Knowledge Bank

The intramolecularly hydrogen-bonded molecule methylmalonaldehyde has two coupled, large-amplitude vibrations: the internal rotation of the methyl group and a motion of the chelate ring atoms between two equivalent, asymmetric conformations. The two vibration frequencies are determined from Coriolis perturbations in the rotational spectra. Four isotopic species $(OH/CH3,OD/CH3,OH/CD3,OD/CD3)$ have been studied. The large isotope shifts observed (typically changes by factors of 3-8) are correlated by a simple two-coordinate mode. Possible implications of these results for theories of the origin of torsional barriers in this and similar systems will be discussed.