Decoding orbital interactions in H-bonded melamine networks by combining electron spectroscopy and DFT

The effects of hydrogen bonding (HB) on the electronic structure of melamine films are investigated by analyzing the electronic states of hydrogen-bonded aggregates in terms of the molecular orbitals (MOs) of the isolated monomer. This approach provides an orbital description of HB directly connected to the spectroscopic signatures observed in valence-level photoemission spectroscopy (VL-PES) and N 1s near edge X-ray absorption fine structure spectroscopy (NEXAFS). The orbital analysis reveals that the hydrogen bond in melamine is described by the interaction between the triazine nitrogen lone pairs (N_lp) and occupied σ(N-H) orbitals, leading to the formation of bonding and antibonding HB states. The antibonding states acquire additional stabilization through a small admixture of the unoccupied σ*(N-H) orbitals of the monomer. Hydrogen bonding is expected to induce splittings of states with lone-pair and σ(N-H) character. In the valence photoemission spectrum, however, these splittings are too small to be resolved directly. In contrast, clear fingerprints of hydrogen bonding emerge in the N 1s NEXAFS spectrum. The MOs derived from the monomer LUMO+1 and LUMO+2, both characterized by σ*(N-H) character, undergo substantial modifications upon hydrogen-bond formation, including orbital mixing, changes in spatial localization, and a redistribution of spectral intensity over a broad photon-energy range. These effects lead to measurable shifts and intensity changes in the NEXAFS resonances, providing direct spectroscopic evidence of the hydrogen-bond-induced perturbation of the electronic structure and supporting the molecular-orbital description of hydrogen bonding in a complex organic supramolecular network.