N-H stretching frequencies and the conformation of substituted ureas: An ab initio MO study

The dependence of N-H stretching-mode frequencies in representative di- and trialkyl ureas on the conformational state of the ureido group has been studied by ab initio MO calculations using HF/3-21G and HF/6-31G** basis sets. The molecules studied were 1,3-dimethylurea, 1-methyl-3,3-dimethylurea and 1 methyl-3,3-di-iso-propylurea. The principal conclusions from the ab initio results are: 1. the trans-trans conformer (N-H bonds trans to the C=O bond) has N-H stretching bands with about 20-30 cm(-1) higher frequency than the respective cis-cis structure, in accord with earlier literature assignments based on experimental data; 2. the N-H stretching frequency interval in tri-substituted ureas is 15-20 cm(-1) higher than the N-H band position in the 1,3-disubstituted molecule studied, the effect being determined mostly by the higher N-H stretching force constant; 3. in the absence of the steric hindrance the stable rotameric forms of the ureido grouping are almost planar at HF/3-21G level of calculations, while HF/6-31G**: calculations predict a slightly pyramidal structure at the nitrogen atoms in the trans-trans conformer; 4. in 1-methyl-3,3-di-iso-propylurea the steric influence of the two bulky iso-propyl groups cause a deviation from planarity of the N-H bond. The non-planar conformation is accompanied by a shift of the N-H stretching mode frequency towards higher values; and 5. the variations of the theoretically estimated N-H stretching-mode frequencies appear to be principally determined by changes in the N-H stretching force constants in the different molecules. (C) 1997 Elsevier Science B.V.