Fault in PM6 concerning N-H and O-H Groups (Home)

After PM6 was finalized an error was discovered by Prof. Victor I. Danilov concerning the relative energies of the O-H and N-H bonds.  This error can best be seen in the energies of the amino-phenols and their dimers. 

Consider the following reactions:

"Reaction"

DHr DFT

DHr PM6

Error in PM6

2-Pyridone

=>

2-Pyridinol

2.44

6.88

4.44

3-Pyridone

=>

3-Pyridinol

-8.09

-5.03

3.06

4-Pyridone

=>

4-Pyridinol

0.16

-0.82

-0.97

m-Amino phenol

=>

m-Amino phenol Zwitterion

42.42

34.97

-7.45

p-Amino phenol

=>

p-Amino phenol Zwitterion

41.64

28.14

-13.50

o-Amino phenol, dimer, neutral

=>

o-Amino phenol, dimer, Zwitterion

17.69

-3.58

-21.27

o-Amino phenol + water

=>

o-Amino phenol Zwitterion + water

14.50

3.31

-11.19

p-Amino phenol + 2 waters

=>

p-Amino phenol Zwitterion + 2 waters

33.67

19.69

-13.98

DFT = B88-PW91 with the DZVP basis set, all energies are in kcal/mol

From the pyridone - pyridinol tautomers it is apparent that the "-C-N-C(OH)-" <=> "-C-N(H)-C(O)-" tautomerization is reproduced with an accuracy typical of PM6, the AUE being 2.8 kcal/mol.

But for reactions of the type "-C(NH2)-  -C(OH)-" <=> "-C(NH3(+))-  )-C(O(-)-"  (a primary amine plus an alcohol (a hydroxy group) going to the Zwitterionic form), the PM6 energy is systematically low.  This is most dramatically illustrated by the calculated heat of formation of o-amino phenol dimer Zwitterion, where DFT predicts that the Zwitterion should be unstable by 17.7 kcal/mol and PM6 predicts that it should be stable by 3.6 kcal/mol.  Since two molecules are involved, the error is 10.6 kcal/mol. 

The conclusion is that PM6 is inaccurate in predicting the relative energies of amine-hydroxy neutral and Zwitterionic species.

 

Copyright 2007, Stewart Computational Chemistry.