Catalytic effect of water, water dimer, HCOOH and H₂SO₄ on the isomerisation of HON(O)NNO₂ to ON(OH)NNO₂: a mechanism study

In this article, we report a theoretical investigation on the role of several catalysts in the isomerisation mechanisms of HON(O)NNO2 to ON(OH)NNO2 by theoretical method of CBS-QB3. The isomerisation reactions with catalyst X (X = H2O, (H2O)2, HCOOH and H2SO4) are multi-hydrogen atom transfer reactions. Compared to the isomerisation mechanisms and rate constant of HON(O)NNO2 to ON(OH)NNO2 without catalysts, incorporation of the catalyst X shows different positive catalytic effects on affecting the reaction processes, with the H2SO4-assisted reaction being the most favourable. Such different catalytic effects are mainly related to the size of the ring structure in X-assisted transition states and the different values of pKa and proton affinities for HCOOH and H2SO4. Besides, compared with the barrier height of the isomerisation process from HON(O)NNO2 to ON(OH)NNO2 with HN(NO2)2 and HON(O)NNO2, the barrier of H2SO4-assisted reaction is lower by 9.3 and 4.5 kcal ·mol−1, meanwhile, the rate constant of H2SO4 catalyzed is larger than water and water dimer–assisted by 3–5 and 2–3 orders of magnitude, respectively. So, H2SO4-assisted reaction is the most favourable.