We report a quantum chemical characterization of the non-natural (synthetic) H-bonded base pair formed by 6-amino-5-nitro-2(1H)-pyridone (Z) and 2-aminoimidazo[1,2-a]-1,3,5-triazin-4(8H)-one (P). The Z:P base pair, orthogonal to the classical G:C base pair, has been introduced into DNA molecules to expand the genetic code. Our results indicate that the Z:P base pair closely mimics the G:C base pair in terms of both structure and stability. To clarify the role of the NO2 group on the C5 position of the Z base, we compared the stability of the Z:P base pair with that of base pairs having different functional groups at the C5 position of Z. Our results indicate that the electron-donating/-withdrawing properties of the group on C5 have a clear impact on the stability of the Z:P base pair, with the strong electron-withdrawing nitro group achieving the largest stabilizing effect on the H-bonding interaction and the strong electron-donating NH2 group destabilizing the Z:P pair by almost 4 kcal/mol. Finally, our gas-phase and in-water calculations confirm that the Z-nitro group reinforces the stacking interaction with its adjacent purine or pyrimidine ring.