IR spectroscopy and theoretical vibrational calculation of the melamine molecule

We have observed the IR spectra of the melamine molecule and its deuteriated counterpart in the gas phase at ca. 150 degrees C and in a solid argon-matrix at 10 K. The assignment of the vibrations of melamine has been facilitated by the calculated thirty nine normal modes using several ab initio and density functional methods. By scaling the calculated vibrational frequencies, the theoretical computations have been demonstrated to be in good agreement with the experimental observations. The optimized equilibrium structure of melamine has been shown to be a planar but distorted-hexagonal triazine ring with three pyramidal amino groups, which result in different conformers. This has been supported by the comparison between the observed and the calculated spectra for non-planar conformers 1 and 2 vs. the planar D-3h structure 3. in view of the small energy differences between the calculated conformers 1 and 2 and the 'transition state' 3 (corresponding to a third-order saddle point on the potential-energy hypersurface), the melamine molecule has a flat potential-energy hypersurface near the equilibrium structures and the conformers can rapidly rearrange.