Insight into the acidic group-induced nitration mechanism of 2-methyl-4,6-dihydroxypyrimidine (MDP) with nitronium

Abstract

The strong desire for developing promising candidates of insensitive nitro energetic materials has spurred numerous attempts to discern the nitration details. The nitration mechanism of 2-methyl-4,6-dihydroxypyrimidine (MDP) with nitronium (NO₂⁺) to form 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione is studied via DFT-B3LYP/6-311G(d,p) method. The possible nitration pathways are excavated and illustrated. Herein, a unique/incredible induction/enhancement of the co-existing acidic group of HSO₄⁻ to the targeted nitration is definitively proposed. The impact of the introduction order of four nitro groups (–NO₂) on the title nitration is systematically demonstrated. It is suggested that the proposed induction of HSO₄⁻ may effectively promote/catalyze not only the NO₂⁺ attack, but the H-transfer and H-abstraction as well, and thus dramatically decreases the activation free energy of the nitration system. Moreover, the NO₂⁺ attack may be dynamically affected by the pre-introduced –NO₂ and the resulted fluctuation of charge distribution in the pre-intermediates, which has been primarily supported via the calculated atomic charge, Coulomb attraction and Fukui function. It is indicated that the preferential dinitration on –CH₃ with the induction of HSO₄⁻ (path A) is the most likely pathway. It is optimistically expected that the present study may provide a theoretical basis to the research and engineering tests of the title nitration, and promote the exploration of related insensitive energetic materials.