activation energy for SiH2 and to a statistical factor.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The bridge function of the one-dimensional lattice gas with interactions between nearest-neighbor and next-nearest-neighbor particles is determined exactly. This result is compared with the recently proposed hypothesis of universality of the bridge function of simple classical fluids. It is found that in contrast to three-dimensional systems previously studied, the bridge function of the one-dimensional lattice gas is a strongly nonmonotonic function of the interparticle distance, being negative at very short distances and vanishing positively at infinity. Consequently, the one-parameter family of positive and monotonic bridge functions of the hard-sphere (rod) system, recently suggested to be universal, cannot be used for the one-dimensional lattice gas. Dedicated to Thomas Leland.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Molecular beam electric resonance spectroscopy has been used to determine the radiofrequency and microwave spectrum of H2S⋅Ar, HDS⋅Ar, and D2S⋅Ar. The electric dipole moment and nuclear hyperfine interactions reflect an angular structure in which the H2S C2 axis is nearly perpendicular to the Ar...S axis, and the H2S and argon are nearly coplanar. The effective argon–H2S center-of-mass distance is 3.977 A(ring).

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A previously formulated Monte Carlo transition-state theory approach to unimolecular reactions has been extended to the calculation of microcanonical rate coefficients for specific angular momentum states. The method is applied to a study of two-center dissociation of CH4 on the Duchovic–Hase–Schlegel (DHS) ab initio potential-energy surface. The rotationally averaged microcanonical rate coefficients on the DHS surface are found to be consistently less than those previously calculated on a semiempirical surface due principally to the larger C–H bond dissciation energy on the DHS surface. The rate coefficients are found to be relatively insensitive to the other topographical features of the potential-energy surface. Angular momentum is found to reduce k(E;J) by an amount in excess of the centrifugal effect. The magnitude of the angular momentum effect is found to decrease as the ratio of the rotational energy to the energy in excess of threshold decreases. Overall, the MCTST procedure is found to be computationally efficient for such calculations.

Electronic Resource

0022-3697

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0022-3697

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0022-3697

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0022-3697

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource

0021-9614

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Physics

Electronic Resource