Molecular hydration of propofol dimers in supersonic expansions: formation of active centre-like structures

Spectroscopic studies of molecular aggregates are a powerful tool to understand the weak interactions between molecules. Here, propofol2(H2O)6,7 clusters were formed in supersonic expansions and their electronic and infrared spectroscopy was explored using several mass-resolved laser-based spectroscopic techniques. Using REMPI, their S1 ← S0 electronic spectrum was obtained with vibrational resolution, while the UV/UV hole burning revealed the presence of a single isomer of propofol2(H2O)6 and of two isomers of propofol2(H2O)7. Employment of IR/UV double resonance yielded the IR spectrum in the OH stretch region. Comparison with the spectra predicted for the structures calculated at the M06-2X/6-31+G(d) level demonstrated that the two propofol molecules interact mainly through C-Hπ contacts between the lipophilic sides of the molecules, while the hydroxyl moieties are in close contact, forming a kind of "active centre" with which the water molecules interact, forming polyhedral structures.