Site and isotope effects on the molecular hydrogen elimination from ethylene at 157 nm excitation

Article abstract of DOI:10.1063/1.476888

Site and isotope effects on the molecular hydrogen elimination from ethylene have been studied from the photodissociation of ethylene at 157 nm excitation. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and 1,2-trans elimination. These elimination processes show significantly different translational energy distributions. Isotope effect on the dynamics of these molecular hydrogen elimination processes has been also investigated carefully.

Full text of DOI:10.1063/1.476888

Site and isotope effects on the molecular hydrogen elimination from ethylene at 157 nm
excitation
Jim J. Lin, Dennis W. Hwang, Yuan T. Lee, and Xueming Yang
Citation: The Journal of Chemical Physics 109, 2979 (1998); doi: 10.1063/1.476888
View online: http://dx.doi.org/10.1063/1.476888
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 109, NUMBER 8
22 AUGUST 1998
Site and isotope effects on the molecular hydrogen elimination
from ethylene at 157 nm excitation
Jim J. Lin and Dennis W. Hwang
Department of Chemistry, National Taiwan University, Taiwan, Republic of China
Yuan T. Lee
Department of Chemistry, National Taiwan University and Institute of Atomic and Molecular Sciences,
Academia Sinica, Taipei, Taiwan, Republic of China
Xueming Yanga)
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, Republic of China
͑Received 6 March 1998; accepted 19 June 1998͒
Site and isotope effects on the molecular hydrogen elimination from ethylene have been studied
from the photodissociation of ethylene at 157 nm excitation. Experimental results show that there
are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and
1,2-trans elimination. These elimination processes show significantly different translational energy
distributions. Isotope effect on the dynamics of these molecular hydrogen elimination processes has
been also investigated carefully. © 1998 American Institute of Physics. ͓S0021-9606͑98͒01332-4͔
Molecular elimination from highly excited molecules is
an important and also interesting class of unimolecular reac-
tions since it is related not only to simple bond rupture pro-
cesses but also bond formation processes during the elimina-
tion. However, there has been lack of systematic studies on
the dynamics of the molecular hydrogen elimination so far,
especially the precise translational and internal energy distri-
butions of the photodissociation products. Recently, a new
universal crossed beam apparatus has been established in our
laboratory.1 Because of its extremely low background of H2
in the detector, this new apparatus provides us a unique and
excellent tool to study the interesting molecular hydrogen
elimination process.
tion ͑see Fig. 2͒. In this work, photodissociation of different
ethylene isotopomers at 157 nm excitation are systematically
investigated in order to examine the site and isotope effects
on the dynamics of molecular hydrogen elimination from
ethylene. At 157 nm excitation, the ethylene molecule is
mainly excited to the 3px,y,z Rydberg states.11 The equilib-
rium geometries of the 3px,y,z Rydberg states are all near
planar from a recent theoretical calculation,12 implying that
the 1,2 cis and trans molecular eliminations can be dynami-
cally different.
The apparatus used in this experiment is a newly built
universal crossed beam machine. A detailed description of
this apparatus can be found somewhere else.1 Briefly, the
new crossed molecular beam apparatus consists of a source
chamber, a main chamber and a rotating detector. The most
crucial part of the apparatus is its ultraclean UHV universal
detector. The ionization detection region can be maintained
to a vacuum of 1ϫ10Ϫ12 Torr or lower in the new apparatus.
The H2 background in the detector is about 2 orders of mag-
nitude lower in comparison with any similar apparatus. This
made the detection of molecular hydrogen, therefore atomic
hydrogen, much easier than before. The 157 nm photolysis
laser beam from a Lambda Physik LPX 210i laser is focused
into the ethylene molecular beam at a distance of 10 mm
away from the nozzle. In a typical experiment, a 50 Hz
pulsed ethylene beam is produced by expanding the neat
ethylene from a pulsed valve ͑General Valve͒ through a 1
mm diam nozzle at a pressure of 50 Torr behind the nozzle.
In this work, the dynamics of the photodissociation of three
di-deuterated ethylene isotopomers: 1,1-CH2CD2, 1,2 cis-
CHDCHD, and 1,2 trans-CHDCHD are investigated. In or-
der to make meaningful comparisons across the three ethyl-
ene isotopomers, all experimental conditions such as
molecular number density, and laser intensity in the interac-
tion area are controlled carefully.
Previous experimental studies2–7 show that the dissocia-
tion of ethylene under the excitation of a high energy photon
involves four different chemical dissociation channels,
C2H4ϩh␯→HCwCHϩH2
→H2CvC:ϩH2
͑1͒
͑2͒
͑3͒
͑4͒
→H2CvCHϩH
→HCwCHϩ2H.
The first two channels are the molecular hydrogen elimina-
tion processes, while the third and the fourth channels are the
atomic hydrogen elimination processes. Molecular hydrogen
elimination has been studied in the photodissociation of eth-
ylene at 193 nm excitation.8–10 Site and isotope effects on
the dynamics of molecular hydrogen elimination from ethyl-
ene have been investigated qualitatively.
The molecular elimination process in ethylene can be
divided into three types of elimination: 1,1 elimination
(11E), 1,2 cis elimination (12cE) and 1,2 trans elimination
(12tE). In each of the three types of elimination, there are
two possible pairs of hydrogen atoms for molecular elimina-
a͒
Time of flight spectra of the photodissociation products
at m/eϭ1,2,3,4 from the ethylene isotopomers at 157 nm
Author to whom correspondence should be addressed. Electronic mail:
xmyang@po.iams.sinica.edu.tw
0021-9606/98/109(8)/2979/4/$15.00
2979
© 1998 American Institute of Physics
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2980
J. Chem. Phys., Vol. 109, No. 8, 22 August 1998
Communications
TABLE I. Relative yields of H2, HD, and D2 eliminations from the three
ethylene isotopomers.
Molecule
D2
HD
H2
Total
1,2 trans-CHDCHD
1,2 cis-CHDCHD
1,1 CH2CD2
1.00
1.30
2.29
8.58
8.18
5.20
1.90
1.90
4.12
11.48
11.38
11.61
to be small since the shape of the total signal of the three
compounds are quite similar, the integrated signals from the
three di-deuterated ethylenes were all within 5% to each
other, indicating that the photodissociation of three deuter-
ated ethylene molecules can be compared quantitatively. It is
necessary to point out that the anisotropy differences in the
dissociation processes might also cause some differences in
the branching ratio. However, experimental results here
show that molecular hydrogen elimination from ethylene
most likely occurs on the ground potential surface through
internal conversion from the initially excited state. This will
probably wash out most of the anisotropy in the dissociation
processes.
Since the dissociative ionization of the hydrogen mol-
ecule can be neglected, the observed signals at m/e
ϭ1,2,3,4 should be the photodissociation products of H, H2
and D, HD and D2, respectively. Therefore from the experi-
mentally measured time of flight spectra for m/eϭ1,2,3,4,
the translational energy distribution of different elimination
channels, atomic and molecular eliminations, can be ob-
tained using a previously developed software package,
CMLAB2.12 The detection efficiencies for the H2, HD, and D2
products are all assumed to be the same in the analysis. By
fitting the time of flight spectra of m/eϭ2,3,4 from all three
isotopomers, the translational energy distributions and yields
for each channel have been derived. Figure 3 shows the ex-
perimental data and simulated fits to the time-of-flight spec-
tra of the D2 products from the photodissociation of the three
isotopomers using the translational energy distributions
shown in Fig. 4͑a͒. It is clear, from Fig. 2, that the H2 and D2
eliminations from all three deuterated ethylenes are site-
specific. The H2 and D2 eliminations from 1,2 trans-
CHDCHD are all due to site specific 1,2 trans elimination
(12tE), the H2 and D2 eliminations from 1,2 cis-CHDCHD
are all due to site specific 1,2 cis elimination (12cE), and the
H2 and D2 eliminations from 1,1-CH2CD2 are all due to site
specific 1,1 elimination (11E). While H2 and D2 elimina-
tions are all site specific, the HD eliminations from the three
deuterated compounds are the combination of two different
site eliminations ͑see Fig. 2͒. As shown in Fig. 2, the HD
elimination from 1,2 trans-CHDCHD is the combination of
the 1,1 HD elimination and the 1,2 cis HD elimination; the
HD elimination from 1,2 cis-CHDCHD is the combination of
the 1,1 HD elimination and the 1,2 trans HD elimination;
and the HD elimination from 1,1-CH2CD2 is the combina-
FIG. 1. TOF spectra for photodissociation products at m/eϭ1,2,3,4 from
the three ethylene isotopomers. All spectra were taken by averaging over
25 000 laser shots.
excitation were recorded at the perpendicular direction of the
molecular beam using the experimental method described
above. Figure 1 shows the time of flight spectra of m/e
ϭ1,2,3,4 from the photodissociation of the three di-
deuterated ethylene isotopomers. In order to know if a mean-
ingful comparison can be made across all isotopomers, the
total photodissociation signals ͑from m/eϭ1 to 4͒ from the
three ethylene isotopomers were compared. Without consid-
ering the transformation ͑lab to center-of-mass frame͒ differ-
ences between different isotopomers, which are considered
TABLE II. Relative yields of specific site H2, HD, and D2 eliminations.
Specific site elimination
D2
HD
H2
1,2 trans elimination ͑12tE͒
1,2 cis elimination ͑12cE͒
1,1 elimination ͑11E͒
1.00
1.30
2.29
1.20
1.40
2.89
1.90
1.90
4.12
FIG. 2. Molecular elimination channels from the three ethylene isoto-
pomers. ͑a͒ 1,2 trans-CHDCHD; ͑b͒ 1,2 cis-CHDCHD; ͑c͒ 1,1-CH2CD2.
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J. Chem. Phys., Vol. 109, No. 8, 22 August 1998
Communications
2981
YHD 1,1-CH CD ͒ϭ2y
12cE͒ϩ2y
12tE͒,
͑
HD
͑7͒
͑
͑
HD
2
2
where YHD is the total yield of HD elimination, and yHD is
the yield of site specific HD elimination. Solving the above
equations, the relative yields for the site specific HD elimi-
nations can be obtained. Table II lists the relative yields of
the H2, HD, and D2 site specific eliminations.
The translational energy distributions for the site specific
eliminations of H2, HD, and D2 are also obtained through the
simulation using the CMLAB2 program. Since the H2 and D2
eliminations are all site specific, the translational energy dis-
tributions obtained for these processes are all site specific.
However, the translational energy distributions for site spe-
cific HD eliminations from the three deuterated compounds
can be obtained using a similar method to the calculations of
the site specific HD yields. Here, the translational energy
distribution of the HD elimination from a di-deuterated eth-
ylene can be viewed as the summation of the contributions of
the two site specific HD eliminations,
FIG. 3. Experimental and simulated TOF spectra of the D2 products from
photodissociation of the three isotopomers. The translational energy distri-
butions used in the simulation are shown in Fig. 4͑a͒.
tion of the 1,2 cis HD elimination and the 1,2 trans HD
elimination.
From the simulation of the time of flight spectra, the
relative yields of the H2, HD, and D2 eliminations from the
three isotopomers can be obtained, which are all listed in
Table I. Since the H2 and D2 elimination processes are site
specific, the relative yields listed for the H2 and D2 elimina-
tions in Table I are also site specific. The HD elimination
from each ethylene isotopomer, however, is a combination of
two specific site eliminations as pointed above. Therefore the
total HD yields listed in Table I are not site specific. In order
to determine the site specific HD yield, the total HD yield
from each deuterated ethylene can be viewed as the summa-
tion of the two site specific HD yields, and each specific site
HD yield from different deuterated ethylene molecules is as-
sumed to be the same,
P12t E͒ϭ2p
E͒ϩ2p
E͒,
E͒,
E͒,
͑8͒
͑9͒
͑
͑
͑
͑
͑
11E
12cE
12tE
12cE
P12c E͒ϭ2p
E͒ϩ2p
͑
͑
11E
P11 E͒ϭ2p
E͒ϩ2p
͑
͑10͒
͑
12tE
where P12t(E), P12c(E), and P11(E) are the translational
energy distribution for the total HD elimination from 1,2
trans-CHDCHD, 1,2 cis-CHDCHD, 1,1-CH2CD2 respec-
tively, while p12tE(E), p12cE(E), and p11E(E) are the trans-
lational distribution for each specific site HD elimination: 1,2
trans elimination, 1,2 cis elimination and 1,1 elimination,
respectively. From the measured translational distributions
P
12t(E), P12c(E), and P11(E), the translational energy dis-
tributions p12tE(E), p12cE(E), and p11E(E) for the site spe-
cific HD elimination can be determined by solving the above
equations. Up until now, all translational energy distributions
for different sites and isotopes have been determined. The
YHD 1,2 trans-CHDCHD͒ϭ2y
11E͒ϩ2y
12cE͒,
͑
HD
͑
͑
HD
͑5͒
͑6͒
YHD 1,2 cis-CHDCHD͒ϭ2y
11E͒ϩ2y
12tE͒,
͑
HD
͑
͑
HD
FIG. 4. ͑a͒ Site effect on the P(E)’s for D2, HD, and H2 elimination; ͑b͒ Isotope effect on the P(E)’s for specific site elimination.
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2982
J. Chem. Phys., Vol. 109, No. 8, 22 August 1998
Communications
specific site effect on the translational energy distribution of
the H2, HD, and D2 elimination is shown in Fig. 4͑a͒, while
the isotope effect on the translational energy distribution of
specific site eliminations is shown in Fig. 4͑b͒. For conve-
nience of comparison, all translational energy distributions
shown in Figs. 3͑a͒ and 3͑b͒ are normalized to the same
height.
channel ͑1,1 molecular hydrogen elimination͒, the transla-
tional energy distributions for H2, HD, D2 are almost exactly
the same ͓see Fig. 4͑b͔͒ implying that the isotope effect on
the dynamics of the 1,1 elimination channel are very small.
The isotope effect on the translational energy distributions
for the four center elimination channels, especially the 1,2
cis elimination, is much more significant. From Table II, it is
easy to see that the relative yields for any specific site elimi-
nation increase as the eliminated species (D2, HD, and H2)
gets lighter.
From this study, complete and important information on
the site and isotope effects on the molecular hydrogen elimi-
nation processes from ethylene has been derived. Using the
new improved experimental technique, dynamical differ-
ences between different microchannels of molecular hydro-
gen elimination processes from ethylene can be detected.
The results of this work provide an excellent test ground for
further theoretical investigations of molecular elimination
processes of ethylene.
From the translational energy distributions and yields for
the specific site H2, HD, and D2 elimination ͓see Fig. 4͑a͒
and Table II͔, it is very clear that the 1,1, 1,2 cis and 1,2
trans D2 elimination processes show significant differences
in their kinetic energy distributions, indicating that the dy-
namics of D2 elimination from different sites are quite dif-
ferent. The difference between the 1,2 cis and 1,2 trans D2
elimination is especially interesting because all previous
studies at different wavelength excitations show no differ-
ences between these two microchannels. The HD elimina-
tions from different sites also show significant differences in
their translational energy distributions. For the H2 elimina-
tion processes, however, the differences between the 1,2 cis
and trans H2 eliminations have diminished. It is clear that the
four center molecular elimination requires a 1,2 hydrogen
atom migration. The transition state of the 1,2 elimination is
believed to be the so-called ethylidene radical
(:CHCH3).13–17 Recent theoretical studies on the ethylene
photodissociation show that the structures of the transition
state of the 1,2 trans and the 1,2 cis elimination processes are
slightly different.17 This will certainly cause some differ-
ences between the dynamics of the 1,2 cis and trans elimi-
nation processes. The main difference shown in the dynam-
ics of 1,2 cis and trans eliminations cannot, however, be
solely explained by the differences in the transition states.17
Other dynamical processes such as 1,2 concerted molecular
hydrogen elimination, which does not require 1,2 hydrogen
atom shift, might be responsible for the main difference be-
tween the cis and trans molecular hydrogen eliminations. It
is certainly conceivable that the concerted molecular hydro-
gen eliminations from cis and trans configurations might be
quite different because the distances between the two hydro-
gen atoms are significantly different in the two type elimina-
tions in which tunneling effects might be important.
This work is supported by the National Research Coun-
cil and Academia Sinica of the Republic of China.
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The isotope effect on the translational energy distribu-
tion is also quite interesting. For the three center elimination
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