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

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 degrees 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 ([C(6)N(7)(NH(2))(NH)](n), plane group p2 gg, a=16.7, b=12.4 A, gamma=90 degrees, Z=4), is composed of layers made up from infinite 1D chains of NH-bridged melem (C(6)N(7)(NH(2))(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 A from X-ray powder diffraction. The presence of heptazine building blocks, as well as NH and NH(2) groups was confirmed by (13)C and (15)N solid-state NMR spectroscopy using (15)N-labeled melon. The degree of condensation of the heptazine core was further substantiated by a (15)N direct excitation measurement. Magnetization exchange observed between all (15)N 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-C(3)N(4) and portend its use as a promising inert material for a variety of applications in materials and surface science.

Tackling the stacking disorder of melon—structure elucidation in a semicrystalline material

“…3,8 Since most synthetic routes invoke the pyrolysis of molecular precursors, [9][10][11][12] moreover, the incorporation of additional hetero atoms or hydrogen can hardly be prevented. This was demonstrated recently by Thomas et al who reported on the synthesis of a microcrystalline, largely hydrogen free graphitic carbon nitride-type material by ionothermal treatment of dicyandiamide.…”

“…X-ray powder patterns usually indicate a pronounced stacking disorder of neighboring layers with an interlayer distance amounting to about 3.2 Å . 4,9,13,23,42 Besides X-ray diffraction techniques, 15,[27][28][29][30][31]34,40,[42][43][44] infrared (IR) and Raman spectroscopy 15,20,27,28,32,33,40,45 as well as X-ray photoelectron spectroscopy (XPS) 16,19,26,29,30 are the most commonly employed characterization techniques. As these mostly probe the local environment rather than intermediate or long range properties, no information about the stacking disorder of melon-type materials have been obtained.…”

“…46 As a case study we used Liebig's melon, 47 which is a 1D carbon amide imide nitride polymer with an ideal composition C 6 N 9 H 3 . 9 The nanocrystalline material was recently investigated by X-ray and electron diffraction, solid-state NMR spectroscopy, and theoretical calculations. 9 Melon consists of a layer structure well-ordered in two dimensions, but with a pronounced stacking disorder in the third.…”

“…9 Melon consists of a layer structure well-ordered in two dimensions, but with a pronounced stacking disorder in the third. The structure of the layers was solved by projection from electron diffraction data (plane group p2gg with a = 16.7 Å , b = 12.4 Å , g = 901 and Z = 4) 9 and is shown in Fig. 1.…”

“…The missing cone-like smearing in the hk0 plane implies disorder originating from translational planar defects rather than rotational ones. 9 Therefore, energetically similar stacking modes with various, yet not statistically distributed translational layer offsets are likely to contribute to the overall 3D structure.…”