Isomerisation of decatetraenes and dimethyldecatetraenes via pentadienyl and 3-methylpentadienyl radicals

The thermal isomerisations of E,E-deca-l,3,7,9-tetraene and E,E-3,7-dimethyldeca-l,3,7,9-tetraene take place via the intermediacy of pentadienyl and 3-methylpentadienyl radicals, respectively, rather than by concerted Cope type rearrangements. The pentadienyl radicals isomerise to pairs of E- and Z-pentadienyl radicals which recombine by end to end and end to centre, but not centre to centre, coupling to give mixtures of isomeric decatetraenes. The relative free energies of formation of these decatetraenes were derived from their equilibrium proportions and compared with relative enthalpies of formation calculated by the empirical MM2(87) method and the semiempirical AM1 and PM3 SCF MO methods. None of these theories was successful at predicting and rationalizing the experimentally observed enthalpy changes. However, they all were somewhat better in dealing with branching than with cis/trans isomerisation. Frontal strain in the decatetraene plays an important part in influencing their stability.     

The free-radical reaction of tri-n-butyltin hydride with peroxides

Rate constants for the tri-n-butyltin radical ( Sn · ) induced decomposition of a number of peroxides have been measured in benzene at 10°C. The values range from ～100 M^?1 sec^?1 for di-t-butyl peroxide to 2.6 × 10⁷ M^?1 sec^?1 for di-t-butyl diperoxyisophthalate. The majority of the peroxides, including diethyl peroxide, diacetyl peroxide, and t-butyl peracetate, have rate constants of ～10⁵ M^?1 sec^?1. It is shown that di-n-alkyl disulfides are ten times as reactive toward Sn · as di-n-alkyl peroxides, although the exothermicities of these reactions are ～15 and ～39 kcal/mole, respectively. The enhanced reactivity of the disulfides is attributed to the easier formation of an intermediate or transition state with 9 electrons around sulfur, compared with an analogous species with 9 electrons around oxygen. The following bond strengths (kcal/mole) have been estimated: D[ Sn ? OR] = 77; D[ Sn ? H] = 82; D[ Sn ? SR] = 83; and D[ Sn ? OC(O)R] = 86, where R = alkyl. Rate constants for reaction of Sn · with some benzyl esters have also been measured. It has been found that t-butoxy radicals can add to benzene and abstract hydrogen from benzene at ambient temperatures.     

Two different sources of fast forward-emittedα-particles in heavy-ion reactions

For the reaction 290 MeV²⁰Ne+¹⁹⁷Au the number of neutrons emitted in coincidence withα-particles is used to discriminate between two equally important but qualitatively different mechanisms for the emission of fastα-particles. They are connected with low and high inelasticity, respectively, and the resultingα-particles differ in mean velocity and velocity distribution.     

Bimolecular homolytic substitution of dialkyl selenides and tellurides with tri-n-butyltin radicals

The rate constants for the S_H2 reactions of tri-n-butyltin radicals with several dialkyl selenides and tellurides have been measured by EPR spectroscopy.     

Residual conductance of correlated one-dimensional nanosystems: A numerical approach

We study a method to determine the residual conductance of a correlated system by means of the ground-state properties of a large ring composed of the system itself and a long non-interacting lead. The transmission probability through the interacting region, and thus its residual conductance, is deduced from the persistent current induced by a flux threading the ring. Density Matrix Renormalization Group techniques are employed to obtain numerical results for one-dimensional systems of interacting spinless fermions. As the flux dependence of the persistent current for such a system demonstrates, the interacting system coupled to an infinite non-interacting lead behaves as a non-interacting scatterer, but with an interaction dependent elastic transmission coefficient. The scaling to large lead sizes is discussed in detail as it constitutes a crucial step in determining the conductance. Furthermore, the method, which so far had been used at half filling, is extended to arbitrary filling and also applied to disordered interacting systems, where it is found that repulsive interaction can favor transport.Received: 19 January 2004, Published online: 18 June 2004PACS: 73.23.-b Electronic transport in mesoscopic systems - 71.10.-w Theories and models of many-electron systems - 05.60.Gg Quantum transport - 73.63.Nm Quantum wires