In the present paper we perform direct molecular simulations (DMS) in a constant-volume reactor under adiabatic conditions. Unlike in previous DMS studies, where the heat bath was kept at a constant translational temperature, in the present case no energy is added to-or removed from the system between collision steps. For our trajectory calculations we use the ab inito potential energy surface (PES) for N2-N2 and N2-N interactions developed by the computational chemistry group at the University of Minnesota. In addition to verifying that our routines conserve overall momentum and energy during the entire simulation, we examine rotational and vibrational energy distributions of the N2-molecules while internal energy exchange and dissociation processes occur in the gas. Our immediate goal is to assess whether higher-lying rovibrational levels are depleted during dissociation and to which degree this depletion is balanced out by excitation from lower-lying levels. Such a quasi-steady state (QSS) state has been observed in isothermal heat bath simulations, both in master equation studies performed by other groups, as well as our own DMS simulations. In our current study we confirm that a transient QSS-like condition is established when the reactor operates at constant energy.