Journal of Molecular Spectroscopy 278 (2012) 35–40
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Journal of Molecular Spectroscopy
journal homepage: www.elsevier.com/locate/jms
Fourier transform microwave spectroscopy of ScS (X2R+) and YS (X2R+)
G.R. Adande, D.T. Halfen, L.M. Ziurys ⇑
Departments of Chemistry and Astronomy, Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, United States
a r t i c l e
i n f o
Article history:
Received 24 May 2012
In revised form 5 July 2012
Available online 22 July 2012
Keywords:
FTMW spectroscopy
Scandium sulfide (ScS)
Yttrium sulfide (YS)
Hyperfine structure
Laser ablation
a b s t r a c t
The pure rotational spectra of the transition metal sulfide radicals ScS and YS in their 2R+ ground states
have been measured in the range 8–48 GHz using Fourier transform microwave (FTMW) spectroscopy.
The radicals were synthesized from the reaction of metal vapor, produced by laser ablation, and H2S
gas, heavily diluted in argon. A DC discharge was needed in the case of ScS. Four rotational transitions
were recorded for each molecule, in which multiple fine and hyperfine components were resolved. The
spectra were analyzed with a case (b) Hamiltonian, and rotational, fine, and hyperfine constants were
determined for both molecules, improving the precision of previous parameters established from optical
and double resonance data. The quadrupole coupling constant eQq has been accurately established for ScS
for the first time, as well. From the rotational constants, the bond lengths were determined to be 2.1288 Å
for ScS and 2.2614 Å for YS. The hyperfine parameters suggest that, although ScS and YS are principally
ionic molecules, they are more covalent than their oxygen analogs.
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
Transition metal sulfides play an important role in many areas of
scientific research. For example, such sulfides have numerous applications in catalysis [1], as well as in the semiconductor industry [2].
They also are relevant in biology, being linked to the primitive development of autotrophic life [3]. Because of their chemical importance,
numerous theoretical investigations have been conducted in order
to understand the structural, electronic, and thermodynamic properties of transition metal sulfides [4–6]. Such studies, however, can
be problematic because of the presence of low-lying electronic
states. High resolution spectroscopic data are therefore necessary
to benchmark and complement such calculations.
Two interesting transition metal sulfides are yttrium monosulfide (YS) and scandium monosulfide (ScS). Their relatively simple
electronic structure provides a good starting point for computational models investigating transition metal bonding. Scandium
and yttrium both belong to the group III transition metals, which
have the simplest open d shell configuration (s2d1). Both molecules
have been studied spectroscopically at optical wavelengths, as well
as by Fourier transform infrared and optical double resonance
methods [7–13]. These studies have shown that both species have
2 +
R ground states, with the unpaired electron likely situated in a r
hybridized molecular orbital, predominantly located on the metal
atom [4]. From the experimental determination of magnetic hyperfine parameters, it had been suggested that the unpaired electron
in ScS occupies an orbital centered on the scandium atom with
⇑ Corresponding author. Fax: +1 520 621 5554.
E-mail address: [email protected] (L.M. Ziurys).
0022-2852/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jms.2012.07.009
57% s character [10], while the equivalent orbital in YS is 53% s
in composition [9].
Here we present the first measurements of the pure rotational
spectra of YS and ScS in their 2R+ ground states, recorded using
Fourier transform microwave (FTMW) techniques. These radicals
were produced using a laser ablation source to generate metal vapor. For both molecules, the fine and hyperfine structures were resolved in multiple rotational transitions, allowing for improved
determination of the spectroscopic parameters, as well as an accurate measurement of the quadrupole coupling constant of ScS. In
this paper we describe these results and their analysis, and give
an interpretation of the fine and hyperfine constants for the two
radicals.
2. Experimental
The pure rotational spectra of YS and ScS were measured using
the Balle–Flygare type Fourier transform microwave (FTMW) spectrometer of the Ziurys group, described in detail elsewhere [14].
Briefly, the instrument consists of a vacuum chamber containing
a Fabry–Perot cavity with two spherical aluminum mirrors in a
near confocal arrangement. The system is maintained at an unloaded pressure of 108 Torr by a cryopump. Microwave radiation
is launched into the cavity either through an antenna (4–40 GHz)
or waveguide (40–60 GHz: see [15]) embedded in one mirror. Molecules of interest are introduced into the chamber via a pulsed
supersonic nozzle. Molecular emission is collected by an antenna
or waveguide embedded in the opposite mirror and detected as a
function of time with a low noise amplifier, the so-called Free
Induction Decay (FID). The time domain signal is digitized and
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G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40
converted by Fast Fourier Transform (FFT) to generate a spectrum,
which appears as Doppler doublets, resulting from the jet expansion of the mixture relative to the electric field in the cavity. The
transition frequency is taken to be the average of the doublets.
The resolution of the FTMW spectrometer is 4 kHz.
Both ScS and YS were created by the reaction of H2S with metal
vapor produced by laser ablation. A gas mixture of 0.1% H2S in
200 psi of argon was introduced into the cavity by a pulsed valve
(General Valve, 0.8 mm nozzle orifice), to which the ablation
source was attached such that the gas mixture and the metal vapor
were injected approximately at the same time into the cavity. A
pulsed Nd:YAG laser beam (200 mJ/pulse) was used to ablate the
metal, contained in the form of a rotating, translating rod. In the
case of ScS, a DC discharge (0.6 kV, 20 mA) was also necessary for
molecule production, applied to the metal/gas mixture immediately following the ablation source. (Yttrium reacted with H2S
spontaneously without the need of a discharge.) The details of
the discharge assisted laser ablation source, called DALAS, can be
found in Sun et al. [16].
3. Results
The rotational measurements were based on the constants obtained by Stringat et al. [13] and Azuma and Childs [8] for YS
and Steimle et al. [10] for ScS. Because the magnetic moment of yttrium is relatively small, the hyperfine pattern in YS follows a classic bbJ coupling scheme, such that J = N + S and F = J + I. The nuclear
spin of yttrium is I = 1/2. Four rotational transitions of YS
(N = 1 ? 0 to N = 4 ? 3) were recorded over the range 8–34 GHz;
see Table 1. Frequency predictions using the previous spectroscopic parameters for YS were typically reliable to ±15 MHz. Fifteen hyperfine components of this radical were recorded in total.
Representative spectra of YS are shown in Fig. 1. In the upper
panel, two hyperfine components in the N = 2 ? 1 rotational transition near 16.6 GHz are displayed, labeled by quantum number F,
with one from each spin–rotation doublet, indicated by J. Similarly,
the lower panel shows two hyperfine components of the N = 3 ? 2
transition neat 24.9 GHz, one from each spin–rotation component.
All lines exhibit Doppler doublets, indicated by brackets.
In contrast to yttrium, scandium has a large magnetic moment
and a nuclear spin of I = 7/2. As a consequence, the Fermi contact
term in ScS is very large relative to the spin–rotation interaction.
The hyperfine structure is therefore of the same order of magnitude as the fine structure, following a bbs coupling scheme as opposed to bbJ, as for YS. Four rotational transitions were recorded
for ScS in the range 11–50 GHz, each consisting of numerous
Fig. 1. Representative FTMW spectra recorded for YS (X2R+). In the upper panel,
two hyperfine components of the N = 2 ? 1 rotational transition near 16.6 GHz are
displayed, indicated by quantum number F, each arising from a different spin
doublet, labeled by J. In the lower panel, two hyperfine lines from the N = 3 ? 2
rotational transition near 24.9 GHz are shown, also originating in separate spin–
rotation components. There is a frequency beak in each spectrum in order to show
the two spectral features. Doppler doublets are indicated by brackets. Each spectral
feature shown was measured in one 600 kHz wide scan, with 1000 pulses per scan.
hyperfine components: see Table 2. For comparison with YS, the
bbJ notation was used for labeling the transitions. The transition
frequencies were typically ±3 MHz away from predictions, based
on the previous constants.
Table 1
Observed transition frequencies of YS (X2R+).
a
mobsa
mobs mcalca
N0 ? N00
J0 ? J00
F0 ? F00
1?0
1/2 ? 1/2
3/2 ? 1/2
3/2 ? 1/2
1?1
1?0
2?1
8296.712
8327.470
8348.735
0.003
0.003
0.001
2?1
3/2 ? 1/2
3/2 ? 1/2
5/2 ? 3/2
5/2 ? 3/2
1?0
2?1
2?1
3?2
16624.146
16649.810
16654.901
16674.080
0.003
0.002
0.001
0.006
3?2
5/2 ? 3/2
5/2 ? 3/2
7/2 ? 5/2
7/2 ? 5/2
2?1
3?2
3?2
4?3
24955.916
24974.668
24982.239
24999.912
0.006
0.002
0.003
0.002
4?3
7/2 ? 5/2
7/2 ? 5/2
9/2 ? 7/2
9/2 ? 7/2
3?2
4?3
4?3
5?4
33282.711
33299.468
33309.440
33325.797
0.009
0.001
0.002
0.004
In MHz.
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G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40
Table 2
Observed transition frequencies of ScS (X2R+).
a
N0 ? N00
J0 ? J00
F0 ? F00
mobsa
mobs mcalca
1?0
3/2 ? 1/2
3/2 ? 1/2
3/2 ? 1/2
3?3
5?4
4?4
11805.146
11849.830
11898.629
0.001
0.005
0.006
2?1
3/2 ? 3/2
3/2 ? 1/2
3/2 ? 3/2
3/2 ? 1/2
5/2 ? 1/2
5/2 ? 3/2
5/2 ? 3/2
3/2 ? 1/2
5/2 ? 3/2
5/2 ? 1/2
5/2 ? 3/2
3/2 ? 1/2
3?4
2?3
4?4
5?4
4?4
2?2
5?4
3?3
6?5
3?4
5?5
4?3
23504.279
23576.110
23607.576
23629.356
23646.380
23664.364
23669.405
23686.733
23696.680
23699.674
23718.195
23790.044
0.005
0.009
0.002
0.001
0.002
0.001
0.000
0.009
0.001
0.005
0.002
0.002
3?2
5/2 ? 3/2
5/2 ? 3/2
5/2 ? 3/2
5/2 ? 3/2
7/2 ? 5/2
7/2 ? 5/2
5/2 ? 5/2
7/2 ? 5/2
5/2 ? 3/2
7/2 ? 5/2
7/2 ? 3/2
7/2 ? 5/2
7/2 ? 5/2
7/2 ? 5/2
7/2 ? 5/2
7/2 ? 5/2
5/2 ? 3/2
1?2
4?4
3?3
6?5
5?5
3?2
5?4
4?3
2?2
6?5
5?4
4?4
2?2
7?6
6?6
3?3
4?3
35416.208
35442.239
35454.586
35457.035
35461.236
35470.531
35471.275
35472.333
35480.732
35512.429
35523.071
35525.635
35530.564
35532.415
35533.948
35535.677
35545.540
0.001
0.001
0.003
0.001
0.004
0.004
0.000
0.002
0.008
0.000
0.003
0.003
0.008
0.002
0.005
0.002
0.001
4?3
7/2 ? 5/2
7/2 ? 5/2
7/2 ? 5/2
9/2 ? 7/2
9/2 ? 7/2
9/2 ? 7/2
7/2 ? 5/2
9/2 ? 7/2
7/2 ? 5/2
9/2 ? 7/2
4?4
7?6
6?5
5?4
4?3
6?5
5?4
7?6
4?3
8?7
47259.004
47285.741
47298.714
47305.885
47311.688
47345.299
47347.847
47348.429
47349.957
47364.951
0.001
0.013
0.004
0.002
0.003
0.002
0.004
0.007
0.001
0.005
In MHz.
In case bbs, S couples with I instead of N to give the intermediate
quantum number G, taking the place of J, i.e. I + S = G. G then couples with N to create F. In this notation, all the transitions reported
in Table 2 would either be assigned to G = 3 or G = 4. Adding N0 or
N00 to the respective G generates the F0 or F00 , as given in the table.
Representative spectra of ScS are shown in Fig. 2. The top panel
displays two hyperfine components of the N = 2 ? 1 rotational transition near 23.6 GHz, both arising in the J = 5/2 ? 3/2 fine structure
doublet (or G = 3, F = 3 ? 2 and G = 4, F = 6 ? 5). Three hyperfine
lines of the N = 3 ? 2 transition near 35.5 GHz are shown in the lower panel, one arising from the J = 5/2 ? 3/2 doublet and the other
two from the 7/2 ? 5/2 doublet (all G = 4). Hyperfine transitions
are labeled by F, and the Doppler doublets are indicated by brackets.
4. Analysis
Both molecules were fit with the non-linear least-squares analysis program SPFIT [17]. The following bbJ effective Hamiltonian
was used [18]:
Heff ¼ Hrot þ Hsr þ Hmhf þ HeQq þ Hnsr
ð1Þ
Rotational, spin–rotation, magnetic hyperfine, electric quadrupole and nuclear spin–rotation interactions were considered in
the analyses.
The fitted spectroscopic parameters for YS and ScS are presented in Table 3. Also given in the table are the constants previously obtained from the optical and double resonance data. In
the analysis of YS, only five parameters were necessary to obtain
an rms of 4 kHz, the experimental precision. In contrast, in the
double resonance study of YS [8], both CI and cD were additionally
used in the spectral fitting. Both parameters did not significantly
improve the fit in this work and therefore were not included. As
Table 3 shows, the FTMW study has improved the precision of
the rotational constants, and the fine and hyperfine parameters
are consistent with the past double resonance work. In the case
of ScS, the FTMW data has increased the accuracy of the spectroscopic constants by factors of 10–100, including the first reliable
determination of the quadrupole coupling constant eQq. In this
analysis, CI was necessary to achieve an rms comparable to the
experimental precision (4 kHz).
5. Discussion
5.1. Hyperfine and fine structure interactions
The electronic configuration of ScS is postulated to be: (core)
10r2 4p4 11r1 [4,10,19]. The unpaired electron in ScS thus lies in
the 11r orbital, which is thought to be centered on the scandium
38
G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40
be used to evaluate the amount of s character in a given orbital
by comparing it to the atomic value of scandium, 2823 MHz [20].
The ratio is then [bF(ScS)/bF(Sc)] = |c1|2 0.58, such that the unpaired electron in ScS is in an orbital with 58% s character. This value is smaller than that found for ScO, where the analogous
electron has 69% s character [20]. The decrease in s character upon
replacement of O by S suggests that the atomic 3d orbital is better
stabilized by the less electronegative sulfur atom.
Similarly, the YS electronic configuration is thought to be (core)
13r2 6p4 14r1 [11]. As with scandium, the 5p orbitals in yttrium lie
more than 16 000 cm1 above the 5s level [21], and are unlikely to
contribute appreciably to the 14r orbital. This orbital is thus likely
to be sd hybridized, with a small sulfur 3p contribution. The atomic
value of the Fermi contact term for the yttrium atom is 1250 MHz
[8]. Therefore, the unpaired electron in YS is 53% s in character, as
opposed to 62% for YO [22]. Again, the amount of s character
decreases with replacement of the oxygen atom with sulfur.
The dipolar hyperfine constant c is defined as [23]:
c¼
3
1 X ð3 cos2 hi 1Þ
g s lB g N lN
2
n
r 3i
s
ð2Þ
Assuming a negligible role for the sulfur 3p orbital, contributions to c must come principally from the metal d orbital that is
hybridized with the metal s orbital, as the angular expectation value for an s electron is zero. Assuming pure sd hybridization, the
dipolar hyperfine parameter reduces to
c¼
2
+
Fig. 2. Representative FTMW spectra recorded for ScS (X R ). In the upper panel,
two hyperfine components of the N = 2 ? 1 rotational transition near 23.6 GHz are
presented, both arising in the J = 5/2 ? 3/2 fine structure doublet and labeled by the
F quantum number. In the lower panel, three hyperfine lines of the N = 3 ? 2
transition near 35.5 GHz are displayed, one arising from the J = 5/2 ? 3/2 doublet
and the others from the 7/2 ? 5/2 component. There are frequency breaks in each
spectrum in order to display multiple hyperfine transitions. Doppler doublets are
indicated by brackets. Each spectral feature shown was measured in one 600 kHz
wide scan, with 1500–2000 pulses per scan.
nucleus [10]. The orbital composition is primarily a mixture of metal 4s and 3d character, because the 4p orbitals of scandium lie
much higher in energy (20 000 cm1 above the 4s level) [19]. In
analogy to the metal oxides, the 11r orbital is thought to be mostly
non-bonding, with slight bonding character due to a small admixture of the sulfur 3p orbital [19]. Because the Fermi contact term
only arises from the contribution of electrons in s orbitals, it can
3
3 cos2 h 1
g s lB g N lN
2
r3
dr
ð3Þ
For dr electrons, h3 cos2 hi 1i = 4/7. Using this factor, h1/r3i can
be calculated for the unpaired electron in both scandium and
yttrium sulfide. For ScS, h1/r3i 1.013 a.u3 for the unpaired electron. This value can be compared to the atomic value for the scandium d electron of h1/r3i 0.911 a.u3 [10] and for the ion Sc+:
h1/r3i 1.851 a.u3 [24]. The magnitude of h1/r3i of the unpaired
electron is significantly closer to the neutral value, suggesting a
considerable degree of covalency in the Sc–S bond. For YS, the
unpaired electron has h1/r3i 1.887 a.u3. The analogous atomic
values are h1/r3i 2.373 for the Y+ ion [24] and h1/r3i 1.711 a.u3
for the neutral atom [25]. The trend found in scandium for h1/r3i is
repeated in yttrium, indicating some fraction of covalent character
also in this molecule.
In this work, the quadrupole coupling constant for ScS has been
accurately determined for the first time. In the context of a
Townes–Dailey analysis, the quadrupole coupling constant eQq0
Table 3
Spectroscopic constants for ScS (X2R+) and YS (X2R+).a
Parameter
ScS
ScS (optical)b
YS
YS (optical)
B
D
bF
c
CI
eQq
5915.2294(12)
0.002873(50)
96.3356(73)
–
1671.2(2.4)
112.558(12)
0.01665(98)
55.709(54)
5914.72(39)
0.002893(23)
92.8(2.2)
–
1673.9(6.2)
116(41)
–
63(159)
56(27)e
4163.0992(21)
0.001331(79)
42.252(15)
–
667.8(1.2)
42.470(94)
4160.2(2.7)c
0.0011(6)c
42.2382(6)d
1.8243(21) 104d
667.479(60)d
42.684(54)d
0.0046(6)d
rms of fit
r0 (Å)
0.004
2.128824(2)
c
cD
a
b
c
d
e
Constants in MHz unlesss specified. Errors quoted are 3r.
From [12], unless otherwise specified.
From [13].
From [8].
From [10].
2.13750(6)
0.004
2.261416(1)
G.R. Adande et al. / Journal of Molecular Spectroscopy 278 (2012) 35–40
ScS and YS, respectively. The values of the spin–rotation parameters in the corresponding oxides are c = 3.2175 MHz (ScO) and
9.2254 MHz (YO). Very small or negative values of the spin–rotation constant in the oxides have been attributed to second-order
spin–orbit coupling from unobserved low-lying electronic 2P
states, arising from the promotion of one electron from the HOMO
p shell [22]. In the metal sulfides, other electronic states must be
contributing to the second-order spin–orbit interaction, generating
net positive spin–rotation constants.
Table 4
Quadrupole coupling constants of scandium species.
Species
eQq (MHz)
ScS
ScO
ScCl
ScF
55.709(54)
72.240(15)
68.2067(90)
74.09(15)
39
5.2. Periodic trends in 3D-transition metal monosulfides
Fig. 3. A comparison of experimentally-determined and theoretical [4] ground state
bond lengths for the 3d transition metal sulfides and oxides. The experimental bond
length values are r0 and the theoretical ones are re. The experimental and theory
values are in relatively good agreement. The sulfides and oxide bond lengths show
subtle variations across the periodic table, with notable differences at scandium and
zinc, and from iron to nickel.
From the measured rotational constant of ScS in its ground
state, a r0 bond length of 2.1288 Å has been determined. In Fig. 3,
the experimentally-determined bond lengths for the 3d transition
metal oxides and sulfides are plotted, as well as the values
obtained from theory using DFT methods [4]. The agreement
between theory and experiment is rather good for ScS and ScO.
Nonetheless, there are two notable differences between the oxide
and sulfide series. First, the bond length of ScS is greater than that
of ZnS by almost 0.1 Å. In contrast, that of ScO is smaller than the
bond distance of ZnO by about 0.03 Å. Secondly, while FeO, CoO
and NiO have similar bond lengths, the bond lengths decrease
steadily from FeS to NiS.
This effect can be qualitatively explained by comparing the
atomic orbitals. The energy separation between the 4s and 3d orbitals of the transition metals and the 2p orbital of oxygen is generally larger than the separation with the 3p orbital of sulfur by
about 34,000 cm1 [27]. Consequently, there is more valence orbital overlap between the atoms in the monosulfides (i.e. increased
bonding character), while in the oxides, these orbitals are predominantly non-bonding. Therefore, addition of electrons into the valence orbitals partly stabilizes the monosulfide molecules,
shortening the bond lengths, as noted by Bridgeman and Rothery
[4]; this stabilization is not as significant for the monoxide species.
can be expressed in terms of eQq320, the quadrupole coupling created by a 3d orbital of scandium [26]:
6. Conclusion
1
eQq0 ¼ eQq320 ndr þ ndp ndd
2
The pure rotational spectra of ScS and YS in their 2R+ ground
states have been measured using FTMW spectroscopy, in combination with laser ablation. Spectroscopic constants have been improved for both radical species. Analysis of the hyperfine
parameters indicates that both YS and ScS are somewhat more
covalent than their oxygen analogs. In addition, YS is slightly less
ionic than ScS. These data support the theoretical prediction that
transition-metal bonds to sulfur are different than those to oxygen.
ð4Þ
Here ni are the orbital populations. Assuming the only contribution to eQq in ScS is the unpaired electron in the hybridized sd orbital, ndr = 1 and the other populations are zero. The term eQq320 can
be evaluated using the formula [26]:
eQq320 ¼ 2:353
3 2lðl þ 1Þ
a0
Q
ð2l þ 3Þð2l 1Þ
r
ð5Þ
For a scandium nucleus, Q = 23.1 fm2 [26], l = 2 for a d electron, and h1/r3i can be obtained from c, as discussed. The coupling
constant eQq is then calculated to be 31.46 MHz. This value is
about 56% of the experimental constant of 55.709 MHz, which suggests that core electrons in scandium sulfide also contribute to eQq.
Some insight into the bonding in ScS can be gleamed from a
comparison of quadrupole parameters among scandium species,
as listed in Table 4. For ScO, eQq = 72.24 MHz [20], while the
respective values are 74.09 MHz and 68.21 MHz for ScF and ScCl.
[26]. For ScS, the eQq = 55.709 MHz. Thus, while the quadrupole
constants indicate that the electronic distribution is similar in
these molecules, there are some differences. ScS is apparently the
most covalent of the four molecules, while ScF is the more ionic.
This result is perhaps expected as oxygen and the halogens are
more electronegative than sulfur.
Finally, the values of the spin–rotation parameters should be
noted. These constants are c = 96.3356 MHz and 42.252 MHz for
Acknowledgment
This work was supported by NSF Grant CHE-1057924.
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