Iodine recombination

Figure 20. Iodine recombination rate in units of cm mol s versus relative friction i/n in units of s" . From Troe and coworkers ( ) theoretical prediction (-).

A termolecular reaction step involves the simultaneous collision of three molecules, which is a much less likely event. An example is the recombination of iodine atoms in the gas phase to form iodine molecules. So much energy is released in forming the I—I bond that the molecule would simply fly apart as soon as it was formed if the event were a binary collision. A third atom or molecule is necessary to take away some of the excess energy. If iodine recombination takes place in the presence of a sufficiently high concentration of an inert gas such as argon, termolecular reactions... 

Fig. 12.1. Representation of some of the principal features of the iodine recombination process following photodissociation. The upward arrow indicates excitation to a bound electronic state. After predissociation to a repulsive excited state, transitions to the ground state may occur at a variety of intemuclear separations. Other potential energy surfaces not explicitly shown may also participate in the recombination process.

D. J. Nesbitt and J. T. Hynes, /. Chem. Phys., 77,2130 (1982). Slow Vibrational Relaxation in Picosecond Iodine Recombination in Liquids. 

For iodine recombination with NO as third body the complex mechanism can be written... 

Such complexes seem to form in the case of iodine recombination (see below). They are also generated through recombination of OH radicals [44]. 

The treatment of diatomic recombination and dissociation using the theory of section IV has been previously applied, at least for impulsive collisions [21]. In the low bath density regime, an adiabatic correction factor C has been used [9] based on the form for B for an exponentially repulsive potential [26]. The important result was that the iodine recombination rate could be accounted for over the entire range of bath densities using as input the diatomic potentials and the atomic diffusion... 

Although the transition to difhision control is satisfactorily described in such an approach, even for these apparently simple elementary reactions the situation in reality appears to be more complex due to the participation of weakly bonding or repulsive electronic states which may become increasingly coupled as the bath gas density increases. These processes manifest tliemselves in iodine atom and bromine atom recombination in some bath gases at high densities where marked deviations from TronnaF behaviour are observed [3, 4]. In particular, it is found that the transition from Lto is significantly broader than... 

Otto B, Schroeder J and Tree J 1984 Photolytic cage effect and atom recombination of iodine in compressed gases and liquids experiments and simple models J. Chem. Phys. 81 202... 

Harris A L, Berg M and Harris C B 1986 Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution J. Chem. Phys. 84 788... 

Marcus R A and Rice O K 1951 The kinetics of the recombination of methyl radicals and Iodine... 

The first reaction filmed by X-rays was the recombination of photodisso-ciated iodine in a CCI4 solution [18, 19, 49]. As this reaction is considered a prototype chemical reaction, a considerable effort was made to study it. Experimental techniques such as linear [50-52] and nonlinear [53-55] spectroscopy were used, as well as theoretical methods such as quantum chemistry [56] and molecular dynamics simulation [57]. A fair understanding of the dissociation and recombination dynamics resulted. However, a fascinating challenge remained to film atomic motions during the reaction. This was done in the following way. 

A dilute I2/CCI4 solution was pumped by a 520 nm visible laser pulse, promoting the iodine molecule from its ground electronic state X to the excited states A,A, B, and ti (Fig. 4). The laser-excited I2 dissociates rapidly into an unstable intermediate (I2). The latter decomposes, and the two iodine atoms recombine either geminately (a) or nongeminately (b) ... 

M. Wulff, S. Bratos, A. Plech, R. Vuilleumier, F. Mirloup, M. Eorenc, Q. Kong, and H. Ihee,. Recombination of photodissociated iodine a time-resolved X-ray diffraction study. J. Chem. Phys. 124(3), 034501 (2006). 

Marcus, R. A and Rice, O. K., The kinetics of the recombination of methyl radicals and iodine atoms.. /. Phys. Chem 55, 894 (1951). Marcus, R. A., Unimolecular dissociations and free radical recombination reactions. J. Chem. Phys. 20, 359 (1952). 

The formation of l from 1 had been postulated by Vetter [8a]. From the above information we can make the following conclusions (a) l" (atom-free radical) can be produced electrochemically, (b) l" does react with pyridine and may react with similar compounds and (c) recombination of l" may be slow in solution phase. Molecular iodine (di-iodine) the radio-isotope, is being used in the treatment of thyroid disorder. One can ask the question is there any biologically beneficial or toxic effect of iodine atom. There has been no study [8b]. 

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