IR SPECTROSCOPY AND THEORETICAL VIBRATIONAL CALCULATION OF THE MELAMINE MOLECULE

We have observed the IR spectra of the melamine molecule and its deute riated counterpart in the gas phase at ca. 150 degrees C and in a soli d argon-matrix at 10 K. The assignment of the vibrations of melamine h as been facilitated by the calculated thirty nine normal modes using s everal ab initio and density functional methods. By scaling the calcul ated vibrational frequencies, the theoretical computations have been d emonstrated 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 a mino groups, which result in different conformers. This has been suppo rted 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 conformer s 1 and 2 and the 'transition state' 3 (corresponding to a third-order saddle point on the potential-energy hypersurface), the melamine mole cule has a flat potential-energy hypersurface near the equilibrium str uctures and the conformers can rapidly rearrange.