Molecular aggregates in cryogenic solutions

Spectra of pyrazine, benzene, and osmium tetroxide have been obtained in cryogenic solutions. Data indicate that mixtures (∼ 10 ppm solute in solvent) deposited at low temperatures (∼ 100 K) yield clusters of solute molecules. The size of these clusters are estimated from light scattering data to be of the order of 103 Å. In particular, molecular aggregates appear in pyrazine/C3H8, benzene/NF3, CF4, OsO4/C3H8, NF3 but not in benzene/C3H8. The major factor governing the appearance of aggregates at low temperature is solubility. Upon warming these solutions to ∼150 K only solution spectra are observed; subsequent equilibrium cooling, of course, yields only precipitate.