Millimeter-wave spectrum of MgSH

Article abstract of DOI:10.1016/S0009-2614(00)01396-8

The pure rotational spectrum of MgSH was observed for the first time, using millimeter-wave absorption spectroscopy in the 280-365 GHz region. This short-lived gas phase free radical was produced in a high-temperature cell by the reaction of Mg metal vapor with H₂S in the presence of an electrical discharge. The spectrum was analyzed using an S-reduced asymmetric rotor Hamiltonian to obtain molecular parameters, including rotational, centrifugal distortion, and spin-rotation constants. The analysis of the spectrum indicated that MgSH is bent, in agreement with an ab initio calculation. The ∠Mg-S-H angle obtained from the analysis was 87.5 ± 6.7°, as compared to a calculated ab initio value of 91.1°.

Full text of DOI:10.1016/S0009-2614(00)01396-8

2 February 2001
Chemical Physics Letters 334 (2001) 195±199
www.elsevier.nl/locate/cplett
Millimeter-wave spectrum of MgSH
Amine Taleb-Bendiab *, Darren Chomiak 1
Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6
Received 11 October 2000; in ®nal form 16 November 2000
Abstract
The pure rotational spectrum of MgSH was observed for the ®rst time, using millimeter-wave absorption spec-
troscopy in the 280±365 GHz region. This short-lived gas phase free radical was produced in a high-temperature cell by
the reaction of Mg metal vapor with H2S in the presence of an electrical discharge. The spectrum was analyzed using an
S-reduced asymmetric rotor Hamiltonian to obtain molecular parameters, including rotational, centrifugal distortion,
and spin-rotation constants. The analysis of the spectrum indicated that MgSH is bent, in agreement with an ab initio
calculation. The \Mg±S±H angle obtained from the analysis was 87.5 Æ 6.7°, as compared to a calculated ab initio
value of 91.1°. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
studies it was shown that, unlike the alkaline-earth
monohydroxides, CaSH is nonlinear, as indeed
predicted by ab initio calculations [20]. The bent
structure is indicative of a signi®cant amount of
covalent bonding in Ca±SH.
To gain more insight into the structure and
bonding of alkaline-earth monohydrosul®des, we
have investigated the pure rotational spectrum of
MgSH using millimeter-wave absorption spec-
troscopy. We have also performed ab initio cal-
culations in order to assist in the analysis of the
spectrum and shed further light on the geometry of
MgSH. In this Letter, we present the measurement
of the millimeter-wave spectrum and its analysis in
terms of spectroscopic constants, including the
spin-rotation parameters. Preliminary structural
results are also given.
Alkaline-earth
monohydroxides
(M±OH,
M ˆ Mg, Ca, Sr, Ba) have been thoroughly in-
vestigated by high-resolution optical [1±9] and
millimeter-wave [10±15] spectroscopic techniques.
The studies clearly demonstrated that these radi-
cals, with the exception of MgOH, are linear, as is
inherent in the ionic character of the M±OH bond.
MgOH was instead shown to be quasi-linear, as
characterized by the presence of a quartic bending
potential [10,11,15]. In contrast to these hydrox-
ides, very little is known about the alkaline-earth
monohydrosul®des, M±SH. The only such radical
that has been the subject of several high-resolution
spectroscopic studies is CaSH [16±19]. From these
* Corresponding author. Present address: North American
Lighting, Inc., 38900 Hills Tech Dr., Farmington Hills, MI
48331, USA. Fax: +1-613-991-2648.
2. Experimental
E-mail address: robert.mckellar@nrc.ca (A. Taleb-Bendiab).
Present address: Coherent Laser Group, 5100 Patrick Henry
Drive, Santa Clara, CA 95054, USA.
1
The rotational spectrum of MgSH was
measured using a millimeter-wave absorption
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 9 6 - 8

196
A. Taleb-Bendiab, D. Chomiak / Chemical Physics Letters 334 (2001) 195±199
spectrometer [21] consisting of a tunable source of
millimeter-wave radiation, an electric discharge
high-temperature absorption cell, and a helium-
cooled InSb detector. The millimeter-wave radia-
tion was obtained by harmonic generation from a
phase-locked Gunn oscillator (80±110 GHz) and
signals were detected using tone burst modulation
[22]. The absorption cell was similar to that used in
the study of CaSH [19].
The MgSH radical was produced by heating
magnesium metal in the high-temperature cell, and
reacting the vapor with H2S in the presence of a
DC electrical discharge. The cell pressure was
about 75 mTorr, with a mixture of magnesium
vapor and 10% H2S in argon carrier gas. The best
absorption signals were obtained at a temperature
of 600°C and a discharge current of 80 mA.
of Ka ˆ 0 and Ka P 2 transitions. The general
pattern was similar to that observed in the case of
CaSH.
A total of 86 a-type transitions with Ka 6 6 were
measured and assigned, and these are listed in
Table 1. Although not listed in Table 1, some
transitions with Ka > 6 were also observed. Tran-
sition frequencies were measured by averaging two
scans taken up in frequency together with two
scans down. The observed linewidths were typi-
cally 1.5±2.0 MHz, and the frequencies are be-
lieved to be accurate to Æ30 kHz. A sample
spectrum is shown in Fig. 1. The transitions listed
in Table 1 showed no evidence of hyper®ne split-
ting or broadening. For the case of MgOH, the
hyper®ne structure was observed in the N ˆ 3 2
transition [10], and it is likely that hyper®ne
structure for MgSH would be observable for ro-
tational transitions with much lower N-values than
those in Table 1.
3. Results and discussion
The transitions in Table 1 were analyzed using
the S-reduced e€ective Hamiltonian of Brown and
Sears [24]. Spectroscopic parameters obtained
from the ®t are given in Table 2. For comparison,
the rotational and quartic centrifugal constants
obtained from the ab initio calculation are also
listed in Table 2. The observed A constant is in
good agreement with that obtained from the cal-
culation. The relatively large uncertainty in the
experimental A constant is due to the absence of b-
type transitions in the ®t, and the fact that MgSH
is an only slightly asymmetric top. The observed B
and C constants are each about 100 MHz larger
than the ab initio values. This di€erence meant
that our observed R-branch transitions were
shifted up in frequency by ꢁ5.5 GHz from the
original predicted positions. The observed dis-
tortion parameters ꢀDN; DNK; d1; d2† are in rea-
sonable agreement with the ab initio values, as
calculated using the Kivelson and Wilson method
[25]. We were unable to extract the parameter DK
from the ®t for the same reasons that resulted in
the poor determination of A, and we therefore
®xed it to the ab initio value of 15.1 MHz, as
shown in Table 2. Two sextic distortion con-
stants, HNK and HKN, were also derived from the
®t. The zero-point inertial defect, derived from
the observed rotational constants, was found to
The spectroscopic studies of CaSH [16,19]
indicated that this radical is bent, with
\Ca±S±H ˆ 91°, and it seemed reasonable to as-
sume a similar geometry for MgSH as a starting
point for spectral searches. However, for a more
precise prediction of the millimeter-wave spectrum
of MgSH we carried out an ab initio geometry
optimization calculation. This was done at the
MP2/6-311+G(3df,2pd) level using the GAUS-
SIAN92 program [23]. The result was indeed a
ꢀ
nonlinear structure, with Mg±S ˆ 2.333 A, S±H ˆ
ꢀ
1.339 A, and \Mg±S±H ˆ 91:1°.
Using the above results, we searched for the
R-branch of the a-type spectrum of MgSH in the
345±361 GHz region. Several transition doublets
with splittings varying between 50 and 60 MHz
were identi®ed. These doublets are very likely to
arise from the spin-rotation interaction, in analogy
with CaSH, for which the average splitting in the
spin-rotation doublets was about 42 MHz [19].
Additional members of the series were found using
the simple ꢀN ‡ 1†=N ratio of frequencies, which is
appropriate in this case because the above ab initio
results indicate that MgSH is a nearly prolate
asymmetric top, with j ˆ 0:9989. We then
identi®ed the Ka ˆ 1 asymmetry doublets which
are more widely split away from the main cluster

A. Taleb-Bendiab, D. Chomiak / Chemical Physics Letters 334 (2001) 195±199
197
Table 1
Observed rotational transitions (in MHz) of MgSH
N0 Ka0 Kc0 ± N00 Ka00 Kc00
J ˆ N 1=2
J ˆ N ‡ 1=2
mobs
mobs
Dma
Dma
0.085
21 0 21 ± 20 0 20
21 1 20 ± 20 1 19
23 1 22 ± 22 1 21
24 0 24 ± 23 0 23
24 1 24 ± 23 1 23
24 1 23 ± 23 1 22
25 0 25 ± 24 0 24
25 1 25 ± 24 1 24
25 1 24 ± 24 1 23
25 2 24 ± 24 2 23
25 2 23 ± 24 2 22
25 3 23 ± 24 3 22
25 3 22 ± 24 3 21
25 4 22 ± 24 4 21
25 4 21 ± 24 4 20
25 5 21 ± 24 5 20
25 5 20 ± 24 5 19
25 6 20 ± 24 6 19
25 6 19 ± 24 6 18
26 0 26 ± 25 0 25
26 1 26 ± 25 1 25
26 1 25 ± 25 1 24
26 2 25 ± 25 2 24
26 2 24 ± 25 2 23
26 3 24 ± 25 3 23
26 3 23 ± 25 3 22
26 4 23 ± 25 4 22
26 4 22 ± 25 4 21
26 5 22 ± 25 5 21
26 5 21 ± 25 5 20
26 6 21 ± 25 6 27
26 6 20 ± 25 6 27
27 0 27 ± 26 0 26
27 1 27 ± 26 1 26
27 1 26 ± 26 1 25
27 2 26 ± 26 2 25
27 2 25 ± 26 2 24
27 3 25 ± 26 3 24
27 3 24 ± 26 3 23
27 4 24 ± 26 4 23
27 4 23 ± 26 4 22
27 5 23 ± 26 5 22
27 5 22 ± 26 5 21
27 6 22 ± 26 6 21
27 6 21 ± 26 6 20
±
281643.993
283491.552
310418.027
321735.158
319812.203
323874.166
±
283431.656
310358.280
321680.825
319761.209
323814.577
335032.727
333045.598
337265.804
335098.555
335292.670
335028.047
335029.679
334841.837
334841.837
334611.911
334611.911
334335.256
334335.256
348378.210
346325.129
350711.914
348460.631
348678.826
348392.034
348394.119
348197.811
348197.811
347958.366
347958.366
347670.762
347670.762
361716.924
359599.535
364152.641
361817.730
362061.867
361751.549
361753.957
361549.085
361549.085
361300.368
361300.368
361001.708
361001.708
)0.115
)0.048
)0.041
)0.041
0.018
0.074
)0.079
0.018
0.046
0.027
)0.024
)0.028
)0.011
0.016
333096.522
337325.369
335154.394
335349.508
±
0.026
0.014
0.032
)0.057
)0.034
)0.015
0.012
)0.146
±
334899.962
334899.962
334671.532
334671.532
334396.710
334396.710
348432.115
346375.936
350771.362
348516.398
348735.877
348448.932
348450.986
348255.709
348255.709
348017.834
348017.834
347731.890
347731.890
361770.792
359650.321
364211.984
361873.351
362118.942
361808.381
361810.818
361606.888
361606.888
361359.449
361359.449
361062.441
361062.441
)0.028
)0.033
)0.003
)0.003
)0.042
)0.042
)0.066
)0.006
)0.026
)0.002
0.002
0.007
0.051
0.051
0.046
0.046
0.052
0.021
0.027
)0.014
0.022
)0.066
0.056
0.054
)0.090
)0.020
)0.012
)0.019
0.050
0.047
)0.062
)0.062
)0.040
)0.040
0.027
0.050
0.009
0.009
)0.027
0.019
0.031
0.074
)0.045
)0.036
0.013
0.035
0.048
)0.042
)0.005
)0.000
)0.009
0.012
)0.040
0.003
0.017
0.008
0.021
0.012
)0.034
)0.034
0.021
0.031
0.031
a Dm ˆ mobs mcalc
.
ꢀ2
ꢀ2
be D0 ˆ 0:158ꢀ4† u A . This value can be mod-
eled fairly accurately using a simple formula
due to Watson [26], which gave in this case D0 ˆ
0:019 u A between the observed and calculated
inertial defects was also obtained for CaSH [19].
The analysis also yielded the principal compo-
nents of the spin-rotation tensor, eaa, ebb, and ecc,
ꢀ2
0:139 u A . Interestingly, a similar discrepancy of

198
A. Taleb-Bendiab, D. Chomiak / Chemical Physics Letters 334 (2001) 195±199
can calculate the g tensor according to Curl's
formula [27]. The result (with ge ˆ 2:00232) for
MgSH is: gaa ˆ 2:00241, gbb ˆ 1:99761 and
gcc ˆ 1:99880. These data will be useful for studies
of the Zeeman e€ect in MgSH.
In the case of CaSH, an o€-diagonal spin-ro-
tation parameter, jeab ‡ ebaj=2 ˆ 3:38 MHz, could
be determined through small perturbations of
transitions involving (J ˆ N ‡ 1=2, Ka ˆ 1,
Kc ˆ N 1) and (J ˆ N 1=2, Ka ˆ 0) energy
levels [19]. In the present case of MgSH, we might
expect a similar e€ect around J ˆ 20:5, as deter-
mined from the approximate expression
Fig. 1. An example showing one spin component of an R(25)
pure rotational transition of MgSH. The spectrum is the aver-
age of 100 scans over a 10 MHz wide region.
J ꢂ ꢀA
B
C†=ꢀB ‡ C† [28]. However, no evi-
dence of such a perturbation was observed here.
We estimate that an upper value of 60 kHz can be
assigned to the parameter jeab ‡ ebaj=2 for MgSH,
based on the precision of our millimeter-wave
measurements.
Table 2
Spectroscopic constants of MgSH
Millimeter-wave
ab initio
A (MHz)
B (MHz)
C (MHz)
289001.8(250)a
6799.698(3)
6629.659(5)
289019.8
6679.4
6528.5
We also searched for b-type transitions, but no
obvious candidates were found. The search range
took into account the uncertainties of the spec-
troscopic constants in Table 2. The absence of b-
type transitions is likely a result of a relatively
small lb dipole moment component. This would
be consistent with the values of la and lb deter-
mined in our ab initio calculation, 2.92 and 0.47 D,
respectively.
DN (kHz)
DNK (kHz)
DK (MHz)
d1 (kHz)
7.618(1)
506.00(11)
15.1b
7.24
472.77
15.1
)0.199(1)
)0.0263(2)
)0.167
)0.018
d2 (kHz)
HNK (kHz)
HKN (kHz)
0.00332(8)
0.0876(9)
The assignment of the MgSH millimeter-wave
spectrum has enabled us to produce an approxi-
mate r0 structure. The least-squares ®t of the three
observed rotational constants yielded the struc-
tural parameters given in Table 3. We have also
repeated the least-squares ®t of two rotational
constants at a time, namely (A; B) and (A; C) pairs.
The ®t of each of these two pairs of constants led
to identical r0 structures within the uncertainty
limits given in Table 3. Due to the absence of
eaa (MHz)
ebb (MHz)
ecc (MHz)
jeab ‡ ꢀbaj=2
(MHz)
)51.2(6)
64.028(43)
46.652(45)
<0.060
ꢀ2
D0 (u A )
0.158(4)
nc
86
47
d
Dmrms (kHz)
a Uncertainties are 1r.
b Fixed to the ab initio value (see text).
c Number of transitions ®tted.
d Dm ˆ mobs mcalc
.
Table 3
Observed and calculated ab initio structures of MgSH
as shown in Table 2. The quantity ꢀebb ‡ ecc†=2,
which correlates with the spin-rotation parameter
c for a linear molecule, is 55.34 MHz for MgSH
compared to 41.93 MHz for CaSH. This is similar
to the trend observed for MgOH (c ˆ 37:56 MHz)
and CaOH (c ˆ 34:76 MHz). Having determined
the three spin-rotation parameters for MgSH we
Observed (r0)
ab initio ꢀre†
2.333
1.339
91.1
ꢀ
Mg±S (A)
2.316(5)
1. 339a
87.5(67)
0.025
ꢀ
S±H (A)
\Mg±S±H (deg)
ꢀ2
b
DIrms (u A )
a Fixed to the ab initio value (see text).
b DI ˆ Iobs Icalc
.

A. Taleb-Bendiab, D. Chomiak / Chemical Physics Letters 334 (2001) 195±199
199
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isotopic data, we were forced to constrain the S±H
bond length to the ab initio value (Table 3). The
observed Mg±S bond length and \Mg±S±H angle
are in fairly good agreement with the ab initio
values. The right angle shape of MgSH is very
similar to that of CaSH [19], and this explains the
fact that the observed A rotational constants for
MgSH and CaSH are of the same order of mag-
nitude. Future investigation of various isotopic
species of MgSH would be useful in obtaining a
more precise r0 structure.
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We wish to thank J.K.G. Watson for useful
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computer program, A.R.W. McKellar for com-
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built the spectrometer used in this study.
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