Synchrotron X‐ray Electron Density Analysis of Chemical Bonding in the Graphitic Carbon Nitride Precursor Melamine. Issue 54 (3rd August 2022)

• Record Type: Journal Article
• Authors: Vosegaard, Emilie S.; Thomsen, Maja K.; Krause, Lennard; Grønbech, Thomas B. E.; Mamakhel, Aref; Takahashi, Seiya; Nishibori, Eiji; Iversen, Bo B.
• Is Part Of: Chemistry. Volume 28:Issue 54(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-03
• Subjects: chemical bonding -- electron density -- graphitic carbon nitride -- melamine -- synchrotron X-ray diffraction; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202201295
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Melamine is a precursor and building block for graphitic carbon nitride (g‐CN) materials, a group of layered materials showing great promise for catalytic applications. The synthetic pathway to g‐CN includes a polycondensation reaction of melamine by evaporation of ammonia. Melamine molecules in the crystal organize into wave‐like planes with an interlayer distance of 3.3 Å similar to that of g‐CN. Here we present an extensive investigation of the experimental electron density of melamine obtained from modelling of synchrotron radiation X‐ray single‐crystal diffraction data measured at 25 K with special focus on the molecular geometry and intermolecular interactions. Both intra‐ and interlayer structures are dominated by hydrogen bonding and π‐interactions. Theoretical gas‐phase optimizations of the experimental molecular geometry show that bond lengths and angles for atoms in the same chemical environment (C−N bonds in the ring, amine groups) differ significantly more for the experimental geometry than for the gas‐phase‐optimized geometries, indicating that intermolecular interactions in the crystal affects the molecular geometry. In the experimental crystal geometry, one amine group has significantly more sp 3 ‐like character than the others, hinting at a possible formation mechanism of g‐CN. Topological analysis and energy frameworks show that the nitrogen atom in this amine group participates in weak intralayer hydrogen bonding. We hypothesize that melamine condenses to g‐CN within the layers and that the unique amine group plays a key role in the condensation process. Abstract : Odd one out : Melamine is the precursor for the polycondensation reaction to graphitic carbon nitride materials with a range of important applications. It is found that one amine group in melamine is significantly different from the others, and based on extensive analysis of the experimental and theoretical electron density this particular amine group is suggested to be the origin of the condensation process.