The mechanism and rate parameters for the pyrolysis of n-hexane in the range 723–823 K

The pyrolysis of n-hexane has been investigated in the ranges 723–823 K and 10–100 Torr at up to 3% decomposition. The reaction is homogeneous and free from the self-inhibition by olefin products observed for several other alkanes. The products of the reaction are hydrogen, methane, ethane, ethene, propene, but-1-ene, and pent-1-ene, with smaller amounts of propane. It is shown that the results are in quantitative agreement with a conventional Rice-Herzfeld chain mechanism terminated by the combination and disproportionation of ethyl radicals, but with the mechanism extended so as to include the unimolecular isomerizations via a six-membered cyclic transition state between 1-hexyl and 2-hexyl (1-methylpentyl) radicals. The overall rate constant of initiation is estimated to be given by The rate constant for the reaction is given by which when combined with published data gives an Arrhenius plot curved upwards at low values of 1/T as has been observed for several other hydrogen abstraction reactions of methyl and of ethyl. Estimates are made of rate constants and ratios of rate constants for several reactions of the free radicals involved in the reaction. It is suggested that the minor product propane arises mainly from a hydrogen abstraction by 1-propyl from hexane with a contribution from a minor termination process involving ethyl and methyl.     

Dynamical Windings of Random Walks and Exclusion Models. Part I: Thermodynamic Limit in ℤ2

We consider a system consisting of a planar random walk on a square lattice, subjected to stochastic elementary local deformations. Both numerical and theoretical results are reported. Depending on the deformation transition rates, and specifically on a parameter η which breaks the symmetry between the left and right orientation, the winding distribution of the walk is modified, and the system can be in three different phases: folded, stretched and glassy. An explicit mapping is found, leading to consider the system as a coupling of two exclusion processes: particles of the first one move in a landscape defined by particles of the second one, and vice-versa. This can be viewed as an inhomogeneous exclusion process. For all closed or periodic initial sample paths, a convenient scaling permits to show a convergence in law (or almost surely on a modified probability space) to a continuous curve, the equation of which is given by a system of two non linear stochastic differential equations. The deterministic part of this system is explicitly analyzed via elliptic functions. In a similar way, by using a formal fluid limit approach, the dynamics of the system is shown to be equivalent to a system of two coupled Burgers equations.     

Sterische Einflüsse auf die Dissoziationskonstanten von acyclischen,mono- und bicyclischen Carbonsäuren

Steric effects on the dissociation constants of acyclic, mono- and bicyclic carboxylic acids Apparent pK values for the series of acyclic, mono- and bicyclic acids 1–14 have been measured in 50% (W/W) ethanol (see the Table). The dissociation constants of these acids decrease as the access of the solvent to the carboxylate group is hindered by the rest of the molecule. These results emphasize the importance of solvation in determining acid strength. New or improved syntheses of several of the acids investigated are reported.     

Determination des angles d'obliquite d'un cristal liquide en phase nematique au voisinage d'une surface

Using several laser sources the dispersion of the quadrupole nonlinear tensor d⁽²⁾(CaCO₃) has been determined in the ultraviolet and visible range 265 – 592 nm. The measured dependence is in a good agreement with the predicted dispersion d⁽²⁾(CaCO₃) ~ λ^-1.     

Acetylene chemistry. Part 31. Imidazoquinazolinones derived from the reaction products of 2,4-dichloroquinazoline and acetylenic amines

The reaction of 2,4-dichloroquinazolines with acetylenic amines afforded 2-chloro-4-(1,1-disubstituted-N-prop-2-inyl)quinazolines which on boiling in formic acid yeilded the corresponding imidazoquinazolones.     

Die Cyclopropylcarbinyl-Cyclobutyl-Homoallyl-Umlagerung II. Teil. Produkte und Geschwindigkeit der Umlagerung von Tricyclo[3.2.1.02,7]octan-3-ol-, endo- und exo-Tricyclo-[3.2.1.03,6]octan-4-ol und exo-Bicyclo[3.2.1.]oct-2-en-7-ol-Derivaten

Solvolyses of the 2,4-dinitrobenzoates of the cyclopropylcarbinol 1a , the cyclo-butanol 2a and the homoallylic alcohol 3a in buffered 70% aqueous dioxane lead to the same product mixture consisting of 78% cyclopropylcarbinol 1a and 22% homoallylic alcohol 3a . Approximately the same product mixture is optained when the homoallylic chloride 3b and the cyclopropylcarbinyl p-nitrobenzoate 1c are solvolyzed. A common cationic intermediate is indicated in these kinetically controlled rearrangement reactions. Under conditions of thermodynamic control only homoallylic products are formed. The 2,4-dinitrobenzoates 1d and 2d react 7.2 × 10³ and 7.7 times, respectively, as fast as the homoallylic isomer 3d . Since the homoallylic chloride 3b reacts ca. 10⁵ times as fast as its saturated analogue, 7-exo-bicyclo[3.2.1]octyl chloride 4 , all three esters react with the enhanced rates which are characteristic of these structural types. The endo-cyclobutyl dinitrobenzoate 2d reacts more than 60 times as fast as its exo-isomer 6d . The rate difference is ascribed to a stereoelectronically more favorable orientation of the participating σ bonds in the puckered cyclobutane ring in the endo isomer 2d .     

Rare earth substituted YBa2Cu3O x

We present some selected results obtained using rare earth Mössbauer spectroscopy, on YBa₂Cu₃O _x partially and/or totally substituted by Yb³⁺, Er³⁺ or Dy³⁺. We report measurements of the single ion crystal field anisotropy and of the size, direction and ordering temperature of the rare earth sublattice magnetic moments. We use dilute¹⁷⁰Yb³⁺ ions to probe both the Cu(2) derived internal (molecular) fields and the fluctuation frequencies of the short range correlated Cu(2) moments. We monitor these frequencies as the carrier level is changed up towards the threshold needed to stabilise superconductivity.     

Mössbauer measurements on170Yb3+ substituted into YBa2Cu3O7

We present some results obtained by Mössbauer spectroscopy on Yb³⁺ when substituted into YBa₂Cu₃O _x -like compounds. For the fully substituted compound YbBa₂Cu₃O₇, we describe the properties of the rare earth sublattice magnetic ordering which coexists with superconductivity. From measurements at the dilute substitution level, we study the Yb³⁺ crystal field properties and the thermal dependence of the Yb³⁺ paramagnetic relaxation rate. By making use of the molecular field produced on the Yb³⁺ probe by the magnetically correlated Cu(2), we examine the evolution of the Cu(2)-based magnetism as a function of carrier density and study the phase separation which occurs in the superconducting samples with intermediate oxygen levels. We also examine how the properties of the Yb³⁺ probe are influenced by the substitution of Pr³⁺ or Zn²⁺ (these substitutions are known to considerably influence the superconducting properties).