AB-INITIO STUDIES OF THE CONFORMATIONS OF METHYLAMINE AND ETHYLENEDIAMINE - INTERACTION FORCES AFFECTING THE STRUCTURAL STABILITY

The structures and conformational energies of methylamine and ethylene diamine have been studied extensively with ab initio molecular orbital theory. For methylamine, there are good linear relationships among th e C-N bond length, cosine of the H-C-N-H dihedral angle, and conformat ional energy. For ethylenediamine which has numerous multiminima, we s tudied the stereoelectronic effect, steric effect, intramolecular hydr ogen bonding, repulsion between hydrogens, and repulsion between lone- pair electrons. The major factor determining the conformational stabil ities of the multiminima is found to be the stereoelectronic effect. T he partial hydrogen bonding, though weak, contributes to the structura l stability so that two isoenergetic gauche conformers become the lowe st energy structures.