The first volume profiles for complex formation of α-cyclodextrins (α-CD) with diphenyl azo dyes (S) are presented as a new approach in understanding inclusion phenomena. The following dyes were selected:  sodium 4-(4-diethylaminophenylazo)benzenesulfonate (1), sodium 4-(3-carboxy-4-hydroxy-5-methylphenylazo)benzenesulfonate (2), sodium 4-(4-hydroxy-3,5-dimethylphenylazo)benzenesulfonate (3), and sodium 2-hydroxy-3-methyl-5-(4-sulfamoylphenylazo)benzoate (4). The behavior of the dyes alone were first studied in aqueous solutions to rule out any competition reaction. Under the experimental conditions used for the stopped-flow kinetic studies, it has been proved that only monomeric species are present (no aggregation of the dye is formed by π−π stacking interactions). NMR experiments and kinetic evidences have shown that only directional binding of the dye via the sulfonate/sulfonamide group through the wide rim of the α-cyclodextrin was possible. The 1:1 complex was the only stoichiometric species formed. The inclusion reactions for the four selected dyes were characterized by a two-step kinetics described by a first fast step that yields the intermediate, S·α-CD*, followed by a slower rearrangement to form the final complex, S·α-CD. 2D NMR experiments served for a molecular dynamics calculation leading to a structural representation of the intermediate and final complexes. An interpretation of the volume profiles obtained from high-pressure stopped-flow kinetic experiments have not only confirmed the so far proposed mechanisms based on “classical” kinetic investigations but offered a new focus on the inclusion process. The inclusion mechanism can be summarized now as follows:  the complexation begins with an encounter of the dye and α-cyclodextrin mainly due to hydrophobic interactions followed by a partial desolvation of the entering head of the dye. The latter interacts with the two “activated” inner water molecules of the free host and their complete release is delayed by the primary hydroxy group barrier of the α-CD. At this first transition state, a squeezed arrangement develops inside the cavity inducing a negative activation volume (ΔV 1,f ⧧ ≈ −8 to −24 cm3 mol-1). The subsequent intermediate is characterized by a total release of the two inner water molecules and interactions of the dye head with the primary hydroxy groups of the host in a trapped-like structure (ΔV 1° ≈ −11 to −4 cm3 mol-1). The latter interactions and concurrent tail interactions with the secondary hydroxy groups of the host lead at different extents to a strained conformation of the host in the second transition state (ΔV 2,f ⧧ ≈ −2 to −16 cm3 mol-1). In the final complex, the head of the dye is totally rehydrated as it protrudes from the primary end of the host cavity which can now adopt a released conformation (ΔV 2° ≈ +3 to +6 cm3 mol-1 vs +17 cm3 mol-1 for 1).

The first variable-temperature and variable-pressure stopped-flow spectrophotometric study of the sequential threading of alpha-cyclodextrin (alpha-CD) onto the guest dye Mordant Orange 10, S, is reported. Complementary (1)H one-dimensional (1D) variable-temperature kinetic studies and two-dimensional (2D) rotating-frame nuclear Overhauser effect spectroscopy (ROESY) and EXSY NMR studies are also reported. In aqueous solution at 298.2 K, the first alpha-CD threads onto S to form a 1:1 complex S.alpha-CD with a forward rate constant k(1,f) = 15 200 +/- 200 M(-1) s(-1) and dethreads with a reverse rate constant k(1,r) = 4.4 +/- 0.3 s(-1). Subsequently, S.alpha-CD isomerizes to S.alpha-CD (k(3,f) = 0.158 +/- 0.006 s(-1), k(3,f) = 0.148 +/- 0.006 s(-1)). This process can be viewed as a thermodynamically controlled molecular shuttle. A second alpha-CD threads onto S.alpha-CD to form a 1:2 complex, S.(alpha-CD)(2), with k(2,f) = 98 +/- 2 M(-1) s(-1) and k(2,r) = 0.032 +/- 0.002 s(-1). A second alpha-CD also threads onto S.alpha-CD to form another 1:2 complex, S.(alpha-CD)(2), characterized by k(4,f) = 9640 +/- 1800 M(-1) s(-1) and k(4,r) = 61 +/- 6 s(-1). Direct interconvertion between S.(alpha-CD)(2) and S.(alpha-CD)(2) was not detected; instead, they interconvert by dethreading the second alpha-CD and through the isomerization equilibrium between S.alpha-CD and S.alpha-CD. The reaction volumes, DeltaV(0), were found to be negative for the first three equilibria and positive for the fourth equilibrium. For the first three forward and reverse reactions, the volumes of activation are substantially more negative, indicating a compression of the transition state in comparison with the ground states. These data were used in conjunction with DeltaH, DeltaH degrees, DeltaS, and DeltaS degrees data to deduce the dominant mechanistic threading processes, which appear to be largely controlled by changes in hydration and van der Waals interactions, and possibly by conformational changes in both S and alpha-CD. The structure of the four complexes were deduced from (1)H 2D ROESY NMR studies.

Oxygen-17 NMR was utilized in aqueous medium (1.2-2.4 m KNO3) to study the oxygen exchange kinetics in the irans-dioxotetracyanometalate complexes of Re(V), Tc(V), W(IV), and Mo(IV). The kinetics are described by the two-term rate law R = (fcaq[MOH2] + &oh[MOH])/2, where MOH2 and MOH represent the diand monoprotonated forms, [MO(OH2)(CN)4]"~a nd [MO(OH)(CN)4](n+, |~, of the dioxotetracyanometalate complexes, respectively. The aqua complexes are by far more reactive toward exchange (kaq values of (9.1 ± 0.1) x 10"2 and (137 ± 5) s~' at 25.0 °C for Re(V) and W(IV), respectively), compared to the [MO(OH)(CN)4]("+1)~i ons under the same conditions (£0h: Re(V), (2.6 ± 0.3) x 10-3; W(IV), (6.5 ±0.1) x 10-4 s™1), with the kon value for the Tc(V) hydroxo oxo complex at 25 °C obtained as 13 ± 1 s_1. The activation parameters are as follows [AH* (kJ mol"1), AS* (J K"1 mor1)]• Re(V):

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