The mechanism and the nature of the dynamically-determined product selectivity in Diels-Alder cycloadditions of 3-methoxycarbonylcyclopentadienone (2) with 1,3-dienes was studied by a combination of product studies, experimental kinetic isotope effects, standard theoretical calculations, and quasiclassical trajectory calculations. The low-energy transition structures in these reactions are structurally balanced between [4π diene + 2π dienone ] and the [2π diene + 4π dienone ] modes of cycloaddition. The accuracy of these structures and their bispericyclic nature is supported by the experimental isotope effects. Trajectories passing through these transition structures can lead to both [4π diene + 2π dienone ] and [2π diene + 4π dienone ] cycloadducts, and the mixture of products obtained varies with the structure of the diene. The factors affecting this selectivity are analyzed. The geometry of the transition structure is a useful predictor of the major product, but the selectivity is also guided by the shape of the energy surface as trajectories approach the products and by how trajectories cross the transition state ridge.

The free scission energy is the thermodynamic parameter that governs the contour length of wormlike micelles (WLMs). It is the contour length and the propensity to coil and entangle that determine the viscoelastic properties of this commercially important substance class. The free scission energy ΔF sc and the associated change in enthalpy ΔH sc and entropy ΔS sc on scission have been determined for a mixed anionic/ zwitterionic surfactant system (sodium laureth sulfate and cocamidopropyl betaine) at various salt concentrations (3−5 wt % NaCl). Both enthalpy ΔH sc and entropy ΔS sc changes decrease linearly with increasing NaCl concentration. At NaCl concentrations above 4 wt %, ΔS sc even adopts negative values. The term TΔS sc decreases more rapidly than ΔH sc around room temperature and causes the observed elongation of WLMs upon addition of NaCl. It is suggested that ΔS sc is initially positive due to fewer bound counterions per surfactant molecule at end caps compared to the intact, cylindrical parts before scission, leading to a net release of ions upon scission. Negative values of ΔS sc are attributed to hydrophobic hydration occurring at the end caps at high salt concentrations. 23 Na NMR measurements indicate the presence of immobilized ions, supporting a previously proposed ion-cloud model based on neutron scattering results.