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 wel l in excess of the threshold is examined. This behavior is sometimes r eferred to as ''intrinsically non-Rice-Ramsperger-Kassel-Marcus'' (RRK M). It is well known that microcanonical unimolecular rates computed b y using classical mechanics can deviate from the predictions of statis tical theories, particularly at high energies. The simplest manifestat ion of this behavior is that rate constants as a function of energy ca nnot 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 numbe r of degrees of freedom s that an significantly smaller than the ''the oretical'' values 3N-6. In the present study, rates were calculated fo r the unimolecular dissociation of dimethylnitramine, (CH3)(2)NNO2, by simple N-N bond rupture over wide energy ranges by using classical tr ajectories and Monte Carlo transition-state theory. The formalism of a diffusional theory of chemical reactions is used to develop a model t hat relates classical reaction rates to intramolecular vibrational ene rgy redistribution (IVR). This model is based on the assumption that t he molecular modes can be separated into reaction coordinate and energ y reservoir modes. It is shown how this model can be used to extrapola te high-energy, nonstatistical classical trajectory rates to the low-e nergy, statistical region. (C) 1996 American Institute of Physics.