This kinetic and thermodynamic study is concerned with a comparison of binding of simple anions, azo-dyes and longchain surfactants to acyclodextrin. Two competition reactions have been employed. (i) In order to obtain information on the association of simple anions and surfactants with a-cyclodextrin, the competitive binding of an azo-dye with a-cyclodextrin has been used in stopped-flow experiments. (ii) In order to study the binding of the azo-dye pyridine-2-azo-p-dimethylaniline (PADA) to acyclodextrin, the competition reaction used is that between PADA and aquo-metal ions. Ni2+(aq) and Zn2+(aq) are used as competitors for PADA in stopped-flow and Joule-heating temperature-jump experiments.It is confirmed that when azo-dyes interact with a-cyclodextrin, they do so by a two-step mechanism. The first step involves the formation of an intermediate complex in a fast pre-equilibrium step. The second step, which is rate-determining, results in the formation of the final stable inclusion complex. Studies with I-(as) as a competing ion suggest that the intermediate is associated with dye binding to the inside rather than the outside of the cyclodextrin.Small anions in general have low stability constants for binding to a-cyclodextrin. In marked contrast, surfactants of general formula C,H,,+,SO; (where n > 8) interact strongly with a-cyclodextrin, and inclusion compounds are formed in which one surfactant binds two cyclodextrin molecules. Materials and MethodsThe dyes MY7 and PADA were supplied by Aldrich and Sigma, respectively. Elemental analysis was used to confirm purity. The structures are shown below.

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In the solid state ethylenediamine-bisborane is an open-chain molecule which exists in the trans conformation. The molecule contains a center of symmetry and all nonhydrogen atoms lie within 0.005 A of a least-squares fitted plane. The compound crystallizes in the space group Pbca and the cell dimensions are a = 10.761 (3) A, b = 8.172 (8) A, and c = 8.131 (14) A. The observed and calculated densities are 0.83 and 0.82 g cm-3, respectively. The intensities were collected by counter techniques and the model was refined to a conventional R factor of 0.062. The C-C distance is 1.514 (7) A, the C-N distance is 1.477 (5) A, and the N-B distance is 1.600 (7) A. The molecule essentially has Cih symmetry in the solid state.

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