Investigating conformation dependence and nonadiabatic effects in the photodissociation of allyl chloride at 193 nm

Article abstract of DOI:10.1063/1.471784

The experiments presented here investigate the competing photodissociation pathways for allyl chloride upon excitation of the nominally ππ*(C=C) transition at 193 nm. The measured photofragment velocity distributions evidence C-Cl bond fission and HCl elimination. The recoil kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for HCl elimination. The experimental measurements show C-Cl bond fission dominates, giving an absolute branching ratio of HCl:C-Cl=0.12±0.03 when the parent molecule is expanded through a nozzle at 200°C. The branching ratio depends on the nozzle temperature; at 475°C, the absolute branching ratio measured is HCl:C-Cl=0.24±0.03. We analyze the experimental results along with Supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to examine the potential influence of nonadiabaticity along the C-Cl fission reaction coordinate and its dependence on molecular conformation.

Full text of DOI:10.1063/1.471784

Investigating conformation dependence and nonadiabatic effects in the
photodissociation of allyl chloride at 193 nm
T. L. Myers, D. C. Kitchen, B. Hu, and L. J. Butler
Citation: The Journal of Chemical Physics 104, 5446 (1996); doi: 10.1063/1.471784
View online: http://dx.doi.org/10.1063/1.471784
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/104/14?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Photodissociation dynamics of OClO
J. Chem. Phys. 105, 8142 (1996); 10.1063/1.472700
Photodissociation dynamics of A
deuterated isotopomers
J. Chem. Phys. 104, 6472 (1996); 10.1063/1.471368
̃ state ammonia molecules. II. The isotopic dependence for partially and fully
Photodissociation dynamics of à state ammonia molecules. I. State dependent μv correlations in the NH2(ND2)
products
J. Chem. Phys. 104, 6460 (1996); 10.1063/1.471367
Photodissociation spectroscopy of MgCH+ 4
J. Chem. Phys. 104, 6452 (1996); 10.1063/1.471366
Photodissociation dynamics of ferrocene at 193 NM
AIP Conf. Proc. 191, 642 (1989); 10.1063/1.38595
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

Investigating conformation dependence and nonadiabatic effects
in the photodissociation of allyl chloride at 193 nm
T. L. Myers, D. C. Kitchen, B. Hu, and L. J. Butler
The James Franck Institute and Department of Chemistry, The University of Chicago, Chicago,
Illinois 60637
͑
Received 10 November 1995; accepted 4 January 1996͒
The experiments presented here investigate the competing photodissociation pathways for allyl
chloride upon excitation of the nominally ␲␲*͑CvC͒ transition at 193 nm. The measured
photofragment velocity distributions evidence C–Cl bond fission and HCl elimination. The recoil
kinetic energy distribution for the HCl products is bimodal, indicating two primary processes for
HCl elimination. The experimental measurements show C–Cl bond fission dominates, giving an
absolute branching ratio of HCl:C–Clϭ0.12Ϯ0.03 when the parent molecule is expanded through
a nozzle at 200 °C. The branching ratio depends on the nozzle temperature; at 475 °C, the absolute
branching ratio measured is HCl:C–Clϭ0.24Ϯ0.03. We analyze the experimental results along with
supporting ab initio calculations and earlier photodissociation studies of vinyl chloride in order to
examine the potential influence of nonadiabaticity along the C–Cl fission reaction coordinate and its
dependence on molecular conformation. © 1996 American Institute of Physics.
͓
S0021-9606͑96͒01514-7͔
I. INTRODUCTION
The first band in the ultraviolet absorption spectrum of
allyl chloride, a broad band peaking at 171 nm, is assigned to
6
We chose to investigate the photodissociation pathways
of allyl chloride at 193 nm in order to investigate a special
class of organic reactions, those allowed by conservation of
total electronic symmetry but Woodward–Hoffmann
the ␲␲* transition. Although no collisionless gas-phase
CvC
photolysis experiments on allyl chloride in this absorption
7
band have been published, the photolysis of gaseous allyl
chloride at 229 nm, which accesses the rising edge of the
1
forbidden. In both vinyl chloride and the cis conformer of
*
␲
␲
absorption band, has been studied by Sears and
CvC
8
allyl chloride, the potential energy surface reached with the
Volman. They used chromatographic analysis to identify the
products resulting from secondary reactions of the photodis-
sociated products and concluded that the only primary disso-
ciation process is C–Cl fission.
␲
␲* absorption has a barrier resulting from an avoided
crossing between the ␲␲*͑CvC͒ and the n͑Cl͒␴*͑C–Cl͒
configurations. Since the pathway to C–Cl fission on the
excited state potential energy surface involves a change in
individual orbital symmetry along the adiabatic reaction co-
ordinate, C–Cl fission is Woodward–Hoffmann forbidden.
This work measures the photofragment velocities and
angular distributions of allyl chloride at 193 nm with a
crossed laser-molecular beam apparatus and determines the
absolute branching ratios between HCl elimination and C–Cl
fission at two nozzle temperatures. These results are com-
2
Previous work in our group has indicated that the reaction
rates in Woodward–Hoffmann forbidden reactions are re-
duced due to nonadiabatic recrossing of the barrier. We have
pared to a similar system with no CH spacer between the
2
3
also shown that increasing the distance between the orbitals
C–Cl and CvC chromophores, vinyl chloride. Analysis of
our results allows us to investigate whether the excited state
C–Cl reaction coordinate is traversed adiabatically or not in
allyl chloride as compared to vinyl chloride by considering
the AЈ adiabatic Born Oppenheimer potential energy surface
involved in the avoided crossing decreases the electronic
coupling and thus increases the probability of nonadiabati-
cally recrossing the barrier. Thus the comparison between the
photodissociation dynamics of allyl and vinyl chloride can
test our understanding of this class of reactions.
formed from the avoided crossing of the ␲␲
*
and the
CvC
Previous experiments4 on the photodissociation of vi-
nyl chloride at 193 nm have identified both C–Cl fission and
HCl elimination as the primary reaction pathways. They pro-
posed that the pathway to C–Cl fission is a predissociation
,5
n ␴
*
states. Preliminary ab initio electronic structure cal-
Cl C–Cl
culations of the potential energy surfaces are also presented
to help interpret the experimental results by providing the
energetic splittings between the adiabatic excited electronic
states at the avoided crossing in the C–Cl bond fission chan-
nel.
from the photoprepared ␲␲*
state to the repulsive
CvC
n ␴* state whereas the channel for HCl elimination is a
Cl C–Cl
5
ground state process following internal conversion. The two
processes compete with a branching ratio reported by Ume-
moto et al., C–Cl: HClϭ1.1:1.0. Furthermore, Mo et al.
have observed two other minor primary reaction channels
attributed to internal conversion, C–H bond fission and an-
other pathway for C–Cl fission producing low energy Cl
fragments.
II. EXPERIMENT
5
4
A. Molecular beam experiments
The experiments here measure the velocity and angular
distributions of the photofragments from the photodissocia-
tion of allyl chloride, CH vCHCH Cl, at 193 nm using a
2
2
5446
J. Chem. Phys. 104 (14), 8 April 1996
0021-9606/96/104(14)/5446/11/$10.00
© 1996 American Institute of Physics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

Myers et al.: Photodissociation of allyl chloride
5447
crossed laser-molecular beam apparatus. After excitation
with a pulsed excimer laser, the fragments photodissociate
from the crossing point of the laser and molecular beam with
velocities determined by the vector sum of the molecular
beam velocity and the recoil velocity imparted in the disso-
ciation. The fragments that scatter into the differentially
pumped detector travel 44.1 cm to an electron bombardment
ionizer where they are ionized by 200 eV electrons. The ions
are mass selected with a quadrupole mass filter and counted
with a Daly detector and multichannel scalar with respect to
their time of flight ͑TOF͒ from the interaction region after
the dissociating laser pulse.
The molecular beam was formed by bubbling He carrier
gas through the liquid allyl chloride, cooled to Ϫ25 °C to
maintain a vapor pressure of roughly 20 Torr, to give a total
stagnation pressure of 300 Torr. The 0.076 mm diam nozzle
was heated to 220 °C in order to prevent cluster formation
during the supersonic expansion. For the branching ratio
data, we used nozzle temperatures of 200 °C and 475 °C.
The velocity of the parent molecular beam was measured by
rotating the molecular beam source to point into the detector
and raising a chopper wheel into the beam.
ϩ
FIG. 1. The laboratory TOF spectrum of the photofragments detected at Cl
from allyl chloride photodissociated at 193 nm with a nozzle temperature of
20 °C and a source angle of 15°. The contribution from Cl fragments to the
Cl signal was determined by fitting the signal with the P(ET) for primary
C–Cl fission in Fig. 3. Similarly, the contribution from HCl fragments was
determined by fitting the signal with the P(E )’s for primary HCl elimina-
2
ϩ
T
tion in Fig. 4.
A Lumonics PM-848 excimer laser with an ArF fill was
used to produce the photolytic wavelength. The unpolarized
laser power was typically 25 mJ/pulse and focused to a 5
evant excited electronic states. These CIS excitation energies
are added to the ground-state Morse oscillator energies to
construct the excited electronic state surfaces.
2
mm spot size in the interaction region.
The source angle was maintained at 15° with respect to
ϩ
the detector axis. The strong signal observed at m/e ϭ35,
ϩ
Cl , after 300 000 shots corresponds to predominantly C–Cl
III. RESULTS AND ANALYSIS
ϩ
fission but also has a significant contribution from the Cl
A. Identification of primary product channels
daughter ion of HCl photofragments. The momentum-
matched fragment for the Cl atom, CH CHCH , was de-
2
2
The data for allyl chloride excited at 193 nm evidence
three competing channels, C–Cl bond fission and two differ-
ent pathways for HCl elimination. Figure 1 shows the TOF
ϩ
tected at the parent ion, m/e ϭ41, after 1 500 000 laser
ϩ
shots. The signal observed at m/e ϭ36 after 1 000 000 shots
ϩ
ϩ
was attributed to HCl elimination. The momentum-matched
spectrum taken at m/e ϭ35, Cl , attributed to signal from
partner for HCl, C H , was observed at the parent ion,
3
4
primary C–Cl fission as well as HCl cracking in the ionizer
ϩ
ϩ
m/e ϭ40, after 2 500 000 shots, but the signal also has a
contribution from a daughter ion of the momentum-matched
fragment of the Cl atom. Signal was also observed at
to give Cl as a daughter ion. The contribution from C–Cl
fission is determined by forward convolution fitting of the
ϩ
2
TOF for the momentum-matched fragment, CH CHCH , in
2
ϩ
m/e ϭ39, 38 and 37 and, although not shown, were ad-
Fig. 2 to derive the center-of-mass product translational en-
equately fit with contributions from the above dissociation
ergy distribution, P(E ), for C–Cl fission. This P(E ) is
T
T
ϩ
ϩ
ϩ
channels. No signal was seen at m/e ϭ49, CH Cl , after
2
then used to fit the TOF spectrum at m/e ϭ35. The remain-
ing signal in the Cl spectrum is fit by the two pathways for
ϩ
500 000 shots.
HCl elimination. The relative scaling of the two contribu-
ϩ
tions was retained from the m/e ϭ36 data presented later.
B. Computational method
Figure 3 shows the P(E ) for C–Cl fission, which peaks at
T
To help interpret the experimental results, we also
45 kcal/mol and extends out to near 60 kcal/mol, two-thirds
of the maximum 80 kcal/mol11 of available energy. The
translational energy distribution for C–Cl fission indicates
there is a significant exit channel barrier along the reaction
coordinate since it peaks well away from zero. Thus, the
pathway to C–Cl fission must occur along an excited state
surface since the barrier to the reverse reaction on the ground
state surface is typically negligible. Although some Cl atoms
with low translational energies were observed for vinyl
chloride and attributed to an internal conversion
present ab initio electronic structure calculations for vinyl
9
͑a͒
9͑b͒
chloride
and cis- and gauche-allyl chloride
using the
GAUSSIAN 92 system of programs10 with a 6-311G* basis
set. Configuration interaction with single and double excita-
tions ͑CISD͒ calculations provide electronic ground state en-
ergies in the harmonic region of the C–Cl stretching poten-
tials. These energies are then fit to Morse potentials with the
correct dissociation energies, 80 kcal/mol for vinyl chloride
and 68 kcal/mol for allyl chloride.11 Configuration interac-
tion with single excitations ͑CIS͒ calculations provide exci-
tation energies from the ground electronic state to the rel-
4
mechanism, no evidence for this pathway in allyl chloride is
found.
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

5448
Myers et al.: Photodissociation of allyl chloride
FIG. 2. The laboratory TOF spectrum of the photofragments detected at
FIG. 4. The laboratory TOF spectrum of the photofragments detected at
HCl from allyl chloride photodissociated at 193 nm with a nozzle tempera-
ϩ
ϩ
CH CHCH from allyl chloride photodissociated at 193 nm with a nozzle
2
2
temperature of 220 °C and a source angle of 15°. The signal is fit with the
ture of 220 °C. The source angle was 15°. The fastest portion of the signal is
fit with the P(ET) shown in the top frame of Fig. 5, and the slowest portion
of the signal is fit with the P(ET) shown in the bottom of Fig. 5, both
corresponding to primary HCl elimination. The fast peak cannot be fit by
P(E ) in Fig. 3 showing the required momentum match between the Cl and
T
the CH CHCH fragments for primary C–Cl fission.
2
2
ϩ
assuming it results from the unlikely C3 daughter ion of the CH CHCH
2
2
ϩ
ϩ
product of C–Cl fission.
As shown in the TOF spectrum for m/e ϭ36, HCl , in
Fig. 4, two different pathways for HCl elimination occur at
1
93 nm. One channel yields fragments with high transla-
4
Since studies of the photodissociation channels of vinyl
chloride at 193 nm have detected a slow kinetic energy C–Cl
fission channel attributed to internal conversion, we took
care to analyze whether our data might indicate this channel.
ϩ
tional energies; the fast component of the HCl spectrum is
fitted with the P(E ) shown in the top frame of Fig. 5. This
P(E ), which peaks at 34 kcal/mol and extends out to 76
kcal/mol, evidences a large barrier to reverse reaction. The
other channel produces low energy HCl fragments, and the
T
T
ϩ
We must assign the slow signal at Cl in Fig. 1 between 160
and 400 ␮s to HCl elimination rather than a slow C–Cl
channel for two reasons. First, the data is fit very well by
assigning all of the slow signal to a daughter ion of HCl; it
has the same velocity distribution as the slow signal at
m/e ϭ36 for HCl shown in Fig. 4. Second, the slow HCl
products in the m/e ϭ36 data increase markedly with tem-
perature, and the slow Cl data shows the same increase.
This can easily be seen by the similar cracking pattern of
HCl calculated from the high vs the low temperature data
P(E ) in the bottom frame of Fig. 5 fits the slow component
in the HCl spectrum. This P(E ) indicates smaller repul-
T
ϩ
T
sive exit channel forces since it peaks at 3.6 kcal/mol. The
two pathways could result from either HCl elimination oc-
curring from different potential energy surfaces or from two
different mechanisms on the same surface. In Sec. IV, we
analyze the possible assignments for the two observed HCl
elimination channels.
ϩ
ϩ
ϩ
ϩ
͑
the cracking pattern of HCl, calculated from the ratio of the
ϩ
ϩ
slow HCl to slow Cl signal in the next section, does not
change with nozzle temperature͒. As a further check, since
the secondary dissociation of very slow allyl radicals from
C–Cl fission is possible ͑so one would not see evidence of a
slow C–Cl fission channel at m/e ϭ41͒, we present in Fig.
6
ϩ
ϩ
the m/e ϭ40 signal which can be adequately fit with con-
tributions from the assigned dissociation channels.
Two other possible primary photodissociation channels,
C–H bond fission and H elimination, are not investigated in
2
these experiments since the momentum-matched fragments
would be too heavy to recoil out to the detection angles
ϩ
ϩ
sampled. Data at H2 and H was not taken due to the high
background at these masses and the low ionization cross sec-
tions of H atom and H , which prohibit their efficient detec-
2
tion in our apparatus.
FIG. 3. The center-of-mass product translational energy distribution, P(E ),
B. Product branching ratios
T
for the C–Cl fission channel in allyl chloride at 193 nm. The P(E ) is
T
ϩ
2
With careful analysis, the data allows the determination
of the absolute branching ratio of the HCl elimination pro-
derived from forward convolution fitting the CH CHCH signal in Fig. 2
2
which results from primary C–Cl fission.
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

Myers et al.: Photodissociation of allyl chloride
5449
FIG. 6. The laboratory TOF spectrum of the photofragments detected at
ϩ
4
C H from allyl chloride photodissociated at 193 nm with a nozzle tempera-
3
ture of 200 °C and a source angle of 15°. The signal is fit with the P(E )’s
T
for primary HCl elimination in Fig. 4 showing the required momentum
match between the HCl and the C H fragments and with the daughter ion of
3
4
the momentum-matched fragment for C–Cl fission.
tern of HCl. ͑The angular distributions of the HCl elimina-
tion pathways are assumed to be isotropic.͒ Assuming that
the cracking pattern of the HCl fragment is identical for the
two pathways and does not change with nozzle temperature,
we integrate the slow signal for HCl elimination at the higher
ϩ
ϩ
FIG. 5. The center-of-mass product translational energy distributions for the
temperature from 248–400 ␮s for both the HCl and Cl
TOF spectra to determine the fragmentation pattern of HCl
HCl elimination channels in allyl chloride at 193 nm. The P(E ) in the top
T
frame is derived from forward convolution fitting the fastest portion of the
ϩ
ϩ
ϩ
to HCl /Cl . After correcting the integrated signal for the
HCl signal in Fig. 4, and the P(E ) in the bottom frame is derived from
T
ϩ
forward convolution fitting the slowest portion of the HCl signal in Fig. 4.
The very high energy portion ͑Ͼ30 kcal/mol͒ of the low energy P(ET)
cannot be uniquely determined because the signal overlaps strongly with the
high energy channel at these energies.
cesses and the C–Cl bond fission pathway. To average out
ϩ
systematic errors, we integrated the TOF spectra at Cl ͑Fig.
ϩ
7
͒ and HCl ͑Fig. 8͒, taken with a nozzle temperature of
2
00 °C, for a total of 210 000 and 1 050 000 shots respec-
ϩ
ϩ
tively, changing the mass from m/e ϭ35 and m/e ϭ36
every 30 000 and 150 000 shots respectively for a total of 7
ϩ
ϩ
scans each. The TOF spectra at Cl ͑Fig. 9͒ and HCl ͑Fig.
1
5
0͒ at 475 °C nozzle temperature, were also recorded for
0 000 and 100 000 shots respectively for a total of four
scans each in order to determine if there was a temperature
dependence. Using the measured kinetic energy distributions
for C–Cl fission and HCl elimination in Figs. 3 and 5, we fit
ϩ
ϩ
the Cl and HCl TOF data by adjusting the relative prob-
abilities until a good fit is obtained. As previously noted, we
retain the relative scaling between the two HCl elimination
pathways used to fit the TOF spectra in Fig. 8 and Fig. 10
ϩ
FIG. 7. The laboratory TOF spectrum of the photofragments detected at Cl
from allyl chloride photodissociated at 193 nm with a nozzle temperature of
200 °C and a source angle of 15°. The contribution from Cl fragments to the
Cl signal was determined by fitting the signal with the P(E ) for primary
C–Cl fission in Fig. 3. Similarly, the contribution from HCl fragments was
ϩ
T
ϩ
when we fit the corresponding Cl spectra in Fig. 7 and Fig.
determined by fitting the signal with the P(E )’s for primary HCl elimina-
T
9
. The absolute branching ratio between both pathways for
tion in Fig. 4 in which the relative scaling of the two pathways was retained
from the HCl TOF in Fig. 8. This data was used to determine the absolute
branching ratios at 200 °C. The percent contributions from each channel
shown are the relative probabilities that already include kinematic factors
but do not yet account for ionization cross sections and cracking probabili-
ties.
ϩ
HCl elimination and C–Cl fission can be calculated from the
relative probabilities, which already incorporate corrections
for kinematic factors, used to fit the Cl spectrum at 200 °C
ϩ
by simply accounting for the daughter-ion fragmentation pat-
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

5450
Myers et al.: Photodissociation of allyl chloride
FIG. 8. The laboratory TOF spectrum of the photofragments detected at
HCl from allyl chloride photodissociated at 193 nm with a nozzle tempera-
ture of 200 °C and a source angle of 15°. The contribution from HCl frag-
FIG. 10. The laboratory TOF spectrum of the photofragments detected at
HCl from allyl chloride photodissociated at 193 nm with a nozzle tempera-
ture of 475 °C and a source angle of 15°. The contribution from HCl frag-
ϩ
ϩ
ments was determined by fitting the signal with the P(E )’s for primary
T
ments was determined by fitting the signal with the P(E )’s for primary
T
HCl elimination in Fig. 4. The percent contributions from each channel
shown are the relative probabilities that already include kinematic factors
but do not yet account for ionization cross sections and cracking probabili-
ties.
HCl elimination in Fig. 4. The percent contributions from each channel
shown are the relative probabilities that already include kinematic factors
but do not yet account for ionization cross sections and cracking probabili-
ties.
number of laser shots, we calculate the experimentally deter-
this value in our branching ratio calculations. We then cor-
rect the branching ratio for the relative ionization cross sec-
tions of the Cl and HCl fragments, which are estimated from
ϩ
ϩ
mined fragmentation pattern of HCl to HCl /Cl to be 0.68
Ϯ0.08 ͑all error bars are reported as 2␴͒. We also calculate
the fragmentation pattern of HCl from the data at 200 °C and
their polarizabilities to be ␴ ϭ2.18 Å and ␴ ϭ2.60 Å3
Cl
3
HCl
ion
ion
ϩ
ϩ
12
obtain HCl /Cl ϭ0.71Ϯ0.19 in order to verify that the ratio
respectively.
is the same at the two temperatures; however, we do not use
total HCl elimination
C–Cl fission
ͩ
ͪ
2
00 °C
Cl
ϩ
͑
%HClfastϩ%HClslow͒
␴
HCl
ion
ϭ
• ␴
•
.
ϩ
͑1͒
HCl
ion
%C–Cl
Cl
The final result is a primary product branching ratio of
HCl:C–Cl͒200 °Cϭ0.12:1.0Ϯ0.03. Similarly, we determined
the branching ratio for the data with a nozzle temperature of
75 °C, obtaining a primary product branching ratio of
HCl:C–Cl͒475 °Cϭ0.23:1.0Ϯ0.03. This result shows the
͑
4
͑
branching ratio between HCl elimination and C–Cl fission
increases when the nozzle temperature is raised.
C. Analysis of the temperature dependence of the
branching ratios with respect to conformer
populations
As shown in the section above, the HCl elimination/
C–Cl fission branching ratio depends strongly on the nozzle
temperature. Since negligible conformer relaxation in the ex-
pansion occurs for allyl chloride,13 the fraction of the higher
energy cis conformer is increased upon raising the nozzle
temperature. Thus, we now analyze the data to investigate
whether the change in the branching ratio results from the
change in the conformer populations. Although gas phase
studies on the conformational properties of allyl chloride
ϩ
FIG. 9. The laboratory TOF spectrum of the photofragments detected at Cl
from allyl chloride photodissociated at 193 nm with a nozzle temperature of
75 °C and a source angle of 15°. The contribution from Cl fragments to the
Cl signal was determined by fitting the signal with the P(E ) for primary
C–Cl fission in Fig. 3. Similarly, the contribution from HCl fragments was
determined by fitting the signal with the P(E )’s for primary HCl elimina-
tion in Fig. 4 in which the relative scaling of the two pathways was retained
from the HCl TOF in Fig. 10. These data were used to determine the
absolute branching ratios at 475 °C. The percent contributions from each
channel shown are the relative probabilities that already include kinematic
factors but do not yet account for ionization cross sections and cracking
probabilities.
4
ϩ
T
T
ϩ
14,15
have been investigated,
a reliable energy difference has
not been reported. Schei and Shen14 used electron diffraction
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

Myers et al.: Photodissociation of allyl chloride
5451
FIG. 12. The two conformers of allyl chloride; cis ͑left͒ and gauche ͑right͒.
slow HCl elimination
C–Cl fission
8.7%
ͩ
ͩ
ͩ
ͩ
ͪ
ͪ
ͪ
ͪ
71.5%
4
2
75 °C
00 °C
475 °C
200 °C
ϭ
ϭ3.4.
slow HCl elimination
C–Cl fission
3.0%
83.1%
͑
4͒
FIG. 11. The laboratory angular distribution of the Cl atom product from
allyl chloride photodissociated at 193 nm with linearly polarized light. ⌰LAB
is the angle of the laser electric vector with respect to detector axis. The data
points show the signal integrated between 144–170 ␮s. Line fits show the
predicted change in detected scattered signal intensity with laser polarization
angle obtained, after transformation from the c.m. to the laboratory frame,
with three trial anisotropy parameters; ␤ϭ0.0, Ϫ0.1, and Ϫ0.2. The inte-
grated signal also includes a minor contribution from the overlapping fast
HCl elimination signal. The effect on the anisotropy parameter is probably
minimal.
Similarly, the relative change in the HCl elimination produc-
ing fragments with high kinetic energy and C–Cl bond fis-
sion is determined.
͑
͑
fast HCl elimination/C–Cl fission͒475 °C
fast HCl elimination/C–Cl fission͒200 °C
͑
19.8%/71.5%͒475 °C
13.9%/83.1%͒200 °C
ϭ
ϭ1.7.
͑5͒
͑
A marked increase in the slow HCl elimination channel oc-
curs with a raise in the nozzle temperature and is too large to
attribute to the change in the conformer populations. The
increase in the fast HCl elimination channel, on the other
hand, is less and may result from a conformation depen-
dence. These results are further considered in Sec. IV.
to determine the amount of the gauche conformer at both
20 °C and 90 °C which was 82%Ϯ9% and 78%Ϯ15%, re-
spectively. From this data, they estimate an energy difference
of 0.62 kcal/mol. The large error bars on the populations
results in great uncertainty in the calculated energy differ-
ence. Using this gauche/cis energy difference of 0.62 kcal/
mol, we can determine the relative change in conformer
16
population upon heating the nozzle from 200 to 475 °C:
D. Angular distribution of the C–Cl fission product
Figure 11 shows the integrated Cl fragment signal from
C–Cl bond fission vs. ⌰LAB , the angle between the electric
vector and the detector axis. The best fit is obtained by vary-
ing the anisotropy parameter ␤ in this expression for the
angular distribution of the fragments17
͑
͑
cis/gauche)475 °C 0.87
ϭ
ϭ1.3.
͑2͒
cis/gauche͒200 °C 0.68
If only the cis conformer contributed to HCl elimination and
only the gauche conformer contributed to C–Cl fission, then
the relative change in the conformer population would pre-
dict an increase in the HCl/C–Cl branching ratio of 1.3. The
experimentally observed change in the branching ratio, how-
ever, is significantly higher than the predicted change.
I͑␪c.m.͒ϭ1/4͓1ϩ␤P ͑cos ␪c.m.͔͒
͑6͒
2
in which ␪c.m. is the angle between the electric vector and the
recoiling fragment in the center-of-mass reference frame.
Since the photofragment angular distribution is measured in
the laboratory frame, fitting the data involves converting be-
tween the center-of-mass and laboratory frames using the
͑
HCl elimination/C–Cl fission͒475 °C 0.23
ϭ
ϭ1.9. ͑3͒
͑
HCl elimination/C–Cl fission͒200 °C 0.12
Although the conformational energy difference may not be
exact, an energy difference of 1.67 kcal/mol would be re-
quired to obtain a relative branching ratio change of 1.9.
Thus, the increase in the HCl elimination/C–Cl fission
branching ratio is not completely due to the change in con-
former population.
measured molecular beam velocity and the P(E ) deter-
T
mined from the unpolarized data. The data in Fig. 11 shows
the photofragment anisotropy for C–Cl bond fission to be
slightly perpendicular with ␤ϭϪ0.1.18 If we assume distor-
tion of the molecule upon photoexcitation does not alter the
C–Cl bond direction before dissociation and the transition
dipole moment is parallel to the CvC bond, the predicted
We can also examine the competition between the two
pathways for HCl elimination with C–Cl fission separately.
The experimentally observed change in the HCl elimination
channel leading to low recoil energies with C–Cl bond fis-
sion is determined from the relative probabilities used to fit
9͑b͒
␤ ϭ2P ͑cos 63.1°͒ϭϪ0.4. The gauche conformer, how-
cis
2
ever, is more complicated.9 The value predicted is
͑b͒
␤gaucheϭ2P ͑cos 127.4°͒ϭ0.11. . If the angle distorts by
2
just 4°, the predicted ␤ changes to Ϫ0.1. At a nozzle tem-
perature of 220 °C, the molecular beam mixture is 79%
ϩ
the Cl spectra in Figs. 7 and 9.
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

5452
Myers et al.: Photodissociation of allyl chloride
FIG. 13. Cuts through the calculated ab initio electronic surfaces at equilibrium CvC bond lengths, for vinyl chloride with R͑CvC͒ϭ1.332 Å ͑left͒ and
cis-allyl chloride with R͑CvC͒ϭ1.316 Å ͑right͒. The four lowest singlet excited electronic states are shown. The two AЈ adiabatic states ͑– • –͒ are involved
in the avoided electronic configuration crossing. The two AЉ adiabatic states ͑– – –͒ are shown only because they fall in the same energy region; in planar
symmetry, they do not interact with the AЈ states. The boxed-in portions are enlarged in the insets above to show the splitting between the adiabats at the
Ϫ1
avoided crossing. For the particular cut along the avoided crossing seam presented here, the splitting at the avoided crossing to C–Cl bond fission is 261 cm
Ϫ1
for vinyl chloride and 982 cm for cis-allyl chloride. For the cis conformer of allyl chloride, the inset also includes the Franck–Condon region ͑indicated by
the arrow͒ and shows the potential is relatively flat.
gauche and 21% cis using the energy difference reported by
excited state reaction coordinate for C–Cl fission in vinyl
chloride. Umemoto et al.5 characterized the pathway to
C–Cl fission in vinyl chloride as a transition to the dissocia-
tive n␴* state which crosses the photoprepared ␲␲* state in
1
4
Schei and Shen. If only one conformer contributes to C–Cl
fission, the expected ␤ would correspond to the value pre-
dicted for that conformer. If, however, both conformers con-
tribute equally to C–Cl fission, then the expected ␤ would be
a weighted average, ␤ϭ0.79͑0.1͒ϩ0.21͑Ϫ0.4͒ϭ0.0. Since
the predicted angular distribution for both conformers are
similar and very sensitive to geometry change in the excited
state, the angular distribution measurement is not sensitive
enough to determine which conformer or weighted distribu-
tion of conformers undergoes C–Cl fission.
the C–Cl coordinate. In C symmetry, the out-of-plane ␲ and
s
␲
*
orbitals are aЉ and the in-plane nCl and ␴C*–Cl orbitals
CvC
are aЈ; therefore, both the ␲␲* and the n ␴* configu-
CvC
Cl C–Cl
rations are AЈ so that they mix and split forming a barrier
along the lower adiabatic excited state potential energy sur-
face. If the molecule traverses the barrier and accesses the
region of the surface which is n ␴* in character, C–Cl
Cl C–Cl
fission results. If, however, the splitting between the adia-
batic electronic surfaces is small reflecting weak configura-
tional mixing, a nonadiabatic transition to the upper adiabat
can occur where the molecule retains its ␲␲* configuration
and turns back toward the Franck–Condon region instead of
evolving to products along the lower surface. Thus, the mag-
nitude of the splitting between the adiabats at the barrier
reflects the probability that the barrier is traversed adiabati-
cally.
IV. DISCUSSION
The primary purpose of these experiments was to deter-
mine if the CH spacer in allyl chloride as compared to vinyl
2
chloride affects the probability of nonadiabatic recrossing of
the barrier to C–Cl fission. The CH spacer both further
2
separates the CvC and the C–Cl chromophores and results
in two molecular conformers, one of which does not retain a
plane of symmetry. To illustrate why we expect the C–Cl
bond fission pathway to be strongly influenced by nonadia-
batic dynamics in these two systems, we first consider the
The splitting is notably smaller for Woodward–
Hoffmann forbidden reactions than for reactions where the
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

Myers et al.: Photodissociation of allyl chloride
5453
2
individual orbital symmetries are conserved. Since the indi-
vidual orbital symmetry changes from aЉ to aЈ along the
excited C–Cl reaction coordinate in vinyl chloride and cis-
allyl chloride, C–Cl fission is Woodward–Hoffmann forbid-
den. Thus, we expect the splitting to be small and nonadia-
batic barrier recrossing to reduce the rate constant for C–Cl
fission. To argue qualitatively why we expect Woodward–
Hoffmann forbidden reactions to have anomalously smaller
splittings than reactions which conserve individual orbital
symmetry, we calculate the configuration interaction matrix
elements which mix and split the ␲␲* and the n␴* configu-
rations at the barrier to C–Cl fission. In a simple two-state
system, the dominant electronic configuration contributing to
the wave function on the reactant side of the barrier, ⌿ , is
R
2
1
1
0
{
•••L(n ) (␲) (␲*) (␴* ) } and for the wave function
Cl
C–Cl
on the product side of the barrier, ⌿P , is
1
2
0
1
{
•••L(n ) (␲) (␲*) (␴* ) }. If no orthogonality is as-
Cl
C–Cl
sumed between the reactant and product molecular orbitals
or between ⌿ and ⌿ , then the splitting between the two
R
P
19
adiabatic surfaces at the barrier is
FIG. 14. Cuts through the calculated ab initio surfaces for gauche-allyl
chloride at equilibrium CvC bond length, R͑CvC͒ϭ1.318 Å. In the
gauche conformer, only the three lowest singlet excited electronic states are
2
͑␤Ϫ␣S͒
1ϪS2͒
Ϫ1
splittingϭ
,
͑7͒
shown ͑the next singlet excited state is above 70 000 cm ͒. Since the sym-
͑
metry is broken, the ␲␲* character, in principle, can mix into all three
electronic states so that a two-state approximation may no longer be com-
pletely valid. By tracking the oscillator strengths ͑predominantly from ␲␲*
character͒ and the dominant electronic configurations of the excited state
potential energy surfaces along the RC–Cl coordinate, we can determine that
primarily the first and third excited states ͑– • –͒ exhibit an avoided crossing
around 1.84 Å. ͑The second excited state surface ͑- - -͒ does not participate
significantly until 1.86 Å.͒ For the particular cut along the avoided crossing
seam presented here, the splitting at the avoided crossing between the first
where ␣ is the energy at which the diabats cross, S is the
overlap integral ⌿R ⌿P /C, and ␤ is the interaction, reso-
nance, or exchange energy ͗⌿ ͉H͉⌿ ͘/C; C corrects for
͗
͉
͘
R
P
unnormalized wave functions. For Woodward–Hoffmann
forbidden reactions, the product and reactant molecular or-
bitals are orthogonal by symmetry. As a result, the overlap
integrals and all one-electron integrals that contribute to the
resonance and exchange energy represented by ␤ are zero so
that only two-electron integrals mix and split the adiabats at
the avoided crossing, resulting in a small splitting between
the adiabats for this class of reactions.
Previous experiments in our lab have also demonstrated
that increasing the distance between the orbitals involved in
the avoided crossing increases the probability of nonadiabati-
cally recrossing the barrier.1 For example, the branching to
C–Br bond fission upon ͓n͑O͒␲*͑CvO͔͒ excitation de-
creased by an order of magnitude in bromopropionyl chlo-
ride compared to bromoacetyl chloride.3 The extra CH2
Ϫ1
and third adiabats is 2626 cm . A larger splitting at the barrier between the
two adiabats involved in the avoided electronic configuration crossing is
seen here as compared to the cis conformer because C–Cl bond fission is
now Woodward–Hoffmann allowed. The boxed-in portion is enlarged in the
inset to the right to show the Franck–Condon region ͑indicated by the ar-
row͒. The inset has the same x/y aspect ratio as the inset for cis-allyl
chloride in Fig. 13 in order to illustrate the different forces experienced in
the Franck–Condon region for the two conformers. The repulsive forces in
the C–Cl bond upon photoexcitation allow C–Cl fission to dominate in the
gauche conformer.
2,3
ever, is planar so C–Cl fission is Woodward–Hoffmann for-
bidden. Thus, we expect C–Cl fission to be suppressed in the
cis conformer, and the change in the HCl elimination/C–Cl
fission branching ratio should follow the change in the frac-
tion of the cis conformer. The data does show that the
branching ratio increases with the raise in nozzle tempera-
ture, but the increase is stronger than that presented by the
conformation dependence alone.
spacer between the n ␲*
and the n ␴*
orbitals in
O
CvO
Br C–Br
bromopropionyl chloride reduces the electronic interaction
matrix elements between the two configurations resulting in
a smaller splitting between the adiabats at the barrier. Thus,
C–Br fission is further suppressed by nonadiabatic barrier
recrossing. Similarly, the branching to C–Cl fission might
also decrease in allyl chloride compared to vinyl chloride.
Unlike vinyl chloride, allyl chloride exists as two con-
formers ͑Fig. 12͒. In the predominant gauche conformer, the
Cl atom is out of the molecular plane breaking the plane of
symmetry; therefore, C–Cl fission is no longer Woodward–
Hoffmann forbidden, and we expect the splitting between the
adiabats at the barrier to C–Cl fission to increase. As a result,
the probability that the barrier to C–Cl fission is traversed
adiabatically should be greater for gauche-allyl chloride than
for vinyl chloride. The higher energy cis conformer, how-
We present GAUSSIAN 92 calculations in order to investi-
1
gate the energetic splittings between the AЈ excited potential
energy surfaces at the avoided crossing for vinyl chloride
and cis- and gauche-allyl chloride. Figure 13 shows cuts
along the C–Cl stretch of the calculated ab initio electronic
surfaces for vinyl and cis-allyl chloride. Although the four
lowest singlet excited electronic surfaces are shown in Fig.
13, two are AЉ so they do not interact with the AЈ surfaces in
planar symmetry. The AЈ potential energy surface clearly
evidences an avoided electronic curve crossing between the
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

5454
Myers et al.: Photodissociation of allyl chloride
cidate the electronic character of the excited states in the
␲
␲* and n ␴* states, forming the adiabatic reaction
CvC
Cl C–Cl
Franck–Condon region.20 The emission spectrum for vinyl
chloride evidences the dominant ␲␲* character of the initial
excitation by the strong emission to the CvC stretching and
twisting modes. The ab initio calculations presented in that
work and augmented in this paper agree with the emission
spectrum; they show the electronic character of the bright
coordinate for C–Cl fission. Thus, the lower AЈ potential
energy surface has a barrier along the forward and reverse
reaction coordinate. As determined by the dominant elec-
tronic configuration in the GAUSSIAN 92 output, the electronic
character changes from predominantly ␲␲*
to n ␴*
CvC
Cl C–Cl
across the barrier in both systems. In vinyl chloride, the split-
ting between the adiabats at the barrier to C–Cl fission is
very small which supports the model that nonadiabatic bar-
rier recrossing can reduce the rate constant for C–Cl fission
and allow HCl elimination to compete. The branching ratio
measured by Mo et al.4 for vinyl chloride, HCl:C–Cl
ϭ1.0:1.1, shows the two pathways are competing. The split-
ting at the barrier calculated for vinyl chloride is surprisingly
1
AЈ potential energy surface in vinyl chloride is ␲␲* in the
Franck–Condon region ͑Fig. 15͒. Upon electronic excitation
of allyl chloride, however, enhanced emission into the C–Cl
stretch eigenstates appear ͑although the CvC stretch still
dominates͒, revealing an admixture of ␲
*
and ␴
*
char-
CvC
C–Cl
acter in the Franck–Condon region. The emission spectrum
for allyl chloride reflects the initial motion in the predomi-
nant gauche conformer of allyl chloride upon photoexcita-
tion. The calculations also show an increased contribution of
smaller than for cis-allyl chloride even though a CH spacer
2
is inserted between the CvC and C–Cl chromophores in the
latter. This may result from an increase in the overlap densi-
ties between the chromophores since in the cis conformer,
the Cl atom eclipses the double bond. As expected for the
gauche conformer in which C–Cl fission is no longer
Woodward–Hoffmann forbidden, the splitting is much larger
␴
*
character to the optically bright state ͑Fig. 16, bottom͒.
C–Cl
Similar to vinyl chloride, the calculations for cis-allyl chlo-
ride reflect the pure ␲␲* character in the Franck–Condon
region ͑Fig. 16, top͒. The broken symmetry in the gauche
conformer permits extensive coupling between the ␲␲* and
͑see Fig. 14͒, and C–Cl fission can proceed adiabatically and
dominate HCl elimination. Figure 14 also shows that in the
Franck–Condon region for the gauche conformer, the forces
are repulsive in the C–Cl bond along the C–Cl bond fission
reaction coordinate. In the cis conformer, the contour is rela-
tively flat in the Franck–Condon region along the C–Cl
bond fission pathway. The repulsive forces in the Franck–
Condon region for the gauche conformer should allow C–Cl
fission to occur more readily. The experiments confirm that
the branching to C–Cl fission has increased; the measured
branching ratio for allyl chloride is ͑HCl:C–Cl͒200 °C
ϭ0.12:1.0. Further calculations are planned which will con-
sider torsion about the double bond and CvC stretching
since these motions are important upon excitation.20
Our laboratory has also measured the emission spectra at
199 nm for vinyl chloride and allyl chloride in order to elu-
FIG. 16. The top frame shows the excited state molecular orbital for cis allyl
chloride from the bright state in the Franck–Condon region. The orbital
illustrates the essentially pure ␲* character. The bottom frame shows the
CvC
FIG. 15. The excited state molecular orbital for vinyl chloride from the
bright state in the Franck–Condon region. The orbital shows essentially pure
dominant excited state molecular orbital for gauche-allyl chloride from the
bright state in the Franck–Condon region. The orbital has an admixture of
␲
2
*
0.
character. Figure reproduced with permission of the authors from Ref.
␲* and ␴C*–Cl character. Figures reproduced with permission of the au-
thors from Ref. 20.
CvC
CvC
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

Myers et al.: Photodissociation of allyl chloride
5455
n␴* electronic configurations and results in repulsive forces
along the C–Cl reaction coordinate in the Franck–Condon
region, reflected also in the slope of the potential in Fig. 14,
which allows C–Cl fission to dominate HCl elimination.
We now discuss likely explanations for the two observed
pathways for HCl elimination in allyl chloride. In vinyl chlo-
ride, HCl elimination is believed to occur following internal
conversion to the ground electronic state in which two dif-
ferent mechanisms were elucidated: a three-center, ␣␣ elimi-
nation of HCl to produce vinylidene and a four-center, ␣␤
elimination of HCl to produce acetylene with a product ratio
of ␣␣:␣␤ϭ3:1.2 The H atom transfer in vinylidene has a
low barrier and occurs fast on the timescale of the reaction.
The exit barrier for the four-center elimination is 52.6 kcal/
mol and the exit barrier for the three-center elimination and
subsequent H atom migration to produce acetylene is 44.1
˙
˙
H CvCHCH Clϩ193 nm→HCvCHCH ϩHCl,
Eavailϭ73 kcal/mol.
2
2
2
͑
10͒
Another possibility is hydrogen atom migration to produce
H CCCH Cl or H CCHCH Cl which subsequently under-
¨
¨
3
2
2
2
goes HCl elimination. The present experiment does not pro-
vide enough information to definitively identify the mecha-
nism involved in both HCl elimination pathways. However,
the very high energies of 76 kcal/mol detected in product
recoil for the HCl elimination channel in the upper frame of
Fig. 5 suggest this channel is most likely due to the mecha-
nism presented in reaction ͑8͒. The low energy signal prob-
ably results from reaction ͑9͒ or ͑10͒. More information is
needed about the exit barriers for these reactions in order to
form any definite conclusions.
The temperature dependence of the HCl elimination
channels provide a further clue as to their mechanism. As
shown in Sec. III C, the branching to the HCl elimination
producing fragments with low kinetic energy increases more
than the channel producing the fast HCl fragments with a
raise in nozzle temperature. When we attempted to attribute
all of the HCl elimination to the cis conformer and all of the
C–Cl fission to the gauche conformer, the observed increase
in the HCl/C–Cl branching ratio upon heating the nozzle
from 200 °C to 475 °C was higher than the relative change in
the cis/gauche conformer population. Instead, let us now as-
sume that the fast HCl channel occurs predominantly from
the cis conformer whereas the slow HCl channel and C–Cl
fission occur predominantly from the gauche conformer.
Then, the change in the relative branching between
HCl͑fast͒/͓HCl͑slow͒ϩC–Cl͔ should follow the change in
the conformer population upon heating the nozzle.
1
9
kcal/mol. Since the three-center elimination dominates over
the four-center elimination and both channels have similar
exit barriers, the HCl elimination pathway in vinyl chloride
was fit using only one broad translational energy distribution.
In allyl chloride, however, two different translational en-
ergy distributions must be used to fit the data. The HCl elimi-
nation could occur from dynamics either on the ground state
potential energy surface after internal conversion or on an
excited state potential energy surface. Several different
2
2
mechanisms for the HCl elimination are possible:
H CvCHCH Clϩ193 nm→H CvCvCH ϩHCl,
2
2
2
2
͑
8͒
9͒
Eavailϭ124 kcal/mol,
H CvCHCH Clϩ193 nm→H CvCHCHϩHCl,
¨
2
2
2
͑
Eavailϭ95 kcal/mol,
͑
͑
fast HCl elimination/͑slow HCl eliminationϩC–Cl fission͒͒475 °C ͑19.8%/͑8.7%ϩ71.5%͒͒475 °C
ϭ
ϭ1.5.
fast HCl elimination/͑slow HCl eliminationϩC–Cl fission͒͒200 °C ͑13.9%/͑3.0%ϩ83.1%͒͒200 °C
7
This result is similar to the relative change in the conformer
population in heating the nozzle from 200 to 475 °C, which
was previously shown to be 1.3.
that Lee et al. have also recently investigated the photodis-
sociation of allyl chloride at 193 nm. They report the major
channel is C–Cl fission, but they only report one channel for
HCl elimination with an average translational energy of 9
kcal/mol accounting for 5% of the total product yield.
We present a plausible explanation for the large increase
in the slow HCl elimination channel with a raise in nozzle
temperature. Going from a 200 °C to 475 °C nozzle expan-
sion results in increased initial excitation of low energy vi-
ACKNOWLEDGMENTS
5
brational motion. Umemoto et al. claim that out-of-plane
This work was supported by the National Science Foun-
dation under Grant No. CHE-9307500. T.L.M. was sup-
ported by a Department of Education GAANN fellowship.
motion in vinyl chloride increases the coupling between the
␲
␲* and the ␲␴* state leading to efficient internal conver-
sion to the ␲␴* state which they propose opens up the door-
way to the ground state. In allyl chloride, the increased bend-
ing motion at the higher temperature increases the coupling
with the predominantly ␲␴* state which may also serve as
the intermediate state for internal conversion to the ground
state.
1
R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry
͑
Verlag-Chemie, Weinheim, 1970͒; S. S. Shaik, J. Am. Chem. Soc. 103,
3692 ͑1981͒.
2
3
G. C. G. Waschewsky, P. W. Kash, T. L. Myers, D. C. Kitchen, and L. J.
Butler, J. Chem. Soc. Faraday. Trans. 90, 1581 ͑1994͒.
P. W. Kash, G. C. G. Waschewsky, L. J. Butler, and M. M. Francl, J.
Chem. Phys. 99, 4479 ͑1993͒.
In the final stages of preparation of this work, we learned
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15

5456
Myers et al.: Photodissociation of allyl chloride
4
15
Y. Mo, K. Tonokura, Y. Matsumi, M. Kawasaki, T. Sato, T. Arikawa, P. T.
A. Reilly, Y. Xie, Y. A. Yang, Y. Huang, and R. J. Gordon, J. Chem. Phys.
A. J. Barnes, S. Holroyd, W. O. George, J. E. Goodfield, and W. F. Mad-
dams, Spectrochim. Acta 38A, 1245 ͑1982͒.
16
9
7, 4815 ͑1992͒.
Although the gauche populations of 82% and 78% at 20 °C and 90 °C
respectively would give an energy difference of 0.75 kcal/mol ͑and a
nonzero entropy change͒, the authors from Ref. 14 report an enthalpy
difference of 0.62 kcal/mol. The value of 0.62 kcal/mol was probably
derived from the populations before they were rounded to two significant
figures ͑for example, percentages of 81.6 and 78.3 give 0.62 kcal/mol͒;
thus we retain the 0.62 kcal/mol value here. If the entropy change is
considered to be negligible, then the calculated energy difference becomes
approximately 0.43 kcal/mol. If we recalculate the relative change in con-
former population upon heating the nozzle from 200 °C to 475 °C with an
energy difference of 0.44Ϯ0.03 kcal/mol, the ratio changes only slightly
to 1.2.
5
M. Umemoto, K. Seki, H. Shinohara, U. Nagashima, N. Nishi, M. Ki-
noshita, and R. Shimada, J. Chem. Phys. 83, 1657 ͑1985͒.
C. W. Worrell, J. Electron. Spectrosc. 3, 359 ͑1974͒.
Private communication of recent results by Y. R. Lee, F. T. Chen, and S.
M. Lin, submitted for publication in J. Chem. Phys.
6
7
8
9
T. S. Sears and D. H. Volman, J. Photochem. 26, 85 ͑1984͒.
͑a͒ The equilibrium geometry for vinyl chloride is taken from J. F. Riehl
and K. Morokuma, J. Chem. Phys. 100, 8976 ͑1994͒; ͑b͒ The equilibrium
geometries for cis- and gauche-allyl chloride are taken from J. R. Durig,
D. T. Durig, M. R. Jalilian, M. Zhen, and T. S. Little, J. Mol. Struc. 194,
2
59 ͑1989͒. The values calculated with the 6-31G* basis set were used.
10
GAUSSIAN 92, Revision C, M. J. Frisch, G. W. Trucks, M. Head-Gordon, P.
M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel,
M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari,
J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker,
J. J. P. Stewart, and J. A. Pople ͑Gaussian, Inc., Pittsburgh, PA, 1992͒.
The bond dissociation energies were taken from A. Streitwieser, Jr., and C.
H. Heathcock, Introduction to Organic Chemistry ͑Macmillan, New York,
17
R. N. Zare, Mol. Photochem. 4, 1 ͑1972͒.
18
The measured angular distribution may depend on the nozzle temperature
if both conformers contribute to C–Cl fission due to the change in the
conformer population. Results by Y. R. Lee, F. T. Chen, and S. M. Lin,
submitted to J. Chem. Phys. measure ␤ϭ0.2 at 170 °C; this result suggests
C–Cl fission occurs predominantly from the gauche conformer.
For a more detailed explanation of this argument see Ref. 2. Also, see D.
M. Silver, J. Am. Chem. Soc. 96, 5959 ͑1974͒.
P. W. Browning, D. C. Kitchen, M. F. Arendt, and L. J. Butler, J. Phys.
Chem. ͑in press͒.
Y. Huang, Y. A. Yang, G. X. He, and R. J. Gordon, J. Chem. Phys. 99,
11
1
2
2
2
9
0
1
2
1
985͒, 3rd ed.
12
We estimated the ionization cross sections using the empirical relationship
obtained in R. E. Center and A. Mandl, J. Chem. Phys. 57, 4104 ͑1972͒,
Ϫ16
2
1/2
3
␴ion͑10
Å ͒ϭ36͑␣ ͒Ϫ18. The atomic polarizability of 2.18 Å for Cl
is given in T. M. Miller and B. Bederson, Adv. At. Mol. Phys. 13, 1
3
2752 ͑1993͒.
͑
1977͒, and the molecular polarizability of 2.60 Å for HCl is given in N.
J. Bridge and A. D. Buckingham, Proc. R. Soc. London, Ser. A 295, 334
1966͒.
The heats of formation for reaction ͑8͒ were taken from R. C. Weast,
Handbook of Chemistry and Physics ͑Chemical Rubber, Boca Raton, FL,
1983͒, Vol. 64. Reactions ͑9͒ and ͑10͒ were estimated from the bond
strengths in D. F. McMillen and D. M. Golden, Annu. Rev. Phys. Chem.
33, 493 ͑1982͒ and exclude any resonance stabilization energy.
͑
1
1
3
4
R. S. Ruoff, T. D. Klots, T. Emilsson, and H. S. Gutowsky, J. Chem. Phys.
3, 1 ͑1990͒.
S. H. Schei and Q. Shen, J. Mol. Struc. 128, 161 ͑1985͒.
9
J. Chem. Phys., Vol. 104, No. 14, 8 April 1996
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.22.67.123 On: Sat, 22 Nov 2014 01:05:15