Certain room-temperature ionic liquids exhibit hypergolic activity as liquid bipropellants. Understanding the chemical pathways and reaction mechanisms associated with hypergolic ignition is important for designing new fuels. It has been proposed (J. Phys. Chem. A 2008, 112, 7816) that an important ignition step for the hypergolic ionic liquid bipropellant system of dicyanamide/nitric acid is the activation and dissociation of the 1,5-dinitrobiuret anion DNB-. For the work reported here, a quasiclassical direct dynamics simulation, at the DFT/M05-2X level of theory, was performed to model H+ + DNB- association and the ensuing unimolecular decomposition of HDNB. This association step is 324 kcal/mol exothermic, and the most probable collision event is for H+ to directly scatter off of DNB-, without sufficient energy transfer to DNB- for H+ to associate and form a highly vibrationally excited HDNB molecule. Approximately 1/3 of the trajectories do form HDNB, which decomposes by eight different reaction paths and whose unimolecular dynamics is highly nonstatistical. Some of these paths are the same as those found in a direct dynamics simulation of the high-temperature thermal decomposition of HDNB (J. Phys. Chem. A 2011, 115, 8064), for a similar total energy.