Persistence rewarded: Successful observation of the B 2Σu+-X 2∏ g electronic transition of jet-cooled BS2

Article abstract of DOI:10.1063/1.1644097

The electronic transition of jet cooled BS₂ is discussed. Laser-induced fluorescence techniques are used for the purpose. It is found that by using a mixture of BCl₃ and CS₂ in high-pressure argon as the precursor, boron disulfide radical is produced in a pulsed electric discharge jet. Results show that the spectrum consists of a strong 0₀⁰ band with short progressions in the symmetric stretching.

Full text of DOI:10.1063/1.1644097

Persistence rewarded: Successful observation of the B̃ 2 Σ u + –X̃ 2 Π g electronic
transition of jet-cooled BS 2
Sheng-Gui He and Dennis J. Clouthier
Citation: The Journal of Chemical Physics 120, 4258 (2004); doi: 10.1063/1.1644097
View online: http://dx.doi.org/10.1063/1.1644097
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 120, NUMBER 9
1 MARCH 2004
¿
2
2
˜
˜
Persistence rewarded: Successful observation of the B ⌺u –X ⌸g
electronic transition of jet-cooled BS2
Sheng-Gui He and Dennis J. Clouthiera)
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
͑Received 6 November 2003; accepted 4 December 2003͒
˜ ˜
The B–X electronic transition of jet-cooled BS2 has been observed using laser-induced fluorescence
techniques. The boron disulfide radical was produced in a pulsed electric discharge jet using a
mixture of BCl3 and CS2 in high-pressure argon as the precursor. The spectrum consists of a strong
0
00 band with the ⌺–2⌸3/2 component at 24 393.2 cmϪ1 and short progressions in the symmetric
stretching (␯Ј1ϭ506.7 cmϪ1) and bending (␯Ј2ϭ303.2 cmϪ1) modes. A rotational analysis of both
spin–orbit components of the 000 band gave an upper state B value of 0.093 277 9͑19͒ cmϪ1 and a
ground-state spin–orbit coupling constant of AϭϪ405.163(4) cmϪ1. The ground-state bond length
2
˜ ˜
of 1.6649͑2͒ Å increases to 1.6812͑1͒ Å on ␴u→␲ electronic excitation. The B–X data have been
g
used to further refine the Renner–Teller analysis, which is in good agreement with our previous
work ͓J. Chem. Phys. 119, 2047 ͑2003͔͒. © 2004 American Institute of Physics.
͓DOI: 10.1063/1.1644097͔
many hot bands built on the 211 band, were assigned in the
LIF spectrum. Due to the restrictive ⌬Pϭ0 selection rule,
the spin–orbit splittings could not be measured directly for
these parallel transitions. Instead, the upper-state spin–orbit
parameter was obtained indirectly, through the variation of
the energy level splittings as a function of the vibrational
quantum numbers, by fitting excited-state vibrational combi-
nation differences. Subsequently, the ground-state spin–orbit
splitting was obtained by rotational analysis of the 000 band
with the upper-state splitting fixed at the value obtained from
the vibrational analysis. Vibrational and rotational analysis
showed that the ground state exhibits a substantial Renner–
Teller effect with ⑀ϭϪ0.2012(34) and a zero-point level
I. INTRODUCTION
Boron disulfide is the least studied of a series of isoelec-
tronic 15 valence electron radicals (BO2 , CO2ϩ, NCO,
NCS͒ which all have ⌸ ground states that exhibit a signifi-
2
cant Renner–Teller effect. A variety of low-resolution studies
have shown that BS2 has two electronic band systems in the
UV–visible range.1–3 An extensive, vibrationally, and rota-
tionally discrete band system in the 720–500-nm region has
been shown to be due to the A ⌸u –X ⌸g transition.3,4
A
2
2
˜
˜
further group of three bands near 410 nm has been assigned
ϩ
to the B ⌺u –X ⌸g electronic transition,1–3 although no ro-
2
2
˜
˜
tational analysis has previously been reported.
spin–orbit coupling constant of A0ϭϪ404.683(3) cmϪ1
.
˜ ˜
The A–X band system was thoroughly studied in matri-
ces by Brom and Weltner3 in 1973. BS2 was produced by
vaporizing zinc sulfide and boron mixtures at high tempera-
tures and trapping the products in neon matrices at 4 K. Their
analysis identified the 000 band at 13 766 cmϪ1 and led to the
Angular momentum coupling is negligible in the excited
state, which has a much smaller spin–orbit coupling constant
of A0ϭϪ259.15(12) cmϪ1, in agreement with the previous
matrix work. The 11BS2 vibrational frequencies are ␻Љ
1
ϭ542.8, ␻Љϭ285.0, ␻Љϭ1010.5, ␻Јϭ514.1, ␻Јϭ309.7,
conclusion that there was little or no Renner–Teller effect in
2
3
1
2
and ␻3Јϭ1543.6 cmϪ1. High-resolution spectra of the 000
band of 11BS2 were rotationally analyzed to obtain effective
molecular structures of r0Љ(BS)ϭ1.6648(4) Å and r0Ј(BS)
ϭ1.7127(5) Å. The spectroscopic data clearly establish that
the radical has the linear SBS structure in both combining
state. Ab initio methods were used to predict the BS2 spec-
troscopic parameters and good agreement with experiment
2
the upper
⌸
electronic state. Taking advantage of the
u
2
weakly allowed ⌸1/2–2⌸3/2 bands observed in the matrix,
˜
they obtained ground- and A-state spin–orbit splittings of
Ϫ440 cmϪ1 and Ϫ263 cmϪ1, with vibrational frequencies of
␯Љϭ510 cmϪ1, ␯Љϳ120 cmϪ1, and ␯3Љϭ1015 cmϪ1 and ␯Ј
1
2
1
ϭ506 cmϪ1, ␯Јϭ311 cmϪ1, and ␯3Јϭ1535 cmϪ1
.
2
Recently,4 we obtained the first electronic spectra of jet-
cooled BS2 , using the pulsed discharge technique with a
gaseous mixture of BCl3 and CS2 in high-pressure argon as
˜ ˜
was found. Several attempts to observe the 415-nm B–X
band system of jet-cooled BS2 using LIF techniques were
2
2
˜
˜
˜
the precursor. The A ⌸u –X ⌸g electronic band system was
studied by a combination of laser-induced fluorescence ͑LIF͒
and wavelength-resolved emission techniques. A long pro-
unsuccessful, suggesting that the B state has a low quantum
yield of fluorescence, although this is unlikely for such a
small molecule.
Believing in the old adage that if at first you don’t suc-
ceed, try, try, try again, we persisted in attempts to observe
gression of strong bands involving the ␯Ј ͑B–S stretching͒
1
mode and combinations involving 2␯Ј and 2␯Ј , as well as
2
3
˜ ˜
the B–X transition and have finally succeeded in doing so,
as we report in this work. By simultaneous rotational analy-
a͒
Author to whom correspondence should be addressed.
0021-9606/2004/120(9)/4258/5/$22.00
4258
© 2004 American Institute of Physics
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4259
J. Chem. Phys., Vol. 120, No. 9, 1 March 2004
Electronic transition of BS2
sis of both spin–orbit components we have obtained a direct
and precise measure of the ground-state spin–orbit splitting
and have derived the effective excited-state molecular struc-
ture. These results have been further used to refine the mo-
2
lecular constants for the X ⌸g state of 11BS2 .
˜
II. EXPERIMENT
In our previous study,4 BS2 was obtained from a gas-
phase precursor mixture of 3% BCl3 and 1% CS2 in argon,
contained in a stainless steel cylinder equipped with a regu-
lator, which was delivered at a pressure of 40 psi into a
pulsed discharge at the exit of a molecular beam valve ͑Gen-
eral Valve, series 9͒. Although this method gave strong spec-
˜ ˜
tra of the A–X band system, it did not allow us to obtain
˜ ˜
spectra of the B–X bands at 415 nm. Subsequent experimen-
ϩ
u
2
2
˜
˜
⌸
FIG. 1. Portion of the low-resolution B
⌺
–X
spectrum of jet-cooled
g
tation showed that a gas-phase mixture of 3% BCl3 in argon
flowed at 40 psi pressure over the surface of room-
temperature, liquid CS2 gave much stronger BS2 spectra and
permitted us to obtain LIF spectra of the UV band system.
This precursor mixture was injected into a Delrin flow chan-
nel attached to the exit of the molecular beam valve. At the
appropriate time in the gas pulse, a pulsed electric discharge
was struck between two stainless steel ring electrodes
mounted in the flow channel, which fragmented the precur-
sor molecules and formed BS2 by subsequent chemical reac-
tions.
Low-resolution LIF spectra were obtained by crossing
the supersonic expansion of the discharge products 1.5 cm
downstream of the discharge apparatus with the beam of a
tunable dye laser ͑Lumonics HD-500͒. The resulting fluores-
cence was imaged by a 5-cm-focal-length lens through an
appropriate cutoff filter onto the photocathode of a high-gain
photomultiplier tube ͑EMI 9816QB͒. Low-resolution ͑0.1
cmϪ1͒ LIF spectra were excited in the 420–390-nm range
and calibrated using optogalvanic lines from an argon-filled
hollow cathode lamp.
BS2 with the assignments of some of the stronger features. The vertical
2
leaders denote the two spin–orbit components (2⌸1/2 on the left,
⌸
on
3/2
the right͒ of the intense 000 and 101 bands. All four components of the 211
sequence band are identified, with the lower-state symmetries in parenthe-
ses, along with the 2⌺–2⌸1/2 component of the 220 band.
III. RESULTS AND ANALYSIS
The ground-state electron configuration of linear BS2 is
2
...(␴g)2 (␴u)2 (␲u)4 (␲g)3 X ⌸g ͑inverted͒, with excited
˜
2
states ...(␴g)2 (␴u)2 (␲u)3 (␲g)4 A ⌸u ͑inverted͒ and
˜
ϩ
u
2
1
4
4
2
˜
...(␴g) (␴u) (␲u) (␲g) B ⌺ . The ␴ and ␲ orbitals
u
u
are bonding and the ␲ orbitals are essentially pure 3p␲
g
nonbonding orbitals on the sulfur atoms. The vibrational fre-
quencies are conventionally labeled as ␯ (␴g , symmetric
1
BS stretch͒, ␯ (␲g,u , bend͒, and ␯ (␴u , antisymmetric BS
2
3
stretch͒. The vibronic selection rules are ⌬ 1ϭ0,Ϯ1,Ϯ2,...
v
and ⌬v2 or ⌬v3ϭ0,Ϯ2,... and transitions from both spin–
orbit components of the zero-point level of the ground state
ϩ
2
2
˜
˜
are allowed for the B ⌺u –X ⌸g electronic transition.
High-resolution ͑0.04 cmϪ1͒ spectra of the 000 band at
410 nm were obtained in the same discharge jet apparatus. In
order to obtain laser radiation in the near ultraviolet that
could be accurately wavelength calibrated, we used a variant
of the Raman shifting technique previously described.5 A
308-nm pumped Lambda-Physik ScanMate2E dye laser
A portion of the low-resolution LIF spectrum of the
˜ ˜
dominated by an intense band at 24 393.2 cmϪ1, which is
B–X transition of BS2 is shown in Fig. 1. The spectrum is
readily assigned as the 2⌺–2⌸3/2 component of the 000 band;
2
the corresponding ⌺–2⌸1/2 component occurs at 23 988.0
cmϪ1, giving a ground-state spin–orbit splitting of Ϫ405.2
cmϪ1, in agreement with the Ϫ404.7 cmϪ1 value obtained
previously.4 We assign the major cold band features in the
spectrum to a short progression (1n0 ,nϭ0–2) involving the
excited-state symmetric stretch, with a fundamental fre-
´
equipped with an intracavity etalon and operated with Cou-
marin 503 laser dye gave narrow-bandwidth ͑0.035 cmϪ1͒,
high-energy ͑10–15 mJ͒ pulses in the 490–505-nm region.
This radiation was focused into a Raman shifting cell filled
with hydrogen at a pressure of 200 psi. The Raman shifted
beams generated in this process were separated by a prism
and the weak first anti-Stokes beam ͑407–417 nm͒ was used
to excite the BS2 LIF spectrum while the first Stokes beam
͑615–639 nm͒ was used to excite I2 LIF spectra. The LIF
and calibration spectra were recorded simultaneously with a
LABVIEW-based digital data acquisition system of our
design.6 As the hydrogen Raman shifts can be very accu-
rately calculated as a function of pressure,5 and the iodine
absorption lines have been accurately measured,7,8 we were
able to calibrate the BS2 LIF spectra to an estimated accu-
quency of ␯Јϭ506.7 cmϪ1. This assignment is in accor-
1
dance with the conclusions of Brom and Weltner,3 allowing
for an approximately Ϫ100 cmϪ1 matrix shift. We also de-
tected weak bands at 24 999.0 and 25 039.5 cmϪ1, not found
in previous studies, which we assign as two components of
the 220 band, originating from the
⌸
level. The corre-
2
3/2
sponding transitions from the upper spin–orbit component
(2⌸1/2) were also identified and the pattern of transitions is
repeated for the 110202 band.
The remaining bands in the spectrum were readily as-
signed as hot bands, based on our previous Renner–Teller
analysis of the ground state.4 All four components of the 211
racy of Ϯ0.003 cmϪ1
.
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4260
J. Chem. Phys., Vol. 120, No. 9, 1 March 2004
S.-G. He and D. J. Clouthier
ϩ
u
2
2
˜
˜
⌸
TABLE II. Molecular constants ͑in cmϪ1͒ for the vibrational zero-point
TABLE I. LIF bands observed in the B
⌺
–X
spectrum of jet-cooled
Maximum
g
levels of the known electronic states of 11BS2 .
11BS2 .
ϩ
u
2
2
2
Parameter
˜
X
˜
A
˜
B ⌺
Maximum
⌸
⌸
i
i
Assignment
͑cmϪ1
͒
Assignment
͑cmϪ1
͒
0.0951053(25)a
Ϫ405.1637(4)
—
Ϫ0.872(14)
0.250͑25͒
0.0
B
A0
␥
0.0898546͑81͒
Ϫ259.6310(2)
—
0.0932779͑19͒
0
22(2⌺–␮
⌸
⌸
)
)
110211(2⌸–2⌬3/2
)
2
23804.0
23828.9
23883.5
23917.6
23923.6
23988.0
24003.5
24027.2
24310.8
24393.2
24413.1
24420.2
24435.4
24489.8
24494.1
24501.7
24525.5
24594.0a
24634.5a
24899.9
24911.4
24918.3
24927.3
24999.0
24999.0
25039.5
25086.3
25120.5?
25404.1
25407.7
25492.5
—
3/2
0
22(2⌺–␮
220(2⌺–2⌸1/2
220(2⌬–2⌸1/2
110(2⌺–2⌸3/2
)
)
)
0.007429(10)
2
1/2
101(2⌺–2⌸3/2
)
104Ad
102(pϩ2q)
T0
0.0b
—
—
101211(2⌸–␮2⌺)
101211(2⌸–2⌬5/2
000(2⌺–2⌸1/2
0.508͑29͒
13839.3919(2)
87
24189.5337(2)
87
)
110211(2⌸–2⌬5/2
)
c
#
135
0.0045
)
110211(2⌸–␮ 2⌺)
␴ ͑cmϪ1
͒
0.0033
0.0037
d
1
2
211(2⌸–␬ 2⌺)
1022(2⌺–␮
⌸ )
2
1/2
211(2⌸–2⌬3/2
)
220(2⌺–2⌸3/2
120(2⌺–2⌸1/2
220(2⌬–2⌸3/2
)
)
)
1
0
2
aThe numbers in parentheses are 3␴ error limits and are right justified to the
last digit on the line; sufficient additional digits are quoted to reproduce the
original data to full accuracy.
1022(2⌺–␮
⌸
)
3/2
000(2⌺–2⌸3/2
211(2⌸–2⌬5/2
)
)
110220(2⌺–2⌸1/2
110220(2⌬–2⌸1/2
)
)
bParameter fixed in the final least-squares analysis.
cNumber of combination differences or transitions fitted.
dOverall standard deviation of fit.
211(2⌸–␮ 2⌺)
2
22(2⌺–␮
101220(2⌺–2⌸3/2
110(2⌺–2⌸1/2
⌸
)
)
120(2⌺–2⌸3/2
)
2
1/2
120211(2⌸–2⌬5/2
110220(2⌺–2⌸3/2
)
)
)
110211(2⌸–␬ 2⌺)
components of the band were readily assigned as branches of
a 2⌺–2⌸i vibronic band with Hund’s case ͑a͒ coupling in the
lower state and Hund’s case ͑b͒ coupling in the upper state.
The sulfur atoms have zero nuclear spin and the nuclear sta-
tistical weights are such that alternate ⌳-doubling levels are
missing, so that only one level exists for each J value. This
leads to a staggering in the appearance of some of the
aThe following 10BS2 isotope bands were assigned: 220(2⌺–2⌸1/2) at
24 618.9 cmϪ1 and 202(2⌬–2⌸1/2) at 24 658.8 cmϪ1
.
sequence band are readily apparent as shown in Fig. 1, con-
sistent with our previous studies that showed substantial
population of Љϭ1 in the discharge jet expansion. This pat-
2
branches in the ⌺–2⌸1/2 component.
v2
tern of four features is repeated for the 110211 hot band and the
0
˜ ˜
Although the B–X 00 band could be satisfactorily fitted
2
⌬
5/2 component was found for the 120211 band. The 101, 101211,
0
˜ ˜
by itself, we elected to include data from the A–X 00 band
and 101220 hot bands were also assigned, giving a ground-state
symmetric stretching frequency, uncorrected for Fermi reso-
nance, of 509.7 cmϪ1. Finally, five very weak bands were
as well, in order to obtain the best set of molecular constants
and improve on the assumptions made in our earlier
analysis,4 where no direct measure of the spin–orbit split-
assigned as transitions from Љϭ2 to the various upper-state
v2
˜
˜
tings in the X and A states was possible. First, for each band,
including both spin components, a data set of strong, sym-
metric, unblended lines was chosen and all possible ground-
state combination differences were formed. The ground-state
constants were refined by fitting the combination differences
levels identified from the strong cold bands. The vibrational
assignments are summarized in Table I.
We have recorded both spin–orbit components of the
0
B–X 00 band of 11BS2 at high resolution; the stronger
˜ ˜
2⌺–2⌸3/2 band is shown in Fig. 2. Four of the six branches
of the band have been resolved at our resolution of 0.04
cmϪ1—the R11 and Q21 branches are blended together form-
ing the strong unresolved feature near the band center. Both
2
to the lower-state ⌸i effective Hamiltonian in the R2 for-
mulation using the matrix elements tabulated previously.9
This gave the ground-state constants summarized in Table II,
which are in generally good agreement with those obtained
previously.4 The only substantial difference is in the
⌳-doubling constant pϩ2q, which is smaller than previ-
ously determined. This difference can be traced to a substan-
˜
˜
tial correlation between the X- and A-state ⌳-doubling con-
0
˜ ˜
stants obtained from the previous analysis of the A–X 00
band. It is especially gratifying that the present spin–orbit
coupling constant A0ϭϪ405.164(4) cmϪ1 agrees so well
with the Ϫ404.683͑3͒ value determined previously by fixing
the upper-state spin–orbit interval at the value obtained from
the Renner–Teller analysis.
˜
˜
Subsequently, the A- and B-state molecular constants
were determined by fitting the 000 band transition frequen-
cies, with the ground-state constants fixed at the values given
˜
in Table II. The A-state Hamiltonian was the same as that of
˜
the ground state, whereas the B-state energy levels were
FIG. 2. High-resolution spectrum of the 2⌺–2⌸3/2 component of the 000
modeled using only the band origin T0 , the rotational con-
stant B, and the spin–rotation coupling constant ␥ contained
in the 2⌺ϩ energy level expressions given in Eqs. ͑1͒ and ͑2͒
ϩ
2
2
˜
˜
–X ⌸
band of the B
⌺
system of BS2 showing the various branches and
g
u
some of the rotational assignments.
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4261
J. Chem. Phys., Vol. 120, No. 9, 1 March 2004
Electronic transition of BS2
TABLE III. Vibrational parameters ͑in cmϪ1͒ for the ground state of 11BS2 .
due to the inclusion of gK and the refined value of the spin–
1
˜ ˜
orbit splittings in the model. Using the B–X 21 band transi-
2
˜
X
⌸
g
tion frequencies and the lower-state bending mode energy
from the Renner–Teller analysis, we calculate the excited-
state bending frequency to be 303.2 cmϪ1, only slightly
Parameter
Present
Previousa
␻
544.806(47)b
283.549(27)
1010.718(15)
Ϫ414.416(23)
Ϫ0.2098(17)
2.9337(39)
0.828(13)
19.2322(72)
1.006(8)
Ϫ8.098(116)
Ϫ1.886(66)
6.218(42)
542.767(61)b
284.962(46)
1010.510(11)
Ϫ413.802(74)
Ϫ0.2012(34)
2.7854(54)
—
19.0202(88)
1.031(11)
Ϫ7.829(89)
Ϫ1.853(51)
6.244(34)
1
larger than half the 2␯Ј interval of 605.8 cmϪ1
.
␻
2
2
␻
3
A
⑀
g4
gK
W1
W2
x13
x23
x33
IV. AB INITIO CALCULATIONS
We have used ab initio theory to predict the molecular
properties of BS2 in the various electronic states to see how
well they agree with the experimental data. The GAUSSIAN 98
programs10 were used in conjunction with a variety of levels
of theory and a range of basis sets to calculate the bond
length, vibrational frequencies, and energies of the three ob-
served electronic states of 11BS2 . In general, density func-
tional theory ͑B3LYP͒ with Dunning’s correlation-consistent
basis sets augmented by diffuse functions ͑aug-cc-pVTZ and
aug-cc-pVQZ͒ ͑Ref. 11͒ gave reasonable agreement with ex-
periment, but the best results were obtained with coupled
cluster theory and only these are reported in this work, as
summarized in Table IV.
aReference 4.
bThe numbers in parentheses are 1␴ error limits and are right justified to the
last digit on the line; sufficient additional digits are quoted to reproduce the
original data to full accuracy.
cThe A values in Tables II and III are different. In the Renner–Teller analysis
the spin–orbit coupling constant is defined by AeffϭA 1Ϫ1⑀2K(Kϩ1) for
͓
͔
8
the unique levels so that the variation with K is included explicitly in the
model whereas in Table II it is A␯ϭ0 , obtained from the rotational analysis,
which is quoted.
V. DISCUSSION
˜
This work reports the vibrational and rotational analysis
of Ref. 9. The constants are collected in Table II. The A-state
ϩ
2
2
˜
˜
values are in good agreement with those obtained in the pre-
of the B ⌺u –X ⌸g band system of boron disulfide in the
blue region of the spectrum. We have clearly established by
rotational analysis that the molecule is linear in both the
ground and excited states. The vibrational analysis of the
vious work.4
Our precise determination of the ground-state spin–orbit
splitting and the new ground-state vibrational combination
differences available from the hot bands in Table I have per-
ϩ
2
2
˜
˜
B ⌺u –X ⌸g bands of BS2 is unequivocal and is supported
by a variety of ancillary lines of evidence. The only bands
which exhibit assignable 10BS2 features are two components
of 220, as documented in the footnote to Table I. Theory gives
the isotope effect as12
mitted us to refine the BS2 ground-state Renner–Teller
1
˜ ˜
analysis. In particular, the B–X 21 hot bands allowed direct
determination of the relative energies of the four levels of Љ
v2
as ␮ 2⌺ϭ0.0,
⌬
5/2ϭ6.7 cmϪ1
,
⌬
3/2ϭ393.7 cmϪ1, and
2
2
␬ 2⌺ϭ417.0 cmϪ1, which made it possible to determine the
gK parameter. We have used the Renner–Teller model de-
scribed previously4 in a weighted least-squares analysis to fit
␻
␻
10BS2͒
11BS2͒
␻
␻
10BS2͒
11BS2͒
1ϩ2mS /m 10
͑
͑
͑
͑
3
3
2
2
͑
B͒
ϭ
ϭ
ͱ
ϭ1.04158, ͑1͒
1ϩ2mS /m 11
͑
B
͒
while experiment gives a 2␯Ј ratio ͑assuming negligible 000
˜ ˜
a total of 89 combination differences obtained from the B–X
2
band isotope effect͒ of 1.041 for the 2⌺ component. The
upper-state vibrational frequencies calculated at the
CCSD͑T͒/aug-cc-pVTZ level of theory ͑Table IV͒ are also in
˜ ˜
LIF spectra and previously studied A–X emission spectra to
an overall weighted standard deviation of 2.03 cmϪ1. The
resulting constants are given in Table III, along with previ-
ously determined values. Both sets of parameters are in gen-
eral agreement, although there are some noticeable changes
excellent agreement with the values of Ј and Ј derived
v1 v2
from experiment. The rotational analysis shows that the as-
TABLE IV. Vibrational parameters ͑in cmϪ1͒, bond lengths ͑Å͒, and energies ͑cmϪ1͒ for the ground and
electronic excited states of 11BS2 .
ϩ
u
2
2
2
˜
X
˜
A
˜
B ⌺
⌸
⌸
g
u
Parameter
Experimenta,c
Ab initiob
Experimentc
Ab initiob
Experimenta,d
Ab initiob
544.8
283.5
1010.7
1.6649
0.0
550.4
282.7
944.1
1.671
0.0
514.1
309.7
1543.6
1.7129
13839.392
514.4
316.8
1569.7
1.718
14269
506.7
303.2
—
1.6812
24189.534
516.6
299.0
1052.0
1.687
24148
␻
␻
␻
r0
1
2
3
T0
aThis work.
bCCSD͑T͒/aug-cc-pVQZ calculations, frequencies with aug-cc-pVTZ basis set.
cReference 4.
d
˜
The B-state vibrational frequencies are the fundamental rather than the harmonic frequencies.
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4262
J. Chem. Phys., Vol. 120, No. 9, 1 March 2004
S.-G. He and D. J. Clouthier
signed 000 band does originate from the same vibrational
of the experimental r0 values. The calculated vibrational fre-
quencies differ from experiment, where available, by a maxi-
mum of 6.6%, although the majority of the differences are
below 2%. The zero-point-corrected electronic excitation en-
0
˜ ˜
level of the ground state as the 00 band of the A–X transi-
tion. The pattern of hot bands substantiates the cold band
assignments and the lower-state intervals are in accordance
with those derived in our previous Renner–Teller analysis of
the ground state.
ergies differ from experiment by 430 cmϪ1 (A–X) and Ϫ41
˜ ˜
cmϪ1 (B–X), which is very good agreement.
˜ ˜
It is apparent from Fig. 2 that there are no 10BS2 spectral
The present analysis confirms all aspects of our previous
Renner–Teller analysis4 of the ground state of 11BS2 . The
data in Table III show that the vibrational parameters change
only slightly when the more precise zero-point spin–orbit
splitting and lower-bending-level intervals are included in
the least-squares analysis.
lines evident in the high-resolution spectrum of the 000 band.
˜
A calculation using the known ground- and B excited-state
11BS vibrational frequencies and the ab initio value for Ј
v3
2
along with the theoretical isotope ratio in Eq. ͑1͒ gives a 000
band isotope shift (10BS2 –11BS2) of ϳ1.2 cmϪ1. Since the B
values are the same in both isotopomers and the rotational
spacings are of the order of 0.5 cmϪ1, the rotational lines of
11BS2 and 10BS2 must be accidentally coincident, although
shifted by one or two units of the angular momentum quan-
tum number J. This also accounts for the broader than ex-
pected width of the overlapping Q21 and R11 branch feature.
The effective (r0) B–S bond lengths calculated from the
ACKNOWLEDGMENTS
The authors thank Brandon Tackett for assistance in op-
erating the instrumentation and Dr. Corey Evans for early
attempts to observe the spectrum. This research was sup-
ported by the National Science Foundation.
˜
˜
rotational constants are Xϭ1.6649(2) Å, Aϭ1.7129(1) Å,
1 V. A. Koryazhkin and A. A. Mal’tsev, Vestn. Mosk. Univ., Ser. 2: Khim.
21, 6 ͑1966͒.
˜
and Bϭ1.6812(1) Å. The bond length changes are as ex-
pected from qualitative molecular orbital theory. Excitation
of an electron from the bonding ␲ orbitals to the partially
2 A. G. Briggs and R. E. Simmons, Spectrosc. Lett. 19, 953 ͑1986͒.
3 J. M. Brom, Jr. and W. Weltner, Jr., J. Mol. Spectrosc. 45, 82 ͑1973͒.
4 S. G. He, C. J. Evans, and D. J. Clouthier, J. Chem. Phys. 119, 2047
͑2003͒, and references therein.
u
˜ ˜
filled nonbonding ␲ orbitals (A–X transition͒ elongates the
bond by 0.048 Å, whereas promotion of an electron from the
g
5 J. Karolczak and D. J. Clouthier, Rev. Sci. Instrum. 61, 1607 ͑1990͒.
6 R. H. Judge and D. J. Clouthier ͑unpublished͒.
˜ ˜
7 S. Gerstenkorn and P. Luc, Atlas du Spectre D’Absorption de la Molecule
d’Iode ͑Editions du C.N.R.S., Paris, 1978͒.
slightly bonding ␴ orbital to the ␲ orbitals (B–X transi-
tion͒ only increases the bond length by 0.016 Å. The
u
g
8 S. Gerstenkorn and P. Luc, Rev. Phys. Appl. 14, 791 ͑1979͒.
9 T. C. Smith, H. Li, D. J. Clouthier, C. T. Kingston, and A. J. Merer, J.
Chem. Phys. 112, 3662 ͑2000͒.
˜ ˜
Franck–Condon activity in the B–X spectrum, with most of
the intensity concentrated in the 000 band, is entirely consis-
tent with the small structural change in electronic excitation.
Coupled cluster theory with large basis sets does an ex-
cellent job of predicting the molecular properties of BS2 , as
shown in Table IV. Theory consistently slightly overesti-
mates the bond lengths, although they are all within 0.006 Å
10 M. J. Frisch et al., computer code GAUSSIAN 98, revision A.11, Gaussian,
Inc., Pittsburgh, PA, 1998.
11 T. H. Dunning, Jr., J. Chem. Phys. 90, 1007 ͑1989͒.
12 G. Herzberg, Molecular Spectra and Molecular Structure II. Infrared and
Raman Spectra of Polyatomic Molecules ͑Van Nostrand, New York,
1945͒, p. 230.
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