Pyrazine (${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$)

A planar six-membered aromatic ring with nitrogen at opposite ring sites. Its molecular point group is the D2h point group. The six ring $p_z$ atomic orbitals form a six-electron conjugated system; the in-plane nitrogen lone pairs are outside this $\pi$ basis.