Raman spectroscopic evidence for the most stable water/ethanol dimer and for the negative mixing energy in cold water/ethanol trimers.

Spontaneous Raman scattering in supersonic jet expansions is used to prove that the mixed dimer of ethanol and water (corresponding to a volume fraction of 79% ethanol in the liquid) prefers ethanol in a gauche conformation as the hydrogen bond acceptor. This represents a particularly simple case of adaptive aggregation. Furthermore, it is shown experimentally that the isolated cold trimer built from one ethanol and two waters (corresponding to 64% ethanol in the liquid) has a significantly negative excess enthalpy, in line with the thermodynamic bulk observation at room temperature.