= Pyrimidine
<Pyrimidine> is the aromatic <organic compound> $\mathrm{C_4H_4N_2}$ with a six-membered ring containing <nitrogen> at positions $1$ and $3$. It is therefore a <diazine>, specifically 1,3-diazine.<PubChem Compound database by National Center for Biotechnology Information>[Pyrimidine, CID9260; formula and connectivity]{ref}
Changing the nitrogen positions gives different <molecules>: <pyridazine> has the $1,2$ arrangement, while <pyrazine> has the $1,4$ arrangement. The same elemental formula therefore does not uniquely specify ring connectivity. In the $1,3$ arrangement, the two <nitrogen> atoms have one ring <carbon> between them along the shorter path; in the $1,2$ arrangement they are adjacent.<PubChem Compound database by National Center for Biotechnology Information>[Pyridazine, CID9259; compare pyrimidine and pyrazine structures]{ref}
<Cytosine>, <thymine> and <uracil> contain a pyrimidine-derived ring framework, but their added groups and bonding make them chemically distinct from unsubstituted <pyrimidine>. This distinguishes a parent ring structure from the biological <nucleobases> named by association with that structure.
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