Vibration-rotation emission spectra of gaseous ZnH2 and ZnD 2

Article abstract of DOI:10.1021/ja046050b

Gaseous ZnH₂ and ZnD₂ have been discovered in an emission source that combines an electrical discharge with a high-temperature furnace. High-resolution infrared emission spectra of ZnH₂ and ZnD₂ have been recorded with a Fourier transform spectrometer, and the antisymmetric stretching fundamental bands of ⁶⁴ZnH₂ and ⁶⁴ZnD₂ were detected near 1889.4 and 1371.6 cm^-1, respectively. Rotational analysis of the bands yielded r₀ bond distances of 1.535271(1) and 1.531836(9) A for linear ⁶⁴ZnH₂ and ⁶⁴ZnD₂, respectively. Copyright

Full text of DOI:10.1021/ja046050b

Published on Web 10/16/2004
Vibration-Rotation Emission Spectra of Gaseous ZnH2 and ZnD2
Alireza Shayesteh, Dominique R. T. Appadoo, Iouli E. Gordon, and Peter F. Bernath*
Department of Chemistry and Department of Physics, UniVersity of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Received July 2, 2004; E-mail: bernath@uwaterloo.ca
Gaseous ZnH2 and ZnD2 have been discovered in an emission
source that combines an electrical discharge with a high-temperature
furnace. High-resolution infrared emission spectra of ZnH2 and
ZnD2 have been recorded with a Fourier transform spectrometer.
The antisymmetric stretching fundamental bands were rotationally
analyzed, and the r0 bond distances for linear 64ZnH2 and 64ZnD2
were determined.
Metal hydrides are among the simplest inorganic molecules. They
are favorite target molecules for ab initio quantum chemists because
of the simplicity of metal-hydrogen bonding. Theoretical calcula-
tions can predict reliable molecular properties for metal hydrides
because they have a relatively small number of electrons. Experi-
mental data on these molecules serve as a test for the accuracy of
ab initio methods.
The reaction of excited zinc atoms with molecular hydrogen in
the gas phase was studied by Breckenridge et al.1 using laser-
pump-probe techniques. Zinc atoms in 3P or 1P excited states can
activate the H-H bond, and it was assumed that gaseous ZnH2 is
a reaction intermediate in the formation of ZnH and H products.2
Solid ZnH2 was synthesized by Barbaras et al.3 from the reaction
of dimethylzinc with a diethyl ether solution of lithium aluminum
hydride. Watkins and Ashby4 found an alternative and convenient
method for the preparation of solid zinc dihydride, using the reaction
of alkali metal hydrides with zinc halides in tetrahydrofuran. The
only spectroscopic studies on the zinc dihydride molecule are the
infrared spectra of ZnH2, ZnHD, and ZnD2 trapped in argon and
krypton matrices5,6 at 10-12 K. ZnH2 has been the subject of a
few ab initio calculations, predicting the equilibrium geometry and
the Zn-H bond energy.7-9 The average Zn-H bond energy was
recently calculated9 to be 51.24 kcal/mol, while the H-H bond
energy is 103.3 kcal/mol.10 Therefore, the overall reaction is nearly
ergoneutral.
Figure 1. The infrared emission spectrum of ZnH2 contained several atomic
lines as well as ZnH and CO lines. The baseline of this spectrum was
corrected using the Bruker OPUS program, and the absorption lines are
from atmospheric water vapor.
Zn(g) + H2(g) f ZnH2(g)
However, there was no observation of gaseous ZnH2 prior to this
work. The only other metal dihydrides known in the gas phase are
FeH2,11 BeH2,12,13 and MgH2.14,15
Figure 2. This expanded view of the ZnH2 spectrum shows a 3:1 intensity
ratio alternation of adjacent rotational lines. The three marked lines
corresponding to each rotational transition are from 64ZnH2, 66ZnH2, and
68ZnH2 (see Table 1).
The discharge furnace source used for the production of ZnH2
is described in detail in our earlier papers on BeH213 and MgH2.14
About 100 g of zinc was placed in an alumina tube and heated to
470 °C using a CM Rapid Temp furnace. Pure hydrogen or
deuterium (0.5-2.5 Torr) flowed slowly through the tube, and a
dc discharge (3 kV, 333 mA) was applied between two stainless
steel electrodes located inside the water-cooled ends of the alumina
tube. The tube was sealed with BaF2 windows, and a BaF2 lens
was used to focus the emitted light onto the entrance aperture of a
Bruker IFS HR 120 Fourier transform spectrometer. The infrared
spectra of ZnH2 and ZnD2 were recorded in the 800-2200 cm-1
region using a KBr beam splitter and a HgCdTe (MCT) detector.
The instrumental resolution was 0.01 cm-1, and several hundred
scans were co-added to improve the signal-to-noise ratio. An
overview of the ZnH2 spectrum, which also contains strong ZnH
and CO lines, is shown in Figure 1. The WSPECTRA program
was used to determine the line positions. The spectra were calibrated
using the impurity CO lines, and the absolute accuracy of the line
positions is better than 0.001 cm-1
.
The lines of Table 1 are assigned to the antisymmetric stretching
fundamental band, 001 (Σ+) f 000 (Σ+), of ZnH2 and ZnD2 for
u
g
the following reasons:
(1) The observed band origins of 1889.4 and 1371.6 cm-1 for
gaseous 64ZnH2 and 64ZnD2, respectively, match the values observed
9
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J. AM. CHEM. SOC. 2004, 126, 14356-14357
10.1021/ja046050b CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
a
Table 1. Line Positions (cm-1) of the Antisymmetric Stretching
Table 2. Spectroscopic Constants of ZnH2 and ZnD2 (cm-1
)
Fundamental Band, 001 (Σ+) f 000 (Σ+), of ZnH2 and ZnD2
a
u
g
constant
64ZnH2
66ZnH2
64ZnD2
b
J
′
J
′′ b
64ZnH2
66ZnH2
64ZnD2
B000
3.548227(6)
4.914(3)
3.506608(7)
4.917(3)
3.548233(11)
4.915(5)
3.506676(11)
4.919(6)
1.783449(21)
1.235(10)
1.768021(23)
1.232(13)
105 × D000
12
11
10
9
8
7
6
5
4
3
2
1
0
1
2
3
4
5
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
1791.1174(2)
1790.2893(0)
1798.2912(0)
1806.2243(2)
1814.0870(2)
1821.8782(1)
1829.5968(-1)
1837.2421(1)
1844.8123(0)
1852.3064(0)
1859.7231(-2)
1867.0621(2)
1874.3212(4)
1881.4988(-2)
1895.6075(-9)
1902.5378(5)
1909.3813(6)
1916.1374(0)
1922.8062(-1)
1929.3865(4)
1935.8758(-1)
1942.2739(-1)
1948.5796(-2)
1954.7918(1)
1960.9085(0)
1966.9295(2)
1322.9638(-1)
1326.8777(0)
1330.7645(3)
1334.6233(1)
1338.4546(2)
1342.2576(2)
1346.0313(-6)
1349.7778(2)
1353.4936(-6)
1357.1814(-2)
1360.8387(-5)
1364.4676(8)
1368.0615(-28)
1375.1678(7)
1378.6723(3)
1382.1457(4)
1385.5863(-7)
1388.9965(-2)
1392.3725(-14)
1395.7189(4)
1399.0305(2)
1402.3095(6)
1405.5540(1)
1408.7654(2)
1411.9402(-23)
B001
1799.1204(-2)
1807.0547(-1)
1814.9187(0)
1822.7112(1)
1830.4308(-1)
1838.0768(0)
1845.6480(3)
1853.1426(0)
1860.5598(-2)
1867.8985(-3)
1875.1581(0)
1882.3362(-1)
1896.4452(-4)
1903.3744(2)
1910.2174(2)
1916.9735(0)
1923.6419(2)
1930.2209(1)
1936.7095(-1)
1943.1068(-1)
1949.4111(-4)
1955.6220(-1)
1961.7379(1)
1967.7571(0)
105 × D001
ν˜3
1889.4326(1)
1.535271(1)
1888.5953(1)
1.535269(2)
1371.6311(3)
1.531836(9)
r0 (Å)
a Numbers in parentheses are one standard deviation uncertainties.
030 level. The largest perturbation was at J′ ) 18 for ZnH2 and at
J′ ) 22 for ZnD2. Therefore, only rotational lines corresponding
to J′ ) 0-12 were used in our analysis for the determination
of r0.
Several hot bands of ZnH2 and ZnD2 involving all three
vibrational modes were also identified in the spectra (i.e., 002 f
001, 003 f 002, 101 f 100, 011 f 010, 021 f 020, and so forth).
Small local perturbations were observed in almost all of the hot
bands, and in all cases, the perturbation effects in ZnD2 were smaller
than those in ZnH2. A complete analysis of all of the bands,
including the perturbation effects, is deferred to a later paper. Using
the rotational constants of the 000, 100, 010, and 001 levels, the
equilibrium rotational constant (Be) and the equilibrium bond
distance (re) will be determined for both ZnH2 and ZnD2.
In summary, gaseous ZnH2 and ZnD2 were discovered, and their
vibration-rotation emission spectra were recorded with a Fourier
transform spectrometer. Rotational analysis of the antisymmetric
stretching fundamental bands confirmed the linear structure and
yielded r0 bond distances of 1.535 271(1) and 1.531 836(9) Å for
64ZnH2 and 64ZnD2, respectively.
6
7
8
9
10
11
12
a Numbers in parentheses are the observed - calculated values (in units
of 1 × 10-4 cm-1) computed with the constants of Table 2. b J′ and J′′ are
the rotational angular momentum quantum numbers of the 001 and 000
levels, respectively.
in the argon (1870 and 1357 cm-1) and krypton (1861 and 1351
cm-1) matrices5,6 if the matrix shifts are taken into account.
(2) The adjacent rotational lines for ZnH2 and ZnD2 have
alternating 3:1 and 1:2 intensity ratios, respectively, because of the
ortho-para nuclear spin statistical weights associated with hydrogen
and deuterium nuclei.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council (NSERC) of Canada.
References
(3) The r0 Zn-H bond distance of 1.535 Å determined in this
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study is close to the calculated re value9 of 1.542 Å.
Lines corresponding to the three naturally abundant isotopes of
zinc (64Zn, 66Zn, and 68Zn) were found in both ZnH2 and ZnD2
spectra, but the signal-to-noise ratio of the ZnD2 spectrum was less
than that of ZnH2. The customary energy expression for a linear
triatomic molecule, E(v1,v2,v3, J) ) G(v1,v2,v3) + BvJ(J + 1) -
Dv[J(J + 1)]2, was used in our least-squares fitting program, and
the spectroscopic constants in Table 2 were determined for the 000
64
ground state and the 001 excited state of 64ZnH2, 66ZnH2, and
-
ZnD2. The r0 bond distances were calculated for these isotopologues
using the ground-state rotational constant (B000). The r0 bond
distance of 64ZnD2 is considerably smaller than that of 64ZnH2
(12) Bernath, P. F.; Shayesteh, A.; Tereszchuk, K.; Colin, R. Science 2002,
297, 1323-1324.
64
because the zero-point level of 64ZnD2 lies lower than that of
ZnH2 in the anharmonic potential-energy surface.
-
(13) Shayesteh, A.; Tereszchuk, K.; Bernath, P. F.; Colin, R. J. Chem. Phys.
2003, 118, 3622-3627.
(14) Shayesteh, A.; Appadoo, D. R. T.; Gordon, I.; Bernath, P. F. J. Chem.
Phys. 2003, 119, 7785-7788.
The wavenumber of the antisymmetric stretching mode is close
to three times the wavenumber of the bending mode (i.e., ν3 ≈
3ν2, for both ZnH2 and ZnD2).6 We observed perturbations in the
001 (Σ+u ) level at high J values, which are caused by the nearby
(15) Shayesteh, A.; Appadoo, D. R. T.; Gordon, I.; Bernath, P. F. Can. J. Chem.
2004, 82, 947-950.
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