The structure of mono- and di-amino derivatives of cyclohexane: Energetic and Natural Bond Orbital approaches

A full exploration of the conformational space of mono and di-amino derivatives of cyclohexane have been performed using the second-order Møller-Plesset approximation method (MP2) and the aug-cc-pVDZ basis set. These data were further complemented by the application of the Natural Bond Orbital (NBO) theory. The population of the equatorial conformers was found to be 84% in cyclohexylamine (CHA), varying from 94% to 99% in the cyclohexyldiamine isomers (CHDA). Based on the weighed mean energies computed for the different CHDA isomers, the following energetic sequence has been established: t12 < c12 < t14 ≈ c13 < c14 ≈ t13. The results given by the MP2 calculations were discussed in terms of pairwise steric repulsions between filled orbitals and electron delocalization from filled to vacant orbitals provided by the NBO analysis. With the exception of the vicinal isomers, the former interactions were found to be the most important factor for the conformers/isomers stabilization. Attention was also given to the role played by the syn-1,3-di-axial repulsions in the destabilization of the axial conformers of CHA. An intramolecular hydrogen bond was found to stabilize the di-axial conformers of c13CHDA.