Microwave spectrum of NCl in the second electronically excited state (b 1∑+)

Article abstract of DOI:10.1021/jp962121k

The pure rotational transitions of N³⁵Cl in the second electronically excited state, b ¹∑⁺, were detected together with those of the isotopic species N³⁷Cl by microwave spectroscopy. The NCl radical in the b ¹∑⁺ state was generated by de-glow discharge of a N₂/Cl₂ mixture. The rotational constant, centrifugal distortion constant, and quadrupole coupling constants of the Cl nucleus were determined. The observation of rotational transitions in the vibrationally excited v = 1 state made it possible to derive equilibrium structural parameters. A discussion on the internuclear potential of the three low-lying electronic states of NCl is given.

Full text of DOI:10.1021/jp962121k

1068
J. Phys. Chem. A 1997, 101, 1068-1071
1
Microwave Spectrum of NCl in the Second Electronically Excited State (b Σ+)
Kaori Kobayashi† and Shuji Saito*,‡
Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444, Japan
ReceiVed: July 16, 1996; In Final Form: December 2, 1996X
The pure rotational transitions of N35Cl in the second electronically excited state, b 1Σ+, were detected together
with those of the isotopic species N37Cl by microwave spectroscopy. The NCl radical in the b 1Σ+ state was
generated by dc-glow discharge of a N2/Cl2 mixture. The rotational constant, centrifugal distortion constant,
and quadrupole coupling constants of the Cl nucleus were determined. The observation of rotational transitions
in the vibrationally excited V ) 1 state made it possible to derive equilibrium structural parameters. A
discussion on the internuclear potential of the three low-lying electronic states of NCl is given.
Introduction
Coombe18 studied collisional quenching rates of NCl(a 1∆) with
1
several molecules. NCl(b Σ+) quenching rate constants were
The difficulty in producing molecules in metastable electronic
states has prevented the observation of pure rotational spectra
of such species. This is especially true for molecules in the
second electronically excited state since microwave spectra have
only been observed for CO(b 3Σ+)1 and SO(b 1Σ+)2. Other than
these examples, some reports on molecules in the first electroni-
cally excited state, that is, SO(a 1∆)3-5, O2(a 1∆g)6, CO(a 3Π)7,
determined by several groups.19,20 The molecular properties of
NCl concerning its geometrical structure,21 electronic struc-
ture,22,23 and radiative lifetime24,25 were studied by several ab
initio MO calculations.
In the present study we have detected the pure rotational
1
spectra of NCl(b Σ+) in the V ) 0 and V ) 1 states and
determined molecular constants and a precise equilibrium
structure. This report completes microwave spectra of the NCl
radical in the three low-lying states. The result is a good
example of how the molecular structure and potential energy
surface change between electronic states.
1
1
NF(a ∆)8, and NCl(a ∆)9, are known. In these electronic
systems, i.e. X3Σ-, a 1∆, and b 1Σ+, these results clearly reflect
that b 1Σ+-X3Σ- transition is weakly allowed via the magnetic
dipole selection rule whereas the a 1∆-X3Σ- transition is spin
and orbital forbidden.
1
1
The b Σ+ state of NCl was first identified in the b Σ+-
X3Σ- electronic spectrum.10 From the b-X electronic emission
spectrum, vibrational and rotational constants of both states were
determined and the low-lying electronic state, b 1Σ+ was shown
to be 15 038.94 cm-1 above the X3Σ- ground state. Yamada
and his collaborators11-13 studied the microwave spectrum of
NCl(X3Σ-) by using dc-glow discharge of a nitrogen and
chlorine gaseous mixture. They determined its detailed mo-
lecular constants and derived the spin densities of the unpaired
electrons for the nitrogen and chlorine atoms. The data of the
b-X electronic transition obtained by Colin and Jones10 were
reanalyzed with the precise molecular constants of NCl(X3Σ-).
Experimental Section
The source-modulated microwave spectrometer attached to
a 2 m free-space cell at the Institute for Molecular Science was
used for the experiment. The experimental setup has been
described in a previous report.26 The experimental conditions
were similar to those used for observing the microwave spectrum
of NCl(a 1∆). NCl(b 1Σ+) was produced by a dc-glow discharge
of nitrogen gas (7 m Torr) and chlorine gas (9.98% diluted in
helium gas, 20 mTorr). The discharge current was 200 mA.
The background pressure was 3 mTorr, and the 2 m free-space
cell was maintained between 180 and 200 K by circulating liquid
nitrogen in the cooling jacket during the measurements.
The radiative life time of NCl(b Σ+) was reported to be 0.25
1
ms when generated by mild microwave discharge,14 which was
1
First we searched for the N ) 9-8 diamagnetic transition
line in 368 GHz region predicted from the rotational constant
and the centrifugal distortion constant obtained from the
reanalysis11 of the b-X electronic transition. A diamagnetic
line was detected near predicted frequency. Since NCl in the
shorter than that (2 ms) of NCl(a ∆). These lifetimes are
consistent with the nature of the electronic transitions, as
described above.
The azide radical reaction
b
1Σ+ state is diamagnetic, the Zeeman effect can not be
N3 + Cl f NCl(a 1∆, b 1Σ+) + N2
applicable to the discrimination of lines due to various dia-
magnetic reaction intermediates and products. The line im-
mediately disappeared when discharge was turned off or nitrogen
gas or chlorine gas flow was cut off. Eight series lines were
found in the 164-450 GHz region in total. The lowest two
rotational lines with N ) 4-3 and N ) 5-4 show hyperfine
structure due to the chlorine nucleus, as shown in Figure 1.
The observation of the N ) 9-8 transition of the isotopic
species, N37Cl(b 1Σ+), in the 363 GHz region in natural
abundance confirmed that these series lines are due to N35Cl(b
1Σ+). Four rotational lines of N37Cl(b 1Σ+) were also measured.
is an efficient method of generating metastable NCl radical,15,16
which was thought to be an important point. The a-X emission
spectra were measured in the near infrared region using this
reaction17 and many studies on the kinetics of NCl were carried
1
out. Our recent microwave study of NCl(a ∆)9, however,
showed that dc-glow discharge of an N2 and Cl2 mixture is also
an effective production method. Recent development of chemi-
cal laser systems utilizing transient species like NCl(a 1∆)
forwarded research on the quenching constants. Ray and
1
For a further confirmation of the spectral lines in the b Σ+
† Email: kaori@ims.ac.jp.
1
state, spectral lines of N35Cl(b Σ+) in the V ) 1 vibrationally
‡ Email: saito@ims.ac.jp.
X Abstract published in AdVance ACS Abstracts, January 15, 1997.
excited state were searched for. A somewhat weaker dia-
S1089-5639(96)02121-4 CCC: $14.00 © 1997 American Chemical Society

Microwave Spectrum of NCl
J. Phys. Chem. A, Vol. 101, No. 6, 1997 1069
1
TABLE 1: Observed Rotational Transitions of NCl(b Σ+)a
∆ν(obs - calc)
N′-N
4-3
F′-F
ν
obs (MHz)
N35Cl V ) 0
(MHz)
weight
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
163 688.416(29)
163 686.946(50)
204 602.392(14)
204 601.452(44)
-0.004
0.025
1.0
1.0
1.0
1.0
0.0d
1.0
1.0b
1.0
1.0
1.0
5-4
0.010
-0.044
6-5
7-6
245 510.950(20)
286 413.376(31)
327 308.398(15)
368 195.041(19)
409 072.104(9)
449 938.688(9)
0.083
0.034
-0.016
0.011
-0.030
0.017
8-7
9-8
10-9
11-10
V ) 1
6-5
7-6
243 236.282(25)
283 759.465(48)
324 275.668(25)
364 783.162(24)
405 281.272(42)
445 768.833(11)
0.033
-0.182
0.014
-0.056
-0.014
0.030
1.0
0.0b,d
1.0
Figure 1. The J ) 5-4 transition line in the V ) 0 state of NCl(b
1Σ+). The splitting due to the quadrupole interaction of the chlorine
nucleus was observed. The NCl radical was generated by a dc-glow
discharge in a gaseous mixture of N2 and Cl2 (9.98% dispersed in helium
gas) at a total pressure of 30 mTorr. The integration time was 80 s.
8-7
9-8
10-9
11-10
1.0
1.0
1.0
N37Cl V ) 0
282 000.635(48)
c
362 522.988(20)
402 770.875(18)
443 008.553(22)
7-6
8-7
0.029
1.0
0.0d
1.0
1.0
1.0
magnetic line compared with that of the ground vibrational state
was observed. By the difference of these two lines, Re was
estimated and five additional V ) 1 lines were observed on the
basis of the calculated frequencies. The estimated vibrational
temperature was 1800 K under the experimental conditions.
9-8
-0.027
-0.022
0.024
10-9
11-10
a The numbers in parentheses represent the variances of the observed
value in units of the last significant digits. b Not accurately measured
due to the interfering line. c Not observed due to the interfering line.
d Not included in the fit.
Results and Discussion
a and b electronic states. This picture is also consistent with
the decrease in the quadrupole coupling constant of the C1
nucleus if the amount of π electron back-donation increases
The observed lines were analyzed by using a conventional
Hamiltonian for a diatomic molecule in the 1Σ+ state with
electronic quadrupole coupling interaction of the chlorine
nucleus. The observed lines of both isotopic species are listed
in Table 1. The molecular constants of N35Cl and N37Cl
obtained from the least-squares fit are summarized in Table 2
with the data of N35Cl obtained from the study on electronic
emission spectroscopy. The molecular constants obtained in
the present study agree well with the constants obtained from
the electronic spectroscopy. However, our results provide
improved accuracy. The standard deviations of the final fit of
N35Cl in the V ) 0 and V ) 1 states and N37Cl in the V ) 0
state were 30, 43, and 36 kHz, respectively, which fall within
the experimental errors, as shown in Table 2.
1
from the X3Σ- state to the b Σ+ state. This back-donation is
supported by the net Mulliken charge of the nitrogen nucleus
obtained by Papakondylis and his co-workers in their ab initio
MO study.23 They showed the decrease in the charge of the N
1
nucleus from -0.08(ø3Σ-) to -0.20(b Σ+) at the CISD level
of theory. The change in the bond length and vibrational
3
frequency in the single bond of NCl from the Σ- state to the
1Σ+ state makes a distinct contrast with the one in the double-
bonded radicals such as SO, which has an isoelectronic system.2
The vibrational frequency given by electronic spectra also agrees
well with our more accurate value of 936.75(99) cm-1
.
1
In the Dunham expansion, the internuclear potential is
assumed to be of the form
The equilibrium parameters of N35Cl(b Σ+) derived from
the molecular constants of the V ) 0 and V ) 1 states are shown
in Table 3. For comparison equilibrium parameters of the other
two low-lying electronic states are also shown in Table 3. The
equilibrium bond length, re, is determined to be 1.568 006 9(20)
Å, where the precision of the calculated bond length is limited
by the experimental accuracy of Planck’s constant and the
atomic mass unit.27 On the other hand, the bond length obtained
from ab initio MO calculation23 by the full configuration
interaction (F-CI) method is 1.598 Å, which is slightly larger
than our value. The bond length of the b 1Σ+ state obtained by
U ) a0ê2(1 + a1ê + a2ê2 + ...)
(3)
where
r - re
re
ê )
(4)
From the observed equilibrium constants, Dunham expansion
coefficients are evaluated. These parameters are tabulated in
Table 4 with the data of the SO radical.2,5,28,29 The shape of
potential is governed by a1 and a2. Near the equilibrium
geometry, we can see that the shapes of potential resemble each
other which suggests that the change of spin states does not
affect the shape of each potential. The strength of the bond is
indicated by a0. It is interesting to notice that the change of
spin orientation decreases the total energy of the NCl and SO
1
the F-CI method has the same value as that of the a ∆ state.
However, the results of other calculation methods showed that
1
the bond length of the b Σ+ state is always smaller than that
1
of the a ∆ state. The decrease in bond length from the X3Σ-
state to the b 1Σ+ state is the same as the result of previous ab
initio MO calculations and well corresponds to the increasing
tendency of vibrational frequency. These tendencies suggest
that the NCl bond acquires more double-bond character in the

1070 J. Phys. Chem. A, Vol. 101, No. 6, 1997
Kobayashi and Saito
1
TABLE 2: Molecular Constants of NCl(b Σ+) (MHz)a
present study
electronic transitionb
N35Cl
N37Cl
0
N35Cl
0
V
0
1
BV
DV
eQq(Cl)
rms
20 462.412 4(53)
0.044 026(30)
-51.7(36)
20 272.855 8(115)
0.043 955(61)
-51.7c
20 147.0853(119)
0.042 702(60)
0c
20 462.84(153)
0.044 25(144)
d
0.030
0.043
0.036
a The numbers in parentheses represent 3 standard deviations in units of the last significant digits. b Reference 11. Converted from cm-1 units
to MHz units. c Fixed value. d Not determined.
TABLE 3: Equilibrium Molecular Constants of N35Cla
NCl(X3Σ-)b
NCl(a 1∆)c
NCl(b 1Σ+)d
NCl(b 1Σ+)e
Be (MHz)
Re (MHz)
De (MHz)
âe (MHz)
19 481.734(144)
192.297(21)
20 290.635 3(27)
188.673 4(29)
0.045 345 2(187)
-0.000 214(20)
905.498(187)f
5.274(50)g
1.578 272 6(20)
1.72(46)
-52.54(68)
20 557.190 7(98)
189.556 6(127)
0.044 062(93)
-0.000 071(68)
936.75(99)f
5.777(181)g
1.568 006 9(20)
i
0.047 912(64)
-0.000 050(32)
827.957 8(26)f
5.300 2(21)g
1.610 705(19)
1.842(96)
ωe (cm-1
)
935.6
5.4
ωeøe (cm-1
re (Å)
)
eQq(N) (MHz)h
eQq(Cl) (MHz)h
-63.13(18)
-51.7(36)
a The numbers in parentheses represent 3 standard deviations in units of the last significant digits. b References 11 and 13. Be, Re, De, âe, ωe, and
ωeøe were assumed to be Y01, -Y11, Y02, Y12, Y10, and -Y20 respectively. Some constants were converted from cm-1 units to MHz units. c Reference
9. d Present study. e Reference 10. Electronic transition. Derived by the following relation, De ) 4Be3/ωe2. g Derived by Dunham expansion coefficients
f
shown in Table 4. h These values are those of the ground vibrational state. i Not determined.
TABLE 4: Potential Parameters of N35Cl and SOa
1
has higher quenching rate than NF(a ∆).32 Especially the
quenching rate of NF(b 1Σ+) with F2 is larger than that of NCl-
NCl(X3Σ-)b,c
NCl(a 1∆)b,d
NCl(b 1Σ+)e
(b Σ+) with Cl2 by 2 orders of magnitude. This fact might
1
a0 (cm-1
a1
a2
)
263 723.88(162)
-3.096 02(23)
6.544 2(27)
302 859(125)
-3.073 37(43)
6.612(49)
319 922(676)
-3.099 5(22)
6.392(176)
explain our successful detection of NCl(b Σ+) and failure to
1
detect NF(b 1Σ+) even though a strong signal was observed for
1
NF(a ∆).
SO(X3Σ-)f
SO(a 1∆)b,g
SO(b 1Σ+)b,h
a0 (cm-1
a1
a2
)
459 187(26)
-3.122 22(15)
6.312 7(141)
435 965(190)
-3.209 91(52)
6.945(188)
406 385(257)
-3.299 13(73)
6.733(69)
Acknowledgment. This study was supported by Grant-in-
Aid from Ministry of Education, Science, Sports and Culture
(No. 04233107).
a The numbers in parentheses represent three standard deviations in
units of the last significant digits. b These parameters are derived from
the experimental values taken from the references by using the Dunham
expansion. c Reference 11. d Reference 9. e Present study. f Reference
28. g References 5 and 29. h Reference 2.
References and Notes
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radicals from the b 1Σ+ state to the X3Σ- state, while it decreases
the bond strength of NCl and increases that of SO. This may
be concerned with the picture that unpaired electrons mainly
occupy 2px and 2py orbitals of the N atom in NCl while they
mainly occupy hybrid orbitals between S and O in SO. For
further understanding of bond nature and quadruple coupling
constants, more accurate quantum chemical calculation are
required.
Although the radiative lifetime of the b Σ+ state (0.25 ms)
1
is about one-eighth of that of the a 1∆ state, the intensity ratio
of NCl(b Σ+) for N ) 9-8 to NCl(a ∆) for J ) 9-8 was
0.05-0.1 depending on its production condition. This result
suggests that the microwave line intensity does not depend only
on the radiative lifetime as Yamamoto pointed out in the report
1
1
on SO(b Σ+)2 but also on the collisional decay process. The
1
decay kinetics of NCl(a ∆, b Σ+) with several gases was
1
1
thoroughly studied18-20 showing large dependency on the
1
quenching gas. Comparing the quenching rates of NCl(a ∆)
and NCl(b Σ+) with several molecules which are thought to
1
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have been present in the discharge cell, NCl(a 1∆) has a higher
quenching rate than NCl(b 1Σ+) in most cases. The NF(b 1Σ+)
radical was searched for on the basis of the production
conditions used in the study of its electronic emission spec-
troscopy.30 However, no signal was observed by dc-glow
discharge of NF3 and H2 or Ar. In case of NF, NF(b Σ+)31
1

Microwave Spectrum of NCl
J. Phys. Chem. A, Vol. 101, No. 6, 1997 1071
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