On the Relationship between Ring Strain Energies and 'atoms-in-molecules' Properties in N2P2 Rings

[eng] Ring strain energies of several N2P2 rings have been computed using the B3LYP/6-311+G⁄ level of theory. We have studied if the use of the Bader's theory of 'atoms-in-molecules' and particularly the properties at ring critical points is an adequate methodology to measure the ring strain. In particular, we analyze the suitability of the utilization of the electron density computed at the ring critical point to estimate the ring strain energies in biradicaloid and closed-shell N2P2 heterocycles. Those rings having a P@P double bond have very small ring strain energies and have been studied using NBO analysis