Reaction coordinate photochemical

A photochemical reaction coordinate has two branches an excited state branch and a ground state branch that is reached after decay at a conical intersection. Thus a conical intersection between ground and excited states of a molecule is a precursor to ground state reactivity, and conforms to the above definition of a reactive intermediate. The main focus of our article will be to develop this idea. In Figure 9.1b, we show the energy profile for a photochemical reaction with a conical intersection... 

We hope the reader has been convinced that it is technically feasible to describe a photochemical reaction coordinate, from energy absorption to photoproduct formation, by means of methods that are available in standard quantum chemistry packages such as Gaussian (e.g., OPT = Conical). The conceptual problems that need to be understood in order to apply quantum chemistry to photochemistry problems relate mainly to the characterization of the conical intersection funnel. We hope that the theoretical discussion of these problems and the examples given in the last section can provide the information necessary for the reader to attempt such computations. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Solid state photochemical reactions of transition metal coordination compounds. E. L. Simmons and W. W. Wendlandt, Coord. Chem. Rev., 1971, 7.11-27 (88). 

Mechanistic aspects of the photochemical reactions of coordination compounds. M. S. Wrighton, Top. Curr. Chem., 1976, 65, 38-104 (196). 

The hexatriene is polarized by unsymmetrical substitution with the C=0 group, and further activated by coordination with Lewis acid. The catalyzed reaction is polar. The similarity between the catalyzed and the photochemical reactions can be understood if polar reactions belong to the pseudoexcitation band as has been proposed in Sect 1. 

Figure 3.25 Schematic reaction coordinates for the photochemical ring opening reactions of 1,3-cyclohexadiene. (Reprinted with permission from reference [122]. Copyright (1994) American Chemical Society.)...

Figure 7.24. Solid-state photochemical decarbonylation model for ketones. The dashed path corresponds to the experimentally determined energies of acetone (in kcal/mol). The effects of substituents with radical stabilizing energies (RSEs) are illustrated by the solid line in the reaction coordinate. See color insert.

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

F ure 9.26. Energy profile along (a) the reaction coordinate at an avoided crossing for a photochemical reaction and (h) an electron transfer process. 

Wrighton, M. S. Mechanistic Aspects of the Photochemical Reactions of Coordination Compounds. 65,37-102 (1976). 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

In this article we have summarized the use of both photochemical and more classical thermal kinetics techniques to deduce the nature of intermediates in the ambient temperature, fluid solution chemistry of several triruthenium clusters. In some cases the photochemically generated intermediates appear to be the same as those proposed to be formed along thermal reaction coordinates, while in other cases unique pathways are the results of electronic excitation. The use of pulse photolysis methodology allows direct observation, and the measurement of the reaction dynamics of such transients and provides quantitative evaluation of the absolute reactivities of these species. In some cases, detailed complementary information regarding... 

W. Rettig and G. Wermuth, The kinetics of formation of twisted intramolecular charge transfer (TICT) states in p-substituted dialkylanilines Consequences of conical intersections along the reaction coordinate, /. Photochem. 28, 351 (1985). 

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