State-Specific Reactivity Studies

In subsequent work, Bowers and coworkers used this separation technique to examine state-specific reactions of Co+ with CjHg, CH3I, and rare gases, °- as well as Fe+ with and NF and CF with rare gases, all at thermal energies. Likewise 

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To date the only state-specific reactivity study for the second- and third-row transition metal cations that utilizes ion mobility state separation is the reaction of Au ( S, D) with CHjBr as conducted by Taylor et al. At room temperature, they observed association products and formation of AuCH2 for the S(5d °) state, whereas the... 

This reaction profile also illustrates one of the other important challenges in the study of transition metal systems, namely that the metal-containing active site often has several accessible spin states. Specifically in the case of Fe(IV)=0, the triplet, quintet, and septet spin states. Consequently, the reaction can, in principle, proceed on different electronic potential energy surfaces and it is necessary to test all possibilities when exploring a reaction surface. This has been labeled two-state reactivity and has been elaborated by Shaik, Schwarz, Schroder, and co-workers (36—40). In the case of TauD, the results show that the reaction is only feasible on the quintet surface, in agreement with earlier DFT studies (11,41 —45). 

Chemical kinetics is a powerful tool that provides unique mechanistic information and deep insight into the activation process that is at the heart of every chemical transformation. This chapter is structured around some of the most important types of information obtained from kinetic studies. The rate law provides the composition of the transition state (TS), kinetic isotope effects (KIEs) can establish whether a specific bond is involved in the activation process, and activation parameters provide information about the energy and entropy requirements. Independent generation, characterization, and reactivity studies of potential intermediates allow one to search for and identify such intermediates in multistep reactions by spectroscopic means or by use of chemical traps. 

Dramatic effects of electronic excitation on the reaction mechanisms have been demonstrated in several cases. One of the first reported examples must be recalled here also as it falls outside the scope of this chapter. Electronically excited 0( D) is much more reactive than ground-state 0( P) and inserts into the C-H bonds of methane [162]. Similar state specificity in the reactivity has also been encountered in electron-transfer reactions and seems to be the rule in light systems. Its origin has been explored systematically in alkali and alkaline earth metal atom reactions. Before discussing some of the studies, it is appropriate to survey a much simpler situation where electronic excitation affects the dynamics of the reaction just by changing the location of the electron-transfer region. 

Collisions of highly vibrationally excited molecules is commonly a topic in the field of nnimolecular reactions [1,2,3,4,5]. Collisions of vibrationally excited molecules in that field are studied in the context of energy transfer, namely, when nomeactive but inelastic collisions change the energy content of the molecule that may undergo a nnimolecular reaction. The possibility that the colhsions of the excited molecules in the gas phase with reactive partners can show specific phenomena has traditionally not been considered, except for diatomic molecules. Bimolecular reactions of vibrationally excited molecules have become a subject of intense study recently, when experimental work indicated that a remarkable speed-up and state specificity can be observed when simple molecules in vibrationally excited states collide with reactive partners [6,7,8, 9]. 

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 effect of conical intersections on the state-specific and state-to-state reactive and nonreactive scattering attributes was demonstrated with the aid of an extended two coordinate quasi Jahn-Teller (JT) model. In recent years, the photodissociation dynamics of triatomic molecules, for example O3 and H2S, have been studied by calculating the diabatic electronic states and their couplings employing an ah initio approach. The reactive scattering dynamics of insertion reactions, for example, C - - H2, ... 

Mode-selective reactivity has been a subject of considerable interest due to recent developments of laser techniques. It is possible to prepare molecules in specific initial internal state and to study the detailed effect of internal energy and its role on chemical reactions. 

Fig. 7.10. The solid state reactivity of shock-modified zirconia with lead oxide as studied with differential thermal analysis (DTA) shows both a reduction in onset temperature and apparent increase in reaction rate. The shock-modified material has a behavior much like the much higher specific surface powder shown in B (after Hankey et al. [82H01]).

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