Department of chemistry, Ferdowsi University, Mashhad, I.R. IRAN
On the basis of 102 assigned frequencies to in plane vibrations of pyrimidine, pyrimidine-2-d2, pyrimidine-2, 5-d2, pyrimidine-4,6-d2, pyrimidine-2,4,6-d3 and pyrimidine-d4, 35 valance force field constants including 14 principals and 21 interaction force constants are calculated. New assignments are suggested for n18b and n19b normal modes in pyrimidine-2,5-d2 and n14 and n18b normal modes in pyrimidine-d4. With this new assignments the agreement between observed and calculated product rule and sum rule has improved.