J. Phys. Chem. 1996, 100, 16169-16174
16169
Displacement Dynamics of Fluorine Atoms Reacting with Bromobenzene and Iodobenzene Molecules J. J. Wang, D. J. Smith, and R. Grice* Chemistry Department, UniVersity of Manchester, Manchester, M13 9PL, U.K. ReceiVed: February 29, 1996X
Reactive scattering of F atoms with C6H5Br and C6H5I molecules leading to both hydrogen and halogen atom displacement has been studied at an initial translational energy E ∼ 38 kJ mol-1 using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distributions of C6H5F reactive scattering show symmetrical forward and backward peaking, which is consistent with a long-lived collison complex, but the product translational energy distributions peak at a fraction f′pk ∼ 0.18 of the total available energy and lie well above the predictions of phase space theory. The branching ratio ∼4 favors halogen atom over hydrogen atom displacement. The H atom displacement pathway is impeded by a potential energy barrier and occurs in competition with atom migration around the benzene ring. The halogen atom displacement occurs directly an F or H atom becomes bonded to the carbon atom adjacent to the halogen atom, following F atom or possibly H atom migration from other locations on the ring. The H atom displacement pathway for both FC6H4Br and FC6H4I reaction products shows a nominally isotropic angular distribution and a product translation energy distribution peaking at a fraction f′pk ∼ 0.2 of the total available energy, in line with the H atom displacement dynamics previously observed for F + C6H5Cl.
Introduction The I atom abstraction reaction of F atoms with C6H5I molecules has been studied recently1 and shows long-lived complex behavior associated with a C6H5IF intermediate. The F atom is bonded to the I atom in this intermediate and the observed IF reactive scattering shows no evidence for F atom migration from the I atom to the π system of the benzene ring. The corresponding Br atom abstraction from C6H5Br molecules is substantially endoergic and has not been observed. In order to investigate the alternative isomers of these reaction intermediates, the reactive scattering arising from the displacement pathways which follow F atom addition to the benzene ring is reported here. The displacement reactions of F atoms with C6H5Cl molecules2 exhibit long-lived complex behavior with H atom displacement being inhibited by a potential energy barrier of height Eb ∼ 20 kJ mol-1 with respect to reaction products, while Cl atom displacement involves no intervening potential energy barrier
F + C6H5X f FC6H5X f C6H5F + X f FC6H4X + H
(1a) (1b)
where X ) Br, I. Experimental Method The apparatus was the same as that previously employed in studies of F atom reactive scattering with C3H5I molecules3 using a high-pressure microwave discharge source4 with an alumina discharge tube. The velocity distribution of the beam of F atoms seeded in He buffer gas was measured by a beam monitor mass spectrometer yielding the parameters listed in Table 1. The bromobenzene and iodobenzene beams issued from a glass nozzle of ∼0.15 mm diameter. Both the bromobenzene and iodobenzene molecules were seeded in ∼60 mbar of N2 buffer gas, since a reservoir temperature ∼50 °C sustains vapor pressures of only 5-10 mbar. The halobenzene beam velocity distributions were measured by the rotatable mass X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)00636-3 CCC: $12.00
Figure 1. Laboratory angular distribution (number density) of C6H5F reactive scattering from F + C6H5Br (upper panel) and C6H5I (lower panel) at initial translational energies E ∼ 38 and 39 kJ mol-1. Solid line shows the fit of the kinematic analysis.
spectrometer detector using cross-correlation time-of-flight analysis5 to yield the parameters listed in Table 1. © 1996 American Chemical Society
16170 J. Phys. Chem., Vol. 100, No. 40, 1996
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Figure 3. Laboratory velocity distributions (flux density) of reactively scattered C6H5F from F + C6H5Br at an initial translational energy E ∼ 38 kJ mol-1. Solid lines show the fit of the kinematic analysis. Only representative samples of the distributions measured are shown.
Figure 2. Laboratory angular distributions of FC6H4Br reactive scattering from F + C6H5Br (upper panel) and FC6H4I from F + C6H5I (lower panel).
TABLE 1: Beam Velocity Distributions, Peak Velocity Wpk, Full Width at Half-Maximum Intensity Wwd, and Mach Number M beam
Vpk/m s-1
Vwd/m s-1
M
F(He) C6H5Br C6H5I
1980 720 780
730 250 230
5 5 7
Results Angular distribution measurements of C6H5F reactive scattering from the pathway of eq. 1a yield ∼170 and 120 counts s-1 against backgrounds ∼ 35 counts s-1, while the FC6H4X scattering yields ∼190 and 420 counts s-1 against backgrounds ∼3 and 20 counts s-1 for bromobenzene and iodobenzene, respectively. The laboratory angular distributions of C6H5F number density shown in Figure 1 extend over a wide range of laboratory scattering angles, while the angular distributions of FC6H4X number density shown in Figure 2 exhibit narrow peaks located in the direction of the laboratory centroid. The angular distribution of Figure 2a was measured on the FC6H4Br+ mass peak and the same result was also obtained measuring on the FC6H4+ mass peak. The angular distribution of Figure 2b was measured on the FC6H4+ mass peak. The laboratory velocity distributions of C6H5F flux shown in Figures 3 and 4 were measured using integration times ∼3000 s to gain signal to noise ratios ∼10 at the peaks of the distributions, while the distributions of FC6H4X flux shown in Figure 5 used integration times ∼300 s to gain signal to noise ratios ∼30. Kinematic analysis
Figure 4. Laboratory velocity distributions of reactively scattered C6H5F from F + C6H5I at an initial translational energy E ∼ 39 kJ mol-1.
of these data was performed using the stochastic method6 with the center of mass differential cross section expressed as a product of an angular function T(θ) and a velocity function U(u)
Icm(θ,u) ) T(θ) U(u)
(2)
The center of mass angular distributions for C6H5F scattering in Figures 6 and 7 show forward and backward peaking which is symmetrical about θ ) 90°, with the peaks being sharper for F + C6H5Br than F + C6H5I. In both cases the angular distributions show a relative intensity ∼0.6 in the sideways direction. The product translational energy distributions P(E′)
Displacement Dynamics of Fluorine Atoms
J. Phys. Chem., Vol. 100, No. 40, 1996 16171
Figure 5. Laboratory velocity distributions of reactively scattered FC6H4I from F + C6H5I (upper curve) and FC6H4Br from F + C6H5Br (lower curve).
Figure 7. Angular function T(θ) and translational energy distribution P(E′) for C6H5F reactive scattering from F + C6H5I at an initial translational energy E ∼ 39 kJ mol-1.
total available energy with only modest tailing toward higher energy, consistent with the lower exoergicity of the reaction pathway of eq 1b. The backfits of the kinematic analysis are shown by solid curves in Figures 1-5. The peak E′pk and average E′av product translational energies are given in Table 2 together with the initial translational energies E and reaction exoergicities ∆D0 calculated from the heats of formation of C6H5F, C6H5Br, and C6H5I from Pedley et al.7 The ratio of the total reaction cross sections for the pathways of eqs 1a and 1b have been determined by the integration over scattering angle and product velocity
Q ) 2π ∫0 T(θ) sin θ dθ ∫0 π
umax
U(u) du
(3)
This yields ratios Q(X)/Q(H) ∼ 3 and 5 for X ) Br, I, when the fragmentation patterns8 of the respective product molecules in the mass spectrometer ion source are taken into account. Discussion
Figure 6. Angular function T(θ) and translational energy distribution P(E′) for C6H5F reactive scattering from F + C6H5Br at an initial translational energy E ∼ 38 kJ mol-1. Dashed curve shows the distribution of initial translational energy.
peak at a fraction f′pk ∼ 0.18 of the total available energy with tails extending up to higher energy. The angular distributions of FC6H4Br scattering in Figure 8 and FC6H4I scattering in Figure 9 are both nominally isotropic. The product translational energy distributions both peak at a fraction f′pk ∼ 0.2 of the
Fluorine atom addition to the π system of the benzene ring is expected9 to involve perpendicular approach of the F atom to the plane of the ring and may give rise to four isomers of the FC6H5X free-radical intermediate according to whether the F atom is bonded to the carbon atom adjacent to the halogen atom or to the ortho, meta, or para carbons. As shown in Figure 10, the stability10 of the FC6H5X radical ∼100 kJ mol-1 with respect to reactants leaves the radicals formed by addition to carbons adjacent to H atoms stable with respect to H atom displacement, which also involves a potential energy barrier. However, the radical formed by F atom addition to the carbon atom adjacent to the halogen is strongly unstable with respect to halogen atom
16172 J. Phys. Chem., Vol. 100, No. 40, 1996
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Figure 8. Angular function T(θ) and translational energy distribution P(E′) for FC6H4Br reactive scattering from F + C6H5Br at an initial translational energy E ∼ 38 kJ mol-1.
Figure 9. Angular function T(θ) and translational enegy distribution P(E′) for FC6H4I reactive scattering from F + C6H5I at an initial translational energy E ∼ 39 kJ mol-1.
TABLE 2: Reaction Energetics (kJ mol-1), Initial Translational Energy E, Peak Product Translational Energy Epk, Average Product Translational Energy Eav, and Reaction Exoergicity ∆Do reaction path
E
E′pk
E′av
∆Do
F + C6H5Br (eq 1a) F + C6H5Br (eq 1b) F + C6H5I (eq 1a) F + C6H5I (eq 1b)
38 38 39 39
39 19 52 19
50 27 72 28
189 63 253 63
displacement for X ) Br, I and marginally unstable2 for X ) Cl where ∆D0 ) 126 kJ mol-1. The angular distributions of C6H5F reactive scattering in Figures 6 and 7 show symmetrical forward and backward peaking which is characteristic of a longlived collision complex with a lifetime of many rotational periods. Phase space theory2,11-13 provides a method of calculating the product translational energy distribution for dissociation of a long-lived collision complex when this is unimpeded by an intervening potential energy barrier. All accessible quantum states of the reaction products are assumed to be populated with equal probability, subject only to the conservation of energy and total angular momentum in the reactive collision. The product translational energy distributions calculated from phase space theory using the full 30 vibrational degrees of freedom of the C6H5F molecule2,14 with initial bm ) 3 Å and final b′m ) 1.5 Å maximum impact parameters, shown in Figure 11, peak well below the experimental distributions and are much narrower for X ) Br, I. This contrasts with previous measurements2 on the chlorobenzene reaction X ) Cl where agreement was found with phase space theory using a reduced number of 13 vibrational degrees of freedom. However, the discrepancy between the phase space predictions and the experimental distributions is not eliminated by employing 13
Figure 10. Potential energy profiles for the reaction pathways of F + C6H5Br leading to Br atom and H atom displacements and to Br atom abstraction (upper panel) and the corresponding pathways of F + C6H5I (lower panel), showing the free-radical intermediates.
vibrational degrees of freedom for X ) Br, I. This indicates that halogen atom displacement may follow directly from the F atom bonding to the carbon atom adjacent to the halogen for
Displacement Dynamics of Fluorine Atoms
J. Phys. Chem., Vol. 100, No. 40, 1996 16173 TABLE 3: Initial Impact Parameters b/Å for F Atom Addition to Different Locations on the Benzene Ring of C6H5X Molecules X
b (adjacent)
b (ortho)
b (meta)
b (para)
Br I
0.3 0.8
1.6 1.9
2.7 3.1
3.1 3.6
is predicted to lie lower in energy than the C6H5F + Cl products in line with the agreement found2 between the observed product translational energy distribution and the phase space prediction for this reaction. Consequently, the possibility of H atom migration leading to halogen atom displacement cannot be excluded even though only F atom migration is predicted16 for the reaction of ethene molecules. The observation of branching ratios favoring halogen atom displacement over H atom displacement imply a rate of atom migration which is more rapid than the rate of H atom displacement but slower than the frequency of rotation of the FC6H5X radical. A rotational period τ ∼ 5 ps may be estimated
τ ) 2πI*/Lm
Figure 11. Product translational energy distribution for C6H5F reactive scattering shown by a solid curve compared with the phase space prediction shown by a broken curve for F + C6H5Br (upper panel) and F + C6H5I (lower panel).
X ) Br, I, while dissociation of this radical for X ) Cl exerts less influence on the energy disposal due to its marginal instability with respect to reaction products. Thus, energy randomization over the internal degrees of freedom of the C6H5F product molecule remains incomplete for X ) Br, I and the lifetimes of these radicals are expected to be short compared with their rotational periods. Hence the observation of C6H5F product scattering which is consistent with a complex lifetime of many rotational periods suggests that these radicals may be formed by F atom migration around the benzene ring from other FC6H5X isomers formed by F atom addition to other carbons of the benzene ring. Since the experimental data does not identify the migrating atom directly, the alternative possibility of H atom migration following F atom addition to the benzene ring also requires consideration. An intermediate FC6H5Br radical with an H atom bonded to the carbon atom adjacent to the Br atom is likely to be more stable by ∼35 kJ mol-1 than the radical with the F atom bonded to the adjacent carbon as shown in Figure 10, due to the changes in hybridization of the C-H and C-F bonds.10,15 Hence the potential energy released in the final displacement of the Br atom would be reduced correspondingly over that for a migrating F atom as shown in Figure 10. Even though a higher proportion of the total available energy would be randomized over internal modes of the FC6H5Br radical in this case, the resulting product translational energy distribution might still have a discrepancy with phase space theory comparable to that shown in Figure 11. Indeed the corresponding FC6H5Cl radical formed by H atom migration in the F + C6H5Cl reaction
(4)
for a moment of inertia I* ∼ 1.4 × 10-44 kg m2 of the FC6H5X radical and a maximum initial orbital angular momentum Lm ∼ 165p. Addition to different locations on the benzene ring is associated with different initial impact parameters as listed in Table 3 for F atom approach in a direction perpendicular to the plane of the C6H5X molecule. Addition to the para carbon is associated with the largest initial impact parameters b ∼ 3.3 Å and hence the highest initial orbital angular momentum L ∼ 170p, while addition to the ortho carbon is associated with smaller impact parameters b ∼ 1.7 Å and lower initial orbital angular momentum L ∼ 80p. Addition to the meta carbons is associated with intermediate impact parameters b ∼ 2.7 Å. High values of the initial orbital angular momentum may result in sharp forward and backward peaking in the C6H5F product angular distribution while smaller initial orbital angular momentum values result in milder peaking. The sharp forward and backward peaks superimposed on substantial isotropic scattering of the F + C6H5Br angular distribution suggest that F atom addition leading to halogen atom displacement occurs both at the para and meta carbons as well as the ortho carbons. While the product halogen atom is expected to depart in a direction perpendicular to the plane of the C6H5F molecule in the product transition state, secondary interaction with the π system of the benzene ring17 may result in transfer of C6H5F rotational angular momentum into product orbital angular momentum and thus sharp forward and backward scattering for a small minority of trajectories. This sharpness of forward and backward scattering is less pronounced for the more exoergic dissociation of the F + C6H5I transition state. The overall forward and backward symmetry of the C6H5F angular distributions for F + C6H5Br and C6H5I suggests that atom migration round the benzene ring plays a more important role in halogen atom displacement than does direct F atom addition to the carbon atom adjacent to the halogen atom, which would result in direct dissociation of the product transition state as shown in Figure 10. The H atom displacement pathways yielding FC6H4Br and FC6H4I products have isotropic angular distributions and product translational energy distributions which peak at a fraction f′ ∼ 0.2 of the total available energy. This is in line with the H atom displacement dynamics previously observed2 for F + C6H5Cl, which involve a potential energy barrier ∼20 kJ mol-1 with respect to reaction products as shown in Figure 10. The F +
16174 J. Phys. Chem., Vol. 100, No. 40, 1996 C6H5Cl reaction2 exhibits a rather higher branching ratio Q(Cl)/ Q(H) ∼ 16 at lower initial translational energy E ∼ 10 kJ mol-1 than those of the F + C6H5Br, C6H5I reactions. In all cases the branching ratios fall far below the values Q(X)/Q(H) > 104 predicted2 by RRKM theory for random displacement via the pathways of eqs 1a and 1b. Hence it appears that the rate of atom migration around the benzene ring leading to halogen atom displacement compared with the rate of H atom displacement is similar for all three halobenzene molecules. Consequently, atom migration between adjacent carbons round the benzene ring appears to be more facile than that previously estimated18 in the F + C2H3Br reaction, where a branching ratio Q(Br)/ Q(H) ∼ 10 is observed and F atom addition to the CH2 group of C2H3Br leaves the CH2F group free to rotate about the C-C bond in contrast to the planar structure of the cyclohexadienyl radicals considered here. Acknowledgment. Support of this work by EPSRC and the European Commission is gratefully acknowledged. References and Notes (1) Wang, J. J.; Zhu, Z. Z.; Smith, D. J.; Grice, R. J. Phys. Chem. 1994, 98, 10787.
Wang et al. (2) Shobatake, K.; Lee, Y. T.; Rice, S. A. J. Chem. Phys. 1973, 59, 1435. (3) Harkin, J. J.; Smith, D. J.; Grice, R. Mol. Phys. 1991, 72, 763. (4) Gorry, P. A.; Grice, R. J. Phys. E 1979, 12, 857. (5) Nowikow, C. V.; Grice, R. J. Phys. E 1979, 12, 515. (6) Entemann, E. A.; Herschbach, D. R. Discuss. Faraday Soc. 1967, 44, 289. (7) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986. (8) Cornu, A.; Massot, R. Compilation of Mass Spectral Data, 2nd ed.; Heyden: London, 1979. (9) Smith, D. J.; Grice, R. Mol. Phys. 1991, 73, 1371. (10) Grover, J. R.; Wen, Y.; Lee, Y. T.; Shobatake, K. J. Chem. Phys. 1988, 89, 938. (11) Pechukas, P.; Light, J. C.; Rankin, C. J. Chem. Phys. 1966, 44, 794. (12) Liu, J.; Light, J. C. J. Chem. Phys. 1966, 59, 2545. (13) Light, J. C. Discuss. Faraday Soc. 1967, 44, 14. (14) Estimated from the vibrational frequencies of C6H6 given by Herzberg: Herzberg, G. Electronic Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966. (15) Tsang, W. J. J. Phys. Chem. 1986, 90, 1152. (16) Engels, B.; Peyerimhoff, S. D. J. Phys. Chem. 1989, 93, 4463. (17) Rodgers, A. S.; Golden, D. M.; Benson, S. W. J. Am. Chem. Soc. 1967, 89, 4578. (18) Zhu, Z. Z.; Smith, D. J.; Grice, R. J. Phys. Chem. 1994, 98, 4003.
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