Showing papers in "ChemInform in 1975"

Complex Formation in Reactive and Inelastic Scattering: Statistical Adiabatic Channel Model of Unimolecular Processes III

Abstract: The statistical adiabatic channel model developed previously [25] for unimolecular dissociation reactions is extended to describe bimolecular collisions proceeding via a strongly bound intermediate complex. As examples three cases are considered, in which the intermediate complex corresponds to a known stable molecule: O + O2 O3*, O + NO NO2* and Cl + NO ClNO*. Using known molecular properties and the experimental high pressure recombination rate constants, the specific dissociation rate constants k(E,J), state to state inelastic scattering cross sections σ(v,j v',j') and center of mass translational energy distributions are calculated with the same simple potential model. Rate constants kvv, for vibrational relaxation are computed and found to be in good agreement with experimental results available for the O + O2 vibrational relaxation. The dependence of cross sections and final state distributions as a function of initial state is illustrated in some detail. Even within the statistical model, there is found to be a marked memory of initial conditions. The range of validity of statistical models is discussed. Das in einer vorgehenden Arbeit [25] fur unimolekulare Dissoziationsreaktionen entwickelte statistische Modell adiabatischer Reaktionskanale wird hier erweitert auf bimolekulare Reaktionen, die uber einen gebundenen Zwischenkomplex verlaufen. Drei Beispiele werden behandelt, in denen der Zwischenkomplex einem bekannten stabilen Molekul entspricht: O + O2 O3*, O + NO NO2* und Cl + NO ClNO*. Mit Hilfe bekannter Molekularkonstanten und der Hochdruckrekombinationskonstanten wird ein einfaches Potentialmodell festgelegt, das es dann ohne weitere Parameter ermoglicht, spezifische Zerfallskonstanten der Zwischenkomplexe, k(E,J), inelastische Streuquerschnitte σ(v,j v',j') und damit auch die Translationsenergieverteilungen nach dem Stos zu berechnen. Weiterhin werden Geschwindigkeitskonstanten fur Schwingungsrelaxation (kvv) berechnet, die in guter Ubereinstimmung mit experimentellen Werten fur das O + O2-System stehen. Es werden an Beispielen die Abhangigkeit der Endzustandsverteilungen vom Anfangsquantenzustand der Systeme erlautert. Auch mit den statistischen Annahmen wird ein bemerkenswertes “Gedachtnis” an den Ausgangszustand gefunden. Es werden Kriterien fur den Gultigkeitsbereich statistischer Theorien diskutiert.

Synthesis of olefins. Cross-coupling of alkenyl halides and Grignard reagents catalyzed by iron complexes

Abstract: Grignard reagents are coupled with alkenyl halides such as 1-bromopropene and P-bromostyrene in the presence of catalytic amounts of iron(II1) complexes to afford alkenes. This cross-coupling reaction can be employed as a synthetic route for alkenes, in which primary, secondary as well as tertiary alkyl groups like isopropyl, cyclohexyl, and tert-butyl Grignard reagents are utilized. The reaction is stereospecific since trans-1-bromopropene affords only transbutene-2 with methylmagnesium bromide and iron(II1) pivalate. Furthermore, the rearrangement of branched alkyl groups such as tert-butyl has not been observed with an iron catalyst. Among various iron(II1) complexes examined, tris(dibenzoylm'ethido)iron(III) is the most effective from the standpoint of rates and deactivation. Product and spectral studies suggest that the active catalyst is a labile iron species derived by reduction of iron(II1) in situ by the Grignard component. High rates of cross coupling are limited by deactivation of the catalyst due to an aging process attributed to aggregation of the active iron species. Several mechanistic schemes are considered for cross coupling including (a) oxidative addition of alkenyl halide to a low valent alkyliron species followed by reductive elimination of the cross-coupled product and (b) assistance by reduced iron in the concerted displacement of halide at the alkenyl center by the Grignard reagent.

Mixed valence complexes of ruthenium ammines with 4,4'-bipyridine as bridging ligand

Abstract: Die Reaktion von Aquopentaamminruthenium(II)-hexafluorophosphat mit 4,4′-Bipyridyl im Verhaltnis 2 : 1 unter Argon liefert den Komplex (Ia), der durch Oxidation mit Brom in (Ic) ubergeht, das als Tosylat gefallt wird.

The standard thermodynamic functions for the formation of electrons and holes in ge, si, gaas, and gap

Abstract: The forbidden energy gaps of Ge, Si, , and have been used to obtain the standard Gibbs energy, enthalpy and entropy of formation of electrons and holes for each semiconductor up to the melting points. The forbidden energy gap is the standard Gibbs energy of formation of electrons and holes and the enthalpy and entropy have been obtained from the energy gap as a function of temperature and familiar thermodynamic relationships. Energy gaps as a function of temperature, available in the literature, have been fit to the semiempirical equation of Varshni and used to extrapolate the energy gaps and thereby the three thermodynamic functions to the melting points. It is well known that the energy gaps, i.e., the Gibbs energies, decrease with increasing temperature but it is not well known that the enthalpy of formation increases with temperature and that it is proportional to the slope of the familiar logarithmic plot of the intrinsic carrier concentration over vs. . Examples of the utility of the enthalpy function are given. It is the entropy that leads to the decrease in energy gap with increasing temperature and its magnitude is large near the respective melting points (10–13 cals/deg, i.e.,) arising from the interactions of electrons and holes with the lattice. The intrinsic carrier concentrations were calculated from the forbidden energy gaps and the average effective masses which were estimated for the higher temperatures.

Reactions of Atomic Hydrogen with Ketene and Acetaldehyde

Abstract: The reaction of hydrogen atoms with ketene yields predominantly CH3 radicals and CO. The associated rate coefficient can be expressed by k3 = (6.0 ± 2.2) · 10−12 exp [-(2340 ± 200) cal/mol · RT] cm3/molecule · sec. Acetyl radicals resulting from the reaction of hydrogen atoms with acetaldehyde react with H atoms to produce ketene and molecular hydrogen with 63 percent yield: a second reaction channel produces CH3 and HCO radicals with 37 percent yield. Die Reaktion von Wasserstoffatomen mit Keten ergibt uberwiegend CH3-Radikale und CO. Der zugehorige Geschwindigkeitskoeffizient last sich darstellen durch k3 = (6,0 ± 2,2) · 10−12 exp [-(2340 ± 200) cal/mol · RT] cm3/Molekul · sec. Acetyl-Radikale, die bei der Reaktion von Wasserstoffatomen mit Acetyldehyd gebildet werden, reagieren mit H-Atomen zu 63% unter Bildung von Keten und molekularem Wasserstoff; ein zweiter Reaktionsweg bildet CH3- und HCO-Radikale mit 37% Ausbeute.

Prediction of fracture energy and flaw size in glasses from measurements of mirror size

Abstract: Fracture strengths (δ), fracture-initiating flaw sizes, and mirror radii (r), as outlined by either the mist or the hackle boundary, were measured for silicate and nonsilicate glasses (e.g. As2S3 and glassy carbon). For all glasses, δr1/2= constant. The average ratios of inner and outer mirror radii to flaw size were ∼10:1 and ∼13:1, respectively, for most of the glasses. Critical fracture energies calculated from either flaw or mirror size agreed very well with those obtained by double-cantilever-beam measurements.