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Phys. Chem. Chem. Phys.
,
202
4,
26
, 27669
Insights of the peroxychloroformyl radical
ClC(O)OO
via
microwave spectrum
†
Ching-Hua Chang,
a
Wen Chao,
b
Cheng-Han Tsai,
a
Mitchio Okumura,
b
Frank A. F. Winiberg
c
and Yasuki Endo
*
a
The exploration of Venus has received much attention in the past and will keep growing due to the
starting of the NASA DAVINCI project. To explain the extremely low O
2
: CO ratio observed in Venus’
atmosphere, a chlorine-initiated CO oxidation catalytic cycle has been proposed. However, relevant
studies on the key intermediates, such as the peroxychloroformyl radical (ClC(O)OO), are rare. In this
study, the ClC(O)OO radical was observed using Fourier-Transform Microwave (FTMW) spectroscopy
under the supersonic expansion condition. Two conformers,
trans
-ClC(O)OO and
cis
-ClC(O)OO, and
their chlorine isotopologues were detected. The molecular constants including the fine and hyperfine
constants, were determined. Based on the experimental and the
ab initio
calculations, the unpaired
electron is mostly located at the terminal oxygen atom, supported by the small magnetic hyperfine
constants of chlorine. In addition, the angles between the Cl–C bond and the
a
-axis of the
trans
-
35
ClC(O)OO and
trans
-
37
ClC(O)OO are similar, but these angles are different for
cis
-ClC(O)OO,
making the quadrupole coupling tensors in the inertial axes disagree with the ratio of the quadrupole
moments of
35
Cl and
37
Cl. Finally, we concluded that the ClC(O)OO radicals should behave similarly to
other peroxyl radicals, as assumed in the current photochemical model of Venus.
Introduction
As one of the nearest planets from the Earth, observations of
Venus have been ongoing for more than hundreds of years,
from spectroscopic measurements by eye and telescope to
physical probing by sending orbiters and probes that reach
the Venusian atmosphere and surface.
1
An understanding of
the evolution of Venus in our solar system could help to
elucidate the formation of exoplanetary systems and the habit-
ability of exoplanets.
2
The current atmospheric composition
provides essential information to verify photochemical models
of the Venusian atmosphere and to trace back its history.
3
However, puzzles remain,
4
leading to significant gaps between
the results of model simulations and real-world observations.
Venus’ atmosphere consists of 96.5% CO
2
, 3.5% N
2
and
trace amounts of CO, H
2
O, Ar, SO
2
and HCl.
1,5
The extremely
low O
2
level (
o
3ppm)
6
contrasts with the observations of O
2
night
glow of the A
1
D
g
–X
3
S
g
transition.
7
The O
2
molecules, produced
from the photolysis of CO
2
to generate O atoms and the subsequent
oxygen atom recombination (CO
2
+
h
n
-
CO+O;O+O+M
-
O
2
* + M), could accumulate in the Venus mesosphere.
A chlorine atom initiated catalytic cycle of CO oxidation has
been proposed to explain the low [O
2
] and [CO], and the high [CO
2
]
through the formation of the chloroformyl radicals (ClCO, (R1))
and the peroxychloroformyl radical (ClC(O)OO, (R2)).
8
The
ClC(O)OO radical could react with either O atoms (R3) or itself
(R4) to form CO
2
and O
2
, releasing Cl atoms to complete the
catalytic cycle (R3a) and (R4a).
9
However, the CO
2
/ClO product
channels (R3b) and (R4b) reduces the active Cl atom concentration
and are estimated to contribute as much of a chlorine reservoir as
HCl,
10
decreasing the efficiency of the Cl catalytic effect.
Cl + CO
-
ClCO
(R1a)
ClCO
-
Cl + CO
(R1b)
ClCO + O
2
-
ClC(O)OO
(R2)
O + ClC(O)OO
-
Cl + CO
2
+O
2
(R3a)
-
ClO + CO
2
+ O
(R3b)
2ClC(O)OO
-
2Cl + 2CO
2
+O
2
(R4a)
-
2ClO + 2CO
2
(R4b)
a
Department of Applied Chemistry, Science Building II, National Yang Ming Chiao
Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30093, Taiwan.
E-mail: endo@nycu.edu.tw
b
Division of Chemistry and Chemical Engineering, California Institute of
Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
c
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
Drive, Pasadena, CA 91109-8099, USA
†
Electronic supplementary information (ESI) available: The transition frequen-
cies and the assignments of all the observed transitions, is available free of charge
at the journal website. See DOI:
https://doi.org/10.1039/d4cp03506b
Received 9th September 2024,
Accepted 22nd October 2024
DOI: 10.1039/d4cp03506b
rsc.li/pccp
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To accurately model the Cl catalytic effect of CO oxidation,
critical parameters including the ClCO equilibrium constant,
the ClCO + O
2
reaction rate coefficient, the reaction rate
coefficients involving ClC(O)OO and the product branching
ratios, are necessary. However, relevant kinetic studies are
sparse. Current Venus photochemical models estimate these
parameters by adopting the values of their isoelectronic
analogs,
5,11
e.g.
ClSO by ClOO and ClC(O)OO by FC(O)OO, but
recent results from the pure rotational spectrum of ClSO
radicals have shown that the reactivities of ClOO and ClSO
are significantly different.
12
In addition to kinetic data, spectroscopic measurements can
provide hints to evaluate the reliability of the rate coefficients
and the product branching ratio in model simulations. For the
ClC(O)OO radical, Francisco and Williams
13
investigated the
ground and the first excited state
via ab initio
calculations and
predicted the X
2
A
00
ground state character at the UHF/6-31G**
level. Pernice
et al.
14
generated the ClC(O)OO radicals from
pyrolysis of ClC(O)OONO
2
and recorded infrared (IR) and UV-
vis spectra in Ar, Ne and O
2
matrices. Recently, Bahera and Lee
reported the gas phase IR spectrum of the ClC(O)OO radicals.
15
They pointed out that the O–O stretching frequency is about
1100 cm
1
, similar to those of other peroxyl radicals.
The reactivity of the ClC(O)OO radical strongly depends on
the distribution of the unpaired electron,
i.e.
the shape of the
singly occupied molecular orbital (SOMO). High resolution
spectroscopic measurements can provide the fine and hyper-
fine structure constants that elucidate the location of the
unpaired electron. In this work, we investigated the high
resolution pure rotational spectrum of the ClC(O)OO radical
in the frequency range of 4–40 GHz using a Fourier-transform
microwave (FTMW) spectrometer. The fine and hyperfine struc-
tures were analyzed to obtain molecular constants involving the
unpaired electron spin and the nuclear spin of the chlorine
atom. High level
ab initio
calculations were conducted for
comparison, and insights obtained from the molecular con-
stants were discussed.
Quantum chemical calculations
The ClC(O)OO radical is considered to have two conformers,
trans
-ClC(O)OO and
cis
-ClC(O)OO, depending on the direction
of terminal OO. Geometries of the two conformers were opti-
mized using the Molpro program package at the RCCSD(T)-
F12A/cc-pVTZ-f12 level of theory.
16
Their optimized structures
are shown in Fig. 1. The calculated molecular constants
and dipole moments are summarized in Table 1.
As shown in Fig. 2, the energy of
trans
-
35
ClC(O)OO is
4.4 kJ mol
1
lower than that of
cis
-
35
ClC(O)OO with a barrier
between both conformers about 22 kJ mol
1
by calculations at
the RCCSD(T)/cc-pVTZ level of theory. The energy difference is
2.65 kJ mol
1
at the RCCSD(T)-F12A/cc-pVTZ-f12 level of theory
with the zero-point energy correction at the RCCSD(T)/cc-pVTZ
level of theory. This barrier indicates that the interconversion
between the
trans
- and
cis
-conformers is slow below room
temperature, and the two conformers may therefore be
observed. Indeed, the FTIR measurements under gas-phase
flow cell conditions at 298 K have identified these two con-
formers from the photolysis of the (ClCO)
2
/N
2
/O
2
mixtures.
15
For
trans
-
35
ClC(O)OO, the dipole moment along the
a
-axis
(
m
a
= 0.91 debye) is close to that along the
b
-axis (
m
b
= 1.13
debye). In contrast, for
cis
-
35
ClC(O)OO,
m
a
(0.27 debye) is a
factor of six smaller than
m
b
(1.83 debye). Larger microwave
power is required to polarize the a-type transitions of
cis
-
35
Cl-
C(O)OO relative to the b-type transitions. The calculated rota-
tional constants are
A
e
= 9899.3 MHz,
B
e
= 2871.9 MHz, and
C
e
=
2226.1 MHz for
trans
-
35
ClC(O)OO, indicating that the
trans
conformer is close to a prolate top molecule (
A
4
B
E
C
,
k
=
0.83). On the other hand, the rotational constants of
cis
-
35
Cl-
C(O)OO are
A
e
= 5629.7 MHz,
B
e
= 4406.7 MHz, and
C
e
=
2471.8 MHz and the geometry is close to an oblate top molecule
(
A
E
B
4
C
,
k
= 0.22). The fine and hyperfine parameters
required to assign the lines observed in this experiment were
calculated using the Gaussian 16 program package.
17
The
calculations were carried out at the B3LYP/aug-cc-pVTZ level
of theory at the geometry optimized by the Molpro program
package.
16
The molecular constants of
37
ClC(O)OO were calculated
using the same method as that of
35
ClC(O)OO. It is important
to properly scale the calculated molecular constants of the
37
Cl
isotopologues to obtain accurate predictions since the intensi-
ties of the spectral lines are a factor of three weaker than those
for the
35
Cl isotopologue due to their natural abundances. The
magnetic hyperfine constants and the quadrupole coupling
constants were scaled using the ratios of the magnetic
moments and the quadrupole moments of
35
Cl and
37
Cl.
18
The scaled parameters for
37
ClC(O)OO are shown in Table 2.
Experimental methods
The ClC(O)OO radical was investigated using FTMW spectro-
scopy, which has been described in detail previously.
19
The
operating frequency of the spectrometer is 4–40 GHz. In brief,
Fig. 1
Schematic pictures of the two conformers, (left)
trans
-ClC(O)OO
and (right)
cis
-ClC(O)OO. The geometries were optimized at RCCSD(T)-
F12A/cc-pVTZ-f12 with the
cis
-conformer about 2.65 kJ mol
1
higher
than the
trans
-conformer with the zero-point energy corrections at
RCCSD(T)/cc-pVTZ.
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a gas mixture with 0.2% CCl
4
, 2% CO, and 2% O
2
diluted in Ne
was flowed into the cavity as a supersonic expansion through
a pulsed-solenoid valve, and its rotational temperature was
cooled down to nearly 2.5 K with a backing pressure of 3 atm.
After the exit of the valve, the gas passed through two electro-
des, separated by a boron nitride insulator, where the
ClC(O)OO radical was generated by a pulsed electronic dis-
charge (
B
1.2 kV). The discharge nozzle was located at the
center of one of the Fabry–Perot cavity mirrors, where
the outer electrode is exposed on the mirror surface. During
the experiment, the pressure of the vacuum chamber was about
8
10
5
Torr. The microwave pulse was sent into the cavity and
polarized the radical through rotational electric resonances.
The microwave pulse, generated by a primary synthesizer
(Rohde & Schwarz, SMB 100A, 1–40 GHz) was emitted into the
cavity by an antenna located at the center of the mirror opposite
to the discharge nozzle.
In this setup, the jet propagates along parallel and anti-
parallel components of the microwave standing wave field in
the cavity, resulting in the Doppler doubling for a transition
line as shown in Fig. 3. The free induction decay signal emitted
from the polarized radical was detected by the same antenna
emitting the polarizing microwave pulse. A duration of 0.3
m
s
was required for the microwave pulse to detect the
trans
-
ClC(O)OO radical. However, about 0.7
m
s of duration was
required to detect a-type transitions of the
cis
-ClC(O)OO radical
because of its small
m
a
. Three sets of Helmholtz coils were
Table 1
Molecular constants of
35
ClC(O)OO. (in MHz)
trans
-
35
ClC(O)OO
cis
-
35
ClC(O)OO
Experimetal
c
Theoretical
a
Experimetal
c
Theoretical
|
m
a
|/debye
0.91
0.27
|
m
b
|/debye
1.13
1.83
|
m
c
|/debye
0.00
0.00
E
/hartree
723.16487
723.16382
D
E
/kJ mol
1
0.00
b
2.65
A
9845.42070(44)
9899.27
5620.75213(56)
5629.72
B
2864.08677(19)
2871.94
4383.39228(58)
4406.68
C
2217.79780(15)
2226.11
2461.93103(21)
2471.84
D
N
/10
3
0.3843(15)
0.38
1.857(23)
3.11
D
NK
/10
3
1.073(36)
1.16
3.916(87)
7.77
D
K
/10
3
16.59
16.59
4.94
4.94
d
N
/10
3
0.0962(14)
0.097
0.795(11)
0.21
d
K
/10
3
1.688
1.688
0.644(56)
5.00
e
aa
943.0312(24)
628.33
119.33402(88)
98.78
e
bb
31.4980(14)
28.48
315.08782(77)
214.60
e
cc
0.90782(91)
1.26
0.70265(57)
1.97
e
ab
48.6318(39)
26.60
126.509(18)
82.41
a
F
(Cl)
0.6353(25)
0.23
1.46513(82)
1.49
T
aa
(Cl)
2.5957(43)
3.09
0.8961(14)
1.27
T
bb
(Cl)
0.9539(33)
1.63
0.0310(13)
0.09
T
ab
(Cl)
0.113(17)
0.38
0.329(14)
0.29
w
aa
(Cl)
57.6002(70)
54.50
0.0084(26)
0.089
w
bb
(Cl)
29.3558(65)
29.34
27.1927(25)
24.26
w
ab
(Cl)
38.64(14)
36.44
58.626(51)
56.53
C
aa
(Cl)/10
3
1.6
1.6
1.9
1.9
C
bb
(Cl)/10
3
1.3
1.3
1.3
1.3
C
cc
(Cl)/10
3
0.9
0.9
0.8
0.8
s
fit
/10
3
2.5
2.0
a
The rotational constants were derived from the geometry calculated at the RCCSD(T)-f12a/cc-pVTZ-f12 level of theory. Other parameters were
calculated at the B3LYP/cc-pVTZ level of theory at the optimized structures calculated by the Molpro program.
b
The zero-point energies of the two
conformers were corrected at the RCCSD(T)/cc-pVTZ level of theory by the Molpro program.
c
The values in parentheses denote one standard
deviation of the fit and apply to the last digits. The values without uncertainty are kept fixed to those obtained theoretically.
Fig. 2
The potential energy surface rotating the OCOO dihedral angle,
f
(ClCOO), calculated at the RCCSD(T)/cc-pVTZ level of theory. The
energy shown in the parenthesis was calculated at the RCCSD(T)-F12A/
cc-pVTZ-f12 level of theory with the zero-point energy correction calcu-
lated at RCCSD(T)/cc-pVTZ.
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utilized in the experiments to compensate the Earth’s magnetic
field avoiding the energy level splitting of open-shell species
due to the Zeeman effect.
Analysis
The Hamiltonian,
H
=
H
rot
+
H
cd
+
H
nsr
+
H
sr
+
H
hf
+
H
Q
, was
used to analyze the transition frequencies of the ClC(O)OO
radical. The
H
rot
denotes the rotational Hamiltonian. The
H
sr
,
H
hf
, and
H
Q
denote the electron spin-rotation interaction term
(
e
ab
), the magnetic hyperfine term (
a
F
and
T
), and the nuclear
quadrupole coupling term (
w
ab
), respectively. In addition to the
fine and hyperfine interactions, minor corrections (magnitude
in a few kHz) from the centrifugal distortion term,
H
cd
in
Watson’s A-reduced form (
D
N
,
D
NK
,
D
K
,
d
N
and
d
K
), and the
nuclear spin-rotation interaction term,
H
nsr
(
C
aa
), were
considered.
For a non-linear molecule, the orbital angular momentum,
L
, is usually quenched, leaving the electron spin-rotation
interaction,
H
sr
, the most pronounced factor for the fine
structure. The Hund’s case (b) formula was used to analyze
the fine structure. In this approximation, the total angular
momentum,
J
, can be expressed as
J
=
N
+
S
, where
N
denotes
the sum of the rotational angular momentum and the orbital
angular momentum, and
S
denotes the electron spin angular
momentum. The magnitude of the electron spin-rotation con-
stants ranges from a few tens of MHz to hundreds of MHz.
The interactions involving the nuclear spin angular momen-
tum,
I
, result in the hyperfine structures. Due to the lack of the
orbital angular momentum, we can ignore the interaction
between
L
and
I
. On the other hand, the magnetic hyperfine
interaction between
S
and
I
have to be considered and can be
expressed as
H
hf
¼
a
F
I
S
þ
I
T
S
;
T
¼
T
aa
T
ab
0
T
ba
T
bb
0
00
T
cc
2
4
3
5
;
where
T
denotes a second-rank traceless tensor with three
independent components for planar radicals, and
a
F
is the
Fermi contact interaction constant. The total angular momen-
tum including the nuclear spin angular momentum,
F
,is
written as
F
=
J
+
I
. For the ClC(O)OO radicals,
S
= 1/2 and
I
(Cl) = 3/2; thus, each rotational level splits into 8 fine and
hyperfine levels for most of the cases.
Results
Tests of the gas mixture recipes
To detect the ClC(O)OO transitions, we tried a few different
recipes for optimizing the a-type transition lines of the ClCO
radical.
20
Initially, a gas mixture with 10% of O
2
(0.2% CCl
4
,2%
CO) was tested. We observed very strong signals of the ClOO
radical and the ClCO signals were hardly detected. Although
the Cl–CO bond energy (
D
H
Cl+CO
(0 K) = 28
3 kJ mol
1
)
21
is
larger than the Cl-OO bond energy (
D
H
Cl+OO
(0 K) = 19.6
0.42 kJ mol
1
)
22
and the energy of ClC(O)OO is about
138 kJ mol
1
lower than that of ClOO + CO, most of the Cl
atoms react with O
2
and the ClC(O)OO radical is hardly formed.
We also tested the gas mixture of CCl
4
:CO:O
2
= 0.3% : 2% : 1%
in Ne. As expected, we observed stronger ClCO signals and
Table 2
Molecular constants of
37
ClC(O)OO. (in MHz)
trans
-
37
ClC(O)OO
cis
-
37
ClC(O)OO
Experimetal
b
Scaled
a
Experimetal
b
Scaled
A
9816.4384(15) 9814.05 5554.96351(73) 5554.45
B
2787.75518(16) 2787.11 4292.30194(33) 4290.57
C
2170.32793(30) 2169.77 2420.49990(38) 2419.89
D
N
/10
3
0.3708(74)
0.36
1.934(18)
3.00
D
NK
/10
3
0.970(85)
1.07
4.338(74)
7.53
D
K
/10
3
16.79
16.79
4.80
4.80
d
N
/10
3
0.09
0.09
0.22
0.22
d
K
/10
3
1.62
1.62
1.18(16)
4.71
e
aa
941.6597(71)
941.17
101.3454(23)
104.91
e
bb
30.2644(22)
30.45
321.3447(31)
320.76
e
cc
0.8966(15)
0.88
0.6934(10)
0.69
e
ab
45.219(61)
43.92 108.179(56)
108.77
a
F
(Cl)
0.5274(17)
0.56
1.2228(41)
1.22
T
aa
(Cl)
2.1510(47)
2.15
0.6992(52)
0.75
T
bb
(Cl)
0.8023(55)
0.79
0.0132(31)
0.03
T
ab
(Cl)
0.12*
0.09
0.429(61)
0.27
w
aa
(Cl)
45.822(11)
45.39
5.7959(86)
0.01
w
bb
(Cl)
23.5516(77)
23.14
15.6387(71)
21.43
w
ab
(Cl)
30.49(38)
30.45
47.23(17)
7.09
C
aa
(Cl)/10
3
1.3
1.3
1.5
1.5
C
bb
(Cl)/10
3
1.0
1.0
1.1
1.1
C
cc
(Cl)/10
3
0.7
0.7
0.7
0.7
s
fit
/10
3
1.5
1.9
a
All parameters were scaled with the experimental and calculated
values of
35
ClC(O)OO, and the ratios of the magnetic moments, and
the quadrupole moments of the isotopes for the hyperfine constants.
b
The values in parentheses denote one standard deviation of the fit
and apply to the last digits. The values without uncertainty are kept
fixed to those obtained theoretically.
Fig. 3
Representative microwave spectrum of the 4
04
-3
03
transition of
the
trans
-
35
ClC(O)OO radical. The upper panel shows the predicted fine
and hyperfine structures. The lower panel shows the experimental spec-
trum. The predictions are very close to the experimental result with only
the 70 MHz frequency difference. The two colored regions mark the
splitting due to the electron spin-rotation interaction. In each region, four
transitions were observed due to the hyperfine splitting. Each hyperfine
component has a doublet line shape due to Doppler splitting.
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much weaker ClOO signals. In addition, the ClC(O)OO signals
were observed for this mixture. Eventually, the gas mixture of
0.3% of CCl
4
, 2% of CO, and 2% of O
2
balanced with Ne to 10
atm was used to get the optimized ClC(O)OO signals.
Microwave spectral measurements
We first searched for the a-type transitions for the
trans
-
ClC(O)OO radical. Usually, the prediction of the
K
a
= 0 a-type
transitions is more accurate than other transitions since the
B
and
C
rotational constants contribute the most. The calculated
B
e
and
C
e
rotational constants are slightly (
B
some tens MHz)
larger than the measured values because the vibrationally
averaged bond lengths are longer than the equilibrium bond
lengths. By assigning the a-type transitions, better estimations
of the
B
and
C
rotational constants and improved predictions of
the b-type transition frequencies were obtained. For the
cis
-
ClC(O)OO radicals, b-type transitions were searched for first
since the calculated dipole moment
m
a
is small. In this case we
had to scan a wider region to detect the lines of
cis
-ClC(O)OO.
Fig. 3 shows the a-type 4
04
-3
03
transition of the
trans
-
35-
ClC(O)OO radical. The observed fine and hyperfine patterns
agreed with the predictions being observed at transition fre-
quencies roughly 70 MHz lower. Intensities were usually stron-
ger using Ne as the carrier gas than the intensities using Ar. For
example, the 4
04
-3
03
transition of
trans
-
35
ClC(O)OO was too
weak to be observed when Ar was used as a carrier gas.
For the
cis
-ClC(O)OO radical, one a-type transition and two
b-type transitions were observed in the 22 190–22 230 GHz
window as shown in Fig. 4. Intensities of the 4
04
-3
03
and
2
20
-1
11
transitions have different responses to the duration of
the polarizing microwave pulse since |
m
b
| is about a factor of six
larger than |
m
a
|. The transitions were assigned by comparing
the predicted hyperfine patterns and the power dependence of
each line. The relative intensities of the 4
14
-3
03
hyperfine
components agreed with the predicted pattern. However, the
absolute intensities were larger than the 2
20
-1
11
transition.
This discrepancy was probably caused by the instability of
the experimental conditions since they were observed on
different days.
We observed a total of 14 a-type and 8 b-type transitions (123
hyperfine components) for
trans
-
35
ClC(O)OO, and 10 a-type and
6 b-type transitions (116 hyperfine components) for
cis
-
35
Cl-
C(O)OO. These sets of data allowed us to determine totals of 17
and 18 molecular constants for
trans
-
35
ClC(O)OO and
cis
-
35
Cl-
C(O)OO, respectively, where a few centrifugal distortion and
nuclear spin-rotation constants were fixed to the calculated
values in the fitting.
The pure rotational transitions for
37
ClC(O)OO were also
searched for. The calculated rotational constants of
37
ClC(O)OO
were scaled, based on the ratio of the calculated and measured
35
ClC(O)OO rotational constants, for predicting the initial
hyperfine transition frequencies. The observed fine and hyper-
fine structures for both
trans
-
37
ClC(O)OO and
cis
-
37
ClC(O)OO
were similar to
35
ClC(O)OO as expected. The relative intensities
for
35
ClC(O)OO and
37
ClC(O)OO followed the 3 : 1 natural
abundance ratio of the chlorine atoms. Due to the small signal
intensities, only 43 and 57 hyperfine components were
observed for
trans
-
37
ClC(O)OO and
cis
-
37
ClC(O)OO, respectively.
The determined molecular constants are summarized in Tables
1 and 2, and the observed transitions frequencies are summar-
ized in Tables S1–S4 (ESI
†
).
Discussions
In this study, two conformers of the ClC(O)OO radical and their
chlorine isotopologues were observed by detecting their pure
rotational transitions in a supersonic expansion of a gas
mixture containing CCl
4
, CO and O
2
diluted in Ne, from a
discharge nozzle. Both
cis
-ClC(O)OO and
trans
-ClC(O)OO con-
formers were observed with similar intensities. This observa-
tion agrees with the insights obtained from the
ab initio
calculations. First, the energy barrier between the
cis
- and
trans
-conformers is indeed about ten times larger than the
thermal energy at 298 K, and, therefore, the interconversion is
slow below the room temperature. Second, both conformers
have similar stability since the energy of
trans
-
35
ClC(O)OO is
only 2.65 kJ mol
1
lower than the energy of
cis
-
35
ClC(O)OO.
Although the rotational temperature in the jet is only about
2.5 K, the two conformers are produced at much higher
temperature. Both of the conformers are thus produced almost
equally, and the population ratio is preserved even when they
are cooled in the supersonic expansion.
The electron spin-rotation constants
As shown in Fig. 5 calculations predict that the unpaired
electrons of the two conformers occupy the out-of-plane p
c
orbital of the terminal oxygen with the
2
A
00
electronic ground
states. Since the electron spin-rotation interaction is a second
order term, the perturbational approach by the second order
Fig. 4
The observed spectral pattern of the
cis
-
35
ClC(O)OO radical in the
22 190–22 230 MHz region. The upward black sticks are the observed
spectral lines. The downward sticks are the predictions of 4
04
-3
03
(red),
2
20
-1
11
(green) and 4
14
-3
03
(blue) transitions.
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correction indicates that the first excited state contributes the
most. For both conformers, the first excited state corresponds
to a transition of the beta electron from the in-plane p orbital to
the out-of-plane p
c
orbital, corresponding to the negative
e
aa
as
shown in Table 1.
For the
trans
-ClC(O)OO, the direction of the orbital angular
momentum produced by the electron excitation is almost
parallel to the
a
-axis, resulting in a large contribution to the
a
-axis components for the spin-rotation interaction. On the
other hand, for
cis
-ClC(O)OO, the direction of the induced
orbital angular momentum upon the excitation of the unpaired
electron is almost parallel to the
b
-axis, resulting in a large
absolute value for
e
bb
.
The magnetic hyperfine constants
The magnetic hyperfine constants (
a
F
and
T
) of the two con-
formers are small since the unpaired electron is mainly located
on the terminal oxygen atom of the COO moiety where the spin
density on the chlorine atom is smaller than 1%. The small
negative Fermi contact constants of the chlorine nucleus in
both conformers could be rationalized by the spin polarization
effect of the Cl–C bond from the unpaired electron on the
central carbon atom. This contrasts with the case of the ClSO
radicals, where some amount of the unpaired electron distrib-
uted in the out-of-plane
p
-orbital on chlorine and the spin
polarization effect of the Cl–S sigma bond results in a small
positive Fermi contact constant.
12
For the dipolar interaction constants, a radical with an
unpaired electron in the out-of-plane
p
radical usually satisfies
a ratio
T
aa
:
T
bb
:
T
cc
=
2/5 :
2/5 : 4/5.
23
However, both the
trans
- and
cis
-ClC(O)OO radicals do not follow this relation
due to the very weak interaction between the unpaired electron
on the terminal oxygen nucleus and the Cl nuclear spin. The
experimental results indicates that the Fermi contact constants
and the dipolar interaction constants satisfy the ratio of the
magnetic moments of the two isotopes 0.82 : 0.68
18
for the two
conformers and their
37
Cl isotopologues.
Quadrupole coupling constants
The quadrupole coupling constants of the
trans
-
35
ClC(O)OO
and
37
ClC(O)OO radicals satisfy the ratio of the quadrupole
moment of the two isotopes, 8.24 : 6.49,
18
while in the case of
the
cis
-ClC(O)OO radical, the constants do not agree with this
relation. This discrepancy is due to a small change of the
molecular axes between the two chlorine isotopologues. For
trans
-ClC(O)OO,
ab initio
calculations showed that the angles
between the
a
-axis and the Cl–C bond are 21.7
1
and 21.3
1
for
the
35
Cl and
37
Cl isotopologues, respectively. On the other
hand, the angles for the
cis
-
35
ClC(O)OO and
37
ClC(O)OO radi-
cals are 54.8
1
and 50.8
1
. The 4
1
difference indicates that the
quadrupole coupling constants projected to the two different
molecular axes are quite different for the
cis
-ClC(O)OO isoto-
pologues and direct comparison becomes meaningless.
By diagonalizing the fitted quadrupole coupling tensor as
shown in Table 3, the rotational angle of the quadrupole
interaction tensor between the
a
-axis and the
x
-axis in the
Fig. 5
The singly occupied molecular orbitals of each conformer in its
electronic ground state and the first excited state. The geometries of the
radicals were optimized using the Molpro program package at the
RCCSD(T)-F12A/cc-p-VTZ-f12 level of theory.
Table 3
Inertial axis and principal axis values of
T
and
w
tensors of ClC(O)OO. (in MHz)
trans
-
35
ClC(O)OO
trans
-
37
ClC(O)OO
cis
-
35
ClC(O)OO
cis
-
37
ClC(O)OO
a
F
(Cl)
0.64
0.53
1.46
1.22
T
aa
(Cl)
2.60
2.15
0.90
0.70
T
bb
(Cl)
0.95
0.80
0.03
0.01
T
ab
(Cl)
0.11
0.12
0.33
0.43
y
T
a
/degree
3.80
5.04
17.68
25.23
T
xx
(Cl)
2.61
2.16
0.14
0.90
T
yy
(Cl)
0.94
0.79
1.01
0.21
T
zz
(Cl)
3.55
2.95
0.87
1.11
w
aa
(Cl)
57.60
45.82
0.01
5.80
w
bb
(Cl)
29.36
23.55
27.19
15.64
w
ab
(Cl)
38.64
30.49
58.63
47.23
y
w
b
/degree
20.81
20.66
51.53
47.97
w
xx
(Cl)
72.39
57.32
73.78
58.21
w
yy
(Cl)
44.05
35.05
46.58
36.77
w
zz
(Cl)
28.24
22.27
27.2
21.44
a
The rotation angle from
T
aa
to
T
xx
, in degree.
b
The rotation angle from
w
aa
to
w
xx
, in degree.
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principal axis system of the tensor is 20.8
1
and 51.5
1
for
trans
-
35
ClC(O)OO and
cis
-
35
ClC(O)OO, which are close to the
angles between the
a
-axis and the Cl–C bond. This suggests that
the principal axes of the quadrupole interactions are directing
almost toward the Cl–C bond. The values for the principal axis
system are almost axially symmetric,
i.e
.
w
xx
E
2
w
yy
E
2
w
zz
.
23
Furthermore the ratios of the principal axis system values for
the two isotopologues of
cis
-ClC(O)OO satisfy the expected ratio
for the quadrupole moments.
Since the
x
-axis lies close to the Cl–C bond, the ionic
character (
i
c
) of the Cl–C bond can be calculated from
w
xx
as
below.
i
c
¼
1
þ
w
xx
eQq
n
10
;
eQq
n
10
¼
109
:
74 MHz for
35
Cl
:
where,
eQq
n
10
is the coupling constant of an atomic p electron
in the orbital
c
n
10
. The
w
xx
of the
trans
-ClC(O)OO and
cis
-
ClC(O)OO radicals are
72.39 and
73.78 MHz as shown in
Table 3. The ionic characters of the Cl–C bond are thus 0.34 and
0.33 for
trans
-
35
ClC(O)OO and
cis
-
35
ClC(O)OO, respectively,
suggesting that the Cl–C bonds possess stronger covalent
character in both conformers.
Insights from the molecular geometry
With very good agreement between the experimental results
and the theoretical predictions for the molecular constants, we
believe that the calculated geometries of the two conformers of
the ClC(O)OO radical are very close to the true values. The O–O
bond lengths of
trans
- and
cis
-
35
ClC(O)OO are 1.326 Å and
1.318 Å, respectively. These bond length are similar to the
O–O bond length of the HOO radical (1.329 Å)
24
and lie between
the bond lengths of O
2
(1.208 Å) and H
2
O
2
(1.475 Å),
25
suggest-
ing that the O–O bonds of the ClC(O)OO radicals have similar
reactivity as those of typical peroxyl radicals. The C
Q
O bond
lengths are 1.174 Å and 1.178 Å for the
trans
-ClC(O)OO and
cis
-
ClC(O)OO conformers, respectively, which are close to the C
Q
O
bond of Cl
2
CO (1.177 Å).
26
This fact indicates that the unpaired
electron barely occupies the CO moiety of the ClC(O)OO radi-
cals and suggests that the CO moiety is not the reactive center
toward other species. Thus, it is possible to say the ClC(O)OO
radical is a –ClCO derivative of the HOO radical.
Conclusions
Pure rotational spectra of both
trans
-ClC(O)OO and
cis
-ClC(O)O
conformers, and their
35
Cl/
37
Cl isotopologues were observed
using an FTMW spectrometer under supersonic expansion. The
observed fine and hyperfine splittings were well-assigned and
the molecular constants, including the electron spin-rotation
constants, the Fermi contact constant, the dipolar interaction
constants, the quadrupole coupling constants and the centri-
fugal distortion constants were determined precisely.
The electron spin-rotation constants and the hyperfine con-
stants agree well with the molecular orbital picture that the
unpaired electron mainly occupies the out-of-plane
p
c
orbital of
the terminal oxygen atom of the COO moiety with a small
distribution to the adjacent oxygen atom, making the terminal
oxygen atom the reactive center with other species.
We also found that the C
Q
O bond lengths of ClC(O)OO are
very close to that of phosgene, indicating the unpaired electron
is localized on the terminal oxygen and has little effect on the
ClC(O) moiety. Indeed, the Cl–C bonds in the ClC(O) moiety
have large covalent characters for both
cis
- and
trans
-
conformers, suggesting that the ClC(O) moiety possesses a
small reactivity with other species in the gas phase. However,
the reactivity with alkaline species,
e.g.
NH
3
, could still be
strong because alkaline species tend to donate the lone pair
electron to the C
Q
O
p
anti-bonding orbital, especially to the
central carbon.
The hyperfine structures of the
35
Cl/
37
Cl isotopologues of
trans
-ClC(O)OO are similar; the values are proportional to the
ratios of the magnetic moment and the quadrupole moment of
the
35
Cl/
37
Cl nuclei. However, in the case of the isotopologues
of
cis
-ClC(O)OO, clear differences have been observed for their
quadrupole coupling constants, which was attributed to the
difference of the angles between the principal axes of the
quadrupole tensor and the inertia axes.
Author contributions
Ching-Hua Chang: data curation, formal analysis, investiga-
tion, writing – review & editing. Wen Chao: conceptualization.
Cheng-Hua Tsai: formal analysis, investigation. Mitchio Oku-
mura: conceptualization, writing – review. Frank A. F. Wini-
berg: conceptualization, writing – review. Yasuki Endo:
conceptualization, data curation, formal analysis, funding
acquisition, methodology, software, writing – review & editing.
Data availability
All the date in the manuscript is included in the ESI.
†
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
The research was supported by the National Science and
Technology Council of Taiwan (grants MOST108-2113-M-009-
025, MOST109-2113-M-009-024). The work of WC and FW was
supported in part by Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National
Aeronautics and Space Administration (80NM0018D0004). The
authors thank Prof. Yuan-Pern Lee for sharing necessary lab
supplies.
References
1 J. G. O’Rourke, C. F. Wilson, M. E. Borrelli, P. K. Byrne,
C. Dumoulin, R. Ghail, A. J. P. Gu
̈
lcher, S. A. Jacobson,
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O. Korablev, T. Spohn, M. J. Way, M. Weller and F. Westall,
Venus, the Planet: Introduction to the Evolution of Earth’s
Sister Planet,
Space Sci. Rev.
, 2023,
219
, 10.
2 F. Westall, D. Ho
̈
ning, G. Avice, D. Gentry, T. Gerya,
C. Gillmann, N. Izenberg, M. J. Way and C. Wilson, The
Habitability of Venus,
Space Sci. Rev.
, 2023,
219
, 17.
3 T. Widemann, S. E. Smrekar, J. B. Garvin, A. G. Straume-
Lindner, A. C. Ocampo, M. D. Schulte, T. Voirin, S. Hensley,
M. D. Dyar, J. L. Whitten, D. C. Nunes, S. A. Getty, G. N.
Arney, N. M. Johnson, E. Kohler, T. Spohn, J. G. O’Rourke,
C. F. Wilson, M. J. Way, C. Ostberg, F. Westall, D. Ho
̈
ning,
S. Jacobson, A. Salvador, G. Avice, D. Breuer, L. Carter,
M. S. Gilmore, R. Ghail, J. Helbert, P. Byrne, A. R. Santos,
R. R. Herrick, N. Izenberg, E. Marcq, T. Rolf, M. Weller,
C. Gillmann, O. Korablev, L. Zelenyi, L. Zasova, D. Gorinov,
G. Seth, C. V. N. Rao and N. Desai, Venus Evolution Through
Time: Key Science Questions, Selected Mission Concepts
and Future Investigations,
Space Sci. Rev.
, 2023,
219
, 56.
4 F. P. Mills and M. Allen, A review of selected issues con-
cerning the chemistry in Venus’ middle atmosphere,
Planet.
Space Sci.
, 2007,
55
, 1729–1740.
5 C. J. Bierson and X. Zhang, Chemical Cycling in the Venu-
sian Atmosphere: A Full Photochemical Model From the
Surface to 110 km,
J. Geophys. Res.: Planets
, 2020,
125
, E006159.
6 F. P. Mills, A spectroscopic search for molecular oxygen in
the Venus middle atmosphere,
J. Geophys. Res.: Planets
,
1999,
104
, 30757–30763.
7 G. Piccioni, L. Zasova, A. Migliorini, P. Drossart, A. Shakun,
A. Garcı
́
a Mun
̃
oz, F. P. Mills and A. Cardesin-Moinelo, Near-
IR oxygen nightglow observed by VIRTIS in the Venus upper
atmosphere,
J. Geophys. Res.: Planets
, 2009,
114
, E00B38.
8 W. B. DeMore and Y. L. Yung, Catalytic Processes in the
Atmospheres of Earth and Venus,
Science
, 1982,
217
,
1209–1213.
9 X. Zhang, M. C. Liang, F. P. Mills, D. A. Belyaev and
Y. L. Yung, Sulfur chemistry in the middle atmosphere of
Venus,
Icarus
, 2012,
217
, 714–739.
10 B. J. Sandor and R. Todd Clancy, First measurements of ClO
in the Venus atmosphere – Altitude dependence and tem-
poral variation,
Icarus
, 2018,
313
, 15–24.
11 Y. L. Yung and W. B. DeMore,
Photochemistry of planetary
atmospheres
, Oxford University Press, New York, 1999.
12 C. H. Chang, C. H. Tsai, Y. T. Liu and Y. Endo, Fourier-
transform microwave spectroscopy of the ClSO radical,
Phys.
Chem. Chem. Phys.
, 2024,
26
, 4922–4928.
13 J. S. Francisco and I. H. Williams, Theoretical characteriza-
tion of formylperoxy and (haloformyl)peroxy radicals,
J. Phys. Chem.
, 1988,
92
, 5347–5352.
14 H. Pernice, P. Garcia, H. Willner, J. S. Francisco, F. P.
Mills, M. Allen and Y. L. Yung, Laboratory evidence for a
key intermediate in the Venus atmosphere: peroxychloro-
formyl radical,
Proc. Natl. Acad. Sci. U. S. A.
, 2004,
101
,
14007–14010.
15 B. Behera and Y.-P. Lee, Infrared spectra of gaseous peroxy-
chloroformyl
radical ClC(O)OO: a key intermediate in the
conversion of CO to CO
2
in the Venus atmosphere,
J. Mol.
Spectrosc.
, 2023,
393
, 111771.
16 H.-J.Werner,P.J.Knowles,G.Knizia,F.R.Manby,M.Schu
̈
tz,
P. Celani, W. Gyo
̈
rffy, D. Kats, T. Korona, R. Lindh,
A. Mitrushenkov, G. Rauhut, K.
R. Shamasundar, T. B. Adler,
R.D.Amos,S.J.Bennie,A.Bernhardsson,A.Berning,
D.L.Cooper,M.J.O.Deegan,A.J.Dobbyn,F.Eckert,
E. Goll, C. Hampel, A. Hesselmann, G. Hetzer, T. Hrenar,
G. Jansen, C. Ko
̈
ppl,S.J.R.Lee,Y.Liu,A.W.Lloyd,Q.Ma,
R.A.Mata,A.J.May,S.J.McNicholas,W.Meyer,T.F.MillerIII,
M. E. Mura, A. Nicklass, D. P. O’Neill, P. Palmieri, D. Peng,
K. Pflu
̈
ger,R.Pitzer,M.Reiher,T.Shiozaki,H.Stoll,A.J.Stone,R.
Tarroni, T. Thorsteinsson, M. Wang and M. Welborn,
1, a package
of ab initio programs edn
, 2022, see
https://www.molpro.net
.
17M.J.Frisch,G.W.Trucks,H.B.Schlegel,G.E.Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A.
Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich,
J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci,
H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg,
D.Williams-Young,F.Ding,F.Lipparini,F.Egidi,J.Goings,
B.Peng,A.Petrone,T.Henderson,D.Ranasinghe,V.G.
Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada,
M.Ehara,K.Toyota,R.Fukuda,J.Hasegawa,M.Ishida,
T.Nakajima,Y.Honda,O.Kitao,H.Nakai,T.Vreven,
K.Throssell,J.A.Montgomery,Jr.,J.E.Peralta,F.Ogliaro,
M.J.Bearpark,J.J.Heyd,E.N.Brothers,K.N.Kudin,
V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand,
K.Raghavachari,A.P.Rendell,J.C.Burant,S.S.Iyengar,
J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo,
R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma,
O.Farkas,J.B.ForesmanandD.J.Fox,
Gaussian
, Wallingford,
CT, 2016.
18 N. J. Stone, Table of nuclear magnetic dipole and electric
quadrupole moments,
At. Data Nucl. Data Tables
, 2005,
90
,
75–176.
19 C. Cabezas, J.-C. Guillemin and Y. Endo, Fourier-transform
microwave spectroscopy of a halogen substituted Criegee
intermediate ClCHOO,
J. Chem. Phys.
, 2016,
145
, 184304.
20 C. H. Chang, C. H. Tsai and Y. Endo, to be published.
21 J. M. Nicovich, K. D. Kreutter and P. H. Wine, Kinetics and
thermochemistry of ClCO formation from the Cl + CO
association reaction,
J. Chem. Phys.
, 1990,
92
, 3539–3544.
22 K. Suma, Y. Sumiyoshi, Y. Endo, S. Enami, S. Aloisio,
S. Hashimoto, M. Kawasaki, S. Nishida and Y. Matsumi,
Equilibrium Constants of the Reaction of Cl with O
2
in the
Formation of ClOO,
J. Phys. Chem. A
, 2004,
108
, 8096–8099.
23 E. Hirota,
et al.
,
High-resolution spectroscopy of transient
molecules
, Springer-Verlag, Berlin, New York, 1985.
24 A. Charo and F. C. De Lucia, The millimeter and submillimeter
spectrum of HO
2
: the effects of unpaired electronic spin in a
light asymmetric rotor,
J. Mol. Spectrosc.
, 1982,
94
, 426–436.
25 K. P. Huber and G. Herzberg,
Molecular Spectra and Mole-
cular Structure IV. Constants of Diatomic Molecules
, Van
Nostrand Reinhold, New York, 1979.
26 K. Kuchitsu,
Structure of Free Polyatomic Molecules
, Springer
Berlin, Heidelberg, 1998.
Paper
PCCP
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