Reaction dynamics of Cl+CH3SH: Rotational and vibrational distributions of HCl probed with time-resolved Fourier-transform spectroscopy

Article abstract of DOI:10.1063/1.1634558

Rotationally resolved infrared emission of HCl was detected with a step-scan Fourier-transform spectrometer in a laser-initiated reaction of Cl+CH₃SH. The average rotational energy and its fraction of partition derived, E_r=8.2±0.9 kJ mol^-1 and _r=0.10±0.01, were much greater than the previous results. The difference in vibrational excitation was consistent with a difference in predicted H-Cl bond distances in transition structures of these reactions. The time-resolved Fourier-transform spectra demonstrated improved temporal resolution and detection sensitivity in determining internal-state distributions of products from bimolecular reactions.

Full text of DOI:10.1063/1.1634558

Reaction dynamics of Cl + CH 3 SH : Rotational and vibrational distributions of HCl
probed with time-resolved Fourier-transform spectroscopy
Shin-Shin Cheng, Yu-Jong Wu, and Yuan-Pern Lee
Citation: The Journal of Chemical Physics 120, 1792 (2004); doi: 10.1063/1.1634558
View online: http://dx.doi.org/10.1063/1.1634558
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 4
22 JANUARY 2004
Reaction dynamics of Cl¿CH3SH: Rotational and vibrational distributions
of HCl probed with time-resolved Fourier-transform spectroscopy
Shin-Shin Cheng and Yu-Jong Wu
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Yuan-Pern Leea)
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
͑Received 16 September 2003; accepted 27 October 2003͒
Rotationally resolved infrared emission spectra of HCl( ϭ1–3) in the reaction of ClϩCH SH,
v
3
initiated with radiation from a laser at 308 nm, are detected with a step-scan Fourier-transform
spectrometer. Observed rotational temperature of HCl( ϭ1–3) decreases with duration of reaction
v
due to collisional quenching; a short extrapolation to time zero based on data in the range 0.25–4.25
␮s yields a nascent rotational temperature of 1150Ϯ80 K. The rotational energy averaged for
HCl( ϭ1–3) is 8.2Ϯ0.9 kJ molϪ1, yielding a fraction of available energy going into rotation of
v
HCl, f ϭ0.10Ϯ0.01, nearly identical to that of the reaction ClϩH S. Observed temporal profiles
͗ ͘
r
2
of the vibrational population of HCl( ϭ1–3) are fitted with a kinetic model of formation and
v
quenching of HCl( ϭ1–3) to yield a branching ratio ͑68Ϯ5͒:͑25Ϯ4͒:͑7Ϯ1͒ for formation of
v
HCl( ϭ1):( ϭ2):( ϭ3) from the title reaction and its thermal rate coefficient k2aϭ(2.9Ϯ0.7)
v
v
v
ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1. Considering possible estimates of the vibrational population of
HCl( ϭ0) based on various surprisal analyses, we report an average vibrational energy 36Ϯ6
v
kJ molϪ1 for HCl. The fraction of available energy going into vibration of HCl is
f ϭ0.45
͘
v
͗
Ϯ0.08, significantly greater than a value
f ϭ0.33Ϯ0.06 determined previously for ClϩH S.
͘
v
2
͗
Reaction dynamics of ClϩH2S and ClϩCH3SH are compared; the adduct CH3S͑Cl͒H is likely
more transitory than the adduct H2SCl. © 2004 American Institute of Physics.
͓DOI: 10.1063/1.1634558͔
I. INTRODUCTION
reported distributions of rotational energies of HX typically
have large uncertainties.
In previous work,6 we employed step-scan time-resolved
Fourier-transform spectroscopy ͑TR-FTS͒ to investigate the
reaction
The energy disposal into HX ͑XϭF,Cl,Br͒ from reac-
tions of halogen atoms with hydrogen-containing species
maintains interest because these reactions generate vibra-
tionally excited HX products.1,2 In these reactions, the newly
formed H–X bond carries most of the vibrational excitation,
leaving the other product vibrationally cold.
ClϩH S→HCl ,J͒ϩHS,
͑v
͑1͒
2
Typical experiments for studying internal energies of HX
are performed either with a fast-flow reactor3 or with a cold-
wall flow reactor using the arrested-relaxation technique.4 A
conventional Fourier-transform infrared ͑FTIR͒ spectrometer
replaces previously employed monochromator or infrared
͑IR͒ interference filters to provide improved sensitivity and
spectral resolution, but the temporal resolution has been typi-
cally in the ms range. In these experiments of bimolecular
reactions, rotational quenching is generally non-negligible so
that direct detection of products in their nascent rotational
distribution is difficult. Observed rotational distributions
typically contain substantial low-J components due to
quenching; hence the nascent rotational distribution was de-
rived by assuming a Boltzmann distribution and transform-
ing the observed distribution to the high-J envelope to cor-
rect for quenching effects.5 Because of such a limitation,
and derived a nascent rotational distribution of HCl product
by a small extrapolation from results determined 0.5–4.0 ␮s
after initiation of the reaction with a laser. Our results show
a rotational energy of 8.3Ϯ1.5 kJ molϪ1 HCl, much greater
than a value 4.5 kJ molϪ1 reported previously using the ar-
rested relaxation method.7 The fraction of available energy
going into rotation of HCl is frϭ0.12Ϯ0.02. With added
carrier gas to facilitate rotational thermal equilibrium, the
temporal evolution of vibrational populations of HCl( ) re-
v
corded with TR-FTS provides information on rates of forma-
tion and quenching of HCl( ), hence the vibrational distri-
v
bution of the HCl product. A branching ratio of 0.14Ϯ0.01
for formation of HCl( ϭ2)/HCl( ϭ1) was determined;
v
v
combining an estimate of the vibrational population of
HCl( ϭ0) based on a surprisal analysis of previous inves-
v
tigations on the reaction ClϩD2S,8 we reported an average
vibrational energy of 23Ϯ4 kJ molϪ1 for HCl. The fractions
a͒
of available energy going into vibration of HCl is
f
͘
v
͗
Author to whom correspondence should be addressed. Electronic mail:
yplee@mx.nthu.edu.tw; Fax: 886-3-5722892.
ϭ0.33Ϯ0.06.
0021-9606/2004/120(4)/1792/9/$22.00
1792
© 2004 American Institute of Physics
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J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Reaction dynamics of ClϩCH3SH
1793
II. EXPERIMENTS
According to chemical dynamics of the reaction type
XϩHY with a heavy–light–heavy mass relationship,9 the
fractions of available energy transforming into vibrational
The apparatus employed to obtain time-resolved IR
emission spectra has been described previously,6,14,15 only a
brief summary is given here. The photolysis beam from a
XeCl laser ͑308 nm, 41 Hz repetition rate͒ with a fluence
ϳ80 mJ cmϪ2 was employed to photodissociate S2Cl2 to pro-
duce Cl atoms to initiate the reaction. Emission of reaction
products were collected with a Welsh cell and detected with
a step-scan Fourier-transform IR spectrometer equipped with
a CaF2 beamsplitter and an InSb detector. The transient sig-
nal of the InSb detector ͑risetime 0.22 ␮s͒ was amplified
͑bandwidth 1 MHz͒ before being digitized with either an
internal digitizer ͑16-bit digitizing resolution, 5 ␮s temporal
resolution͒ or an external data-acquisition board ͑12-bit digi-
tizing resolution, 25 ns temporal resolution͒. To decrease the
duration of data acquisition, a filter passing 2010–3310 cmϪ1
for detection of HCl was used to facilitate undersampling of
interferograms. At each scan step data were typically aver-
aged over 60 laser pulses; 6450 scan steps were performed to
yield an interferogram resulting in a spectrum of resolution
and rotational energies of HX,
f
and f , respectively,
͘ ͗ ͘
r
͗
v
should be nearly independent of Y. On the other hand, if
available energy were statistically distributed, Y of a more
complex structure would compete successfully for energy,
leading to HX with less internal energy. In order to under-
stand the effect of the structure of Y on reaction dynamics,
we extend investigation of reaction ͑1͒ to the reaction
ClϩCH SH→HCl ,J͒ϩCH S ,J͒.
͑v ͑v
͑2a͒
3
3
Dill and Heydtmann employed the arrested relaxation
method to observe IR chemiluminescence of HCl from reac-
tions ͑1͒ and ͑2a͒ with a FTIR spectrometer; Cl atoms were
produced from microwave-discharged Cl2 .7 They observed
for reaction ͑1͒ emission from only HCl( ϭ1) with a rota-
tional distribution peaked near J ϭ4, but for reaction ͑2a͒
v
Ј
emission from HCl( ϭ1 and 2͒ with rotational distributions
v
peaked at J ϭ2 and 1, respectively. For reaction ͑2a͒, they
Ј
reported a rotational energy of 5.3 kJ molϪ1 and a ratio of
0.5 cmϪ1
.
vibrational distributions 0.10:0.90 for HCl( ϭ2):HCl(
v
v
Typical flow rates and partial pressures are 0.07–0.24
STP cm3 sϪ1 and 0.022–0.075 Torr for CH3SH, 0.22–0.45
STP cm3 sϪ1 and 0.070–0.142 Torr for S2Cl2 , and 0.32–4.14
STP cm3 sϪ1 and 0.10–1.30 Torr for Ar; STP implies a stan-
dard temperature of 273 K and pressure of 1 atm Ar ͑Scott
Specialty Gases, 99.9995%͒ and CH3SH ͑AGA Specialty
Gases, 99.5%͒ were used without purification. S2Cl2
ϭ1), corresponding to an average vibrational energy of 38.3
kJ molϪ1 if the population of HCl( ϭ0) is assumed to be
v
negligible. The internal energy of the CH3S product was un-
investigated.
Nesbitt and Leone10 detected IR chemiluminescence of
HCl to determine the rate coefficient k2aϭ(1.8Ϯ0.4)
ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1 for reaction ͑2a͒; they used
S2Cl2 as a source of Cl and initiated the reaction with laser
irradiation at 300 nm. More recent measurements on the rate
¨
͑Riedel–de Haen, 99%͒ was degassed at 180 K before use.
III. RESULTS AND DISCUSSION
coefficient of reaction ͑2a͒ range from 1.1ϫ10Ϫ10 to 2.0
11,12
ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1
.
Nesbitt and Leone10 used
We employed S2Cl2 rather than Cl2 as a source of Cl
atoms because CH3S reacts with Cl2 to propagate chain
reactions.10,16 Photodissociation of S2Cl2 in a molecular
beam at 308 nm has been extensively investigated with frag-
mentation translational spectroscopy.17 As discussed previ-
ously, the average translational energy of Cl atoms immedi-
ately after photolysis at 308 nm is 64 kJ molϪ1, yielding
an average collisional energy of 38 kJ molϪ1 between Cl
and CH3SH.6 At a pressure of 0.32 Torr, there are more
than 12 collisions within 1 ␮s; hence most Cl atoms are
thermalized within 1 ␮s. At 0 K reactions ͑2a͒ and ͑2b͒ have
enthalpies of reaction ⌬HؠϭϪ68.2 kJ molϪ1 and ⌬Hؠ
ϭϪ42.0 kJ molϪ1, respectively, derived from enthalpies of
also Cl2 as a source of Cl and observed a chain reaction with
a length smaller than that expected for reaction ͑2a͒, fol-
lowed by
CH3SϩCl2→CH3SClϩCl.
͑3͒
They also proposed a possible involvement of the reaction
ClϩCH3SH→HClϩCH2SH,
͑2b͒
and later determined the rate coefficient to be k2bϭ(4.3
13
Ϯ1.0)ϫ10Ϫ12 cm3 moleculeϪ1 sϪ1
.
Nicovich et al. inves-
tigated reaction ͑2͒ using Cl2CO as a source of Cl; they
reported a small negative temperature dependence of k2 and
found no kinetic isotope effect when they replaced CH3SH
with CD3SD, indicating that formation of an energized ad-
duct is rate determining.11 Although both reactions ͑1͒ and
͑2a͒ have small negative activation energy, only the former
shows a substantial kinetic deuterium-isotope effect;11 hence
reaction mechanisms of these two reactions might be differ-
ent. Comparison of internal state distributions of reaction
products from these reactions might provide further informa-
tion on the details of their reaction mechanism.
formation ͑in kJ molϪ1
͒
of Cl ͑119.62͒,18 CH3SH
(Ϫ12.12),19 CH3S (131.42),20 CH2SH (157.59),21 and HCl
͑Ϫ92.13͒.18 Hence the available energies for reactions ͑2a͒
and ͑2b͒ at 298 K are ϳ79.4 and 53.2 kJ molϪ1, respectively,
after taking into account translational and rotational energies
of Cl and CH3SH; theoretical calculations predict no barrier
for reaction ͑2a͒.22 An available energy used for reaction ͑2a͒
in previous work was 75 kJ molϪ1 7
Park et al. used tunable infrared diode laser to probe the
.
transition Cl(2P1/2
)
Cl(2P3/2) and reported that the relative
Here we report measurements of internal-energy distri-
butions of HCl produced from reaction ͑2a͒ by means of
TR-FTS in emission mode and compare with those deter-
mined for HCl from reaction ͑1͒.6
yield of Cl(2P1/2) is 0.48Ϯ0.06 from photolysis of S2Cl2 at
308 nm.23 The rate coefficient of quenching of Cl(2P1/2) by
Ar is small,24 (3.0Ϯ1.0)ϫ10Ϫ16 cm3 moleculeϪ1 sϪ1, but
that by S2Cl2 is much larger. Based on the data of Park
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1794
J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Cheng, Wu, and Lee
FIG. 1. Infrared emission spectra of HCl in spectral region 2400–3200 cmϪ1 recorded after irradiation of a flowing mixture of S2Cl2 (0.142 Torr),
CH3SH (0.075 Torr), and Ar ͑0.102 Torr͒ with a XeCl excimer laser at 308 nm. Spectral resolution is 0.5 cmϪ1; 60 laser pulses were averaged at each scan
step. ͑A͒ 0.25–0.75 ␮s; ͑B͒ 0.75–1.25 ␮s, ͑C͒ 1.25–1.75 ␮s, and ͑D͒ 0.25–6.25 ␮s after laser irradiation. Assignments are shown as stick diagrams; J values
Љ
are listed in parentheses.
et al.,23 with 10 mTorr of S2Cl2 and 1.99 Torr of Ar in the
system, 90% of Cl(2P1/2) is quenched within 75 ␮s after
production. In our experiments we employed 142 mTorr of
S2Cl2 ; hence most Cl(2P1/2) is expected to be thermalized
within 5 ␮s.
vibration-rotational lines in the P branch, as described
previously.6 Representative semilogarithmic plots of
P (J )/(2J ϩ1) versus J (J ϩ1) for HCl( ϭ1) are
v
Ј
Ј
Ј Ј
v
shown in Fig. 2. Data of overlapped lines P (J ) for ϭ1,
v
Ј
v
J ϭ3, 4, 6, 10, 11, and 13, are not shown. Fitted Boltzmann-
Ј
By decreasing the pressure of the reagent as much as
possible while maintaining a satisfactory ratio of signal to
noise, we recorded emission of HCl with a fast external digi-
tizer at 25-ns resolution, followed by averaging every 20
consecutive time-resolved spectra to yield spectra with tem-
poral resolution of 0.5 ␮s; a nascent rotational population of
HCl was subsequently derived by short extrapolation to t
ϭ0 based on observed population. To determine the vibra-
tional distribution of HCl, we added about 1.3 Torr of Ar to
thermalize the rotational distribution and used an internal
16-bit digitizer at 5-␮s resolution to obtain temporal profiles
type rotational distributions yield rotational temperatures
1110Ϯ90 K ͑0.25–0.75 ␮s͒, 1100Ϯ70 K ͑0.75–1.25 ␮s͒,
of HCl( ) up to 500 ␮s after initiation of reaction. Under
v
experimental conditions (tр5 ␮s) for measurements of ro-
tational distribution of HCl, the presence of excessive
Cl(2P1/2) might have a small effect on the rotational distri-
bution, whereas for measurements of vibrational distribu-
tions (tу5 ␮s), Cl(2P1/2) and Cl(2P3/2) are thermalized.
A. Nascent rotational distribution of HCl„v…
Figure 1 shows emission spectra of HCl recorded 0.25–
0.75, 0.75–1.25, 1.25–1.75, and 0.25–6.25 ␮s after photoly-
sis of a flowing mixture containing S2Cl2 (0.142 Torr),
CH3SH (0.075 Torr), and Ar ͑0.102 Torr͒ at a spectral reso-
lution of 0.5 cmϪ1. Assignments based on spectral param-
eters reported by Arunan et al.25 and Coxon and
Roychowdhury26 are shown as stick diagrams; notations
P(J ) and R(J ) are used. Trace C of Fig. 1 shows emission
Љ
Љ
of HCl( ϭ1) with rotational levels J up to 16 and weak
v
Ј
FIG. 2. Semilogarithmic plots of relative rotational populations of HCl(
v
emission of HCl( ϭ2 and 3͒ with J up to 14 and 10, re-
spectively, but at shorter time ͑such as traces A and B͒ the
v
ϭ1) after irradiation of a flowing mixture of S2Cl2 (0.142 Torr),
Ј
CH3SH (0.075 Torr), and Ar ͑0.102 Torr͒ with a XeCl excimer laser at 308
nm. ͑A͒ 0.25–0.75 ␮s; ͑B͒ 0.75–1.25 ␮s, and ͑C͒ 1.25–1.75 ␮s after laser
irradiation; data reported by Dill and Heydtman ͑Ref. 7; symbol ᭺͒ are
included for comparison. Solid lines represent least-squares fits. For traces
͑B͒–͑D͒ the y-axes are displaced vertically for clarity.
poorer signal to noise ratio allows measurements of J up to
Ј
only 15, 12, and 8 for HCl( ϭ1–3), respectively. A relative
v
population P (J ) was derived from intensities of observed
Ј
v
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J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Reaction dynamics of ClϩCH3SH
1795
TABLE I. Rotational temperature (T ), rotational energy (E ), and nascent rotational energy (E0) of HCl( )
v
r
r
r
produced from ClϩCH3SH in various reaction periods.
Pϭ0.32 Torr
Pϭ0.81 Torr
E0( )b
v
E0( )b
Time/␮s
Tr /K
͚JPv(J)a
v
͚JPv(J)a
v
E ( )
Tr /K
E ( )
v
r
r
r
r
v
E0r ϭ8.9Ϯ0.4 kJ molϪ1
8.1Ϯ0.7
HCl( ϭ1)
0.25–0.75 1110Ϯ 90 2933
8.4
8.3
9.1
9.3
8.9
1070Ϯ 60
1050Ϯ 60
920Ϯ 90
890Ϯ 40
840Ϯ 30
2792
4367
4907
5712
6686
7.9Ϯ0.5
7.9Ϯ0.5
7.4Ϯ0.8
6.9Ϯ0.5
6.8Ϯ0.5
8.5
8.6
9.3
8.9
9.2
0.75–1.25 1100Ϯ 70 4691
8.0Ϯ0.6
7.8Ϯ0.6
7.4Ϯ0.5
7.0Ϯ0.2
1.25–1.75
1.75–2.25
2.25–2.75
990Ϯ 70 5942
920Ϯ 60 6625
890Ϯ 40 7197
v
E0r ϭ7.1Ϯ0.5 kJ molϪ1
6.0Ϯ0.7
HCl( ϭ2)
0.25–0.75 1080Ϯ110 1011
0.75–1.25 1040Ϯ150 1381
6.3
6.4
6.2
6.6
7.5
1020Ϯ210
1010Ϯ140
870Ϯ 80
850Ϯ110
790Ϯ 40
960
1210
1390
1765
2077
6.0Ϯ1.4
6.1Ϯ1.0
5.7Ϯ0.6
5.3Ϯ0.8
5.4Ϯ0.6
6.8
6.9
7.5
7.2
7.7
5.8Ϯ1.0
5.4Ϯ0.8
5.5Ϯ0.6
5.8Ϯ0.2
1.25–1.75
1.75–2.25
2.25–2.75
990Ϯ110 1606
850Ϯ 80 1815
880Ϯ 30 1823
v
E0r ϭ4.5Ϯ0.4 kJ molϪ1
3.9Ϯ0.5
HCl( ϭ3)
0.25–0.75 1100Ϯ180
0.75–1.25 1060Ϯ170
207
232
272
294
308
4.0
3.9
4.2
4.8
4.8
1040Ϯ200
970Ϯ150
900Ϯ140
850Ϯ100
790Ϯ 80
173
212
215
255
354
3.2Ϯ1.0
3.7Ϯ0.8
3.4Ϯ0.6
3.7Ϯ0.6
3.6Ϯ0.9
3.5
4.3
4.3
4.9
5.2
3.6Ϯ0.5
3.6Ϯ0.6
3.8Ϯ0.3
3.7Ϯ0.3
1.25–1.75
1.75–2.25
2.25–2.75
960Ϯ160
910Ϯ 90
880Ϯ 70
aPv(J)ϭ͑relative integrated emittance͒/͓͑instrumental response factor͒ ͑Einstein coefficient͔͒; arbitrary unit.
bE0r ϭErϫ(Tr0/Tr); T0r ϭ1150Ϯ80 K.
similar; an average value T0r ϭ1145Ϯ30 K is thus obtained.
Considering possible errors, we report T0r ϭ1150Ϯ80 K.
and 990Ϯ70 K ͑1.25–1.75 ␮s͒ for HCl( ϭ1); unless oth-
erwise noted, the uncertainties represent one standard devia-
v
tion in fitting. Also shown in Fig. 2 is the rotational distribu-
Average rotational energies E for HCl( ϭ1–3) are de-
rived on summing a product of level energy and normalized
v
r
tion of HCl( ϭ1) reported by Dill and Heydtmann;7 their
v
results contain two distributions corresponding to tempera-
tures ϳ300 K and ϳ1270 K, with the high-J component
having a temperature similar to our measurements. Emission
population Pv(J) for all observed rotational level in each
vibrational state; for overlapped lines, the populations are
estimated with interpolation based on Boltzmann plots such
as the ones in Fig. 2. Nascent average rotational energies Er0
are derived on multiplying experimental rotational energies
lines associated with HCl( ϭ2,3) are treated similarly to
v
derive corresponding rotational temperatures; results are
summarized in Table I. Overlapped P(J ) lines are J ϭ4, 5,
Ј
Ј
6, and 11 for ϭ2, and J ϭ4, 6, and 7 for ϭ3.
v
v
Ј
Rotational distribution of HCl produced via channel ͑2b͒
has little effect on our measurements because its rate coeffi-
cient is Ͻ3% of that of reaction ͑2a͒. Under our experimental
conditions for measurements of rotational distribution of
HCl, with a reaction duration of less than 5 ␮s, nearly all
HCl is produced from reaction ͑2a͒.
Derived rotational temperatures of HCl( ϭ1) as a func-
v
tion of reaction periods are shown in Fig. 3 for two total
pressures ͑0.32 and 0.81 Torr͒. The rotational temperature is
smaller at a greater pressure or for a larger reaction period
because of quenching. We fitted all data to an exponential
decay
T ϭ298ϩ T0Ϫ298͒exp Ϫkt͒
͑4͒
͑
͑
r
r
in which T0r is the nascent rotational temperature and k is a
decay coefficient that varies linearly with pressure. Values
T0ϭ1180Ϯ20 and 1150Ϯ20 K were derived for HCl(
v
r
ϭ1) in experiments with pressures of 0.32 and 0.81 Torr,
respectively. Similarly, values T0r ϭ1140Ϯ10 and 1110Ϯ30
K were derived for HCl( ϭ2), and T0ϭ1170Ϯ20 and
v
r
FIG. 3. Variation of rotational temperatures Tr of HCl( ϭ1) as a function
of period after irradiation; solid lines represent least-squares fits of Eq. ͑4͒;
v
1120Ϯ10 K for HCl( ϭ3) in experiments at pressures 0.32
v
and 0.81 Torr, respectively. All rotational temperatures are
see text.
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1796
J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Cheng, Wu, and Lee
FIG. 4. Infrared emission spectra of HCl in spectral region 2400–3200 cmϪ1 recorded at 16.9 ␮s ͑A͒ and 201.9 ␮s ͑B͒ after photolysis of a flowing mixture
of S2Cl2 (0.072 Torr), CH3SH (0.022 Torr), and Ar ͑1.29 Torr͒ with a XeCl excimer laser at 308 nm. Spectral resolution is 1.0 cmϪ1; 60 laser pulses were
averaged at each scan step. Assignments are shown as stick diagrams.
Er at each interval by a correction factor T0r /Tr . As Table I
HCl( ϭ1–3) obtained upon photolysis of the flowing mix-
v
shows, this small correction for quenching yields consistent
nascent average rotational energies. We average values listed
ture yielding spectra in Fig. 4. We employed a commercial
kinetic modeling program FACSIMILE ͑Ref. 27͒ to fit all three
temporal profiles collectively according to a model contain-
ing the following reactions:
in Table I to obtain E0( )ϭ8.9Ϯ0.4, 7.1Ϯ0.5, and 4.5Ϯ0.4
v
r
kJ molϪ1 for ϭ1–3, respectively; the decrease in E0( ) as
v
v
r
increases is because less rotational levels were observed.
v
ClϩCH3SH→HCl͑vϭ1͒ϩCH3S,
An averaged nascent rotational energy E0r ϭ8.2
Ϯ0.9 kJ molϪ1 for HCl( ϭ1–3) was derived on multiply-
rateϭkI2a͓Cl͔, branching ␥1 ,
͑5͒
͑6͒
͑7͒
v
ing E0( ) by its corresponding vibrational population, to be
v
r
discussed later. The only previous report7 of Er
ClϩCH SH→HCl ϭ2͒ϩCH S,
͑v
3
3
ϭ5.3 kJ molϪ1 is about 65% of our value.
rateϭkI2a͓Cl͔, branching ␥2 ,
The highest rotational level (J ϭ8) observed for
Ј
HCl( ϭ3) lies 9024 cmϪ1 above the ground state. With the
v
ClϩCH SH→HCl ϭ3͒ϩCH S,
͑v
3
3
same energy, J ϭ24 and 17 of HCl( ϭ1) and HCl(
v
v
Ј
rateϭkI2a͓Cl͔, branching ␥3 ,
ϭ2), respectively, might be populated. The absence of
higher levels in the spectra might reflect limited detectivity.
If we assume a Boltzman distribution and associate extrapo-
lated population with unobserved lines for ϭ1 and 2, we
v
derive average rotational energies E0( ϭ1)ϭ9.6Ϯ0.3 and
v
r
E0( ϭ2)ϭ8.7Ϯ0.4 kJ molϪ1. The average rotational en-
v
r
ergy of HCl( ϭ1–3), E0ϭ9.1Ϯ0.9 kJ molϪ1, should be
v
r
considered as an upper limit.
B. Vibrational distribution and rate coefficient
Experiments were carried out with flowing gaseous
mixtures containing S2Cl2 (ϳ0.070 Torr), CH3SH (0.022–
0.042 Torr), and Ar ͑ϳ1.3 Torr͒; HCl emission was recorded
at 5-␮s intervals. Figure 4 shows emission spectra of HCl at
a spectral resolution of 1.0 cmϪ1 recorded at 16.9 and 201.9
␮s after photolysis of a flowing mixture containing
S2Cl2 (0.072 Torr), CH3SH (0.022 Torr), and Ar ͑1.29 Torr͒.
The signal to noise ratios of these spectra are superior to
those recorded for rotational distribution measurements be-
cause a 16-bit ADC was used and because more HCl was
produced at a later reaction period. We confirmed a nearly
thermal ͑ϳ350Ϯ50 K at 5 ␮s͒ distribution for rotation under
these experimental conditions. We averaged results from se-
lective rovibrational P(J ) lines to yield relative vibrational
Љ
FIG. 5. Temporal profiles of HCl( ) recorded after photolysis of a flowing
mixture of S2Cl2 (0.072 Torr), CH3SH (0.022 Torr), and Ar ͑1.29 Torr͒ with
v
populations; J ϭ2, 4–8 for ϭ1, J ϭ2–4, 7, 8 for ϭ2,
v
v
Ј
Ј
and J ϭ1, 2, 5 for ϭ3 were selected.
v
Ј
a XeCl excimer laser at 308 nm. ᭺, HCl( ϭ1); ᮀ, HCl( ϭ2); छ,
v
v
Figure 5 shows representative temporal profiles of
HCl( ϭ3).
v
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J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Reaction dynamics of ClϩCH3SH
1797
TABLE II. Experimental conditions and fitted rate coefficients and branching ratios for the reaction ClϩCH SH→HCl( ,J)ϩCH S.
v
3
3
Expt. No.
1
2
3
4
5
6
PCH SH /mTorr
22
72
1290
77
25
68
1320
80
29
70
1300
79
35
69
1280
78
39
68
1320
79
42
71
1310
80
3
PS
P
/mTorr
Cl
2
A2r /mTorr
Laser fluence/mJ cmϪ2
% S2Cl2 photolyzed
3.0
3.1
3.1
3.0
3.1
3.1
k
k
k
q1/104 sϪ1
q2/104 sϪ1
q3/104 sϪ1
0.80Ϯ0.02
1.29Ϯ0.09
3.7Ϯ0.9
10.8Ϯ0.8
1.09Ϯ0.03
1.55Ϯ0.10
5.2Ϯ1.4
12.6Ϯ1.8
1.31Ϯ0.02
1.82Ϯ0.12
7.6Ϯ1.3
25.0Ϯ3.2
1.58Ϯ0.07
2.22Ϯ0.19
6.8Ϯ2.9
1.63Ϯ0.04
2.64Ϯ0.10
7.7Ϯ3.3
1.59Ϯ0.05
2.92Ϯ0.23
8.2Ϯ3.4
kI2a/104 sϪ1
28.8Ϯ3.8
40.0Ϯ4.3
40.2Ϯ4.1
100␥1 /(␥1ϩ␥2ϩ␥3)
100␥2 /(␥1ϩ␥2ϩ␥3)
66Ϯ4
27Ϯ3
66Ϯ5
26Ϯ3
69Ϯ9
26Ϯ3
68Ϯ5
25Ϯ3
70Ϯ5
23Ϯ4
71Ϯ6
22Ϯ3
ClϩCH SH→HCl ϭ1͒ϩCH SH, rateϭkI2b͓Cl],
͑v
in which fv is the ratio of the vibrational energy of HCl to
the total available energy, we used a surprisal analysis with a
3
2
͑2b͒
model to estimate the vibrational distribution of HCl( ϭ0)
v
HCl ϭ3͒→HCl ϭ2͒, rateϭkq3 HCl ϭ3͒ , ͑8͒
͑v ͑v ͑v
͓
͔
for reaction ͑2a͒ from the observed distribution of HCl(
v
ϭ1–3). HCl( ϭ0) / HCl( ϭ1) Х0.22 is derived to
͓
v
͔ ͓
v
͔
HCl ϭ2͒→HCl ϭ1͒, rateϭkq2 HCl ϭ2͒ , ͑9͒
͑v ͑v ͑v
͓
͔
yield a distribution of HCl( ϭ0):( ϭ1):( ϭ2):( ϭ3)
v
v
v
v
HCl ϭ1͒→HCl ϭ0͒, rateϭkq1 HCl ϭ1͒ ,
͑v ͑v ͑v
ϭ(13Ϯ2):(59Ϯ2):(22Ϯ3):(6Ϯ1). The distribution yields
͓
͔
an average vibrational energy of 41.2Ϯ1.0 kJ molϪ1
.
͑10͒
Hossenlopp et al.8 used time-resolved infrared diode la-
ser absorption spectra to probe DCl produced from the reac-
tion
in which kI2aϭk2aϫ CH SH and kI ϭk2bϫ CH SH are
͓
͔
͓
͔
3
3
2b
pseudo-first-order rate coefficients, and kq1 –kq3 are rate co-
efficients of vibrational quenching. Reaction ͑2b͒ is exother-
mic to populate only HCl( р1). We found that if reaction
͑2b͒ were not included in the model, satisfactory fitting for
v
ClϩD S→DCl ,J͒ϩDS ,J͒
͑12͒
and determined a vibrational distribution of DCl( ϭ0):(
͑v ͑v
2
v
v
the temporal profile of HCl( ϭ1) could not be obtained.
v
ϭ1):( ϭ2)ϭ(33Ϯ7):(56Ϯ7):(11Ϯ3). If we assume that
v
Typically we fit the later portion of temporal profiles
with a single exponential decay to derive estimates of
the dynamics of reactions ͑1͒, ͑2a͒, and ͑12͒ are similar and
have similar surprisal plots with the cold-DS model ͑assum-
ing that available energy does not distribute into vibration of
k
q1 –kq3 , followed by fitting all temporal profiles collec-
tively with the above model by keeping kI2bϭ4.3
ϫ10Ϫ12 cm3 moleculeϪ1 sϪ1 and estimated kq1 –kq3 invari-
ant to derive estimates of Cl ͑relative value͒, kI , ␥2 /␥1 ,
DS͒, we derive a ratio HCl( ϭ0) / HCl( ϭ1) Х0.75
͔ ͓
͓
v
v
͔
which yields a vibrational distribution of HCl( ϭ0):(
v
v
͓
͔
0
2a
ϭ1): ( ϭ2): ( ϭ3)ϭ(34Ϯ2): (45Ϯ2): (17Ϯ3): (4Ϯ1);
v
v
and ␥3 /␥1 . All parameters except kI2b were further opti-
mized to provide the best fit. Values of kI2a , ␥1 /(␥1ϩ␥2
ϩ␥3), ␥2 /(␥1ϩ␥2ϩ␥3), and kq1 –kq3 thus derived under
various experimental conditions are listed in Table II; we
report an averaged ratio ␥1 :␥2 :␥3ϭ(68Ϯ5):(25Ϯ4):(7
an average vibrational energy of 31.4Ϯ1.0 kJ molϪ1 for HCl
is thus derived.
The population of HCl( ϭ0) is clearly critical to the
v
determination of average vibrational energy of HCl, but we
are unable to obtain this information with the TR-FTS tech-
nique. Results from the last two estimates of HCl( ϭ0)
͔
serve as possible ranges and we take the average of these
values, 36Ϯ6 kJ molϪ1, as our estimate, with the error limit
covering both values.
Ϯ1), corresponding to the branching HCl( ϭ1) : HCl(
͔ ͓ v
͓
v
͓
v
ϭ2) : HCl( ϭ3) . Our observed distribution corresponds
͔ ͓
v
͔
to a vibrational temperature greater than that reported by Dill
and Heydtmann with HCl( ϭ1) : HCl( ϭ2) ϭ9:1.7
͓
v
͔ ͓
v
͔
There is no information on the relative population of
HCl( ϭ0). Dill and Heydtmann7 assumed that HCl(
v
͓
v
ϭ0) ϭ0 and derived an average vibrational energy of 38
͔
C. Transition state and reaction dynamics
of Cl¿CH3SH and Cl¿H2S
kJ molϪ1, corresponding to a fraction of available energy ͑75
kJ molϪ1 used by them͒ going into the vibration of HCl to be
With the B3LYP/aug-cc-pVTZ density-functional
theory28,29 using the GAUSSIAN 98 program,30 we performed
calculations to locate structures of a transition state ͑TS1͒
and an adduct CH3S͑Cl͒H with Cl attaching to the S atom,
for the reaction ClϩCH3SH. Geometries of the adduct and
TS1, and displacement vectors corresponding to imaginary
vibrational wave numbers of TS1 predicted with the B3LYP
method are shown in Fig. 6. The transition state has a H–Cl
bond length of 1.675 Å, smaller than a value of 1.72 Å
previously reported by Wilson and Hirst,22 who employed
f
ϭ0.51. If we also assume HCl( ϭ0) ϭ0, we derive
͓ v ͔
͗
͘
v
an average vibrational energy of 47.4 kJ molϪ1, about 25%
greater than the previous report. However, we think that this
assumption likely underestimates the population of HCl(
ϭ0).
Assuming that available energy does not distribute into
vibration of CH3S so that the reaction is similar to Cl
ϩH2S, and a prior function
v
P0 f ͒ϭ 1Ϫf ͒3/⌺ 1Ϫf ͒3,
͑11͒
͑
͑
͑
v
v
v
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1798
J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Cheng, Wu, and Lee
FIG. 7. Potential energies ͑in kJ molϪ1͒ for reactions ClϩCH3SH ͑A͒ and
ClϩH2S ͑B͒ calculated with MP4/6-311ϩϩG(3df,2p)//B3LYP/
aug-cc-pVTZ.
combination is a major factor in determining the internal
energies of HCl. The large internal energy of the product
HCl is consistent with rapid motion of the reacting H atom,
which leaves without interacting with the remainder of the
radical fragment CH3S. Our observation is consistent with
such a model. With an available energy of 79 kJ molϪ1, the
FIG. 6. Geometry of adduct ͑A͒ and transition state ͑B͒ and its associated
vector displacements ͑C͒ for the reaction ClϩCH3SH predicted with the
B3LYP/aug-cc-pVTZ method. Displacement vectors corresponding to
imaginary wave numbers are shown with a solid arrow. Bond lengths have
unit of Å. Results from Wilson and Hirst ͑Ref. 22͒ are listed in parentheses.
fraction of energy that leads to rotational energy,
f , is
͗ ͘
r
0.10Ϯ0.02. This value is nearly identical to that determined
for the reaction between ClϩH2S, indicating perhaps a simi-
lar kinematic constraint in both reactions.31 As in the case of
ClϩH2S, the modified impulse model failed to predict the
rotational energy of HCl produced from ClϩCH3SH. The
small value of ␣ϭ1.1° calculated for the vector of reaction
coordinate for TS1 implies little rotational energy for HCl
from reaction ͑2a͒, in contrast to the experimental observa-
tion. The impulse model also fails if we assume that the
impulse is exerted along the H–S bond of the TS1; according
to Eq. ͑10͒ in Ref. 6, the torque angle of ␣ϭ41.4° yields a
rotational energy ϳ33 kJ molϪ1 for HCl.
Kinetic measurements on reaction ͑2a͒ showing the ab-
sence of a deuterium kinetic isotope effect11 indicate that the
formation of the adduct is rate-determining and the adduct
further undergoes rapid reaction to form products. If an ad-
duct CH3S͑Cl͒H is formed initially, rearrangement of the ad-
duct to form CH3S and HCl is expected to provide greater
rotational excitation of HCl than direct abstraction. As a
large fraction of available energy is deposited into HCl, we
however expect such an adduct to be ephemeral so that sub-
stantial redistribution of energy does not occur.
**
the MP2͑full͒/6-311G
method to predict the transition
structure. Their reported molecular parameters are listed par-
enthetically in Fig. 6 for comparison. The angle of Cl–H–S,
138.6°, predicted in this work is greater than a value 122.6°
predicted previously. The displacement vector for motion of
the reacting H atom in the reaction coordinate is only 1.1°
from the H–Cl band. The adduct has an S–Cl bond length
2.644 Å, slightly greater than a previously reported value of
2.61 Å.22 Predicted vibrational wave numbers are 37.9,
115.1, 223.6, 698.2, 820.3, 933.6, 1035.2 1323.7, 1346.0,
1459.7, 1463.4, 3051.7, 3132.6, 3147.5, and 769.5i cmϪ1 for
TS1 and 103.3, 139.4, 230.6, 410.0, 692.0, 794.8, 962.9,
1096.8, 1351.5, 1466.0, 1473.2, 2683.2, 3061.6, 3149.4,
3164.9 cmϪ1 for CH3S͑Cl͒H.
Rate coefficients of both reactions ClϩH2S and Cl
ϩCH3SH show a small negative temperature dependence,11
indicating that these reactions proceed via adduct formation.
Energies calculated for both reactions with MP4/6-311ϩ
ϩG(3df,2p) at geometries optimized with B3LYP/aug-cc-
pVTZ are shown in Fig. 7. Calculated exothermicity of Ϫ61
and Ϫ46 kJ molϪ1 for reactions ͑2a͒ and ͑1͒ at 0 K, respec-
tively, are slightly smaller than experimental values of Ϫ68
and Ϫ58 kJ molϪ1. At this level of calculation, we could not
locate transition states for the direct abstraction path.
We expect that the kinematic effect of the Cl–H–S mass
In contrast, even though the rate coefficient of the reac-
tion ClϩH2S also has a small negative temperature depen-
dence, it exhibits a substantial kinetic isotope effect with rate
coefficient decreasing by a factor of 2 when D2S replaces
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J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Reaction dynamics of ClϩCH3SH
1799
H2S.11 To explain this effect, Nicovich et al. suggested that
dissociation of the energized adduct back to reactants com-
petes successfully with the H-transfer reaction; when D2S
replaces H2S the rate of the reactive path slows down con-
siderably, whereas the dissociation rate of energized adduct
changes little. Our calculations showing TS2ϳ11 kJ molϪ1
above the energy of ClϩH2S and TS1ϳ7 kJ molϪ1 below the
energy of ClϩCH3SH are consistent with this model.
With an available energy of 79 kJ molϪ1, the fraction of
energy that produces vibrational excitation of HCl is
f
͘
v
͗
ϭ0.45Ϯ0.08, greater than a value of
f ϭ0.33Ϯ0.06 re-
͘
v
͗
ported for reaction ͑1͒;6 the error limits barely overlap. That
reaction of Cl with CH3SH produces more vibrational exci-
tation of HCl than that from the reaction of Cl with H2S does
not conform to a statistical model because CH3SH has many
more internal degrees of freedom and is hence expected to
retain most available energy if the energy were distributed
statistically. Our observation of a greater value of
f
for
͘
͗
v
reaction ͑2a͒ is consistent with the model in which the ad-
duct in reaction ͑2a͒ is more transitory than that in reaction
͑1͒; energy redistribution is less facile in reaction ͑2a͒, hence
the other fragment CH3S gains little energy. As described in
the previous paragraph, our calculations also support that
TS1 is more transitory than TS2. Another aspect about the
enhanced vibrational excitation of HCl in the reaction Cl
ϩCH3SH is that the distance between H and Cl in the tran-
sition state, predicted to be 1.675 Å, is greater than the value
1.619 Å predicted for the reaction ClϩH2S; the former de-
viates more from HCl in its equilibrium structure and is ex-
pected to be associated with greater vibrational excitation.
To provide more detailed information on this reaction
and to compare with experimental results, further theoretical
studies on the potential energy surfaces and associated reac-
tion dynamics are needed.
FIG. 8. Plot of pseudo-first-order rate coefficient kI2a as a function of
CH SH ; the solid line represents least-squares fitting of data to Eq. ͑13͒.
͓
͔
3
greater than values (1.1–2.0)ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1
reported previously,10–12 but with overlapping error limits.
IV. CONCLUSION
Rotationally resolved infrared emission of HCl(
v
ϭ1–3) is observed with a step-scan Fourier-transform spec-
trometer in laser-initiated reaction of ClϩCH3SH. The aver-
age rotational energy and its fraction of partition derived in
this work, Erϭ8.2Ϯ0.9 kJ molϪ1 and f ϭ0.10Ϯ0.01, are
͗ ͘
r
much greater than in a previous report, but nearly identical to
our previous determination for the reaction of ClϩH2S. The
similarity in rotational energies of HCl product from both
reactions indicates that it might be kinematically constrained;
the structural complexity of the molecular reactant has little
effect on the rotational distribution of HCl produced from its
reaction with Cl. The observed vibrational distribution of
D. Rate coefficient of Cl¿CH3SH\HCl¿CH3S
Pseudo-first-order rate coefficients kI2a derived with
model fitting of temporal profiles of vibrational populations
of HCl( ϭ1–3) are listed in Table II and plotted as a func-
v
tion of CH SH in Fig. 8. Although kI may be derived by
͓
͔
3
2a
fitting the rise of the population of HCl ( ϭ3), model fitting
v
with all data provides rate coefficients with less uncertain-
ties. Bimolecular rate coefficients k2a were derived on fitting
an equation
HCl,
( ϭ1):( ϭ2):( ϭ3)ϭ(68Ϯ5):(25Ϯ4):(7Ϯ1),
v
v
v
and an estimate of
f
ϭ0.45Ϯ0.08 indicate more vibra-
͗
͘
v
tional excitation of HCl than previously reported for this
reaction and also for the reaction of ClϩH2S. The difference
in vibrational excitation is consistent with a difference in
predicted H–Cl bond distances in transition structures of
these reactions. The pronounced vibrational excitation is also
consistent with a more transitory adduct for the reaction Cl
ϩCH3SH: kinetic isotope effects observed only for the re-
action ClϩH2S but not for ClϩCH3SH ͑Ref. 11͒ are consis-
tent with this picture. Potential energy of the transition state
of the reaction ClϩCH3SH, but not of ClϩH2S, calculated
to be smaller than the initial energy of the reactants supports
this model. Our time-resolved Fourier-transform spectra
have demonstrated improved temporal resolution and detec-
tion sensitivity in determining internal-state distributions of
kI2aϭk2aϫ CH SH ϩintercept.
͑13͒
͓
͔
3
A value of (2.93Ϯ0.13)ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1 is de-
rived. When we fit the data with zero intercept in Eq. ͑13͒,
we derive k2aϭ(2.99Ϯ0.03)ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1
.
Considering that errors of kI2a in model fitting are ϳ15% and
in measurements of flow rates and pressure are ϳ3% and
1%, respectively, and that these experiments were not spe-
cifically designed for accurate kinetic measurements so that
only a small range of CH SH was employed, we report a
͓
͔
3
rate coefficient k2aϭ(2.9Ϯ0.7)ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1
for reaction ͑2a͒, with listed uncertainties representing esti-
mated errors. The value of k2a derived in this work is slightly
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1800
J. Chem. Phys., Vol. 120, No. 4, 22 January 2004
Cheng, Wu, and Lee
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nascent rotational distributions that could previously not be
determined directly and accurately.
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ACKNOWLEDGMENTS
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We thank the National Science Council of Taiwan ͑Grant
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moting Academic Excellence of Universities ͑Grant No. 89-
FA04-AA͒ for support.
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