Atomistic study of dislocation loop emission from a crack tip

We report the first atomistic calculation of the saddle-point configuration and activation energy for the nucleation of a 3D dislocation loop from a stressed crack tip in single crystal Cu. The transition state is found using reaction pathway sampling schemes, the nudged elastic band, and dimer methods. For the (111)[110] crack, loaded typically at 75% of the athermal critical strain energy release rate for spontaneous dislocation nucleation, the calculated activation energy is 1.1 eV, significantly higher than the continuum estimate. Implications concerning homogeneous dislocation nucleation in the presence of a crack-tip stress field are discussed.     

Multiple bifurcation of rotating pipe flow

Viscous rotating Hagen-Poiseuille flow is shown to contain a multiple bifurcation point at which two Hopf bifurcations coalesce. The double Hopf bifurcation has nonsemisimple 1:1 resonance. The full problem in the neighborhood of this point may be reduced to a four-dimensional system of weakly nonlinear amplitude equations using an expansion of the center manifold type. An efficient numerical procedure to find the coupling coefficients in the amplitude equations is employed which avoids the explicit need to compute adjoint eigenfunctions. Numerical investigation of the amplitude equations has lead to the discovery of several new states of the fluid motion in this system, including stable rotating traveling and modulated traveling waves, a Feigenbaum period-doubling cascade, and chaotic trajectories. Homoclinic loops are found leading to dynamics which may be associated with bursting events in the physical problem under consideration.This work was supported by the Air Force Office of Scientific Research under Contracts AFOSR-89-0346, monitored by Dr. L. Sakell, and AFOSR-89-0226, monitored by Dr. J. McMichael. A. Mahalov is supported by an IBM Watson Fellowship.     

Kinetics and thermochemistry of the reaction of acetylene and nitric oxide

The shock tube data of Ogura [5] on the pyrolysis of C₂H₂/NO mixtures (1100–1500 K) is shown to be consistent with a simple mechanism whereby radicals are both initiated and terminated by NO (Scheme I). The scheme accounts for the rate of formation of the main product, vinylacetylene (VA), the lesser products CO and HCN and a very minor product, propionitrile. It is also shown to be consistent with other studies below 900 K and observation at 300 K on the reactions of vinyl radicals with NO. The substantial inhibition of vinyl acetylene formation by 5% NO makes untenable any substantial role of vinylidene in the C₂H₂ pyrolysis above 1000°K. The reaction of NO with acetylene is an efficient source of HCN. It appears to be a general reaction of NO with substituted acetylenes and below 900 K a mechanism is presented to account for the production of acrylonitrile (AN) from the reaction of NO with VA. Thermochemical data are estimated on Δ_fH°₂₉₈ and S°₂₉₈ for some alkyl-NO, vinyl NO, and acetylene NO compounds and radicals and some new and some revised group values are estimated for estimating Δ_fH°₂₉₈ of derivatives of hydroxyl amines, imines, and isoxazolines. © 1994 John Wiley & Sons, Inc.     

Hydrogen bonded complexes—II: The lactam-phenol ratios and structures in lactam-phenol complexes

Lactams form solid, isolable complexes with the three dihydroxybenzenes, 0,0'-biphenol and 2,3-naphthalenediol, in which lactam-phenol ratios of 1:1, 2:1, 3:1 and even 4:3 were observed. These ratios can be determined with precision by a gas chromatography method which is described. IR spectroscopy indicates that the bonds linking lactam and phenol are O-H...O and/or N-H...O hydrogen bonds Structures for the complexes are suggested.     

Liquid-phase oxidation of isobutane—a reanalysis of the data

Data on the liquid-phase oxidation of isobutane at 50 and 100°C have been reexamined, using a modified mechanism to take into account the termination by isobutylperoxy radicals. Algebraic expressions are derived from steady-state methods. Using Arrhenius parameters fitted by transition-state A factors and activation energies derived from observed “best” rate constants, new sets of parameters are derived for the rate constants for propagation by t? BuO₂ + t? BuH → t-BuO₂H + t? Bu^?: where θ = 2.303RT in kcal/mol. This, together with new values for the termination parameters and rates of i-butyl production by k_4B, is shown to give good agreement with the published data. An important reaction: is shown to quench the possible contributions to termination of adventitious radicals such as CH₃O^?₂.     

Analysis of flow dynamics in a new,very low pressure reactor. Application to the reaction: Cl + CH4⇌ HCl + CH3

A new, improved variant of the very low pressure reactor (VLPR) system with interchangeable discharge orifices was used for studying the compatibility of the chemical and physical processes occurring simultaneously. It is shown that the ratio of calculated and effective escape rate constants is a complex function of the reactor cell geometry for reactions of non-spherical symmetry. The test reaction of atomic chlorine with methane proved to be a pure chemical process free of side reactions and was used to calibrate the system. The measured rate constant is k₁ = (0.993 ± 0.013) × 10^-13 cm³/molec-s at 25°C. A new procedure is outlined for measuring the equilibrium constant by changing the concentrations of all three components resulting in a remarkable accuracy of K₁ = 1.406 ± 0.034 for the test reaction at 25°C.     

Regiospecific oxidation of substituted 1-benzyl-3,4-dihydro-isoquinolines using singlet oxygen

Singlet oxygen regiospecifically oxidizes 1-benzyl-3,4-dihydroisoquinoline. Mechanistic studies do not distinguish between a dioxetane or the alternative zwitterionic peroxide intermediate.