Koosolekul

Koosolekul is a term used in chemistry to describe a transient, non-covalent association of two or more molecules in a solvent, forming a discrete supramolecular entity smaller than typical aggregates but larger than a single molecule. The term derives from Estonian koos, meaning together, and molekul, meaning molecule. In practice, a koosolekul is a dynamically exchanging assembly whose constituents retain identity but are held together by non-covalent interactions.

These assemblies arise when solute molecules experience favorable interactions such as hydrogen bonding, dipole–dipole interactions, π–π stacking, ionic contacts, or hydrophobic effects in a given solvent. Common stoichiometries include bimolecular and pseudo-tetrameric complexes; the number and arrangement depend on molecular geometry, solvent polarity, temperature, and concentration. Koosolekul complexes are typically labile and exist in reversible equilibrium with free components, with lifetimes ranging from picoseconds to milliseconds in simple solvents.

Characterization relies on techniques that probe non-covalent binding: NMR spectroscopy can reveal slow exchange, diffusion-ordered spectroscopy (DOSY) estimates of size, mass spectrometry detects non-covalent adducts, and IR or UV-Vis spectra may show shifts upon complexation. Crystallization can yield co-crystals that represent the assemblies in solid state.

Koosolekul concepts inform studies in solution chemistry, crystallization control, catalysis, and drug delivery, where co-solute interactions modulate solubility, stability, and reactivity. While not a fixed molecular species, koosolekul serves as a useful descriptor for understanding subtle intermolecular associations in complex systems.