The form of the normal modes of s-triazine: infrared and Raman spectral analysis and ab initio force field calculations

Abstract

In this study, ab initio force field calculations with 6-31G basis sets are used in a comprehensive examination of the Cartesian displacement vectors of the normal modes of s-triazine. Assignments are verified for the observed IR and Raman active vibrations of s-triazine and the inactive triazine ν 4 and ν 5 modes are assigned based on analysis of gas phase IR and condensed phase Raman combination bands. We use a standing wave description of the 12 elementary vibrations of a six-membered ring to aid analysis of the complex ring vibrations of triazine and melamine. Vibrational coupling of these ring modes with the rocking and bending vibrations involving the triazine hydrogen atoms are discussed. Furthermore, we identify several ring modes where decreased vibrational coupling with the substituents explains why certain bands make good group frequencies in related triazine derivatives.