Spectroscopic Fingerprints of Intermolecular H‐Bonding Interactions in Carbon Nitride Model Compounds

The effect of intermolecular H‐bonding interactions on the local electronic structure of N‐containing functional groups (amino group and pyridine‐like N) that are characteristic of polymeric carbon nitride materials p‐CN(H), a new class of metal‐free organophotocatalysts, was investigated. Specifically, the melamine molecule, a building block of p‐CN(H), was characterized by X‐ray photoelectron (XPS) and near edge X‐ray absorption fine structure (NEXAFS) spectroscopy. The molecule was studied as a noninteracting system in the gas phase and in the solid state within a H‐bonded network. With the support of DFT simulations of the spectra, it was found that the H‐bonds mainly affect the N 1s level of the amino group, leaving the N 1s level of the pyridine‐like N mostly unperturbed. This is responsible for a reduction of the chemical shift between the two XPS N 1s levels relative to free melamine. Consequently, N K‐edge NEXAFS resonances involving the amino N 1s level also shift to lower photon energies. Moreover, the solid‐state absorption spectra showed significant modification/quenching of resonances related to transitions from the amino N 1s level to σ* orbitals involving the NH 2 termini.