Study of the Reaction Rate,in the Critical Regime,of a Solute Embedded in a Lennard-Jones Crystal

In a recent study, we showed that the Grote-Hynes theory for the chemical reaction rate exhibits critical phenomena near a critical coupling strength and large solvent response times. Therefore it is possible to write (he rate in a scaling form with a universal scaling function that depends only on the short-time behavior of the memory friction kernel. This study involved a friction kernel involving only one relaxation time. In this paper, we apply the same techniques to a crystal friction involving multiple timescales. We show that the rate shows critical behavior and obtain the universal scaling function. We find the asymptotic behavior of the rate in different regimes of interest. In this way, we uncover the rich mathematical structure of the reaction rate associated with the crystal friction with multiple timescales.     

Optically active organoaluminum based inclusion compounds. Synthesis and characterization of (S)-(−)-α-[(C6H5)CH(CH3)N(CH3)3][Al2R6I] (R=CH3, C2H5)

The optically active quaternary ammonium salt (S)-(?)-α-[(C₆H₅)CH(CH₃)N(CH₃)₃I] reacts with AlR₃ to afford optically active organoaluminum based inclusion compounds, liquid clathrates, of the formula (S)-(?)-α-[(C₆H₅)CH(CH₃)N(CH₃)₃][Al₂R₆I] (R=CH₃, C₂H₅). Specific rotation ([α] ₂₅ ^D ) for the Al(CH₃)₃ compound was determined to be ?13.19° while that for the Al(C₂H₅)₃ analog was determined to be ?14.30°. There are 13.8 toluene molecules per anionic moiety for the trimethylaluminum based liquid clathrate while there are 15.0 toluene molecules per anion for the corresponding triethylaluminum inclusion compound.     

An automated fuel element scanning system

This paper describes an automated fuel element scanning system, based upon gamma-ray spectroscopy, that has been developed at the Oak Ridge Research Reactor. The scanning system is located in the reactor pool and allows fuel elements to be scanned nondestructively at various intervals during their core life. Fuel elements are located 430 cm beneath the pool. Gamma-ray measurements (aided by a collimating assembly) are made using a GeLi detector positioned above the pool water. Measurements of¹³⁷Cs count rates from relatively cold elements indicate that the counting data obtained using this apparatus is reproducible within 5%. Power distribution in the reactor's core (element by element) has been derived from¹⁴⁰La counting data. The method of determining power is discussed briefly and some example results are presented.     

The crystal structures of Ti2O3, a semiconductor,and (Ti0.900V0.100)2O3, a semimetal

The crystal structures of the semiconductor Ti₂O₃ and the semimetal (Ti_0.900V_0.100)₂O₃ were determined from X-ray diffraction data collected from single crystals. The compounds are isostructural with Al₂O₃ of rhombohedral unit cell dimensions of a = 5.4325(8) Å and α = 56.75(1)° for Ti₂O₃, and a = 5.4692(8) Å and α = 55.63(1)° for the doped system. The effect of substitution of V⁺³ is to increase the metal-metal distance across the shared octahedral face from 2.579 Å in Ti₂O₃ to 2.658 Å in (Ti_0.900V_0.100)₂O₃, while decreasing the metal-metal distance across the shared octahedral edge from 2.997 to 2.968 Å. The metal-oxygen distances exhibit only small changes. These structural changes are consistent with the band theory proposed by Van Zandt, Honig, and Goodenough (9) to explain changes in electrical and other properties with increasing vanadium content in (Ti_1?xV_x)₂O₃.     

Electron Domains and the VSEPR Model of Molecular Geometry

The valence shell electron pair repulsion (VSEPR) model—also known as the Gillespie–Nyholm rules—has for many years provided a useful basis for understanding and rationalizing molecular geometry, and because of its simplicity it has gained widespread acceptance as a pedagogical tool. In its original formulation the model was based on the concept that the valence shell electron pairs behave as if they repel each other and thus keep as far apart as possible. But in recent years more emphasis has been placed on the space occupied by a valence shell electron pair, called the domain of the electron pair, and on the relative sizes and shapes of these domains. This reformulated version of the model is simpler to apply, and it shows more clearly that the Pauli principle provides the physical basis of the model. Moreover, Bader and his co-workers' analysis of the electron density distribution of many covalent molecules have shown that the local concentrations of electron density (charge concentrations) in the valence shells of the atoms in a molecule have the same relative locations and sizes as have been assumed for the electron pair domains in the VSEPR model, thus providing further support for the model. This increased understanding of the model has inspired efforts to examine the electron density distribution in molecules that have long been regarded as exceptions to the VSEPR model to try to understand these exceptions better. This work has shown that it is often important to consider not only the relative locations and sizes, but also the shapes, of both bonding and lone pair domains in accounting for the details of molecular geometry. It has also been shown that a basic assumption of the VSEPR model, namely that the core of an atom underlying its valence shell is spherical and has no influence on the geometry of a molecule, is normally valid for the nonmetals but often not valid for the metals, including the transition metals. The cores of polarizable metal atoms may be nonspherical because they include nonbonding electrons or because they are distorted by the ligands, and these nonspherical cores may have an important influence on the geometry of a molecule.     

Buffer Standards for the Physiological pH of Zwitterionic Compounds,MOBS and TABS from 5 to 55°C

The values of the second dissociation constant, pK₂, and related thermodynamic quantities of 4-(N-morpholino)butanesulfonic acid (MOBS) and N-tris(hydroxymethyl)-4-aminobutanesulfonic acid (TABS) have already been reported over the temperature range 5–55°C including 37{°}C. This paper reports the pH values of twelve equimolal buffer solutions at designated pH (s) with the following compositions: (a) mixtures of MOBS (0.05 mol-kg^–1) + NaMOBS (0.05 mol-kg^–1); (b) MOBS (0.08 mol-kg^–1) + NaMOBS (0.08 mol-kg^–1); (c) MOBS (0.08 mol-kg^–1) + NaMOBS (0.08 mol-kg^–1) + NaCl (0.08 mol-kg^–1); (d) TABS (0.05 mol-kg^–1) + NaTABS (0.05 mol-kg^–1); and (e) TABS (0.08 mol-kg^–1) + NaTABS (0.08 mol-kg^–1); and (f) TABS (0.08 mol-kg^–1) + NaTABS (0.08 mol-kg^–1) + NaCl (0.08 mol-kg^–1). Two buffer solutions have ionic strengths I= 0.05 mol-kg^–1, another two have I=0.08 mol-kg^–1, and the remaining two buffer solutions have I= 0.16 mol-kg^–1, which is close to that of the clinical fluids (blood serum). These buffers have been recommended as a useful pH standard for the measurements of physiological solutions. Conventional pH values of all six buffer solutions from 5–55°C, as well as those obtained from the liquid junction potential correction at 25 and 37{°}C have been calculated. The flowing-junction calomel cell has been utilized to measure E_j, the liquid junction potential.     

Electrically ‘wired’ tyrosinase enzyme inhibition electrode for the detection of respiratory poisons

The use of the solution redox species, [Os(bpy)₂Cl₂]^+/0, [Os(bpy)₂(MeIm)Cl]^2+/+ and [Fe(CN)₆]^4−/3−, where bpy is 2,2-bipyridine and MeIm is N-methylimidazole, as electron mediators in the enzymatic reduction of oxygen by tyrosinase is investigated. Co-immobilization of both enzyme and an osmium redox mediator in a hydrogel on glassy carbon electrodes results in a biosensor for the ‘reagentless’ addressing of enzyme activity, consuming only oxygen present in solution. Immobilized enzyme inhibition biosensors can thus be constructed for the detection of tyrosinase inhibitors, such as sodium azide, using this approach. The enzyme inhibition biosensor can detect levels of azide as low as 5 × 10⁻⁶ mol dm⁻³ in solution and may be useful in environmental monitoring applications and as an early warning poison sensor.     

Enhanced and normal Raman scattering from pyridine adsorbed on rough and smooth silver electrodes

A multiplex spectrograph has been used to record potential difference and modulation Raman spectra of pyridine adsorbed on silver electrodes in an electrochemical cell. Spectra have been obtained from rough silver surfaces which give SERS and from surfaces where SERS has been diminished by prolonged cathodic polarisation (DSERS). Raman scattering from pyridine at smooth silver surfaces in potassium perchlorate and fluoride solutions has been distinguished from solution scatter by a potential modulation technique. The results show that the enhanced scattering caused by silver atom or cluster sites is respresentative of the surface as a whole as similar Raman spectra are obtained on smooth surfaces at a count rate as low as ?1.4 photons s^?1 (incident laser power 500 mW).Correlation of simultaneous differential capacitance data and “snapshot” SER spectra indicate that pyridine molecules in aqueous chloride ion solutions adsorb on silver in a flat π-bonded configuration at potentials markedly positive to the point of zero charge and exhibit specific reorientation at ?0.3 V and ?0.45 V (vs. SCE) to become N-bonded, perpendicular to the surface. Results also show that the adsorption behaviour of pyridine in chloride and fluoride ion solutions is largely similar.     

A DFT study of the thermal, orbital symmetry forbidden, cyclophanediene to dihydropyrene electrocyclic reaction. Predictions to improve the dimethyldihydropyrene photoswitches

The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPD<==>DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TSs) for these reactions could not be modeled using restricted DFT (RB3LYP) but were located using unrestricted DFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, S2 not = 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/DDPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.     

Construction of demountable hollow-cathode lamps for stimulating emission of organic compounds

A demountable hollow cathode has been constructed which provides emission spectra of organic compounds. The construction and operation of the hollow cathode is described. The emission spectra of organic and inorganic phosphates are illustrated and discussed.