Iodine atom recombination

The photo-dissociation of iodine molecules (dissociation energy e 150kJmol 1) in solution has been widely studied by Noyes (see ref. 5) and has provided some of the most interesting evidence for the importance of the solvent cage. Usually, Noyes and co-workers have photo-dissociated 

Nesbitt and Hynes [57] have pointed out that much of the oscillator strength of the iodine molecule s optical absorption arises from the lowest few vibrational states of the ground electronic state (X). It is necessary for the iodine atoms to recombine and then relax to these low vibrational 

Escape probability of iodine atoms formed by photodissociation of iodine molecules with laser light pulses ( 530 nm wavelength, 30 ps duration) in various solvents. 

Possibly the only safe conclusion with respect to the iodine atom recombination work is that it is a far from ideal system for recombination probability studies and considerable further effort will be required to understand it. 

A recent study of iodine atom recombination in solution by Luther et al. [294] used a dye laser (wavelength 590nm, pulse duration 1.5ps) to photodissociate iodine molecules in n-heptane, -octane or methyl cyclohexane at pressures from 0.1 to 300 MPa. Over this pressure range, the viscosity increases four-fold. The rate of free-radical recombination was monitored and the second-order rate coefficient was found to be linearly dependent on inverse viscosity. This provides good reason to believe that the recombination of free iodine atoms is diffusion-limited, especially as the rate coefficient is typically 10 °dm mol s . The recombination of primary and secondary pairs is too rapid to be monitored by such equipment as was used by Luther etal. [294] (see below). Instead, the depletion of molecular iodine absorption just after the laser pulse was used to estimate the yield of (free) photodissociated iodine atoms in solution. They found that the photodissociation quantum yields (survival probability) were about 2.3 times smaller than had been measured by Noyes and co-workers [291, 292] and also by Strong and Willard [295]. This observation raises doubts as to the accuracy of the iodine atom scavenging method used by Noyes et al. or perhaps points to the inherent difficulties of doing steady-state measurements. In addition, Luther et al. 

Kelley and Rentzepis [297] have recently studied the recombination of iodine atoms in liquid and fluid xenon over times to 150 ps after photolysis. The iodine molecule can be biphotonically dissociated through the state to produce geminate pairs with larger initial separations. Some degree of spin relaxation of excited iodine atoms ( Pi/2) produced by biphotonic excitation may occur and reduce the probability of recombination. There is also evidence that the 11 state of I2 may be collisionally predissociated and that recombination may be more rapid than the rate of vibrational relaxation of the excited 12 state in polyatomic solvents (see also ref. 57). Despite these complications, several workers have attempted to model the time dependence of the recombination (or survival) probability of iodine atom reactions. The simple diffusion equation analysis of recombination probabilities [eqn. 

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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) ... 

From studies by Chuang et al. [266] of iodine atom recombination, the probability of recombination is large. The probability would be even greater if the iodine atoms were formed as an encounter pair. Accordingly, a is near to unity. Furthermore, Noyes suggests / /a 10 for iodine atoms and so 0> 0.9. Hence, q(r0) R/r0, as might be seen from the long-time limit of eqn. (134). 

By now the opinions of the author on the direction of future work should begin to have become apparent. Above all else, more detailed and careful experimental work is needed. Much progress has been made since the time of Noyes early studies on iodine atom recombination. There are so many holes in our understanding of experimental systems that many of the articles, and especially those on nanosecond or picosecond time-resolved studies of reaction rates which have been published so far, are important events for a kineticist It is to be hoped that the increasing interest in this experimental field will continue to grow. 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

There have been several studies of the iodine-atom recombination reaction which have used numerical techniques, normally based on the Langevin equation. Bunker and Jacobson [534] made a Monte Carlo trajectory study to two iodine atoms in a cubical box of dimension 1.6 nm containing 26 carbon tetrachloride molecules (approximated as spheres). The iodine atom and carbon tetrachloride molecules interact with a Lennard—Jones potential and the iodine atoms can recombine on a Morse potential energy surface. The trajectives were followed for several picoseconds. When the atoms were formed about 0.5—0.7 nm apart initially, they took only a few picoseconds to migrate together and react. They noted that the motion of both iodine atoms never had time to develop a characteristic diffusive form before reaction occurred. The dominance of the cage effect over such short times was considerable. 

Shin and Kapral have applied the kinetic theory of reactions in solution to the case of two radicals (e.g. iodine atoms) recombining with one another [286]. As it is the behaviour of both radicals which is of interest, Shin and Kapral seek to evaluate the doublet density of A and B, t), rather than the singlet density as used in the case of homogenous reactions of the type [eqn. (306)] where one species is not transformed. The doublet density changes as a result of collision with the solvent and so the triplet density, / BS(123, f), is of concern and the equation for the doublet density is like that of eqn. (295) with a = A, j3 = B and p = S. The triplet density, /f 8, itself depends on a quartet distribution, that of the radical reactants A and B and any two solvent molecules. The second solvent molecule can collide with A, B or the first solvent molecule and thereby change f BS. Following the usual procedure, the triplet density... 

Returning to the survival probability, in Fig. 57, the kinetic theory and diffusion equation [cf. eqn. (132)] predictions are compared. Three values of the activation rate coefficient are used, being 0.5, 1.0 and 2.0 times the Smoluehowski rate coefficient for a purely diffusion-limited homogeneous reaction, 4ttoabD. With a diffusion coefficient of 5x 10 9 m2 s1 and encounter distance of 0.5 nm, significant differences are noted between the kinetic theory and diffusion equation approaches [286]. In all cases, the diffusion equation leads to a faster rate of reaction. In their measurements of the recombination rate of iodine atoms in hydrocarbon solvents, Langhoff et al. [293] have noted that the diffusion equation analysis consistently predicts a faster rate of iodine atom recombination than is actually measured. Thus there is already some experimental support for the value of the kinetic theory approach compared with the diffusion equation analysis. Further developments cannot fail to be exciting. 

Fig. 5-2. Dependence of bimolecular rate constant kobs of iodine atom recombination at 333°K on [NO] in the high-pressure region. The curve is calculated and the points are experimental (from Porter, Szab6, and Townsend350 with permission of the Royal Society).

It was realized that another reaction, the unimolecular formation of the olefine with elimination of hydrogen iodide would also take place, but it was thought that measurement of the iodine would be a good measure of equation 4.2,4.1. This simple view depends upon the assumption that iodine atoms recombine to give iodine more rapidly than they take part in the reverse of the reaction of equation 4.2,4.1. This seems rather unlikely, because the recombination of iodine atoms requires three body collisions and the recombination of radicals with iodine atoms may well be a bimolecular process. Under these circumstances, it seems more likely that the prevention of complete reformation of the alkyl iodide is due to further reactions of the alkyl radical, such as... 

In the course of investigating the eifect of various third bodies on the rate of iodine atom recombination, Engleman and Davidson studied the nitric oxide-iodine atom system by flash photolysis. Runs were made at 50 and 200 °C and they found the recombination rate was too rapid to be measured with their apparatus A lower limit for the rate coefficient of... 

R, (iii) the possible participation of excited electronic states and (iv) the density dependence of After these have been dealt with adequately, it can be shown that for many solvent bath gases, the phenomenon of the turnover from a molecular reaction into a diffusion-controlled recombination follows equation (A3.6.26) without any apparent discontinuity in the rate coefficient k at the gas-liquid phase transition, as illustrated for iodine atom recombination in argon [36, 37]. For this particular case, is based on and extrapolated from experimental data, R is taken to be one-half the sum of the Lennard-Jones radii of iodine atom and solvent molecule, and the density-dependent contribution of excited electronic states is implicitly considered by making the transition from the measured vin dilute ethane gas to in dense liquid ethane. 

Of these rate constants, all except the last cited have been determined in discharge-flow systems. Bromine and iodine atom recombination rates have been measured using flash photolysis over a wide temperature range, and for a wide variety of third bodies M. Temperature coefficient measurements using discharge-flow methods have been reported for N, H and Cl atom recombinations. Data on Br atom recombination at 298 K have also been obtained using these methods. Limited data on the effect of third body on the rate constants have been obtained in these cases. However, there is much scope for the acquisition of good data with systematic variation of T and M, as well as extensions to hetero-atom recombination reactions. At present, quantitative kinetic data are available for only one reaction of the latter type... 

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