Iodine, dissociation

Without doubt, the iodine molecule, I2, dissociates into atoms I2=I+I. The state of the system in equilibrium will be represented by kCi2=k Ciz. If x denotes the proportion of iodine dissociated, and v the volume of the iodine vapour, then, Bince v volumes of iodine vapour become 2v volumes of dissociated iodine vapour, it follows that the concentration of the dissociated iodine will be xjv, and of the iindissociated iodine (1—x)/v. Hence for equilibrium... 

The decay of the parent is biexponential with a short time constant, typically 250 fs, and a longer one, 800 fs. The kinetics appear to be parallel, i.e., correspond to different and competing decay channels. Their pre-exponential factors are different, with the short one dominating. The short decay has been assigned to an ionic to covalent back electron transfer pictured in Figure 17, resulting in iodine dissociation. A back transfer from the iodine n orbital to the half-filled benzene n orbital leaves iodine in a dissociative state. The reaction channel forms (benzene) - -I-1, and corresponds to the minor decay component of the parent. 

For many years the reaction was thought to be a prime example of an elementary reaction. However, later work by J. H. Sullivan in 1967 (entitled. Mechanism of the bimolecular hydrogen-iodine reaction) showed that iodine atoms were involved in the reaction so that it must be composite. A two-step reaction mechanism can be proposed, in which molecular iodine dissociates, and the iodine atoms then react with a hydrogen molecule... 

The iodine dissociation process is poorly understood and provides the greatest difficulty in modeling the gas-phase kinetics of the COIL device. The energetics of the dissociation process is shown in Fig. 7. The second electronically excited state of oxygen, 02(b S), is produced from the energy pooling reaction ... 

A simplified kinetic mechartism that retains the essential features of the iodine dissociation process is provided in Table II. 

Iodine dissociation. Compute the dissociation constant Kp for iodine at T = 300 K. 

Figure 7.10 Iodine dissociation and recombination, I2 21 (1), Snapshots of iodine-argon clusters in the first picosecond of the reaction. Models showing the movements of iodine atoms in an iodine-argon cluster, during the first few picoseconds after a femtosecond pump pulse, as calculated by molecular-dynamics simulation, (a) For a cluster with 17 Ar atoms (l2Ari7) initially one of the iodine atoms is not capped by argon atoms the two atoms separate by more than an atomic diameter, and the subsequent recombination takes more than 4 ps. (b) For a cluster of the same size (l2Ari7) but with the iodine molecule fully enclosed the iodine atoms separate by about one atomic diameter and the recombination is direct, taking < 1 ps. (c) For a larger cluster with 44 argon atoms or more, the iodine molecule is almost always fully enclosed and recombination takes < 1 ps. From Ref. [17,b].

Figure 7.11 Iodine dissociation and recombination, 21 (2). Sub-picosecond transient in an argon cluster. Laser-induced-fluorescence transient after excitation of iodine molecules to the A state by a pump pulse of 614 nm at a series of pump-probe delay times. Upper panel Molecular-dynamics simulation for an hAr44 cluster with an initial temperature of 30 K prohe wavelength 307 nm. Lower panel Experimental transient from an iodine-argon molecular beam. The simulation reproduces the initial peak a at time zero, the decrease and recovery during the first picosecond b, c the subsequent slower rise, and some of the observed modulations, indicated by vertical arrows. See text. After Ref. [17,b],...

Figure 7.12 Iodine dissociation and recombination, I2 21 (3). The course of the reaction in argon. Time-dependence, calculated by molecular-dynamical simulation, of the changes in the first few picoseconds after a femtosecond pump pulse of the average I-I distance. From Ref. [17,b, p. 11327].

Figure 7.13 Iodine dissociation and recombination (4). Comparison of (a) recombination rates and (b) quantum yields in helium, neon, argon and krypton. See text. From Ref. [17,cJ.

Molecular iodine dissociates into atomic iodine at relatively moderate temperatures. At 1000 K, for a 1.00-L system that has 6.00 X 10 moles of I2 present initially, the final equilibrium pressure is 0.750 atm. Determine the equilibrium amounts of I2 and atomic I, calculate the equilibrium constant, and determine if the relevant equilibrium is... 

Molecular iodine dissociates at 625 K with a first-order rate constant of 0.271 s What is the half-life of this reaction ... 

In the gas phase, the quantum 3deld of dissociation of a molecule to radicals (atoms) is equal to unity. In the liquid phase it is much lower than unity because the radicals that formed partially recombine in the cage. For example, for iodine dissociation in CCI4 at 298 K the quantum yield 4> = 0.14, and for bromine under the same conditions cj) = 0.22. For the photodissociation of azo compounds in a solution = 0.25-K).10. 

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