High Activation Free Energy Research Articles

1H NMR study of the structure and dynamics of water confined between wetting solids

1H NMR relaxation measurements in a broad range of frequencies give information on the dynamical behaviour of water molecules in dilute suspensions of swelling clay (Hectorite). The logarithmic frequency variation of the longitudinal relaxation rate suggests a 2D diffusion of the water molecules adsorbed on the clay particles, with a residence time as long as 10 −6 s. The corresponding high activation free energy (40 kJ/mol) originates mainly from the clay-water interactions, as shown by Monte Carlo simulations of clay hydration.

Homogeneous and heterogeneous electron transfer to benzylphenylsulfide

The thermodynamic and kinetic parameters of the electron transfer (ET) to benzyl phenyl sulfide in DMF have been determined by cyclic voltammetry. The heterogeneous ET has been studied at three different electrodes: Hg, Pt, and glassy carbon. The activation parameters, namely the standard rate constant and the transfer coefficient, corrected for the double-layer contribution, show no significant difference for the three materials. The rate constants for the homogeneous ET to benzyl phenyl sulfide from eight electrogenerated anion radicals have been determined and compared with the corresponding reaction free energies, according to the current theories of the ET process. The results indicate that the ET is an endergonic process driven by a fast bond breaking of the ET product. Both the homogeneous and the heterogeneous reactions are consistent with an outer-sphere adiabatic electron-exchange process, characterized by a relatively high activation free energy. To account for the latter, inner reorganization energies must be considered, together with solvent reorganization. This is in agreement with the easy breaking of the C-S bond in the primary ET product.

Statistical thermodynamics of chain-folded polymer crystals

A simple molecular theory is developed in order to discuss the thermodynamic stability of chain-folded crystals. The partition function for a crystal containing different fold loops is formulated for a monodisperse polymer in terms of the lattice model; the analytical evaluation is made by using the maximum term method. The free energy calculated decreases linearly with decreasing the number of fold loops, i.e., its minimum corresponds to the extended-chain-type crystal. However, a lamellar-type crystal with long fold period and tight fold loops is also actually stable owing to the existence of high activation free energy barriers between them. The average number of monomer units comprised in the loop is estimated to be 5–7 irrespective of the lamellar thickness. The irreversible lamellar thickening and melting behavior of folded-chain crystals can be explained from this model. The results are also valid for the polydisperse system having a chain length distribution; it gives the crystal of nonuniform but well-defined lamellar thickness.