The microwave spectrum of the NCI radical in the electronically excited (a1Δ) state

Article abstract of DOI:10.1063/1.471621

The pure rotational spectrum of the ¹⁴N³⁵Cl radical in the first electronically excited a ¹Δ state was detected by microwave spectroscopy. The NCl radical was produced by a de-glow discharge of an N₂ and Cl₂ mixture between 175-210 K. Seven rotational transitions for ν=0 and five for ν=l, showing hyperfine structures due to the nitrogen and chlorine nuclei, were observed in the 162-404 GHz region. The rotational, centrifugal distortion, and hyperfine coupling constants including nuclear spin-rotation coupling constant of the chlorine nucleus were determined accurately by a least-squares analysis of the measured frequencies. The equilibrium structural parameters were derived and discussed.

Full text of DOI:10.1063/1.471621

The microwave spectrum of the NCl radical in the electronically excited (a 1Δ) state
Kaori Kobayashi, Masahiro Goto, Satoshi Yamamoto, and Shuji Saito
Citation: The Journal of Chemical Physics 104, 8865 (1996); doi: 10.1063/1.471621
View online: http://dx.doi.org/10.1063/1.471621
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Published by the AIP Publishing
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The microwave spectrum of the NCl radical in the electronically
1
excited (a ⌬) state
Kaori Kobayashi and Masahiro Gotoa)
Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444, Japan
Satoshi Yamamoto
Department of Physics, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Shuji Saito
Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444, Japan
͑
Received 12 February 1996; accepted 8 March 1996͒
The pure rotational spectrum of the 14N Cl radical in the first electronically excited a ⌬ state was
detected by microwave spectroscopy. The NCl radical was produced by a dc-glow discharge of an
N and Cl mixture between 175–210 K. Seven rotational transitions for ␷ϭ0 and five for ␷ϭ1,
35
1
2
2
showing hyperfine structures due to the nitrogen and chlorine nuclei, were observed in the 162–404
GHz region. The rotational, centrifugal distortion, and hyperfine coupling constants including
nuclear spin–rotation coupling constant of the chlorine nucleus were determined accurately by a
least-squares analysis of the measured frequencies. The equilibrium structural parameters were
derived and discussed. © 1996 American Institute of Physics. ͓S0021-9606͑96͒02222-2͔
INTRODUCTION
troscopy. The first spectroscopic identification of the NCl
radical was made by Milligan observing the infrared bands
8
The reports on microwave spectra of the electronically
excited states have been rather limited since its first report on
of NCl produced by photolysis of ClN trapped in an argon
3
3
Ϫ
matrix at 4.2 K. The microwave spectrum of NCl in the ⌺
ground electronic state was studied by Yamada and his
1
1
the SO͑a ⌬͒ radical in 1970. One of the reasons is a diffi-
culty in producing electronically excited molecules efficient
enough to be detected by microwave spectroscopic method.
collaborators9
–11
using a glow discharge of a nitrogen and
chlorine gas mixture. They determined its detailed molecular
constants and derived the spin densities of unpaired electrons
for nitrogen and chlorine atoms. The electronic spectra of
1
2
3
3
ϩ
3,4
1
5
So far studies on O ͑a ⌬ ͒, CO͑a ⌸, b ⌺ ͒, NF͑a ⌬͒,
2
g
1
ϩ
and SO͑b ⌺ ͒ ͑Ref. 6͒ molecules were reported in addition
to the 1970 report on SO͑a ⌬͒. Generally, electronic transi-
1
3
3
Ϫ
1
ϩ
3 Ϫ
NCl were studied for A ⌸–X ⌺ as well as b ⌺ –X ⌺
in the sixties,1
2,13
but it was in 1980 that the a ⌬ state of
1
tions between the states studied and the ground electronic
states are strongly forbidden by spin and/or orbital angular
momentum conservation rules, that is, the states studied are
metastable states. The excitation energy of the above meta-
NCl was made clear through observations of emission spec-
tra in the near infrared region.14 Pritt and his collaborators
found that the reaction of the chlorine atom with the molecu-
15
7
1
stable states ranges from 0.8 to 7 eV. The delta singlet oxy-
lar azide efficiently generates the NCl radical in the a ⌬
1
ϩ
1
gen is efficiently generated in discharged plasma of oxygen,
state as well as the b ⌺ state, and observed the a ⌬–X
3
3
ϩ
3
Ϫ
1
ϩ
3 Ϫ
CO is easily excited to the a ⌸ or b ⌺ state by the energy-
⌺ transition and the b ⌺ –X ⌺ transition in emission.
1
exchange reaction of CO with the excited helium atom, NF͑a
They determined the vibrational constants for the a ⌬ state
1
Ϫ1
⌬
͒ is produced by exothermic reaction of the hydrogen atom
and T to be 9260 cm . However, so far no molecular con-
e
1
1
with the NF radical, and SO is excited into the a ⌬ or b ⌺
stants related to rotational motion and hyperfine interaction
have been available for the a 1⌬ state. Several quantum
chemical calculations were made to predict the molecular
2
3
Ϫ
state by a spin-exchange reaction of SO͑X ⌺ ͒ with O ͑a
2
1
⌬g͒. The detailed molecular constants of electronically ex-
16–18
cited states make clear how the molecular structure changes
by an electronic excitation in the molecule. The changes in
the molecular constants could give us clues to understanding
changes in chemical reactivity of the molecules in the ground
electronic state and the electronically excited state.
properties of NCl including its geometrical structure,
electronic structure,19 and radiative lifetime.
20,21
In the present study, we have observed the pure rota-
1
tional spectrum of NCl͑a ⌬͒ in the ␷ϭ0 and ␷ϭ1 states by
microwave spectroscopy, determined its precise molecular
constants, and we derived the equilibrium structure.
The NCl radical, similar to the NF radical, has the first
electronically excited state at about 1 eV above the ground
electronic state, and the electronic transition between the a
1
3 Ϫ
⌬
state and the X ⌺ state is strongly forbidden. Therefore
EXPERIMENT
1
the NCl radical in the a ⌬ state has a relatively long lifetime
1
and is a good candidate to be studied by microwave spec-
The spectral lines of NCl in the a ⌬ state were first
detected by using a dc-glow discharge in a mixture of AlCl3
and N2 with a millimeter-wave spectrometer at Nagoya
University. The identification of the spectral lines was based
a͒Present address: National Industrial Research Institute of Nagoya, Hirate-
cho, Kita-ku, Nagoya Aichi 462, Japan.
4
J. Chem. Phys. 104 (22), 8 June 1996
0021-9606/96/104(22)/8865/6/$10.00
© 1996 American Institute of Physics
8865
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8866
Kobayashi et al.: Microwave spectrum of the NCl radical
TABLE I. Observed rotational transitions of NCl ͑a 1⌬͒ in the ␷ϭ0 state.
⌬
␯͑obs.–calc.͒
͑MHz͒
JЈ–J
G
FЈ–F
␯obs ͑MHz͒a
Weight
1.0
4
4
4
4
5
5
5
5
5
5
5
–3
–3
–3
–3
–4
–4
–4
–4
–4
–4
–4
2.5
2.5
1.5
0.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
0.5
0.5
2.5
2.5
2.5
2.5
1.5
2.5
2.5
1.5
1.5
1.5
0.5
0.5
2.5
2.5
2.5
0.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
0.5
6.5–5.5
5.5–4.5
5.5–4.5
4.5–3.5
7.5–6.5
6.5–5.5
5.5–4.5
4.5–3.5
3.5–2.5
2.5–1.5
6.5–5.5
5.5–4.5
4.5–3.5
3.5–2.5
5.5–4.5
4.5–3.5
8.5–7.5
7.5–6.5
6.5–5.5
5.5–4.5
4.5–3.5
4.5–3.5
3.5–2.5
7.5–6.5
6.5–5.5
5.5–4.5
6.5–5.5
5.5–4.5
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
6.5–5.5
5.5–4.5
4.5–3.5
8.5–7.5
7.5–6.5
6.5–5.5
5.5–4.5
7.5–6.5
161 514.573͑6͒
161 562.446͑19͒
161 532.878͑24͒
161 551.625͑27͒
201 912.416͑15͒
201 939.580͑20͒
201 962.391͑26͒
201 979.138͑36͒
201 989.791͑41͒
201 994.475͑62͒
201 924.924͑19͒
201 946.292͑21͒
201 963.951͑50͒
201 974.150͑11͒
201 937.438͑15͒
201 954.479͑34͒
242 298.360͑15͒
242 315.429͑18͒
242 330.366͑7͒
Ϫ0.018
Ϫ0.257
0.002
b
0.0
1.0
0.0
b
0.077
Ϫ0.033
Ϫ0.004
0.020
Ϫ0.023
Ϫ0.024
0.019
Ϫ0.011
Ϫ0.004
Ϫ0.029
Ϫ0.006
0.033
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1
5–4
FIG. 1. The Jϭ9–8 transition in the ␷ϭ0 state of NCl͑a ⌬͒. The NCl was
5
5
5
5
6
6
–4
–4
–4
–4
–5
–5
generated by a dc-glow discharge in a 1:1 mixture of N2 and Cl ͑9.98%
2
dispersed in helium gas͒ at a total pressure of 40 mTorr. The integration
time was 20 s.
Ϫ0.006
0.111
Ϫ0.035
0.006
bd
0.0
1.0
1.0
on production chemistry indicating the necessity of chlorine
and nitrogen atoms, and we tentatively derived a rotational
constant from the observation of the complicated spectral
patterns. Then, all the spectral lines in the millimeter- and
submillimeter-wave regions were measured by a spectrom-
6–5
6
6
6
6
–5
–5
–5
–5
0.0bc
242 341.168͑29͒
Ϫ0.187
242 349.398͑31͒
242 354.193͑26͒
242 307.352͑13͒
242 321.078͑20͒
242 333.155͑18͒
242 316.252͑16͒
242 328.002͑21͒
282 674.241͑5͒
282 686.027͑11͒
Ϫ0.006
0.001
0.002
Ϫ0.006
0.049
0.009
0.006
0.039
0.018
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
6–5
6–5
6–5
eter combined with a 2 m free-space cell at the Institute for
Molecular Science.22
6
6
7
–5
–5
–6
The production of NCl was optimized by monitoring its
9
spectral lines of the ground electronic state. The measure-
ments were carried out by cooling the cell to the temperature
of 175–210 K with liquid nitrogen. For the precise frequency
measurements of the spectral line frequencies, a dc-glow dis-
charge in a 1:1 mixture of N and Cl ͑9.98% dispersed in
7–6
7–6
1.0c
282 696.512͑10͒
0.005
7
7
7
7
–6
–6
–6
–6
282 704.580͑10͒
282 710.485͑18͒
282 714.864͑12͒
282 681.160͑12͒
282 690.681͑20͒
282 699.278͑9͒
282 705.771͑6͒
282 687.783͑6͒
323 040.697͑2͒
323 049.289͑12͒
323 057.049͑6͒
323 063.187͑6͒
323 067.710͑4͒
323 071.590͑11͒
323 046.158͑12͒
323 053.129͑9͒
323 059.505͑9͒
323 064.958͑9͒
323 051.279͑9͒
323 058.133͑5͒
Ϫ0.060
0.011
0.001
0.021
0.014
0.004
0.035
Ϫ0.001
0.001
0.028
Ϫ0.029
Ϫ0.016
0.004
Ϫ0.015
0.002
Ϫ0.017
Ϫ0.006
Ϫ0.005
Ϫ0.023
0.016
Ϫ0.003
0.003
Ϫ0.013
0.001
Ϫ0.005
Ϫ0.023
Ϫ0.011
0.025
0.0b
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2
2
helium gas͒ was used, because this production system gave a
smaller number of disturbing lines due to diamagnetic mol-
7–6
7–6
1
ecules, and also reasonably strong signals of NCl͑a ⌬͒. A
7
–6
larger discharge current ͑150 mA͒ and a higher pressure ͑40
9
7–6
mTorr͒ than the condition of NCl in the ground electronic
7
8
–6
–7
1
state were required to obtain sufficient intensity of NCl͑a ⌬͒
2.5 10.5–9.5
in the ␷ϭ0 state transition. Observed line widths range from
8–7
8–7
2.5
2.5
2.5
2.5
2.5
1.5
1.5
1.5
1.5
0.5
0.5
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
5.5–4.5
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
8.5–7.5
7.5–6.5
0.5 to 0.7 MHz. The ⌳-type doubling splitting was not ob-
8
8
8
8
–7
–7
–7
–7
served. For the ground vibrational state, we observed seven
rotational transitions from Jϭ4–3 to 10–9 in the 162–404
GHz region. Each rotational transition splits into 12 compo-
nents due to hyperfine interaction of the nitrogen and chlo-
rine nuclei so that in total 70 spectral lines in the ␷ϭ0 state
were measured. The lines with Jϭ4–3 state were so weak
that only four components were measured, and with high J
some components overlapped with each other. Figure 1
shows an example of the whole spectral pattern for the
Jϭ9–8 transition. The observed spectral lines in the ␷ϭ0
state are listed in Table I.
8–7
8–7
8
8
8
9
–7
–7
–7
–8
2.5 11.5–10.5 363 397.381͑8͒
2.5 10.5–9.5
2.5
2.5
2.5
2.5
9–8
9–8
363 403.871͑4͒
363 409.889͑26͒
363 414.688͑10͒
363 418.247͑12͒
363 421.660͑21͒
363 401.778͑6͒
363 407.164͑5͒
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
9
–8
9–8
–8
1
The transition frequencies of NCl͑a ⌬͒ in the ␷ϭ1 state
9
2
3
were estimated from the anharmonicity constant, ␹ , given
e
9–8
9–8
9–8
1.5 10.5–9.5
15
by infrared emission spectroscopy. The Jϭ9–8 transition
frequency in the ␷ϭ1 state is thus expected to be lower than
that of the ground state by 2.7 GHz. Hence, we have
searched for them in the 361 GHz region. Complicated spec-
tral lines similar to the ground vibrational state lines were
1.5
1.5
0.5
1.5
0.5
9.5–8.5
8.5–7.5
8.5–7.5
7.5–6.5
9.5–8.5
1.0c
363 411.706͑13͒
0.009
9
9
9
–8
–8
–8
363 416.629͑8͒
363 405.856͑2͒
Ϫ0.035
Ϫ0.032
1.0
1.0
J. Chem. Phys., Vol. 104, No. 22, 8 June 1996
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Kobayashi et al.: Microwave spectrum of the NCl radical
8867
TABLE I. ͑Continued.͒
TABLE II. Observed rotational transitions of NCl ͑a 1⌬͒ in the ␷ϭ1 state.
⌬
␯͑obs.–calc.͒
͑MHz͒
JЈ–J
G
FЈ–F
␯obs ͑MHz͒a
⌬␯͑obs.–calc.͒ ͑MHz͒ Weight
JЈ–J
G
FЈ–F
␯obs ͑MHz͒a
Weight
6
–5 2.5
8.5–7.5
7.5–6.5
6.5–5.5
240 034.474͑14͒
240 051.506͑26͒
240 066.382͑6͒
Ϫ0.006
Ϫ0.042
0.018
1.0
1.0
1.0
10–9
10–9
10–9
10–9
10–9
10–9
10–9
10–9
10–9
10–9
10–9
10–9
2.5
2.5
2.5
2.5
2.5
1.5
2.5
1.5
1.5
0.5
1.5
0.5
12.5–11.5 403 743.588͑10͒
11.5–10.5 403 748.686͑5͒
0.001
0.026
Ϫ0.012
0.010
1.0
1.0
1.0
1.0
6–5 2.5
6–5 2.5
6–5 2.5 5.5 –4.5
10.5–9.5
9.5–8.5
8.5–7.5
8.5–7.5
7.5–6.5
11.5–10.5 403 747.172͑16͒
10.5–9.5
10.5–9.5
9.5–8.5
9.5–8.5
403 753.441͑15͒
403 757.309͑8͒
2
40 078.266͑9͒
0.949
0.0bcd
6–5 1.5
6–5 2.5
6–5 1.5
6–5 1.5
6–5 1.5
6–5 0.5
6–5 0.5
7–6 2.5
7–6 2.5
4.5–3.5
3.5–2.5
7.5–6.5
6.5–5.5
5.5–4.5
6.5–5.5
5.5–4.5
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
6.5–5.5
5.5–4.5
4.5–3.5
8.5–7.5
7.5–6.5
6.5–5.5
5.5–4.5
7.5–6.5
240 090.374͑47͒
240 043.508͑15͒
240 057.153͑27͒
240 069.010͑22͒
240 052.444͑32͒
240 064.102͑21͒
280 033.178͑31͒
280 044.814͑38͒
0.269
Ϫ0.052
0.027
0.018
0.009
0.009
0.026
Ϫ0.038
0.0b
1.0
1.0
1.0
1.0
1.0
1.0
1.0
4
03 759.789͑13͒
0.064
0.0bc
403 763.195͑4͒
0.001
Ϫ0.038
1.0
1.0
4
03 751.043͑11͒
03 755.193͑11͒
0.049
0.009
1.0c
4
1.0c
7
–6 2.5
280 055.385͑10͒
0.036
1.0c
aThe numbers in parentheses represent the variances of the observed value
7–6 0.5
in units of the last significant digits.
7
7
7
7
7
7
–6 2.5
–6 2.5
–6 2.5
–6 1.5
–6 1.5
–6 1.5
280 063.363͑21͒
280 069.226͑30͒
280 073.679͑33͒
280 040.111͑52͒
280 049.449͑26͒
280 057.917͑19͒
280 064.560͑45͒
280 046.742͑17͒
320 022.399͑18͒
320 030.916͑14͒
320 038.622͑14͒
320 044.752͑10͒
320 049.171͑14͒
320 053.126͑4͒
320 027.866͑14͒
320 034.769͑13͒
320 041.079͑20͒
320 046.573͑12͒
320 033.014͑14͒
320 039.803͑9͒
Ϫ0.003
0.093
0.097
1.0
0.0
0.0
b
Not included in the fit.
b
c
Overlap of two NCl spectral lines.
b
d
Not accurately measured due to the disturbance.
0.029
1.0
1.0
1.0
1.0
1.0
1.0
1.0
Ϫ0.027
Ϫ0.024
0.014
0.025
Ϫ0.016
0.018
found at about 360 GHz, which is lower than the ␷ϭ0 line
frequency by 3.4 GHz. Then, other lines were found readily.
In this way five rotational transitions were observed. This
vibrational excited state species was generated by dc-glow
discharge ͑150–300 mA͒ of N ͑7 mTorr͒ and Cl diluted
with helium gas ͑20 mTorr͒ between 175–210 K. In this
temperature range, the intensity does not depend dramati-
cally on the temperature though the optimized temperature
was about 195 K. Around 170 K, the lines disappeared
which corresponds to the freeze out of chlorine gas ͑m.p.
72.17 K͒. While searching the optimum condition for the
ϭ1 state, we noted that cleanliness of the cell affected the
production efficiency of the metastable molecules, because
the transitions of using NCl͑a ⌬͒ were not observed by
using a cell which had been used in the prior experiment
using Si compound. We observed five rotational transitions
of Jϭ6–5 to 10–9 in ␷ϭ1 state, 49 components in total. The
intensity ratio of the ␷ϭ1 state to the ␷ϭ0 state was evalu-
ated to be roughly 0.3 for the Jϭ9–8, Gϭ2.5, and Fϭ11.5–
7–6 1.5
7
8
8
8
–6 0.5
–7 2.5 10.5–9.5
–7 2.5
–7 2.5
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
5.5–4.5
9.5–8.5
8.5–7.5
7.5–6.5
6.5–5.5
8.5–7.5
7.5–6.5
b
Ϫ0.062
Ϫ0.005
Ϫ0.029
Ϫ0.023
Ϫ0.009
0.015
0.0
2
2
8–7 2.5
8–7 2.5
1.0
1.0
1.0
1.0
1.0
8
8
8
8
–7 2.5
–7 1.5
–7 1.5
–7 1.5
b
0.075
0.0
8–7 1.5
8–7 0.5
Ϫ0.011
0.000
Ϫ0.004
0.008
0.026
0.025
1.0
1.0
1.0
1.0
1.0
1.0
1
␷
8
9
9
9
–7 0.5
–8 2.5 11.5–10.5 360 001.929͑6͒
–8 2.5 10.5–9.5
–8 2.5
360 008.365͑11͒
360 022.641͑16͒
360 026.009͑6͒
360 006.335͑10͒
360 011.638͑5͒
360 021.111͑21͒
360 010.391͑4͒
1
7.5–6.5
6.5–5.5
b
9–8 2.5
9–8 1.5 10.5–9.5
Ϫ0.075
0.006
0.050
Ϫ0.013
Ϫ0.032
Ϫ0.027
0.023
Ϫ0.101
0.012
0.0
1.0
1.0
1.0
1.0
1.0
1.0
9
9
9
–8 1.5
–8 1.5
–8 0.5
9.5–8.5
7.5–6.5
9.5–8.5
10–9 2.5 12.5–11.5 399 970.965͑15͒
10–9 2.5 11.5–10.5 399 976.036͑13͒
1
0.5 transition with a discharging current at 300 mA. The
b
vibrational temperature was roughly estimated to be 1100 K
from the intensity ratio of two vibrational states, assuming
that the dipole moments in both states are the same. The
observed transition frequencies for the ␷ϭ1 state are listed in
Table II.
10–9 2.5 10.5–9.5
399 980.691͑32͒
399 984.621͑31͒
399 990.460͑21͒
0.0
10–9 2.5
9.5–8.5
7.5–6.5
1.0
1.0
0.0
10–9 2.5
Ϫ0.031
Ϫ0.127
b
10–9 1.5 11.5–10.5 399 974.493͑20͒
10–9 1.5 10.5–9.5
3
3
99 978.372͑13͒
99 982.500͑33͒
0.007
1.0c
10–9 0.5 10.5–9.5
1
1
0–9 1.5
0–9 0.5
9.5–8.5
9.5–8.5
Ϫ0.012
1.0c
ANALYSIS
aThe numbers in parentheses represent the variances of the observed value
in units of the last significant digits.
In the initial stage of the analysis, we used the coupling
scheme of a sequential addition of the angular momenta as
F ϭJϩI and FϭF ϩI , and evaluated the matrix ele-
b
Not included in the fit.
c
Overlap of two NCl spectra lines.
1
Cl
1
N
d
5
Not accurately measured due to the disturbance.
ments of the appropriate Hamiltonian given as
HϭH ϩH
,
hfs
͑1͒
͑2͒
rot
where
H ϭB͓J͑Jϩ1͒Ϫ⌳ ͔ϪD͓J͑Jϩ1͒Ϫ⌳ ͔ ,
Hhfsϭa͑Cl͒ICl–Lϩa͑N͒IN–LϩHQ͑Cl͒ϩHQ͑N͒
2
2 2
ϩC ͑Cl͒I –͑JϪL͒ϩC ͑N͒I –͑JϪL͒.
͑3͒
rot
I
Cl
I
N
and
Here HQ represents nuclear-quadrupole interaction. How-
J. Chem. Phys., Vol. 104, No. 22, 8 June 1996
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8868
Kobayashi et al.: Microwave spectrum of the NCl radical
TABLE III. Matrix elements for the hyperfine Hamiltonian of the ⌬ state.a
1
F
GЈ JЈ I1 GЈ I2
JϩI ϩI ϩGЈϩGϩFϩ1
͓I1͑I1ϩ1͒͑2I1ϩ1͒͑2GЈϩ1͒͑2Gϩ1͒͑2JЈϩ1͒͑2Jϩ1͔͒1/2
1
2
͗
͗
͗
͗
͗
͗
I1I2GЈJЈ⌳FMF
͉
a͑1͒I1•L
͉
I1I2GJ⌳FMF
͘
ϭ͑Ϫ1͒
ͭ
ͮͭ
ͮ
1
J
G
G
I1
1
JЈ
1
J
JЈϪ⌳
ϫ͑Ϫ1͒
ͩ
ͪ
⌳a͑1͒
Ϫ⌳
0
⌳
F
GЈ JЈ I2 GЈ I1
JϩI ϩI ϩ2GЈϩFϩ1
1
2
I1I2GЈJЈ⌳FMF
I1I2GЈJЈ⌳FMF
I1I2GЈJЈ⌳FMF
I1I2GЈJЈ⌳FMF
I1I2GЈJЈ⌳FMF
͉
͉
͉
͉
͉
a͑2͒I2•L
͉
I1I2GJ⌳FMF
͘
ϭ͑Ϫ1͒
ͭ
ͮͭ
ͮ
͓I2͑I2ϩ1͒͑2I2ϩ1͒͑2GЈϩ1͒͑2Gϩ1͒
1
J
G
G
I2
1
JЈ
1
0
J
ϫ͑2JЈϩ1͒͑2Jϩ1͔͒1/2͑Ϫ1͒
JЈϪ⌳
⌳a͑2͒
ͩ
ͪ
Ϫ⌳
⌳
1/2
F
GЈ JЈ I1 GЈ
I
͑I ϩ1͒͑2I ϩ1͒͑2I ϩ3͒
2
1
1
1
1ϩI2ϩGЈϩGϩF
HQ͑1͒
HQ͑2͒
͉
͉
I1I2GJ⌳FMF
I1I2GJ⌳FMF
͘
͘
ϭ͑Ϫ1͒JϩI
ͭ
ͮͭ
ͮ
ͫ
ͬ
͓͑2GЈϩ1͒͑2Gϩ1͒
2
J
G
G
I1
2
I1͑2I1Ϫ1͒
JЈ
2
0
J
ϫ͑2JЈϩ1͒͑2Jϩ1͔͒1 ͑Ϫ1͒
/2
JЈϪ⌳
eQq͑1͒
4
ͩ
ͪ
Ͳ
Ϫ⌳
⌳
1/2
F
GЈ JЈ I2 GЈ
I
͑I ϩ1͒͑2I ϩ1͒͑2I ϩ3͒
1
2
2
2
ϭ͑Ϫ1͒JϩI
1ϩI2ϩ2GЈϩF
͓͑2GЈϩ1͒͑2Gϩ1͒
ͭ
ͮͭ
ͮ
ͫ
ͬ
2
J
G
G
I2
2
I2͑2I2Ϫ1͒
JЈ
2
J
ϫ͑2JЈϩ1͒͑2Jϩ1͔͒1/2͑Ϫ1͒JЈϪ⌳
eQq͑2͒
4
ͩ
ͪ
Ͳ
Ϫ⌳
0
⌳
F
GЈ JЈ I1 GЈ I2
1ϩI2ϩGЈϩGϩFϩ1
1/2
Hnsr͑1͒
Hnsr͑2͒
͉
͉
I1I2GJ⌳FMF
͘
͘
ϭ͑Ϫ1͒JϩI
ͭ
ͮͭ
ͮ
͓I1͑I1ϩ1͒͑2I1ϩ1͒͑2GЈϩ1͒͑2Gϩ1͔͒
1
J
G
G
I
1
1
JЈ
1
0
J
͓J͑Jϩ1͒͑2Jϩ1͔͒1 Ϫ͓͑2JЈϩ1͒͑2Jϩ1͔͒ ͑Ϫ1͒
/2
1/2
JЈϪ⌳
⌳
C ͑1͒
ϫ
ͫ
␦
ͩ
ͪ
ͬ
JЈJ
I
Ϫ⌳
⌳
F
GЈ JЈ I2 GЈ I1
JϩI ϩI2ϩ2GЈϩFϩ1
1/2
1
I1I2GJ⌳FMF
ϭ͑Ϫ1͒
ͭ
ͮͭ
ͮ
͓I2͑I2ϩ1͒͑2I2ϩ1͒͑2GЈϩ1͒͑2Gϩ1͔͒
1
J
G
G
I2
1
JЈ
1
0
J
͓J͑Jϩ1͒͑2Jϩ1͔͒1 Ϫ͓͑2JЈϩ1͒͑2Jϩ1͔͒ ͑Ϫ1͒
/2
1/2
JЈϪ⌳
⌳
C ͑2͒
ϫ
ͫ
␦
ͩ
ͪ
ͬ
JЈJ
I
Ϫ⌳
⌳
a
Coupling scheme of the angular momenta is as follows: GϭI ϩI , FϭJϩG. For NCl ͑a 1⌬͒, I1 and I2 corresponds to ICl and IN , respectively.
1
2
ever, we found that the above coupling scheme does not hold
spectral patterns. Finally, the 70 observed spectral lines for
␷ϭ0 and 49 lines for ␷ϭ1 were analyzed to determine mo-
lecular constants by the least-squares method. The nuclear
spin–rotation constant of the nitrogen nucleus was found to
be smaller than that of the chlorine nucleus by one order of
magnitude so that it was fixed to be zero in the final analysis.
The standard deviations of the final fit for the ␷ϭ0 and the
␷ϭ1 transitions were 21 and 24 kHz, respectively, which
could be compared with frequency measurement errors, as
shown in Table I and Table II. The molecular constants for
the ground vibrational state and the first excited state are
listed in Table IV.
1
well for the a ⌬ state of NCl, and F is not a good quantum
1
number. In fact, we had difficulty in assigning several weak
components of the hyperfine structure. This is because hy-
perfine interactions due to the nitrogen and chlorine nuclei
have nearly the same orders of magnitude, as suggested by
the hyperfine spectral pattern shown in Fig. 1. Therefore, we
adopted the following coupling scheme of the angular mo-
menta:
GϭI ϩIN
͑4͒
Cl
and
FϭJϩG.
͑5͒
DISCUSSION
The matrix elements of the Hamiltonian given above were
derived by the standard spherical-tensor formalism.24 The
derived elements were summarized in Table III. In this cou-
pling scheme, ⌬Gϭ0 and ⌬FϭϮ1,0 transitions are allowed.
A least-squares fit program was written for the ⌬ state hav-
ing two nuclei with different spins.
In the previous studies it was reported that the reaction
of the chlorine atom with the molecular azide was essential
to produce the NCl radical in the a ⌬ state. However, we
1
1
found that a simple dc-glow discharge of a mixture of Cl2
1
and N gives a reasonable amount of the radical in the a ⌬
2
1
An initial guess of hyperfine coupling constants and
nuclear electric quadrupole coupling constants was made
from a comparison of the corresponding constants for NCl͑X
state in the plasma. The NCl͑a ⌬͒ radical is generated from
2
2
N͑ D͒ and Cl͑ P͒ when produced by three-body reaction of
3
Ϫ
4
2
19
atoms, while NCl͑X ⌺ ͒, from N͑ S͒ and Cl͑ P͒. Larger
currentand higher pressure may be linked with the sufficient
production of N͑ D͒ other than the production by a simple
3
Ϫ 9
1
5
⌺
͒ and NF͑a ⌬͒. Then, definite assignments for hyper-
2
fine components were made on the basis of the predicted
J. Chem. Phys., Vol. 104, No. 22, 8 June 1996
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Kobayashi et al.: Microwave spectrum of the NCl radical
TABLE IV. Molecular constants of NCl ͑a 1⌬͒ ͑MHz͒.a TABLE V. Equilibrium structural parameters of NCl ͑a 1⌬͒.a
8869
␷
0
1
This study
Infrared emissionb
B
D
a͑Cl͒
a͑N͒
eQq͑Cl͒
eQq͑N͒
C ͑Cl͒
20 196.298 60͑160͒
0.045 238 1͑112͒
102.515͑152͒
91.43͑21͒
Ϫ52.54͑68͒
1.72͑46͒
20 007.6252͑24͒
0.0450 240͑166͒
101.48͑36͒
91.53͑38͒
Ϫ54.07͑144͒
2.07͑84͒
Be ͑MHz͒
De ͑MHz͒
␣e͑MHz͒
20 290.6353͑27͒
0.045 345 2͑187͒
188.6734͑29͒
Ϫ0.000 214͑20͒
905.498͑187͒
␤e ͑MHz͒
Ϫ1 c
␻ ͑cm
͒
904͑18͒
4.7͑36͒
e
Ϫ1 d
␻e␹e ͑cm
re ͑Å͒
͒
6.392 99͑178͒
1.578 272 6͑20͒
0.0196͑99͒
0.0159͑165͒
I
aThe numbers in parentheses represent three standard deviations in units of
the last significant digits.
aThe numbers in parentheses represent three standard deviations in units of
the last significant digits.
b
Reference 15.
c
3
2
Derived by the following relation, D ϭ4B /␻ .
e
e
e
d
Derived by the following relation, ␣ ϭ(6B /␻ )[(␻ ␹ /B )1/2Ϫ1] by as-
2
3
Ϫ
1
e e e e e e
excitation of NCl͑X ⌺ ͒ to NCl͑a ⌬͒ by discharge. Better
production with a shorter cathode length indicates that an
even smaller area of the metal electrode may somewhat dis-
turb the recombination of the nitrogen and chlorine atoms or
suming Morse potential.
value.27 It is to be noted that the total spin density of NCl͑a
1
1
1
the quenching of NCl͑a ⌬͒ to the ground electronic state.
⌬
͒ is less than 90%. The spin densities of the a ⌬ state and
Using the following relation23 by assuming Morse func-
tion as an internuclear potential:
3
Ϫ
the X ⌺ state are quite similar, though these evaluations by
the orbital average and the spin average include at least 15%
difference.28 This hampers a detailed discussion on the spin
density change in NCl by electronic excitation. We can only
3
2
␹eBe 6Be
␣eϭ6
ͱ
Ϫ
,
͑6͒
␻e
␻e
1
say that the spin density of NCl͑a ⌬͒ is nearly the same as
we estimated the rotation–vibration coupling constant ␣ to
that of NCl in the ground electronic state.
e
search for lines in the ␷ϭ1 state with ␻ ␹ obtained from
The ionic character of Cl was estimated from the quad-
rupole coupling constant of the Cl nucleus. The increase in
e
e
infrared emission spectroscopy.15 However ␣ was found to
e
2
e
2
e
3
Ϫ
1
be underestimated. On the other hand, since B /␻ is small
the ionic character from NCl͑X ⌺ ͒ to NCl͑a ⌬͒ is consis-
tent with the decrease in the bond length: reϭ1.610 705͑19͒
enough, Dunham expansion25 is applicable. By assuming Y11
and Y20 to be Ϫ␣ and Ϫ␻ ␹ , cubic and quartic internu-
3
Ϫ
1
Å for X ⌺ ͑Ref. 11͒ to reϭ1.578 272 6͑20͒ Å for a ⌬.
The quadrupole coupling constant of the N nucleus is equal
to that of the ground state within experimental uncertainties.
This is because the change in the small absolute value is
difficult to distinguish. The differences in the hyperfine pa-
rameters between ␷ϭ0 and ␷ϭ1 were small and marginal.
While the nuclear spin rotation coupling constant of the
Cl nucleus, CI͑Cl͒, was determined, that of the N nucleus,
CI͑N͒, could not be determined in the present study. The
nuclear spin rotation coupling constant are estimated by the
e
e e
clear potential constants, a and a were determined to be
1
2
1
Ϫ3.1 and 7.2 for NCl͑a ⌬͒. By these constants, ␤ was
e
calculated to be Ϫ0.000 44 MHz. The experimental value of
our study was Ϫ0.000 214͑20͒ MHz which is the same order
of magnitude and thought to be reasonable. This is caused by
relatively large value of a to a . The small difference be-
2
1
tween a and a decides the sign of ␤ . If Morse potential
1
2
e
was assumed, ␤ should be a positive value. Both results, the
e
underestimation of ␣ and the negative value of ␤ , indicate
e
e
1
5
that the Morse potential is not a good approximation in this
case.
second-order perturbation in the same manner as NF͑a ⌬͒.
4
Ba
1
Since molecular constants of NCl͑a ⌬͒ in the ␷ϭ0 and
C ϭ
I
.
͑7͒
⌬
E
␷ϭ1 levels were determined in the present study, equilibrium
1
parameters are derived as shown in Table V where vibra-
⌬E represents an energy difference between the a ⌬ state
tional parameters by infrared emission spectroscopy15 are
and the nearest perturbing state. From C ͑Cl͒, this energy
I
also given. The equilibrium bond length, r , is determined to
difference is calculated to be ca. 1.8 eV. To the best of our
e
be 1.578 272 6͑20͒ Å. The three standard errors in Planck’s
constant and the atomic mass unit26 are included in error
estimation. The bond length from an ab initio MO
TABLE VI. Hyperfine constants of each nucleus and derived parameters.
1
7
calculation at the estimated full configuration interaction
level, is 1.598 Å, which is slightly larger than our result. The
vibrational frequency given by infrared emission
NCl͑a 1⌬͒␷ϭ0
3 Ϫ
NCl͑X ⌺ ͒
a
Spin density on the N atom
Spin density on the Cl atom
Electron transfer ⌬p
eQq͑N͒ ͑MHz͒
eQq͑Cl͒ ͑MHz͒
Ionic character ic
0.659
0.234
0.47
1.72
Ϫ52.54
0.29
0.758
a
0.219
1
5
spectroscopy also agrees well with our more accurate value
0.44
Ϫ1
of 905.498͑187͒ cm derived by a well-known relation,
1.842
Ϫ63.13
0.21
3
2
D ϭ4B /␻ .
e
e
e
The spin density and hyperfine constants of each nucleus
are summarized in Table VI together with those of
C ͑Cl͒ ͑MHz͒
0.0196
This study
0.0152
Ref. 9
I
3
Ϫ 9
NCl͑X ⌺ ͒. The spin density of each nucleus was evalu-
ated from the ratio of hyperfine constants, a to the atomic
aCalculated from Ϫ͑5/3͒c.
J. Chem. Phys., Vol. 104, No. 22, 8 June 1996
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141.217.58.222 On: Wed, 26 Nov 2014 04:14:37

8870
Kobayashi et al.: Microwave spectrum of the NCl radical
8
knowledge, no corresponding electronic state is reported
even in the references on quantum chemical calculations. By
D. E. Milligan, J. Chem. Phys. 35, 372 ͑1961͒.
9
0
1
C. Yamada, Y. Endo, and E. Hirota, J. Chem. Phys. 79, 4159 ͑1983͒.
C. Yamada, Y. Endo, and E. Hirota, J. Mol. Spectrosc. 115, 105 ͑1986͒.
C. Yamada, Y. Endo, and E. Hirota, J. Mol. Spectrosc. 117, 134 ͑1986͒.
A. G. Briggs and R. G. W. Norrish, Proc. R. Soc. London Ser. A 278, 27
1
1
using this value, C ͑N͒ is calculated to be 0.017 MHz. As
I
our fitting results suggested that C ͑N͒ is smaller than C ͑Cl͒
I
I
12
by one order of magnitude, estimated C ͑N͒ is too large
I
͑
1964͒.
compared to our result. This discrepancy may be due to a
cancellation by the second term of nuclear-spin rotation in-
teraction which is a term of charge distribution about the
nucleus concerned.25
13
14
R. Colin and E. W. Jones, Can. J. Phys. 45, 301 ͑1967͒.
A. T. Pritt, Jr. and R. D. Coombe, Int. J. Chem. Kinet. 12, 741 ͑1980͒.
A. T. Pritt, Jr., D. Patel, and R. D. Coombe, J. Mol. Spectrosc. 87, 401
͑1981͒.
J. Lievin and J-Y. Metz, Theor. Chim. Acta͑Berlin͒ 67, 391 ͑1985͒.
K. A. Peterson and R. C. Woods, J. Chem. Phys. 93, 1876 ͑1990͒.
A. Papakondylis, A. Mavridis, and A. Metropoulos, J. Phys. Chem. 99,
10759 ͑1995͒.
15
1
1
1
6
7
8
ACKNOWLEDGMENTS
The authors thank M. Tanimoto for his critical reading
of the manuscript. Calculations were carried out at the Com-
puter Center of the Institute for Molecular Science. This
study was supported by Grant-in-Aid from Ministry of Edu-
cation, Science, and Culture ͑Grant Nos. 03249211 and
1
2
9
0
M. Bettendorff and S. D. Peyerimhoff, Chem. Phys. 104, 29 ͑1986͒.
M. Bettendorff, R. Klotz, and S. D. Peyerimhoff, Chem. Phys. 110, 315
͑
1986͒.
2
2
1
2
D. Yarkony, J. Chem. Phys. 86, 1642 ͑1987͒.
S. Saito and M. Goto, Astrophys. J. 410, L53 ͑1993͒.
C. L. Pekeris, Phys. Rev. 45, 98 ͑1934͒.
A. R. Edmonds, Angular Momentum in Quantum Mechanics ͑Princeton
University, Princeton, 1960͒.
C. H. Townes and A. L. Schawlow, Microwave Spectroscopy ͑Dover,
New York, 1975͒, Chap. 8.
I. Mills, T. Cvista sˇ , K. Homann, N. Kallay, and K. Kuchitsu, Quantities,
Units and Symbols in Physical Chemistry ͑Blackwell Scientific, Oxford,
1988͒.
J. R. Morton and K. F. Preston, J. Magn. Reson. 30, 577 ͑1978͒.
E. Hirota, High-Resolution Spectroscopy of Transient Molecules
͑Springer-Verlag, Berlin, 1985͒.
04233107͒.
23
24
1
2
S. Saito, J. Chem. Phys. 53, 2544 ͑1970͒.
G. Cazzoli, C. D. Esposti, and P. G. Favero, Chem. Phys. Lett. 100, 99
25
26
͑
1983͒.
3
4
5
6
7
C. S. Gudeman and R. C. Woods, Phys. Rev. Lett. 48, 1344 ͑1982͒.
S. Yamamoto and S. Saito, J. Chem. Phys. 89, 1936 ͑1988͒.
S. Yamamoto and S. Saito, J. Chem. Phys. 86, 102 ͑1987͒.
S. Yamamoto, Chem. Phys. Lett. 212, 113 ͑1993͒.
K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure
IV: Constants of Diatomic Molecules ͑van Nostrand Reinhold, New York,
2
2
7
8
1
979͒.
J. Chem. Phys., Vol. 104, No. 22, 8 June 1996
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