Hydrogen abstraction dynamics in solution studied by picosecond infrared spectroscopy

Hydrogen abstraction dynamics in solution studied by picosecond infrared spectroscopy

Volume 201, number 5,6 CHEMICAL PHYSICS LETTERS 8 January 1993 Hydrogen abstraction dynamics in solution studied by picosecond infrared spectroscop...

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Volume 201, number 5,6

CHEMICAL PHYSICS LETTERS

8 January 1993

Hydrogen abstraction dynamics in solution studied by picosecond infrared spectroscopy D. Raftery, M. Iannone, C.M. Phillips and R.M. Hochstrasser Department

of Chemistry. UniversityofPennsylvania, Philadelphia. PA 19104, USA

Received 2 1October 1992

Solution-phase bimolecular reactions are observed directly for the first time using transient infrared absorption spectroscopy. Laser photolysis of Cl2using a WVpulse at 355 nm creates chlorine radicals in neat cyclohexane solution and initiates a hydrogen abstraction reaction to form HCl or DC1 (in deuterated solvent) as products. Picosecond infrared pulses at 3.3 or 5 pm probe the vibrational state populations to determine the time development of the product formation. The observed reaction rate of HCI formation is ( 5.6 f 1) x 10’ M- ’ s-l, which is approximately I4 + 6 times slower than previously reported gas-phase kinetic results.

1. Introduction The application of microscopic gas-phase kinetic theories to reactions in solutions has become a topic of increasing interest. In particular, the range of applicability of such theories to the condensed phase needs to be investigated more fully. In this Letter, we report our first in a series of experiments designed to probe the state to state dynamics of condensedphase reactions using transient infrared spectroscopy. Gas-phase studies have relied on collisionless conditions that allow evaluation of product states long after the reaction has taken place. Recent experience has shown that vibrational relaxation in the condensed phase is often slow (tens to hundreds of picoseconds) , so that evaluation of vibrational product states is indeed possible. Over the past several years, methods in transient IR spectroscopy have progressed to the point where it allows the sensitivity and time resolution necessary to probe these distributions before they thermalize and to measure reaction rate constants. In this first study we report on the reaction of chlorine atoms with cyclohexane solvent molecules. Further studies will probe the more highly exoergic reactions of fluorine atoms and CN radicals with solvent molecules where the products are expected to have much higher vibrational temperatures. Using transient IR spectroscopy, we hope

to provide the quantitative state to state information that will allow a careful comparison of gas- and condensed-phase reactions. It is clear from evidence gathered on barrier crossing processes in liquids [ 1] that such dynamics are critically dependent on the ability of the solvent to dissipate energy from the reactive coordinates near the transition state. A typical classical time constant T, for translational energy loss due to Stokes translational friction is m/6xaq, so that for low viscosity (q) liquids such as hexane this time constant is z 100 fs for a chlorine atom (of mass m= 35 g/mol) having a radius a of 1 A. It would not therefore be surprising to find that reactions involving chlorine atoms are solvent sensitive when the molecular potential is such that the time spent in the neighborhood of the transition state is much longer than z 100 fs. This classical time T=is even longer for heavier and larger molecular reactants. The characteristic times for passing through the transition state are greatly dependent on the barrier height and shape [ 2 1. A sharp barrier implies stronger forces on the particles and a shorter period r, for reactive motion over a barrier of a given height. This qualitative description clearly indicates that gas-phase reaction dynamical theories should not be assumed to have predictive value in solution phases, particularly in the range r, 5 r,.

0009-26I4/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

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A more quantitative description of solution-phase reactions should ultimately be obtainable from molecular dynamics calculations. For instance, recent simulations [ 3 ] concerning the Cl + C&Cl,+ Cl reaction in argon solution suggest that gas-phase kinetics, and transition state theory in particular, may be successfully carried over (after appropriate modification of the activation free energies to account for solvation) to describe solution-phase reaction rates and product state energy partitioning. Apparently in that case the barrier frequency is much larger than the frequency of effective solvent collisions and the solvent has little effect. Again, the range of validity of such arguments applied to real situations requires experimental verification. Recent developments in transient infrared absorption spectroscopy have allowed the study of the dynamics of vibrational energy transfer [4] and detailed studies of recombinations [ 5 ] of small solute molecules on the picosecond and femtosecond timescales. Transient infrared methods have the advantage of providing relatively well resolved spectra as compared with visible spectroscopy, where the electronic transitions are often severely lifetime broadened, and where fast vibrational redistribution, dephasing or inhomogeneous broadening in the excited electronic state can limit resolution. In this Letter we describe experiments that are aimed at determining the chemical dynamics of bimolecular reactions in solution. In particular, we have determined the reaction rate constant of hydrogen abstraction by chlorine atom in neat cyclohexane solution, and we have probed the vibrational state populations of the products, HCl and DCl, by means of transient IR spectroscopy. Fig. 1 shows the energetics relevant to our experiments. A 355 nm photon excites the chlorine molecule from its ground state to the ‘II state. The transition to the % excited state is spin-forbidden and therefore its potential is omitted for clarity even though it may assume a role in the cage dynamics of C12.In the gas phase, photolyzing chlorine atoms at 355 nm would result in 11 kcal/M of translational energy per atom, which would easily be enough to surmount the small activation barrier for hydrogen abstraction from the reactant. However, as will be discussed, our results in solution indicate that most of the reaction is thermal, and thus the chlorine at514

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2

3

4 Distance

-I

0

1

(A)

Fig. 1.Combined potential energy diagram of chlorine molecule and the reaction coordinate for hydrogen abstraction. The 355 nm transition is shown from the ground state to the dissociative ‘KIstate with excess energy I1 kcal/M per Cl atom. The vertical dotted line indicates the minimum necessary separation (4.6 A) at which Cl atoms may go on to react with hydrogen. The potential energy (dotted line) along the reaction coordinate (hydrogen asymmetric stretch) is also shown, along with the transition state.

oms lose their kinetic energy before they react with the solvent. Fig. 1 also shows the potential energy surface (dotted line ) along the reaction coordinate (the hydrogen asymmetric stretch), along with the Cl-H-R transition state. The activation barrier (exagerated in the figure) is estimated to be 0.5 to 1 kcal/M in the gas phase, and the exoergicity is 8 + 0.5 kcal/M. The dotted vertical line that separates the chlorine molecular potential from the reaction potential indicates the “point of no return” for the chlorine atoms. At this atomic separation, of approximately 4.6 A, chlorine atoms feel an attractive energy kT. At distances less than 4.6 A, chlorine atoms recombine due to the strong attractive force along the ground state potential. Beyond this distance they can go on to react with the solvent. One or both chlorine atoms will have left the original solvent cage in order to achieve this large separation. The hydrogen abstraction from deuterated cyclohexane by chlorine has recently been studied in the gas phase by Flynn and co-workers [6 ] where it was found that energetic chlorine atoms react to produce DC1 whose ro-vibrational product states were monitored with high resolution using a cw diode laser.

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The pseudo-first-order rate constant for the reaction of chlorine atoms with deuterated cyclohexane was measured to be (1.3kO.S) x lO-LDcm3 molecule-’ s-‘. Their experiments showed that the DC1 products were created with relatively little rotational energy, with some vibrational energy, and with considerable translational energy, as could be expected from the heavy-light-heavy mass combination on a repulsive, exoergic potential energy surface and a colinear transition state geometry [ 7 1. The observed v= 1 excited vibrational state population for DCl was about So/b,and would be expected to be lower for HCl product, due to the higher vibrational frequency of HCl. In contrast with these results, our experiments on the reaction of chlorine with cyclohexane give a relatively slow reaction rate constant of (9 &2) X 1O- I2 cm3 molecule- ’ s- ‘, which is more than an order of magnitude slower than the results of Flynn and coworkers. Our results also show that the reaction rate of Cl with deuterated cyclohexane is the same within our errors as compared to the reaction of protonated cyclohexane. Finally, although we were not yet able to detect any excited vibrational state population, we can place an upper limit of about 101 for the DC1 excited state population.

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2. Experimental A schematic drawing of the experimental apparatus is shown in fig. 2. An active-passive modelocked Nd: YAG laser operating at 10 Hz produced a train of about 50 pulses. One pulse was selected by a KD*P Pockels cell and amplified in a three-stage YAG amplifier to produce 6-10 mJ pulse energies with duration of approximately 35 ps. The 1.06 pm pulses were doubled in KDP to produce 532 nm light. This light was then mixed with residual 1.06 pm light to produce the 355 nm UV pump radiation with pulse energies of 100-200 pJ. The UV light was passed through a computer controlled optical delay and focused to a 150 pm diameter spot size at the sample. The 532 nm beam was split into three parts, first to pump a traveling wave dye laser [ 8 ] that was then amplified (using a second portion of the green), and finally combined with the remaining green (about 50%) in LiI03 to produce infrared via difference frequency generation. Mixing 532 nm green with 594 nm amplified light from the TWDL (Kiton Red in MeOH) produced 5 pm infrared light with an energy of approximately 100 nJ per pulse. DCM dye dissolved in DMSO was used to make 3.3 pm infrared light for the HCl experiments. The infrared light was then spatially filtered using a 200 pm diameter pinhole, then passed through a 50% beam splitter to cre-

n

activtipassive ML Nd:YAG with 3 stage amplification

HgCdTe IR detectors

/f\ I delay line ~ ~

~

+ sample

355 ma

Fig. 2. UV pump and IR probe transient absorption apparatus. See text for details.

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ate a reference beam. The remaining IR ( 50°h) was focused at the sample to a size of 150 pm diameter. The transmitted and reference IR beams were focused onto two HgCdTe IR detectors cooled to 77 K, and the signals were then digitized. The absorbance change was computed by taking the log of the ratio of IR intensity (normalized to the reference IR intensity) with and without the UV pump beam hitting the sample. A shutter blocked the UV pump light on alternate pulses. Optimization of the UV and IR beam overlap was carried out using the signal from a thin silicon wafer. The silicon acts as a strong attenuator of IR immediately following the absorption of UV or visible photons as a result of the promotion of electrons to the conduction band where they have a broad IR absorption. A tit of the silicon data to a Gaussian pulse shape convoluted with a step function gave an excellent fit to the silicon data risetimes and is therefore used to determine the instrument function. It is well established that the risetime of the silicon IR transient absorption is subpicosecond. The zero time of the experiment (where pump and probe beam overlap temporally) is also determined in the fit. The fit gives a typical instrument function value of a=39+3 ps (fwhm). A cross correlation of the UV and IR pulses allowed an independent measure of the instrument function. Difference frequency mixing of the UV and IR pulses in a LiI03 crystal produced light at 380 nm that was detected using a monochromator and a phototube. A Gaussian tit to the signal intensity gave an instrument function of 0~42 5 3 ps (fwhm), in agreement with the silicon rise times obtained. The sample cell consisted of a teflon cell with two CaF, windows separated by 150 or 250 pm teflon spacers. It was important to avoid all contact of the chlorine in solution with any metal in the sample region that could easily catalyze the thermal reaction of chlorine. Stray light was also reduced for the same reason. The cyclohexane solvent was pumped through the sample cell by a peristaltic pump at a rate that was sufficient to sweep the sample volume past the laser beam path within a time between laser shots, approximately 100 ms. Chlorine was introduced slowly into a flask and flowed through the system before starting data acquisition. The chlorine concentration was monitored by measuring the absorption 516

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of 380 nm light from a narrow band filtered white light source. The light was passed through a 0.5 mm thick cell and was detected by a phototube. The absorbance was then calibrated using a UV-visible spectrophotometer. Typical chlorine concentrations were 0.5 to 1 M such that the optical density at 355 nm was about 1 OD. Up to 500 laser shots per time point were signalaveraged by adding together several consecutive runs in both forward (increasing time delay) and backward scan directions. This ameliorated variations in the signal intensity due to changes of the laser power and alignment caused by slow temperature drifts.

3. Results Fig. 3 shows a portion of the infrared absorption spectrum of HCl in C6DLZ,which consists of a prominent collision-induced Q branch with a band center that is shifted about 120 cm-’ to lower frequency compared with the gas-phase vibrational frequency, and one of the two smaller shoulders that are primarily P- and R-like branches. The quasi-rotational band structure and relaxation properties of such solution-phase diatomic spectra have been well studied [ 9 1. An IR spectrum (circles) of photolyzing products created by Cl2 in cyclohexane 400 picoseconds

:4 2 a : -8 B m

-20 2900

2800

2700 frequency

2600

2500

(cm-‘)

Fig. 3. HCI transient spectrum at 400 ps delay (open circles) shown with the FTIR spectrum ofHC1 in C& (solid line). The negative absorbance spectrum of C6H12is also shown, scaled to the data at 2690 cm- I, and using the HCl spectrum as the baseline.

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after the photolysis pulse is also shown in fig. 3. This clearly verifies that HCl is produced as the product in the reaction in solution. Unfortunately, the HCl Q&R branches are completely obscured by the C-H solvent stretching band in cyclohexane. Part of the cyclohexane absorption spectrum is shown (dashed line) to indicate the regions which could be probed, and to show the possible origin of the negative absorption signals. Small bleaching signals (negative OD changes) are observed in the spectrum that occur where there are narrow absorption bands in the solvent. These bleaching signals correspond to the small decreases in the concentration of the pure solvent as it undergoes reaction to form cyclohexyl radical, or chlorocyclohexane. The infrared transient absorption signals of the reaction products, HCl (at 2760 cm-‘) and DC1 (at 2020 cm-‘) are shown in figs. 4a and 4b (respectively) as functions of the delay time. Silicon transient absorption signals, taken under precisely the same experimental conditions - in fact, immediately before recording the reported kinetics - are also shown in fig. 4. In all the experiments performed (at several concentrations), the time development of the

10 ’ -150

nohlinear function

of the chlorine

I. -70

I

10

90

170

250

170

250

time (ps)

-10

I



-150

product in these reactions is noticeably slower than the instrument functions. For the HCl reaction, a tit

to an increasing exponential convoluted with a Gaussian pulse with width a obtained from the cross correlation or silicon experiments yields the exponential reaction time constant ‘I= 20 f 4 ps with the errors indicating a 50%increase in the x2 of the fit. For DCl, the tit gives r= 19 ? 4 ps, although the data are somewhat noisier than for HCl at short delay times. In both of these experiments, the Clz concentration was 0.5 M at which point the results seemed independent of concentration. We found that at Cl2 concentrations above 1 M, there occurred the onset of other reactions. These signals were strong at times exceeding 1 ns. The initial risetime was an increasing

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-70

I

10

90

time(ps) Fig. 4. (a) HCI and (b) DC1 transient absorption data (solid circles) with tits to the data (dashed lines) giving reaction times of approximately 2Ok 4 ps. Silicon pump-probe signals (open circles), with tits to the data (solid lines) giving instrument functions of approximately 42 ps, are also shown.

DCl. Using the gas-phase anharmonicity of 102 cm-’ the expected shift of u= 1 to v=2 transition in solution is to 1960 cm- ‘. No change in the transient at 1960 cm-’ was observed as compared to the data taken at 2020 cm-‘, except for a small decrease in overall magnitude of the signal.

4. Discussion

concentration,

which may be due to clustering of the Cl2 molecules that impeded the release of atoms into the solvent. HCl data were obtained at IR wavenumbers between 2780 and 2620 cm-‘. The strong solvent absorption limited our experiments to energies below 2780 cm-’ for HCl and 2020 cm-’ for DCl. The excited vibrational transition v= 1 to v=2 was also probed for

The transient spectrum shown in fig. 3 is convincing proof that we are observing the expected reaction. In addition, the absorption signal intensity observed is consistent with the expected quantity of HCl molecules produced in the reaction. A 100 uJ pulse at 35 5 nm corresponds to about 1.7x 1Oi4photons which could potentially produce approximately 517

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3.4 x 1014chlorine atoms, or a concentration of 0.16 M, assuming an optically thin (70% absorption) sample volume of 0.0026 ~1. The maximum signal intensity observed, 0.014 OD units, indicates that the HCl concentration is only 0.031 M (assuming the gas-phase extinction coefficient ~30 II mol-’ cm-‘), so that only about 19% ofthe chlorine atoms actually react with cyclohexane to form product. The rest of the chlorine atoms presumably recombine very rapidly due to the case effect [ lo] where, due to the small dimensions of the cyclohexane solvent cage, the Cl atoms are under the influence of one another unless they quickly escape the solvent cage and become separated atoms. In studies on IZ recombination in Ccl, solution [ 111 and in photoacoustic measurements on CIZrecombination [ 12] quantum efficiencies (radical atom productions) were found to be in the range of 13%19%. There is excellent agreement between these results and the present study. The possibility that effects such as rotational relaxation might influence the observed signal needs to be considered. The HCl IR spectrum is expected to change with time since the molecules emerge from the reaction with rotational angular momentum which is subsequently quenched. However, the reorientation and rotational relaxation times for HCl are expected to be quite rapid in solution [ 131, occurring within a time of less than 1 ps corresponding to a few collisions, and this would not explain the observed dynamics. At higher concentrations, the appearance of secondary reactions also can become important:

R’+Cl,+Cl’+R-Cl, since Cl’ can produce more HCl. This is a much slower reaction at our concentration because it depends on the diffusion of’R’ and Cl*. The concentration of R’ is low in our experiments, typically less than 0.1 M. However, we have observed a continuous evolution of HCl at much longer times, perhaps as a result of this diffusion controlled reaction. The occurrence of such reactions dramatizes the desirability of carrying out these experiments at the lowest possible concentrations with maximum sensitivity. What then, are the factor which could give rise to such a slow reaction rate that is approximately 14 518

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times slower than gas-phase results of Flynn and coworkers? Clearly a large fraction of the chlorine atoms generated by photolysis do not react during their first encounter with a solvent molecule. From kinematic considerations, after z 3 collisions, occurring within a time of x 1 ps, chlorine atoms would lose the 11 kcal/M of initial translational energy (imparted by the 355 nm photolysis pulse) and thus become thermalized. Experiments and simulations by Andresen and Luntz [ 141 have shown that in the similar abstraction reactions of O(3P) with hydrocarbons, the reaction rate has a very large dependence on the reaction geometry. In their simulations, colinear reaction geometries give rise to activation barriers of approximately 5 kcal/M, while 90” geometries have activation barriers of about 55 kcal/ M. Such highly selective geometry constraints would increase the likelihood that chlorine atoms would thermalize their kinetic energy before reacting with cyclohexane. However gas-phase experiments indicate that Cl atoms react with unit probability upon collisions with an isolated cyclohexane molecule, so this effect alone would not explain the change in reaction rate. One possible explanation of the slow reaction rate is that chlorine atoms are complexed by cyclohexane. This is somewhat unlikely, since competitive kinetic studies show that cyclohexane does not form complexes with chlorine atoms to a large extent, especially in comparison to solvents such as cyclooctane or benzene [ 15 1. However, molecular modelling indicates that chlorine molecules may be sandwiched between cyclohexane molecules in such a way that the photolysis pulse would cause chlorine atoms to collide, not with the closest cyclohexane molecules, but with cyclohexane some (small ) distance away. The two photolyzed chlorine atoms are under the influence of each other until they escape the solvent cage, which as indicated above, must occur quickly, otherwise they will recombine. Two possible scenarios of thermalized Cl atoms come’ to mind. First, cyclohexane cages could absorb the excess kinetic energy as the Cl atoms pass from the photolysis cage to find a reactant solvent molecule. Second, Cl atoms that do go on to react are released slowly from the cage. Based on analogy with I2 dissociation in which I is released from the cage within a few ps [ 16 ] ; this second possibility seems unlikely.

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An interesting question is whether there is a subpicosecond, ballistic component in the HCl product formation dynamics. Femtosccond experiments are currently underway to help elucidate the chlorine dynamics. It is expected that the mean force potential for the bimolecular process will be modified from the gasphase potential by solvation. The resulting change in activation free energy will cause the rates in gas and solution to be different. Therefore the differences in our reaction rates compared to those observed by Flynn and co-workers could simply be caused by an increased activation barrier in solution. Although no direct measurement of the barrier has been reported for this reaction, there are several gas-phase studies of the reaction of chlorine with cyclopentane that give an activation barrier of about 0.5 kcal/M [ 171. A change in activation free energy of approximately 1.1 kcal/M would be required to account for the factor of 14 observed in the rates, assuming we are measuring essentially the thermal rate constant. One might expect the increased barrier height to be due to the free energy of solvated Cl atoms being lowered more than in the transition state. There is also information which could imply that part of the reduction in the rate is due to entropic factors caused by the solvent caging effect on Cl [ 181. Assuming the rather high barrierof 1.6 kcal/M and our present signal/noise, observation of an appreciable change in the rate would require experiments to be conducted around 220 K. We hope soon to address this interesting issue by determining the temperature dependence of the reaction rate with an improved apparatus. Finally, the LEPS potential surface used by Flynn and co-workers allows us to estimate our expected isotope effect in the reaction rate. Assuming an inverted harmonic potential to describe the reaction coordinate around the transition state region of their LEPS potential would give rise to a barrier crossing frequency of about 400 cm-‘. One would then predict that the isotope effect, in substituting D for H, would reduce the reaction rate by approximately 1.3, a value that is within Flynn and co-workers’ error bars and slightly outside of ours. Also, we would not expect a large isotope effect due to the isotope spectator solvent molecules. The pre-exponential factor predicted by transition state theory [ 19] using Flynn and co-workers LE6PS surface gives a result that is

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within a factor of two of the gas-phase rate they measured. Unfortunately, the LEPS surface in their paper does not seem to satisfactorily describe the activation barrier, since the LEPS parameters they provide result in a barrier of over 5 kcal/M. Since their experimental reaction energies are 5.3 and 14.3 kcal/M, it was still possible for them to use such a potential surface to advantage in their trajectory calculations. However our results are certainly not consistent with such a high barrier.

5. Conclusions We have measured the time development of hydrogen abstraction reactions in solution using transient infrared spectroscopy. The observed reaction rate constant is slower than previous gas-phase results for chlorine atoms reacting with cyclohexane. We do not see an isotope effect on the reaction rate. We are currently improving these experiments to probe the excited state populations with higher sensitivity and extending them to study other bimolecular reactions in solution. It should be possible in the near future to determine the reactant to product energy redistribution on a state to state basis in solution for simple reactions, as has been done for many gas-phase reactions, thereby allowing direct comparisons between isolated molecule and solution-phase dynamics.

Acknowledgement DR gratefully acknowledges the National Science Foundation for a postdoctoral fellowship in chemistry. This work is supported by the National Institute of Health and the National Science Foundation.

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[ 5 ] J.N. Moore, P.A. Hansen and R.M. Hochstrasser, J. Am. Chem. Sot. 111 ( 1989) 4563. [ 61 J. Park, Y. Lee, J.F. Hershberger, J.M. Hossenlopp and G.W. Flynn, J. Am. Chem. Sot. 1I4 ( 1991) 58. [ 71 C.A. Parr, J.C. Polanyi and W.H. Wang, J. Chem. Phys. 58 (1973) 5; R.L. Johnson, KC. Kim and D.W. Setser, J. Phys. Chem. 77 (1973) 2499. [8] J. Hebling, J. Klebniczki, P. Heszler, Zs. Bor and B. R&z, Appl. Phys. B 48 (1989) 401. [9] D. Richon and D. Patterson, Chem. Phys. 24 (1977) 235; B.C. Sancturary and D. Richon, J. Chem. Phys. 69 ( 1978) 3782. [ lo] D. Booth and R.M. Noyes, J. Am. Chem. Sot. 82 ( 1960) 1868.

[ 111R.L. Strongand I.E. Willard, J. Am. Chem. Sot. 79 ( 1957) 2098; R. Marshall and N. Davidson, J. Chem. Phys. 21 ( 1953) 2086.

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[ 121K.B. Clark and D. Griller, Chem. Phys. Letters 168 (1990) 477.

[ 131R.G. Gordon, J. Chem. Phys. 44 (1966) 1830. [ 141 P. Andresen and AC. Luntz, J. Chem. Phys. 72 (1980) 5842. [15 E.S. Huyser, in: Advances in free-radical chemistry, Vol. 1. Solvent effects in free radical reactions, ed. G.H. Williams (Logos Press, London, 1965). ]16 J.K. Brown, C.B. Harris and J.C. Tully, J. Chem. Phys. 89 (1988) 6687. 117 V.N. Kondratiev, Constants of gas phase reactions reference book, ed. R.M. Fristrom (NBS, Washington, 1972); D.D. Davis, W. Braum and A.M. Bass, Intern. J. Chem. Kinetics 2 (1970) 101. fJ8 F. Galiba, J.M. Tedder and J.C. Walton, J. Chem. Sot. B ( 1966) 604. [19 R.D. Levine and R.B. Berstein, Molecular reaction dynamics and chemical reactivity (Oxford Univ. Press, Oxford, 1987).