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I am trying to figure out which tautomer of creatinine:

two tautomers of creatine

is more stable.

We see that the enol tautomer is more highly conjugated: the $\pi$ electrons from the alcohol to the double bond to the $\pi$ electrons of the Nitrogen to the imine to the final $\pi$ electrons of the amine.

In the keto form, we have the $\pi$ electrons from the carbonyl to the imine to the two amines.

I know the enol form is more conjugated, but in general I also know that the keto form is more stable. How do I reconcile this?

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    $\begingroup$ I feel that you should also look at another possibility, that of having an aromatic pyrrole ring. I guess that's the most stable form. Nevertheless the real molecule will be a hybrid of all these. $\endgroup$
    – Esha Manideep
    Nov 30, 2018 at 15:57

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The NMR spectrum seems to show that the tautomeric state of creatinine is that of the keto-form (bottom structure).

If the ketone is the only functional group, the keto form is more stable than the enol form. One the other hand, if the carbon-carbon bond is part of a conjugated system, the enol form might be more stable (e.g. acetylacetone or phenol).

You also suggest either the imino or the amino tautomer. There is a Raman study that addresses this question and also raises the dependence on pH.

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