ABINITIO STUDY OF THE FORCE-FIELD, GEOMETRY AND VIBRATIONAL ASSIGNMENT OF UREA

The optimized geometry and the complete harmonic force field of urea have been calculated at the ab initio Hartree-Fock level using 3-21G as well as 6-31G basis sets. On this basis, a thorough discussion is presented on the previously observed infrared spectra of urea and urea-d, in an argon matrix. The accuracy of the force constants was tested by calculating the N-15-isotopic shifts for the vibrational frequencies of urea and urea-d4, and the agreement between the calculated and observed shifts is remarkably good. The calculated force fields are utilized to critically examine the published experimental assignments of urea and some revisions are suggested. Evidence is presented for large frequency shifts in carbonyl and hydrogen-involving vibrations due to intermolecular hydrogen bonding in the solid state. The theoretical geometry and the ab initio force constants of urea are compared with the results of the experimental study. The theoretical infrared and Raman intensities of the normal modes together with qualitative experimental band intensities are presented. The predicted relative intensities compare well qualitatively with experiment for the matrix isolated species.