Normal co-ordinates for the planar vibrations of thiourea, and frequency assignment for selenourea

Abstract

A normal co-ordinate analysis of the planar vibrations of thiourea has been carried out, with reference to the results obtained for urea using ¹⁸O and ¹⁵N frequency shift data in addition to ¹H and ²H frequency data. The 21-parameter force field obtained is very similar to that for urea, or follows predictable trends for the co-ordinates involving the CS bond. The resulting normal co-ordinates are considered to be reliably determined, and are given in terms of the potential energy distribution in Table 3. By extending the calculations to [¹H₄]- and [²H₄]-selenourea, an assignment of the planar vibrations is made. It seems that the vibration band at 735 cm^–1 is due to an out-of-plane vibration and does not belong to the A₁ species, as has been suggested by analogy with the almost coincident vibration band in thiourea. Our calculations favour assignment of the A₁ species vibration to the band at 640 cm^–1. This assignment is supported by the Raman spectrum of [¹H₄]selenourea.