Unmasking Melon by a Complementary Approach Employing Electron Diffraction, Solid‐State NMR Spectroscopy, and Theoretical Calculations—Structural Characterization of a Carbon Nitride Polymer

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Abstract
Poly(aminoimino)heptazine, otherwise known as Liebig's melon, whose composition and structure has been subject to multitudinous speculations, was synthesized from melamine at 630 °C under the pressure of ammonia. Electron diffraction, solid‐state NMR spectroscopy, and theoretical calculations revealed that the nanocrystalline material exhibits domains well‐ordered in two dimensions, thereby allowing the structure solution in projection by electron diffraction. Melon ([C6N7(NH2)(NH)]n, plane group p2 gg, a=16.7, b=12.4 Å, γ=90°, Z=4), is composed of layers made up from infinite 1D chains of NH‐bridged melem (C6N7(NH2)3) monomers. The strands adopt a zigzag‐type geometry and are tightly linked by hydrogen bonds to give a 2D planar array. The inter‐layer distance was determined to be 3.2 Å from X‐ray powder diffraction. The presence of heptazine building blocks, as well as NH and NH2 groups was confirmed by 13C and 15N solid‐state NMR spectroscopy using 15N‐labeled melon. The degree of condensation of the heptazine core was further substantiated by a 15N direct excitation measurement. Magnetization exchange observed between all 15N nuclei using a fp‐RFDR experiment, together with the CP‐MAS data and elemental analysis, suggests that the sample is mainly homogeneous in terms of its basic composition and molecular building blocks. Semiempirical, force field, and DFT/plane wave calculations under periodic boundary conditions corroborate the structure model obtained by electron diffraction. The overall planarity of the layers is confirmed and a good agreement is obtained between the experimental and calculated NMR chemical shift parameters. The polymeric character and thermal stability of melon might render this polymer a pre‐stage of g‐C3N4 and portend its use as a promising inert material for a variety of applications in materials and surface science.