On the electronic origin of strain energy: QTAIM study of perfluorocycloalkanes

The strain energies (SE) of the five smallest perfluorocycloalkanes (c-CnF2n; n=3, 4, 5, 6, and 7) were calculated by means of several homodesmotic processes using B3LYP/6-31++G(d,p) optimized molecular energies. These values were compared with the energy difference between the linear and cyclic CF2 groups calculated by means of the quantum theory of atoms in molecules (QTAIM) applied on charge densities obtained at the same computational level. The differences between the values computed with both methods vary from 255 (n=3) to 629 (n=7) kJ mol-1. These differences arise because QTAIM-computed SE contain the energy involved in opening the ring to give rise to nearly transferable central CF2 fragments of linear perfluoroalkanes, whereas homodesmotic energies contain energy terms corresponding to transformation of nontransferable linear CF2 fragments and a ring-opening energy, which depending on the process, transforms CF2 cyclic compounds into nearly transferable or nontransferable linear CF2 groups.