Investigation of local density of monomer units and molecular dynamics in solutions of poly-4-vinyl-pyridine by the spin label technique

The local density of monomer units of the macromolecule (local density of the host molecule, ?_host) in poly-4-vinyl-pyridine solutions in ethanol was determined by the spin label technique. In dilute solutions, ?_host is considerably greater than the mean density of monomer units in the volume of the polymer coil; when the polymer concentration rises from 0.5 to 65 wt%, its increase does not exceed 30%. This indicates that the differences between polymer coil microstructure (mutual positions of monomer units close to a chosen unit) in dilute and concentrated solutions is small. The local density of monomer units of neighbouring macromolecules (guest macromolecules, ?_guest) is strongly dependent on the polymer concentration in solution. In dilute solutions, ?_host ? ?_guest; for polymer concentrations above 2–3 wt%, overlapping and interpenetration of macromolecular coils take place, local density of monomer units of neighbouring macromolecules rises and monotonously increases as polymer concentration grows. The concentration dependence of the local rotational and translational mobility of monomer units is determined by the local density of monomer units of the neighbouring macromolecules.     

Chemically induced magnetic isotope effect on the tin nuclei during the photolysis of (1-fluorenyl)trimethyltin

A magnetic isotope effect on the ¹¹⁷Sn and ¹¹⁹Sn nuclei, accompanied by the fractionation of magnetic and nonmagnetic tin isotopes, was observed during the photolysis of (9-fluorenyl)trimethyltin. The magnetic and nonmagnetic isotopes were accumulated, respectively, in the initial compound and a photolysis product (hexamethyldistannane).     

NMR in paramagnetic complexes of radicals with organic ligands: III. Method of identification of electronic structure of complexes — Experiment

Paramagnetic shifts and broadenings of the NMR lines of different organic molecules which result from their complexation with stable nitroxide radical are given and analyzed in terms of the theory reported previously. The main problem — that is, what orbitals of the partners are involved in the formation of the complex — is solved. Different types of complexes are identified and their electronic structure is established.     

Polyferroorganosiloxanes as catalysts of oxidation processes

Polyferroorganosiloxanes were studied as catalysts for homogeneous oxidation of alkanes by hydrogen peroxide tinder mild conditions. In the oxidation of cyclohexane the catalysts are characterized by high efficiency (conversion of hydrogen peroxide is 25%) and stability Kip to 80 cycles per gat NJ The min product of the oxidation W do presence a 2,4,6-tri-tert-btitylphenol is cyclohexanol (tip to 35% per H₂O₂).     

Radical reactions in organic crystals under pressure: Experiment and theory

The kinetics of radical transformations in gamma-irradiated dicarboxylic acid single crystals at a high pressure was studied by EPR spectroscopy. The activation parameters of the detachment of the hydrogen and reaction coordinates were determined. The dependence of the rate constant for the reaction on the pressure and crystal physical parameters was studied. The compression of crystals decelerated the detachment of hydrogen (the activation volumes were positive), and the rate-determining stage of the process was free volume fluctuations responsible for the approach of reagent active centers to each other and their arrangement and orientation along the reaction coordinate on the potential energy surface. The kinetic parameters of the reaction are closely related to the crystal characteristics such as the molar volume, compressibility, and thermal expansion coefficient. A theory describing this relation is suggested. Only part of the free volume was shown to be used for “reaction” fluctuations. The types of reagent motions responsible for the molecular organization of the transition state were identified.     

Modeling the H5+ potential-energy surface: a first attempt

 Ab initio molecular electronic structure calculations are performed for H₅ ⁺ at the QCISD(T) level of theory, using a correlation-consistent quadruple-zeta basis set. Structures, vibrational frequencies and thermochemical properties are evaluated for ten stationary points of the H₅ ⁺ hypersurface and are compared with previous calculations. The features of the H₃ ⁺…H₂ interaction at intermediate and large intermolecular distances are also investigated. Furthermore, an analytical functional form for the potential-energy surface of H₅ ⁺ is derived using a first-order diatomics-in-molecule perturbation theory approach. Its topology is found to be qualitatively correct for the short-range interaction region. Received: 15 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001