Femtochemistry reactions studied

The IHgl system, representing this class of reactions, was one of the first reactions studied in femtochemistry [5]. The activated complexes [IHgl], for which the asymmetric (translational) motion gives rise to vibrationally cold (or hot) nascent Hgl, were prepared coherently at the crest of the energy barrier (Fig. 14). The barrier-descent motion was then observed using series of probe pulses. 

The fastest that anything happens in a chemical reaction is on a time scale of approximately 1 femtosecond (1 fs = 10 15 s). That is the time it takes for a bond to stretch or bend, and perhaps break. If we could follow atoms on that time scale, we could make a movie of the changes in molecules as they take part in a chemical reaction. The new field of femtochemistry, the study of very fast chemical processes, is bringing us closer to realizing that dream. Lasers can emit very short but intense pulses of electromagnetic radiation, so they can be used to study processes on very short time scales. 

Despite their transient existences, it is possible to study transition states of certain reactions in the gas phase with a technique called laser femtochemistry Zewall, A.H. Bernstein, R.B. Chem. Eng. News, 1988, 66, No. 45 (Nov. 7), 24. For another method, see Ceilings, B.A. Polanyi, J.C. Smith, M.A. Stolow, A. Tarr, A.W. Phys. Rev. Lett., 1987, 59, 2551. See Smith, M.B. Organic Synthesis, McGraw-Hill NY, 1994, p. 601. 

A. H. Zewail, Femtochemistry Recent Progress in Studies of Dynamics and Control of Reactions and Their Transition States, J. Phys. Chem. 1996, 100, 12701. 

In a classical Bohr orbit, the electron makes a complete journey in 0.15 fs. In reactions, the chemical transformation involves the separation of nuclei at velocities much slower than that of the electron. For a velocity 105 cm/s and a distance change of 10 8 cm (1 A), the time scale is 100 fs. This is a key concept in the ability of femtochemistry to expose the elementary motions as they actually occur. The classical picture has been verified by quantum calculations. Furthermore, as the deBroglie wavelength is on the atomic scale, we can speak of the coherent motion of a single-molecule trajectory and not of an ensemble-averaged phenomenon. Unlike kinetics, studies of dynamics require such coherence, a concept we have been involved with for some time. 

Diau, E. W. G., Kotting, C., Zewail, A. H., Femtochemistry of Norrish Type I Reactions I. Experimental and Theoretical Studies of Acetone and Related Ketones on the S, Surface,... 

Zewail AH. Femtochemistry recent progress in studies of dynamics and control of reactions and their transition states. J Phys Chem 1996 100 12701-12724. 

O Reading Check Explain the importance of the methods of femtochemistry to the study of reaction mechanisms. 

The talk, which is summarized here, focuses on examples of femtochemical studies from the myriad of applications studies in different phases and around the world. Of particular interest are reactions involving multiple-center transition states, and, of historical interest, those related to Bodenstein and his fellow giants. The experimental approach in femtochemistry is detailed elsewhere [3] and will not be discussed here. 

At present, the direct study of transition state structures has been possible thanks to the enormous advances made in femtochemistry. For instance, the existence of biradical species has been established by direct femtosecond studies of the transition state structures. For the retro-Diels-Alder reaction, femtosecond dynamic studies seem to indicate that both the biradical and the concerted processes take place. ... 

A new area of research, femtochemistry, in the framework of which reactions are studied in the femtosecond time scale, has recently speared along with the term coherent elementary reactions in which phase characteristics of the motion of atoms in the molecular reacting system are taken into account. 

From the experimental side this photoreaction has been recently studied using ultrafast (femtochemistry) techniques. The first results apparently pointed to a slowing down of the ESIPT reaction when HPMO was encapsulated inside j8-CD [56]. However, another work by the same authors pointed to the direct ESIPT reaction being almost unaffected when HPMO is placed inside a CD nanocavity [57]. In this work Douhal et al. observed two groups of time-resolved... 

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