Specific energy disposal

The efficiency of cluster impact in driving four-center reactions is due to a matching between what the cluster can do to the reactants or products and the very selective energy requirements and the specific energy disposal in a concerted reactive collision, as discussed in details in Sec. 3.2. The cluster serves to provide both the steric and energetic conditions necessary for this reaction. In terms of the impact parameter of the relative motion of the two reactants, their confinement by the cluster keeps it low, so that they do not miss one another. This confinement within the cluster favoring low impact parameter collisions is a key ingredient in why such processes are so efficient. Furthermore, both the activation of the reactants before the reaction and the stabilization of the hot product after it, are due to the cluster atoms. 

Strazisar BR, Lin C, Davis HF. 2000. Mode-specific energy disposal in the four-atom reaction OH- -D2 HOD + D . Science 290(5493) 958-961. 

From specific energy disposal to the mode-selective control of chemical reactions... 

The observation of the very specific energy disposal in exoergic reactions (Section 1.2) is often accompanied by a strong preferential angular disposition of the products. For example, in reactions such as... 

Before discussing the details of the product energy distributions for specific reactions, we shall try to identify the important factors that determine the disposal of energy in chemical reactions. Broadly, energy disposal is determined by the nature of the interactions (the reaction potential-energy surface), by dynamical or kinetic effects and by the initial states of the reagents. For a particular reaction, one effect may dominate or the system may be governed by a complex interaction of all three effects. 

In this introduction an attempt has been made to indicate how studies of state-selected processes relate to other areas of interest in modern reaction kinetics. In the remainder of the chapter, the main emphasis will be put on the relationship between the observed results, for systems that have been at least moderately well characterized, and features in the molecular collision dynamics - and ultimately, therefore, in the intermolecular potential - that give rise to different types of detailed behaviour. First, however, it is useful to establish the formal connections between the parameters that define the specificity of energy disposal when reaction proceeds in one direction and those that describe the selectivity of energy consumption for reaction in the opposite direction. This is done in the next section. 

Most of the reactions whose specificity of energy disposal has been studied by the technique of i.r. chemiluminescence are exothermic reactions of the A + BC - AB + C type that proceed directly and have low activation energies. For these reactions the assumption that k v yJ v,J, i) is proportional to k( W l T) seems quite reasonable since the thermal spread of initial energies among the degrees of freedom of A and BC is small relative to the energies released in the reactions themselves. This conclusion has been supported by the results of quasiclassical... 

The greater the specificity of energy disposal in an exothermic reaction, then the larger is its entropy deficiency. Values of A., and KS,. for some well-studied atom-transfer reactions are listed in Table 1 is the av age fraction of available energy that is released as product vibration, i.e. 

In the light of the discussion of microscopic reversibility that was given earlier in this section, it should occasion no surprise that the information-theoretic analysis can be applied to the selective energy requirements for reaction as well as to the specificity of energy disposal. However, the results that are availaUe for this treatment are largely derived either from data obtained for exothermic reactions or firom quasiclassicd trajectory calculations, rather than from direct experimental measurements. 

Studies based on the observation of i.r. chemiluminescence from reactions where the reagents are prepared as uncollimated beams or molecular sprays have provided most of the little available information about how selective excitation of the reactants alters the specificity of energy disposal. In experiments in Polanyi s laboratory the spectra emitted by the reaction products are observed as a... 

On to the specificity of energy disposal and selectivity of energy requirements... 

The prior distribution does not seek to match experimental results. Rather, it provides a reference against which to compare. The (logarithmic) deviation of the actual (observed or computed) distribution Irom the prior one is known as the surprisal. Then, armed with detailed balance, we will also seek to characterize the energy requirements of chemical reactions. Consistent with the arguments already presented, tiie measnre of specificity of energy disposal in the forward reaction turns out to be equal to the measure of selectivity of energy requirement in the reverse reaction. 

We have jnst discnssed a measnre of specificity of energy disposal of chemical reactions. Already in Chapter 1 we drew attention to the implication of microscopic reversibility that energy disposal in a forward reaction and energy requirements of the reversed reaction are one and the same. Here we show that both are characterized by the same surprisal. 

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