Nonthermally activated systems

Distribution Functions and Hydrogen-Deuterium Isotope Effects in Nonthermal Activation Systems. In Sec. II-D, hydrogen-deuterium isotopic rate ratios for monoenergetic systems were discussed. In practice, the measured effects are ratios modified by the energy distribution functions and should be compared to kan/kaD rather than to k,n/ktn. A s appropriate for the system under investigation, one of eqs. (19)-(22) is written for each of the isotopic species and a ratio, kttn/kaD, is thus constructed for comparison of isotope effects. These need not be listed in detail. It should be noted that the distribution function for the normal and isotopically substituted systems will usually be somewhat different (Fig. SB). 

Activated reactions take several different forms. In thermally activated systems, most of the reactions that have been studied via molecular dynamics are isomerizations, dissociations/recombinations, simple bimolecular reactions (such as atom replacement), or electron transfer (which we shall not review here). The simulation of activation in these systems requires techniques different from those for nonthermally activated systems, of which the most common class studied (in a variety of condensed phases) is photodissociation. 

How do chemical reactions happen in solution What are the microscopic processes that lead to a thermal reaction system s climbing a barrier, reaching a transition state, choosing whether or not to go on to products, and then coming back down to equilibrium How does nonthermal activation, such as in photodissociation, change this picture and what other processes are important in such reactions How can we look in detail at these processes, both on the appropriate (angstrom) length scale and (femtosecond/picosecond) time scale ... 

Our primary interest in this review is in those calculations where the dynamics of all atoms, reactant and solvent, are followed in full detail. One will quickly see that the number of studies that have done this is smaller than might have been expected, given the notable increase in computational power since the first simulation of Bunker and Jacobson. We will review both thermally and nonthermally activated reaction systems in solvents as simple as Lennard-Jones models of rare gases to those as complex as fully flexible models of liquid water. 

The velocity side of the initial conditions can be handled by choosing the velocities in all the coordinates from the Boltzmann distribution at the appropriate temperature. For some nonthermally activated processes, for example, overtone-induced dissociations or vibrational excitation, the energy can be placed into the system at this time by adjusting the velocity, and hence the kinetic energy, of the appropriate coordinate. 

A few years ago the concept considered was introduced also in the low-temperature chemistry of the solid.31 Benderskii et al. have employed the idea of self-activation of a matrix due to the feedback between the chemical reaction and the state of stress in the frozen sample to explain the so called explosion during cooling observed by them in the photolyzed MCH + Cl2 system. The model proposed in refs. 31,48,49 is unfortunately not quite concrete, because it includes an abstract quantity called by the authors the excess free energy of internal stresses. No means of measuring this quantity or estimating its numerical values are proposed. Neither do the authors discuss the connection between this characteristic and the imperfections of a solid matrix. Moreover, they have to introduce into the model a heat-balance equation to specify the feedback, although they proceed from the nonthermal mechanisms of selfactivation of reactants at low temperatures. Nevertheless, the essence of their concept is clear and can be formulated phenomenologically as follows the... 

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