Ab initio study of malonaldehyde rotamers

Malonaldehyde rotamer geometries were optimized using ab initio calculation s at the HF level with STO-3G** and 6-21G** basis sets. The most stable rot amer is the omega-shaped one with cyclic structure and intramolecular hydro gen bond. The most unstable rotamer is that obtained by rotation of the ome ga-rotamer around the CO single bond by 180 degrees due to the loss of the additional stabilization contributed by the intramolecular H-bond. The ener gy barriers separating the different rotamers vary between 13 and 233 kJ.mo l(-1). The structure of the transition states is non-planar with rotation a ngles varying between 72 and 98 degrees.