JOURNAL OF CHEMICAL PHYSICS
VOLUME 114, NUMBER 17
1 MAY 2001
Three-center versus four-center elimination in photolysis of vinyl fluoride
and vinyl bromide at 193 nm: Bimodal rotational distribution of HF
and HBr „ v Ï5… detected with time-resolved Fourier transform spectroscopy
Shiaw-Ruey Lin, Shih-Che Lin, Yu-Chang Lee, Yung-Ching Chou,
I-Chia Chen, and Yuan-Pern Leea)
Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu,
Taiwan 30013, Republic of China
共Received 25 September 2000; accepted 1 December 2000兲
Following photodissociation of vinyl fluoride (CH2 CHF) and vinyl bromide (CH2 CHBr) at 193
nm, fully resolved vibration–rotational emission spectra of HF and HBr in spectral regions 3050–
4900 and 2000–2900 cm⫺1 , respectively, are temporally resolved with a step–scan Fourier
transform spectrometer. With a data acquisition window 0–5 ␮s suitable for spectra with
satisfactory ratio of signal-to-noise, emission from HX 共with X ⫽ F or Br兲 up to v ⫽6 is observed.
All vibrational levels show bimodal rotational distributions. For CH2 CHF, these two components of
HF have average rotational energies ⬃2 and 23 kJ mol⫺1 and vibrational energies ⬃83 and 78
kJ mol⫺1 , respectively; the values are corrected for small quenching effects. For CH2 CHBr, these
two components of HBr correspond to average rotational energies ⬃4 and 40 kJ mol⫺1 ,
respectively, and similar vibrational energies ⬃68 kJ mol⫺1 . The separate statistical ensemble
共SSE兲 model is suitable for three-center 共␣, ␣兲 elimination of HX because of the loose transition
state and a small exit barrier for this channel; predicted vibrational energy distributions of HX are
consistent with those observed for the high-J component. An impulse model taking into account
geometries and displacement vectors of transition states during bond breaking predicts substantial
rotational excitation for three-center elimination of HX but little rotational excitation for four-center
共␣, ␤兲 elimination; observed rotational energies of low-J and high-J components are consistent with
those predicted for four-center and three-center elimination channels, respectively. The model also
explains why observed rotational energy of HF produced via three-center elimination of CH2 CHF
is smaller than that of HCl from CH2 CHCl. Ratios of rate coefficients 共0.66:0.34 and 0.88:0.12兲
predicted for three-center or four-center elimination channels based on Rice–Ramsberger–Kassel–
Marcus theory are consistent with estimated branching ratios ⬃0.75:⬃0.25 and ⬃0.81:0.19
determined based on counting vibrational distribution of HF and HBr, respectively, to v ⭐5 for
high-J and low-J components and considering possible quenching effects within 5 ␮s. Hence we
conclude that, similar to photolysis of CH2 CHCl, observed high-J and low-J components
correspond to HX ( v ,J) produced from three-center and four-center elimination channels,
respectively. The results are compared with those from photolysis of vinyl chloride at 193 nm.
© 2001 American Institute of Physics. 关DOI: 10.1063/1.1343079兴
I. INTRODUCTION
ceed on the ground electronic surface; these include elimination of H, H2 , HCl, and Cl 共translationally cold兲. Two secondary channels eliminating H after elimination of Cl, and
Cl after elimination of H from CH2 CHCl are also identified.
Recently we employed a step–scan Fourier transform spectrometer to record fully resolved vibration–rotational emission spectra of HCl during photolysis of CH2 CHCl at 193
nm.2 All vibrational levels 共up to v ⫽6兲 of HCl show bimodal
rotational distributions with low-J components corresponding to HCl ( v , J兲 produced from four-center 共␣, ␤兲 elimination and high-J components corresponding to HCl from
three-center 共␣, ␣兲 elimination, respectively. The branching
ratio of 0.84:0.16 determined for three-center or four-center
elimination is consistent with rate coefficients for unimolecular decomposition predicted for these two channels based on
RRKM 共Rice–Ramsberger–Kassel–Marcus兲 theory.
Trobridge and Jennings3 performed Hg photosensitized
Although photolysis of vinyl halides (CH2 CHX, X⫽F,
Cl, or Br兲 was investigated by numerous researchers, a detailed mechanism of its dissociation is still uncertain partly
because its dissociation proceeds via multiple channels.
Among these halides, vinyl chloride (CH2 CHCl兲 was studied
most extensively. The most detailed investigation is reported
by Blank et al. 1 who employed photofragment translational
spectroscopy 共PTS兲 using VUV synchrotron radiation to ionize products and observed five primary dissociation channels
following excitation of CH2 CHCl at 193 nm. Most Cl atoms
are translationally hot; they originate from dissociation via
an electronically excited state. The remaining channels proa兲
Author to whom correspondence should be addressed. Also at the Institute
of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan.
Electronic mail: [email protected]
0021-9606/2001/114(17)/7396/11/$18.00
7396
© 2001 American Institute of Physics
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J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
decomposition of CH2 CHF and reported that HF and C2 H2
are the sole products. Kirk and Tschuikow-Roux4 identified
reaction products C2 H2 , C2 HF, and 1,1-C2 H2 F2 from photolysis of CH2 CHF at 147 nm. Based on the pressure dependence of the relative yield of 1,1-C2 H2 F2 with respect to
C2 H2 , they proposed that secondary reactions of F atoms
with excited C2 H2 F take place. Berry and Pimentel5 observed laser emission of HF and HCl following UV photolysis of CH2 CHF and CH2 CHCl. Further studies by Sirkin and
Pimentel6 resulted in observation of 33 laser transitions of
HF ranging from J ⬙ ⫽8 – 31 and v ⬙ ⫽0 – 5. They concluded
that HF was mainly produced via four-center elimination
based on a similarity in the rotational pattern of laser emission resulting from photolysis of CH2 CHF and CH2 CF2 . 7
They also collected and analyzed products of flash photolysis
of CH2 CHF 共␭⬎155 nm兲 with gas chromatography.8 Primary branching ratios of 0.82, 0.13, and 0.05 were determined for HF, F, and H2 elimination channels. Caballero and
Wittig9 determined reaction products from infrared multiphoton dissociation 共IRMPD兲 of CH2 CDF with gas chromatography and concluded that the branching ratio for threecenter to four-center elimination is 3:7. Sato et al.10
employed PTS to probe reaction dynamics upon photolysis
of CH2 CHF at 157 nm. They reported elimination channels
producing HF, H, and H2 and proposed that HF is mainly
from four-center elimination because it possesses substantial
共⬃170 kJ mol⫺1 ) translational energy. Three measurements
of vibrational distribution of HF upon photolysis of CH2 CHF
using various methods are reported. Quick and Wittig11 employed a low-resolution monochromator to resolve HF emission produced upon IRMPD of CH2 CHF and determined
relative vibrational population for v ⫽1 – 4 to be
1.00:0.55:0.15:⬍0.05. Watanabe et al.12 carried out Hg sensitized decomposition of CH2 CHF and observed IR emission
of HF by a modulation technique to determine population
ratios 1.00:0.56:0.42:0.24 for v ⫽1 – 4. Donaldson et al.13
investigated the energy distribution of HF produced from the
reaction F⫹C2 H3 with a modified arrested relaxation infrared chemiluminescence technique in which vinyl radicals are
prepared from reaction of F with C2 H4 . They reported
relative population of HF( v ) for v ⫽3 – 7 as
0.44:0.25:0.17:0.10:0.04; HF emission from reaction of F
with C2 H4 interfered with values for v ⫽1 and 2.
Previous kinetic studies14–16 yielded an activation energy
in the range 280–341 kJ mol⫺1 . Several theoretical
calculations10,17,18 predict geometries and energies of transition states of three-center and four-center HF-elimination
channels of CH2 CHF; the two channels are predicted to have
similar barriers in the range 335–360 kJ mol⫺1 . Kato and
Morokuma17 also calculated rate coefficients for unimolecular decomposition by RRKM theory including quantummechanical tunneling effects; they predicted that three-center
elimination is favored by a factor ⬃1.3 over four-center
elimination. Palma et al.19 investigated the role of H migration in dissociation of CH2 CHF and found a barrier 共⬃293
kJ mol⫺1 ) slightly smaller than that for HF elimination.
They also pointed out that, as CHCH2 F is not a true local
minimum, formation of CH3 CF is more facile than CHCH2 F
with respect to HF elimination via H migration.
Photolysis of vinyl fluoride and vinyl bromide
7397
Experimental investigations on dissociation of
CH2 CHBr are sparse. Johnson and Price20 photolyzed
CH2 CHBr with a Xe flash lamp and reported that HBr formation is the major channel. However, Wodtke et al.21 employed PTS to measure distributions of translational energy
and anisotropy parameters of photodissociation products Br
and HBr from CH2 CHBr at 193 nm, and reported a branching ratio of 1.28⫾0.05 between these two channels. Photofragment ion imaging was applied by Katayanagi et al.22 in
photolysis of CH2 CHBr in a molecular beam to determine
production of Br in both spin–orbit ( 2 P 1/2 and 2 P 3/2) states
with a branching ratio of 关Br*兴/关Br兴 of 0.06 ⫾ 0.03. Saito
et al.23 employed a shock tube to investigate thermal decomposition of CH2 CHBr at 1300–2000 K; they derived a barrier height ⬃ 230 kJ mol⫺1 . This value is much smaller than
theoretically predicted barriers of 308 and 340 kJ mol⫺1 for
three-center and four-center HBr-elimination channels,
respectively.24 Experiments on photolysis of HBr and C2 H2
共Ref. 25兲 and of CH2 CHBr 共Ref. 26兲 in a Kr matrix lead to
extensive theoretical investigations on decomposition of
CH2 CHBr. 24,27–29 Abrash et al.24 used a classical trajectory
method on an analytic global potential-energy surface to predict that, at thermal energies, the only brominated decomposition product is HBr. As excitation energy is increased to
619 kJ mol⫺1 共corresponding to excitation at 193 nm兲, reaction channels with elimination of H2 共48%兲, HBr 共44%兲, Br
共5%兲, and H 共3%兲 occur. Abrash et al.24 also predicted that
elimination of H2 or HBr occur almost exclusively via a
three-center mechanism.
In summary, current experimental results indicate that,
upon photodissociation of CH2 CHF, HF is formed via either
exclusively four-center elimination or both three-center and
four-center channels in a ratio of 3:7, even though theoretical
calculations predict a preference for three-center elimination.
No rotational energy distribution of HF is reported; three
reports on vibrational distribution of HF are inconsistent. No
experimental result on the internal energy distribution of
HBr and the branching between three-center and four-center
HBr-elimination paths of CH2 CHBr is available, but theoretical calculations predict a nearly exclusive three-center
elimination channel.
We have demonstrated that step–scan time-resolved
Fourier transform spectroscopy 共TR-FTS兲 provides much
improved resolution and sensitivity over previous IR emission techniques30,31 and reported vibration–rotational emission from HCl (1⭐ v ⭐7, J⭐32) during photolysis of
CH2 CHCl at 193 nm.2 Observation of bimodal rotational distributions of HCl in all vibrational states yields enhanced
understanding of three-center and four-center HCl elimination channels, as described previously. Here we report our
continuing effort to investigate HX elimination channels for
CH2 CHF and CH2 CHBr.
II. EXPERIMENT
The apparatus employed to obtain step–scan timeresolved Fourier transform spectra resembles that described
previously,2,30,31 only variation from Ref. 2 is described. A
telescope served to mildly focus the photolysis beam from an
ArF laser 共193 nm兲 to ⬃10 and 20 mm2 at the reaction center
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7398
Lin et al.
J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
FIG. 1. Infrared emission spectra of HF in spectral region 4410–3100 cm⫺1 recorded 0–5 ␮s after photolysis of CH2 CHF 共180 mTorr兲 in Ar 共270 mTorr兲.
Spectral resolution is 2.0 cm⫺1 ; 100 laser pulses were averaged at each scan step. Four spectra recorded under similar conditions were averaged. Assignments
are shown in stick diagrams.
with fluences ⬃100 and 40 mJ cm⫺2 for experiments with
CH2 CHF and CH2 CHBr, respectively. Filters passing either
3050–4900 cm⫺1 共Corion, RL-2000F and Spectrogon,
SP3056.6, for detection of HF兲 or 2000–2900 cm⫺1 共OCLI,
W04212-4, for detection of HBr兲 were placed in the sample
compartment of the spectrometer. The detected transient signal was amplified with a gain of (5 – 20)⫻106 V/A 共effective
bandwidth 1MHz兲 before being sent to an internal A/D converter 共16 bit, 200 kHz兲 of the spectrometer. Typically 300
time-slices were acquired at 5 ␮s intervals and the signal was
averaged for 60–100 laser pulses at each scan step; 2517 or
4498 scan steps were performed to yield a spectrum with
resolution 2.0 or 0.4 cm⫺1 in the spectral range for detection
of HF and HBr, respectively.
The partial pressure of CH2 CHF 共or CH2 CHBr) was
kept in a range 180–190 共or 135–190兲 mTorr. Ar 共270–320
mTorr, 570 mTorr in one experiment for CH2 CHF) was
added near the entrance photolysis port to suppress formation of brown deposit on the quartz window. CH2 CHF
共Lancaster, ⬎98%兲 and CH2 CHBr 共Merck, ⬎99.5%兲 were
used without purification except for degassing; no impurity
was detected in IR spectra.
III. RESULTS
Although we employed an instrumental configuration capable of measurements at 1 ␮s resolution in our previous
study of photolysis of CH2 CHCl,2 the signal-to-noise 共S/N兲
ratio achievable in this work with such a configuration was
unsatisfactory because emission from HF and HBr are much
weaker. Compared with CH2 CHCl, photolysis of CH2 CHF is
limited by its small absorption cross section at 193 nm,
whereas experiments on CH2 CHBr are limited by the small
Einstein coefficients of HBr. Hence satisfactory data acquisition was achieved only with an internal 16-bit A/D converter of the spectrometer with a temporal resolution of 5 ␮s.
Temporal evolution of emission of HF or HBr up to 1.5 ms
was recorded. In this study, we focus on only the first avail-
able acquisition window 共0–5 ␮s兲 to obtain information on a
nearly nascent vibration–rotational distribution of HX after
photolysis.
A. Photolysis of CH2 CHF
To improve the S/N ratio, we averaged four spectra recorded under identical conditions. Figure 1 is a partial emission spectrum of HF recorded 0–5 ␮s after photolysis of
CH2 CHF 共0.180 Torr兲 and Ar 共0.270 Torr兲 with a resolution
of 2.0 cm⫺1 . Partial assignments, based on spectral parameters reported by Sengupta et al.32 and Ram et al.,33 are
shown as stick diagrams. The spectrum clearly shows emission from high vibration–rotational levels of HF with J ⬘ up
to 17 and v ⬘ up to 6. Each vibration–rotational line in the
R-branch was normalized with the instrument response function, integrated, and divided by its respective Einstein
coefficient34 to yield relative population P v (J). For partially
overlapped lines, deconvolution was applied before integration. Semilogarithmic plots of P v (J)/(2J⫹1) of HF for
v ⫽1–5 produced from CH2 CHF are shown in Fig. 2, in
which v and J indicate vibrational and rotational levels of the
emitting state. Bimodal rotational distributions observed for
all vibrational levels are fitted with biexponential functions
to yield two rotational temperatures, as listed in Table I. We
denote these two components as high-J and low-J components.
B. Photolysis of CH2 CHBr
For improved S/N ratio, we averaged four spectra recorded under nearly identical conditions. Figure 3 shows a
partial emission spectrum of HBr recorded 0–5 ␮s after photolysis of CH2 CHBr with a resolution of 0.4 cm⫺1 . Partial
assignments shown in Fig. 3 are based on spectral parameters reported by Coxon and Hajigeorgious.35 The spectrum
exhibits emission from HBr with J ⬘ up to 32 and v ⬘ up to 6;
R bandheads for v ⭐3 are clearly visible. Each vibration–
rotational line in the R-branch was analyzed similarly to
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J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
Photolysis of vinyl fluoride and vinyl bromide
7399
IV. DISCUSSION
A. Effects of quenching in photolysis of CH2 CHCl
Our previous investigation on photolysis of CH2 CHCl
共Ref. 2兲 showed that rotational quenching of HCl ( v , J兲 is
nonnegligible under our experimental conditions when the
data acquisition window was set at 0–5 ␮s, especially for
v ⬎4. However, the weak signal in this work prevents us
from recording satisfactory data at periods less than 5 ␮s.
Nevertheless, we can estimate errors introduced due to
quenching by comparison of results obtained from 0–1 ␮s
共nearly collisionless兲 and 0–5 ␮s windows after photolysis
of CH2 CHCl at 193 nm. A detailed discussion may be found
in the Electronic Physics Auxiliary Publication Service
共EPAPS兲 of Ref. 2.37 In summary, rotational energy of the
low-J components are nearly unaffected by quenching during
the 0–5 ␮s acquisition period, whereas observed vibrational
energy decreases by ⬃4%. Observed rotational energy and
vibrational energy of the high-J components decrease by
about 20% and 11%, respectively. As a consequence, observed branching ratio for three-center to four-center HCl
elimination varies from 81:19 to 74:26 when the acquisition
window is increased to 0–5 ␮s.
B. HF„ v , J … from photolysis of CH2 CHF at 193 nm
FIG. 2. Semilogarithmic plots of relative rotational populations of HF ( v
⫽1 – 5, symbol •兲 averaged 0–5 ␮s after photolysis of CH2 CHF 共180
mTorr兲 in Ar 共270 mTorr兲 at 193 nm. Data reported by Donaldson et al.
共Ref. 13; symbol ⌬, for v ⫽3 – 6) are also shown. Solid lines represent
least-squares fits of low-J and high-J components.
yield relative population P v (J); Einstein coefficients reported by Malins and Setser were used.36 Semilogarithmic
plots of P v (J)/(2J⫹1) of HBr for v ⫽1 – 5 produced from
CH2 CHBr are shown in Fig. 4. Bimodal rotational distributions observed for all vibrational levels are fitted with biexponential functions to yield two rotational temperatures, as
listed in Table II.
1. Bimodal rotational distribution
According to Fig. 2 and Table I, photolysis of CH2 CHF
produces HF in vibrational states v ⭐5 with bimodal rotational distribution, similar to CH2 CHCl. The rotational temperatures of the high-J and low-J components of HF with
v ⭐4 are ⬃2500⫾350 K and 150⫾20 K, respectively;
the uncertainties only reflect deviations among various vibrational levels. Although Donaldson et al.13 showed stick diagrams for populations of HF( v ,J) with v ⭓3 on reacting F
with C2 H3 共with flow rates F CH2CHF⫽2 and F CF4⫽0.5
␮mol s⫺1 , pressure unspecified兲 using arrested relaxation,
rotational distributions were not analyzed. Their results are
compared with ours in Fig. 2 共symbol ⌬兲; they are nearly
identical to our data for v ⫽ 3 and 4. Our data for v ⫽5 show
TABLE I. Experimental conditions, fitted rotational temperature, and total rotational population of HF( v ) after photolysis of CH2CHF at 193 nm.
Expt.
no
P CH2CHF
/mTorr
P Ar
/mTorr
t/␮s
/␮s
1
180
270
0–5
2
190
570
0–5
Low-J component ./mTorrv
High-J component
v
1
2
3
4
5
1
2
3
4
5
T rot/K
兺 J P v (J)
2800⫾1000
2600⫾700
1900⫾800
2200⫾1400
1000⫾800
2900⫾1200
2600⫾1000
2100⫾800
2800⫾1400
1700⫾1400
590 共545兲
545共510兲
390共370兲
205共180兲
60共60兲
520共480兲
410共390兲
310共280兲
200共160兲
45共40兲
b
a
c
E rot/kJ
23.1 共18.4兲
20.1共16.6兲
14.6共12.5兲
14.4共10.8兲
4.4共4.4兲
23.2共18.6兲
20.3共17.0兲
15.6共11.8兲
16.6共11.7兲
5.3共4.2兲
b
c
T rot/K
兺 J P v (J) a
E rot/kJ
160⫾60
100⫾70
140⫾80
130⫾70
100⫾80
150⫾80
150⫾120
160⫾70
180⫾110
100⫾80
315
290
200
105
30
230
240
185
90
35
1.8
1.3
0.9
1.0
0.5
2.1
1.7
1.0
1.1
0.5
P v (J)⫽共relative integrated emittance兲/关共instrumental response factor兲 共Einstein coefficient兲兴.
Fitted values; extrapolated populations up to J max⫽28 are included. See text.
c
Summed values are listed in parentheses; only observed levels are summed. See text.
a
b
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7400
J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
Lin et al.
FIG. 3. Infrared emission spectra of HBr in spectral region 2820–2150 cm⫺1 recorded 0–5 ␮s after photolysis of CH2 CHBr 共190 mTorr兲 in Ar 共320 mTorr兲.
Spectral resolution is 0.4 cm⫺1 ; 100 laser pulses were averaged at each scan step. Four spectra recorded under similar conditions were averaged. Assignments
are shown in stick diagrams.
significant quenching. Their observation of both low-J and
high-J components of HF in their experiments under
relaxation-free conditions in a flowing system supports our
conclusion that observed low-J components are nascent,
rather than from rotational quenching.
FIG. 4. Semilogarithmic plots of relative rotational populations of HBr ( v
⫽1 – 5, symbol ␱兲 averaged 0–5 ␮s after photolysis of CH2 CHBr 共190
mTorr兲 in Ar 共320 mTorr兲 at 193 nm. Solid lines represent least-squares fits
of low-J and high-J components.
Lines associated with high-J in a state with large v are
more difficult to detect because of limited sensitivity. We
assume a Boltzman distribution for the high-J component
and associate an extrapolated population with unobserved
lines up to J max(v) for each vibrational level. J max(v) are 28,
25, 21, 15, and 7 for v ⫽1 – 5, corresponding to a vibration–
rotational energy ⬃19 100 cm⫺1 for the highest observed
level of HF in v ⫽5. We refer to populations derived from
observed levels as ‘‘summed’’ values, and those from observed and extrapolated values as ‘‘fitted’’ values. Fitted and
summed populations of the high-J component in each vibrational state of HF are listed in column ⌺ J P v (J) of Table I,
with the latter in parentheses. Fitted and summed rotational
populations for the low-J component are nearly identical;
hence, only fitted values are listed in Table I. Rotational
energies of high-J components obtained on summing a product of level energy and normalized population for each rotational level including extrapolated values for J⭐J max are
listed in columns E rot of Table I 共referred to as ‘‘fitted’’
values兲; corresponding values calculated on considering only
observed vibration–rotational lines are listed parenthetically
共referred to as ‘‘summed’’ values兲. Averaged rotational energies for v ⫽1 – 5 are derived on multiplying E rot( v ) in
Table I by its corresponding population; values for the high-J
component of HF are 19⫾1 and 15⫾1 kJ mol⫺1 for fitted
and summed values, respectively; the error limits represent
only deviations between two experiments. Assuming that
quenching effects are similar to those in our previous experiments on CH2 CHCl, we estimate a nascent rotational energy
of 23⫾5 kJ mol⫺1 for the high-J component of HF; the uncertainty covers the observed value before correction. The
average rotational energy of the low-J component is 1.4
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J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
Photolysis of vinyl fluoride and vinyl bromide
7401
TABLE II. Comparison of relative vibrational populations of high-J and low-J components of HF and HBr from photolysis of CH2CHF and CH2CHBr,
respectively, at 193 nm with statistical calculations for three-center elimination.
Species
HF
HBr
Component
Expt. no.
v ⫽0 a
high-J
1–2
0.40⫾0.06
low-J
high-J
PST 共three-center兲
SSE 共three-center兲
1–2
3–4
0.25⫾0.10
0.37⫾0.06
low-J
PST 共three-center兲
SSE 共three-center兲
3–4
0.35⫾0.10
v ⫽1
b
0.340⫾0.010
共0.341⫾0.014兲c
0.470
0.418
0.318⫾0.021b
0.309⫾0.006b
共0.297⫾0.011兲c
0.361
0.326
0.324⫾0.038b
v ⫽2
v ⫽3
v ⫽4
v ⫽5
0.291⫾0.013
共0.297⫾0.009兲
0.262
0.257
0.306⫾0.001
0.274⫾0.003
共0.286⫾0.005兲
0.247
0.238
0.224⫾0.012
0.214⫾0.005
共0.215⫾0.007兲
0.140
0.153
0.225⫾0.012
0.226⫾0.004
共0.225⫾0.005兲
0.166
0.171
0.231⫾0.044
0.124⫾0.010
共0.114⫾0.007兲
0.071
0.087
0.113⫾0.004
0.121⫾0.002
共0.119⫾0.005兲
0.110
0.122
0.159⫾0.021
0.032⫾0.002
共0.033⫾0.005兲
0.034
0.048
0.040⫾0.006
0.071⫾0.003
共0.075⫾0.003兲
0.071
0.085
0.063⫾0.004
a
Estimated value; see text.
Derived from fitted values listed in Table I; uncertainties represent only deviations between two experiments.
c
Derived from summed values listed in Table I and listed in parentheses; uncertainties represent only deviations between two experiments.
b
⫾0.2kJ mol⫺1 . Considering errors propagated from fitting
the high-J components, we estimate that the rotational energy of the low-J component is 1–4 kJ mol⫺1 .
2. Vibrational distribution of HF
Values of ⌺ J P v (J) associated with each vibrational state
were normalized to yield a relative vibrational population
共for both ‘‘summed’’ and ‘‘fitted’’ values兲, as listed in Table
II. Distributions derived from fitted values are used for discussion, unless noted. The vibrational distribution of the
high-J component is compared with those of previous reports
and statistical predictions in Fig. 5A. Our observed distribution agrees well with those reported by Watanabe et al.12
共symbol 䊊, for v ⭐4) and Donaldson et al.13 共symbol ⵜ, for
v ⭓3) except for v ⫽5 in which quenching is significant in
our experiments. A vibrational distribution reported by
Quick and Wittig11 appears to correspond to a much smaller
temperature, presumably due to their use of a 100 kHz RC
filter and possible vibrational quenching in their system.
Based on observed vibrational distribution for v ⫽1 – 3 of the
high-J component, we estimate by linear extrapolation the
relative population of v ⫽0 of HF to be 0.40⫾0.06 in Fig.
5. Because the population in Fig. 5 was normalized for HF
( v ⫽1–5兲, this value indicates that about 29% 共⫽0.4/1.4兲 of
the total population of HF is in its v ⫽0 state.
The vibrational population of the low-J component
关shown in Fig. 5共B兲兴 appears to be nearly inverted at v ⫽1.
The population of v ⫽0 for the low-J component is
estimated by fitting Fig. 5 with a smooth curve; relatively
large uncertainties are associated with this estimate,
0.25⫾0.10.
Averaged vibrational energies for both components are
thus derived to be 70⫾3 kJ mol⫺1 共high-J兲 and 80⫾6
kJ mol⫺1 共low-J兲. Assuming similar quenching effects as in
photolysis of CH2 CHCl under our experimental conditions,
we estimate Evib to be 78⫾11 共high-J兲 and 83⫾9 kJ mol⫺1
共low-J兲; the uncertainties cover observed values before correction.
C. HBr„v, J… from photolysis of CH2 CHBr at 193 nm
1. Bimodal rotational distribution
FIG. 5. Comparison of relative vibrational distributions of HF for both
high-J and low-J components after photolysis of CH2 CHF 共180 mTorr兲 in
Ar 共270 mTorr兲 at 193 nm. 䉱: this work 共from ‘‘fitted population’’兲; ⵜ:
Donaldson et al. 共Ref. 13兲; 䊊: Watanabe et al. 共Ref. 12兲; 䊐: Quick and
Wittig 共Ref. 11兲; solid lines: SSE calculation; dotted lines: PST calculations.
Similar to CH2 CHF and CH2 CHCl, photolysis of
CH2 CHBr produces bimodal rotational distributions of HBr
in vibrational states v ⭐5 共Fig. 4兲. The rotational temperatures of the high-J and low-J components of HBr with
v ⭐3 are ⬃6500⫾900 K and 330⫾50 K 共Table III兲, respectively; the uncertainties only reflect deviations among
various vibrational levels.
Similarly, the population is extrapolated to J max(v) for
each vibrational level. J max(v) are 38, 34, 30, 24, and 18 for
v ⫽1 – 5, corresponding to a vibration–rotational energy of
⬃15 600 cm⫺1 for the highest observed level in v ⫽5. Fitted
and summed populations of the high-J component in each
vibrational state of HBr are listed in column ⌺ J P v (J) of
Table III; only fitted values are listed for the low- J com-
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7402
Lin et al.
J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
TABLE III. Experimental conditions, fitted rotational temperature and total rotational population of HBr( v ) after photolysis of CH2CHBr at 193 nm.
Expt.
no
P CH2CHBr
/mTorr
High-J component
P Ar
/mTorr
t
␮s
3
135
300
0–5
4
190
320
0–5
T rot/K
兺 J P v (J) a
6500⫾1400
6900⫾1000
5800⫾1800
4500⫾1600
3400⫾1400
8200⫾1300
5500⫾1200
6400⫾1600
3700⫾1400
3600⫾1200
4340 共3905兲
3745共3575兲
3070共2790兲
1640共1450兲
1020共970兲
3920共3435兲
3570共3490兲
2960共2750兲
1590共1475兲
880共880兲
v
1
2
3
4
5
1
2
3
4
5
b
Low-J component.
E rot/kJ
c
40.1 共33.1兲
35.3共32.8兲
27.6共23.2兲
20.8共15.7兲
11.3共9.1兲
50.0共39.6兲
33.4共31.8兲
30.9共27.5兲
20.0共17.3兲
12.5共12.5兲
b
c
T rot/K
兺 J P v (J) a
E rot/kJ
270⫾180
400⫾150
320⫾170
420⫾130
270⫾160
260⫾140
310⫾160
410⫾120
370⫾130
220⫾140
1785
1045
920
885
290
1470
1210
1410
710
340
4.7
3.3
2.6
3.1
1.9
3.2
2.3
3.4
2.8
1.3
P v (J)⫽共relative integrated emittance兲/关共instrumental response factor兲 共Einstein coefficient兲兴.
Fitted values; extrapolated populations up to J max⫽38 are included. See text.
c
Summed values are listed in parentheses; only observed levels are summed. See text.
a
b
ponent in Table II. Averaged rotational energies for v
⫽1 – 5 for the high-J components of HBr are 33⫾2 and
28⫾2 kJ mol⫺1 for fitted and summed values, respectively.
Assuming similar quenching effects as in photolysis of
CH2 CHCl, we estimate a rotational energy of 40⫾9
kJ mol⫺1 for the high-J component. The average rotational
energy of the low-J component is 3.2⫾0.4 kJ mol⫺1 . Considering possible errors propagated from fitting the high-J
component, we report a rotational energy 3 – 6 kJ mol⫺1 for
the low-J component.
2. Vibrational distribution of HBr
We performed calculations to optimize structures of
transition states of three-center 共TS3兲 and four-center 共TS4兲
elimination of HF from CH2 CHF with MP2共Full兲/ 6-31G**
and B3LYP/aug-cc-pVTZ density functional theory38,39 using GAUSSIAN 98 programs.40 Geometries of transition states
TS3 and TS4 of CH2 CHF predicted with the B3LYP method
are shown in Fig. 7; structural parameters predicted with
MP2共full兲 are listed parenthetically for comparison. Predicted structures of TS3 and TS4 are similar to previous
reports.10,17 Displacement vectors corresponding to each
imaginary vibrational wave number are shown in the lower
part of Fig. 7; they are similar to those reported previously17
Vibrational distributions of high-J and low-J components are listed in Table II and shown in Fig. 6. To our
knowledge, no experimental data on the rotational or vibrational distribution of HBr are reported.
Based on observed vibrational distribution for v ⫽1 – 3,
we estimate by linear extrapolation the relative population of
v ⫽0 of HBr to be 0.37⫾0.06 and 0.35⫾0.10 in Fig. 6
for high-J and low-J components, respectively. The vibrational distribution of the low-J component has larger errors
and appears to be non-inverted, unlike for photolysis of
CH2 CHF and CH2 CHCl. Averaged vibrational energies thus
derived are similar for both components: 61⫾3 kJ mol⫺1
共high-J 兲 and 65⫾5 kJ mol⫺1 共low-J兲. Considering possible
quenching effects under our experimental conditions, we estimate E vib to be 68⫾10 共high-J兲 and 68⫾8 kJ mol⫺1
共low-J兲; the uncertainties cover observed values before correction.
D. Transition states of CH2 CHF and CH2 CHBr
The ultraviolet absorption of vinyl halides CH2 CHX is
characterized by an intense band near 190 nm dominated by
a ␲ *← ␲ valence transition. Several nearby repulsive surfaces, Ã 1 A ⬙ ( ␲␴ *), b̃ 3 A ⬙ ( ␲␴ *), and c̃ 3 A⬘( ␲␴ *) in the case
of vinyl bromide,27 lead to rapid C – X bond fission either by
surface crossing or direct absorption. On the other hand, relaxation to the ground electronic state leads to elimination of
H, X, H2 , and HX.1,24 Hence, we only investigate the ground
electronic surface for HX elimination channels.
FIG. 6. Relative vibrational distributions of HBr for both high-J and low-J
components after photolysis of CH2 CHBr 共190 mTorr兲 in Ar 共320 mTorr兲 at
193 nm. 䉱: this work 共from ‘‘fitted population’’兲; solid lines: SSE calculation; dotted lines: PST calculations.
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J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
Photolysis of vinyl fluoride and vinyl bromide
7403
FIG. 8. Comparison of energies 共in kJ mol⫺1 ) of transition states 共TS3 and
TS4兲 and dissociation products relative to CH2 CHX for X⫽F 共italicized兲, Cl
共bold兲, and Br 共underlined兲. For X⫽F and Cl, values using QCISD共T兲/6-311
⫹G共d,p兲 at MP2共full兲/6-31G** optimized geometries are used with zeropoint energy corrected; for X⫽Br, values are taken from Ref. 24 共MP4兲.
FIG. 7. Geometries of transition states for three-center elimination 共TS3兲
and four-center elimination 共TS4兲 of HF from CH2 CHF predicted with the
B3LYP/aug-cc-pVTZ method. Parameters predicted with the MP2 共Full兲/631G** method are also listed in parentheses for comparison. Bond lengths
are in Å. Displacement vectors corresponding to imaginary wave numbers
are shown in the lower part.
except that the angle between the motion of H with respect to
the H–F bond decreases from ⬃16° to 12° for TS3 in our
calculations. Predicted vibrational wave numbers for TS3
and TS4 of CH2 CHF are listed in Table IV. Abrash et al.24
reported geometries and energies of TS3 and TS4 of
CH2 CHBr using HF and MP4 method, but we found that
TS3 of CH2 CHBr is difficult to characterize accurately with
the B3LYP method.
Kato and Morokuma17 used singles and doubles configuration interaction 关共S⫹D兲 CI⫹QC兴 with a 6-31G** basis set
to predict barriers 334 and 339 kJ mol⫺1 for TS3 and TS4 of
CH2 CHF, respectively; the zero-point energies were corrected. Sato et al.10 employed MP2/6-31G** method to predict barriers of 357 and 344 kJ mol⫺1 for three-center and
four-center elimination of HF. We performed similar calculations using a QCISD共T兲/6-311⫹G** method with geometry optimized at MP2共full兲/6-31G** and obtained nearly
identical barriers 310 kJ mol⫺1 for both channels. Calculations using the B3LYP/aug-cc-pVTZ method yield barriers
of 295 and 293 kJ mol⫺1 for three-center and four-center
elimination, respectively. We concluded that barrier height
for these two channels are similar.
TABLE IV. Vibrational wave numbers (cm⫺1) of transition states TS3 and
TS4 for CH2CHF predicted with the B3LYP/aug-cc-pVTZ method.
TS3
CH2CHF
3216
1281
519
1160i
3131
897
464
2226
854
230
TS4
1661
824
3395
973
702
1807i
3323
947
523
1828
739
500
1710
736
Relative energies and barriers of three-center and fourcenter HX elimination channels for elimination of HX from
CH2 CHX 共X⫽F, Cl, and Br兲 are compared in Fig. 8; values
predicted with the QCISD共T兲 method are used for X⫽F so as
to compare with values reported previously for
CH2 CHCl.2,41 Energies for X⫽Br are taken from Ref. 24;
transition states were calculated with the MP4 method. Potential energy differences for HX elimination channels of
CH2 CHX are similar for X⫽F, Cl, and Br, with variations in
energy less than 25 kJ mol⫺1 in all cases.
E. Statistical model calculations
Similar to CH2 CHCl,2 we performed statistical theories
such as phase space theory 共PST兲42,43 and separate statistical
ensemble 共SSE兲44 to predict rotational and vibrational distribution of HX products from the three-center elimination
channel which is characterized with a loose transition state
and a small exit barrier. We employed vibrational wave
numbers of vinylidene predicted theoretically by Chang
et al.45 and energies 共corrected for zero-point energy兲 of precursor and products predicted from QCISD共T兲 calculations.
Vibrational wave numbers of 2000 cm⫺1 for HF and 1300
cm⫺1 for HBr were used in the simulation. These values are
taken as half the vibrational wave numbers of HF and HBr.
For comparison, predicted imaginary wave numbers are
1977i关MP2共full兲/6-31G**兴 and 1807i 共B3LYP/aug-ccpVTZ兲 for TS4 and 1191i 关MP2共full兲兴 and 1160i 共B3LYP兲
for TS3 of CH2 CHF, and, according to Ref. 24, 1144i 共TS4兲
and 921i 共TS3兲 for CH2 CHBr.
The SSE model predicts greater vibrational excitation of
products than PST. The vibrational distributions of HF predicted for three-center elimination with both models are
shown in Table II and Fig. 5共A兲; they correspond to average
vibrational energies 57 共PST兲 and 69 kJ mol⫺1 共SSE兲. The
latter value is slightly smaller than experimental observation
of 78⫾11 kJ mol⫺1 , whereas predicted rotational energy
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7404
Lin et al.
J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
TABLE V. Comparison of average internal energies of high-J and low-J components of HX 共X⫽F, Cl, and Br兲
with statistical and impulse model calculations.
Species
Energy
/kJ mol⫺1
High-J
component
Low-J
component
CH2CHF
E rot
23⫾5d
共19⫾1兲e
78⫾11d
共70⫾3兲e
E vib
CH2CHClg
E rot
E vib
CH2CHBr
E rot
E vib
Four-center
eliminationb
Three-center
eliminationa
PST
SSE
Impulsec
Impulsec
1–4d
共1.4⫾0.2兲e
83⫾9d
共80⫾6兲e
38
37
14
共24兲f
9.4
57
69
47⫾2h
共37⫾2兲e
74⫾3h
共66⫾3兲e
3–7h
共3.8⫾0.3兲e
81⫾2h
共78⫾2兲e
37
36
52
2.8
55
66
40⫾9d
共33⫾2兲e
68⫾10d
共61⫾3兲e
3–6d
共3.2⫾0.4兲e
68⫾8d
共65⫾5兲e
36
35
60
71
a
Available energies are 354 (CH2CHF), 341 (CH2CHCl兲, and 337 (CH2CHBr), respectively.
Available energies are 528 (CH2CHF), 515 (CH2CHCl), and 512 (CH2CHBr), respectively.
c
Assuming that the impulse force is along the displacement vector corresponding to the imaginary mode of the
transition state; see text.
d
Corrected for possible quenching effects; see text.
e
Data obtained with a 0–5 ␮s detection window are listed parenthetically.
f
Using displacement vectors in Ref. 17.
g
From Ref. 2.
h
Data obtained with a 0–1 ␮s 共near collisionless兲 detection window.
b
⬃37 kJ mol⫺1 is greater than experimental observation of 23
⫾ 5 kJ mol⫺1 .
The vibrational distributions of HBr calculated for threecenter elimination with both models using ␯ HBr⫽1300
cm⫺1 , shown in Table III and Fig. 6共A兲, correspond to average vibrational energies 60 共PST兲 and 71 kJ mol⫺1 共SSE兲.
The latter is close to experimental observation of 68⫾10
kJ mol⫺1 for both components. However, predicted rotational energy ⬃35 kJ mol⫺1 is consistent with experimental
value of 40 ⫾ 9 kJ mol⫺1 for the high-J component.
Statistical calculations were also performed for fourcenter elimination channel for comparison, even though one
expects unsatisfactory results in view of the tight transition
state and a large exit barrier for this channel. Rotational energy ⬃50 kJ mol ⫺1 and vibrational energy ⬃102 kJ mol⫺1
for both HF and HBr were derived with the SSE model;
predicted vibrational distributions are also shown in Figs.
5共B兲 and 6共B兲.
Observed internal energies of HX 共X⫽F, Cl, and Br兲
from both high-J and low-J components are compared with
those according to PST and SSE calculations for three-center
elimination in Table V. Predicted rotational energies of HX
from three-center elimination match better with those observed for the high-J components. For X⫽Cl and Br, observed rotational energies are ⬃5–11 kJ mol⫺1 共12%–23%兲
greater than statistical prediction, whereas for X⫽F observed
rotational energy is ⬃14 kJ mol⫺1 smaller. Statistical models
fail to explain why rotational energy of HF is much smaller
than HCl or HBr from similar paths. For vibrational energies,
observed values are ⬃8 kJ mol⫺1 greater than those pre-
dicted for X⫽F and Cl, but is nearly identical to statistical
prediction for X⫽Br.
Rotational energies of low-J components are much
smaller than those predicted statistically for either elimination channel. This suggests that, similar to HCl from
CH2 CHCl, the low-J component may be associated with a
dynamically controlled dissociation process that typically associates with a tight transition state.
F. The impulse model
Because dissociation occurs with simultaneous breaking
of two bonds, regular impulsive model is not applicable for
channels involving TS3 and TS4. Hence, we consider motions of the reaction coordinates described by displacement
vectors associated with imaginary modes of transition states
TS3 and TS4 of CH2 CHF. The direction of the repulsive
force is assumed to be parallel to these displacement vectors.
Hence displacement vectors shown in Fig. 7 indicate substantially more rotational excitation of HF in three-center
elimination than in four-center elimination because in the
former case the H atom moves toward the F atom with a
larger impact parameter. If we distribute available energy
between H and C2 H2 according to classical mechanics and
calculate the dynamics after the energized H atom moves
toward the X atom along the displacement vector predicted
with theory, rotational energies are predicted according to
the equation
E rot⫽ 关 m Xm C / 共 m H⫹m X兲共 m H⫹m C兲兴 E avail sin2 ␣ ,
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J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
in which E avail is available energy and ␣ is the angle between
direction of motion of H and that of the H–X bond. With an
available energy of 354 kJ mol⫺1 and ␣ ⫽12.1°, rotational
energy of 13.4 kJ mol⫺1 is predicted for three-center elimination; with ␣⫽16° 共as predicted in Ref. 17兲, the energy
increases to 24 kJ mol⫺1 . The latter value is similar to the
experimental value of 23⫾5 kJ mol⫺1 . If an available energy
of 528 kJ mol⫺1 is used, rotational energies 20 and 34
kJ mol⫺1 for HF is predicted for ␣⫽12.1° and 16°, respectively; these values are maximal because all available energy
from isomerization of vinylidene is utilized. It should be
noted that predicted rotational energy is sensitive to ␣ which
might vary appreciably depending on methods of theoretical
calculations.
For the high-J component, observed rotational energy of
HF 共23⫾5 kJ mol⫺1 兲 is much smaller than that of HCl
共47⫾2 kJ mol⫺1 兲 even though the barrier and exothermicity
are similar for both reactions. This situation may be rationalized on comparison of displacement vectors of TS3 of
CH2 CHF and CH2 CHCl; with ␣⫽12°–16° in the former
case, the H atom moves toward F in TS3 of CH2 CHF with an
impact parameter smaller than that of the motion of the H
atom toward Cl in TS3 of CH2 CHCl with ␣⫽24°.2
In contrast, little rotational excitation is predicted for
four-center elimination according to associated displacement
vectors in Fig. 7 because the H atom is moving toward F
with a small impact parameter. The expectation is consistent
with results of calculations of the intrinsic reaction coordinate 共IRC兲 of CH2 CHF by Kato and Morokuma17 for fourcenter elimination in which HF moves away with a substantial decrease in bond length but little rotational motion. In
contrast, the IRC shows substantial rotation of HF for threecenter elimination. With a maximal available energy of 528
kJ mol⫺1 a rotational energy of 9.4 kJ mol⫺1 is predicted for
this channel, slightly greater than the experimental value of
1–4 kJ mol⫺1 .
Similarly, in the case of CH2 CHBr, three-center elimination is expected to produce HBr with substantial rotational
excitation whereas four-center elimination produces HBr
with little rotational excitation. However, TS3 for CH2 CHBr
cannot be characterized successfully with theory; hence the
rotational energy of HBr in this channel cannot be predicted.
The impulse model using displacement vectors of imaginary frequencies hence describes satisfactorily rotational distributions observed for high-J and low-J components of HX
as resulting from three-center and four-center elimination,
respectively. It also provides a possible explanation for the
reason why observed rotational energy of HF is smaller than
that of HCl in three-center elimination of CH2 CHX.
As a comparison, the statistical model fails to describe a
variation in rotational energies between HF and HCl produced from three-center elimination even though it describes
vibrational distributions of HX satisfactorily. In the case of
photolysis of CH2 CHCl, because observed translational energy of HCl is greater than that predicted statistically for
three-center elimination, a model considering that the exothermicity of isomerization of vinylidene to acetylene may
provide further excitation of HX was proposed
previously.1,46 If such a ‘‘kick’’ for HX from isomerization
Photolysis of vinyl fluoride and vinyl bromide
7405
of vinylidene to acetylene occurs, one expects the effect be
greater for X⫽F than for X⫽Br because of differences in
bond distances and masses. Our data, showing that observed
vibrational energies of HBr for the high-J component are
close to statistical prediction whereas those observed for HCl
and HF are ⬃8 kJ mol⫺1 greater than prediction, are consistent with such a picture.
G. Branching ratio and RRKM rates of dissociation
We estimate the rate of dissociation on the ground electronic surface of vinyl halides irradiated at 193 nm with a
microcannonical transition state theory. The Whitten–
Rabinovitch equations 47 were used to calculate the density
of states and number of transition states. Rates of dissociation for HF elimination depend on the accuracy of predicted
potential energy barrier and wave numbers of the transition
states. With identical energies 309 kJ mol⫺1 for transition
states TS3 and TS4, and vibrational wave numbers predicted
with B3LYP/aug-cc-pVTZ 共Table V兲, rates of dissociation
for three-center and four-center elimination are calculated to
be 7.8 and 4.0⫻1011 s⫺1 , respectively. Accordingly, the
branching ratio for formation of HF according to three- and
four-center processes is estimated to be 0.66:0.34.
Taking into account of estimated population at v ⫽0 as
described previously, we determine the branching ratio for
the high-J and low-J components of HF with 0⭐v ⭐5 to be
0.68⫾0.03:0.32⫾0.03. The actual branching ratio is estimated to be ⬃0.75:⬃0.25 if quenching effects similar to
those observed for HCl from CH2 CHCl 共as discussed in Sec.
IV A兲 are assumed. The estimated value is consistent with
that predicted for three-center and four-center elimination
channels with RRKM theory. The branching ratio is expected to vary only slightly if populations of higher vibrational states are extrapolated because the distributions of v
⬎5 for both channels are similar. Previous work on photolysis of CH2 CDF using IRMPD by Caballero and Wittig9
yielded a branching ratio of 0.3:0.7, inconsistent with our
results and RRKM calculations. A possibility remains that
migration of the H atom plays an important role for the ratio
of 关HF兴/关DF兴, as the barrier for this channel is predicted to be
⬃293 kJ mol⫺1 by Palma et al.19 Further experiments on
internal energy distributions of HF and DF from photolysis
of CH2 CDF and CD2 CHF are needed to understand the role
of H migration.
Rates of dissociation for three-center and four-center
HBr elimination with energies 318 and 284 kJ mol⫺1 共Ref.
24兲, respectively, at the transition state are calculated to be
1.8 and 0.23⫻1012 s⫺1 , respectively. Accordingly, the
branching ratio for formation of HBr following three- and
four-center processes is estimated to be 0.88:0.12.
The branching ratio for the high-J and low-J components
of HBr with 0⭐ v ⭐5 is determined to be 0.74⫾0.04:0.23
⫾0.04. Considering possible errors and vibrational quenching as discussed in Sec. A, the revised branching ratio of
⬃0.81:⬃0.19 is consistent with that predicted for threecenter and four-center elimination channels with RRKM
theory.
Based on discussions of Secs. IV D–IV G and similarities in internal energy distributions of HF 共HBr兲 to those of
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7406
HCl from photolysis of CH2CHCl, we conclude that observed high-J and low-J components correspond well with
HF 共HBr兲 produced via three-center and four-center elimination, respectively.
V. CONCLUSION
Rotationally resolved emission from HF and HBr up to
v ⫽6 is observed after photolysis of vinyl fluoride and vinyl
bromide, respectively, at 193 nm. Similar to the case of
CH2 CHCl, all vibrational levels show bimodal rotational distributions with low-J and high-J components corresponding
to HX ( v , J兲, X⫽F or Br, produced from four-center and
three-center elimination channels, respectively. The SSE statistical model predicts satisfactorily vibrational energy distribution of HX from three-center elimination, but does not
explain a rotational energy of HF much smaller than HCl or
HBr. An impulse model considering geometries and displacement vectors of transition states during bond breaking
predicts the rotational distributions of both channels satisfactorily and provides a possible explanation for rotational excitation of HF less than that of HCl and HBr produced from
three-center elimination. The branching ratios determined for
three-center:four-center elimination are consistent with predicted rate coefficients for RRKM unimolecular decomposition through these two channels.
ACKNOWLEDGMENTS
We thank the National Science Council of the Republic
of China 共Grant Nos. NSC89-2113-M-007-076 and NSC892119-M-007-003兲 for support and the National Center for
High-Performance Computing for computer time.
1
Lin et al.
J. Chem. Phys., Vol. 114, No. 17, 1 May 2001
D. A. Blank, W. Sun, A. G. Suits, and Y. T. Lee, J. Chem. Phys. 108,
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