Urea and Urea Nitrate Decomposition Pathways:  A Quantum Chemistry Study

Electronic structure calculations have been performed to investigate the initial steps in the gas-phase decomposition of urea and urea nitrate. The most favorable decomposition pathway for an isolated urea molecule leads to HNCO and NH₃. Gaseous urea nitrate formed by the association of urea and HNO₃ has two isomeric forms, both of which are acid−base complexes stabilized by the hydrogen-bonding interactions involving the acidic proton of HNO₃ and either the O or N atoms of urea, with binding energies (D₀°, calculated at the G2M level with BSSE correction) of 13.7 and 8.3 kcal/mol, respectively, and with estimated standard enthalpies of formation (Δ_fH₂₉₈°) of −102.3 and −97.1 kcal/mol, respectively. Both isomers can undergo relatively facile double proton transfer within cyclic hydrogen-bonded structures. In both cases, HNO₃ plays a catalytic role for the (1,3) H-shifts in urea by acting as a donor of the first and an acceptor of the second protons transferred in a relay fashion. The double proton transfer in the carbonyl/hydrogen bond complex mediates the keto−enol tautomerization of urea, and in the other complex the result is the breakdown of the urea part to the HNCO and NH₃ fragments. The enolic form of urea is not expected to accumulate in significant quantities due to its very fast conversion back to H₂NC(O)NH₂ which is barrierless in the presence of HNO₃. The HNO₃-catalyzed breakdown of urea to HNCO and NH₃ is predicted to be the most favorable decomposition pathway for gaseous urea nitrate. Thus, HNCO + NH₃ + HNO₃ and their association products (e.g., ammonium nitrate and isocyanate) are expected to be the major initial products of the urea nitrate decomposition. This prediction is consistent with the experimental T-jump/FTIR data [Hiyoshi et al. 12th Int. Detonation Symp., Aug 11−16, San Diego, CA, 2002].