Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4'-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R = butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4'-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers-Grote-Hynes model or by the Agmon-Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 10(4)-10(7) s(-1).
The complex formation of crown ethers with ammonium ions chemically bonded on silica gel has been chromatographically investigated. Although the selectivity in the complex formation of crown ethers is basically the same as in solution, a counter anion existing in the diffuse double layer of the silica gel also plays an important role, even when a polar solvent with hydrogenbond formation ability such as methanol, is used as a medium. The overall complex formation at the silica gel surface can be divided into three processes: (i) the dissociation of a counter anion from its ion-pair with the ammonium ion; (ii) complex formation of a crown ether with the ammonium ion; (iii) formation of an ion-pair between the anion and the ammonium ion complexed by the crown ether. This mechanism is evaluated from thermodynamic viewpoints including the solvation of anions, the ion-pair formation of anions with ammonium, etc.Much attention has been paid to the complex formation of polyethers with hard cations by a number of researchers in various branches of fundamental and applied chemistry since the discovery of crown ethers in 1967 by Pedersen.' One of the main interests is what is the thermodynamic origin of the selectivity in polyether complexation. To seek the answer to this question, a number of approaches have been attempted, including spe~trometry,~-~ ~alorimetry,',~ X-ray crystallo-graph^,^.* electrochemical methods,g-' ' computer simulation,12-15 etc.'Size-fit' theory is one of the most well known concepts used to explain the selectivity of macrocyclic compounds; i.e. the fit of the cavity size of a macrocycle to the crystalline size of a cation is thought to be the most important factor according to this concept. The size-fit theory is an idea based on the rigid structures of macrocycles, and does not necessarily govern the overall free energy of complex formation if conformational changes are taken into account. However, a suitable arrangement of donor atoms around a cation is necessary for stable bond formation; 'preorganization' of a ligand is a preferable term rather than ' size-fit '. ' Preorganization' results
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