Intrinsic non-RRK behavior: Classical trajectory, statistical theory, and diffusional theory studies of a unimolecular reaction

The nonstatistical behavior of a unimolecular reaction at energies well in excess of the threshold is examined. This behavior is sometimes referred to as ''intrinsically non-Rice-Ramsperger-Kassel-Marcus'' (RRKM). It is well known that microcanonical unimolecular rates computed by using classical mechanics can deviate from the predictions of statistical theories, particularly at high energies. The simplest manifestation of this behavior is that rate constants as a function of energy cannot be represented by simple expressions such as the RRK equation, k(E) = nu(1-E*/E)(s-1) with a single set of parameter values over a wide energy range; more specifically, fits of the classical RRK expression to trajectory results frequently yield values for the effective number of degrees of freedom s that an significantly smaller than the ''theoretical'' values 3N-6. In the present study, rates were calculated for the unimolecular dissociation of dimethylnitramine, (CH3)(2)NNO2, by simple N-N bond rupture over wide energy ranges by using classical trajectories and Monte Carlo transition-state theory. The formalism of a diffusional theory of chemical reactions is used to develop a model that relates classical reaction rates to intramolecular vibrational energy redistribution (IVR). This model is based on the assumption that the molecular modes can be separated into reaction coordinate and energy reservoir modes. It is shown how this model can be used to extrapolate high-energy, nonstatistical classical trajectory rates to the low-energy, statistical region. (C) 1996 American Institute of Physics.