4-Oxo- or 1-oxo-N7O+? A computational and experimental study

Abstract

In a previous paper [Inorg. Chem., 2010, 49, 1245], we studied the reaction of F₂NO⁺ with an excess of HN₃ which led to the quantitative formation of N₅⁺ and N₂O. Based on ¹⁵N-labeling experiments and theoretical calculations, the formation of a 4-oxo-N₇O⁺ intermediate with a decomposition energy barrier of about 40 kcal mol⁻¹ was proposed. Since this relatively high barrier disagreed with our failure to experimentally observe this cation, a thorough theoretical study of the isomerization, dissociation and formation pathways of N₄FO⁺ and N₇O⁺ was carried out at the B3LYP and G3B3 levels at 240 K. It was found that the self-decomposition of 4-oxo-N₇O⁺ to NO⁺ and N₂ has a considerably lower barrier of only 19.6 kcal mol⁻¹ and, therefore, would be more likely than a self-decomposition to N₅⁺ and N₂O. Additional calculations also showed that alternate reaction pathways between the stable and well-characterized z-N₄FO⁺ intermediate product and HN₃ involving 7- or 9-membered cyclic transition states, can lead to the observed N₅⁺ and N₂O products with the observed ¹⁵N distribution and barriers as low as 20.7 kcal mol⁻¹. The transition states for these reactions contain a 1-oxo-N₇O⁺ component which can decompose without a barrier to N₅⁺ and N₂O. These alternative pathways involving an unstable 1-oxo-N₇O⁺ cation are in better agreement with experiment than the one involving 4-oxo-N₇O⁺. The correctness of this re-interpretation was experimentally verified by a ¹⁵N-labeling experiment between α- and γ-¹⁵N-labeled HN₃ and unlabeled N₄FO⁺ which resulted exclusively in unlabeled N₂O and α- and γ-¹⁵N-labeled N₅⁺. Therefore, we conclude that in the reaction of NF₂O⁺ with excess HN₃ the experimental and theoretical evidence supports only the formation of an unstable 1-oxo-N₇O⁺ cation, and that for the preparation of the symmetric 4-oxo-N₇O⁺ cation different synthetic approaches will be required.