Proton transfer in an ammonia-nitric acid system is studied by ab initio calculations in this work. Proton transfer does not occur in the globalminimum structure of a single ammonium nitrate unit (monomer). The compulsory occurrence of proton transfer in the monomer causes the formation of a large dipole moment and the total energy is destabilized. However, with the application of an external electric field, proton transfer spontaneously occurs in the monomer; moreover, the total energy becomes lower than that of the global-minimum structure owing to the dipole- electric field interaction energy. In two ammonium nitrate units (dimer), proton transfer does occur to enhance internal dipole moments. On the basis of these results, it is revealed that the dipole-dipole interaction between the monomers in the dimer not only promotes the occurrence of proton transfer but also contributes to the stabilization of the total energy of the system.
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