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What is a molecular system in general terms?

Could the solution of sugar in water be called a molecular system, if we were not interested in sugar-water interaction?

Today Jean Marie Lehn told me that bulk water is classical supramolecular system, but I cannot understand why.

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Molecular chemistry is based predominantly on interactions through covalent bonds. It looks at molecules as individual entities, and is limited in its scope of accomodating intermolecular interactions. Your solution of sugar in water could easily be studied as a molecular system, if all you are interested in is the sugar molecule itself. Subtle differences in molecular conformation of the sugar in different solvent systems, for instance, would simply be written off as 'solvent effects'.

Supramolecular chemistry is based on intermolecular interactions. Typically, these are based on weak interactions (non-covalent forces), and include:

  • ion-ion interactions
  • ion-dipole interactions
  • dipole-dipole interactions
  • hydrogen bonding
  • van der Waals forces
  • pi stacking
  • hydrophobic interactions
  • and probably some others

It is through these types of interactions of that we can explore phenomena such as host-guest association, self-assembly, metal-ligand chelation, micelles, and even the structure of DNA, as all of these systems rely on the weak intermolecular interactions outlined above. You could also study your aqueous sugar system as a supramolecular system, and here you could consider what predominant intermolecular interactions are at work, and how they affect the overall chemistry of the bulk system.

Water is supposed to exist as water clusters, weak oligomeric and polymeric forms of $\ce{(H2O)_{n}}$

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