Showing papers by "Sebastião J. Formosinho published in 1986"
TL;DR: In this paper, an intersecting-state model has been applied to the calculation of activation energies in elementary gas-phase reactions, and a hierarchy of electronic factors for chemical reactions has been found and associated with the overall bond order of the activated complexes, which is considered to vary over the reaction path.
Abstract: An intersecting-state model has been applied to the calculation of activation energies in elementary gas-phase reactions. For thermoneutral reactions the sum of the bond distensions, d, at the transition state is found to depend on the sum of the equilibrium lengths of the reactive bonds and on the bond order at the transition state. For non-thermoneutral reactions d is found to have a quadratic dependence on the enthalpy of the reaction. Such dependence is affected by the so-called configuration entropy. A hierarchy of electronic factors for chemical reactions has been found and associated with the overall bond order of the activated complexes, which is considered to vary over the reaction path. Transition-state bond orders higher than those of the reactants or products are interpreted in terms of the rearrangement of a pair of electrons from occupied antibonding or non-bonding orbitals which acquire a bonding character at the transition state. The model appears to be general and encompasses earlier models relating thermodynamic and kinetic parameters of chemical reactions.
53 citations
TL;DR: In this paper, an intersecting-state model was used to analyze the spin crossover rate of some complexes of iron(II), iron(III), and cobalt(II) and showed that the main energy barrier involves the internal reorganization of the coordination shell of the complex.
Abstract: Rates of spin exchange (spin crossover) of some complexes of iron(II), iron(III) and cobalt(II) have been analysed by an intersecting-state model. Similarities in parameters with those observed in elementary gas-phase reactions suggests that the main energy barrier involves the internal reorganization of the coordination shell of the complex. Whilst the expected transition-state bond orders of one are observed with FeII and most CoII complexes, bond orders greater than one are observed in bis(terpyridyl)cobalt(II) and in FeIII complexes. This is explained in terms of involvement of non-bonding electrons in the transition state. In contrast to previous suggestions, nuclear tunnelling does not appear to be an important mechanism in spin exchange.
13 citations
TL;DR: In this paper, a new general expression relating the bond distensions in the transition state with the energy of reaction is proposed; the relationship is devised using the bond energy-bond order method, based on a modified Pauling relationship and using the assumption that the bond order may vary along the reaction co-ordinate.
Abstract: From data for 25 different hydride reactions in the vapour phase a new general expression relating the bond distensions in the transition state with the energy of reaction is proposed; the relationship is devised using the bond energy–bond order method, based on a modified Pauling relationship and using the assumption that the bond order may vary along the reaction co-ordinate.
11 citations
TL;DR: In this paper, it was shown that the apparent rates of the fluorescence quenching of aromatic molecules by the uranyl ion in ethanol solutions are considerably higher than the rate of diffusion.
10 citations
TL;DR: In this article, the potential energy along a reaction coordinate can be calculated in terms of the reaction energy, the force constants and the equilibrium bond lengths of the reactive bonds, the bond order at the transition state and, for very exothermic or endothermic reactions, the so called "configuration entropy".
Abstract: The potential energy along a reaction coordinate can be calculated in terms of the reaction energy, the force constants and the equilibrium bond lengths of the reactive bonds, the bond order at the transition state and, for very exothermic or endothermic reactions, the so called "configuration entropy". Such potential energy curves allows one to assess the importance of quantum mechanical tunnelling in Habstraction, a-cleavage and cycloaddition photochemical reactions of excited lzetones. Experimental recognition of tunnelling can he made by the study of the efficiency of the photoreactions as a function of molecular parameters and medium properties. The theory allows the establishement of structure-efficiency relationships and criteria to distinguish between the mechanisms of nuclear tunnelling and thermal activat ion.
9 citations
TL;DR: The effect of binding by α-alumina on the luminescence spectrum of the uranyl ion is examined in this article, where the presence of uranyl ions leads to a considerable reduction in the intensity of this scattering, and a comparison of emission spectra of this sample with those of alumina, uranyl nitrate and an alumina/uranyl ion suspension is performed.
5 citations