On the low volatility of cyclic esters: an infrared spectroscopy comparison between dimers of gamma-butyrolactone and methyl propionate

The dramatically lower volatility of gamma-butyrolactone compared to its open chain analog methyl propionate is analyzed at the molecular dimer level using FTIR spectroscopy in supersonic jets. It is found that the spectral shifts from the monomer to the dimer are about three times more pronounced in the lactone at low temperatures. The spectra are consistent with sandwich-like dimers optimizing their strong dipole-dipole interaction, possibly augmented by specific C-H center dot center dot center dot O=C hydrogen bond contacts. The spectra show significant evolution from the dimer to the condensed phase, indicative of secondary interactions with the ester oxygen and long range forces. The reduced dipole moment in the open chain ester leads to less specific interactions, unless a trans conformation of the ester group as in the lactones is enforced. The latter is not energetically accessible in open chain esters because it would bring the molecular C=O and C-O-C dipole moments into an unfavorable near-parallel orientation, thus their higher volatility.