THEORETICAL-STUDY OF THERMAL-DECOMPOSITION MECHANISM OF OXALIC-ACID

Density functional theory B3LYP/6-31G(*) and ab initio MP2/6-31G(**) and MP4(SDQ)/6-311++G(*) calculations were carried out to study the s tructures and isomerization and decomposition mechanisms of oxalic aci d. The B3LYP structures and relative energies of the rotational isomer s of oxalic acid are found very similar to MP2 results, confirming tha t the most stable isomer is the doubly intramolecular hydrogen-bonded C-2h structure E1, with four other planar isomers within 6 kcal/mol. I t is predicted that unimolecular formation of carbon dioxide and dihyd roxycarbene (DHC) from oxalic acid has an activation barrier of 31 kca l/mol and that unimolecular formation of HCOOH from DHC has an activat ion barrier about 31 kcal/mol higher. The unimolecular formation of CO 2, CO and H2O from oxalic acid via a concerted transition state has an activation barrier of only 42 kcal/mol, indicating it is a more favor able unimolecular decomposition channel. On the other hand, hydrogen m igration from oxygen to carbon of DHC to produce HCOOH can be accompli shed through a hydrogen exchange with H2O (a model for oxalic acid) wi th an activation barrier of less than 37 kcal/mol. Transition state th eory calculations indicate that this bimolecular channel might be resp onsible for the rapid formation of CO2 and HCOOH in gas phase oxalic a cid thermal decomposition, thus confirming the proposal of Beck and Re dington. With increasing temperature the unimolecular channel to produ ce CO2, CO, and H2O might become significant.