

Dear Jeremy:

Interesting problem.  I'm surely no expert in this field, but
based on limited experience you might wish to consider MP2/6-31G* 
optimizations.  The system is pretty small, so I think this would be
possible with gaussian-92 running on (say) an IBM550 or greater.
You will need lots of disk space; 2 Gb should do.  

If the MP2 optimization is not affordable, the other approach (which
makes sense to do anyway) is a series of opts with increasing basis 
sets to see how your results change from one to the next.  Start with 
6-31G* then 6-31+G* then 6-31++G* ... etc.  You might also look at DH 
basis sets.

The energy differences between tautomers is often quite small and
correlation seems like something you definitely want to include,
so follow the HF opts with some kind of correlated single points.
You also could follow the MP2/6-31G* opts (if you do them) with
higher basis-set correlated single points calcs.  I will avoid the 
whole religious issue of which correlation method would be best 
for your system.

I have no idea whether density functional is appropriate for this
kind of system.  Semi-empirical is a complete waste of time, IMHO.

Finally, I gather the formula is C4H4N2O2.  What is your experimental
evidence for the hydroxy form?  Is it the diol or the mono-ol?  What
is the pKa?  Pyridones (sometimes called 2-hydroxy pyridines) can
be drawn as hydroxy compounds, but in fact they exist as the lactams.
The proton is on the nitrogen.  So if you actually have the alcohol
form, then somehow this system's extra nitrogen atom has switched
the tautomeric preference.  That would be interesting to understand.
(I freely confess I don't understand tautomerization very well.)

J: Actually the formula of the structures I'm dealing with is
J: C5H7N3O2, containing an aminomethyl substitution. The literature
J: sugests that in gas, liquid, and solid phases, for pyridazine-3,6-diones,
J: the oxo/hydroxy (mono-ol, mono-one) tautomers are preferred. This
J: is indeed interesting, and somewhat different from other hydroxy-
J: nitrogen heterocycles. Pyridazine-3,6-dione itself is strongly acidic,
J: pKa = 1.00

I would love to learn more about your system, and of course look 
forward to your summary!

/ Mark
([email-masked])

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