Theoretical study of isomerism in protonated forms of N₂O, NPO, and P₂O

Abstract

The geometric parameters of the isomers HN₂O⁺, HPNO⁺, and HP₂O⁺ were calculated by the nonempirical SCF/3-21G* method and their relative energies were determined with consideration of the electronic correlation in the MP3/DEHD + PS approximation. According to the calculations, protonation of N₂O, PNO, and P₂O molecules should preferably take place at the oxygen atom. Isomers with a quasilinear NNO and PNO backbone are most advantageous in HN₂O⁺ and HPNO⁺, and cyclic isomers are ∼60 and ∼30 kcal/mole less stable, respectively. On the contrary, the cyclic form is more stable for HPO ⁺₂ (by ∼10 kcal/mole). The bond at the attacked atom usually weakens (breaks) and the neighboring (opposite) bonds are strengthened in protonation. Protonation of P₂O stabilizes the cyclic isomer by 15 kcal/mole more strongly than the "open" isomer, resulting in inversion of their position on the energy scale. In the case of N₂O and PNO, the relative position of the cyclic and basic isomers virtually does not change, but the linear NPO isomer is destabliized. The stability of the cyclic isomers in comparison to the "open" isomers increases on substitution of N atoms by P atoms in both molecules of N₂O, PNO, and P₂O and in their ions HN₂O⁺, HPNO⁺, and HP₂O⁺. This tendency probably holds in subsequent transition to As and Sb atoms.