J. Phys. Chem. 1994, 98, 10787-10793
10787
Halogen Atom Abstraction Dynamics of Fluorine Atoms Reacting with Allyl Bromide and Iodobenzene Molecules J. J. Wang, Z. Z. Zhu, D. J. Smith, and R. Grice* Chemistry Department, University of Manchester, Manchester, MI3 9PL, U.K. Received: June 2, 1994; In Final Form: August 8, 1994@
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Reactive scattering of F atoms with C3H5Br and C6H51molecules leading to Br and I atom abstraction has been studied at an initial translational energy E 40 kJ mol-' using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distributions of BrF and IF scattering show peaking in the forward and backward directions, which is consistent with reaction via persistent C3H5-BrF and C6H5-1-F complexes with lifetimes greater than two rotational periods. The sharply peaked angular distribution observed for F C3HsBr is consistent with a microcanonical description, whereby reactive scattering arises from a product transition state which approximates to a strongly prolate symmetric top. The mildly peaked angular C6H51is consistent with a phase space description whereby unconstrained distribution observed for F rotation is established between the nascent molecules in the product transition state. The product translational energy distributions are both consistent with randomization of energy over internal modes of the collision complex. The lifetime of the C3H5BrF collision complex greatly exceeds that of the FCH2--'CH-CH2Br free radical intermediate in the Br atom displacement pathway, showing that these radicals are not coupled via a four-membered-ring transition state. Similarly, the rate of coupling to the 'CH2-CHF-CH2Br radical must be slower than the rate of dissociation of the C3H5BrF radical, and the rate of migration of the F atom to the n system of the very long-lived C6H5IF radical must also be slower than its rate of dissociation.
+
+
Introduction The reactive scattering of fluorine atoms with allyl bromide molecules yielding bromine atom displacement has been studied recently' and exhibits a stripping mechanism which is similar to the corresponding reaction of allyl iodide molecules.2 The reaction of F atoms with C3HsI molecules also follows an I atom abstraction pathway3which exhibits a short-lived collision complex mechanism. This pathway has been studied4 using laser-induced fluorescencedetection to determine the vibrational state distribution of the IF reaction products, where it was suggested that reaction may involve an addition-elimination mechanism. However, the observed differences2s3 in the mechanism of the displacement and abstraction pathways suggest that these pathways are independent. The abstraction of Br atoms from C3H5Br is much less exoergic than I atom abstraction from CsHsI and should provide a much more decisive test of the independence of the displacement and abstraction pathways in the reactions of F atoms with allyl halide molecules.
F
-
+ C3H5Br
BrF
+ C,H,
(1)
The abstraction of I atoms from C&I molecules has a similarly low exoergicity and should provide a test of the independence of displacement and abstraction pathways in the reaction of F atoms with aryl halide molecules.
F
+ C6H5I - IF + C6H5
(2)
Detection of IF product from the pathway of eq 2 provides a stringent test for the absence of F atom migration in the C&IF intermediate complex, which is otherwise expected to persist for a very long time before dissociation. Experimental Section The apparatus was the same as that previously employed in studies of F atoms with C3H5I molecules3 using a microwave @Abstractpublished in Advance ACS Abstracts, October 1, 1994.
0022-365419412098-10787$04.50/0
discharge source5with an alumina discharge tube. The F atom beam velocity distribution peaked at -2030 m s-' with a full width of -900 m s-l at half-maximum intensity corresponding to a Mach number M of -3.5. The allyl bromide beam issued from a glass nozzle of diameter -0.15 mm with a stagnation pressure of -100 mbar maintained by a reservoir at -30 "C. The iodobenzene was seeded in 60 mbar of N2 buffer gas, since a reservoir temperature of -45 "C sustains a vapor pressure of only -5 mbar. The C3H5Br and C6H51beam velocity distributions measured by the rotatable mass spectrometer detector using cross-correlation time-of-flight analysis6 peaked at -625 and -785 m s-l with full widths of -270 and -280 m s-' at halfmaximum intensity, corresponding to Mach numbers M of -4 and 5 . Results
Angular distribution measurements of BrF reactive scattering yield -11 counts s-l against a background of -5 counts s-' and IF scattering of -14 counts s-l against a background of -2 counts s-'. The BrF laboratory angular distribution shown in Figure 1 exhibits two peaks symmetrically placed about the centroid vector, while the IF distribution shown in Figure 2 exhibits only a single peak located at the centroid vector. Laboratory velocity distributions of BrF flux shown in Figure 3 and IF flux in Figure 4 were measured using integration times -8000 and 1500 s to gain signal to noise ratios -10 at the peaks of the distributions. Kinematic analysis of these data has been performed by the stochastic method7 with the differential cross section expressed as a product of an angular function I"(@ and a velocity function U(u,O)which is parametrically dependent on the scattering angle:
The resulting angular distribution and product translational energy distributions P(F) are shown for BrF in Figure 5 and 0 1994 American Chemical Society
Wang et al.
10788 J. Phys. Chem., Vol. 98, No. 42, 1994
F(He) + C3H,Br -> FBr + C,H,
I
Lab Angle 8 /degrees
Figure 1. Laboratory angular distribution (number density) of BrF reactive scattering from F C3HSBr at an initial translational energy E 37 kl mol-'. Solid line shows the fit of the stochastic kinematic analysis.
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3.0 5.0 7.0 9.0 11.0 13.0 15.0 17.0 19.0
[
F(He) + C,H,I -> IF + C,H,
1
LAB vELocITY/1ooms~'
1.0
I
2
0.2 i F
J
, a
C6H5'
f
Lab Angle 0 /degrees
-
Figure 2. Laboratory angular distribution of IF reactive scattering from F C&5I at an initial translational energy E 42 kl mol-'.
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IF in Figure 6 together with the initial translational energy distributions. The angular distribution for BrF shows sharp peaking in the forward and backward directions, with the backward peak being lower than the forward peak by a factor -0.9. The IF angular distribution shows milder peaking in the forward and backward directions with intensities which are equal within experimental uncertainty. The product translational energy distribution for BrF is shifted to higher energy for scattering in the forward and backward directions than for the sideways scattering. The IF distribution peaks at lower translational energy and is independent of scattering angle. The peak E'pk and average E'av product translational energies are listed in Table 1 together with the initial translational energies E and the reaction exoergicities calculated from the BrF and IF bond energies of Huber and Herzberg,8the C3HSBr bond energy of Traeger? and the C&I bond energy of McMillen and Golden.lo The total reaction cross section has been calculated for each reaction by integration over center-of-mass scattering angle and velocity Qr
LxT(8)sin 8 d6J ~ U ( Udu )
-
(4)
giving an estimated ratio Qr(IF)/Qr(BrF) 2/3. Experiments were also undertaken with a supersonic beam of FZmolecules seeded in He buffer gas produced from a heated
3.0 5.0 7.0 9.0 11.0 13.0 15.0 17.0 19.0
LAB VELOCITY/~O~~S-'
-
Figure 3. Laboratory velocity distributions (flux density) of reactively scattered BrF from F C3HsBr at an initial translational energy E 37 kl mol-'. Solid line shows the fit of the stochastic kinematic analysis.
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alumina nozzle. No product corresponding to the endoergic reaction
F,
-
+ C,H,Br
-
C,H,BrF
+F
(5)
could be detected up to a maximum initial translational energy E 65 kJ mol-'. Experience with previous experiments'l on the analogous reactions of alkyl and allyl iodide molecules suggests that the translational energy threshold for the reaction of eq 5 must lie above &h ;2 75 kJ mol-', which corresponds to an upper limit to the bond energy for F atom addition to the Br atom of C3HsBr of Do(C3H5Br-F) 5 80 kJ mol-', which in
Halogen Atom Abstraction Dynamics
J. Phys. Chem., Vol. 98, No. 42, 1994 10789 FWe) + C,HJBr -> FBr t C,HJ
FWs) + C,H,I ->IF + C,H,
-
0.0 0
20
40 60 80 100 120 140 160 180
CM Angle, e
3.0
5.0
7.0
9.0
11.0
13.0
15.0
LAB VELOCITY/IOO~S" Figure 4. Laboratory velocity distributions of reactively scattered IF from F C6HJ at an initial translational energy E 42 kJ mol-[.
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turn corresponds to Do(C3H5-BrF) 5 64 kJ mol-'. The experimental bond energy" for F atom addition to the I atom O f C6H51 Of Do(C6H5I-F) 120 kJ mol-' corresponds to Do(CrjH5-F) 115 kJ mol-'.
-
"r
Discussion The angular distribution of reactive scattering shown in Figure 5 is characteristic of reaction via a long-lived C3H5BrF collision complex with a lifetime'? greater than two rotational periods z 2 6 ps. The product transition state shown in Figure 7 has a similar structure to that previously proposed for F C3H51, with a range of configurations corresponding to rotation about the extended C-Br bond. The principal moments of inertia listed in Table 2 for the limiting extended and contracted configurations of the transition state both approximate well to prolate symmetric tops. The ratio of the decay lifetime z to the rotational period" for a C3H5BrF collision complex formed in large impact parameter collisions may be approximated by
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Trans. Energy, E'W mol"
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Figure 5. Angular function T(8)and translational energy distributions P(B)for F C3H5Br at an initial translational energy E 37 kJ mol-[. Left-most translational energy distribution corresponds to 8 = 90", and right-most distribution to 8 = 0", 180". Dashed energy curve shows the distribution of initial translational energy.
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ratios in braces, a nominal bond energy EO 55 kJ mol-' may be estimated for F C3HsBr from the previously measured" bond energy of F C3H7I. This value is consistent with the approximate upper limit Do(C3Hs-BrF) 64 kJ mol-' estimated above. The microcanonical theory14 for angular distributions of reactive scattering arising from the dissociation of a long-lived collision complex via a prolate symmetric top transition state predicts a probability distribution for the cosine of the helicity angle a' lying between the L and the final relative velocity v' of the form
+ +
cos a')
E+hDo+E0-E',,
-
= ~ ( -1a2 cos2
(7)
where where L,,,denotes the maximum initial orbital angular momentum, P the mean vibrational frequency, Z*2 the larger moment of inertia of the complex, and EOthe bond energy of the complex with respect to reaction products. The angular distribution of IF scattering previously measured13 for F C3H7I is rather similar to that determined here for F C3H5Br, indicating that each reaction has a similar complex decay lifetime and similar moments of inertia for the complex and product transition state. The reactions differ mainly in their exoergicities ADOand the bond dissociation energies EO of the complexes with respect to reaction products. If it is assumed that all other parameters in eq 6 are the same for each reaction, thereby equating the energy
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and s denotes the number of internal degrees of freedom of the transition state, with I+l, Z+2 being the moments of inertia parallel and perpendicular to the symmetry axis of the transition state and N the normalization function. The angular distribution of reactive scattering for a specific value of the parameter u2 is given by integration over the probability distribution for the cosine of the helicity angle min (1 - a2 COS' ar)s-1/2 2 - 2 d cos a' I(e,a ) - ;
s,
(sin2 e - cos2 a')''?
(9)
Wang et al.
10790 J. Phys. Chem., Vol. 98, No. 42, 1994 FWe) + C,H,I -> IF + C,H5
I
1
o.2 0.04 0
1
I
40
60
1
20
I
1
~
:
4
:
80 1M) 120 140 160 180 CM Angle, e
0.0
20
0
40
80
60
100
120
140
160
180
CM Angle 8 p Figure 8. Comparison of the F C3H5Br angular distribution averaged over impact parameters predicted by the microcanonical theory (solid curve) with the experimental angular distribution (broken curve).
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1.o
-
''
TABLE 1: Reaction Energeticsfld mol-', Peak Product Translational Energy E'pk, Average Product Translational Energy E'av, and Reaction Exoergicity ALh forward-backward sideways
0.8
vi
n
0.6
2
product BrF IF
v h
W
p'
5
E
0.4
0.2 0.0
Trans. Energy, E/kJ mol.'
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Figure 6. Angular function T(0) and translational energy distribution P(E') for F 3- C&I at an initial translational energy E 42 kJ mol-'.
E
Fpk
Ea,
E'pk
37 42
13 3
16 6
9 3
E'av 13 6
ADO 16 5*10
TABLE 2: Estimated Principal Moments of Inertia for Extended and Contracted Configurations of the C$IS-BrF Product Transition State with rCBR = 2.5 A, b B , F = 1.8 A, rcc = 1.5 A, and rCH = 1.1 A, the C&BrF Four-Membered-RingTransition State with rCBr = 1.9 A, and the C&IF Complex with rm = 1.9 configuration 1,/10-46 kg m2 Zt,/10-46 kg m2 ZJ10-46 kg m2 extended 5.6 78.6 83.6 contracted 10.8 67.1 11.9 50.3 ring 14.3 40.3 complex 14.9 146.2 161.1
The resulting expression,
/
/ b
is then employed in calculating the observed angular distribution by integration over the distribution of initial orbital angular momentum arising from the distribution of impact parameters L = pvb.
'
I I B
Figure 7. Limiting extended configuration for the product transition state of the F + C3H5Br reaction.
where min = llu or sin 8, which ever is less. Explicit formulas have been t a b ~ l a t e d ' ~ -for ' ~ the resulting angular distributions and normalization functions for values of the number of internal degrees of freedom up to s = 1512. The ratio I+2lI+l 14.5 estimated from the moments of inertia for the extended configuration of the product transition state in Table 2 and Figure 7 is used in determining the dependence of the parameter u2 upon impact parameter by equating the maximum centrifugal barrier17 with the rotational energy of the transition state precessing in the plane of collision.
-
B', = @I$ = Lm2/2p,
The resulting angular distribution calculated for s = 1512 and multiplied by an exponential decayI2 function exp( -818*) with 8* = 6800" shows excellent agreement in Figure 8 with the experimental angular distribution. This corresponds to only a limited number of internal degrees of freedom being classically e ~ c i t e d in ~ ~transition J~ states formed with high initial orbital angular momentum L Z 150 h. The allyl radical has only five vibrational with frequencies < 1000 cm-I, while the energy which is free to be distributed over internal degrees of freedom of the transition state E -t ADO- L2/21+2 37 kJ mol-' corresponds to only -5 kJ mol-' per degree of freedom. Hence, only low-frequency modes of the transition state are expected2' to contribute significantly to the vibrational density
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Halogen Atom Abstraction Dynamics
J. Phys. Chem., Vol. 98, No. 42, 1994 10791
awBriF HE SEBDED BrF
gE
-
0.4-
z 0.2-
0.0i
+
Figure 11. Comparison of the F Cas1 angular distribution predicted by phase space theory (solid curve) with the experimental angular
distribution (broken curve).
Trans.Energy, E/W mor' Figure 9. Product translational energy distribution for F iC3H5Br sideways scattering (solid curve) compared with the phase space model (broken curve). C6HJI+FOie)->IF
RRKM.AM.2.Gauram MODEL
phase space calculation yields23the distribution of the cosine of the angle ( d 2 ) - a between the initial and final orbital angular momenta, which has been fitted by least squares to a three-term expansion.
1
P(sin a ) = co
I
+ c2 sin2a + c4 sin4 a
(13)
The corresponding distribution of the cosine of the helicity angle between the final relative velocity and the initial orbital angular momentum is then given byz3 P(cos a') = co
I
20 40 Trans. Energy, E'/kJ mol-' Figure 10. Product translational energy distribution for F + sideways scattering (solid curve) compared with the phase space model (broken curve).
1 3 + -c2 sin2 a' + -c4 sin4 a' 2 8
(14)
The angular distribution of reactive scattering may then be calculated, including the effect of the C a 5 1 reactant angular momentum in tilting the total angular momentum away from the initial orbital angular momentumz4represented by the most probable value of the cosine of the reactant helicity angle cos y = J m d L when Jmp
