The many forms of alpha-methoxy phenylacetic acid in the gas phase: flexibility, internal dynamics, and their intramolecular interactions.

We present a rotational spectroscopy study of alpha-methoxy phenylacetic acid in the gas phase. This acid is a derivative of mandelic acid and is used in various organic reactions. The conformational landscape of alpha-methoxy phenylacetic acid was explored to gain insight into its intramolecular dynamics. A rich rotational spectrum was obtained using chirped-pulse Fourier transform microwave spectroscopy in the 2-8 GHz range. Five conformers out of six calculated low-energy forms were identified in the spectrum, and the assignment of the 13 C singly substituted isotopologues for the lowest-energy conformer led to its accurate structure determination. Splitting patterns were analyzed and attributed to the internal rotation of a methyl top. The analysis of the non-covalent interactions within the molecule highlights the subtle balance in the stabilization of the different conformers. We thus provide high-level structural and intramolecular dynamics information that is also used to benchmark the performance of quantum-chemical calculations.