Infrared chemiluminescence studies of the H+(CH3)3COCl and H+RC(O)SCl (R = Cl, F, OCH3) reactions: Observation of OCS infrared chemiluminescence

Article abstract of DOI:10.1021/jp001042c

Infrared chemiluminescence from a room-temperature flow reactor was used to study the reactions of H atoms with (CH₃)₃COCl, ClC(O)SCl, FC(O)SCl, and CH₃OC(O)SCl. Infrared emission spectra were recorded from the HCl, HF, and OCS products. The anharmonic shifts from bands involving ν₁, ν₂, and ν₃ excitation are too small to obtain information about bending vs stretch excitation of OCS from the Δν₃ = -1 spectra; however, a computer simulation method was developed to analyze the Δν₃ = -1 transition to assign the average total vibrational energy of OCS, 〈E_v(OCS)〉. The enthalpy changes for the carbonylsulfenyl chloride reactions were estimated from ab initio calculations. The proposed mechanism for the carbonylsulfenyl chlorides includes two reaction pathways: one involves interaction with the S-Cl bond to give HCl; the second involves an RC(O)SCl·H adduct that subsequently gives RH and OCS (+Cl). The 〈E_v(OCS)〉 values were 17.2, 14.6, and 8.4 kcal mol^-1 from FC(O)SCl, CH₃OC(O)SCl, and ClC(O)SCl, respectively. The fraction of the available energy released as HCl vibrational energy, {f_v(HCl)〉, from reaction with the S-Cl bond was approximately 0.3 for all three reactions. The reaction mechanism for H+(CH₃)₃COCl, which was employed as a reference reaction, is thought to be direct abstraction and 〈f_v(HCl)〉 is 0.23.

Full text of DOI:10.1021/jp001042c

J. Phys. Chem. A 2000, 104, 11013-11024
11013
ARTICLES
Infrared Chemiluminescence Studies of the H + (CH3)3COCl and H + RC(O)SCl (R ) Cl,
F, OCH3) Reactions: Observation of OCS Infrared Chemiluminescence
G. C. Manke, II* and D. W. Setser
Department of Chemistry, Kansas State UniVersity, Manhattan, Kansas 66506
ReceiVed: March 20, 2000; In Final Form: September 26, 2000
Infrared chemiluminescence from a room-temperature flow reactor was used to study the reactions of H
atoms with (CH
from the HCl, HF, and OCS products. The anharmonic shifts from bands involving ν
are too small to obtain information about bending vs stretch excitation of OCS from the ∆V
however, a computer simulation method was developed to analyze the ∆V ) -1 transition to assign the
average total vibrational energy of OCS, 〈E (OCS)〉. The enthalpy changes for the carbonylsulfenyl chloride
3 3
)
COCl, ClC(O)SCl, FC(O)SCl, and CH
3
OC(O)SCl. Infrared emission spectra were recorded
, ν , and ν excitation
) -1 spectra;
1
2
3
3
3
v
reactions were estimated from ab initio calculations. The proposed mechanism for the carbonylsulfenyl chlorides
includes two reaction pathways: one involves interaction with the S-Cl bond to give HCl; the second involves
an RC(O)SCl‚H adduct that subsequently gives RH and OCS (+Cl). The 〈E
v
(OCS)〉 values were 17.2, 14.6,
OC(O)SCl, and ClC(O)SCl, respectively. The fraction of the available
energy released as HCl vibrational energy, 〈f
(HCl)〉, from reaction with the S-Cl bond was ∼0.3 for all
three reactions. The reaction mechanism for H + (CH COCl, which was employed as a reference reaction,
is thought to be direct abstraction and 〈f (HCl)〉 is 0.23.
-1
and 8.4 kcal mol from FC(O)SCl, CH
3
V
3 3
)
V
I. Introduction
bend-stretch distributions impossible, even by computer simu-
lation. An additional problem for OCS is the close energy match
among several vibrational levels and the expected rapid
equilibration of bend-stretch levels of a similar energy.1 For
these reasons, only the average total energy and the general
shape of the vibrational distribution can be deduced from the
observed ∆V3 ) -1 spectra. As far as we know, the only other
The infrared chemiluminescence (IRCL) technique is an
established method to measure vibrational, and even rotational,
product state distributions and rate constants from reactions
giving HX products (X ) O, F, Cl, Br).1,2 This laboratory has
employed infrared emission from a flow reactor to characterize
the reaction kinetics and the vibrational distributions of many
primary and secondary reactions involving both uni- and
bimolecular elementary steps giving HX products. More re-
cently, the flow reactor technique was extended to observe
infrared spectra from triatomic product molecules, including
0,11
12
report of IRCL from OCS has been from the O + CS2 reaction.
However, the emission was not consistently observed12 and the
main products are CS + SO.
The H + carbonylsulfenyl chloride reactions were selected
as chemical systems that might give OCS as a product. By
3
-5
3a
6
7
H2O, HOD, D2O,
CO2, HNO, HCN, and HNC. The
1
3
14,15
vibrational distributions for the molecules just mentioned were
assigned by computer simulation of the spectra obtained at
experimental conditions that were chosen to avoid vibrational
relaxation. By taking advantage of anharmonic shifts, the
assignment of specific vibrational distributions to bend and
stretch modes from the strongest emission band, e.g., ∆V3 )
analogy to H + RSCl and ROCl
reactions, we anticipated
rapid abstraction of a Cl atom followed by decomposition of
1
6
the RC(O)S radical (as for HCO2) or a secondary reaction of
H atoms with RC(O)S followed by HR elimination with con-
comitant formation of OCS. The H + ClC(O)SCl, FC(O)SCl,
and CH OC(O)SCl reactions were examined, and infrared
3
3
a
-
1 for CO2, generally was possible. One of the major goals
emission was observed from the HCl, HF, and OCS products.
of the present work was to examine the IRCL technique for
reactions that give OCS as a product. The large Einstein
For purposes of comparison, we also studied the H + (CH ) -
3
3
COCl reaction, which proceeds via direct Cl atom abstraction.
-
1 8
14,15
emission coefficient (A ) 385 s ) for the asymmetric stretch
Direct Cl atom abstraction reactions
normally give inverted
-
1
mode, ν3 ) 2071 cm , makes OCS a favorable candidate.
distributions with 〈fV(HCl)〉 ≈ 0.3, whereas unimolecular HCl
Unfortunately, the modest, and similar, anharmonicity con-
elimination processes give 〈fV(HCl)〉 e 0.2 and noninverted
9
-1
-1
stants, ø13 ) -6.46 cm , ø22 ) -7.55 cm , and ø33 ) -11.46
17-19
distributions.
-
1
cm , make assignment of the ∆V3 ) -1 spectra to specific
The principal experimental data are the HCl(∆V ) -1),
HF(∆V ) -1), and OCS(∆V3 ) -1) infrared emission spectra
from which vibrational distributions and relative product
concentrations are deduced. The room-temperature rate constants
*
To whom correspondence should be addressed. Current address: Bldg.
6
5
19, AFRL/DELC, 3500 Aberdeen Ave. SE, Kirtland AFB, NM 87117-
776.
1
0.1021/jp001042c CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/01/2000

11014 J. Phys. Chem. A, Vol. 104, No. 47, 2000
Manke and Setser
for HX production also were measured by comparison to the H
vapor pressure of 8 Torr at room temperature, and a flow of Ar
was passed over the surface of the liquid maintained at 0 °C to
introduce CH3OC(O)SCl into the reactor. The flow rate was
determined by collecting CH3OC(O)SCl in a liquid-nitrogen-
cooled trap and weighing the contents of the trap.
+
Cl2 reaction, and the rate constants for HCl (or HCl + HF)
-
12
-12
formation are: (3.0 ( 1.0) × 10 , ∼1 × 10 , and ∼1.7 ×
-
12
3
-1 -1
1
0
cm molecule
s
for R ) CH3O, Cl, and F, respec-
tively. The total H atom removal rate constant by ClC(O)SCl
-
12
3
-1 -1
was (3.7 ( 0.5) × 10
cm molecule s , indicating the
Experiments were performed with one reagent concentra-
tion held constant while the other was varied. For example,
possible presence of one or more dark reactions. The primary
reaction mechanism for the carbonylsulfenyl chlorides appears
to be a combination of addition and abstraction processes. The
addition step occurs at either the O or S position (although the
C position cannot be entirely ruled out), and it is followed by
HR (or HCl) elimination:
1
2
-3
constant [H] was maintained, usually 8.0 × 10 cm for
1
3
H + CH3OC(O)SCl and 2.0 × 10 for H + ClC(O)SCl and
FC(O)SCl, while [RC(O)SCl] was varied from 0.5 to 2.0 ×
1
3
-3
10 cm . Alternatively, [RC(O)SCl] was held constant,
1
3
-3
typically 0.7-1.0 × 10 cm , while [H] was varied from 0.5
1
3
-3
to 3.0 × 10 cm .
H + R-C(O)S-Cl f HCl + R-C(O)S
R-C(O)S f R + OCS
(1a)
(1b)
Methoxycarbonylsulfenyl chloride, ClC(O)SCl, and (CH3)3-
COCl were purchased (98% or better purity) from either Aldrich
or TCI America and loaded into the reservoirs after degassing
by a single distillation step. Fluorocarbonylsulfenyl chloride was
synthesized by the reaction of ClC(O)SCl with SbF3 in
H + R-C(O)S-Cl f [R-C(O)S-Cl‚H]
R-C(O)S-Cl‚H] f R-C(O)SH + Cl
(2a)
20
tetrahydrothiophene dioxide. Several distillations were required
to separate pure FC(O)SCl from the solvent and ClC(O)SCl.
Infrared absorption spectra confirmed the presence of FC(O)SCl
and the absence of ClC(O)SCl. The H + Cl2 reaction was used
as a reference reaction for rate constant measurements; the
research grade Cl2 was purchased from Matheson.
[
(2b.1)
f R-C(O)S + HCl
f OCS-Cl + HR
(2b.2)
(2b.3)
(2c)
OCS-Cl f OCS + Cl
Infrared emission from the reactor was collected through NaCl
windows by a backing mirror plus a 5-cm focal length CaF2
lens and focused into a BioRad Fourier transform infrared
spectrometer (FTIR). The instrument used a liquid-nitrogen-
cooled InSb detector (Infrared Associates Inc., D* ) 2.165 ×
We found no evidence to support a secondary reaction of H
atoms with RC(O)S as the major source of OCS emission. The
direct abstraction of Cl atom from sulfur (eq 1a) and the
addition-elimination step (eq 2b.2) both can lead to HCl
11
0.5
-1
1
0
cm Hz W ), which has a cold filter to block radiation
emission. The OCS emission can arise from either the direct
-1
below ∼1850 cm . Each emission spectrum was acquired by
subtracting a thermal background spectrum from the experi-
mental spectrum. The difference spectrum is very sensitive to
o
(
eq 1b) or addition (eq 2c) pathways. Since the ∆Hf (RC(O)-
SCl) are not available, the thermochemistry of these reactions
was analyzed using ab initio calculations plus standard thermo-
chemical methods. An upper limit measurement also was made
for the S-Cl bond energy from the short wavelength limit of
the XeCl(B-X) fluorescence from reactions of CH3OC(O)SCl
and ClC(O)SCl with excited-state xenon atoms. A few survey
experiments were done with the F + CH3C(O)SH reaction to
investigate the possibility of formation of OCS from decomposi-
tion of the CH3C(O)S radical.
-1
the background spectrum near the 1850 cm cutoff. The OCS
emission is also in this region, and care must be exercised to
acquire a suitable background spectrum. All spectra were
-1
collected with 2 cm resolution and corrected for the wave-
length response of the detector which is approximately 5 times
more sensitive for OCS than for HCl emission. The CH3OC-
(O)SCl reaction gave the most intense emission of the RC(O)-
SCl molecules, and a characteristic spectrum is shown in Figure
1
for a pressure of 0.3 Torr and a reaction time of 0.3 ms. The
-1
II. Experimental Methods
major part of the HCl spectrum extends from 3200 to 2400 cm
-
1
and the strong chemiluminescence in the 1900-2100 cm
II.A. The Flow Reactor. The 0.5-m long, 4.0-cm i.d. Pyrex
range is the OCS(∆ν ) -1) emission. The weak features in
3
-1
3
-7
reactor used in this study already has been described.
The
the 2400-2100 cm range are P branch lines from J g 10
-
1
total Ar flow (typically 4.8 mmol s ) was monitored with a
calibrated Hastings flow meter and directed to various inlet ports
of the reactor by needle valves. Tank grade Ar was purified by
passage through 3 molecular sieve-filled traps cooled to 195
K. A gate valve separating the reactor from the Roots blower
and mechanical pump could be partly closed to reduce the flow
velocity; the operating pressure range was 0.3-2.0 Torr. The
hydrogen flow was introduced at the entrance of the reactor
and the reagent was introduced via a four-jet inlet placed just
upstream of the NaCl observation window, which was located
levels of HCl(V ) 1, 2); the R branch bandheads at 3050 and
-
1
3175 cm also are from these high J levels. These small
populations are the residue of the nascent rotational distribution,
13,21
which must have been highly rotationally excited.
Emission
was observed from HF, HCl, and OCS from the FC(O)SCl
reaction, and the shapes of some of the HF lines are not ideal
(see Figure 1). This is a consequence of a poor choice for the
phase factor by the FT software. Reliable relative HFv,J
concentrations were obtained, as deduced from either its P or
R branch lines, when the peak height was measured from the
minimum point below the baseline to the top of the peak of the
distorted lines.
4
0 cm from the entrance of the reactor. The reaction time, ∆t,
could be changed from 0.2-1.2 ms by the throttling valve. The
H2 flow was passed through a microwave discharge to produce
H atoms; the dissociation efficiency has been measured on
II.B. Data Analysis. The relative concentrations of HCl or
HF can be calculated from the area of each vibrational-
rotational line from a rotationally resolved spectrum:
1
4,15
several occasions
to be ∼50%. Reagent gases, except for
CH3OC(O)SCl, were stored in darkened Pyrex bulbs as 10%
mixtures in argon, and their flow rates were regulated by a
Teflon needle valve and measured by the pressure rise in
calibrated volumes. Methoxycarbonylsulfenyl chloride has a
area(V,J)
Av,JR(ω)
[HX(V g 1)] )
(3)
∑
V
∑
J

Observation of OCS Infrared Chemiluminescence
J. Phys. Chem. A, Vol. 104, No. 47, 2000 11015
di
di
dk
l
2
G(V ,V ,V ) ) ω ν +
+
ø ν +
νk +
+
1
3
∑i
i
(
i
)
∑∑ ik
(
i
2)(
2)
2
i kgi
di
d
d
j
k
2
ø l +
y
νi +
ν +
ν +
(4)
ll
∑∑∑ ijk
(
)(
j
)(
k
)
i
jgi kgj
2
2
2
where di is the degeneracy of the vibration νi (d1 ) d3 ) 1, d2
)
2), and l is the vibrational angular momentum due to the
degeneracy of the bending vibration ν2. The frequencies, ωi,
anharmonicity coefficients, øik, and g22 coefficient were taken
9
from the literature. For simplicity, the third-order terms with
anharmonicity constants, yijk, were truncated. The rotational line
positions for Σ-Σ transitions (l ) 0 both in the upper and lower
state) were given by:
l
2
2
E ) G(V ,V ,V ) + (B′ + B′′)m + (B′ - B′′)m
(5)
v,J
1
3
where m ) J + 1 for the R branch and m ) -J for the P branch.
In the bands with l * 0 (Π-Π, ∆-∆, ...), the Q branch was
added for which:
l
2
2
E ) G(V ,V ,V ) + (B′ - B′′)J + (B′ - B′′)J
(6)
v,J
1
3
In the formula for the line positions, B′ and B′′ are the rotational
constants in the upper and lower vibrational state, equal to:
di
di
dj
B ) B - R ν +
+
γ ν +
νj +
(7)
v
e
∑i
i
(
i
)
∑
ij
(
i
)(
)
2
jgi
2
2
Figure 1. Emission spectra from the H + CH
and ClC(O)SCl reactions for ∆t ) 0.3, 0.6, and 0.3 ms and pressure )
.3, 2.0, and 1.0 Torr, respectively. The strong band at 1900-2100
3
OC(O)SCl, FC(O)SCl,
-1
-5
Rotational constants Be ) 0.203457 cm , R1 ) 66.9499 × 10
-
1
-5
-1
-5
cm , R2 ) -33.8966 × 10 cm , and R3 ) 124.959 × 10
0
-
1
-1
cm is the ∆V
3
) -1 emission of OCS. The emission from high J
cm were taken from ref 9. Einstein emission coefficients for
the relative intensities of individual rotational lines were
calculated by the expression:
levels of HCl(V ) 1 and 2) from the H + CH
3
OC(O)SCl reaction is
labeled in both the P and R branches. These spectra have not been
corrected for detector response, which increases by a factor of ∼2 from
-
1
4
100 to 3000 cm and by another factor of ∼5 from 3000 to 2000
3
I ∝ V (2J + 1) exp(-B′J(J + 1)/kT)S S
(8)
-
1
IJ
v
cm . The weaker signal from the ClC(O)SCl reaction illustrates its
-
1
slower reaction rate. Note the scale changes at ∼2100 cm for
-
1
where SIJ is the rotational line intensity or H o¨ nl-London factor
and Sv is the square of the rotationless transition moment, which
is equal to 0.374 D for V3 ) 1. The Einstein coefficients were
assumed to scale with V3 according to the harmonic oscillator
approximation, but to be independent of V1 and V2.
The P and R branch rotational lines are separated by only
0.4 cm , and each line was given a width corresponding to
the spectrometer resolution and co-added to simulate a spectral
band. Calculated spectra from the (2,0,1), (0,2,1), (0,4,1), and
3
CH OC(O)SCl and 3200 cm for FC(O)SCl.
8
The area(V,J) is the product of the peak height and ∆ω, where
ω is the half-width and both P and R branch areas are needed;
∆
R(ω) is the detector response function. The majority of the
rotational distribution is in the 300 K Boltzmann component,
and we normally found the relative vibrational distribution, Pv,
as described below and then summed over the V levels to obtain
relative HX(V g 1) concentrations. The Pv values for HF or
HCl were calculated by dividing the response corrected peak
height by Av,J and the Boltzmann fraction in level J for all
nonoverlapped rotational lines from a given V level. These
numbers were averaged to give Pv. This procedure was repeated
for each V level to get a relative distribution. The total relative
HX concentration was obtained by including P0 from an
empirical estimate for the HX(V ) 0) contribution based on the
-
1
(
0,8,1) levels are shown in Figure 2. The ∆V3 ) -1 spectra do
shift to smaller wavenumbers with increasing vibrational energy,
but a spectrum cannot be assigned to a specific (V1,V2,V3)
distribution because the bands from several OCS(V1,V2,1) levels
are overlapped.
Strong, and resolved, Q branch features from highly excited
CO2 bending levels were the key to assigning the distribution
from the ∆V3 ) -1 spectra from the decomposition of acetic
acid. Although Q branch structure is evident in the calculated
spectra form high V levels (see Figure 2), the experimental OCS
spectra do not show resolved Q branches. Bending excitation
of OCS is expected, since all of the expected precursor radicals
have a bent structure. Because of the nearly commensurate
nature of the ν1(859 cm ), ν2(520 cm ), and ν3(2071 cm )
frequencies, numerous near energy resonances exist. The
vibrational relaxation rates
3a
22
P1-P4 distribution. The ∆V ) -1 Einstein coefficients of HCl
2
-
1
for V ) 1-4 are 40, 70, 90, and 100 s , and those for HF are
-
1
1
95, 334, 423, and 467 s .
The observed OCS emission may be described by the overlap
-
1
-1
-1
of the transitions with ∆V3 ) -1 with combination and hot
l
2
l
2
bands: (V1,V ,V3) - (V1,V ,V3 - 1). The band centers were
10,11
suggest that nearly isoenergetic
calculated using the conventional formula for the vibrational
energy levels:
vibrational levels of OCS will tend to equilibrate in 0.4 ms at
1 Torr Ar. Because of the expected collisional mixing of
9

11016 J. Phys. Chem. A, Vol. 104, No. 47, 2000
Manke and Setser
Figure 3. (A) Schematic presentation of the vibrational levels of OCS.
The division of the levels into boxes (see text) is illustrated for the
(
0,2,1), (0,4,1), and (0,8,1) boxes. (B) Simulations of the ∆V ) -1
3
Figure 2. (A) Calculated ∆V
3
) -1 spectra from four vibrational levels
of OCS at 2 cm resolution. Note the Q branch in the (0,8,1) band.
B) Calculated OCS basis spectra from the (0,8,1), (0,4,1), and (0,2,1)
OCS emission spectrum from the H + CH OC(O)SCl reaction for the
two vibrational distributions shown in panel C. The flat distribution
gives the narrower emission spectrum.
3
-
1
(
energy boxes with normalization to the same height of the strongest
feature of each spectrum.
TABLE 1: Summary of OCS(W ,W ,W ) Energy Box
Description
1
2
3
(
V1,V2,V3) populations of the same total energy, the energy range
average
was divided into boxes, with each (0,V2,1) bending level defining
the center of a box and extending (200 cm . The vibrational
boxa,b
emitting fractionb
Einstein coeff (s-1)c
〉 cm-1
b
〈E
-
1
(
(
0 0 1)
0 1 1)
(0 2 1)
(0 3 1)
0 4 1)
0 5 1)
0 6 1)
0 7 1)
0 8 1)
0 9 1)
(0 10 1)
(0 11 1)
(0 12 1)
0.08
0.11
0.22
0.28
0.31
0.41
0.39
0.45
0.34
0.38
0.46
0.52
0.57
0.53
0.60
0.65
0.68
0.67
0.69
0.67
0.69
0.66
385
385
385
385
415
428
397
454
565
576
595
618
660
599
601
646
663
739
752
816
875
830
2000
2525
3037
3527
4060
4606
5126
5640
6185
6640
7224
7734
8239
8784
9358
9886
-
1
levels and the boxes up to 7000 cm are illustrated in Figure
; emission can arise from V3 g 2, as well as from V3 ) 1 for
3
the (V1,V2 g 4,1) boxes. A basis spectrum from each box was
generated for the purpose of simulating experimental spectra.
The statistical weight of each (V1,V2′,V3) level within a box,
relative to the (0,V2,1) level, was calculated according to eq 9
with Tvib ) 300 K:
(
(
(
(
(
(
N1 g1
)
exp[-(E - E )/kT]
(9)
1
0
N0
g0
(
0 13 1)
(0 14 1)
where N0, N1, g0, g1, E0, and E1 are the concentrations,
degeneracies, and vibrational energies of the OCS(0,V2,1) and
OCS(V1,V2′,V3) levels, respectively, in a box. For example, the
(
(
(
(
0 15 1)
0 16 1)
0 17 1)
0 18 1)
(0 19 1)
(0 20 1)
10355
10901
11444
11967
12519
13048
(0,4,1) box has four radiating states [(0,4,1), (1,2,1), (2,1,1),
(0,0,2)] and six dark states [(0,8,0), (1,6,0), (2,4,0), (3,3,0),
(4,1,0), (5,0,0)]. The populations for the (1,2,1), (2,1,1), (0,0,2),
(
0 21 1)
and (0,4,1) levels are 0.16, 0.02, 0.02, and 0.11, respectively,
and the remaining population is in the dark states. The spectra
from each of the four emitting levels were co-added with their
relative weights and with the appropriate Einstein coefficients
to form the basis spectrum of the (0,4,1) box, which is shown
in the lower panel of Figure 2. The weighted energies of all
levels in a box were combined to give the average vibrational
energy, E(b)i, of a box. Table 1 lists the average Einstein
a
The six energy levels below (0,0,1) are not included in this table.
b
-1 c
See Figure 2 for a listing of levels below 7000 cm
coefficients were assumed to scale with V according to the harmonic
oscillator approximation but to be independent of V and V
. The Einstein
3
2
1
.
coefficients ( Ah i), E(b)i, and the fraction of emitting levels for
the “boxes”. Basis spectra are all normalized to unity. The
assumptions are that levels within each box are equilibrated with

Observation of OCS Infrared Chemiluminescence
J. Phys. Chem. A, Vol. 104, No. 47, 2000 11017
was obtained by extrapolation of the fitted box distribution. A
calculated OCS spectrum from the H + CH3OC(O)SCl reaction
is shown in Figure 3B for a distribution that monotonically
-1
declines with an average energy of 5691 cm (Figure 3C). The
overall shape and width of the spectrum is matched, but the
simulation has slightly more structure (due to Q branches from
the high bending levels) than the experimental spectrum, which
suggests that our model may still slightly overestimate the
contribution from the bending levels. While this distribution is
not unique, a flat distribution (Figure 3C) with nearly the same
-
1
average energy (5898 cm ) does not fit the experimental
spectrum, as shown in Figure 3B. Thus, we conclude that
simulation of the ∆V3 ) -1 spectra can provide 〈Ev(OCS)〉 and
the general shape of the distribution.
It is difficult to assign an uncertainty to the calculated
〈
Ev(OCS)〉. Fits to spectra from nominally identical experiments
-1
for the CH3OC(O)SCl reaction gave uncertainties of (200 cm .
The most serious experimental problem is the proximity to the
cutoff limit of the detector and the associated poor baseline for
some spectra from background subtraction; see the three spectra
shown in Figure 4. However, the model adopted for the
simulation probably introduces the largest uncertainty for
〈
Ev(OCS)〉. We first attempted simulations using just the (0,V2,1)
levels of OCS. These simulations showed too much structure
from Q branches, and it was necessary to use contributions from
(
0,V2,2) or (V1,V2,1) levels of a similar energy but with reduced
ν2 excitation. Such simulations gave nearly the same average
energy as the model that finally was adopted.
2
3
The possible loss of vibrational energy by downward
relaxation from adjacent boxes must be considered. Three
spectra from the CH3OC(O)SCl reaction are shown in Figure 4
that were obtained with pressures of 0.5 or 0.8 Torr and ∆t )
Figure 4. Three representative OCS emission spectra from the H +
3
CH OC(O)SCl reaction showing the reproducibility for nominally
identical conditions (A) and (B) at 0.5 Torr and the small effect of
increased pressure (C) at 0.8 Torr. The spectrum in panel C also was
chosen to illustrate the necessity of obtaining a satisfactory thermal
background spectrum for subtraction.
0
.25 ( 0.05 ms. The band center does slightly shift and the
bandwidth is narrower in the 0.8 Torr spectrum. A small
reduction in the 〈Ev(OCS)〉 is noticeable at 0.8 Torr, although
-1
the trend is barely outside the (200 cm statistical uncertainty.
The spectra for reaction times longer than 0.3 ms or pressures
higher than 0.8 Torr gave 〈Ev(OCS)〉 values that were signifi-
a temperature of 300 K but that relaxation between boxes is
negligible are somewhat arbitrary. The model was chosen to
reflect the expectation that nearly isoenergetic vibrational levels
will be partly equilibrated. Experimental justification of the
model is that the spectra did not change significantly from 0.4
to 0.8 Torr of Ar and the absence of Q branch structure from
high V2 level excitation. The spectra did change at higher
pressure (or longer reaction times), vide infra, as expected from
loss of total vibrational energy of OCS.
2
3
cantly reduced.
In addition to the average vibrational energies, we also wish
to compare the relative concentrations of HF, HCl and OCS
from intensities measured in the same spectrum. For OCS, the
response-corrected total area is related to the total number of
emitting molecules:
total corrected area R∑i
N Ah f (e)
(12)
i
i i
The basis spectra from each box were co-added with
appropriate weights to fit a given experimental spectrum. The
relative intensities from each box, Ii, are obtained from eq 10:
The relative concentration of OCS is the sum, Ntot ) ∑iNi.
Equation 12 can be rewritten in terms of N1, the concentration
for the first box (0,0,1), and xi, which is the ratio Ni/N1:
I ) P f (e) Ah
i
(10)
i
i
i
total corrected area
where Pi is the relative contribution of the ith box to the total
distribution and fi(e) is the fraction of levels that radiate. The
relative contributions from each box were changed until a
satisfactory visual fit to an experimental spectrum was achieved.
The average vibrational energy for the distribution above the
OCS(0,0,1) level is given by eq 11:
N R
(13)
1
x Ah f (e)
∑i
i
i i
and
N ) N xi
(14)
tot
1
∑i
〈
Ev(OCS)〉 ) Pi E(b)i
(11)
∑i
Scaling the N1 value by the sum of xi gives the OCS
concentration in terms of the total band area.
In computing the global average vibrational energy, the six
levels below the energy of the (0,0,1) box must be included.
These six levels were considered as one additional box with an
III. Results
III.A. Rate Constants and Relative Product Concentra-
tions. III.A.1. CH3OC(O)SCl. This reaction could be studied
-
1
average energy of 3.4 kcal mol , and the relative contribution

11018 J. Phys. Chem. A, Vol. 104, No. 47, 2000
Manke and Setser
from the OCS levels below the first box as 0.1, see section
III.B.1, gives [HCl]/[OCS] ) 9.0.
III.A.2. ClC(O)SCl. The emission from this reaction was the
weakest of the three RC(O)SCl reactions, and high H atom and
ClC(O)SCl concentrations were necessary to collect spectra with
acceptable signal-to-noise ratios. A spectrum is shown in Figure
1
3
13
-3
1
∆
for [ClC(O)SCl] ) 1.6 × 10 , [H] ) 3.0 × 10 cm , and
t ) 0.3 ms. The formation of HCl and OCS products were
first-order with respect to [ClC(O)SCl] according to tests for
the differential rate law. Based on intensity measurements
relative to H + Cl2, the rate constant for HCl(V g 0) formation
-
12
3
-1 -1
is ≈1 × 10
cm molecules s . These HCl spectra data
had poor signal-to-noise ratios, and this rate constant is uncertain
to a factor of 2. The IOCS /IHCl(Vg1) ratio was 2.4 ( 0.9, which
corresponds to an average [HCl(V g 0)]:[OCS] ratio of 7.6 (
2
.1. Inclusion of the estimate (25%) for the contribution of the
lowest six levels of OCS reduces the [HCl]/[OCS] ratio to 6.1.
The total rate constant for H atom removal was measured
using the integrated rate law for pseudo-first-order kinetics: e.g.
[ClC(O)SCl] > [H]. The relative [H] at the observation window
was measured by monitoring the relative HF(∆V ) -1)
chemiluminescence generated by adding F2 at the reagent inlet.
The ClC(O)SCl or Cl2 reagent was added at the entrance to the
flow reactor; the total reaction time was ∼4 ms according to
the plug-flow approximation with ideal mixing. The HF(∆V )
-
1) signal was reduced as [Cl2] or [ClC(O)SCl] was increased,
as shown in Figure 5B. The ratio of the slopes of the plots in
Figure 5B is 5.2 ( 0.7. This ratio can be used to obtain a rate
-
12
3 -1
constant of (3.7 ( 0.5) × 10 cm s for removal of H atom
by ClC(O)SCl; the actual reaction time cancels from the ratio.
The difference of 3.7 between the HCl(V g 0) formation rate
constant and the total H atom removal rate constant is considered
significant, and other reaction channels may exist that consume
H and do not give chemiluminescence. However, secondary
reactions involving radicals also may consume some H atoms.
Figure 5. Experimental kinetic data from the H + CH
ClC(O)SCl reactions: (A) plot of [HCl(V g 0)] vs [Cl
O)SCl] for differential rate law conditions (∆t ) 0.3 ms, P ) 0.5
3
OC(O)SCl and
2
] and [CH OC-
3
(
1
2
-3
Torr, and [H] ) 8.0 × 10 cm ); (B) plot of ln [H] vs [Cl
2
] and
[ClC(O)SCl] for integrated pseudo-first-order rate law conditions. The
reaction time corresponds to 4 ms for plug flow approximation to the
flow conditions. The relative [H] was monitored from the HF emission
intensity arising from the H + F reaction; see text for the experimental
2
description.
III.A.3. FC(O)SCl. Plots of [HF(V g 1)], [HCl(V g 1)], and
[
OCS(V3 g 1)] vs [FC(O)SCl] at constant [H] and vs [H] at
constant [FC(O)SCl] were consistent with a first-order depend-
ence of these products on the H and FC(O)SCl concentrations.23
The HCl emission was weak, see Figure 1; however, after
adjustment for Einstein coefficients the [HF(V g 0)]/[HCl(V g
for short ∆t and with [H] and [CH3OC(O)SCl] low enough that
relaxation of HCl(V) was not serious. The differential rate law
was used to examine the dependence of each product on [H]
and [CH3OC(O)SCl]. The data of Figure 5A demonstrate the
first-order dependence of [HCl(V g 1)] on [CH3OC(O)SCl] for
0
)] ratio was equal to 0.47 ( 0.11 for a range of conditions.
The stronger HF(V) emission was used to estimate the HF
formation rate constants by comparison of [HF(V g 1)] to the
1
2
-3
[HCl(V g 1)] from H + Cl in a plot like Figure 5A. The HF(V
∆
t ) 0.24 ms and [H] ) 8.0 × 10 molecules cm . Similar
2
-
13
3 -1
23
g 1) formation rate constant is 4.1 × 10 cm s . The P :P
data show that [OCS] formation also was first-order with
respect to [CH3OC(O)SCl], [H], and ∆t. The HCl(V g 1)
formation rate constant, relative to H + Cl2, was obtained as
0
1
-
13
ratio was estimated as 0.9, and k(HF(V g 0)) is 5.4 × 10
3
-1
cm s . Since the [HF(V g 0)]:[HCl(V g 0)] ratio is 0.47, the
-
12
3 -1
-
12
3
-1
total rate for [HX(V g 0)] formation is ∼1.7 × 10
cm s ;
2
.6 × 10
cm s from the ratio (7.8) of the slopes of the
-
11
the estimated uncertainty was assigned as a factor of 2 largely
because the lack of sample limited the number of experiments.
The average IHF/IOCS and IHCl/IOCS ratios were 2.5 ( 0.8 and
plots in Figure 5A and the recommended value (2.06 × 10
3
-1
24
cm s ) for k(H + Cl2). Adjustment for the unobserved
molecules in V ) 0 (P0 ) P1 ) 0.25), see section III.B.1,
increases [HCl] by a factor of 1.25, and since P0 from H + Cl2
is negligible, the rate constant for HCl(V g 0) formation is 3.4
0
.95 ( 0.16, respectively. Converting these numbers to con-
centration ratios gave [HF(V g 0)]/[OCS] ) 2.5 ( 0.9 and
[HCl(V g 0)]/[OCS] ) 4.8 ( 0.9; assignment of a 10%
contribution to OCS levels below the first box, see section
III.B.3, lowered these ratios to 2.3 and 4.4.
-
12
3 -1
×
10
cm s . This rate constant corresponds to the sum of
eqs 1a and 2b.2. Based on three such experiments, the rate
-
12
3 -1
constant for HCl(V g 0) formation is (3 ( 1) × 10 cm s .
The product intensity ratio adjusted for response, IOCS/IHCl(Vg1)
was 2.3 ( 0.9 and, within the experimental uncertainty,
independent of [H], [CH3OC(O)SCl], and ∆t. The kinetic data
imply that OCS formation is a primary reaction. After adjust-
ment for the difference in Einstein coefficients and inclusion
of HCl(V ) 0), the intensity ratio gives a concentration ratio of
III.A.4. (CH3)3COCl. Even for the fastest pumping speed and
low [H], a strong HCl signal was observed from the H + (CH3)3-
COCl reaction. Tests of the differential rate law showed first-
order dependence of the HCl emission intensity on [H] and
12
-3
[(CH3)3COCl] for concentration ranges of 2-19 × 10 cm .
Comparison of the HCl emission intensity with those of the H
+ Cl2 reaction gave a rate constant for HCl(V g 0) formation
[
HCl(V g 0)]/[OCS] ) 9.9 ( 3.6. Estimation of the contribution

Observation of OCS Infrared Chemiluminescence
J. Phys. Chem. A, Vol. 104, No. 47, 2000 11019
TABLE 2: Summary of Results for H + RC(O)SCl
〈
E
v
(HCl)〉e
〈E
v
(OCS)〉
d
a
3
(kcal mol-1)
-1
reactant
OC(O)SCl
ClC(O)SCl
FC(O)SCl
rate constant (cm s-1)
[HCl]:[OCS]
(kcal mol
)
-
12
9.9 ( 3.6 (9.0)d
d
CH
3
3.0 ( 1.0 × 10
12.6
9.5
12.7c
17.6
12.7
15.7 (14.6)
9.7 (8.4)
18.6 (17.2)
-
-
-
12 b
12 b
12 c
d
d
≈1.0 × 10
∼1.2 × 10
7.6 ( 2.1 (6.1)
d
d
4.8 ( 0.9 (4.4)
c
d
∼
0.54 × 10
2.5 ( 0.9 (2.3)
-
11
(
CH
3
)
3
COCl
1.5 ( 0.2 × 10
a
Rate constant for formation of HCl(V g 0). These rate constants have an uncertainty of a factor of 2 because of the weak signals. c For HF(V
b
d
g 0) formation. These entries include only the emitting levels of OCS. Estimates (see text) for the contributions of the six nonemitting levels to
e
the OCS concentration give the numbers in parentheses. For HCl(V g 0) and HF(V g 0); see Table 3 for the distributions and the text for the
0
estimation of P .
a
TABLE 3: Summary of Thermochemistry and the HCl(W) and HF(W) Distributions from the H + RC(O)SCl Reactions
oa
HC(V) or HF(V) distribution
∆
H
0
reactant
OC(O)SCl
product
(kcal mol-1)
P
0
P
1
P
2
P
3
P
4
CH
3
f CH
f CH
f CH
3
3
3
OC(O)S + HCl
O + OCS + HCl
OH + OCS + Cl
-43
-26
-28
-44
-46
-43
-12
-40
-55
25 ( 4
25 ( 4
24 ( 3
17 ( 2
9 ( 2
ClC(O)SCl
FC(O)SCl
f ClC(O)S + HCl
f Cl + OCS + HCl
f FC(O)S + HCl
f F + OCS + HCl
f Cl + OCS + HF
38 ( 5
25 ( 5
29 ( 5
25 ( 5
18 ( 3
23 ( 3
9 ( 2
6 ( 2
17 ( 2
10 ( 2
23 ( 5
15 ( 3
26 ( 5
32 ( 3
27 ( 4
38 ( 2
15 ( 3
13 ( 2
9 ( 2
2 ( 1
(
CH
3
)
3
COCl
3 3
f (CH ) CO + HCl
a
o
+ 4.5RT. Since the uncertainty in ∆H o
The available energy is usually defined as 〈E〉 ) ∆H
0
+ E
a
0
is comparable to the thermal energy and
o
the E
a
is unknown, we just used the ∆H
0
values as the available energy.
of (1.5 ( 0.2) × 10-11 cm s ; the P0 value was taken as 0.5P1.
These experiments were much easier than for the RC(O)SCl
molecules, because the (CH3)3COCl rate constant is an order
of magnitude larger, with a correspondingly higher HCl(V)
emission intensity.
3
-1
distribution declines with Ev, but it extends to ∼13000 cm
-1
-
1
-1
(37 kcal mol ) and gives 〈Ev(OCS)〉 ) 15.7 kcal mol . The
relative contribution from the box corresponding to the six
lowest levels was estimated as 0.1 and the global 〈E (OCS)〉
becomes 14.6 kcal mol , which is just slightly larger than
v
-
1
III.B. Vibrational Distributions. III.B.1. CH3OC(O)SCl.
〈E (HCl)〉. As discussed previously, 〈E (OCS)〉 was invariant
v
v
Collisions with hydrogen atoms are the principal relaxation
for a change in Ar pressure of a factor of 2 for ∆t ) 0.3 ms.
process for HCl(V) in the flow reactor,1
4,15,19,25
and the best
The OCS spectrum also was independent of [CH OC(O)SCl]
3
1
2
-3
distribution was determined from spectra obtained at the lowest
from 0.7 to 8.0 × 10 cm . We believe that the 〈E (OCS)〉
v
1
2
combination of [H] and ∆t for [CH3OC(O)SCl] e 8.0 × 10
should be close to the nascent vibrational energy.
-
3
cm . The average HCl(V ) 1-4) distribution from 14
independent spectra was 33 ( 4:32 ( 4:23 ( 2:12 ( 2. These
experiments included a set with constant [H] ) 7.0 or 8.0 ×
12
After the box distribution for OCS is assigned, the relative
concentrations of HCl and OCS can be estimated from the
experimental IHCl/IOCS intensity ratio of 2.3 ( 0.9. As noted in
section III.A.1, this ratio corresponds to [HCl]/[OCS] ) 9.0.
The “average” Einstein coefficient for OCS is ∼5 times larger
than for HCl and, irrespective of the simulation model chosen
for OCS, the [HCl] is greater than the [OCS].
1
2
-3
10
cm and variable [CH3OC(O)SCl] from 0.7 to 8.0 × 10
3
-
cm ; the HCl(V ) 1-4) distribution was invariant. The reaction
time, ∆t, was 0.22 or 0.24 ms. The measured HCl(V g 1)
distribution for H + Cl2 for the same [H] and approximately
equivalent [Cl2] was 12:39:40:9, which matches the accepted
nascent distribution1
4,15
A few experiments were done with the D + CH3OC(O)SCl
reaction. The DCl emission was not very useful, because it was
weak and partly overlapped by the OCS emission. The OCS
band, however, was strong and matched that obtained from H
to within the combined uncertainties.
This comparison shows that relaxation from the [H] was not
important and the HCl(V) distribution from H + CH3OC(O)SCl
should be the nascent distribution. Since the nascent HCl(V g
+
CH3OC(O)SCl, which provides confirmation of the 〈Ev(OCS)〉
given in Table 2.
1
) distribution from reaction 1 is rather flat, we set P0 ) P1.
This distribution, which is given in Table 3, corresponds to
-
1
〈
Ev(HCl)〉 ) 12.6 kcal mol .
III.B.2. ClC(O)SCl. Experiments with the lowest [H] and ∆t
gave a HCl(V) distribution of P1-P4 ) 47:28:15:10. The HCl(V)
spectrum from H + Cl2 under similar conditions was nearly
nascent, and this HCl(V) distribution should be close to the
nascent distribution. Since the vibrational distribution declines
with increasing V, P0 was set equal to 1.3P1 and the P0-P4
distribution is 38:29:18:9:6, which gives 〈Ev(HCl)〉 ) 9.5 kcal
The HCl spectrum in Figure 1 shows the presence of high
rotational lines, up to J ) 25 in the P branch, and the R branch
bandheads were also observed. These features were consistently
observed from experiments with pressure e 0.6 Torr, while at
higher pressures only emission from the Boltzmann component
was observed. From averaging over several experiments, the
population in the high J component (J ) 10-25) represented
-
1
mol . The HCl(V) distributions were invariant for [ClC(O)-
+
4
2
21
12
5
% of the total HCl(V g 1) distribution. It seems likely that
SCl] ) 8.0-16.0 × 10 and ∆t ) 0.15-0.25 ms; however,
-
1
3
-3
the nascent HCl(V) distribution had a much higher population
for J g 10, and the reaction must release considerable rotational
energy to the HCl(V) product.
A representative OCS spectrum and the corresponding box
distribution from CH3OC(O)SCl are shown in Figure 3B,C. The
increasing [H] from 0.8 to 3.0 × 10 cm gave some
relaxation. High rotational lines of HCl(V ) 1,2) were not
observed for any conditions from the ClC(O)SCl reaction. The
absence of the high rotational levels and the lower 〈Ev(HCl)〉,
relative to the CH3OC(O)SCl reaction, suggests that interaction

11020 J. Phys. Chem. A, Vol. 104, No. 47, 2000
Manke and Setser
0
.5 [H2] or ∆t > 0.3 ms. For the purpose of calculating the
〈
0
Ev(HF)〉 and 〈Ev(HCl)〉 in Table 2, we assumed P0(HF) )
.9P1(HF) and P0(HCl) ) P1(HCl). These distributions cor-
-
1
respond to 〈Ev(HCl)〉 ) 12.7 and 〈Ev(HF)〉 ) 17.6 kcal mol .
These 〈Ev〉 are considered lower limits; however, relaxation will
affect a flat distribution to a lesser degree than the HCl(V)
distribution from H + Cl2.
The OCS spectra from the FC(O)SCl reaction were the most
difficult to analyze, because the spectra extended closer to the
cutoff limit of the detector, indicating a high 〈Ev(OCS)〉.
Although the distribution extends to higher Ev, the shape of the
distribution is similar to that of the CH3OC(O)SCl reaction,
compare Figures 6A (lower panel) and 3C. A flat distribution
is not suitable for the FC(O)SCl reaction as shown in Figure
6
A. Extrapolation suggests that ∼10% of the distribution could
be in the six nonemitting levels, and the 〈Ev(OCS)〉 is lowered
from 18.6 to 17.2 kcal mol . Since some relaxation may have
-
1
occurred based on the H + Cl2 experiment, the 〈Ev(OCS)〉,
〈
Ev(HF)〉, and 〈Ev(HCl)〉 values are considered to be lower limits.
Nevertheless 〈Ev(OCS)〉 is larger than 〈Ev(HCl)〉 and comparable
to 〈Ev(HF)〉. Although the OCS band appears very strong in
Figure 1, adjustment for the Einstein coefficients gives a smaller
OCS yield than for HF or HCl, and the global [HF]/[OCS] and
[HCl]/[OCS] ratios are 2.5 and 4.8, respectively.
III.B.4. (CH3)3COCl. The average P1-P4 distribution from
seven experiments at low [H] and short ∆t was 38 ( 4:45 (
2
:15 ( 2:2 ( 1. This distribution showed no dependence on
1
2
-3
[
(CH3)3COCl] from 4-19 × 10 cm . The vibrational
distribution from H + Cl2 for the same experimental condition
Figure 6. Comparison of the experimental and simulated ∆V
OCS emission spectra for the (A) H + FC(O)SCl and (B) H + ClC-
O)SCl reactions. The vibrational energy distributions for each reaction
are shown in the lower panels; in each case the flat distribution gives
the narrower emission spectrum.
3
) -1
15a
was nascent and the HCl(V) distribution should be reliable.
The P0 contribution was set as 0.5P1 and the overall P0-P4
distribution becomes 15 ( 3:32 ( 3:38 ( 8:13 ( 2:2 ( 1 with
(
-
1
15a
〈
Ev(HCl)〉 ) 12.7 kcal mol . The H + CF3OCl reaction had
a similar, but more sharply peaked, distribution with P1-P4 )
3:47:26:4.
with the C-Cl bond contributes to HCl formation, as well as
the interaction with the S-Cl bond.
2
The OCS spectrum from ClC(O)SCl is much narrower and
shifted to higher wavenumbers than the spectra from either
CH3OC(O)SCl or FC(O)SCl; compare Figure 6B to Figures 3B
and 6A. The OCS distribution does not extend beyond 6000
IV. Discussion
IV.A. Thermochemistry. The available energy for the H +
RC(O)SCl reactions cannot be calculated in the usual way
because the ∆Hf (RC(O)SCl) values have not been reported.
-
1
-1
o
cm (17 kcal mol ), and it has higher populations in the first
few boxes than for the other two reactions. A flat distribution
also could be used to simulate the spectrum from ClC(O)SCl,
but we chose the same shape as for the CH3OC(O)SCl reaction.
Our attempt to estimate the available energies for these reactions
is summarized in the Appendix; the results are tabulated in Table
3 and shown in Figure 7. Two available energies are listed in
Table 3 for formation of HCl: the first corresponds to removal
of the Cl atom from the sulfur site, and the second is for HCl
formation with dissociation of the RC(O)S radical. These
radicals can be bound by a few kcal mol and the overall ∆H°
for formation of OCS + R + HCl can be less than for reaction
1a. Even if the subsequent dissociation of the RC(O)S radicals
is exoergic, the energy available to HCl formed by direct
abstraction will be released in the first step. According to our
calculation, the C(O)SCl radical is not bound, and formation
of HF from FC(O)SCl has only one entry in Table 3. Formation
of HCl from the carbon site of ClC(O)SCl has the same ∆H°
as the second entry (HCl + OCS + Cl) in Table 3, but the
mechanism is different from reaction at the sulfur site. The
expectation of a weakly bound RC(O)S radical is supported by
-
1
The 〈Ev(OCS)〉 of the emitting levels is 9.7 kcal mol ; the
contribution from the six nonemitting levels was set as 25%,
-
1
which reduces 〈Ev(OCS)〉 to 8.4 kcal mol . Within their
combined uncertainties, 〈Ev(OCS)〉 is equal to 〈Ev(HCl)〉.
III.B.3. FC(O)SCl. The presence of the HF(V) and HCl(V)
emissions demonstrates that reaction can occur with halogens
at both the carbon and sulfur atoms. From limited data due to
restriction on the available sample, the average HF(vs) distribu-
tion was P1-P4 ) 33 ( 3:35 ( 5:20 ( 3:12 ( 3, while the
average HCl(V) distribution was P1-P4 ) 35 ( 3:31 ( 2:21 (
-
1
2
:13 ( 2. The HF and HCl distributions were investigated over
13
-3
a moderate range of [H] ) 2.0-4.0 × 10 cm , [FC(O)SCl]
1
2
-3
)
6.0-11.0 × 10 cm for ∆t ) 0.3 ms. In general, HF(V)
distributions are less affected by high [H] than are HCl(V)
distributions25 and the HF(V) distribution should be close to the
nascent distribution. However, the HCl(V) distribution, P1-P4
-1
the ab initio calculations for FC(O)S (31 kcal mol ) and
-
1
CH3OC(O)S (17 kcal mol ), but not for ClC(O)S. The latter
did have a stable structure and apparently a potential energy
barrier exists for exoergic dissociation to Cl + OCS. Although
the calculations may overestimate the difference between the
dissociation energies of FC(O)S and ClC(O)S, a similar trend
)
25:43:27:5, from H + Cl2 for these conditions was somewhat
relaxed. The [H] was slightly larger than for the best conditions
used for the ClC(O)SCl reaction, and apparently the difference
was enough to cause observable relaxation from the strongly
inverted HCl(V) from H + Cl2. It is also possible that [H] >
2
6
was reported for D(Cl-CO2) vs D(F-CO2).

Observation of OCS Infrared Chemiluminescence
J. Phys. Chem. A, Vol. 104, No. 47, 2000 11021
IV.B.2. RC(O)SCl Molecules. The arguments below suggest
that the event associated with HCl formation from reaction at
the sulfur site is not responsible for formation of the vibrationally
excited OCS molecules.
(1) A set of experiments in which F atoms were reacted with
thioacetic acid, CH3C(O)SH, gave very strong HF emission plus
weak OCS emission from only low vibrational levels.23 The
estimated [OCS]-to-[HF] ratio from the interaction of F atoms
with just the S-H bond was 1:20. Even in this favorable case,
hydrogen abstraction followed by dissociation of CH3C(O)S to
-
1
give OCS with 10-12 kcal mol of vibrational energy does
not occur.
o
(2) If the calculated ∆H rxn values are correct for FC(O)SCl
and CH3OC(O)SCl, the available energy is much too small for
both HCl and OCS to be excited to the levels cited in Table 3
for reactions 1a and 1b. Furthermore, a potential energy barrier
probably exists for dissociation of these RC(O)S‚ radicals, thus
requiring even more energy.
(3) The OCS yield is consistently much smaller than the HCl
yield. The total rate constant associated with loss of [H] seems
to be larger than the rate constant for just HCl formation, and
other reaction pathways exist. However, the experimental data
for the latter claim are based only on the ClC(O)SCl reaction.
(4) The observation of HF from FC(O)SCl shows that the H
atom can interact with a halogen attached to either carbon or
sulfur. Direct attack by H atoms on C-Cl or C-F bonds usually
has a large activation energy, and the HF formation presumably
proceeds by H atom addition followed by unimolecular re-
arrangement from one or more intermediates.
(5) The ab initio calculations predict that the RC(O)S radicals
are bound, whereas the C(O)SCl radical is dissociative.
Figure 7. Schematic representation of the calculated energetics for
the carbonylsulfenyl reactions (see Appendix for details). No attempt
For these reasons, we will discuss the reaction mechanism
as an interaction with the S-Cl site that can be either direct or
proceed by an intermediate, RC(O)SCl‚H. The RC(O)SCl‚H
intermediate subsequently can dissociate to HCl and RC(O)S
or it can rearrange to give RH + OCSCl; the OCSCl radical
will dissociate to OCS + Cl. The ab initio D(Cl-SC(O)R)
values are all similar, and the available energy for formation of
HCl from the sulfur site is essentially the same for the three
reactions. In our opinion, the most likely sites for H atom
addition to the RC(O)SCl molecules is the O or S atom. Addition
of an H atom to the carbon site would generate an oxygen-
centered radical that would subsequently dissociate by rupture
was made to identify intermediate species for the CH
3
OC(O)SCl + H
system, except for the CH OC(O)S radical. The energy of a third set
3
-
1
of possible products (HF + CO + SCl) lies 25 kcal mol below H +
FC(O)SCl in the bottom panel.
The ∆Hfo values for (CH3)3CO, -21.7 kcal mol , and
-1
-
1
27
(
CH3)3COCl, -40 kcal mol , have been reported. Based on
these values, the available energy for the H + tert-butyl
-
1
o
hypochlorite reaction is ≈56 mol . The ∆Hf ((CH3)3COCl),
which is based upon liquid-phase photoinitiated calorimetry
and may not be very reliable, gives D298((CH3)3CO-Cl) ) 47.6
2
7
-1
29a
kcal mol . Within the experimental uncertainties, the available
of the C-S bond to give a stable aldehyde. A theoretical
energies for the CF3OCl1 and (CH3)3COCl reactions are the
5b
29b
study of H + F2CO favored addition of the H atom to the
-
1
-1
same, and we will use 55 kcal mol as the available energy
for both cases.
oxygen site, but the activation energy was ∼15 kcal mol ; the
FC(OH)F radical decomposes by dissociation back to H + F2CO
or by HF elimination, with the latter having the higher threshold
energy. We attempted to generate HCl emission from reacting
H atoms with CH3C(O)Cl in our flow reactor, but no emission
was observed and an activation energy exists for both abstraction
and addition. Given the small rate constants for the primary H
+ RC(O)SCl reactions and the low yield of OCS relative to
HCl, the generation of some OCS from a secondary reaction of
H with the RC(O)S radicals cannot be excluded, even though
we found no experimental evidence for a second-order depend-
ence of the [OCS] yield on [H].
IV.B. Mechanisms. IV.B.1. (CH3)3COCl. For an available
-
1
energy of 55 kcal mol , the HCl(V) distribution of Table 3
corresponds to a fractional vibrational energy (〈fV(HCl)〉 )
〈
(
Ev(HCl)〉/〈Eavailable〉) of 0.23, which is smaller than for CF3OCl
0.28)1 and Cl2O (0.32). The shape of the HCl(V) distribution
5b
14
is similar to the other abstraction reactions, but the V ) 3 and
populations are slightly smaller than for CF3OCl. As expected
4
for direct halogen atom abstraction reactions by H atoms, the
HCl(V) distributions do not extend to the thermochemical
limit.1 The tert-butoxy radical readily dissociates to acetone
4,15
2
8
and CH3, and the tert-butoxy radical may store some vibra-
tional energy as it relaxes from the equilibrium (CH3)3COCl
geometry to the (CH3)3CO structure. The available evidence
supports a direct Cl abstraction mechanism, but with some
special dynamics associated with the tert-butyl group. It is
interesting that the rate constants for the CF3OCl and (CH3)3-
COCl reactions are nearly the same.
The CH3OC(O)SCl reaction favors HCl over OCS by a factor
-
1
of 9. For an available energy of 43 kcal mol , 〈fV(HCl)〉 ≈
0.3, which is consistent with the H atom reactions with the
inorganic sulfur chlorides (SCl2, SOCl2, S2Cl2), although those
1
3
reactions gave more inverted HCl(V) distributions. Of special
note is the release of high rotational energy to the HCl(V ) 1,
2) product, which also was observed for the H + SCl2 reaction.13

11022 J. Phys. Chem. A, Vol. 104, No. 47, 2000
Manke and Setser
The high rotational energy was associated with a mechanism
involving H addition to the sulfur atom followed by migration
of the H to the chlorine atom with high angular momentum
being acquired in the migratory motion. By analogy, identifica-
tion of a highly rotationally excited HCl(V ) 1, 2) distribution
suggests the addition of H to the sulfur atom in the CH3OC-
0.43, and 0.19 for the CH3OC(O)SCl, FC(O)SCl, and ClC(O)-
SCl reactions, respectively. The second pathway involves
addition, reaction 2a, probably at the sulfur atom, followed by
unimolecular decomposition, reactions 2b.3 + 2c, giving HR
+ OCS (+Cl). Ab initio calculations were employed to estimate
the reaction exoergicity and hence ∆H°f(RC(O)SCl). Interpreta-
tions for the mechanism for OCS formation depend, in part,
upon the bond dissociation energy for the R-C(O)S radicals,
and experimental verification of the thermochemistry of these
interesting radicals is needed. We have argued that the dis-
sociation of R-C(O)S is not the major pathway for formation
of OCS in the H + RC(O)SCl systems, rather OCS arises from
dissociation of OCS-Cl. Although we favor the addition
mechanism, eq 2a followed by eqs 2b.3 + 2c, for the formation
of OCS, a contribution from the H + RC(O)S secondary reaction
cannot be totally excluded. Analogy to the unimolecular
dissociation of the H-C(O)-ONO and CH3-C(O)-ONO
(
O)SCl reaction. The OCS product had an average vibrational
-1
energy of ∼15 kcal mol . If we associate OCS formation with
-
1
the CH3OH + OCS + Cl channel with ≈28 kcal mol of
available energy, 〈fV(OCS)〉 ) 0.52. The mechanism could be
four-centered elimination from the intermediate formed by H
atom addition to the sulfur atom, and the dynamics may
resemble the more completely characterized formation of CH4
+
CO2 from the unimolecular decomposition of acetic acid,
which had 〈fV(CO2)〉 ) 0.32.
3
The HCl(V) distribution from FC(O)SCl corresponds to
-
1
〈
fV(HCl)〉 ) 0.29 for an available energy of 43 kcal mol . The
channel involving HF formation gives 〈fV(HF)〉 ) 0.44 and
molecules formed by HCO and CH3CO reacting with NO2 is
-
1
5,30
〈
fV(OCS)〉 ) 0.43 for an available energy of 40 kcal mol .
interesting.
In this case, dissociation to give NO and HC-
These values seem rather high, but the available energy probably
has been underestimated. A higher 〈fV(OCS)〉, relative to the
CH3OC(O)SCl reaction, is plausible due to the smaller number
of vibrational modes in FC(O)SCl‚H. The ab initio calculations
also identified a bound FC(S)OH intermediate, but formation
of HF + OCS from this intermediate would provide even less
available energy. The more serious problem with the proposed
mechanism is the 2-fold larger yield of HF relative to OCS.
Some possible reasons for the apparent low yield of OCS could
be uncertainty in the intensity measurements, e.g., failure to
record emission below the detection limit or underestimation
of the concentration of the OCS(V3 ) 0) molecules by the
simulations model or because of incorrect Einstein coefficients
for the high energy OCS molecules. Although reservation should
be maintained about the mechanism, other plausible processes,
e.g., the FC(S)OH intermediate or the H + FC(O)S secondary
reaction, also would give [HF] ≈ [OCS]. The available energy
from formation of HFCO + SCl seems too low for subsequent
HF elimination from HFCO to be important.
(O)O, or CH3C(O)O, is followed by rupture of the H, or CH3,
with formation of vibrationally excited CO2 molecules that are
observed by infrared chemiluminescence.
The H + (CH3)3COCl reaction was investigated as a reference
system involving direct Cl atom abstraction from a large
molecule. The 〈fV(HCl)〉 ) 0.23 is somewhat lower than for
chlorine atom abstraction from smaller molecules containing
the S-Cl or O-Cl bond. The rate constant for (CH3)3COCl
was 1 order of magnitude larger than for the carbonylsulfenyl
-
12
3
-1
molecules, which have rate constants of 1-3 × 10
cm s
for HCl formation.
Acknowledgment. This work was supported by National
Science Foundation Grant CHE-9505032. We thank Dr. Pedro
Mui n˜ o (Kansas State University) for extensive discussions and
advice about the ab initio calculations.
Appendix
The S-Cl bond dissociation energies for SCl2, S2Cl2, and
The fraction of the energy released to HCl (0.21) and OCS
-1
S(O)Cl2 have been reported as 45, 46, and 54 kcal mol ,
(0.19) from ClC(O)SCl is the lowest of the three reactions. The
respectively, from the highest observed vibrational energy of
the XeCl(B) and HgCl(B) products from the reactions of
energy available to HCl by direct abstraction from the sulfur
site is similar for all three reactions, and the lower 〈fV(HCl)〉
from ClC(O)SCl probably reflects a contribution to the HCl(V)
yield by formation of HCl from the chlorine atom on the carbon
center. Possible explanations for the low 〈Ev(OCS)〉 might be a
role for other intermediates in OCS formation, such as ClC-
3
31
Xe(6s[3/2]2) and Hg(6s6p, P0) atoms with these molecules.
A similar approach was attempted for ClC(O)SCl and CH3OC-
(O)SCl using the metastable Xe atom reactions, e.g.:
Xe(6s[3/2] ) + ClC(O)SCl f XeCl(B) + ClC(O)S (A-1)
2
(S)OH or ClC(O)SH, which were identified in ab initio
calculations, or dissociation of some of the ClC(O)S radicals.
The XeCl(B-X) emission was strong and the spectra gave
-
1
the same upper limit for D(S-Cl) of 64 ( 2 kcal mol for
each molecule. This value may be too large, but this experiment
suggests that these bonds are stronger than those of the inorganic
sulfur halides. Reactions of H atoms with R-S-Cl compounds
V
V. Conclusions
The major objective of this project was to evaluate the utility
of the OCS(∆ν3 ) -1) infrared chemiluminescence as a probe
of state-resolved reaction dynamics. The emission is relatively
easy to detect since the Einstein coefficient is large and the ∆V3
typically give 〈f (HCl)〉 ≈0.3 and the highest HCl(V,J) level
1
3,14
does not extend to the thermochemical limit.
Thus, observa-
-
1
)
-1 band lies in a sensitive region of the InSb detector. Unlike
tion of HCl(V ) 4), which gives D(S-Cl) e 70 kcal mol ,
does not provide a useful estimate for the bond energies of the
RC(O)S-Cl molecules.
CO2, excitation in the bending mode of OCS cannot be
distinguished from ν3 excitation, and only the total vibrational
energy can be assigned from computer simulation of the ∆V3
Experimental D(R-C(O)S) values would be useful, because
o
o
32
)
-1 spectra. Short times (e0.3 ms) and low pressures (e0.6
f f
the ∆H (OCS) and ∆H (R) values are known and then the
o
Torr of Ar) must be used to avoid vibrational relaxation in a
flow reactor experiment.
f 2
∆H (RC(O)S) could be evaluated. The HCO and HC(O)S
radicals have been studied. The most recently measured D(H-
3
3
-1
The H + RC(O)SCl reactions seem to occur by two pathways.
One involves interaction with the S-Cl end of the molecule to
give HCl with 〈fV(HCl)〉 ≈ 0.3. The 〈fV(OCS)〉 values were 0.52,
CO2) value is -13 ( 3 kcal mol , which agrees with the
result that can be obtained from the thermochemistry of
HC(O)OH and D(HC(O)O-H). The ab initio calculated values
32

Observation of OCS Infrared Chemiluminescence
J. Phys. Chem. A, Vol. 104, No. 47, 2000 11023
for D(H-CO2)34 are approximately 0 kcal mol , and the
calculated D(H-COS)35 is 8.1 kcal mol . According to the
calculations, both radicals have significant barriers, 10-20 kcal
-1
calculated difference between CH3O + OCS + HCl and CH3OH
-1
-1
+ OCS + Cl is 1.3 kcal mol , which matches the experimental
-1
difference of 2 kcal mol . On the basis of the energy difference
-
1
16
mol , for dissociation. Experiments in which HCO2 and
HOCO are generated by the F + HC(O)OH reaction suggest
that HCO2 dissociates whereas HOCO does not. In our labora-
tory, we investigated the F + HCOOD reaction in the flow
reactor and searched for CO2 emission without success. In fact,
the thermochemistry for stepwise formation of DF + (CO2 +
H) is unfavorable for observation of ∆V3 ) -1 infrared emission
from CO2. We can conclude that H-CO2 and H-C(O)S are,
at best, weakly bound but that small barriers may exist for their
dissociation.
calculated with the 6-311++G** basis sets and the energy for
o
the CH3O + OCS + HCl limit, we recommend ∆H f(CH3OC-
-
1
(O)SCl) ) -76 kcal mol ; the uncertainly is expected to be
-
1
between 5 and 10 kcal mol .
The calculated structure and vibrational frequencies for
ClC(O)SCl were in agreement with the experimentally measured
3
8
39
structure and vibrational frequencies. The calculated energy
-
1
difference, 46 kcal mol , between HCl + OCS + Cl and H +
o
ClC(O)SCl using the 6-311++G** basis set gave ∆H f,298(ClC-
-
1
(O)SCl) ) -32 kcal mol . The calculated S-Cl bond energy
-
1
is 51 kcal mol , but using the experimental D(HCl) and the
The CH3-CO2, F-CO2, and Cl-CO2 radicals also have been
-
1
o
43 kcal mol energy difference between H + ClC(O)SCl and
examined. Since D298(CH3C(O)O-H) and ∆H f(CH3C(O)OH)
-
1
o
-1
HCl + ClC(O)S gives D(ClC(O)S-Cl) ) 59 kcal mol .
Searches were made for several intermediates, and bound
structures were found for the radical corresponding to the
addition of H atoms to oxygen, ClC(OH)SCl, the ClC(S)OH
molecule from Cl displacement, and the thiol counterpart,
ClC(O)SH. The ClC(O)S radical had a stable structure, but the
are established, ∆H f(CH3-CO2) ) -50 kcal mol is reliable
-1
32
and D298(CH3-CO2) ) -10 kcal mol can be accepted. This
is in accord with the observation36a that the decomposition of
methyl acetate to give CH3 and CH3CO2 is followed by the
dissociation of CH3-CO2. The decomposition of CH3-CO2 also
has been observed from the reaction of CH3C(O) with NO2,
which proceeds by association followed by dissociation of
-
1
dissociation is 2.4 kcal mol exoergic. The reaction rate for
Cl + OCS is quite slow, as expected since ClC(O)S and
OCSCl are not stable radicals.
4
0
CH3C(O)O-NO.3
0,36b
The D(CH3-C(O)S) can be estimated
from the S-H bond dissociation energy of CH3C(O)SH and
o
o
32
o
The calculated structure of FC(O)SCl is in agreement with
the ∆Hf (OCS) and ∆Hf (CH3). For ∆H f,298(CH3C(O)SH) )
4
1
-1
the experimentally determined geometry. According to the
-
39.7 and D298(H-SC(O)CH3) ) 93 kcal mol , the dissocia-
o
3
1d
results from the 6-31G** basis set, ∆H f(FC(O)SCl) ) -77
tion of CH3C(O)S is thermoneutral. Similar arguments from
-
1
and D(FC(O)S-Cl) ) 52 kcal mol . The calculated energy
differences between HCl + OCS + F and two other sets of
products, HF + CO + SCl and HF + OCS + Cl, were -13.0
dimethyl carbonate, CH3OC(O)OCH3, suggest that D(CH3O-
C(O)O) is approximately 0 kcal mol . According to the ab
initio results, D(Cl-CO2) and D(F-CO2) are -30 and 5 kcal
mol , respectively. This summary suggests that RC(O)S
radicals may have comparable or slightly higher binding energies
than RCO2 radicals; however, the actual energy change in the
dissociation step should be small.
-
1
2
6
-
1
-
1
and -28.3 kcal mol , respectively, which are within 5 kcal
-
1
-1
mol of the experimental values, -17.6 and -33.0 kcal mol ,
respectively.
References and Notes
Since the experimental data were limited, we used the
Gaussian 92 ab initio package to calculate optimized geom-
etries and total energies for the main species of interest
37
(1) Polyani, J. C. Angew. Chem., Int. Ed. Engl. 1987, 26, 952.
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(
reactants, products, and RC(O)S radicals). The geometry of
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geometries were used for single-point calculations at the MP4
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-
1
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-
1
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1
-
1
D(H-Cl) ) 102.2 kcal mol is combined with the calculated
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1
mol , which is close to the value obtained from the Xe(6s)
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(
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(
-
1
-
1
-1
was erroneously listed as 51 kcal mol rather than 55 kcal mol
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(
(
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-
11
3
-1
(
0.46 ( 0.04) × 10
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