Multiple Photon IR Dissociation of ( CF ) CBr and Synthesis of ( CF ) Cl in Laser-Radical Chemical Reactions

CO2-laser induced multiple photon excitation and dissociation of the molecule (CF3)3CBr in the environment of I2 has been studied experimentally and theoretically. The synthesis of the (CF3)3CI molecule in laser-radical chemical reactions with high yield has been demonstrated.


I. INTRODUCTION
Multiple photon (MP) dissociation of molecules under powerful IR laser radiation ,2 is becoming a simple and universal tool for fast production of high concentrations of various free radicals.These radicals can be used for selective synthesis of molecular compounds in radical gas-phase chem- ical reactions.In MP dissociation of molecules the free radicals are gen- erated under highly nonequilibrium conditions.The products of laser- radical chemical reactions may differ greatly from the products of their corresponding thermal radical reactions because under nonstationary con- ditions the temporal factors may be also essential besides the thermodynamic ones.IR laser-radical chemical synthesis of such molecules as BC12H, SFNF2, CF3Br and CF3I has been studied in several works. 3-It is shown fragmentation of the molecules.A classical example of such successive dissociation is the MP dissociation of the SF6 molecules forming the rad- icals SF and then SF4 .7 Besides, situations can take place when in a molecule with similar dissociation energies in different channels the rates of its dissociation in these channels become comparable in magnitude.This leads to competition of different dissociation channels and gives birth to different fragments. 8It is evident that the processes of fragmentation for selective laser-radical chemical synthesis are undesirable.Therefore, a condition of MP dissociation should be chosen in order to eliminate these processes.
This work deals with IR MP excitation and dissociation of the 14-atom molecule (CF3)aCBr in the environment of iodine for the purpose of syn- thesis of the (CF3)3CI molecule in an IR laser-radical chemical reaction.In MP dissociation of the (CF3)3CBr molecule the undesirable fragmen- tation may be rather essential which materially decreases the efficiency of synthesis of (CF3)3CI.But, as our studies show, the fight choice of con- ditions of MP excitation allows a fairly high dissociation yield of (CF3)aCBr and at the same time a low level of undesirable fragmentation to be provided, in this way it makes the process of synthesis of (CF3)3CI sufficiently effective.A simple theoretical model of MP dissociation of (CF3)3CBr enables us to explain quite satisfactorily the experimental data.

EXPERIMENTAL SETUP
MP excitation and dissociation of the (CF3)3CBr molecule was carded out with a TEA CO2 laser (Figure 1) with selection of vibrational-rotational lines.The lines were selected by turning the diffraction grating.With the use of a plane and a concave mirrors the radiation was directed into the cell.The radius of curvature of the mirror was chosen so that the caustic was longer than the cell.Directly before the cell a diaphragm (d 3-5 mm) was placed transmitting only the central part of the beam.In this manner we could produce a beam rather uniform in section and length.
Attenuators made of CaF2 were used to change the pulse energy.A part of radiation was reflected with a plane-parallel plate made of NaC1 to the L CELL

FIGURE
Experimental setup.
IR detector to control the input energy.A calorimeter was used to calibrate it and measure the output energy.The laser pulse was standard in form with its leading spike duration of about 200 ns and a tail duration 0.2 IS.
The effective pulse duration "r is determined by the ratio of energy in its front and tail.It was varied by changing the content of nitrogen in the gas mixture.The pulse shape was determined for each mixture with a "photondrag" detector.The experiments on MP dissociation and synthesis made use of stainless-steel cells 12.2 cm long, with their internal diameter of 1.4 cm and windows of NaC1.Some iodine crystals were placed in a small appendix of the cell.The cell was located in a thermostat; the temperature variation in the thermostat allowed the pressure of vapor in the cell volume to change.The composition of reaction products in the cell was controlled mainly with an IR spectrometer as well as a mass spectrometer.MP ab- sorption in (CFa)aCBr was measured per one pass in cells of 100 cm and 12.2 cm and also with the use of an optical-acoustic detector. 9

MULTIPLE PHOTON ABSORPTION OF (CFa)aCBr
The (CF3)3CBr molecule has two relatively weak absorption bands like 1o in the region of action of CO2 laser: v6 934 cm -1 and v2 963 "l)cc cm-.The spectra of linear IR absorption for (CFa)aCBr and (CF3)3CI at 295 K are given in Figure 2. Figure 3 shows a section of the spectrum of the IR laser radiation energy E absorbed in (CFa)aCBr varying with frequency, for a radiation energy fluence 0.32 J/cm2.The value of E is measured with the optoacoustic method.There is a conventional shift to the long-wave side in the MP absorption spectrum relative to the linear spectrum 9 which is conditioned by vibration anharmonicity.
The dependence of absorbed energy E on radiation energy density (Figure 4) was measured both in pure (CFa)aCBr and in its mixture with iodine for varying duration of IR pulse.The dependence E () observed is nearly linear.The cross-section of MP absorption slightly decreases with an increase in which can be usually observed in complex molecules with a low boundary of quasi-continuum, for example in S2Fo. 1 In pure (CFa)3CBr with < 3 J/cm 2 the increase of E somewhat slows down which seems to be connected with dissociation of a part of molecules during a radiation pulse.As may be seen from Figure 4, the absorbed energy in the presence of 12 Tort iodine is much higher than in pure (CF3)3CBr.This may be , ] , (CS/,CZ 400 I00 I00 II00 I000 900 V(cm-')  Dependence of average absorbed energy on laser energy fluence. 1. P(CF3)3CBr 0.6 Torr, "r 100 ns; 2. P(CF3)3CBr 2 Torr, PI: 12.5 Torr, "r 100 ns; 3. P(CF3)3CBr 2 Torr, P12 12.5 Torr, "r 1.5 txs v 934.90 cm-.
explained by two causes.First, the effect of rotational relaxation increases the fraction q of molecules involved in the process of MP excitation.Second, there may be an effect of vibrational deactivation produced by the iodine molecules.Indeed, since the cross-section of MP absorption decreases with an increase in excitation level, the processes of V-V ex- change and V-T,R relaxation, as the (CF3)aCBr molecules collide with 12, must cause the value of E to increase.
To study the influence of rotational relaxation on MP excitation of (CF3)3CBr we measured the fraction of molecules involved in the process of MP excitation.The value of q was found from the maximum relative increase of when a buffer gas is added.The value of q turned out to be almost independent of ( in the measured range of + from 0.03 to 7 J/cm2. The values of q measured near the maximum of linear absorption bands at the pressure of (CF3)3CBr P 0.14 Torr and "r 100 ns were: with v 967.71 cm-1, q 0.85 0.1; and with v 934.90 crn-1, q 0.7 _ _ .0.1.The values of q measured with P > 0.6 Torr and with the same frequencies were near unity.This means that the main cause of the increase in absorbed energy, when iodine is added (Figure 4), is a con- siderable vibrational deactivation of the (CF3)3CBr molecules during their collisions with 12.This conclusion is also supported by the results of measurements of the dissociation yield of (CF3)3CBr in the atmosphere of iodine.

MULTIPLE PHOTON DISSOCIATION OF (CFa)aCBr AND SYNTHESIS OF (CFa)aCI
A considerable fraction of molecules of (CF3)3CBr dissociates in one ra- diation pulse with > 1-2 J/cm2.As a result of MP dissociation of (CF3)aCBr in the environment of 12, the final products contain the desired product (CF3)3CI and the secondary product CFaI.The yield and com- position of end products in this case depend greatly on the conditions of excitation.Figure 5 shows how the radiation energy fluence affects the 1,0 dissociation yield of 13 of (CF3)aCBr as well as the fraction of dissociated molecules fSd/f5 used for the formation of the desired radical (CF3)3C" which is essential for synthesis of (CF3)3CI in the ensuing reaction with I2.The value of 13 increases rapidly with an increase of ).With + 5.5 J/cm 2 around 50% molecules in the volume under exposure dissociate during one pulse.At the same time the value of fSd/f5 drops monotonically with an increase in +, and with + 5.5 J/cm 2 just about 40% dissociated molecules are spent to form the desired radical (CF3)3C'.
Figure 6 illustrates the dependence of 13 and 3d/3 on the pressure of molecular iodine Pt: controlled by the variations in the temperature of the cell walls.At low PI: the value of 13 increases with an increase of P which is due to prevention of recombination of the radicals (CF3)3C" and Br.
simultaneously with an increase in 13.This is why on the long-wave side of the spectrum we can realize such optimal conditions for synthesis of (CF3)3CI as a high dissociation yield and low losses of original substance in the formation of secondary products.
The process of MP dissociation of (CF3)3CBr is essentially affected by the IR radiation pulse duration.In the table the values of 13 and fSd/ are given for three different values of IR radiation duration.It may be seen that with decreasing "r, the value of 13 the same, the value of d/3 increases considerably, i.e., the process of synthesis of (CF3)3CI becomes more efficient.In qualitative respect this effect can thus be explained: As the pulse duration is reduced, the fraction of the (CF3)3C" radicals formed during an IR radiation pulse and which are able to absorb energy and dissociate, with the CF3" group breaking away, is decreased.For quanti- tative interpretation of the results obtained it is necessary that the dynamics of MP dissociation of (CF3)3CBr should be considered.
First of all, it should be pointed out that the CF3" radical can be formed in two different channels: 1) immediately in dissociation of (CF3)3CBr [reaction (lb), "a parallel channel"]; and 2) in dissociation of the (CF3)3C" radical [reaction (2a), "a successive channel"].We think that in our case the main contribution is made by the successive channel.If CF3" were FIGURE 8 Possible pathways of (CFa)3CBr molecule dissociation.formed in the parallel channel, the both basic products (CF3)3CI and CF3I would result directly from decomposition of (CFa)aCBr [reactions (la) and (lb)], and then their ratio would be determined exclusively by the excitation level of the original molecule.In this case fSd/B would decrease with increasing energy since at low excitation levels the breakaway of the Br atom is preferential due to a smaller bond energy, and at high excitation the preferential breakaway of CF3" is conditioned by statistical factors.
Since the total expenditure of (CFa)3CBr also depends on the level of its excitation, in case of formation of CF3I in the parallel channel one may expect that the ratio of the products will drop monotonically with increasing I and not depend on other factors.From Figure 7 we can see, however, that with excitation at the frequencies of 930 and 951 crn-' at the same yields I 0.2 the product ratios will be radically different: 0.8 and 0.3 respectively.This difference may be explained only if we attribute the main contribution to the formation of (CF3)3CI to the successive channel provided that the (CF3)3C" radical has a much smaller absorption cross- section in the region of 930 cm-1.
The conclusion on a small contribution of the parallel channel is also supported by calculating the decay rate in reactions (2a) and (lb) with the use of the RRKM theory. 12Even though the absolute accuracy of these calculations is not large because of nondeterminacy in the choice of ac- tivated complex parameters, the relative position of the curves given in Figure 9 allows us to conclude that with the real excitation energies E < 60 000 cm -1 reaction (lb) may be neglected as compared to (la).
In Figure 9 we can see another peculiarity characteristic of dissociation of large molecules: their decay rate increases rather slowly with increasing / m q, 36 O0 8 $2 $6 60 Evto,, energy with the result that their strong excitation over the dissociation limit becomes possible.The decay rate of (CF3)aCBr, for example, will be only koo 6.2 105 s -1 with E 2Do 48 000 crn -, and even if it is higher than the dissociation limit by three times (i.e., with E 3D 72 000 cm-1) the decay having a constant koo 5.3 108 smay compete with further acquiring of energy (with "r 100 ns and E 50 000 cm -1 the rate of photon absorption equals 5 108 s -1).So, in a pulsed IR field, rather complex polyatomic molecules can easily acquire energy that ex- ceeds by 2 or 3 times the energy of rupture of the weakest bond.It is a low decay rate of (CF3)aCBr that causes the value of I to drop quickly as Px: increases (Figure 5).Indeed, vibrational deactivation of the molecule during its collisions with the buffer gas occurs in a time "r + 1/kDo.
According to the estimates given above, this time for (CF3)3CBr may exceed considerably the pulse duration.At the same time, in case of such a simple molecule as CF3Br the decay rate is high enough even if its excitation is a little higher than the dissociation limit and vibrational deac- tivation occurs in a time of the order of a pulse duration.As a result, the rate of decrease of 13 with an increase in PI is an order lower for the CFaBr molecule than for (CF3)3CBr. 6aking into consideration the fact that the parallel channel (lb) turns out to be unessential, we want to demonstrate now that the kinetics of formation of the basic [(CF3)3CI] and secondary [CF3I] products agrees with the following simple scheme if the assumption is made that the radicals (CF3)3C" and CFa. are completely bound with iodine.The validity of such interpretation is confirmed by calculations within a simple model when the kinetics of formation of each of the radicals is described by an Arrhenius-like expression dni ni_Ai_exp dt Si-1 Di-1 ) Ei-" Ez,i- where ni (i 0,1,2) are the densities of (CF3)3CBr, (CFa)aC" and (CFa)2C: respectively; Di is the energy of rupture of corresponding bonds; A_I A2 0. The expression exp (-s.,Dd(E + Ez,)) is used as the dependence of the dissociation rate on the energy E, which is contained in s vibrational degrees of freedom of the i-th component in the mixture, Ez,i is the cor- responding energy of zero-order vibrations.This expression follows from the RRKM theory when we use the quasi-classical expression for state density at Boltzmann distribution of vibrational energy.Dissociation no- nequilibrium was allowed for by introducing different vibrational temper- atures Ti Edsi for each component of the mixture.Their variation was described by the equations O'irt'J + si(T Ti)ni-1Ai-exp D + 2 Ti n.,Aiexp -Ei + E,,: The first term describes the absorption of laser radiation of intensity I.'2 This assumption is quite substantiated since, as the number of atoms is large, even after absorbing one quantum the molecules turns out to be in a region with a high density of vibrational states.This can be also confirmed by the fact that the value of q observed for (CF3)3CBr is close to unity.The second term in (6) allows for the arrival of energy to the component "i" together with the dissociation products of the component "i 1 ," the third term allows for the energy expended on dissociation.The rest of the terms are conditioned by collisional transfer of energy between the components of the mixture.The last term allows for the presence of a buffer which basically consists of I2 but, for simplicity, includes both the rest of the components of the mixture not allowed for in (5), ( 6) and the rotational- translational degrees of freedom of (CF3)3Cnr, (CF3)3C" and (CF3)2C:.The buffer heating is described by the equation dTb A si+ ,i (Ti The first term, containing A si+ 1, si+l s < 0, describes the heating of the buffer caused by transfer of some energy during dissociation to rotational-translational degrees of freedom; their corresponding terms are present in (6), too.Equations ( 5)-( 7) contain a large number of parameters the values of which are not known now.Since the processing of our results does not enable us to restore unambiguously the full set of parameters we have assigned some rightful, though arbitrary, values to the most of them (see Table I).The energy of rapture of the C---C bond in the radical (CF3)3C" Do and the rate constants of energy transfer to the buffer (crV)b were considered to be varying parameters.The rates (o'v)ij did not vary since the energy transfer between the dissociating components is unessential due The following parameters were used in calculations: Do 24000 crn-; DI 22000 crn-; Ao 104s-" A 10Ss-; So 36; S 33; $2 27; PI2 12 Torr; tlb(tV)i 5" 10 -1.
to a small difference between their temperatures.For simplicity, we con- sidered the cross-sections tri to be independent of the excitation level and i; in this case the set of equations ( 5)-( 7) has a simple integral that expresses the law of conservation of energy This equation was actually used to control the accuracy of the computer calculations.
The reaction yield was calculated for the instant tcoo 5 txs after the pulse began.In most cases the reaction by this moment was already over and its increase did not change essentially the results.
Before turning to the comparison of this model with experiment we should stress that in deducing ( 5) we used the assumption on Boltzmann (thermal) energy distribution over vibrational degrees of freedom.This assumption in a reaction under high-power pulsed radiation can no doubt be fulfilled just approximately.Moreover, even if the distribution, on the whole, is close to the Boltzmann one, the presence of a chemical reaction will cause the high-energy distribution tail to vary from it.It is known that the main contribution to the reaction at Boltzmann distribution is made by molecules with their energy of the order of E + D. 13 For (5) to be valid, it is necessary that the reaction rate with this energy should be lower than the frequency of collisions.Using the data given in Figure 9, we can see that at a pressure of about 10 Torr these rates become equal with E 40 000 cm -1 which with s 36 corresponds to T 1100 cm-1.Since in our calculations the temperatures were 800 to 900 cm -1 this suggests that the Boltzmann distribution is partially disturbed.Besides, in case of short pulses the rate of laser excitation also becomes sufficiently high which disturbs the Boltzmann distribution.Therefore we should not expect full quantitative agreement of the model with experiment.We are going to use the results of our calculations only for qualitative confirmation of our interpretation of the basic results.
The reaction yield 13 and the product ratio are calculated in the table.The best agreement was attained with the value of relaxation rate no (O'V)io 5 107 cm -1 which corresponds to an average loss of energy by a reacting molecule of approximately 200 crn -1 per one gas-kinetic collision with a molecule of 12. Figure 6 (curve 2) shows the estimated dependence 13(PI) which approximates the experimental one.
The dependence 13 (E) observed turns out to be less sharp than that predicted by our model.The experimental and theoretical dependences can be consistent within this model only due to variations in the energy of rupture of the CmI bond Do.But the calculations performed show that satisfactory agreement can be attained only at inadmissible parameters Do 70 000 crn -1 and Ao 4.3 10 37 s-1.So, then we used the realistic values Do 24 000 cm -1 and Ao 1014 s -1 which did not vary.The slow dependence 13 (E) observed was interpreted as evidence of an increase in the rate of relaxation processes with increasing molecular energy which was not allowed for within the model concerned.
Here we want to point out a factor which at first sight seems unexpected.
To obtain a slower dependence 13 (e--) we must increase the activation energy Do despite the fact that this leads to a sharper dependence of the reaction rate df3/dt exp [-soDo/(E + Ez,o)] on absorbed energy.This result becomes clear if we take into account that Do in this case is also the dissociation energy, and the law of conservation of energy calls for a decrease of 13 with increasing Do.In Ref. 14 it is shown that in such a case the dissociation yield varies linearly with absorbed energy 3 (E Eth)/Do (Eth is a threshold energy), and an increase in Do here really brings about a slower dependence 13 (e--).
Satisfactory agreement with experiment on f3d/f3 for a long pulse ('r 800 ns) has been obtained with D1 22 000 crn-1.This energy turns out to be lower than the energy of the C---C bond (--30 000 cm-1), which may be attributed to a less stable structure of the radical (CF3)3C'.In our model, however, the dependence of fSd/f3 on pulse duration has proved too slight.This is quite clear and again related to the distrubance of the Boltz- mann distribution at high energies.Indeed, the use of (5) presupposes that dissociation products are formed at a temperature close to the temperature of the reagent and at this instant their rate of decay is equal to the equi- librium rate with this temperature.Taking into account the fact that the main contribution to dissociation is made by molecules with their energy of about E + D we may conclude that, for this picture to be valid, two- step dissociation must have Boltzmann distribution in the primary molecule up to energies of about E + 2Do which is hardly probable.In a more realistic model we should allow for the time required for the product of the first stage of decay [(CF3)3C'] to acquire energy which would be sufficient for further dissociation.Though it can not consistently be done in our model limited by Boltzmann distributions, we tried to estimate its influence in a simplest model.We assumed that radicals (CF3)3C" can dissociate only after some decay "tint (incubation time) from their formation.So the dissociation rate of (CF3)3C" has been calculated by substituting into the fight-hand side of (5) the values of concentration of (CF3)3C" at the proceeding instant of time t "fine.The discrepancy with experiment is much less with Xin 100 ns (see Table I).Thus, the basic distinctive features of the process of laser synthesis can be understood within the simple model developed with reasonable param- eters.

CONCLUSION
The studies of IR MP excitation and dissociation of a sufficiently complex molecule (CFa)aCBr in the environment of iodine with the object of syn- thesizing the (CFa)aCI molecule carded out in this work have shown that, with the frequency and duration of IR radiation properly chosen, the process of synthesis can be effective enough.Specifically, with a pulse duration of CO2 laser "r 100 ns, frequency v 931.00 crn -, energy fluence 5.5 J/cm2, the dissociation yield of (CFa)3CBr will be 13 0.7, and no less than 70% of the dissociated molecules are transformed in this case into (CF3)3CI.Further increase in the efficiency of the process of laser- radical synthesis of (CF3)aCI may be connected with the application of the method of combined (thermal + MP) excitation. 6Besides, as it follows from Figure 6 and Table I, if we reduce the frequency and duration of CO2 laser pulse this may also contribute to the growth of the dissociation yield of (CF3)3CBr and the ratio of yield of the desired product (CF3)CI to dissociation yield fd/fS.But, as a rule, this entails a decrease in the CO2 laser efficiency and hence the deterioration of the energetics of synthesis.
The theoretical model of MP dissociation of (CF3)3CBr developed in this work provides quite satisfactory qualitative interpretation of the ex- perimental data obtained.Despite the use of essential simplifying as- sumptions (Boltzmann distribution of energy, independence of relaxation rate on energy, etc.) with reasonable choice of the parameters describing dissociation and relaxation, it was possible to reproduce the data on dis- sociation yield 13, relative yield of desired product fSalf and their depen- dence of iodine pressure.At the same time the comparison of the exper-