Ab Initio Calculation of Solid-State NMR Spectra for Different Triazine and Heptazine Based Structure Proposals of g-C₃N₄

Abstract

We present a comprehensive theoretical study of the structure and NMR parameters of a large number of triazine and heptazine based structure proposals for g-C3N4 in different condensation states. This approach includes a detailed investigation of cyclic melon which tends toward the formation of densely packed hydrogen bonded meshes. In all of the investigated systems, we found planar layers to represent saddlepoints on the energy surface, whereas corrugated species were identified as minima. The corrugation source was linked to the repulsion of nitrogen lone pairs in close NN contacts. A linear dependency of the corrugation energy from the number of NN interactions in the investigated clusters was found. Heptazine based systems gain about twice as much energy per NN close contact in comparison to triazine structures which could be understood in terms of the distortion mechanism in the investigated structures. Furthermore, a full study of the 15N and 13C chemical shift tensors was performed for the different C/N layers. The description of the NMR parameters required dividing the investigated systems into subclusters for which the NMR tensors were calculated with density functional theory (DFT) methods. A statistical analysis of these entities allowed for the investigation of the change in the chemical shift upon corrugation and, in the case of the cyclic melon system, hydrogen bonding. With the here presented study, the most prominent structure models for g-C3N4 are characterized in terms of the 15N and 13C NMR parameters which now can directly be compared to experimental spectra.

Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles

“…The solid-state 13 C CP-NMR spectra (Fig. 2c) further confirms the presence of heptazine tectons in the g-C 3 N 4 solids, and the signals at approximately 155.6 and 164.3 ppm related to the formation of a poly(tri-s-triazine) structure characteristic of melem, melon and the final carbon nitride 21,36,37 . The presence of sharper lines in the NMR spectrum of the HCNS sample than the bulk g-C 3 N 4 could be due to a reduced inhomogeneous broadening of the hollow sphere samples, which would indicate an improved condensation and local organization in the thin shell structure.…”

Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides

“…The problem is that usual characterizations such as IR spectroscopy can not help to make any distinctions between both models [45]. In this way, NMR spectra have been widely recorded [35,37,43,[45][46] or predicted via ab initio calculations [47]. In the Heptazine-based model, the 13 C MAS NMR spectra are expected to present two signal groups.…”

“…In our opinion, the existence of melamine at the beginning of the decomposition mechanism unambiguously proves the existence of triazine cycles in the starting material. However, the presence of melem when starting the decomposition can be either reminiscent from the starting carbon nitride skeleton (according to pressure rules, the system decomposes in order to densify the system by minimizing the vacancies in the graphitic plane) or further formed by condensation of melamine when heated (this thermal condensation being well known [45,47]). …”

Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

“…The calculations indicate that the shape of the melon chain is determined by a competition between the energy gain by extending the p-electron system into a linear chain and the repulsion between the lone-pair electrons at the N atom on the edge of the tri-s-triazine unit, as has been discussed recently. 26,35 The linear chain is therefore strongly buckled, since the tri-s-triazine molecules tries to maximize the distance between the N atoms on the edge of adjacent molecules. It was recently proposed that the melon chain is instead constructed from dimers where the top corner is connected to a side corner, giving the melon chain a staircase shape.…”