Kinetic study of beta-cyclodextrin-dye system by high-pressure temperature-jump method.

Area of Science:

• Physical Chemistry
• Supramolecular Chemistry
• Chemical Kinetics

Background:

• Understanding host-guest complexation is crucial in supramolecular chemistry.
• Beta-cyclodextrin (a cyclic oligosaccharide) is a well-known host molecule.
• Phenolphthalein is a common indicator dye that forms inclusion complexes with cyclodextrins.

Purpose of the Study:

• To investigate the pressure dependence of the formation and dissociation rate constants for the beta-cyclodextrin-phenolphthalein inclusion complex.
• To elucidate the reaction mechanism and transition state properties under high pressure.

Main Methods:

• Utilized a newly constructed high-pressure temperature-jump apparatus.
• Applied temperature jumps of 2.8°C within 1 microsecond.
• Conducted experiments under pressures up to 2,000 kg/cm².

Main Results:

• The inclusion reaction was analyzed as a simple reversible bimolecular reaction.
• Determined activation volumes: 1.78 ± 0.85 cm³/mol for the forward reaction and 10.65 ± 1.40 cm³/mol for the backward reaction.
• Observed significant pressure effects on the reaction kinetics.

Conclusions:

• The activation volume data suggest that desolvation of phenolphthalein and release of water from the beta-cyclodextrin cavity are key events at the transition state.
• High pressure influences the structural changes occurring during complex formation and dissociation.
• Provides insights into the molecular mechanisms governing cyclodextrin inclusion complexation under varying pressure conditions.