Unimolecular dissociation of NO2. I. Classical trajectory and statistical calculations on a global potential energy surface

The title reaction is examined with classical and statistical (RRKM, SACM) mechanics on an ab initio three-dimensional global potential energy surface of the lowest adiabatic electronic state of NO2. The energy-resolved rate constants obtained within different theoretical frameworks are compared wit h each other and with the available experimental data. The question of the transition state definition in this barrierless reaction is analyzed and th e applicability of the statistical hypothesis is discussed. Although the ra te of internal vibrational energy redistribution is estimated to be larger than 5 ps(-1) at the quantum dissociation threshold, which is much larger t han the dissociation rate, a substantial fraction of classical trajectories remains regular. This causes statistical transition-state-like theories to overestimate the actual dissociation rate defined from the full classical dynamics calculations. (C) 1999 American Institute of Physics. [S0021-9606( 99)01425-7].