Reaction rate constant state-resolved products

So far we have shown that the observed reaction rate constant is an average over the rate constants for the selected state reactants. If we do state-resolve the products then... 

Transient, or time-resolved, techniques measure tire response of a substance after a rapid perturbation. A swift kick can be provided by any means tliat suddenly moves tire system away from equilibrium—a change in reactant concentration, for instance, or tire photodissociation of a chemical bond. Kinetic properties such as rate constants and amplitudes of chemical reactions or transfonnations of physical state taking place in a material are tlien detennined by measuring tire time course of relaxation to some, possibly new, equilibrium state. Detennining how tire kinetic rate constants vary witli temperature can further yield infonnation about tire tliennodynamic properties (activation entlialpies and entropies) of transition states, tire exceedingly ephemeral species tliat he between reactants, intennediates and products in a chemical reaction. 

If the rate constants for parallel reactions are to be resolved, then analysis of the products is essential (Sec. 1.4.2). This is vital for understanding, for example, the various modes of deactivation of the excited state (Sec. 1.4.2), Only careful analysis of the products of the reactions of Co(NH3)jH20 + with SCN, at various times after initiation, has allowed the full characterization of the reaction (1.95) and the detection of linkage isomers. Kinetic analysis by a number of groups failed to show other than a single second-order reaction.As a third instance, the oxidation of 8-Fe ferredoxin with Fe(CN)g produces a 3Fe-cluster, thus casting some doubt on the reaction being a simple electron transfer. 

Thus kcai and are a function of all the rate constants in the pathway and any simplifying assumptions concerning individual rate constants are likely to be inaccurate. Moreover, the three reaction pathways shown in Schemes I and 11, and 111 are indistinguishable by steady-state methods. Although product inhibition patterns provide evidence for the E-P state, individual kinetic constants cannot be resolved. Schemes 11 and 111 reduce to Scheme 1 under the conditions where ki, k2- Steady-state kinetics cannot resolve the three reaction mechanisms because the form of the equation for steady-state kinetics is identical for each mechanism (v = rate) ... 

As seen already, Cpd I is a triradicaloid with singly occupied -ir, -ir and aj orbitals and, hence, has a virtually degenerate pair of ground states ( Ajy). As such, it is expected that at least these electronic states will participate in the reactions, and will lead thereby to two-state reactivity (TSR) In TSR, each state may produce its specific set of products with different rate-constants, regio- and stereoselectivities and lead thereby to apparently controversial information when viewed through the perspective of singlestate reactivity (SSR). It is our contention that TSR resolves much of the controversy that has typified the P450 field of reaction mechanism in recent years , and opens new horizons for reactivity studies. 

The overwhelming majority of reactions between small free radicals and molecules involve transfer of a single atom. Such reactions are very important in atmospheric ch nistry and in combustion, and experimental results are regularly evaluated and recommendations made in respect of rate constants and their temperature dependence [3]. The literature on these reactions is vast. Here, I shall only allude to some recent and current experiments in my group as examples of what can now be done. This work relies on the production of OH(OD) or CN radicals by pulsed photolysis and the.determination of kinetic decays of the radicals in either their vibrational ground state, v=0, or in v=l by time-resolved LIF. I shall concentrate on reactions of CN. 

Discussions of relaxation kinetics (see section 6.2) and of transient kinetics, often contain the following general statements. In principle the relaxation spectrum of a reaction contains the necessary information to evaluate all the rate constants of the elementary steps of the reaction. Similarly one can state that in principle the time profile, and its concentration dependence, of the appearance of products during the transient approach to the steady state, contains all the information for the evaluation of the individual rate constants of the formation and interconversion of intermediates. However, in both cases there are important limitations. The theoretical limitations are that the degeneracy of the sequential time constants and the position of the rate limiting step within the sequence of events can reduce the information contents, even if the record of the reaction has an unlimited signal to noise ratio. In real life, noise and restricted time resolution further reduce the number of steps which can be resolved in any particular experiment. The time resolution of different... 

It is quite straightforward to perform quasiclassical trajectory computations (QCT) on the reactions of polyatomic molecules providing a smooth global potential energy surface is available from which derivatives can be obtained with respect to the atomic coordinates. This method is described in detail in Classical Trajectory Simulations Final Conditions. Hamilton s equations are solved to follow the motion of the individual atoms as a function of time and the reactant and product vibrational and rotational states can be set or boxed to quantum mechanical energies. The method does not treat purely quantum mechanical effects such as tunneling, resonances. or interference but it can treat the full state-to-state, eneigy-resolved dynamics of a reaction and also produces rate constants. Numerous applications to polyatomic reactions have been reported. ... 

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