Force field for planar vibrations of urea: use of CNDO/Force MO calculations

Abstract

Symmetry force field calculations have been performed for the planar vibrations of urea using CNDO/Force method. The CNDO/Force calculations predict well the signs and the magnitudes of bending and interaction force constants; the stretching force constants are found to be higher in magnitude. The bending and interaction constants obtained from these calculations and the stretching force constants obtained from the literature are considered for the initial force field. Using the observed frequencies for urea and its isotopic analogues, urea-D₄, urea-¹⁵N₂, urea-¹⁵N₂D₄ and urea-¹⁸O in the solid as well as in the solution phases, the force field is refined by carrying out iterations over the diagonal force constants. In the final stages of the refinement iterations are carried out over all the force constants keeping the signs of the interaction constants unchanged. It is found that the agreement between the calculated and the observed frequencies is excellent. The final force fields in terms of symmetry as well as redundancy free internal valence coordinates are reported. On the basis of the potential energy distribution the vibrational assignments are discussed