Ultrafast reactions

Gruebele M and Zewail A H 1990 Ultrafast reaction dynamics Phys. Today 43 24-33... 

We have seen that 10" M s is about the fastest second-order rate constant that we might expect to measure this corresponds to a lifetime of about 10 " s at unit reactant concentration. Yet there is evidence, discussed by Grunwald, that certain proton transfers have lifetimes of the order 10 s. These ultrafast reactions are believed to take place via quantum mechanical tunneling through the energy barrier. This phenomenon will only be significant for very small particles, such as protons and electrons. 

Fonseca, T. and Ladanyi, B. M., in Ultrafast Reactions and Solvent Effects, Gaudel,... 

Rasaiah, J. and Zhu, J. (1994) Solvent dynamics and electron transfer reactions,in Gauduel, Y. and Rossky, P. J.(eds.), Ultrafast reaction dynamics and solvent effects, AIP Press, New York,pp.421-434. 

The accuracy achieved through ab initio quantum mechanics and the capabilities of simulations to analyze structural elements and dynamical processes in every detail and separately from each other have not only made the simulations a valuable and sometimes indispensable basis for the interpretation of experimental studies of systems in solution, but also opened the access to hitherto unavailable data for solution processes, in particular those occurring on the picosecond and subpicosecond timescale. The possibility to visualize such ultrafast reaction dynamics appears another great advantage of simulations, as such visualizations let us keep in mind that chemistry is mostly determined by systems in continuous motion rather than by the static pictures we are used to from figures and textbooks. It can be stated, therefore, that modern simulation techniques have made computational chemistry not only a universal instrument of investigation, but in some aspects also a frontrunner in research. At least for solution chemistry this seems to be recognizable from the few examples presented here, as many of the data would not have been accessible with contemporary experimental methods. 

In nonpolar solvents, 10-hydroxybenzoquinoline (10-HBQ) is converted from the enol to the keto form in a time as short as 100 fs after photoexcitation [6]. In order to examine the coherent nuclear dynamics in this ultrafast reaction, we excited this molecule by ultrashort uv pulses at 360 nm and measured time-resolved stimulated emission around 620 nm from the reacting excited state with a 27-fs resolution. As shown in Fig. 3, the observed... 

Abstract Ultrafast photoreactions in PNS of PYP have been studied by means of fs fluorescence up conversion method. Conclusions obtained are (a) Photoreaction in PNS (chromophore twisting) occurs from vibrationally unrelaxed fluorescence state and coherent oscillations in the fluorescence decay curves have been observed for the first time, (b) Comparative studies on fluorescence dynamics of mutants and w.-t. PYP have proved that the w.-t. PYP is best engineered for the ultrafast reaction, (c) The coherent oscillations in the fluorescence decay completely disappeared and the reaction was much slower in the denatured state, demonstrating the supremely important role of PNS for the photoreaction. 

H.-H. Limbach, G. Scherer, L. Meschede, F. Aguilar-Parrilla, B. Wehrle, J. Braun, Ch. Hoelger, H. Benedict, G. Buntkowsky, W. P. Fehlhammer, J. Elguero, J. A. S. Smith and B. Chaudret in Ultrafast Reaction Dynamics and Solvent Effects, Experimental and Theoretical Aspects, (Eds. Y. Gauduel and P. J. Rossky), American Institute of Physics, 1993. 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

Gruebele, M. and Zewail, A.H. (1990). Ultrafast reaction dynamics, Physics Today, May issue, 24-33. 

K. Kakitani, N. Matsuda, T. Denda, N. Mataga, and Y. Enomoto, Ultrafast Reaction Dynamics and Solvent Effects, AIP Conf. Proc., New York, Y. GauduelandP. J. Rossky, eds., 1993, p. 298. 

Microreaction technology has already shown a great deal of promises for homogeneous reactions, be thermal, photochemical or electrochemical. Efficient mixing, precise control of reaction temperature and residence time enable one to manipulate the selectivity issue, tame an ultrafast reaction or even conduct a highly exothermic... 

R.A. Marcus. In Femtochemistry <5 Femtobiology Ultrafast Reaction Dynamics at the Atomic-Scale Resolutions. Nobel Symposium 101, V. Sundstrom, ed. (London Imperial College Press, 1997), p. 54. 

As most chemical and virtually all biochemical processes occur in liquid state, solvation of the reaction partners is one of the most prominent topics for the determination of chemical reactivity and reaction mechanisms and for the control of reaction conditions and resulting materials. Besides an exhaustive investigation by various experimental methods [1,2,3], theoretical approaches have gained an increasing importance in the treatment of solvation effects [4,5,6,7,8], The reason for this is not only the need for sufficiently accurate models for a physically correct interpretation of the experimental data (Theory determines, what we observe ), but also the limitation of experimental methods in dealing with ultrafast reaction dynamics in the pico- or even subpicosecond regime. These processes have become more and more the domain of computational simulations and a critical evaluation of the accuracy of simulation methods covering experimentally inaccessible systems is of utmost importance, therefore. 

Kim HJ, Staib A, Hynes JT. In V Sundstrom, ed. Ultrafast Reaction Dynamics at Atomic-Scale Resolution Femtochemistry and Femtobiology. London Imperial College Press, 1998, pp 510-527. 

P. F. Baibara and W. Jaizeba, Adv. Photochem. IS, 1 (1990) Dynamics andMechanizms of Photoinduced Electron Transfer and Related Phenomena, ed. N.Mataga, T. Okada, and H. Masuhaia, (Elsevier, Amsterdam, 1992) " Ultrafast Reaction Dynamics aid Solvent Effects", Y. Gauduel and P. J. Rossky Eds., AH Conference Proceedings 298 (1994). 

The efficiency of mixing is usually determined by a standard chemical method, the so-called Dushman reaction.Mixing of a strong acid (FICl) and a solution of I , lOs , and CFi3C02Na (sodium acetate) leads to a combination of an ultrafast reaction (neutralization of the acid) and a fast reaction (I2 formation by an acid promoted redox reaction between iodide ions and iodate ions) as shown in Scheme 7.1. These two reactions having H as a common reaction component compete in a parallel way. If the mixing is very fast, the ultrafast process masks the slower second... 

Scheme 7.1 Ultrafast reaction and fast reaction promoted by...

V. Sundstrdm, ed., Nobel Symposium Book Femtochemistry and Femtobiology Ultrafast Reaction Dynamics at Atomic Scale Resolution, World Scientific, Imperial College Press, London, 1997. 

The various ultrafast reaction techniques have inevitably been used in a number of the reported investigations on dyes. A study of the saturation absorption dynamics of a cyanovinyldiethylamine dye has yielded a measured lifetime of 3 1 ps, a value which is determined by very rapid internal conversion of the Sj state. ... 

The rest of my review this year will therefore concentrate on this aspect of photochemistry, with the customary apologies to those who would prefer to read about developments in, for example, mechanistic studies, ultrafast reactions or photoaffinity probes. No doubt these themes will be picked up in future volumes. As usual, the chapter and reference numbers of the publications cited in this review can be found by using the Author Index, but I have also included the chapter numbers to aid those who wish to scan for chemical structures. 

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