Chemical Physics Letters 365 (2002) 514–524
www.elsevier.com/locate/cplett
~ 1A0):
The millimeter/submillimeter spectrum of LiSH (X
further investigations of the metal–sulfur bond
A. Janczyk, L.M. Ziurys
*
Department of Chemistry and Department of Astronomy, Steward Observatory, University of Arizona,
933 North Cherry Avenue, Tucson, AZ 85721, USA
Received 28 June 2002; in final form 29 August 2002
Abstract
The pure rotational spectrum of LiSH and its 6 Li and deuterium isotopomers has been recorded in the region 73–521
GHz using millimeter/sub-millimeter direct absorption techniques. These species were created by the reaction of H2 S or
D2 S and lithium vapor in a dc discharge. Extensive Ka ladder structure was observed for all three isotopomers, indi~ 1 A0 electronic state. Rotational parameters have been decating that LiSH is a near-prolate asymmetric top with a X
termined for the three species, enabling the calculation of a rmð1Þ structure. The Li–S–H bond angle was found to be 93°,
indicating a high degree of covalent bonding in this molecule.
Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
While metal oxides and hydroxides species have
been studied extensively by a variety of spectroscopic methods [1–6], many of their sulfide analogs
in comparison have been neglected. Although
metal–oxygen bonds are quite common in multiple
areas such as combustion, corrosive processes and
chemical vapor deposition [7,8], metal-sulfide
bonds also play a relevant chemical role. For example, they are significant in biological systems in
the structures of amino acids and proteins [9].
They additionally are important in such applications as catalysis, pollution control, and in me-
*
Corresponding author. Fax: +520-621-1532.
E-mail address: [email protected] (L.M. Ziurys).
chanical lubricants [10]. It is therefore of interest
to study metal–sulfur bonds and determine their
properties, in particular in relationship to their
oxygen counterparts.
One interesting class of molecules containing a
metal–sulfur bond is the hydrosulfides, with the
structure MSH. These species are thought to be
particularly important because they serve as
models for organic thiols MSR [11]. Also, many
MOH compounds have been investigated spectroscopically, including those where M is an alkali
metal [2,12], an alkaline earth element [1,13,14], a
transition metal [4], and aluminum [3]. Hence, a
good comparison can be made between the metal
hydroxides and hydrosulfides.
Metal hydroxide compounds in themselves exhibit interesting bonding and corresponding structural trends. For example, most metal hydroxides
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 5 0 4 - X
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
are linear species such as CaOH, SrOH, BaOH and
AlOH [1,3,14]. These molecules are thought to be
highly ionic. In contrast, CuOH and AgOH are
bent species, and are speculated to have a greater
degree of covalency [4]. In between lie MgOH,
NaOH and LiOH, which appear to be quasi-linear
molecules with very low barriers to the bending
motion [2,12,13]. In fact, calculations have shown
that LiOH has virtually no barrier to a bent
geometry up to an angle of 90° [12].
The sulfur analogs of these compounds that
thus far have been investigated, on the other hand,
have all proven to be bent. These species include
the alkaline earth radicals MgSH [15], CaSH
[16,17], SrSH [18,19], as well as NaSH [20]. Optical
and pure rotational data have conclusively shown
that these molecules are near-prolate asymmetric
tops with M–S–H angle near 90°. Hence, they
closely resemble H2 S. Therefore, the metal hydrosulfides appear to exhibit a greater degree of
covalent bonding than their hydroxides analogs.
Clearly, it would be of interest to investigate other
metal hydrosulfide species to determine whether
their structures are bent, linear, or even quasi-linear, and examine any geometrical trends.
LiSH is one possible candidate in this respect. A
study of the lithium form of the hydrosulfides is of
particular relevance because this species can be
most directly compared to H2 S. Moreover, lithium-alkyl compounds are extremely well known in
organic synthesis; there has been much speculation
concerning the nature of the Li–R bond [21,22],
which has been consequently extended to Li–SR
systems [11]. Quantum calculations have also been
carried out for LiSH by several authors. The earliest study was by Pappas [11], who suggested that
the molecule is bent with a Li–S–H angle of 97°,
and with a Li–S bond length that is 0.05 A
greater than that found experimentally in the LiS
radical [23]. More recent computations by Remko
and Rode [24] and Magnusson [25] also indicate a
bent geometry, with an angle near 94° and 91°,
respectively. A crystal structure of LiSH has been
published as well [26].
As a continuation of our work on metal–ligand
interactions, we have recorded the pure rotational
~ 1 A0 ground electronic
spectrum of LiSH in its X
state. To our knowledge, this work is the first gas-
515
phase spectroscopic investigation of this species in
its monomeric form. The spectrum of this molecule was measured using millimeter/sub-mm direct
absorption techniques and those of LiSD and
6
LiSH were recorded as well. LiSH is unstable
under ordinary conditions, and rapidly hydrolyses
in air [11]. Hence, it had to be created in a dc
discharge. Here we present our measurements and
analysis, and discuss bonding in metal hydrosulfides based on these new data.
2. Experimental
The rotational spectrum of LiSH and two of its
isotopomers were measured using one of the millimeter-wave spectrometers of the Ziurys group,
which is described in detail elsewhere [27]. Briefly,
the source of radiation consists of a phase-locked
Gunn oscillator combined with a multiplier, which
enable frequency coverage of 65–640 GHz. The
microwave radiation is propagated quasi-optically
through the cell, a double pass system, using a
series of Teflon lenses, a wire grid, and a rooftop
reflector. The radiation is then detected by a helium cooled, InSb hot electron bolometer.
LiSH was created by the reaction of lithium
vapor and H2 S in a dc discharge. The metal vapor
was generated by placing solid lithium in a metal–
lined alumina crucible and heating it in a Broidatype oven. The metal liner was necessary to prevent
the molten lithium from attacking the crucible. The
Li vapor was then reacted with about 4 mTorr of
H2 S, introduced through a tube over the top of
the oven. Approximately 20 mTorr of argon, which
was added through the bottom of the oven, was
used as a carrier gas for the lithium. The optimum
discharge current was 0.21 A at 70 V. Discharging
the reaction mixture resulted in bright pink emission whose intensity depended upon the concentration of lithium vapor. To create LiSD, D2 S
(Cambridge Laboratory) was reacted instead of
H2 S. To preferentially synthesize the 6 Li isotopomer, enriched lithium-6 metal (Aldrich) was
used.
After the initial search using data scans successively covering 100 MHz in range (see Section
3), actual transition frequencies were determined
516
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
from 5 MHz scans, which were always recorded in
pairs, increasing and decreasing in frequency.
Usually only one such pair was needed to obtain a
sufficient signal-to-noise ratio. However, for the
weaker Ka components, as many as 8 pairs were
averaged. The center frequency of all features was
determined by fitting a Gaussian curve to the line
profile.
3. Results
To initially search for the spectrum of LiSH, a
geometry had to be predicted. Because NaSH [20],
MgSH [16] and CaSH [17] are all bent species, the
lithium analog was likely to have a similar structure and a 1 A0 ground electronic state. The Li–S
bond length was assumed to be that of the dia
tomic species, LiS [23], which was rLi–S ¼ 2:164 A
(r0 ); for the S–H or (S–D) bond distance, that of
: [20]).
NaSH (or NaSD) was used (rS–H ¼ 1:396 A
The bond angle was chosen to be that of H2 S (92°:
[28]), because LiSH probably most closely resembles this molecule of all the alkali hydrosulfides.
From the geometric parameters, A, B, and C rotational constants were calculated. The major
centrifugal distortion constants, DJ and DJK , were
estimated by mass-scaling those of NaSH and
NaSD. Using those five spectroscopic parameters,
rotational frequencies were predicted for all three
isotopomers,
assuming
a-type
transitions
(DKa ¼ 0, DKc ¼ 1); a-type lines have been routinely observed for the other metal hydrosulfides
[15,17,19].
The predicted transition frequencies in the end
turned out to be very close to those actually
measured. (Only later were the ab initio calculations found in the literature [11,24,25].) Hence, for
the main LiSH isotopomer, a group of lines resembling an a-type pattern was initially found
within a few GHz (2 GHz) of the predictions,
and then a second transition was discovered. The
Ka ¼ 0 component and the Ka ¼ 1 asymmetry
doublets in both transitions were assigned initially
on the basis of their stronger intensity and their
position in frequency space relative to other observed lines. With two transitions observed, additional ones could be accurately predicted and were
subsequently recorded. Assignment of additional
Ka components was done by first finding the
Ka ¼ 3 lines, identified because of their small
asymmetry splitting which totally collapsed at
lower J. With the Ka ¼ 0, 1 and 3 lines assigned, it
was then relatively easy to predict and identify the
Ka ¼ 2 asymmetry doublets and the Ka P 4 components, which were collapsed into single features.
A similar situation occurred in the case of
LiSD, where predictions were initially good enough to come within 7 GHz of the actual transition frequencies. The Ka ¼ 0 and Ka ¼ 1 lines were
located because of their larger intensities, and the
Ka ¼ 3 doublets were found because of their small
asymmetry splitting. For the 6 Li isotopomer,
Fig. 1. A stick figure showing the progression and relative intensities of the Ka components of the J ¼ 6 ! 7 transition of
LiSH, LiSD and 6 LiSH. These patterns all clearly indicate that
lithium hydrosulfide is a bent molecule. The patterns are similar
for LiSH and 6 LiSH, with Ka ¼ 0 component located roughly in
between the Ka ¼ 1 asymmetry doublets. In contrast, the Ka ¼ 0
line in LiSD is shifted to the left of the Ka ¼ 1 centroid, with
other Ka components lying to the right at higher frequency.
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
candidate transitions were first identified in the
spectrum of 7 LiSH, based on the natural lithium
isotope abundances, and then searched for with
lithium-6 enriched metal. Actual lines arising from
6
LiSH then naturally appeared more intense. The
Ka asymmetry pattern was very easily assigned
because it closely mimicked that of 7 LiSH.
These variations in patterns are shown in Fig. 1,
which displays stick spectra of the J ¼ 6 ! 7
transition of all three isotopomers. The 7 LiSH and
6
LiSH patterns are very similar: the Ka ¼ 0 line is
located near the centroid of Ka ¼ 1 asymmetry
doublets, the Ka ¼ 2 doublets are split by 200
MHz near the Ka ¼ 0 feature, and the rest of the
components are collapsed into single features and
located at lower frequency. In contrast, for LiSD,
the Ka ¼ 2, 3 and 4 components are located to
higher frequency relative to the Ka ¼ 0 line, and
the asymmetry doublets for Ka ¼ 2 and 3 have
significantly larger splittings. These effects are expected, as LiSD is more asymmetric. In general,
these observed patterns clearly indicate a bent
molecule of the Cs point group.
Representative spectra are shown in Figs. 2 and
3. A section of the J ¼ 6 ! 7 transition of the
Fig. 2. Spectrum of a section of the J ¼ 6 ! 7 rotational
~ 1 A0 ) near 256 GHz, showing the Ka ¼ 0
transition of LiSH (X
and Ka ¼ 2 through 5 components. At this frequency, only the
Ka ¼ 2 lines are split because of asymmetry doubling, as illustrated in the spectrum. The lines marked by asterisks are unidentified. This spectrum is a composite of 30, 100 MHz scans,
each lasting 1 min in duration.
517
main lithium isotopomer near 256 GHz is presented in Fig. 2. The quantum number designation
is (Ka00 ; Kc00 )–(Ka0 ; Kc0 ). The Ka ¼ 0, 2, 3, 4 and 5
components are visible in these data; only the
Ka ¼ 2 lines are split by asymmetry doubling. The
features follow the progression of a-type transitions of near-prolate asymmetric top with Cs
symmetry. Asterisks indicate unidentified lines. In
Fig. 3, spectra of a section of the J ¼ 7 ! 8
transitions of both LiSD and 6 LiSH near 281 and
331 GHz, respectively, are displayed. The upper
panel shows the data for the deuterium isotope
over a 1.5 GHz range, and includes the Ka ¼ 0, 4,
Fig. 3. Spectra of a section of the J ¼ 7 ! 8 rotational transitions of LiSD and 6 LiSH near 281 and 331.5 GHz, respectively. In the LiSD spectrum (top panel), the Ka ¼ 4 and 5
asymmetry doublets are collapsed and appear to the higher
frequency side of the Ka ¼ 0 line. For 6 LiSH, only the Ka ¼ 0
and 2 components are visible, the latter which are split by
asymmetry by over 600 MHz. The other Ka components are
located to lower frequency of the Ka ¼ 0 line. Unidentified
features are indicated by asterisks. The LiSD spectrum is a
composite of 15, 100 MHz scans, each lasting about 1 min in
duration, and the 6 LiSH data are a composite of 10 similar
scans.
518
Table 1
~ 1 A0 Þa
Selected observed rotational transitions of LiSH, LiSD, and 6 LiSHðX
2
3
3
3
7
7
7
7
7
7
7
7
7
7
7
7
7
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
9
Ka0
0
1
0
1
1
6
6
5
5
4
4
3
3
2
0
2
1
1
7
7
6
6
5
5
4
4
3
3
2
0
2
1
8
Kc0
2
3
3
2
7
1
2
3
2
4
3
4
5
6
7
5
6
8
1
2
2
3
3
4
5
4
5
6
7
8
6
7
2
J 00
1
2
2
2
6
6
6
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
8
Ka00
0
1
0
1
1
6
6
5
5
4
4
3
3
2
0
2
1
1
7
7
6
6
5
5
4
4
3
3
2
0
2
1
8
Kc00
1
2
2
1
6
0
1
2
1
3
2
3
4
5
6
4
5
7
0
1
1
2
2
3
4
3
4
5
6
7
5
6
1
LiSH
6
LiSD
mobs
mobs mcalc
73286.203
107986.018
109913.610
111801.081
251836.709
253580.778
253580.778
254439.925
254439.925
255156.006
255156.006
255729.134b
255729.134b
256063.948
256172.532
256304.105
260723.499
287757.665
288635.854
288635.854
289775.726
289775.726
290757.602
290757.602
291577.204
291577.204
292236.822b
292236.822b
292594.111
292643.520
292954.113
297907.022
323224.830
)0.005
0.006
)0.005
)0.058
)0.014
0.030
0.030
0.012
0.012
)0.033
)0.034
–
–
0.000
0.017
)0.072
0.005
)0.013
0.043
0.043
0.012
0.012
0.001
0.001
)0.043
)0.047
–
–
)0.010
0.021
)0.040
0.016
0.006
LiSH
mobs
mobs mcalc
mobs
mobs mcalc
239412.422
245227.331
245227.331
245977.302
245977.302
246619.098
246619.098
247195.987
247170.448
247089.235
246041.160
248596.840
254862.750
273461.548
279257.257
279257.257
280248.091
280248.091
281111.120
281111.120
281857.407b
281857.407b
282559.131
282507.928
282268.286
280627.804
284515.280
291086.417
)0.033
)0.026
)0.026
0.015
0.015
0.060
)0.087
)0.056
0.027
0.048
)0.007
)0.056
0.006
0.011
)0.004
)0.004
0.013
0.013
)0.016
)0.015
–
–
0.014
0.017
0.032
0.010
)0.050
0.018
284478.348
286762.674
286762.674
287854.787
287854.787
288767.666
288767.666
289503.386
289502.161
289903.395
289960.028
290300.950
295859.277
325040.196
326245.774
326245.774
327691.669
327691.669
328939.976
328939.976
329985.486
329985.486
330835.011
330831.012
331249.353
331190.770
331845.049
338036.154
0.011
)0.024
)0.024
0.041
0.041
0.014
0.022
)0.163
0.092
0.040
0.019
)0.077
0.025
0.016
0.006
0.006
)0.017
)0.017
0.025
0.025
0.008
0.016
0.116
)0.123
0.031
)0.022
)0.029
0.0131
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
J0
8
1
7
7
6
6
5
5
4
4
3
3
0
2
2
1
8
8
1
7
7
6
6
5
5
4
4
3
3
0
2
2
1
8
8
1
7
7
6
6
5
1
9
3
2
4
3
5
4
6
5
7
6
9
8
7
8
2
3
10
3
4
5
4
6
5
7
6
8
7
10
9
8
9
7
6
14
7
8
9
8
10
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
13
13
13
13
13
13
13
13
8
1
7
7
6
6
5
5
4
4
3
3
0
2
2
1
8
8
1
7
7
6
6
5
5
4
4
3
3
0
2
2
1
8
8
1
7
7
6
6
5
0
8
2
1
3
2
4
3
5
4
6
5
8
7
6
7
1
2
9
2
3
4
3
5
4
6
5
7
6
9
8
7
8
6
5
13
6
7
8
7
9
323224.830
323656.553
324676.775
324676.775
325958.314
325958.314
327062.907
327062.907
327986.554
327986.554
328732.072
328735.745
329064.771
329104.143
329617.887
335065.303
359092.465
359092.465
359530.696
360704.068
360704.068
362126.978
362126.978
363354.308
363354.308
364382.526
364382.526
365216.544
365223.082
365430.319
365591.476
366296.906
372195.109
502401.524
502401.524
502728.693
504646.346
504646.346
506631.291
506631.291
508349.534
0.006
0.012
0.014
0.014
)0.006
)0.006
)0.017
)0.017
)0.025
)0.036
0.126
)0.175
0.025
)0.004
)0.003
0.016
0.001
0.001
0.021
)0.014
)0.014
)0.030
)0.030
)0.021
)0.021
)0.015
)0.038
0.113
)0.158
0.020
)0.037
0.044
0.021
0.020
0.020
)0.022
0.025
0.025
)0.012
)0.012
)0.009
307453.542
314145.762
314145.762
315264.119
315264.119
316242.694
316242.694
317098.489b
317098.489b
317852.372
317946.243
315004.601
317399.708
320580.486
327231.694
0.018
0.008
0.008
0.025
0.025
0.021
0.016
–
–
)0.035
0.065
0.002
0.025
0.005
)0.008
341383.668
349027.651
349027.651
350274.800
350274.800
351371.608
351371.608
352342.435b
352342.992b
353202.277
353362.982
349159.020
352477.604
356796.934
363286.194
)0.005
)0.004
)0.004
)0.014
)0.014
)0.004
)0.017
–
–
)0.126
0.096
)0.011
)0.022
0.029
)0.012
365571.278
366980.201
366980.201
368605.782
368605.782
370010.423
370010.423
371189.418
371189.418
372148.005
372156.501
372345.435
372567.767
373417.480
380177.212
0.014
0.012
0.012
)0.043
)0.043
0.035
0.035
0.017
)0.002
0.012
)0.048
)0.017
)0.043
0.053
0.005
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
9
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
14
14
14
14
14
14
14
14
519
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
and 5 components, as well as unidentified features
marked by asterisks. As the figure illustrates, the
asymmetry doubling is completely collapsed in the
Ka ¼ 4 and 5 lines. In the lower panel, which
covers approximately 1 GHz in frequency, the
Ka ¼ 0 and 2 components of 6 LiSH are visible.
Here the Ka ¼ 2 lines are split by over 0.5 GHz due
to the asymmetry effects.
A subset of the data measured for these three
species are presented in Table 1. (For the complete
data set, please contact the authors.) Altogether,
almost every Ka component (Ka 6 8) of 12 individual rotational transitions was recorded for
7
LiSH in frequency range 73–521 GHz, a total of
104 individual spectral lines. For LiSD, all components with Ka 6 7 were recorded for four transitions, and for 6 LiSH, a similar range of Ka
features were measured for three transitions. The
lowest transition measured for 7 LiSH was for
J ¼ 1 ! 2; although 7 Li has a nuclear spin of
I ¼ 3=2, no evidence of quadrupole hyperfine interactions were found in the single line recorded.
mobs
b
In MHz, for m ¼ 0. For the complete data set, contact L.M. Ziurys.
Blended lines.
4. Analysis
a
5
4
4
0
3
3
2
2
1
9
11
10
14
12
11
13
12
13
13
13
13
13
13
13
13
13
13
5
4
4
0
3
3
2
2
1
8
10
9
13
11
10
12
11
12
508349.534
509802.621
509802.941
510222.581
511004.972
511042.547
511264.227
513188.518
520363.252
)0.009
)0.036
0.025
)0.034
)0.064
0.033
0.027
)0.020
)0.008
mobs
mobs
14
14
14
14
14
14
14
14
14
Kc0
Ka0
J0
Table 1 (continued)
J 00
Ka00
Kc00
LiSH
mobs mcalc
LiSD
mobs mcalc
6
LiSH
mobs mcalc
520
All three data sets were analyzed using a S-reduced Hamiltonian that consisted of terms for
molecular frame rotation and its centrifugal distortion [29]. The Hamiltonian was incorporated
into the non-linear least-squares code, SPFIT,
developed by Pickett [30]. The data were analyzed
by first fitting only the lower Ka components of
each species (Ka ¼ 0, 1 and 3). Spectral lines that
were partially blended (usually Ka ¼ 3 and 4
asymmetry doublets) were not included in the fit.
This analysis produced a preliminary set of constants. Then additional Ka components were subsequently included in the fit, which required
increasingly higher centrifugal distortion corrections. In the final fit for 7 LiSH, 12 such parameters
were found necessary to obtain a reasonable rms,
which is 62 kHz, as shown in Table 2. These
constants included four sixth-order terms (HJJK ,
HJKK , h2 , h3 ), three eighth-order corrections (LJJK ,
LJK , LKKJ ) and one 10th-order term (PKJ ). Use of
such high order constants is justified, given that
components as high as Ka ¼ 8 were included in the
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
521
Table 2
Rotational constants for LiSH, 6 LiSH and LiSDa
A
B
C
DJ
DJK
d1
d2
HJJK
HJKK
h2
h3
LJJK
LJK
LKKJ
l3
PKJ
rms of fit
a
LiSH
LiSD
6
293283(17)
18959.3604(92)
17687.1945(89)
0.077746(11)
5.83133(80)
)0.006309(16)
)0.002428(30)
0.0002416(25)
0.004626(55)
2:8ð1:2Þ 107
3:2ð2:7Þ 108
1:48ð61Þ 108
2:90ð24Þ 107
1:21ð14Þ 105
151540.6(4.4)
18797.388(29)
16582.180(26)
0.065721(61)
4.9052(16)
)0.011131(78)
)0.006615(25)
0.0002489(94)
0.002522(55)
293407(41)
21565.078(44)
19935.302(42)
0.09804(14)
7.4396(23)
)0.00919(15)
)0.003898(36)
0.000354(16)
0.006358(70)
LiSH
1:18ð22Þ 105
3:2ð1:1Þ 108
0.062
3:4ð2:1Þ 107
2:49ð66Þ 106
5:3ð3:9Þ 107
1:258ð77Þ 105
1:10ð22Þ 107
0.036
0.053
In MHz.
analysis, and given the range of rotational quantum number (J ¼ 1 to J ¼ 13), which encompassed transitions over a broad frequency range
(73–521 GHz). Furthermore, a similar set of constants has been used to successfully fit the millimeter-wave spectrum of SrSH [19], which included
a comparable data set. For LiSD and 6 LiSH, fewer
higher order centrifugal distortion parameters
were required to achieve a rms of 36 and 53 kHz,
respectively (see Table 2). These data sets were
considerably smaller. When the partially blended
features were included in the fit, the values of rms
increased to 122, 167 and 53 kHz for 7 LiSH, LiSD
and 6 LiSH.
Two types of structures for LiSH were determined from the rotational constants in Table 2: r0
and rmð1Þ . While a r0 geometry is clearly inferior
because it does not compensate for zero- point
vibrations, such a calculation was carried out
nevertheless for comparison with other molecules.
Because three moments of inertia were experimentally established for each isotopomer, there are
nine separate moments for which to determine
three geometric parameters (rS–H , rLi–S and hLi–S–H ).
A least squares fit to all nine values yielded
, rS–H ¼ 1:353 A
and hLi–S–H ¼
rLi–S ¼ 2:146 A
93:0°, as shown in Table 3. (Fitting two moments
of inertia in the three separate combinations of
pairs, and then taking the average, gave virtually
identical results for the structure.) In contrast, a
rmð1Þ structure attempts to account for at least some
of the contribution of the vibrational motion to
Table 3
~ 1 A0 )
Structures for LiSH (X
rLiS
)
(A
rSH
)
(A
hLi–S–H
(deg.)
Experimental
r0
rmð1Þ
2.146
2.143
1.353
1.348
93.0
93.2
Theoretical
re
2.145a , 2.149b
1.340a , 1.351b
91.4a , 94.6b
a
b
From [25].
From [24].
522
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
Table 4
Comparison of structures (r0 ) for alkali and alkaline earth hydrosulfides
M–SH
rM–S
)
(A
LiSH
NaSHa
MgSH
CaSH
SrSHa
2.146
2.479
2.316
2.564
2.706
a
b
(1)
(1)
(15)
(6)
(3)
rS–H
)
(A
hM–S–H
(deg.)
References
1.353 (1)
1.354 (1)
1.339b
1.357 (5)
1.358 (4)
93.0 (1)
93.1 (1)
87 (20)
91 (5)
91.04 (3)
This work
[20]
[15]
[17]
[19]
Structure refit; see text.
Held fixed.
the equilibrium geometry by introducing a correction term proportional to the square root of the
1=2
moment of inertia, ðI a Þ [31]
1=2
I0a ¼ Ima þ ca Ima
:
ð1Þ
Here I0a refers to the moments of inertia related to
the r0 structure, Ima the corrected moments, and ca
is a correct factor determined from fitting the data.
A linear least-squares fit to the moments of inertia
for all three isotopomers resulted in a geometry
very similar to the r0 structure, namely rLi–S ¼
, rS–H ¼ 1:348 A
and hLi–S–H ¼ 93:2°. These
2:143 A
values are also given in Table
3. (The
pffiffiffiffiffiffiffiffiffi
ffi correction
and cðcÞ ¼
factorspffiffiffiffiffiffiffiffiffi
wereffi cðaÞ ¼ 0:0321 amu A
).
0:0311 amu A
5. Discussion
This study of the pure rotational spectrum of
LiSH has established that this molecule is indeed
bent, with an angle of 93° – very close to that of
H2 S (92.1°). A bent geometry was also predicted
by theory [11,24,25], but two of the three calculated angles were noticeably larger than the experimental values (97° and 94.6°). On the other
hand, the structures obtained by both Remko and
Rode [24] and Magnusson [25] are still reasonably
close to those determined experimentally, as
shown in Table 3. Because the bond angle in LiSH
is so close to that of H2 S, it is quite likely that the
bonding in the hydrosulfide is predominantly covalent. Pappas [11], in fact predicts the Li–S bond
is covalent, based on Mulliken populations, but
then argues against this conclusion, suggesting
strictly an ionic interaction. Magnusson [25] also
attributes the bend geometry to the effects of
covalency.
If LiSH were strictly an ionic molecule, it
should have a linear structure in analogy to LiOH
and the alkaline earth hydroxides [12,14]. In fact,
this linear geometry is expected if ion-dipole interactions dominate the bonding, a situation where
the molecule can be viewed at as a metal cation
interacting with a negative ligand (OH or SH )
[25]. Clearly, metal hydrosulfides prefer the bent
geometry, probably a result of bonding via p2 orbitals. A fair degree of covalent character must
exist in the Li–S bond and the other metal–sulfur
bonds to produce such structures. This property
distinguishes LiSH from LiOH, which is on average, linear [12].
LiSH could also be exceptionally ÔfloppyÕ and
have a low barrier to opening to a linear form, in
the opposite analogy to LiOH. Unfortunately, vibrational excited states of LiSH have not been
observed in this work. However, some information
about the rigidness of a molecule can be obtained
from the inertial defect, D0 , which consists of a
harmonic and Coriolis terms [32]
D0 ¼ Dharm
þ DCor
0
0 :
ð2Þ
The Coriolis term is thought to be small for simple
molecules such that the major contribution to the
inertial defect is Dharm
. This term arises from vi0
brations that distort the structure of molecule.
Hence, the magnitude of D0 reflects on the degree
of Ôfloppiness.Õ
Inertial defects calculated for LiSH, LiSD2 and
6
, reLiSH are 0.194, 0.257, and 0:193 amu A
spectively. These values are relatively large, compared with other triatomic species [32]. Hence,
A. Janczyk, L.M. Ziurys / Chemical Physics Letters 365 (2002) 514–524
there is likely some vibrational distortion of the
molecule. In comparison, the inertial defects for
MgSH and 2 NaSH are D0 ¼ 0:158ð4Þ and
[20]. The value for sodium hydro0:203 amu A
sulfide is comparable to that of lithium, while the
magnesium number is smaller. Because magnesium and sodium hydrosulfides are heavier molecules, it might be expected that their initial defects
would be larger than their lithium counterpart, as
has been found in studies of metal amide species
[33]. The fact that they are not, is, again, suggestive that LiSH does not have a particularly rigid
structure.
If the major contribution to the inertial defect
arises from the harmonic term, then the effective
frequency of vibration which distorts the molecule,
can be estimated. This approximation assumes
that the vibrational frequencies that contribute to
non-planarity can be replaced by an effective value, xeff . Then, according to Watson [32], the following relationship can be derived
2
xeff 3
h
:
hcD0
ð3Þ
Using this formula, the effective vibrational frequency for LiSH was calculated to be xeff 521 cm1 , while that of LiSD is 390 cm1 .
The geometry of LiSH can additionally be
compared to those of NaSH and the alkaline earth
hydrosulfides. The r0 structures for these compounds are all listed in Table 4. (The structure for
NaSH was analyzed using the rotational constants
from [20]. The SrSH geometry was recalculated
using the data in [19].) There are some obvious
trends in this data set. First of all, the metal–sulfur
bond distance increases fairly uniformly with the
size of the metal atom. Secondly, the sulfur–hydrogen bond lengths, when actually fit, have re.
markably close values near 1:355 0:003 A
Within the quoted errors, they are all essentially
the same. In contrast, the hydrosulfide bond angles
vary in the range 87°–93°. The angle for MgSH is
smallest but has a large error; isotopic substitution
was not carried out for this species, so the comparison is not as appropriate. The angles for SrSH
and CaSH are better defined and are about 91°,
while those of LiSH and NaSH are the same with
the quoted uncertainty at a value of 93°. If these r0
523
structures are to be believed, then it appears that
the alkaline-earth hydrosulfides have a slightly
smaller bond angle than the alkali analogs. This
effect likely arises from the presence of the unpaired electron on the alkaline earth species, which
is thought to reside primarily on the metal atom
[19]. This extra electron does not exist for the alkali hydrosulfides. Repulsion likely occurs between this unpaired electron and the two sets of
lone pairs on the sulfur atom, resulting in a slight
closing of the M–S–H angle relative to that in the
alkali hydrosulfides. It would certainly be of interest to study such species as KSH and BaSH to
see if this apparent difference between the alkali
and alkaline-earth species continues down the periodic table.
6. Conclusion
This millimeter-wave study of the pure rotational spectrum of LiSH has shown that indeed
this molecule is bent in its ground electronic state,
in contrast to its oxygen counterpart, LiOH, which
is linear. The bond angle found in LiSH, from a
rmð1Þ calculation, is 93.2° – quite close to the bond
angle in H2 S. Substitution of a lithium atom in the
place of hydrogen therefore did not significantly
change the basic H2 S structure. Consequently,
LiSH appears to be predominantly a covalently
bonded molecule, in contrast to LiOH. This study
is further evidence that metal-sulfide bonds do
vary in their properties from metal–oxide bonds.
Studying additional species containing such bonds
would be useful in further elucidating metal–sulfur
interactions.
Acknowledgements
This research is supported by NSF Grant CHE98-17707.
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