Molecules in Cages Flashcards
What is a carcerand?
A closed molecular container or capsule without portals of significant size through which guests can either enter or leave. Guests are permanently trapped.
What is a hemicarcerand?
Closed molecular container from which guests can enter and exit with a measurable activation barrier.
What is constrictive binding?
Constrictive binding energy represents magnitude of sterically-imposed activation energy barrier to guest entry into the cavity exchange and arises from instability of guest-exchange transition state. Constrictive binding energy generally much larger than intrinsic binding energy.
How can binding to hemicarcerands be monitored?
Proton NMR - magnetic anisotropy of aryl rings that generally form cavity walls give rise to large chemical shift changes. Effect increases as guest penetrates deeper into cavity.
Infrared spectra - significant perturbations, commonly intermediate between liquid and gas phases of pure molecules.
How can acid-base behaviour be affected by hemicarcerands?
Example: pyridine - TFA.
Pyridine is protonated by TFA due to the difference in pKas. In the hemicarcerand, this protonation step does not happen, as TFA is unable to enter the hemicarcerand. In order to protonate the pyridine, the cation and the TFA anion would need to be separated and overcome ionic force.
How can hemicarcerands be used to prepare unstable species?
Example: cyclobutadiene.
Cyclobutadiene very unstable due to high levels of ring strain. Reacts very quickly with oxygen or via dimerisation. Sequestering cyclobutadiene in a hemicarcerand by initially forming it there removes ability to dimerise due to steric effects. Only requirement is to remove oxygen. Cyclobutadiene can be released and allowed to dimerise by heating.
