Tailor-made aggregates of alpha-hydroxy esters in supersonic jets

Hydrogen-bonded clusters of methyl glycolate and methyl alpha-hydroxyisobutyrate are investigated by ragout-jet FTIR-spectroscopy to elucidate the influence of alpha-C hydrogens on the unusual aggregation of alpha-hydroxy esters. Highly structured OH-stretching spectra, stagnation pressure studies, and argon coating experiments show that two alpha-C hydrogens enable the formation of small oligomers such as trimers and tetramers, whereas the lack of alpha-C hydrogens stops selective aggregation beyond dimers. Methyl lactate with a single chiral alpha-C hydrogen is suggested to be intermediate in its clustering behavior, with a homochiral preference for trimers and a strong heterochiral preference for tetramers.