AB-INITIO CONFORMATIONAL-ANALYSIS OF THE CHELATING BIDENTATE LIGANDS ETHYLENEDIAMINE AND 1,3-PROPANEDIAMINE

We report the results of an ab initio investigation of the conformational properties of ethylenediamine and 1,3-propanediamine. We have carried out 6-31++G** optimizations of the geometries of ten ethylenediamine conformers and 4-31+G* optimizations of 25 1,3-propanediamine conformers. In the case of ethylenediamine we find the lowest energy conformers to have a gauche NCCN arrangement, stabilized by internal hydrogen bonding. In the case of 1,3-propanediamine the lowest energy conformer was also found to be stabilized by internal hydrogen bonding. Our results for ethylenediamine agree well with available experimental data. It was also found that the conformational trends in the smaller ethylenediamine molecule are still present in the larger 1,3-propanediamine molecule. The introduction of Moller-Plesset second-order electron correlation strengthens the gauche effect in both molecules.