Nonthermally Activated Reactions

Nonthermally activated reactions in condensed phases, such as photodissociation processes, have been a popular area of study via computer simulation for several reasons. In part these reasons are historical The first computer simulations of reactions in liquids were performed on Ij photodissociation in carbon tetrachloride and in dense rare gases. Almost concurrently with these early simulations, the first experiments on this process with picosecond resolution were being performed.Photodissociation makes for easy comparison between simulation and experiment, because the experimental zero of time, namely the excitation to an upper electronic state, can be easily duplicated in the simulation. As we shall sketch out in the first part of this section, the use of molecular dynamics in helping to understand and explain the I2 

The dynamics of photodissociation in condensed phases is clearly different from that in the gas phase. The early simulations of these processes was 

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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. 

One cannot divorce the computational studies from all that has been done in analytic theory or in experiment (much of which predates the significant increase in the number of computational studies that occurred in the mid-1980s). We will therefore discuss some aspects of the analytic theories that shed light on the interaction between theory and simulation. A number of reviews have concentrated on analytic theories of chemical reactions and reaction rates in solution. In particular, we commend to the reader those of Hynes, Berne et al., and Hanggi et al. These reviews usually contain some discussion of computer simulations. However, here we reverse the priority and concentrate primarily on simulation. In addition, we will describe much of the work that has been done on how reactions climb barriers and what happens as they come off a barrier and return to equilibrium (or in the case of nonthermally activated reactions, how the energy placed into the reaction coordinate by outside means is dissipated into the solvent). Some of these areas have recently been discussed in a review by Ohmine and Sasai of the computer simulation of the dynamics of liquid water and this solvent s effect on chemical reactions. 

Chapter 3 by Robert M. Whitnell and Kent R. Wilson extends some of the concepts delineated in Chapter 2. The chapter on computational molecular dynamics of chemical reactions in solution is a definitive, long-awaited bridge between the organic and chemical physics communities. Techniques for simulating reaction dynamics are covered in nonmathematical language. Work on thermally activated reactions, such as isomerization, atom exchange, 5 2, and S l reactions, as well as ion-pair association, and proton transfers, are reviewed. For nonthermally activated reactions, a variety of photodissociations and isomerizations are discussed. The interplay of computer simulations of solution reaction dynamics and models of the reactions is explained. 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

It is well known that a wide variety of organic reactions are accelerated substantially by microwave irradiation in sealed tubes. These rate enhancements can be attributed to superheating of the solvent, because of the increased pressure generated when the reactions are performed in the a.m. manner. Furthermore several reports have described increased reaction rates for reactions conducted under the action of microwave irradiation at atmospheric pressure, suggesting specific or nonthermal activation by microwaves. Some of these re-studied reactions occur at... 

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 ... 

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. 

However, the nonthermal plasma has a main drawback. Plasma reactions are rather nonselective and during the total oxidation this leads to the formation of undesired reaction by-products as well. A solution to this problem is the combination of plasma and heterogeneous catalysis. In this way, the high efficiency of nonthermal activation provided by plasma combined with the high selectivity offered by catalysis can lead to a synergetic effect [62]. 

Microwave-mediated Leuckart reductive amination of carbonyl compounds was carried out (Loupy et al, 1996). They observed a strong specific (nonthermal) activation effect of microwaves. The yields obtained were excellent (75-97%) within short reaction time compared to conventional harsh conditions. 

The essential questions raised by the assumption of athermal or specific effects of microwaves are, then, the change of these characteristic terms (free energy of reaction and of activation) of the reaction studied. Hence, in relation to previous conclusions, five criteria or arguments (in a mathematical sense) relating to the occurrence of microwave athermal effects have been formulated by the author [25], More details can be found in comprehensive papers which analyze and quantify the likelihood of nonthermal effects of microwaves. This paper provides guidelines which clearly define the character of nonthermal effects. 

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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