Molecules containing a methyl rotor can undergo internal rotation. The methyl's rotation relative to the molecule's frame leads to complicated splitting patterns in the rotational spectra, which are challenging to model and interpret. Researchers readily observe these splitting patterns using microwave spectroscopy, owing to the high resolutions obtainable in a collision-free environment. Still, molecules containing internal rotors have received less study due to their complexity. This research studied molecules that have complex internal dynamics. Specifically, two custom-built microwave spectrometers and high-level quantum chemical calculations enabled measurements and models of methyl cyanoacetate (MCA), 2-methyl-2-oxazoline (2M2O) and 2-ethyl-2-oxazoline (2E2O).

These molecules’ spectra exhibit fine and hyperfine splittings due to the methyl rotor and the 14N quadrupole nucleus. These were analyzed to derive their barriers to internal rotation. This research studied the effects of hyper-conjugative interactions on MCA’s conformational stability and methyl barrier by comparing the derived barriers to those of similar molecules. For 2M2O and 2E2O, this work highlighted that the factors that govern the barriers in methylated molecules containing a partially unsaturated ring are poorly understood, as there has been little study on such molecules to date. As such, this work provides the basis for future studies on molecules with flexible ring systems containing a methyl rotor, which would aid our understanding of these types of molecules and the development of better computational models for analyzing and predicting their spectra.