Structure Determination of Strawberry Aldehyde by Broadband Microwave Spectroscopy: Conformational Stabilization by Dispersive Interactions

The rotational spectrum of ethyl 3-methyl-3-phenylglycidate (C12H14O3, strawberry aldehyde) has been obtained with chirped-pulse Fourier transform microwave spectroscopy. The sample is a mixture of diastereomers, cis and trans, with different relative stereochemistry around the central epoxide. The spectra of five conformers of this molecule (two of cis and three of trans) have been assigned, and carbon backbone structures for the two most populated conformers (one of cis and one of trans) were determined from 13C isotopomers in natural abundance using the Kraitchman relations. Comparisons of experimentally determined structural data to ab initio calculations show that the B3LYP density functional fails to account adequately for a long-range dispersive interaction between the phenyl ring and the terminal ethyl group in cis-strawberry aldehyde. However, calculations performed using both MP2 and the M05-2X density functional are able to capture the effects of this interaction on the molecular geometry.