# Why does the tautomerism of purine favor 9H-purine? [duplicate]

I tried to explain the equilibrium by comparing the basicity of different nitrogen atoms in a purine anion, but I can’t see any obvious difference. Seems like that the 9-N will be the most basic because it can have two adjacent imine structure while keeping the structure of pyrimidine untouched. Computational results will be appreciated.

P.S. I don’t think the question is a duplicate of the “purine nitrogen basicity” one because that one compares the basicity of nitrogens in a neutral purine molecule whereas my question compares the basicity in a purine anion.

## marked as duplicate by Mithoron, Mathew Mahindaratne, Tyberius, Nilay Ghosh, Todd MinehardtAug 4 at 18:44

In $$\ce{9H–Purine}$$ (figure 2), lone pairs are far apart from each other (3 and 7). Comparatively in $$\ce{7H–Purine}$$ (figure 1) lone pairs are very near each other (3 and 9) . Nearness of lone pairs in $$\ce{7H–Purine}$$ could leed to greater repulsions between them. This could make it energetically unfavorable in comparison to $$\ce{9H–Purine}$$.
This could be the reason why $$\ce{9H–Purine}$$ is more favored tautomer compared to $$\ce{7H–Purine}$$.