Fault in PM6 in Prediction of Oxalic Acid Heat of Reaction

 

PM6 predicts the heat of reaction of oxalic acid (HOOC-COOH) decomposing to CO2 + C(OH)2 to be in error by 29.6 kcal/mol. There is no reason to doubt the B88PW91 result, therefore PM6 has a definite error.

Reaction

Oxalic acid

CO2

+

C(OH)2

ΔHr

Ref.

-175.0

-94.1

-41.4

39.5

PM6

-156.3

-84.8

-61.6

9.9

The main source of the error is likely to be in C(OH)2. If that is the case, then it is also likely that divalent carbon is in error.

Reference and PM6 heats of formation (kcal/mol)

Molecule

Ref.

PM6

Diff

Oxalic acid

-175.0

-156.3

18.7

CO2

-94.1

-84.8

9.3

C(OH)2

-41.4

-61.6

-20.2