No abstract

No abstract

No abstract

electric response can be probed for a single magnitude of solvation. Similar models have been proposed by others. Marcus and co-workers1 discuss a similar two-dimensional potential, but the reactive motion is modeled via transition-state theory and although useful for electron-transfer reactions, does not seem appropriate for isomerization reactions that couple to the solvent mechanical friction. Agmon et al.37 consider a two-dimensional model where diffusion is considered in both dimensions. However, their model is for two internal coordinates, and the potential with which they numerically solve the diffusion equation may not be appropriate in describing the polarization coordinate. ConclusionsThis work demonstrates the dramatic importance of both static and dynamical dielectric effects on the photoisomerization of stilbenes. In particular, polar solvents are observed to lower the barrier to isomerization over the nonpolar solvent case, hence increasing the rate dramatically. For the case of «-alkanenitriles, which are unassociated solvents, the dielectric response is rapid enough that the more polar members of the homologous series better solvate the transition state. For the case of «-alkyl alcohols, which are associated solvents, the dielectric response lags the reactive motion and the aviscous activation barrier is higher for the more polar members of the homologous series. These observations are consistent with a two-dimensional view of the reaction, consisting of the twisting coordinate and a solvent polarization coordinate. These observations hold for 4,4'-dihydroxystilbene, 4,4'-dimethoxystilbene, and f/ww-stilbene. Although the above behavior is observed for symmetrically substituted stilbenes, it seems likely that they would be enhanced for asymmetrically substituted stilbenes or any reactive system with an initial dipole moment. Studies of the isomerization of 4-(dialkylamino)-4'-azastilbene and 4-(dialkylamino)-4'-nitrostilbene show clearly that increasing solvent polarity leads to a decrease in isomerization yield,43•44 consistent with the results reported here.

No abstract

Absorption rates of NOin aqueous solutions of Fe(II)-EDTA chelate as well as in mixed solutions of Fe(II)-EDTA and Na2SO3 were measured using a stirred vessel with a free flat gasliquid interface and a bubble column. The rate constants of the complexing reaction of NOwith Fe(II)-EDTA were determined on the basis of the theory of gas absorption accompanied by a reversible reaction. The chemical equilibrium constants were also determined at various pH values. It was found that the rate constant was of the order of 108 //g-mol sec and that the equilibrium constant was about 106 //g-mol at 25°C. These values are muchhigher than the corresponding values of the reaction between NOand Fe(II) in the absence of EDTA.The mechanismof the absorption of NOin mixed solutions of Fe(II)-EDTA and Na2SO3 was deduced from the observation that the absorption efficiency decreased in the early stage of absorption and then increased to some steady value. The absorption rates were satisfactorily explained on the assumption that NO coordinates to Fe(II) (EDTA) (SO^~) irreversibly.It was also found that the absorption rate of NOin the aqueous solution of Fe(II)-EDTA was much higher than those of other liquid absorbents so far investigated.

No abstract

No abstract