J. Higgins et al., THEORETICAL-STUDY OF THERMAL-DECOMPOSITION MECHANISM OF OXALIC-ACID, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(14), 1997, pp. 2702-2708
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.