Zewail studies

In his first experiment (1987) Zewail studied the unimolecular disintegration of iodine cyanide into iodine atom and cyano radical ICN I + CN. They managed to observe a transition state corresponding to the I-C bond breaking. The whole reaction was over in 200 fs. 

Pedersen S, Herek J L and Zewail A H 1994 The validity of the Diradical hypothesis direct femtosecond studies of the transition-state structures Science 266 1359-64... 

Mass spectrometric studies are not limited to the investigation of stable intermediates they have also been carried out on reaction transition states. The ultrafast studies by Zewail, for example, are nominally mass spectrometric based, where photoionization is used to detect reactive species on exceedingly short (femtosecond) time scales.Time resolved studies provide insight into the rates of unimo-lecular reactions, but do not provide direct thermochemical insight. 

In addition to the natural improvements expected in the accuracy of the measurements, and the increased scope in the types of systems examined, new techniques go beyond the issue of thermochemistry to allow for very detailed studies of reaction dynamics. The investigation by Zewail and co-workers of the reactivity of planar COT" on the femtosecond time scale is likely only the beginning. Time-resolved photoelectron spectroscopy, for example, has recently been used to map the potential energy surfaces for the dissociation of simple ions IBr and l2. " Although applications in the field of organic reactive molecules are likely far off, they are now possible. 

Note, however, that the 1999 Nobel Prize for Chemistry was awarded to A. Zewail for his studies of the transition states of chemical reactions using femtosecond spectroscopy (Academy s citation, October 12,... 

Through the use of pump-probe techniques pioneered by Zewail and coworkers,62 it is becoming possible to identify the detailed mechanisms of reactions at the molecular level and follow the actual course of a reaction. The study of ammonia clusters has provided an example of what can be accomplished using these techniques. 

At the beginning of this decade, Zewail and coworkers reported a fundamental work of solvation effect on a proton transfer reaction [195]. a-naphthol and n-ammonia molecules were studied in real-time for the reaction dynamics on the number of solvent molecules involved in the proton transfer reaction from alcohol towards the ammonia base. Nanosecond dynamics was observed for n=l and 2, while no evidence for proton transfer was found. For n=3 and 4, proton transfer reaction was measured at pisosecond time scale. The nanosecond dynamics appears to be related to the global cluster behavior. The idea of a critical solvation number required to onset proton transfer... 

In another important experiment (1989) Zewail and his group studied the dissociation of sodium iodide (Nal) Na+ r Na + I. The pump pulse excites the ion pair Na+r which has an equilibrium distance of 2.8 A between... 

Zachariasse K. A., Kozankiewicz B. and Kuhnle W. (1983) Micelles and Biological Membranes Studied by Probe Molecules, in Zewail A. H. (Ed.), Photochemistry and Photobiology, Vol. II, Harwood, London, pp. 941-960. 

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

The references cited here depend heavily on investigations reported by the Professor Ahmed H. Zewail, the 1999 Nobel Laureate in Chemistry, and his collaborators at the California Institute of Technology. In them, one will find extensive citations of work leading up to recent advances in femtochemistry as well as to contemporary studies from other laboratories. 

Precise measurements of the excited state lifetimes of the DNA constituents were not available till very recently, mainly due to the limited time resolution of conventional spectroscopic techniques. Studying the DNA nucleosides by transient absorption spectroscopy, Kohler and co-workers observed a very short-lived induced absorption in the visible which they assigned to the first excited state [5,6]. The lifetimes observed were all well below 1 picosecond. The first femtosecond fluorescence studies of DNA constituents were performed using the fluorescence upconversion technique. Peon and Zewail [7] reported that the excited state lifetimes of DNA/RNA nucleosides and nucleotides all fall in the subpicosecond time, thus corroborating the results obtained by transient absorption. 

Phase- and Energy-Changing Collisions in Iodine Gas Studies by Optical Multiple-Pulse Spectroscopy, E. T. Sleva and A. H. Zewail, Chem. Phys. Lett. 110, 582 (1984). 

The Validity of the Diradical Hypothesis Direct Femtosecond Studies of the Transition-State Structures, S. Pedersen, J. L. Herek, and A. H. Zewail, Science 266, 1359 (1994). 

A. H. Zewail About this problem, Dr. Gaspard has done very interesting calculations to identify and study the nature of resonances above the saddle point and compared with our experiments on Hgl2 and other systems. I would like to refer to his contribution in this conference. 

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

A. H. Zewail If we solve for the molecular Hamiltonian, we will be theorists I do, of course, understand the point by Prof. Quack and the answer comes from the nature of the system and the experimental approach. For example, in elementary systems studied by femtosecond transition-state spectroscopy one can actually clock the motion and deduce the potentials. In complex systems we utilize a variety of template-state detection to examine the dynamics, and, like every other approach, you/we use a variety of input to reach the final answer. Solving the structure of a protein by X-ray diffraction may appear impossible, but by using a number of variant diffractions, such as the heavy atom, one obtains the final answer. 

A. H. Zewail Prof. Yamanouchi is correct in pointing out the relevance of ultrafast electron diffraction to the studies of vibrational (and rotational) motion. In fact, Chuck Williamson in our group [1] has considered precisely this point, and we expect to observe changes in the radial distribution functions as the vibrational amplitude changes and also for different initial temperatures. The broadening in our radial distribution function presented here is limited at the moment by the range of the diffraction sampled. 

A. H. Zewail For vibrational adiabaticity we must complete the study of correlation of reaction product distribution to the nature of the initial excitation (see reply to Prof. Marcus below). You may be interested to know that for a given energy within our pulse we see tra-... 

A. H. Zewail My answer to Prof. Troe is that, in our experiment, already the cluster with one solvent shows the shift in 0- As for the boiling off of solvent molecules in larger clusters this is a nontrivial problem that we have considered in our paper. Based on the analysis of the translational eneigy and the kinetics, we concluded that the exponential decays (rates) are determined by the isomerization [see Chem. Phys. Lett. 242, 380 (1995)]. In any event, only one solvent molecule (at most) can be evaporated for the available energy studied experimentally recall that the binding energy of hexane is relatively large. 

A. H. Zewail Prof. Chetgui s observations are very interesting. As discussed in this conference, coherent wavepacket motion has been observed in condensed phases in many laboratories. In Prof. Chergui s experiment it is now possible to study the time scale for bubble formation. Molecular dynamics should tell us the nature of forces which maintain any coherence in such systems. 

Wan C, Fiebig T, Schiemann O, Barton JK, Zewail AH (2000) Femtosecond direct observation of charge transfer between bases in DNA. Proc Nat Acad Sci USA 97 14052-14055 Wang J, Biedermann KA, Wolf CR, Brown JM (1993) Metabolism of the bioreductive cytotoxin SR 4233 by tumour cells enzymatic studies. Br J Cancer 67 321-325 Wang W-F, Schuchmann MN, Schuchmann H-P, KnolleW, von Sonntag J, von Sonntag C (1999) Radical cations in the OH-radical-induced oxidation of thiourea and tetramethylthiourea in aqueous solutions. J Am Chem Soc 121 238-245... 

Zewail et al. recently reported femtosecond time-resolved studies on the reduction of T by photoexcited 2-aminopurine in DNA duplexes [7]. 

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