N–H stretching frequencies and the conformation of substituted ureas: an ab initio MO study

Abstract

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. © 1997 Elsevier Science B.V.