Computational methods for conformational analysis of unsymmetrical 1,3-diamines: 3-aminotropanes

Ab Initio and Molecular Mechanics (MM3) Calculations of Protonated−Neutral Diamine Hydrogen Bonds

“…For example, consider the relative energies of β 2g and β 2a , where the structural difference is simply between an axial or equatorial N -methyl group. The energy of the axial N -methyl conformation is approximately 1 kcal/mol greater than the equatorial conformation, which is precisely in line with other observations …”

“…In the MM3 energy breakdown, a significant portion of the boat stabilization is due to internal hydrogen bonding. As discussed in previous work, however, the Hartree−Fock structural data are inconsistent with an internal hydrogen bond, leading to speculation why the boat was favored in ab initio calculations. In addition to the hydrogen bond, the boat is stabilized approximately 8 kcal/mol in the charge−dipole term.…”

“…On the other hand, the MM3(2000) result suggests that the boat/chair difference is only around 0.6 kcal/mol. The small MM3-calculated relative energy difference is striking, especially considering that there is no opportunity for internal hydrogen bonding . The result may be due to a difference in the charge−dipole term of the chair and boat energies; this term is actually about 1 kcal/mol lower in the boat form.…”

“…In previously reported work, we performed a conformational analysis of 3α- and 3β-aminotropanes, 1,3-unsymmetrical diamines, with a variety of computational methods . It was determined that the MM3 force field, where a dielectric continuum of 80 was use to mimic water solvent, did a superior job of reproducing the NMR- determined conformational stabilities of these molecules in dilute aqueous media.…”