Crystal Structure and Spectroscopic Analysis of Melamine Hydrobromide. Evidence for iso-Melamine Cations and Charge-Transfer Complexes in the Solid State

The crystal structure and vibrational analyses of melamine hydrobromide by FTIR and FT- Raman spectra, are presented. The crystallographic data show π-electron delocalization towards the amino substituents with ring nitrogen protonation in the solid state. Additionally, the presence of intermolecular hydrogen bonding interactions occurring between amino substituents and ring nitrogen lone-pairs gives rise to charge-transfer complexes, as demonstrated by UV-VIS reflectance in the solid state. FTIR and FT-Raman spectra allow the assignments of the vibrational modes in melamine hydrobromide by comparison with the corresponding deuteriated molecules, and show the existence of iso-melamine cations in the solid state.