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

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 observe d IR and Raman active vibrations of s-triazine and the inactive triazine v( 4) and v(5) modes are assigned based on analysis of gas phase IR and conden sed phase Raman combination bands. We use a standing wave description of th e 12 elementary vibrations of a six-membered ring to aid analysis of the co mplex ring vibrations of triazine and melamine. Vibrational coupling of the se ring modes with the rocking and bending vibrations involving the triazin e hydrogen atoms are discussed. Furthermore, we identify several ring modes where decreased vibrational coupling with the substituents explains why ce rtain bands make good group frequencies in related triazine derivatives. (C ) 1999 Elsevier Science B.V. All rights reserved.