First observation of the rotational spectrum of the bromomethyl radical, CH2Br

Article abstract of DOI:10.1039/b405115g

The first high resolution rotational spectrum of the bromomethyl radical in the gas phase has been observed. The radical was produced in a fast flow system by hydrogen abstraction of CH₃Br by Cl or F atoms obtained from a 2450 MHz microwave discharge in Cl₂ or in CF₄, respectively. The rotational, quartic centrifugal distortion constants have been obtained as well as the electron spin-rotation interaction constants. The small positive inertial defect, Δ₀ = 0.032 amu A² deduced from the rotational constants of the CH₂⁷⁹Br species clearly indicates that the radical is planar in its ground vibronic state.

Full text of DOI:10.1039/b405115g

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C O M M U N I C A T I O N
First observation of the rotational spectrum of the
bromomethyl radical, CH Br
2
a
a
a
b
a
Stephane Bailleux,* Pascal Drean, Marc Godon, Zdenek Zelinger* and Chuanxi Duan
´ ´
ˇ
a
Laboratoire de Physique des Lasers, Atomes et Mole´cules, CERLA, UMR CNRS 8523,
Universite´ de Lille 1, Villeneuve d’Ascq Cedex F-59655, France. E-mail:
stephane.bailleux@univ-lille1.fr; Fax: þ33 320336463; Tel: þ33 320336464
b
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejsˇkova 3, 18200 Prague 8, Czech Republic. E-mail: zelinger@jh-inst.cas.cz;
Fax: þ420 2 8658 2307; Tel: þ420 2 6605 3046
Received 6th April 2004, Accepted 11th May 2004
First published as an Advance Article on the web 18th May 2004
1
9
The first high resolution rotational spectrum of the bromomethyl
radical in the gas phase has been observed. The radical was
electron impact experiments with CH
Br
2 2
.
An early matrix
infrared study has suggested that this elusive species has a
planar structure because of the large positive quartic term in
produced in a fast flow system by hydrogen abstraction of CH
by Cl or F atoms obtained from a 2450 MHz microwave
discharge in Cl or in CF , respectively. The rotational, quartic
3
Br
2
the out-of-plane bending potential function. This conclusion
0
2
1
was later supported by EPR investigations. Davies et al. have
2
4
centrifugal distortion constants have been obtained as well as the
electron spin–rotation interaction constants. The small positive
recently reported the detection of this radical in the gas phase
2
by far infrared laser magnetic resonance. They have also
2
˚
2
inertial defect, D = 0.032 amu A deduced from the rotational
interpreted the spectra in terms of a planar structure. However,
23
recent ab initio calculations indicate that CH Br is most
2
0
7
2
9
constants of the CH
radical is planar in its ground vibronic state.
Br species clearly indicates that the
probably very slightly non-planar, if not planar. Depending
2
on the structure, the ground electronic state is either A’ (non-
2
planar) or B (planar). This shows that the planarity still needs
It has been well established that halogen atoms and radicals
play an important role in catalytic reactions contributing in the
1
to be confirmed by high-resolution spectroscopic studies.
Furthermore, high quality ground state constants will make
the analysis of vibrational or electronic spectra of this radical
2
significantly easier, and identification of CH Br in such experi-
ments more definitive if the ground state constants agree to
1
–4
stratospheric ozone destruction. In particular, many studies
5
–9
have been devoted to the role of chlorine. Until recently, the
chemistry of bromine-containing molecules and the adverse
impact of bromine on the atmosphere have received little
attention although there has been accumulating evidence for
more than two decades that reactive bromine is also involved in
within error with the millimeter wave values. In this letter, we
report the detection and the identification of the first high
resolution rotational spectrum of CH Br in the gas phase.
1
0–12
the depletion of the ozone layer.
The concentration of
2
bromine in the stratosphere is much lower than that of
chlorine. However several studies have demonstrated that
bromine is much more destructive to the stratospheric ozone
than chlorine. Indeed, Garcia and Solomon have recently
reported that bromine could be up to 100 times more efficient
The millimeter wave (mmw) spectrometer used has been
2
described in detail elsewhere. It was adapted for the produc-
4
tion of the CH Br radical. Briefly, it consists of a single-pass
2
absorption system comprising a tunable mmw source, an
absorption cell and a detector. The mmw radiation was
obtained from two phase-locked backward wave oscillators,
one emitting in the 160–260 GHz frequency region (Istok), the
other one oscillating between 418–470 GHz (Thomson C.S.F.).
The detection of the absorption signals was achieved with a
helium-cooled InSb hot electrons bolometer (Q.M.C. Instru-
ments). For improved sensitivity, source modulation at 70 kHz
was used together with lock-in detection at twice this fre-
quency, resulting in a second-derivative lineshape. Frequency
scanning, data acquisition, signal processing and frequency
measurement were controlled using a PC; the errors on the
frequency measurement were estimated to be less than 50 kHz.
1
3
than chlorine, on an atom-per-atom basis. Methyl bromide is
considered to be the major source of bromine in the atmo-
1
4
3
sphere. CH Br is released into the atmosphere from both
1
5
natural and anthropogenic sources. The main known re-
moval process of this halomethane occurs via hydrogen ab-
straction by its reaction either with the hydroxyl radical or with
chlorine, leading to the formation of the monobromomethyl
radical, CH Br. This radical reacts further with other open
2
shell species to release Br atoms for secondary reactions with
ozone. The monobromomethyl radical is therefore a key
transient for which it is essential to obtain accurate informa-
tion from high-resolution spectroscopy. Such information is
crucial to allow kinetic and photochemical laboratory studies
in order to achieve a better understanding of the role played by
bromomethane in the processes leading to the destruction of
stratospheric ozone. In addition to its importance in atmo-
CH Br was generated in a fast flow system by hydrogen
2
abstraction of monobromomethane by chlorine or fluorine.
The 1 m long absorption cell in Pyrex was equipped with a coil
for creating a magnetic field to confirm that the detected lines
were paramagnetic by on/off measurement. At one end a
vertical Pyrex crosspiece was used to connect to a partially
choked diffusion pump below the cell and to a pressure gauge
mounted above the cell. At the other end, a horizontal Pyrex
crosspiece was used to introduce CH Br vapour and Cl or CF
spheric chemistry, the study of CH
order to compare its structure with those of its well-studied
2
Br is also of importance in
1
fluorine- and chlorine-analogues, CH F and CH Cl.
6
17,18
2
2
These studies have revealed that CH
CH Cl is planar, the difference being attributed to the larger
2
F is nearly planar whereas
3
2
4
2
separately. This method allowed in situ production of the
reactive species. The outer end of each crosspiece was sealed
electronegativity of fluorine. CH
2
Br was first detected in
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 0 4 9 – 3 0 5 1
3049

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8
2
1
by a Teflon window mounted at Brewster angle in order to
improve transmission and to reduce standing waves. Cl
diluted with He (2:100) or CF mixed with Ar (CF
:Ar ¼
:10) were used as a source for atomic chlorine or fluorine,
the CH
Br species was shifted to the lower frequencies by
2
about 1600 ꢂ 40 MHz.
4
4
We then carried out the search for the N ¼ 20 ’ 19 lines of
1
2
CH Br by scanning the 439–452 GHz frequency range. The
respectively. Cl or F atoms were produced by a microwave
discharge in a small quartz tube placed at the entrance of the
second crosspiece and surrounded by a cavity fed by radiation
from a magnetron at 2450 MHz. The power of the microwave
discharge was kept as low as possible (E20 W). The optimised
observed spectrum appeared so congested that we proceeded in
two steps: at first we scanned using the experimental conditions
described above. Subsequently we repeated the measurement
by applying a magnetic field along the cell in order to both
distinguish only the transitions sensitive to a magnetic field and
to eliminate the numerous and often strong absorption lines of
ꢁ
3
partial pressure for CH Br was 7 ꢀ 10 mbar, and the total
3
2
ꢁ
pressure was 1.5 ꢀ 10 mbar.
3
CH Br. This procedure led to the detection of more than 40
Before initiating our search for the spectra, we decided to
unidentified paramagnetic lines, most of them lying in the 443–
447 GHz frequency region. These lines could also be observed
using CF for the production of fluorine instead of chlorine,
3
2
5
37
extend the measurement of the spectra of CH
(
Cl and CH
natural abundances: Cl: 75.77%, Cl: 24.23%) in the 418–470
GHz frequency region. We have used the molecular constants
2
Cl
35
37
4
3 3
but they disappeared when CH Br was replaced with CH Cl.
17
published by Endo et al. to predict the spectra falling within
this range. The analysis of the spectra is now in progress for
publication. The observation of the spectral lines of the mono-
chloromethyl radical has allowed us to optimize the experimental
conditions in order to maximize our chance to detect transitions
of the bromomethyl radical. In particular, we found that the use
of Cl led to a much clearer spectrum than the use of CF .
At this stage, the chemical tests and the comparison with the ab
initio calculations support strongly the detection of the two
isotopomers of CH Br. Moreover, the majority of the transi-
2
tions appeared as partly resolved doublets or triplets, some-
times as singlets. This hyperfine structure could be due to the
7
9
81
nuclear spins of bromine (I( Br) ¼ I( Br) ¼ 3/2) and of
1
2
hydrogen (I(H) ¼ ). By comparison with the predictions, we
2
4
We have afterwards guided our search for CH
to the rotational constants deduced from the ab initio struc-
2
Br according
were quickly able to recognize two sets of lines of nearly equal
intensity, each set displaying an identical pattern. The average
difference between the transitions of these two sets is 1609
MHz, a value very close to the calculated one. A stick spectrum
of the detected lines is shown on Fig. 1.
2
ture of this radical in the ground state together with the
3
1
centrifugal distortion constants obtained for CH Cl. The
7
2
calculated A, B and C rotational constants are listed in Table
7
9
23
1
. The ab initio B þ C value of CH Br, 22 304 MHz, agrees
However, the electron spin–rotation interaction led ob-
viously to a strong mixing of the various K components. It
a
was therefore not possible to readily recognize a typical a-type
R branch pattern of a prolate asymmetric top, preventing us
from assigning the spectrum. We have consequently extended
the measurement in the lower frequency region in order to
2
2
2
with the value reported by Davies et al., 22 305 MHz. More
importantly, this group has also assigned the main rotational
quantum number N ¼ 20 ’ 19 to the unresolved lines lying at
ꢁ1
1
4.88 cm (446 091 MHz). This frequency falls in a range
where our spectrometer has its highest sensitivity. Bromine has
7
two stable isotopes of nearly equal abundances ( Br: 50.69%,
9
observe the various K
a
components of the N ¼ 19 ’ 18 series,
8
1
Br: 49.31%). The spectra of the two isotopomers should then
be detected with the same intensity. Assuming a planar struc-
ture, the dipole moment of CH Br lies along the C symmetry
2
2
axis, which is the axis of the least moment of inertia. The
rotational spectrum should hence display the characteristic
a-type pattern of a slightly asymmetric prolate rotor (k ¼
ꢁ
0.996). In addition, owing to the electron spin–rotation
interaction, each rotational level is split into two, leading
to a fine structure in the spectrum. As a first estimate of the
fine splittings, we have used for the prediction the eii cons-
1
7
tants of CH Cl (see Table 1). The two components of the
2
7
9
N ¼ 20 ’ 19, K
a
¼ 0 lines of CH
2
Br were predicted at
4
44597 and 444680 MHz. The four K ¼ 1 components were
a
79
calculated to lie at 440736, 440772, 450320 and 450767 MHz.
The corresponding calculated set of transition frequencies for
Fig. 1 Stick diagram of the N ¼ 20 ’ 19 transitions for the CH Br
2
81
and CH
2
Br species.
79
81 35
Br and CH
2
Table 1 Molecular constants of CH
2
Br, CH
2
Cl in the ground vibronic state
79
2
81
2
CH Br
CH
Br
a
b
c
b
c
35 cd
Parameter
Ab initio
Experimental
Ab initio
Experimental
2
CH Cl
A
B
C
278041.8
11375.7
10928.6
273774.8(87)
11395.1281(92)
10932.2056(90)
278041.8
11333.8
10889.9
274052.2(113)
11353.1712(75)
10893.5319(73)
274380(31)
15948.0282(50)
15057.0443(49)
0.022550(22)
0.50968(10)
D
D
D
N
0.0121011(25)
0.324795(39)
0.0120101(20)
0.319403(171)
NK
e
e
f
K
23.52
0.4684(30)
0.1880(38)
23.52
0.5149(28)
0.1584(31)
22.85
1.274(40)
3
10 d
N
f
d
K
0.3325
e
e
e
D
D
a
aa
ꢁ12535.06(88)
ꢁ12531.45(89)
ꢁ3149.45(14)
ꢁ237.623(46)
11.814(40)
bb
ꢁ697.684(69)
69.221(56)
0.3270(127)
0.719(42)
ꢁ695.986(67)
cc
S
NK
S
K
69.626(63)
g
0.3270
g
0.719
0.1120(65)
0.2635(77)
b
In megahertz. From ref. 23. One standard deviation in parentheses. Values in parentheses have been converted from ref. 17 to one standard
c
d
e
80
f
g
79
2 2 2
deviation. Fixed to the value of H C Se, from ref. 27. Fixed to the value of H CS. Fixed to the CH Br value.
3
050
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 0 4 9 – 3 0 5 1
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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The fact that reaction of CH
3
Br with Cl or F atoms led to
the observed transitions together with the relatively good
agreement between the experimental and ab initio A, B and C
constants for both isotopic species enables us to ascertain that
we have observed the rotational spectrum of the monobromo-
methyl radical. This is the first high-resolution spectroscopic
identification of this species. Work directed toward the analysis
of the hyperfine structure is in progress. The small positive
2
˚
value of the inertial defect, D
0
¼ 0.032 amu A proves the
planarity of CH Br in the ground vibronic state.
2
Acknowledgements
The CERLA is partly supported by the French Research
Ministry, the Nord-Pas-de-Calais Region, and the European
Funds for Regional Economic Development. Z. Z. thanks the
EGIDE/Barrande program and the projects No. A3040101,
No. A1010110 of the GA AS CR for support. C. D. thanks the
Embassy of France in China for supporting his stay in France
through a ‘‘Bourse Doctorale en Alternance’’.
Fig. 2 Observed spectrum of the 101,10–91,9, J ¼ 9.5–8.5 transition of
2 4 2 3
Br, using (a) CF or (b) Cl for the reaction with CH Br. The
8
1
CH
dashed lines have been obtained with a magnetic field to confirm that
the CH Br lines are paramagnetic. The spectrum was recorded by
2
integrating 20 scans with 1 Hz repetition rate and 10 ms time constant
of the lock-in amplifier.
7
9
focusing our search on the Br species. This led to the
detection of 22 new paramagnetic lines in the 422–425 GHz
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2
51, 39.
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7
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7
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7
9
series of CH2 Br allowed us to assign the K ¼ 3–6 compo-
7
9
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9
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8
1
The identification of the spectrum of CH2 Br was then
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1
1
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a
8
1
r 6 in the 160–470 GHz frequency range. For the CH
2
Br
isotopomer, only 42 transitions corresponding to N ¼ 9, 10 and
1
9 r N r 21, K r 6 have been measured. The average
a
1
1
3
4
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3
46, 717.
r
Hamiltonian in the I representation. Predictions and fittings
25
15 World Meteorological Organisation, 1992 Scientific Assessment of
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2
6
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1
7 Y. Endo, S. Saito and E. Hirota, Can. J. Phys., 1984, 62, 1347.
3
5
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2
9
0
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7
2
9
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2
2
3
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a
a
2
2
ꢁ1
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2
4
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¨
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2
5
6
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order of magnitude than the ones of CH
also identical. Finally, the eaa spin–rotation interaction con-
stant is about four times larger in CH Br than in CH Cl. Such
a difference was already observed between the CH Cl and
2
Cl. Their signs are
2
2
2
2
27 D. J. Clouthier, R. H. Judge and D. C. Moule, J. Mol. Spectrosc.,
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1
7
2
CH F radicals.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 3 0 4 9 – 3 0 5 1
3051