The vibration-rotation emission spectrum of MgH2

Article abstract of DOI:10.1063/1.1609973

The gaseous MgH₂ molecule was discovered in an electrical discharge inside a furnace. The vibration-rotation emission spectrum of MgH ₂ was recorded with a Fourier transform spectrometer. Three hot bands were found and rotationally analyzed. The MgH₂ molecule was found to have a linear structure.

Full text of DOI:10.1063/1.1609973

The vibration–rotation emission spectrum of MgH 2
A. Shayesteh, D. R. T. Appadoo, I. Gordon, and P. F. Bernath
Citation: The Journal of Chemical Physics 119, 7785 (2003); doi: 10.1063/1.1609973
View online: http://dx.doi.org/10.1063/1.1609973
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 119, NUMBER 15
15 OCTOBER 2003
The vibration–rotation emission spectrum of MgH2
A. Shayesteh and D. R. T. Appadoo
Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
I. Gordon
Department of Physics, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
P. F. Bernatha)
Department of Chemistry and Department of Physics, University of Waterloo, Waterloo, Ontario,
N2L 3G1, Canada
͑
Received 27 June 2003; accepted 25 July 2003͒
The gaseous MgH2 molecule has been discovered in an electrical discharge inside a high
2
4
temperature furnace. The vibration–rotation emission spectrum of MgH was recorded with a
2
Fourier transform spectrometer and the antisymmetric stretching mode (␯ ) was detected near 1589
3
Ϫ1
cm . In addition, three hot bands involving ␯ and ␯ were found and rotationally analyzed. The
2
3
MgH molecule has a linear structure with an R Mg–H bond distance of 1.703 327͑3͒ Å. © 2003
2
0
American Institute of Physics. ͓DOI: 10.1063/1.1609973͔
INTRODUCTION
tion of gaseous MgH . MgH and related materials have
2
2
been proposed for hydrogen storage because of their low cost
and large hydrogen-storage capacity.
The reaction of magnesium vapor with hydrogen has
been studied many times in the last two decades.1–6 It is
known that Mg atoms in the 3s3p excited electronic states
15,16
Recently we reported on our detection of gaseous
17,18
and we now present results on MgH . The only
3
1
5
BeH2 ,
2
(
P and P) can activate the H–H bond, and the MgH
17,18
2
known gaseous metal dihydrides are BeH ,
MgH , and
2
2
molecule is assumed to be a reaction intermediate in the
production of MgH in the following reaction:
19
FeH2 .
Mg͑g͒ϩH ͑g͒→MgH͑g͒ϩH͑g͒.
2
EXPERIMENTAL DETAILS
This reaction has been studied extensively by pump–probe
The high resolution vibration–rotation emission spec-
4
–6
laser experiments
scattering. Breckenridge has recently reviewed the experi-
mental and theoretical status of the reactivity of Mg, Zn, Cd,
as well as by far wing laser
trum of MgH was recorded with a Fourier transform spec-
2
2
,3
trometer. The MgH molecule was generated in an emission
2
source that combines an electrical discharge with a high tem-
perature furnace. The central part of an alumina tube (5 cm
ϫ120 cm) was heated to 650 °C by a CM Rapid Temp fur-
nace. The ends of the tube were cooled by water and sealed
5
and Hg with H–H, Si–H, and C–H bonds. The reaction of
1
ground state S magnesium with H is inhibited by a large
2
barrier to insertion. The ground state reaction,
1
with CaF windows. About 20 g of powdered magnesium
Mg͑ S͒ϩH ͑g͒→MgH ͑g͒,
2
2
2
metal was placed in the center of the tube, and a slow flow of
argon ͑1.6 Torr͒ and hydrogen ͑0.9 Torr͒ was passed through
the cell. A dc discharge ͑3 kV, 333 mA͒ was struck between
two stainless steel tube electrodes inside the cool ends of the
alumina tube. The emitted light from the source was focused
7
8
is calculated to be endoergic by 5 kcal/mol or 3 kcal/mol
depending on the level of theory used. In fact, Ahlrichs
8
et al. state that ‘‘... we conclude that MgH is not bound
2
with respect to MgϩH and hence not likely to be a stable
2
species.’’ We report here on the discovery of free gaseous
MgH2 .
with a CaF lens to the entrance aperture of a Bruker IFS 120
2
HR Fourier transform spectrometer. The spectrum was re-
corded using a CaF2 beamsplitter and a liquid nitrogen-
cooled HgCdTe ͑MCT͒ detector at an instrumental resolution
As MgH is a relatively small molecule, it has been stud-
2
7
–11
ied several times by ab initio methods.
For example,
Tschumper and Schaefer calculated the molecular geometry
and the harmonic vibrational frequencies of MgH2 using
high level ab initio methods with large basis sets.11 In spite
Ϫ1
of 0.01 cm . The spectral region was limited to 1200–2200
Ϫ1
Ϫ1
cm by the CaF beamsplitter and a 2200 cm long-wave
2
pass filter. Approximately 550 scans were added during 8 h
of integration. The recorded spectrum contained atomic and
molecular emission lines, as well as absorption lines from
atmospheric water vapor.
of this interest in MgH , the molecule remains unknown
2
1
2
except for the detection of its infrared spectrum in argon,
krypton, and xenon matrices13 at 10–12 K.
1
4
Solid MgH is well known. Molecular hydrogen is eas-
ily released by heating the solid MgH to 85 °C. This heat-
2
1
4
2
RESULTS AND ANALYSIS
ing results in decomposition to the elements, not the produc-
The strongest emission lines in the spectrum are from
a͒Author to whom all correspondence should be addressed. Electronic mail:
bernath@uwaterloo.ca
the vϭ1→0 fundamental band of MgH, for which the band
Ϫ1
head is at 1578.8 cm . In addition, a series of weaker lines
0021-9606/2003/119(15)/7785/4/$20.00
7785
© 2003 American Institute of Physics
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1

7786
J. Chem. Phys., Vol. 119, No. 15, 15 October 2003
Shayesteh et al.
TABLE I. The line positions of the 001→000, ⌺ –⌺g , band of 24MgH2
ϩ
ϩ
u
Ϫ1
͑
in cm ͒.
Assignment
Line position
Assignment
Line position
R͑29͒
R͑27͒
R͑25͒
R͑23͒
R͑22͒
R͑21͒
R͑20͒
R͑19͒
R͑18͒
R͑17͒
R͑16͒
R͑15͒
R͑14͒
R͑13͒
R͑12͒
R͑11͒
R͑9͒
R͑8͒
R͑7͒
R͑6͒
R͑5͒
R͑4͒
R͑3͒
R͑2͒
R͑1͒
1726.1501͑Ϫ43͒a
1719.3386͑5͒
1712.1626͑4͒
1704.6319͑5͒
1700.7389͑43͒
1696.7513͑4͒
1692.6794͑Ϫ17͒
1688.5263͑4͒
1684.2864͑6͒
1679.9607͑Ϫ11͒
1675.5565͑20͒
1671.0645͑Ϫ2͒
1666.4919͑Ϫ14͒
1661.8409͑1͒
1657.1066͑Ϫ15͒
1652.2959͑Ϫ1͒
1642.4374͑3͒
1637.3909͑Ϫ10͒
1632.2703͑Ϫ6͒
1627.0746͑0͒
1621.8047͑6͒
1616.4613͑12͒
1611.0441͑4͒
P͑1͒
P͑2͒
P͑3͒
P͑4͒
P͑5͒
P͑6͒
P͑7͒
P͑8͒
P͑9͒
P͑10͒
P͑11͒
P͑12͒
P͑14͒
P͑15͒
P͑16͒
P͑17͒
P͑18͒
P͑19͒
P͑20͒
P͑21͒
P͑22͒
P͑23͒
P͑25͒
P͑27͒
P͑31͒
P͑33͒
1582.9065͑0͒
1577.0738͑Ϫ7͒
1571.1767͑2͒
1565.2155͑20͒
1559.1867͑4͒
1553.0933͑Ϫ29͒
1546.9445͑8͒
1540.7292͑Ϫ9͒
1534.4569͑5͒
1528.1244͑11͒
1521.7316͑Ϫ3͒
1515.2838͑5͒
1502.2176͑Ϫ4͒
1495.6029͑Ϫ4͒
1488.9360͑7͒
1482.2138͑Ϫ11͒
1475.4443͑13͒
1468.6206͑Ϫ1͒
1461.7492͑2͒
1454.8288͑2͒
1447.8610͑1͒
1440.8472͑7͒
1426.6819͑0͒
1412.3408͑Ϫ18͒
1383.1706͑6͒
1368.3525͑8͒
FIG. 1. An expanded view of the R branch of the 001→000 band of
24
MgH . The MgH lines are marked with asterisks and display a 3:1 in-
2
2
tensity alternation. The absorption lines are from atmospheric water vapor.
with alternating 3:1 intensities were observed to higher wave
numbers ͑Fig. 1͒, which are due to the antisymmetric stretch-
ing fundamental band of 24MgH2 .
1605.5584͑26͒
1599.9963͑Ϫ8͒
The program WSPECTRA written by Carleer ͑Universite´
Libre de Bruxelles͒ was used to determine the line positions.
The spectrum was calibrated using impurity CO lines, and
a
Observed minus calculated values computed with the constants of Table II
in units of 10 cm ).
Ϫ4
Ϫ1
͑
the absolute accuracy of the measured lines is better than
Ϫ1
0
.001 cm . A color Loomis-Wood program was used as an
aid for the assignment of the observed bands.
The next strongest band had a large l-type doubling, and was
assigned to the first bending mode hot band, 011–010,
The lines of Table I are assigned to the ␯ (␴ ) antisym-
3
u
metric stretching mode of the linear MgH molecule for the
2
⌸g –⌸ transition. In this transition, the absolute rotational
following reasons:
u
assignment was made based on the fact that e and f parity
levels have the same band origins. We used the energy level
expression:
͑
1͒ For the linear MgH molecule with Dϱh symmetry, the
2
adjacent rotational lines have a 3:1 intensity ratio due to
the ortho–para nuclear spin statistical weights (Iϩ1)/I
1
20
associated with the Iϭ 2 hydrogen nuclei, which is
consistent with our observation.
E͑v1 ,v2 ,v3 ,J͒ϭG͑v1 ,v2 ,v3͒ϩF͑J͒,
with
F͑J͒ϭBJ͑Jϩ1͒ϪDJ ͑Jϩ1͒ ϩHJ ͑Jϩ1͒
͑1͒
͑
2͒ There are three measurements of the band origin of the
2
2
3
3
␯3
mode of MgH in argon, krypton, and xenon matrices
2
that predict the band origin at 1571.9, 1558, and 1569
1
2
2
Ϫ1
12,13
Ϯ ͓qJ͑Jϩ1͒ϩq J ͑Jϩ1͒ ͔
2
D
͑2͒
cm , respectively.
cm observed in our spectrum matches these measure-
ments if matrix shifts are taken into account.
The band origin of 1588.672
Ϫ1
in our least-squares fitting program. The constants of Table II
were determined for 24MgH2 using Eq. ͑2͒, in which q
11
͑
3͒ A high level ab initio calculation predicted the Mg–H
ϭq ϭ0 for ⌺ states, and the plus ͑minus͒ sign refers to
D
equilibrium distance (R ) at 1.7108 Å. The observed
e
e(f ) parity for ⌸ states. The observed line positions and the
outputs of the least-squares fits for all the bands have been
placed in Electronic Physics Auxiliary Publication Service
value of R in our experiment is 1.703 327͑3͒ Å, which
0
is in good agreement with the theoretical prediction.
͑
EPAPS͒.21 By fitting the 001–000 and 011–010 bands, we
ϩ
In addition to the ␯3 fundamental band, 001–000 ⌺u
determined two vibration–rotation interaction constants, ␣2
ϩ
Ϫ⌺ , three hot bands of MgH were also assigned and
20
g
2
and ␣ , using
3
rotationally analyzed:
1
1
2
Bv1 ,v2 ,v3ϭB Ϫ␣ ͑v ϩ ͒Ϫ␣ ͑v ϩ1͒Ϫ␣ ͑v ϩ ͒.
ϩ
ϩ
e
1
1
2
2
2
3
3
0
0
11–010 ⌸ Ϫ⌸ , 002–001 ⌺ –⌺ ,
g u
g u
͑
3͒
2
2
2 1͑ f ͒–02 0͑ f ͒ ⌬ –⌬ .
u
g
For the other hot bands, 002–001 and 021–020, fewer lines
were observed and the absolute rotational assignments were
more difficult. The B
The absolute rotational assignment of the 001–000,
ϩ
ϩ
g
values of these vibrational levels
v v v
1 2 3
⌺u –⌺ , fundamental band was made based on the ‘‘miss-
ing line’’ at the band origin and the 3:1 intensity alternation.
were predicted from B000 and ␣ values. Different absolute
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7787
J. Chem. Phys., Vol. 119, No. 15, 15 October 2003
Vibration–rotation emission spectrum of MgH2
TABLE II. Spectroscopic constants of 24MgH in cm ͑all uncertainties are 1␴͒.
Ϫ1
2
5
1010
2
6
Level
G ϪZPE
B
10 D
H
10 q
10 q
D
v
ϩ
ϩ
0
0
0
00 ⌺g
01 ⌺u
0.0
1588.671 57͑24͒
3165.420 02͑52͒
a
aϩ1582.695 60(26)
b
2.882 607͑11͒
2.848 667͑11͒
2.815 078͑13͒
2.891 557͑15͒
2.857 499͑15͒
2.900 037͑21͒
2.865 913͑21͒
3.9178͑25͒
3.8919͑28͒
3.8664͑39͒
4.0865͑47͒
4.0581͑50͒
4.1711͑28͒
4.1515͑31͒
6.49͑17͒
6.53͑21͒
6.57͑35͒
7.20͑45͒
6.84͑52͒
ϩ
g
02 ⌺
a
0
10 ⌸
Ϫ5.0460͑19͒
Ϫ4.9976͑20͒
3.239͑29͒
3.212͑33͒
u
0
11 ⌸g
2 0(f ) ⌬
2 1(f ) ⌬
2
0
0
g
u
2
bϩ1576.706 89(63)
a
The estimated value of a (␯ ) is 437 cmϪ1, and the value of b is approximately 2 times ␯2 ͑see the text͒.
2
rotational assignments were attempted for these two bands,
and in each band only one rotational assignment resulted in
͑ZPE͒, G(0,0,0). We estimated the value of
2
4,25
͕
G(0,1,0)–G(0,0,0)
͖
in Table II (␯ ) using
2
B
values close to the predicted ones. The 001–000 and
v v v
2 3
2
2
1
2
Be
4␻2
2
0
02–001 bands were fitted together, and that also confirmed
our assignment.
We observed a large splitting between e and f levels of
q010ϭϪ
1ϩ
.
͑5͒
ͩ
2
2
ͪ
␻2
␻ Ϫ␻
3
Equation ͑5͒ is for equilibrium constants, which are not
available, so we employed B , ␯ , and ␯ instead of B ,
␻ , and ␻ , respectively. Equation ͑5͒ results in a value of
2
2
the 02 0 (⌬ ) and the 02 1 (⌬ ) vibrational levels, which is
due to the l-type resonance with the 02 0 (⌺ ) and the
g
u
0
2
3
e
0
ϩ
g
2 3
0
ϩ
u
22–25
ϩ
Ϫ1
02 1 (⌺ ) levels, respectively.
Since all the ⌺ rota-
437 cm for ␯ which is close to the matrix isolation value
2
Ϫ1 12
tional levels have e parity, they interact only with the ⌬ state
e levels, and therefore the ⌬ ͑f ͒ energy levels are unaffected.
In our previous work on BeH , we performed a more com-
plete analysis of the l-type resonance by using a Hamiltonian
matrix that included the appropriate interaction matrix
of 439.8 cm
.
The Mg–H bond distance (R ) was calcu-
0
lated from B to be 1.703 327͑3͒ Å, in agreement with the ab
0
11
2
initio value of 1.7108 Å. We were also able to determine
x23 and x33 in Eq. ͑4͒ using the 001–000, 011–010, and
002–001 band origins ͑Table III͒.
1
8
elements. However, the number of observed lines for
MgH was smaller and the data were not sufficient to deter-
Although BeH , MgH , and the dihydrides of group 12
2
2
2
6
2
elements (ZnH , CdH , and HgH ) are linear, the heavier
2 2 2
mine the desired constants. Only the f parity of the ⌬ states,
dihydrides of the group 2 elements (CaH , SrH , and BaH )
2 2 2
may be bent. A very recent ab initio calculation has pre-
2
2
27
02 1 (f )–02 0 (f ) band, was fitted using a simple ⌺ state
energy expression, and the constants are presented in Table
II.
dicted the H–M–H bond angles of CaH , SrH , and BaH
2
2
2
9
to be 180°, 136°, and 123°, respectively. There are other ab
initio calculations predicting different bond angles for these
molecules.2 In fact, Xiao et al. recorded the infrared spec-
8,29
DISCUSSION
trum of CaH in krypton and xenon matrices at 12 K, and
2
The vibrational energy G(v ,v ,v ) in Eq. ͑1͒ can be
written as
they assigned a very weak vibrational band to the symmetric
1
2
3
2
0
30
stretching mode of CaH assuming that it is bent.
2
1
2
1
2
We have performed experiments similar to our BeH and
2
G͑v ,v ,v ͒ϭ␻ ͑v ϩ ͒ϩ␻ ͑v ϩ1͒ϩ␻ ͑v ϩ ͒
1
2
3
1
1
2
2
3
3
MgH work trying to record the vibration–rotation emission
2
1
2
2
1
2
ϩx ͑v ϩ ͒ ϩx ͑v ϩ1͒ ϩx ͑v ϩ ͒
spectra of group 2 and 12 dihydrides in the gas phase to
1
1
1
2
22
2
33
3
2
determine their structures. So far we have found gaseous
1
2
1
2
1
2
ϩx ͑v ϩ ͒͑v ϩ1͒ϩx ͑v ϩ ͒͑v ϩ ͒
31
1
2
1
2
13
1
3
ZnH with a linear structure. We also recorded the infrared
2
1
2
2
emission spectra of CaH, SrH, and BaH with high signal-to-
noise ratios, but we did not see any sign of CaH , SrH , or
ϩx ͑v ϩ1͒͑v ϩ ͒ϩg l .
͑4͒
2
3
2
3
22 2
2
2
The vibrational energies in Table II are presented as the dif-
ference between G(v ,v ,v ) and the zero point energy
BaH2 .
1
2
3
In summary, we observed the gaseous MgH molecule
2
for the first time. The antisymmetric stretching mode (␯3)
and a few hot bands were rotationally analyzed. The pre-
TABLE III. Molecular constants of 24MgH in cmϪ1.
2
dicted linear structure of MgH with D
symmetry was
2
ϱh
R0 ͑Å͒
1.703 326 7͑31͒
confirmed and the R Mg–H bond distance was determined.
0
B0
␣2
␣3
2.882 607͑11͒
Ϫ0.008 950͑18͒
0.033 940͑15͒
Ϫ0.050 460͑19͒
ACKNOWLEDGMENT
This work was supported by the Natural Sciences and
Engineering Research Council ͑NSERC͒ of Canada.
q010
a
␯2
␯3
(␲u)
(␴u)
437
1588.671 57͑24͒
Ϫ5.975 97͑35͒
Ϫ5.961 56͑35͒
1
x23
x33
N. Adams, W. H. Breckenridge, and J. Simons, Chem. Phys. 56, 327
͑1981͒.
2
P. D. Kleiber, A. M. Lyyra, K. M. Sando, S. P. Heneghan, and W. C.
aCalculated from the q010 value and Eq. ͑5͒.
Stwalley, Phys. Rev. Lett. 54, 2003 ͑1985͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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J. Chem. Phys., Vol. 119, No. 15, 15 October 2003
Shayesteh et al.
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4
See EPAPS Document No. E-JCPSA6-119-015339 for the observed line
positions and the outputs of the least-squares fits for all the bands. A direct
link to this document may be found in the online article’s HTML reference
section. The document may also be reached via the EPAPS homepage
͑http://www.aip.org/pubservs/epaps.html͒ or from ftp.aip.org in the direc-
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