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 |
Δ Hr |
|
|
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 |