Photodissociation of 1,2-C2H2Br2 at 248 nm: Competition between three-body formation Br+Br+C2H2 and molecular Br2 elimination

Article abstract of DOI:10.1063/1.1387476

The photodissociation of 1,2-C₂H₂Br₂ was studied using product translational spectroscopy. A detector consisting of an electron impact ionizer, quadrupole mass filter and Daly type ion counter was used to measure the dissociation production after travelling a flight path of 365 mm from the reaction zone. Experimental analysis suggested that the dissociation of the molecule into triple products was due to an asynchronous concerted reaction. Behavior of the molecule in the presence of additional bromine atom and the molecular elimination of Br₂ were also studied. The product anisotropy indicated that both Br fragments were produced in a fraction of rotational period.

Full text of DOI:10.1063/1.1387476

Photodissociation of 1,2- C 2 H 2 Br 2 at 248 nm: Competition between three-body
formation Br+Br+C 2 H 2 and molecular Br 2 elimination
Y. R. Lee, C. C. Chou, Y. J. Lee, L. D. Wang, and S. M. Lin
Citation: The Journal of Chemical Physics 115, 3195 (2001); doi: 10.1063/1.1387476
View online: http://dx.doi.org/10.1063/1.1387476
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 115, NUMBER 7
15 AUGUST 2001
Photodissociation of 1,2-C H Br at 248 nm: Competition between
2
2
2
three-body formation Br¿Br¿C H and molecular Br elimination
2
2
2
a)
Y. R. Lee, C. C. Chou, Y. J. Lee, L. D. Wang, and S. M. Lin
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
and Department of Chemistry, National Taiwan University, P.O. Box 23-166, Taipei, Taiwan
͑
Received 30 April 2001; accepted 4 June 2001͒
The photodissociation of 1,2-C H Br at 248 nm has been studied by product translational
2
2
2
spectroscopy. The results show that the molecule dissociates exclusively into the products ͑1͒
Br ϩC H and ͑2͒ Br ͑fast͒ϩBr ͑slow͒ϩC H with a branching ratio ϳ0.2:0.8. While the cleavages
2
2
2
2
2
of the C–Br bonds are not symmetric, producing the Br atoms at unequal velocities, the anisotropy
of the products indicates that both reactions occur in a fraction of a rotational period. Following an
asynchronous concerted reaction, the triple products were simulated with the P͑E ͒ distributions
t
coupled by asymmetric angular distributions. A mechanism consistent with the measured results is
proposed that the Br elimination is a result of a fast intersystem crossing from the ␲␲* pumped
2
state while the triple products occur via a simultaneous asymmetric scission of the C–Br bonds
along the n␴* state. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1387476͔
I. INTRODUCTION
todissociation of 1,2-C H Br at 248 nm by product transla-
2
2
2
tional spectroscopy ͑PTS͒. This molecule has an absorption
The UV photodissociation of vinyl chloride and its de-
rivatives has been a focus of attention for many years. Con-
siderable progress in finding the mechanistic and dynamic
details has been made for a total of five possible dissociation
channels. A fast, anisotropic Cl product was explained to
predissociate from the excited potential energy surface while
continuum around ϳ210 nm, which was primarily assigned
1
10,11
as a ␲*� ␲ transition.
Our interest in this work is to
examine how the molecule behaves when an additional Br
atom is present. Surprisingly, we observed the three-body
formation of Br ͑fast͒ϩBr ͑slow͒ϩC H rather than a simple
2
2
C–Br bond rupture. The product anisotropy indicates that
both Br fragments were produced in a fraction of rotational
the remaining products HCl, slow Cl, H, and H were con-
2
sidered to originate from the ground electronic state after
internal conversion from the ␲␲* state. In the 193 nm pho-
period. In addition, a significant amount of the Br product
2
was measured.
tolysis of dichloroethylenes, however, no Cl elimination
2
2
In the photodissociation of isolated molecules, most ex-
perimental data involve pairs of products. Although dissocia-
tion into three fragments seldom happens, it can occur when
the molecule is photoexcited with sufficient energy. In this
case, a distinction must be made whether it proceeds by a
sequential ͑stepwise͒ reaction or a synchronous concerted
reaction.12 In a sequential reaction, three fragments are pro-
duced consecutively when one of the binary products further
dissociates into a pair of fragments. Because of the pro-
longed reaction time, these secondary products usually show
no angular preference with respect to polarized laser light. In
contrast, a synchronous concerted reaction is defined when
three fragments occur simultaneously. Because each frag-
ment can have a wide range of momentum for a given kinetic
energy release, a full analysis in velocity space is normally
required.13 An asynchronous concerted reaction is another
class of interest in the three-body formation. This was shown
by North et al.14 in the photolysis of azomethane
(CH N CH ) at 351 nm. It differs from a synchronous con-
could be observed. According to ab initio calculations, this
is a difficult process with a large energy barrier.3,4 While the
chloroethylenes have been extensively studied, there are only
5
a few papers concerning the bromides. Wodtke et al. re-
ported that upon excitation at 193 nm, vinyl bromide under-
went the competing dissociation into the products Br and
HBr. From the maximum release of product translational en-
ergy, an upper limit of 77Ϯ3 kcal/mol was determined for
the C–Br bond energy. Using photofragment ion imaging,
6
Katayanagi et al. showed that the Br atom was mainly pro-
2
duced in the ground P3 state with anisotropy parameter ␤
/2
ϭ1.3. This fast component was ascribed from surface cross-
ing between the ␲␲* pumped state and the n␴* or ␲␴*
state. In addition, a low translational energy component of
the Br atom was observed and explained to originate from
the ground state molecule. The results of classical trajectory
7
calculations by Raff and co-workers indicated that several
excited singlet and triplet states were involved in the Br atom
production. Although Br elimination from the ground elec-
tronic state was possible, it was an order of magnitude
smaller than the three-centered HBr elimination.8
3
2
3
certed reaction in that the two CH fragments were produced
3
,9
at unequal velocities. Because the cleavages of the two C–N
bonds are not symmetric, a satisfactory fit to the product
time-of-flight ͑TOF͒ spectra was derived with product trans-
The present work was undertaken to investigate the pho-
a͒Author to whom correspondence should be addressed. Electronic mail:
lational energy distributions P͑E ͒ strongly coupled by asym-
t
smlin@po.iams.sinica.edu.tw
metric angular distributions.
0021-9606/2001/115(7)/3195/6/$18.00
3195
© 2001 American Institute of Physics
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1

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J. Chem. Phys., Vol. 115, No. 7, 15 August 2001
Lee et al.
II. EXPERIMENT
The PTS apparatus used in the present experiment is a
rotating source molecular beam machine that has been pre-
viously described elsewhere.15,16 A continuous reactant beam
was prepared by bubbling helium through a liquid sample
maintained at Ϫ10 °C and expanded into the vacuum cham-
ber from a preheated oven at 190 °C. Commercially available
1
,2-C H Br ͑Fluka, a mixture of cis and trans isomers with
2 2 2
a stated purity of 98%͒ was used without further purification.
As described in the appendix, the cis isomer in the mixture
was 66% ͑by wt͒. The most probable beam velocity was
5
measured at 1.3ϫ10 cm/s with a spread of ϳ8% in full
width at half maximum.
A Lambda Physik EMG 103 MSC Excimer laser oper-
ating on the KrF transition ͑248 nm͒ crossed the molecular
beam at the interaction region. The dissociation products af-
ter traveling a flight path of 365 mm from the reaction zone
were measured with a detector consisting of a 200 eV elec-
tron impact ionizer, quadrupole mass filter, and Daly type ion
counter. The ion flight time constant in the detector was de-
1
/2
termined as 2.9 ␮s per amu .
For the polarization measurements, the laser beam was
linearly polarized with a stacked pile of fused-silica plates at
the Brewster angle followed by a half-wave retarder. In the
present study, the polarized TOF were collected with the
electric field vector ␧ either parallel (␧
␧Ќ) to the detection axis.
ʈ
) or perpendicular
FIG. 1. The experimental evidence for the Br elimination.
(
2
triple products Br͑fast͒ϩBr͑slow͒ϩC H should be consid-
ered. Consequently, we looked into the polarized spectra of
Fig. 4 for more information.
2
2
III. RESULTS AND ANALYSIS
In the present study, unpolarized TOF spectra of m/e
ϩ
ϩ
ϩ
ϩ
These polarized spectra were measured with a laser out-
put energy of ϳ20 mJ/pulse at which no saturation was ex-
perimentally verified. Close inspection of these spectra
shows that the intensity of the relevant ions including the
ϭ158(Br ), 79 (Br ͒, and 26 (C H ) were collected mostly
2
2
2
at a 20° beam angle. Within the limits of apparatus sensitiv-
ϩ
ϩ
ϩ
ity, neither m/e ϭ105 ͑C H Br ͒ nor 80 (HBr ) was ob-
2
2
served.
ϩ
Br2 signal of Fig. 4͑c͒ enhanced ͑open circles͒ when the
These unpolarized spectra were simulated by a forward
convolution method for the determination of product transla-
tional energy distribution P͑Et͒.17 Figure 1 shows the Br2
spectra taken with the laser output energy at ϳ100 mJ/pulse
and ϳ25 mJ/pulse. The circles are experimental data while
the solid curve was derived according to reaction ͑1͒,
electric field vector of polarized laser light was set parallel to
ϩ
the detection axis, ␧ϭ␧
ʈ
. Thus, we expect that reaction ͑1͒
and the reaction channel for the triple products occurred in a
1
8,19
fraction of rotational period.
More importantly, since the
ϩ
first Br component at ϳ200 ␮s has a stronger polarization
C H Br →Br ϩC H .
͑1͒
2
2
2
2
2
2
The corresponding P͑E ͒ distribution of Fig. 2 peaked at 27
t
kcal/mol with a rather narrow breadth.
We then proceeded to analyze the coproduct C H of
2
2
reaction ͑1͒ with this P͑E ͒ distribution. As shown by the
t
dashed curve in Fig. 3͑a͒, it only accounted for the fast
ϩ
2
ϩ
2
C H ions. In search of the precursor for the unfit C H
2
2
ϩ
signal, we looked into the unpolarized Br spectrum of Fig.
͑b͒ for the possible coproduct. This spectrum consists of
3
three components. Apart from the slow signal ͑dashed curve͒
cracked from the Br product by a 200 eV electron impact
2
ϩ
ionizer, there are two fast Br ion components. Since these
ϩ
ϩ
fast Br ions and the unfit C H2 signal are separated by
2
more than ϳ50 ␮s in the appearance time, their precursors
must be totally different. Hence, a possible involvement of
FIG. 2. The P͑E ͒ distribution for the Br elimination of reaction ͑1͒.
t
2
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1

3197
J. Chem. Phys., Vol. 115, No. 7, 15 August 2001
Photodissociation of 1,2-C H Br
2 2 2
ϩ
2
ϩ
ϩ
2
FIG. 4. Polarized TOF of ͑a͒ C H , ͑b͒ Br , and ͑c͒ Br collected at a 20°
2
beam angle with a laser energy of ϳ20 mJ/pulse. These spectra were mea-
sured with ␧ϭ␧ʈ ͑open circles͒ and ␧ϭ␧Ќ ͑solid circles͒ to the detection
axis. Each spectrum was measured with 100 000 laser shots.
ϩ
2
ϩ
ϩ
FIG. 3. Unpolarized spectra of ͑a͒ C H at 20°, ͑b͒ Br at 20°, and ͑c͒ Br
2
at 30° taken at 248 nm with a laser energy of ϳ100 mJ/pulse. The simulated
curves are: -- for C H ϩBr of reaction ͑1͒, ¯ for the Br product of reaction
2
2
2
͑
2a͒, and -•- for C H ϩBr of reaction ͑2b͒.
2
2
values are smaller than the measured data. The discrepancy
was ignored in the present study because the measured sig-
effect than the second one at ϳ250 ␮s, their precursors must
be produced at different stages. Thus, we used an asynchro-
nous concerted reaction model14 for subsequent simulation
of Fig. 3. This was performed by assuming that the first Br
ion peak was cracked from the Br atom of reaction ͑2a͒,
nals were likely due to the thermalized C H2 fragment
2
1
7
bouncing off various surfaces in the detection chamber.
ϩ
With the derived center-of-mass ͑c.m.͒ distributions of
ϩ
Fig. 5, we proceeded to deconvolute the Br spectrum to
determine whether it was free from the second photon effect.
In particular, we must assure that reaction ͑2b͒ was not due
to the absorption of additional photons by the C2H2Br radi-
cal. This is shown in Fig. 6 with a log-log plot for the inte-
grated ion counts vs. the laser output energies of 120, 40, 24,
and 12 mJ/pulse. The results clearly indicate that both the
C H Br →Br ͑fast͒ϩC H Br,
and the slow Br signal and the unfit C H ion were due to
͑2a͒
2
2
2
2
2
ϩ
ϩ
2
2
a rapid dissociation of unstable C H Br,
2
2
C H Br →Br͑slow͒ϩC H .
͑2b͒
Using the standard forward convolution method,17 reaction
2a͒ was calculated and shown by the dotted curves in Figs.
͑b͒–3͑c͒. The resultant P͑E ͒ distribution ͑dotted curve͒ of
2
2
2
2
2
ϩ
fast and the slow Br signals were collected in a single pho-
ton absorption process.
͑
3
We now returned to the polarized TOF of Fig. 4 for the
determination of anisotropy parameter ␤. This parameter is a
function of the angle between the c.m. recoil velocity of the
products and the transition dipole moment in the conven-
t
Fig. 5͑a͒ yielded an average translational energy
Et ϭ19 kcal/mol. The simulation of reaction ͑2b͒ was more
͗
͘
complicated by incorporating asymmetric angular distribu-
tion with respect to the recoil direction of the nascent prod-
uct C H Br with an assumed P͑E ͒ distribution. The compu-
tional expression, 18I(␪)ϳ1ϩ␤P (␪), where ␪ is the angle
2
between the electric field vector ␧ and the product recoil
2
2
t
tation was iterated until a satisfactory fit was obtained. The
results are shown by the dashed-dot curves in Figs. 3 and 5.
The asymmetric angular distributions of Fig. 5͑b͒ indicates
that the slow recoil Br atom and C H are produced in a
direction, and P is the second degree Legendre polynomial.
2
In the limit of instantaneous dissociation, ␤ equals 2 if they
are parallel and Ϫ1 if perpendicular. Because we measured
the polarized spectra with the electric vector ␧ relative to the
detection axis, an offset angle should account for the differ-
ence in the fragmentation between the c.m. and laboratory
coordinates. The results are shown in Fig. 7 with the open
2
2
limited range of angles. This is in agreement with the pre-
diction from the measured anisotropy. As shown in Fig. 3,
ϩ
the overall fit is acceptable except the slowly decaying C H2
2
signal from ϳ200 ␮’s to 400 ␮s over which the calculated
circles for ␧ϭ␧ and the solid ones for ␧ϭ␧ . The fast and
Ќ
ʈ
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3198
J. Chem. Phys., Vol. 115, No. 7, 15 August 2001
Lee et al.
ϩ
2
ϩ
FIG. 7. Polarization dependence of ͑a͒ Br , ͑b͒ the fast Br ion peak, and
͑
ϩ
c͒ the second Br peak at 248 nm. The data are the integrated ion counts of
Figs. 4͑b͒ and 4͑c͒ with the open circles for ␧ϭ␧ and the solid circles for
ϭ␧Ќ . The solid lines represent the best fit to the data with the dotted
curves indicating the limits of uncertainty.
ʈ
FIG. 5. The c.m. distributions for the asynchronous three-body formation of
␧
Br ͑fast͒ϩBr ͑slow͒ϩC H in the 248 nm photolysis. ͑a͒ The P͑E ͒ curves,
2
2
t
¯
and -•-, were derived for reactions ͑2a͒ and ͑2b͒, respectively. ͑b͒ Asym-
metric angular distributions were calculated for reaction ͑2b͒ with respective
to the recoil direction of the nascent fragment C H Br.
2
2
Using the standard procedure20 and the correction
factor21 of 1ϩ␤/4, the branching ratio for reaction ͑1͒/͑2͒
was determined at 0.2:0.8. This is calculated from the spectra
collected at the laser energy of 100 mJ pulse and at a 20°
beam angle.
ϩ
slow Br ions in Figs. 7͑b͒–7͑c͒ were obtained from Fig.
͑b͒ after deconvoluting in accord with reactions ͑2a͒ and
4
͑
2b͒. The best fit led to ␤ϭϪ0.5, 1.5, and 0.4 for reactions
ϩ
͑1͒, ͑2a͒, and ͑2b͒ in sequence. If the C H spectrum of Fig.
2
2
4͑a͒ was used, we obtained ␤ϭ0.6 for reaction ͑2b͒.
IV. DISCUSSION
A. The triple products of Br „fast…¿Br „slow…¿C H
2
2
We started the discussion by examining the thermochem-
istry of reactions ͑2a͒ and ͑2b͒. With estimated enthalpy of
0
22
formation ⌬ H ϭ25 kcal/mol for 1,2-C H Br , these reac-
f
2
2
2
tions are endothermic by a total amount of 83 kcal/mol. If we
neglect the internal energy of the reactant, it yields a total
available energy of 32 kcal/mol for reactions ͑2a͒ and ͑2b͒
upon excitation at 248 nm.23 After subtracting from
͗ ͘
Et
ϭ19 and 8 kcal/mol, we obtained ϳ5 kcal/mol for the prod-
uct internal excitation energy. From the energy consider-
2
ations, we then predict that only the ground state Br ( P )
3
/2
atom was produced. Because reaction ͑2a͒ was measured
with a maximum kinetic energy release Emaxϭ35 kcal/mol,
we placed an upper limit of 80 kcal/mol for the C–Br bond
energy of the reactant molecule. This value is in agreement
with the bond dissociation energy р83.8 kcal/mol deter-
mined from the experimental activation energy for dissocia-
tive thermal electron attachment.24 It follows that the C–Br
bond energy of the ␤-bromovinyl radical •CHϭCHBr
ϩ
FIG. 6. A linear least-square fit for the fast and slow Br signals collected at
four laser energies of 12, 24, 40, and 120 mJ/pulse. They were obtained after
deconvoluting with reactions ͑2a͒ and ͑2b͒. The slopes were calculated for
the first three laser energies with a standard deviation of Ϯ0.01.
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3199
J. Chem. Phys., Vol. 115, No. 7, 15 August 2001
Photodissociation of 1,2-C H Br
2 2 2
should be very weak ϳ3 kcal/mol compared to ϳ11 kcal/mol
the same ␲*� ␲ transition. However, this does not mean
that a ␴*(C–Br)� n ͑Br͒ transition is totally absent in the
1
,2
for the C–Cl bond energy of •CHϭCHCl 5and ϳ45 kcal/
mol for the C–F bond energy of CHϭCHF.2
248 nm photoexcitation.
32
The observation of three-body formation is quite unex-
pected in the present study, not only because of its high
reaction yield, but because it proceeded by asynchronous
concerted reactions ͑2a͒ and ͑2b͒. It differs from the sequen-
tial C–Cl bond ruptures of dichloroethylenes with anisot-
ropy parameter ␤ϭ0.4 derived for reaction ͑2b͒ relative to
V. CONCLUSION
In the present study, we have examined the photodisso-
ciation of 1,2-C H Br at 248 nm by product translational
2
2
2
2
spectroscopy. The results indicate that the molecule dissoci-
ates predominantly into the triple products
Br ͑fast͒ϩBr ͑slow͒ϩC H by an asynchronous concerted re-
action. From the measured anisotropy of the products, the
molecule was considered to dissociate from the repulsive
n␴* state. Because of the weakness of the C–Br bond
strength in the intermediate, a simultaneous asymmetric
scission of the second C–Br bond was proposed for the slow
␤
ϭ1.5 for reaction ͑2a͒. These polarized data indicate that
2
2
reaction ͑2b͒ should occur very rapidly after reaction ͑2a͒ so
that product TOF spectra could be simulated only when the
P͑F ͒ distributions were coupled by asymmetric angular dis-
t
tributions. If we assume that reaction ͑2a͒ is to originate
from the n␴* or ␲␴* state as the C–Br bond rupture of
6
,7
vinyl bromide, a simultaneous elongation of the second
C–Br bond was then expected to proceed when the critical
length for the first C–Br bond cleavage was reached. This is
feasible for the bromovinyl radical with a weak C–Br bond
strength and the cleavage of the second C–Br bond can be
immediately driven by the unpaired electrons to form the
C–C triple bond. This explains why no trace amount of
Br atom. The Br elimination is another important reaction
2
channel measured in the present study. From the Gaussian-
like P͑E ͒ distribution and the anisotropy of the products, it
t
was explained to arise from the excited triplet state by a fast
intersystem crossing from the ␲␲* pumped state. This is
supported from energy considerations the triplet state
3
3
ϩ
A ⌸(1 ) or B ⌸(0 ) was accessible to the Br product.
ϩ
u
u
2
C H Br could be measured in the present study. Therefore,
2
2
no energy barrier is expected for reaction ͑2b͒. This is com-
patible with a negative temperature-dependent rate constant
derived for the addition of Br atom to acetylene at high-
ACKNOWLEDGMENTS
We are grateful to Professor Yuan-Tseh Lee for loaning
us the PTS apparatus to collect the C H2 TOF spectra. We
2
6
ϩ
pressure limit.
2
also thank Dr. Qi Zhao of IAMS for providing the NMR
spectrum and the reviewer for suggesting to run this spec-
trum. This research has been supported, in part, by the Na-
tional Science Council of the Republic of China ͑NSC 88-
B. The molecular elimination of Br2
The Br elimination is another important dissociation
2
2
113-M-001-024͒ and the Chinese Petroleum Corporation
channel detected in the present study. To our knowledge,
such a molecular elimination is without precedent for this
class of molecules.2
͑NSC 89-CPC-7-001-002͒.
,27
APPENDIX: THE CONCENTRATION OF THE CIS AND
TRANS ISOMERS IN THE REAGENT
The Br elimination of reaction ͑1͒ was measured with
2
anisotropy parameter ␤ϭϪ0.5 and a Gaussian-like P͑Et͒
distribution of Fig. 2. These experimental results strongly
suggest that it cannot occur from the ground state potential
energy surface. Hence dissociation through the excited state
must be sought. With Eavlϭ79 kcal/mol for reaction ͑1͒, the
products were internally excited by 52 kcal/mol, which is in
excess of 45 kcal/mol for the dissociation energy of the
ground state Br molecule. If the coproduct C H was not
A carbon-13 with proton coupling NMR spectrum was
measured at 25 °C ͑Bruker MSL-500P͒. Tetramethyl silane
was used as a standard reference for 13C chemical shift ␦.
The shift ␦ for the cis and the trans isomers was identified at
Ϫ115.1 and Ϫ108.1 ppm, respectively. It yielded an intensity
ratio of 0.66:0.34 for the cis/trans isomers. It should be noted
that this ratio may change in the gas phase since the latter has
a slightly higher vapor pressure than the former at the oper-
2
2
2
much excited, this amount of internal excitation is sufficient
3
3
ϩ
u
33
for Br to be formed in the triplet A ⌸(1 ) or B ⌸(0 )
ating temperature Ϫ10 °C.
2
u
2
8
state. If this truly happened, reaction ͑1͒ could arise most
likely from an excited triplet state after a fast intersystem
crossing from the ␲␲* state. For the Br atom with a large
spin-orbit interaction, this crossing rate can be strongly en-
1
D. A. Blank, W. Sun, A. G. Suits, Y. T. Lee, S. W. North, and G. E. Hall,
J. Chem. Phys. 108, 5414 ͑1998͒, and references therein.
K. Sato, S. Tsunashima, T. Takayanagi, G. Fujisawa, and A. Yokoyama, J.
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1