Conformational equilibrium and normal coordinate analyses of ketenecarboxylic acid O=C=CH-COOH

The conformational and structural stability of ketenecarboxylic acid O=C=CH-COOH were investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311+G** basis set. From the calculations ketenecarboxylic acid was predicted to exist predominantly in a mixture of cis (the CO and the CHCO groups eclipse each other) and trans conformations with the trans being the lower energy form. The C-C- and CO rotational barriers in the molecule were calculated to be about 10 and 12 kcal/mol, respectively. The equilibrium constant for the cis ↔ trans conformational interconversion of ketenecarboxylic acid was calculated to be 0.4117 that corresponds to an equilibrium mixture of about 29% cis and 71% trans at 298.15 K. The vibrational frequencies of the molecule in the cis and trans conformations were computed at the DFT-B3LYP level. Complete vibrational assignments were made on the basis of normal coordinate analyses of cis and trans ketenecarboxylic acid.