Chirped-pulse and cavity-based Fourier transform microwave spectroscopy of a chiral epoxy ester: methyl glycidate.

Rotational spectra of a chiral epoxy ester, methyl glycidate, were measured using a chirped-pulse and a cavity-based Fourier transform microwave spectrometer. The two lowest energy conformers where the epoxy oxygen and the ester oxygen atoms are in the syn and anti relative orientation with respect to each other were identified experimentally. Spectra of four (13)C isotopologues of the lowest energy conformer of methyl glycidate were also measured and assigned. All of the observed rotational transitions are split into doublets due to the presence of the ester methyl internal rotor. The rotational constants and the internal rotation barrier height for the ester methyl group were determined for both conformers of methyl glycidate and for the four (13)C isotopologues of the most stable conformer. A value for the interconversion barrier between the two most stable conformers was estimated. Furthermore, comparison to strawberry aldehyde, a larger derivative of methyl glycidate, shows how the syn-anti conformational equilibrium shifts as a result of the additional bulky substituents at the epoxy ring and at the ester oxygen atom.