Volume 205, number 4,5
CHEMICALPHYSICSLETTERS
16April 1993
The millimeter and sub-millimeter spectrum of the BaOH radical
M.A. Andetson I, M.D. Allen, W.L. Barclay Jr. ’ and L.M. Ziurys ’
Department of Chemistry.Arizona State University.Tempe, AZ 85287-1604. USA
Received 2 October 1992;in final form 2 February 1993
The pure rotational spectrum of the X *Z+ ground electronic states of the BaOH and BaOD radicals has been observed using
millimeter/sub-millimeter direct absorption spectroscopy. The lesser abundant isotopically substituted species, ““BaOH and
13’BaOH,have been detected as well.The radicals were created by reacting barium metal vapor, produced in a Broida-type oven,
with either H2& or D202,The rotational and spin-rotation constants were determined for the molecules froma nonlinearleastsquaresfit to the data, using a 2ZHamiltonian. Hyperline constants were also derived for ‘“‘BaOH,the one species where hypertine structure was resolved. ‘Thesemcasurcments wntirm a linear structure for BaOH.
1. IntrodnctloIt
The spectra of the alkaline-earth metal monohydroxide radicals have been investigated in the past
primarily at optical wavelengths. For example, Harris
and co-workers have observed both the A *II-X ‘E+
and B *2+-X ‘IP systems of CaOH [ 11, as well as
the B *x+-X *C+ transition of SrOH and SrOD [ 2 1.
Bernath and Kinsey-Nielsen [3] have also studied
the BZEf-X *C+ system of CaOH, using Fourier
transform
spectroscopy, while Kinsey-Nielsen,
Brazier and Bemath [4] investigated the B 2Z+X *Z+ transition of both BaOH and BaOD. In general, these optical measurements have shown these
radicals to be linear, except for MgOH, which has
some quasi-linear properties [ 5 1. Their linear structure is evidence for chiefly ionic bonding in these
species.
Recently, Ziurys and co-workershave observed the
pure rotational spectrum of the ground electronic
states of several alkaline-earth hydroxide radicals at
high resolution, including CaOH [6], MgOH [ 71,
and SrOH [ 81. These species were investigated using millimeter/sub-millimeter
direct absorption
spectroscopy. The ground (0, 0,O) vibrational state
of these radicals, as well as several of their low-lying
’ NASASpace Grant Fellow.
z Presidential Young Investigator. Presidential Faculty Fellow.
0009-2614/93/S 06.00 0 1993 Elsevier Science Publishem B.V.
bending and stretching modes, were detected. These
data support a linear structure of CaOH and SrOH.
MgOH, however, may be quasilinear, as suggested
by a shorter O-H bond length [ 71 and by the progression of its excited vibrational modes [ 91. The
quasilinear nature of magnesium hydroxide suggests
its bonding is somewhat more covalent than that of
CaOH or SrOH, although the large hypertine structure of 25MgOH[7] does show that the lone pair
electron in this radical still resides primarily on the
magnesium.Thus, th species must have a structure
dominated by an ioni1 M+OH- configuration.
To further investigatethe structure and bonding of
alkaline-earthhydroxides,we have measuredthe pure
rotational spectrum of BaOH and BaOD in their
electronic and vibrational states, using mm/sub-mm
direct absorption spectroscopy. The lesser abundant
isotopically substituted species *36BaOH and
‘.“BaOH,have been observed as well. Twenty-three
rotational transitions have been measured for
‘38BaOHin the frequency range 77-376 GHz, in addition to ten rotational transitions for 13’BaOHand
twelve rotational transitions for ‘38BaODin the region 234-376 GHz. Three rotational transitions have
additionally been detected for 13’BaOH.Splittings
arising from fine structure interactions were resolved in all of these radicals. Also, in 13’BaOH,hyperfimestructure was observed, which arises because
of the barium 137 spin of 3/2. Here we present our
Allrightsreserved.
415
Volume205, number 4,5
CHEMICALPHYSICSLETTERS
measurements, and derive new spectroscopic constants for these molecules.
2. ExperImental
The rotational transitions of BaOH and its isotop
ically substituted specieswere measured using a millimeter/sub-millimeter direct absorption spectrometer operating in the range 65-400 GHz. The details
of this instrument are described elsewhere [ lo]. This
spectrometer consists of a tunable source of millimeter radiation (65-l 15 GHz), Gunn oscillators
(J.E. Carlstrom Co. ) , which are phase-lockedto a 2
GHz signal generator (Fluka 6082A). Higher frequencies are obtained by using Schottky diode multipliers (Millitech Corp. ) . Radiation from the Gunn
is quasi-optically injected into a gas absorption cell
from a scalar feedhorn and several teflon lenses.The
cell is 0.5 m in length and is a double pass system.
After one pass through the cell, the beam is folded
onto itself by a rooftop reflector, which rotates the
linearly polarized radiation by 90”. The radiation is
then passed back through the cell, and is reflected
into a helium-cooled InSb detector (Cochise Instruments) by a wire grid. Phase sensitive detection is
accomplished by FM modulation of the Gunn oscillators at a rate of 25 kHz and by using a lock-in
amplifier (EG&G PAR5301).
The BaOH radical was produced by heating barium metal in a Broida-type oven, and reacting the
vapor with hydrogen peroxide (75% concentration
in water). The vapor was flowed into the absorption
cell with about 5 mTorr of argon carrier gas, where
it was added to &10 mTorr of Hz02. A pale green
chemiluminescencewas observed when the vapor reacted with hydrogen peroxide, which may actually
arise from BaO [ 111. Strong rotational lines of this
molecule were also detected. The barium 136 and
137 isotopically substituted species of BaOH were
observed in their natural abundance ( 13’Ba:71.66%;
13’Ba:11.32%;13SBa:7.81%). In a similar manner,
the BaOD radical was produced by reacting barium
metal vapor with D20Z.The deuterated peroxide was
made by adding D,O to H202 and successively
pumping off the water.
The linewidths of the spectra measured vary from
about 200 to 700 kHz for the frequency range of 77416
16April 1993
376 GHz. At the higher frequencies, the larger linewidths arise primarily from modulation broadening.
A scan 100 MHz in frequency coveragewas typically
used to initially find the BaOH lines. The actual frequenciesof the transitionswere measured from scans
3 MHz in width, using Gaussian fits to the line
profiles.
3. Results
Table 1 lists the transition frequencies measured
for 13*BaOH,the main barium isotope species. Fine
structure splittings, due to spin-rotation interactions, were observed in all 23 transitions studied.
There was no evidence of hypefine structure in any
of these detected lines, which would arise from the
proton spin of l/2. Table 2 gives the ten rotational
transitions measured for “‘BaOH, and table 3 shows
those observed for BaOD. Again,for both these radicals, only fine structure was resolved in the spectra;
proton or deuterium hypertine interactions were not
apparent in any of these data.
In table 4, the frequencies of the three rotational
transitions measured for 13’BaOHare presented. For
this particular species,both fine and hyperfine structures were readily observed in the spectra. The appearance of hyperfine structure is likely due to the
fact that the 137isotope of barium has a nuclear spin
of 3/2, as opposed to the other Ba nuclei which have
no spin. For a spin of 3/2, eight strong hypefine
components should be present in the BaOH spectrum; all eight were detected in each of the three
transitions observed.
Fig. 1 shows representative spectra of the
N=28-+29 rotational transition “‘BaOH, BaOD,
and 136BaOH,near 376, 340, and 376 GHz, respeo
tively. The doublet structure due to spin-rotation interactions is clearly apparent in these data, and illustrates the fact that the fine structure splitting is
somewhat smaller in BaOD than in BaOH. Fig. 2 is
the spectrum of the N=25-+26 rotational transition
of 13’BaOHnear 337 GHz. In fig. 2, the eight hyperfine components, which result from the 137 barium nuclear spin, are clearly apparent. One line appears as a doublet because it is blended with a BaOH
feature arising from an excited bending mode.
Table 5 gives the spectroscopic constants for bar-
CHEMICAL. PHYSICS LETTERS
Volume 205, number 4,5
16 April 1993
Table 1
Observed transition frequencies of ‘%aOH: X %+ (u=O)
N+N
5-6
J-Q
9/2+11/2
11/2+13/2
6-7
7-8
11/2+13/2
13/2+15/2
1312-r 1512
15/2+17/2
9-+10
10-11
11-12
12+13
13-14
14+15
15-16
16417
F+F’
5+6
4-t5
6-7
5-6
6+7
5-6
7-d
6-7
6-7
7+8
7-d
a+9
vh
v&s- %-AC
(MHz)
(MHz)
77885.432
77956.178
37/2-3912
0.057
90870.430
0.015
19-20
37/2-t3912
1
90941.801
39/2-4112
0.027
1
103854.631
o.ocn
20-21
39/2-41f2
I
I
103925.993
129820.097
-0.008
lo+11
9-10 1
129891.492
-0.008
19/2+21/2
lO+ll
9410 1
142601.118
-0.010
19/2+21/2
11-12
10-111 >
142872.557
21/2-+23/2
11-112
IO+11 I
155780.835
-0.015
21/2+23/2
12+13
ll-bl2 >
155852.264
-0.011
2312-2512
2312-2512
12+13
11+12 1
168759.137
-0.015
25/2-27/2
13-14
12-13 1
168830.594
0.000
13-?14
12-113 1
13+14
14-tlS >
181735.933
0.015
2512-2712
27/2+29/2
181807.374
- 0.005
27/2+29/2
13+14
14-bl5 1
194710.993
-0.035
29/2+31/2
14+15
15-116 >
194782.497
-0.012
15-116
14-15 1
15+16
16-17 1
16-17
15+16 I
207684.390
207755 a64
220655.848
3312-3512
;;:;;
33/2+35/2
17+18
16-17 1
3512-3712
IS-19
233696.794
17+18 >
21-22
0.026
22+23
0.017
43/2-45f2
45/2-+47/2
23-24
45/2+47/2
41/2+49/2
24-25
41/2-+49/2
49/2+51/2
25-26
49/2-u/2
51/2+53/2
26427
51/2+53/2
53/2+55/2
27+28
53/2+55/2
55/2-+57/2
-0.003
0.039
41/2+43/2
43/2+45/2
0.020
1
233625.234
41/2+43/2
-0.007
9-tlO
849 >
31/2+33/2
35/2-37f2
I
17/2-+19/2
29/2+31/2
31/2+33/2
18-419
J-4
>
220727.352
17+18
0.061
N+h”
28~29
55/2-57f2
57/2+59/2
F-F’
17418
18+19
18-19
19-20
IS-19
19-20
19420
20+21
19-20
20+21
20-21
21-22
20-21
21+22
21-22
22-23
21+22
22-23
22-23
23-24
22-23
23-24
23-24
24-25
23424
24-25
24-25
25-26
24-25
25-26
25+26
26-27
25426
26-27
26427
27-28
26-21
27-28
27+28
28-29
27428
28-29
28-29
29-30
V.bE
vob- Vd.
(MHz)
(MHz)
246592.547
-0.003
246664.114
-0.012
259551.599
-0.011
259629.194
-0.019
272520.302
-0.004
272591.937
0.000
285480.518
0.000
285552.165
-0.015
298438.128
-0.002
298509.819
-0.004
311393.024
0.000
311464.740
-0.008
324345.069
-0.009
324416.834
-0.003
337294.177
-0.001
331365.979
0.007
350240.214
0.010
350312.037
0.002
363183.031
-0.007
363254.912
0.005
376122.568
0.006
376194.485
0.014
-0.010
0.000
417
Volume 205, number 4,5
CHEMICAL PHYSICS LETTERS
Table 2
Observedtransition frequencies of ‘36BaOH: X ‘Z+ (v=O)
N-rN’
J-d
F-F’
v,
(MHz)
19+20
3712-3912
18-19
vob- V&
(MHz)
-0.005
0.006
20-21
3912-4112
0.010
-0.009
21+22
22-23
4112-4312
4312-4512
vob.
(MHz)
19-+20
37/2+39/2
vdm- V&
(MHz)
-0.01 I
0.003
-0.014
-0.018
-0.006
0.008
-0.006
0.015
0.029
-0.007
0.002
0.002
-0.002
0.015
-0.001
-0.009
27-28
F-rp’
0.008
-0.018
55/2-+57/2
J+J’
0.012
-0.008
28+29
N-N
“‘BaOD:X *Z+(u=O)
0.009
0.005
26-+27
Table 3
Observed transition frequencies of
0.008
0.003
5312-5512
16 April 1993
-0.003
0.000
-0.013
0.018
-0.014
-0.014
-0.026
- 0.009
0.006
-0.002
0.038
-0.011
-0.004
ium hydroxide and its isotopically substituted species determined from our millimeter-wavemeasurements. The constants were derived from a non-linear
least-squares fit to the data, using a ‘Z Hamiltonian.
For 13gBaOH,136BaOH,and BaOD, hyperfine terms
were not included in the Hamiltonian. For 13’BaOH,
magnetic hypefine, electric quadrupole and nuclear
spin-rotation interactions had to be considered in
418
-0.003
0.002
0.006
Volume 205, number4,5
Table 4
Observed transition frequencies of i’%aOH: X *E+ (v-0)
i&N’
25-26
J-4
49/2-+51/2
51/2+53/2
26-27
51/2+53/2
53/2+55/2
27-28
5312-5512
55/2+57/2
16 April 1993
CHEMICAL PHYSICS LETTERS
F-F’
23+24
26-27
25-26
24-25
24-+25
25-26
26-27
27+28
24425
27-28
26+27
25+26
25426
26+27
27-28
28-29
25+26
28+29
27-28
26-27
26+27
27-+28
28-29
29-30
vh
w-w
VobI-V&
(MHz)
331560.942
337570.607
337582.491
337594.512
337599.096
337611.153
337623.093
337632.855
350517.238
350526.626
350538.288
350551.465
350554.665
350567.870
350579.550
350589.045
363470.233
363479.233
363490.888
363505.206
363506.746
363521.388
363532.855
363542.131
0.057
0.037
-0.041
-0.084
0.013
-0.013
-0.052
0.043
0.124
0.060
- 0.004
- 0.056
0.048
0.023
- 0.025
0.020
0.092
-0.134
0.028
-0.038
-0.201
0.069
0.056
0.093
fitting the measurements. In fact, the data were best
tit using both eqQ and C,, although in this case the
hypertine parameter c was fixed to be zero.
Also listed in table 5 are the rotational and spinrotation constants for BaOH and BaOD estimated
from the optical work of Kinsey-Nielsenet al. [ 41.
As table 5 shows, the millimeter-wave data does refine the numbers derived from the optical measure
ments, especiallyfor the spin-rotation constant. AIso,
there were no past estimates of any spectroscopic
constants for ‘36BaOHor 13’BaOH,even for the hyper-fineparameters; our constants are therefore completely new values.
The errors assigned to the constants shown in table 5 are purely statistical and arise from the goodness of the tit to the data in tables 1-4. The constants
do reproduce the measured frequencies quite well,
with typically u,,,,- vulc5 40 kHz for BaOH, BaOD,
and 136BaOH.The difference between vob,and u&~
is larger however, for the N=5-+6 transition of
BaOH, but small hype&e interactions may be re-
BaOH QZZt) : N=28 +29
J,S,Z
‘38BaOH J=s7_ss
2
i
2
2
I
‘I
6,108
1
I
376,158
376,X
J=5_7*S
J,B,Z
2
2
2 2 13*BaOD
339,953
‘%BaOH
2
II
J=Z,S
2
J=LE
376,706
340,053
3Qm33
2
2
I
I
I
I
376,758
376.808
Frequency (MHz)
Fig. 1. Spectra of the N=28+29 rotational transitions of BaOH,
BaOD, and ‘%aOH near 340-377 GHz. Each spectrum covers
afrequencyrangeof100MHz,withascantimeof~4min.Third
order baselines were subtracted to produce these data. Apparent
in each spectrum is a doublet structure arising from spin-rotation interactions. The lines appear in emission because of the
phase-sensitive detection scheme employed.
sponsible for this effect. For i3’BaOH,v,~- v~, is
5200 kHz. There are also uncertainties due to the
absolute frequency stability of the 2 GHz signalgenerator, which are not incorporated into v,,,- vcalc
Consideringerrors arising from this source, absolute
precision on the frequencies measured here are estimated to be + 100 kHz for all barium hydroxide
419
Volume205, number 4,5
16April1993
CHEMICALPHYSICSLETTERS
Table 5
Molecularconstants for BaOHand BaOD
Constant
‘“BaOH
BD
DO
Y
YD
“‘BaOH
Bo
Do
337,550
337,600
337,650
Frequency (MHz)
:
C
Fig. 2. Spectrum of the N=25+26 rotational transition of
‘%aOH near 337 GHz. In this 100 MHz scan, all eight hypertine components arisingfrom this transition are visible. One line
appears as a doublet due to a BaOH feature arising from an excited bending mode.
species except for the measurements of “‘BaOH,
which have an error of ? 250 kHz.
Table 6 givesthe r. and r, bond lengthsderived for
BaOH from the millimeter-wave data presented in
this work. The r. bond lengths were derived from
three separate isotope ratios: BaOH/BaOD,
‘38BaOH/‘36BaOH,and ‘3*BaOH/‘37BaOH.The
bond lengthsagree wellwith those obtained from optical data, when considering the same set of isotopically substituted species (BaOH and BaOD ) . Those
determined from the other isotope sets are somewhat smaller for the Ba-0 bond and significantly
larger for the O-H bond, in comparison with the optical values. This is expected, since the O-D bond is
usuallyshorter than that of O-H. The r, values given
in table 6 are derived from a partial substitution
structure using 13’BaOH,“‘jBaOH, and 138BaOD.
These numbers are fairly close in magnitude to the
r. values.
4. Discussion
The millimeter-wavedata supports the notion that
BaOH is a linear molecule. Not only does the data
fit well to a *C Hamiltonian, but the apparent lack
of hyperfme structure in all but 13’BaOHis also indicative of a linear configuration. If the molecule is
420
G
eqQ
‘=BaOH
&
Do
Millimeter-wave
(MHz) ‘)
6493.77515(39)
0.00492475(34)
71.325(21)
-0.000231(15)
optical
(MHZ)b’
6493(l)
0.0046(4)
81(45)
6498.926( 15)
0.0049396(99)
72.01(13)
2200.2(5.9)
0.0
C)
-0.101(46)
-394.2(1.2)
V
VLJ
6504.14090(63)
0.00494022(48)
71.415(58)
-0.000243(30)
%aOD
Bo
Do
V
VD
5868.49450(42)
0.00378762(29)
64.660(40)
-O.OCOl89(19)
5868(3)
0.003(2)
‘) Errors quoted are 3a statistical uncertainties and apply to the
last quoted digits.
b, Ref. [4].
c, Fixed value.
linear, the lone pair electron resides chiefly on the
barium in a Ba+OH- ionic-type bonding scheme.
Hypefine interactions should thus be large when
there is a nuclear spin on the barium, and smallwhen
the only spin involved arises from hydrogen. This is
exactly what is experimentally found. For the barium hydroxide specieswhere the nuclear spin is only
on the H or D, no hypertine structure was detected.
In contrast, for 13’BaOH,the only case where the
barium nucleus has a spin, quite substantial hyperfine splitting was observed.
Another point supporting ionic bonding for barium hydroxide is the large value of the hyperfine b
parameter for “‘BaOH, which is 2.2 GHz. The magnitude of this constant is comparable to that of another ionic species, 13’BaF[ 121, which has bx 2.3
GHz. In addition, the quadrupole term for 137BaOH
is fairly large as well, having a value of eqQ= - 394
CHEMICALPHYSICSLETTERS
Volume 205, number 4,s
16Aprill993
Table 6
Bond lengths for BaOH (in A)
Optical c’
Millimeter-wave
ro (BaO)
ro (OH)
r, (BaO) d,
r, (OH) d’
r~ (BaO)
ro (OH)
2.200 ”
2.188 b’
2.197”
0.927 ‘)
1.064b’
0.996 ”
2.196
0.930
2.201
0.923
‘I Determined by ‘3BBaOH/‘3BaOD.
b, Determined by “‘BaOH/‘36BaOH.
‘) Determined by “‘BaOH/“‘BaOH.
‘) Calculatedfrom partial substitution structure using ““BaOH, 136BaOH,
and ‘38BaOD.
‘I Ref. 141.
MHz. The constant eqQ is indicative of the magnitude of the field gradient across the molecule; the
larger the gradient, the more ionic the species. Thus,
the large quadrupole constant for 137 barium hydroxide further supports the Ba+OH- structure. The
b hyperline and quadrupole constants for BaOH are
also far larger than those of *‘MgOH, which exhibits
b=-304.4
MHz and eqQ=-40
MHz [7]. The
bonding in magnesium hydroxide is thought to be
far more covalent than that of BaOH, which is consistent with both the smaller quadrupole and magnetic hyperfine coupling.
For a diatomic molecule, the spin-rotation constant y scales approximately as l/,n, where p is the
reduced mass. If the OH- group is considered as a
unit, this constant might follow a similar pattern for
the alkaline-earth hydroxides, provided bond lengths
do not significantly change. The spin-rotation constant, however, varies in a non-uniform way for these
species. For example, y=72.7 MHz for SrOH and
y= 7 1.3 MHz for BaOH, while for MgOH and CaOH,
this constant equals 37.6 and 34.8 MHz. Such variations are likely because the OH- group probably
functions less as a unit for the lighter hydroxides,
where the metal atom is smaller. On the other hand,
the spin-rotation constant for barium hydroxide does
scale approximately as l/p from that of SrOH.
It is interesting to compare the bond distances derived for BaOH with the other alkaline-earth hydroxides. For the metal-oxygen bond, the lengths
appear to increase with increasing size of the metal
atom. Both the r. and r, values for this bond are 1.8,
2.1 and 2.2 A, respectively, for MgOH, &-OH, and
BaOH [7,8]. The O-H bond, however, appears to
be about the same for barium and strontium hydroxide, with ro- 1.O A, not considering the deuterated isotopes, and r,-0.92-0.93 8, [ 81. In contrast,
the O-H bond in MgOH has r. N 0.94 A, derived from
the ratio of 24MgOH/26MgOH, and r,-0.82 A. Thus,
the O-H bond in magnesium hydroxide appears to
be smaller than for either BaOH or SrOH. This may
arise from the quasi-linear nature of MgOH. Unusually short bond distances are often found in quasilinear species, for example for the C-H bond in
HCNO [ 13,141.
Acknowledgement
This research was supported by NFS grants AST90-58467 (Presidential Young Investigator Award)
and AST-10701, and NASA grant NAGW 2989.
MAA and WLB acknowledge the NASA Space Grant
Program at ASU for their fellowships.
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Volume205, number 4,5
CHEMICALPHYSICSLETTERS
[7] W.L. BarclayJr., M.A. Anderson and L.M. Ziurys, Chem.
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