Normal coordinates for the planar vibrations of urea using 15N and 18O frequency shift data

Abstract

A force constant refinement calculation to the observed planar vibration frequencies of OC(NH 2 ) 2 and OC(ND 2 ) 2 and simultaneously to the observed frequency shift data of 18 OC(NH 2 ) 2 and OC( 15 NH 2 ) 2 has enabled the normal coordinates for the planar vibrations of urea to be determined with some degree of precision in terms of twenty-one independent force constants. The accuracy of the determined force field is tested by its ability to reproduce the observed 15 N frequency shifts of OC( 14 NH 2 )( 15 NH 2 ), the lower symmetry of which causes the A 1 and B 1 species vibrations of the symmetric molecule to combine into one symmetry species containing all thirteen vibrations. The observed 15 N frequency shift on the weak absorption at 1066 cm −1 , coupled with the calculations of this paper, confirm that this band arises from the B 1 species NH 2 rocking mode, as originally assigned by Waldron and by Stewart, but reassigned by Yamaguchi et al . The corresponding absorption in urea- D 4 is observed at 850 cm −1 , in excellent agreement with the product rule.