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Graphitic (g)-C_xN_yH_z has become a popular element in photocatalytic water splitting cells. Recently, the crystal structures of completely polymerized g-C₃N₄ and incompletely polymerized g-C₆N₉H₃ crystals based on the triazine and heptazine monomers have been characterized. In this manuscript we evaluate the atomic and electronic nature of these structures using DFT. The study revealed strongly corrugated structures for the fully polymerized g-C₃N₄ and planar structures for the incompletely polymerized g-C₆N₉H₃. The exciton binding energies of the heptazine-based structures are lower than that of their triazine-based analogues and lower for the completely polymerized structures than their incompletely polymerized analogues. The rather low dielectric constant and charge mobilities result in high exciton binding energies and hence low dissociation probabilities for these excitons. This confirms the …