Infrared spectra of aluminum hydrides in solid hydrogen: Al2H4 and Al2H6

Article abstract of DOI:10.1021/ja0353560

The reaction of laser-ablated Al atoms and normal-H₂ during co-deposition at 3.5 K produces AlH, AlH₂, and AlH₃ based on infrared spectra and the results of isotopic substitution (D₂, H₂ + D₂ mixtures, HD). Four new bands are assigned to Al₂H₄ from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH₃ and seven new absorptions for Al₂H₆ in the infrared spectrum of the solid hydrogen sample. These frequencies include terminal Al-H₂ and bridge Al-H-Al stretching and AlH₂ bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al₂H₆, a molecule isostructural with diborane. Annealing these samples to remove the H₂ matrix decreases the sharp AlH₃ and Al₂H₆ absorptions and forms broad 1720 ± 20 and 720 ± 20 cm^-1 bands, which are due to solid (AlH₃)_n. Complementary experiments with thermal Al atoms and para-H₂ at 2.4 K give similar spectra and most product frequencies within 2 cm^-1. Although many volatile binary boron hydride compounds are known, binary aluminum hydride chemistry is limited to the polymeric (AlH₃)_n solid. Our experimental characterization of the dibridged Al₂H₆ molecule provides an important link between the chemistries of boron and aluminum.

Full text of DOI:10.1021/ja0353560

Published on Web 07/04/2003
Infrared Spectra of Aluminum Hydrides in Solid Hydrogen:
Al2H4 and Al2H6
Xuefeng Wang,§ Lester Andrews,*,§ Simon Tam,|,† Michelle E. DeRose,| and
Mario E. Fajardo|,‡
Contribution from the Department of Chemistry, P.O. Box 400319, UniVersity of Virginia,
CharlottesVille, Virginia 22904-4319, U.S. Air Force Research Laboratory, AFRL/PRSP,
Edwards AFB, California 93524-7680
Received March 28, 2003; E-mail: lsa@virginia.edu
Abstract: The reaction of laser-ablated Al atoms and normal-H2 during co-deposition at 3.5 K produces
AlH, AlH2, and AlH3 based on infrared spectra and the results of isotopic substitution (D2, H2 + D2 mixtures,
HD). Four new bands are assigned to Al2H4 from annealing, photochemistry, and agreement with frequencies
calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH3 and
seven new absorptions for Al2H6 in the infrared spectrum of the solid hydrogen sample. These frequencies
include terminal Al-H2 and bridge Al-H-Al stretching and AlH2 bending modes, which are accurately
predicted by quantum chemical calculations for dibridged Al2H6, a molecule isostructural with diborane.
Annealing these samples to remove the H2 matrix decreases the sharp AlH3 and Al2H6 absorptions and
forms broad 1720 ( 20 and 720 ( 20 cm-1 bands, which are due to solid (AlH3)n. Complementary
experiments with thermal Al atoms and para-H2 at 2.4 K give similar spectra and most product frequencies
within 2 cm-1. Although many volatile binary boron hydride compounds are known, binary aluminum hydride
chemistry is limited to the polymeric (AlH3)n solid. Our experimental characterization of the dibridged Al2H6
molecule provides an important link between the chemistries of boron and aluminum.
Introduction
the binding energy for dialane is calculated to be only slightly
smaller than that for diborane but significantly larger than
Boron hydride chemistry has been investigated for a century,
and a large number of distinct boron hydride compounds have
been identified and characterized;1-4 however, aluminum hy-
dride chemistry under normal conditions is limited to the
nonvolatile polymeric solid trihydride (AlH3)n.4-6 The diborane
molecule is fundamentally important, and the textbook example
of hydrogen (µ-hydrido) bridge bonding.4 Although the isos-
tructural dialane molecule is calculated to be stable by several
groups,7-13 molecular Al2H6 has not been isolated even though
computed for digallane.8,10 The failure to observe dialane is even
more surprising in view of the recent synthesis of digallane,
which yielded an isostructural Ga2H6 molecule.14-16 However,
the properties of solid gallane suggest a discrete oligomer, such
as (GaH3)4, which retains terminal M-H bonds,16 unlike solid
(AlH3)n, which contains only Al-H-Al bridge bonds.5 The only
experimental evidence for Al2H6 is mass spectrometric detection
+
of trace Al2H6 cation17-19 and a broad photodetachment
spectrum of Al2H6 anion.20 Surface science investigations
-
suggest that aluminum hydrides such as AlH, AlH3, and Al2H6
desorb at elevated temperatures from Al metal surfaces contain-
ing (AlH3)n or adsorbed hydrogen.18-22
Since the dimerization of AlH3 is exothermic by about 35
kcal/mol depending on the theoretical methods employed,8,10,11,20
the direct Al2H6 preparation involves the dimerization of AlH3.
Alane (AlH3) has been observed by three groups from reactions
* To whom correspondence should be addressed.
§ University of Virginia.
| U.S. Air Force Research Laboratory.
† Present Address: KLA-Tencor Corp., 1 Technology Drive, Milpitas,
CA 95035.
‡ Present Address: U.S. Air Force Research Laboratory, AFRL/MNME,
Eglin AFB, FL 32542.
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9218
J. AM. CHEM. SOC. 2003, 125, 9218-9228
10.1021/ja0353560 CCC: $25.00 © 2003 American Chemical Society

Infrared Spectra of Aluminum Hydrides in Solid Hydrogen
A R T I C L E S
of energetic aluminum atoms with hydrogen in solid argon and
characterized by infrared spectroscopy;23-26 however, the
concentration of AlH3 was not sufficient to form Al2H6 in the
rigid argon matrix. Our successful synthesis of Al2H6 for the
first time involves pure hydrogen as the matrix.27-29 This ensures
the selective formation of the highest monohydride AlH3, and
diffusion on annealing the soft hydrogen matrix to 6.5 K allows
dimerization to Al2H6. Similar laser ablation experiments with
boron and hydrogen give B2H6.30,31 A preliminary account of
this work has been reported.32
Experimental and Theoretical Methods
The experiments for reaction of laser-ablated aluminum atoms with
hydrogen during condensation in excess argon and neon have been
described previously.23,30,33,34 The Nd:YAG laser fundamental (1064
nm, 10 Hz repetition rate with 10 ns pulse width) was focused (10 cm
f.l. lens) onto a rotating aluminum target (Johnson Matthey, 99.998%).
The laser energy was varied from 10 to 20 mJ/pulse at the sample.
Laser-ablated aluminum atoms were co-deposited with 60 STPcc of
pure normal hydrogen or deuterium (Matheson) or 120 STPcc of Ne/
H2 or Ne/D2 onto a 3.5 K CsI cryogenic window for 25-30 min or for
50-60 min. Mixed isotopic HD (Cambridge Isotopic Laboratories),
and H2 + D2 samples were used in different experiments. FTIR spectra
were recorded at 0.5 cm-1 resolution on Nicolet 750 with 0.1 cm-1
accuracy using an MCTB detector. Matrix samples were annealed at
different temperatures using resistance heat, and selected samples were
subjected to filtered broadband photolysis by a medium-pressure mer-
cury arc lamp (Philips, 175W) with globe removed for 20 min periods.
Complementary thermal Al experiments in para-H2 were performed
at Edwards AFB. Samples were prepared by co-deposition of Al atoms
from a commercial thermal effusive source (EPI SUMO) and a fast
flow of precooled para-H2 gas35 onto a BaF2 substrate cooled to T ≈ 2
K in a liquid helium bath cryostat.36 Operation of the ortho/para con-
verter at T ) 15 K yields ≈100 ppm residual ortho-H2 content. Infrared
absorbance spectra were recorded with a Bruker IFS120HR spectrom-
eter at resolutions of 0.02-0.1 cm-1; sample thickness and dopant
concentrations are calculated from these spectra as described previ-
ously.31,37 Ultraviolet absorption spectra (not shown) were recorded
during some sample depositions, resulting in very weak irradiation and
the consequent appearance of minor photolysis products in the deposited
samples. Samples were deliberately photolyzed in the ultraviolet using
an unfiltered 30 W deuterium lamp located ∼8 cm from the deposition
substrate.
Figure 1. Infrared spectra in the 2000-1100 and 900-600 cm-1 regions
for laser-ablated Al co-deposited with normal hydrogen at 3.5 K: (a)
spectrum of sample deposited for 25 min, (b) after annealing to 6.2 K, (c)
after λ > 380 nm photolysis, (d) after λ > 290 nm photolysis, (e) after λ
> 240 nm photolysis, (f) after annealing to 6.5 K, and (g) after second λ
> 240 nm photolysis.
Results
Laser-ablated Al atoms were co-deposited with pure normal
hydrogen, neon/H2, and argon/H2 samples; thermal Al atoms
were reacted with para-H2; and new infrared absorptions will
be presented along with supporting density functional calcula-
tions of aluminum hydrides.
Hydrogen. Aluminum atoms were co-deposited with normal
hydrogen using three different laser energies and different
sample irradiations: spectra from the lowest laser energy invest-
igation are illustrated in Figure 1. The spectrum of the deposited
sample is dominated by the strong AlH absorption at 1598.7
cm-1, which is intermediate between the gas phase (1624.4
cm-1 45,46 and argon matrix (1590.7 cm-1) absorptions23-25,47
)
for diatomic AlH. A weak 3141.4 cm-1 band (1% of 1598.7
cm-1 absorbance) appears to be the overtone. The ωexe value
deduced for AlH in solid hydrogen, 28.0 cm-1, is slightly
smaller than the gas-phase value, 29.1 cm-1, and the ωe value
for AlH in solid hydrogen, 1654.7 cm-1, is 27.9 cm-1 lower
than the gas-phase value.
Density functional theory (DFT) calculations of aluminum hydride
frequencies are given for comparison with experimental values. The
Gaussian 98 program38 was employed with the 6-311++G** basis set
for hydrogen and aluminum.39 All geometrical parameters were fully
optimized with the B3LYP and BPW91 density functionals,40-44 and
analytical vibrational frequencies were obtained at the optimized
structures.
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113, 9067.
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J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9219

A R T I C L E S
Wang et al.
Figure 2. Infrared spectra in the 1960-1760 cm-1 region for laser-ablated
Al co-deposited with isotopic hydrogen samples at 3.5 K: (a) pure H2 and
Al deposited, (b) after λ > 380 nm photolysis, (c) after λ > 240 nm
photolysis, (d) 35% H2 + 65% D2 and Al deposited, (e) pure HD and Al
deposited, and (f) after annealing to 7.3 K.
Figure 3. Infrared spectra in the 4500-500 cm-1 region for laser-ablated
Al co-deposited with isotopic hydrogen samples at 3.5 K: (a) pure H2 and
Al deposited and subjected to λ > 240 nm photolysis, (b) after annealing
to 6.8 K, (c) after annealing to 7.0 K, (d) pure D2 and Al deposited and
subjected to λ > 240 nm photolysis, (e) after annealing to 14 K, (f) 40%
H2 + 60% D2 and Al deposited, photolyzed λ > 240 nm, and annealed to
13 K.
Absorptions are also observed for AlH3 (1883.7, 777.9, 711.3
cm-1) and AlH2 (1821.9, 1787.8 cm-1), which are approxi-
mately 1, -5, 14 cm-1 and 16, 18 cm-1 higher than argon matrix
values.23-26 New absorptions are observed at 1838.4, 1835.8,
1825.5, 747 cm-1 (labeled Al2H4), at 1638.1 cm-1 (labeled
AlH4-),48 at 1156.1 cm-1 (labeled Al2H2)23 with the hint of very
weak absorptions at 1932.3, 1915.1, 1408.1, 1268.2, 835.6,
702.4, 631.9 cm-1 (labeled DA) (Figure 1a). Annealing to 6.2
-
K increased all bands except AlH and AlH4 (Figure 1b).
Photolysis stepwise at λ > 380, 290, 240 nm destroyed the
Al2H2 absorption, decreased the AlH band, and increased the
AlH3 absorptions 2×, 6×, and 13× and the DA band set 2×,
4×, and 16× (Figure 1c,d,e). A subsequent annealing to 6.5 K
increased all but the 1638 cm-1 band (Figure 1f), and a final λ
> 240 nm irradiation increased the DA bands another 25%
(Figure 1g). These experiments were remarkably free of oxide
impurities: Al2O was typically A < 0.001 at 995.2 cm-1 on
deposition,33 and HO2 was not detected.49
Figure 4. Infrared spectra in the 1440-880 and 620-500 cm-1 regions
for laser-ablated Al co-deposited with normal deuterium at 3.5 K: (a)
spectrum of sample deposited for 25 min, (f) after annealing to 7.3 K, (c)
after λ > 290 nm photolysis, (d) after annealing to 7.5 K, (e) after λ > 240
nm photolysis, (f) after annealing to 8.0 K, and (g) after annealing to 9.0
K.
Similar behavior was found in the other hydrogen experi-
ments. Although irradiation at λ > 630 nm had no discernible
effect, λ > 530 nm photolysis destroyed the 1156.1 cm-1 band,
slightly decreased AlH2 bands, slightly increased Al2H4 and DA
bands, and left the sharp 844.1 cm-1 band unchanged. Irradiation
at λ > 380 nm slightly decreased Al2H4 peaks and markedly
decreased AlH2, whereas λ > 240 nm virtually destroyed Al2H4
with significant increases in AlH3 and DA absorptions (Figure
2). Another spectrum after λ > 240 nm photolysis is shown in
Figure 3a including the solid hydrogen absorptions at 4220 and
4149 cm-1. Annealing to 6.8 K allows H2 to evaporate: solid
H2 absorptions, AlH3 bands, and DA absorptions decrease by
80-90%, and broad absorptions appear at 1720 ( 20 and 720
( 20 cm-1 (Figure 3b). Further annealing to 7.0 K allows all
H2 to evaporate, all sharp absorptions disappear, and the broad
absorptions remain (Figure 3c). These broad bands decrease
steadily on annealing to room temperature where 50% of the
broad absorbance remains.
Al atoms and D2 were co-deposited in three experiments, and
spectra from the sample prepared using higher laser energy are
shown in Figure 4. The AlDx product absorptions observed on
deposition include AlD3, Al2D4, AlD2, AlD, and Al2D2 as listed
in Table 1. The DA peaks appear on λ > 290 nm photolysis,
increase markedly on λ > 240 nm irradiation, and increase
slightly on subsequent 8.0 K annealing (Figure 4c,e,f). One
difference: new bands at 1050.0 and 1043.0 cm-1 with D2 have
no H2 counterparts. On λ > 290 nm photolysis, these bands
give way to the 1183.2 cm-1 counterpart (AlD4-) of the 1638.1
cm-1 absorption in solid hydrogen. Further annealing to 9.0 K
increases Al2D4 bands 2-fold with a small decrease in AlD2
absorptions (Figure 4g). Note that solid D2 can be annealed
about 3 K warmer than solid H2 before the solid evaporates.
Annealing to 9.3 K reveals decreased sharp product absorptions,
annealing to 10.2 K markedly decreases solid D2 bands at 3286,
3168, 2982 cm-1, and the new product bands, and produces a
(48) Pullumbi, P.; Bouteiller, Y.; Manceron, L. J. Chem. Phys. 1994, 101, 3610.
(49) (a) Wang, X.; Andrews, L.; Chertihin, G. V.; Souter, P. F. J. Phys. Chem.
A 2002, 106, 6302. (b) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2003,
125, 6581.
9
9220 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

Infrared Spectra of Aluminum Hydrides in Solid Hydrogen
A R T I C L E S
Table 1. Infrared Absorptions (cm-1) Observed from Codeposition
of Laser-Ablated Al Atoms and with Ne/H2, Pure Normal H2, and
Ar/H2 at 3.5 K
Ne/H
Ne/D
H
2
D
2
Ar/H
2
identification
2
2
4061.6
3141.4
1932.3
1927.2
1915.1
1909.1
1883.7
1851.8
1838.4
1835.8
1825.5
1821.9
1787.8
1751.2
1638.1
1610.7
1598.7
1504.9
2919.6
(H2)AlH3
AlH (2ν)
DA
DA site
DA
DA site
AlH3 (ν3, e)
AlxHy
Al2H4
Al2H4
Al2H4
AlH2 (ν3, b2)
AlH2 (ν1, a1)
1931.4
1923.1
1889.1
1840.8
1415.8
1396.1
1381.3
1345.8
1414.9
1413.4
1402.9
1401.0
1378.7
1922.2
1911.4
1905.7
1897.3
1882.6
1346.1
1822.8
1306.4
1337.2
1293.1
1272.4
1183.2
1169.6
1163.1
1812.0
1806.0
1769.6
1743.7
1608.8
1828.6
1794.7
1340.4
1296.0
-
1627.8
1610.8
1186.9
1171.0
AlH4
AlH site
AlH
Figure 5. Infrared spectra in the 1950-1150 cm-1 range for laser-ablated
Al co-deposited with pure HD at 3.5 K: (a) spectrum of sample deposited
for 25 min, (b) after annealing to 6.8 K, (c) after λ > 290 nm photolysis,
(d) after λ > 240 nm photolysis, (e) after annealing to 7.3 K, and (f) after
λ > 240 nm photolysis.
1590.7
-
AlH4-
1050.0
1043.0
1028.5
919.7
852.0
696
AlD2-
AlD2
1404.7
1260.2
1032.5
925.3
1408.1
1268.2
1156.1
924
844.1
835.6
777.9
770.5
770.5
747
1388.0
1243.4
DA
DA
Al2H2
HAl(H)3Al
HAl(H)3Al
DA
835.0
782.9
770.7
607.3
562.9
607.5
561.8
568.2
560.3
563
783.5
766
766
AlH3-(ν4, e)
AlH4
AlH2
Al2H4
712.2
703.1
630.2
523.2
511.6
711.3
702.4
631.9
522.0
510.6
697.7
AlH3 (ν2, a2′′)
DA
DA
broad 1260 ( 20 cm-1 absorption. Further annealing to 14 K
removes D2 and leaves the broad band with weak CH4, CO,
and CO2 peaks (Figure 3e). At about 50 K, CH4 and CO
evaporate, but the broad 1260 ( 20 cm-1 band and CO2 remain.
This broad band decreases about 50% on warming to room
temperature.
Figure 6. Infrared spectra in the 1950-1550 cm-1 region of a 1.45-mm-
thick para-H2 sample doped with 7 ppm Al atoms from a thermal effusive
source at T ) 950 °C; sample deposition time ) 120 min: (a) deposited
sample at T ) 2.4 K, (b) obtained during annealing at T ) 4.8 K for about
60 min, (c) annealed sample cooled back to T ) 2.5 K, and (d) effects of
irradiation with an unfiltered deuterium lamp for 10 min. Spectral resolution
Three experiments were done with different H2 + D2
mixtures, and both Al-H and Al-D regions reveal strong bands
in the spectrum of a 35% H2 + 65% D2 sample; detail from the
Al-H region is shown in Figure 2d. Slight shifts in peaks are
due to the change from pure H2, but several new absorptions
arise from isotopic mixing. As the matrix host evaporates at
9-10 K, the sharp product absorptions decrease, and broad
bands appear, and on annealing to 13 K the broad bands remain
at 1740 ( 20, 1280 ( 20, and 710 ( 20 on the salt window
(Figure 3f). One experiment was performed with pure HD, and
the 1950-1150 cm-1 region is illustrated in Figure 5. More
detail is shown in Figure 2e,f, where four new, sharp bands
is 0.1 cm-1
.
dependences, possibly due to the reversible formation of ortho-
H2/dopant clusters; however, this hypothesis is still under
investigation. Of significance here is the fact that the main
absorption features in solid para-H2 appear within (2 cm-1 of
the corresponding absorptions in a normal-H2 host.
A similar experiment with 100 ppm Al produced stronger
Al2H2 and Al2H4 bands on deposition (Figure 7a). A longer
ultraviolet photolysis produced strong AlH3 and AlH bands and
weak DA bands at 1933.5, 1918.6, 1406.1, 1264.3 (Figure 7b),
and 835.2 cm-1 (the BaF2 substrate reduces the reliability of
spectra below 800 cm-1). Annealing to 4.8 K for 60 min slightly
increased the product absorptions (Figure 7c). Finally, high-
resolution spectra are shown in the 1950-1750 cm-1 region in
Figure 8. Since this thinner sample was deposited in only 30
min, it received much less ultraviolet exposure. Thus much
weaker bands are observed on sample deposition at 2.4 K
(Figure 8a), but growth of the AlH3, AlH2, and AlH bands is
obvious on photolysis (Figure 8b). Of most interest is the “clean”
were observed near 1920 cm-1
.
Thermal aluminum atoms were co-deposited with para-H2 at
2.4 K using Al concentrations ranging from 7 to 100 ppm.
Spectra from the most dilute sample are shown in Figure 6
where the product bands are weak: AlH is dominant at 1606.0
cm-1. Warming to 4.8 K increased weak 1840.9, 1838.3, 1826.8
cm-1 (Al2H4) and 1155.0 cm-1 (not shown) (Al2H2) bands,
and ultraviolet photolysis for 10 min produced new 1885.5,
1825.0, 1822.0, 1790.2, and 1787.9 cm-1 absorptions (labeled
AlH3 and AlH2), markedly increased AlH (1618.4, 1606.0
cm-1), and destroyed the 1155.0 cm-1 band. All of these
absorption features show complicated reversible temperature
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9221

A R T I C L E S
Wang et al.
Figure 9. Infrared spectra in the 2000-1200 cm-1 range for laser-ablated
Al co-deposited with neon + 10% H2 at 3.5 K: (a) spectrum of sample
deposited for 50 min, (b) after annealing to 7.0 K, (c) after λ > 240 nm
photolysis, and (d) after annealing to 7.6 K.
Figure 7. Infrared spectra in the 1950-1100 cm-1 region of a 1.48-mm-
thick para-H2 sample doped with 100 ppm Al atoms from a thermal effusive
source at T ) 1075 °C; sample deposition time ) 120 min: (a) deposited
sample at T ) 2.4 K, (b) obtained at T ) 2.4 K after irradiation with an
unfiltered deuterium lamp for 30 min, and (c) sample cooled back to T )
2.4 K after annealing at T ) 4.8 K for 60 min. Spectral resolution is 0.05
Argon. Argon matrix experiments were done with 10% H2
to compare with the present neon and earlier argon/dilute H2
work.23 Product absorptions in the deposited spectrum (Table
1) are in agreement with previous assignments to AlH, AlH2,
AlH3, and AlH4- in solid argon.23-26,47,48 Neither HO2 nor AlO2
absorptions were detected, but a weak Al2O band was observed
cm-1
.
at 992.4 cm-1 32,49
Satellite absorptions at 1811.8 and 1777.3
.
cm-1 are appropriate for the (H2)AlH2 complex.23,25 Annealing
to 10-12 K had little effect on the spectrum. Irradiation at λ >
290 nm decreased AlH2 and AlH bands but increased AlH3 and
-
AlH4 absorptions. Subsequent annealing to 16 K increased
absorptions at 1822.8 and 1812.0 cm-1 and produced weak new
bands at 1922.2, 1388.0, and 1243.4 cm-1. Subsequent λ > 240
nm irradiation and 23 K annealing continued these trends.
Calculations. Density functional calculations were performed
at the B3LYP/6-311++G** level to provide a consistent set
of frequencies for aluminum hydrides and their partially and
fully deuterated counterparts to assist in the identification of
the new Al2H4 and Al2H6 molecules observed here. We
recomputed Al2H2 for comparison.23,50 Our results are presented
in Tables 2 and 3. The frequencies and bond lengths calculated
for AlH2, AlH3, and AlH4- are in line with previous work.23-26,48
Figure 8. Infrared spectra in the 1950-1750 cm-1 region of a 0.40-mm-
thick para-H2 sample doped with 20 ppm Al atoms from a thermal effusive
source at T ) 1000 °C; sample deposition time ) 30 min: (a) deposited
sample at T ) 2.4 K,(b) obtained at T ) 2.4 K after irradiation with an
unfiltered deuterium lamp for 30 min, (c) obtained during annealing to T
) 4.6 K for 60 min, and (d) sample cooled back to T ) 2.4 K. Spectral
We also find AlH2- to be a stable anion with lower frequencies
-
resolution is 0.02 cm-1
.
than AlH4
.
Our B3LYP calculations reach the same conclusions about
Al2H4 isomers as the previous investigation.51 It is necessary
to have computed infrared intensities in order to identify the
Al2H4 isomer formed here (Table 2).
spectrum recorded during annealing at 4.6 K (Figure 8c), which
reveals a sharp dominant 1885.5 cm-1 AlH3 absorption, sharp
Al2H4 bands at 1840.9, 1838.3, and 1826.8 cm-1, sharp AlH2
bands at 1822.0, 1787.9 cm-1, and DA bands at 1933.6, 1918.6
The important Al2H6 molecule has attracted considerable
theoretical attention, and these calculations reveal a stable molec-
ule of D2h symmetry and frequencies in accord with this struc-
ture.7-13 Table 3 compares the infrared active modes computed
at SCF, CCSD, B3LYP, and BPW91 levels of theory and shows
that the calculated frequencies decrease in this order. Figure 10
illustrates the B3LYP structures for Al2H2, Al2H4, and Al2H6,
which are in agreement with previous reports.7-13,23,50,51 These
structures are useful to picture the vibrational modes. Finally,
cm-1. A sharp Al2H4 band is also observed at 747.8 cm-1
.
Neon. Neon matrix investigations were performed with high
hydrogen concentrations to favor higher hydrides. Figure 9
illustrates spectra from a 10% H2 in neon experiment. The strong
absorptions in the first spectrum at 1889.1 and 1828.6 cm-1
(Figure 9a) are immediately recognized as AlH3 and AlH2 from
their blue shifts relative to hydrogen and argon matrix absorp-
tions for these molecules. Weak absorptions at 1931.4, 1923.1,
1404.7, and 1260.2 cm-1 (labeled DA) in the first spectrum
increase on photolysis and annealing (Figure 9b-d). The
deuterium counterparts from a similar 10% D2 experiment are
listed in Table 1.
(50) Stephens, J. C.; Bolton, E. E.; Schaefer, H. F., III; Andrews, L. J. Chem.
Phys. 1997, 107, 119.
(51) Lammertsma, K.; Gu¨ner, O. F.; Drewes, R. M.; Reed, A. E.; Schleyer,
P. v. R. Inorg. Chem. 1989, 28, 313.
9
9222 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

Infrared Spectra of Aluminum Hydrides in Solid Hydrogen
A R T I C L E S
Table 2. Calculated (B3LYP/6-311++G**) Structures and Frequencies for Aluminum Hydride and AlH2 Molecules
molecule
length (Å); angle (deg)
frequencies, cm-1 (intensities, km/mol)
AlH (1Σ+)
1.666
1.603; 118.1
1642.2 (732)
AlH2 (2A1)
(C2v)
1861.9 (b2, 346), 1817.0 (a1, 91), 768.9 (a1, 278)
2133.5 (σu, 0.4), 2034.9 (σg, 0), 567.5 (πu, 130 × 2)
1474.4 (a1, 1473), 1466.0 (b2, 1466.0), 808.7 (a1, 383)
1948.7 (e, 272 × 2), 1939.1 (a1, 0), 799.4 (e, 233 × 2), 713.1 (a2′′, 380)
1711 (a1, 193), 1698 (e, 677 × 2), 761 (e, 222 × 2), 581 (a1, 319)
1735.4 (a1, 0), 1648.5 (t2, 713 × 3), 783.2 (t2, 535 × 3), 763.0 (e, 0 × 2)
254 (0)
AlH2+ (1Σg
)
1.551
+
a
(D∞h
)
AlH2- (1A1)b
(C2V)
1.699; 94.6
1.584
AlH3 (1A1′)
(D3h)
AlH3- (2A1)c
(C3V)
1.632; 112.7
1.644
AlH4- (1A1)d
(Td)
Al2 (3Πu)
2.762
(D∞h
)
Al2H2 (1Ag)
1.831; 2.976
1359 (ag, 0), 1187 (b1u, 2064), 1070 (b3g, 0), 966 (b2u, 118), 315 (ag, 0), 234 (b3u, 157)
(D2h)
HAl(H)3Al (1A1)e
(C3V)
1.578, 1.676, 1.976;
128.0, 86.0, 70.7
1.585, 1.715, 1.864;
125.5, 81.7, 74.0
1953 (a1, 340), 1694 (a1, 75), 1574 (e, 43 × 2), 936 (a1, 851), 895 (e, 46 × 2),
821 (e, 323 × 2), 411 (a1, 72), 384 (e, 0 × 2)
H2Al(H)2Al (1A1)
(C2V)
1934 (b1, 24), 1921 (a1, 127), 1579 (a1, 461), 1357 (b2, 3), 1198 (a1, 1065),
1085 (b2, 395), 820 (b1, 132), 730 (a1, 341), 672 (a2, 0), 513 (b2, 29), 332 (a1, 40),
127 (b1, 19)
H2AlAlH2 (1A1)
(D2d)
1.594, 2.592; 115.9
1901 (a1, 0), 1899 (e, 272 × 2), 1880 (b2, 409), 819 (a1, 0), 758 (b2, 719),
549 (e, 75 × 2), 340 (a1, 0), 306 (e, 73 × 2), 149 (b1, 0)
+
-
-
a AlH2 is 166 kcal/mol higher energy than AlH2. b AlH2 is 25.3 kcal/mol lower energy than AlH2. c AlH3 is 10.1 kcal/mol lower energy than AlH3.
-
e
d AlH4 is 69.9 kcal/mol lower energy than AlH2 + H2. C3V structure is global minimum: C2V structure is 1.6 kcal/mol higher, and D2d structure is 9.4
kcal/mol higher.
Table 3. Comparison of Calculated and Observed Infrared Active Frequencies (cm-1) for Dibridged Al2H6
b
c
d
f
sym
modea
SCF/TZP
CCSD/DZP
B3LYP
BPW91d
observede
H/D
b1u
8
9
2062 (518)
977 (393)
249 (18)
1350 (544)
694 (353)
2051 (101)
1603 (1399)
766 (890)
2047 (344)
954 (317)
235 (15)
1368 (463)
664 (328)
2024 (88)
1589 (1162)
744 (684)
1989 (419)
866 (244)
223 (13)
1292 (352)
634 (263)
1966 (126)
1483 (1096)
712 (648)
1934 (379)
808 (199)
205 (10)
1275 (291)
607 (230)
1908 (130)
1431 (968)
683 (575)
1932 (0.069)
836 (0.037)
1.366
1.376
10
13
14
16
17
18
b2u
b3u
1268 (0.053)
632 (0.039)
1915 (0.023)
1408 (0.128)
702 (0.048)
1.379
1.364
1.370
1.376
a Symmetry (D2h point group) and mode description from ref 7. b Reference 7; calculated intensities (km/mol) in parentheses. c Reference 11. d This
work, 6-311++G** basis set. e In solid hydrogen, integrated intensities at maximum yield in parentheses. f Hydrogen/deuterium frequency ratio.
we compute Al2H6 in D3h and D3d symmetry to be two unbound
AlH3 subunits.
Discussion
The new absorptions observed here will be assigned to the
Al2H4 and Al2H6 molecules on the basis of their reactions on
annealing, photochemistry, behavior on deuterium substitution,
and comparison to vibrational frequencies predicted by density
functional calculations. This work provides the first experimental
evidence for these two neutral dialane molecules.
Al2H4. The 1840-1780 cm-1 region of our normal hydrogen
matrix spectra contains five sharp peaks. The strongest band at
1821.9 cm-1 is associated with the 1787.8 cm-1 band on
annealing and photolysis throughout these experiments: these
major product bands are 15.9 and 18.2 cm-1 above argon matrix
AlH2 bands23,25 and 6.7 and 6.9 cm-1 below new neon matrix
AlH2 bands observed here. This is the relationship found for
neon, hydrogen, and argon matrix absorptions of the same
species.49 The much larger argon-to-hydrogen matrix shifts for
AlH2 compared to AlH3 (1 cm-1) suggests that AlH2 in solid
Figure 10. Structures calculated (B3LYP/6-311++G**) for the dialumi-
hydrogen may in fact be a weak (H2)AlH2 complex;23,25
num hydrides Al2H2, Al2H4, and Al2H6.
however, the 1821.9 and 1787.8 cm-1 bands are sharp, and we
find no evidence of an associated H-H stretching mode. In
sharp 770.5 cm-1 band appears to be due to the bending mode
of AlH2 observed at 766.4 cm-1 in solid argon.25
para-H2, these bands are at 1822.0 and 1787.9 cm-1. A weaker,
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9223

A R T I C L E S
Wang et al.
The sharp 1838.4, 1835.8, and 1825.5 cm-1 bands behave as
a group on annealing and photolysis. The λ > 530 and 470 nm
photolysis sequence decreased AlH2 absorptions and slightly
increased the above group whereas λ > 290 nm reversed this
trend and λ > 240 nm almost destroyed both band sets. The λ
> 380 photolysis slightly decreased the above group but nearly
destroyed the AlH2 absorptions (Figure 2b). The unique nature
of these sharp bands is demonstrated by their spontaneous
formation on deposition in para-H2 at 1840.9, 1838.3, and 1826.8
cm-1 (Figure 7).
In solid deuterium, the corresponding 1350-1290 cm-1
region contains four sharp peaks (Figure 4). The 1337.2 and
1293.1 cm-1 bands are stronger initially, and they increase on
λ > 290 photolysis, which almost destroys the 1346.1, 1306.4
cm-1 set. The latter pair dominate after 9.0 K annealing (Figure
4g). The 1337.2 and 1293.1 cm-1 bands are 12.9 and 14.8 cm-1
above AlD2 absorptions in solid argon,23,25 but 3.2 and 2.9 cm-1
below new neon matrix AlD2 bands observed here. Thus, the
1337.2 and 1293.1 cm-1 bands are due to AlD2 in solid
deuterium.
metric Al-H2 stretching mode and the latter is an out-of-phase
combination of symmetric Al-H2 stretching modes on the two
AlH2 subunits. Furthermore these modes are predicted 37 and
63 cm-1 above the stretching modes of AlH2, and the 1838.4,
1835.8 and 1825.5 cm-1 bands are 15 and 38 cm-1 higher than
the AlH2 absorptions in solid hydrogen. We also find a
correspondence in the H/D isotopic frequency ratios: for AlH2,
the H/D frequency ratios are 1.3624 and 1.3826 for the
antisymmetric and symmetric modes, respectively, and for the
above group 1837.1 (average)/1346.1 ) 1.3648 and 1825.5/
1306.4 ) 1.3974. Scale factors (observed/calculated) using our
hydrogen matrix and B3LYP frequencies for AlH2 are 0.979
(b2) and 0.984 (a1). Similar scale factors are found for the above
group: the split 1838.4, 1835.8 cm-1 band is assigned to the e
mode and the 1825.5 cm-1 band to the b2 mode of Al2H4, and
0.967 (e) and 0.971 (b2) scale factors result. All of the above
evidence substantiates the present infrared identification of
Al2H4. The stronger e mode of Al2D4 is not split by the D2
matrix. Our B3LYP calculations predict a strong AlH2 bending
mode (b2, out-of-phase scissors) for Al2H4 at 758 cm-1. A broad
747 cm-1 band tracks with the above band group on annealing
and photolysis in normal hydrogen and is assigned to the b2
bending mode of Al2H4. In para-H2 this band is sharp at 747.8
cm-1. Finally, the unique observation of these bands on
deposition of thermal Al atoms with para-H2 at 2.4 K points to
a spontaneous reaction of Al2 (see the Reaction Mechanisms
section).
The mixed isotopic experiments provide further support for
the identification of Al2H4. Our B3LYP calculation for AlH2-
AlD2 predicts two strong absorptions coincident with e modes
of the pure isotopic species plus strong bands just 9 and 25
cm-1 lower in the Al-H and Al-D stretching regions,
respectively. The new bands at 1811.5, 1808.7 cm-1 and 1326.3,
1325.1 cm-1, which are 22 and 20 cm-1 lower than the
corresponding bands for the pure isotopic species, are assigned
to AlH2AlD2. Our calculation for AlHDAlHD predicts four new
observable bands in these regions, and new bands at 1844.5,
1835.2, 1341.5, and 1331.0 cm-1 in pure HD are appropriate
for the AlHDAlHD isotopic molecule.
The H2 + D2 experiments give several new absorptions
(Figure 2d). In the Al-H region, the 1822.7, 1789.4 cm-1 peaks
arise from AlH2 where 0.8 and 1.6 cm-1 differences are due to
the change from pure H2. The new 1807.2 cm-1 band is
appropriate for AlHD observed 18.4 cm-1 lower at 1788.8 cm-1
in solid argon.23,25 New 1811.5, 1808.7 cm-1 bands appear to
track with the 1834.7, 1832.2, 1822.9 cm-1 set, which exhibits
different relative intensities from the analogous pure H2 product
group. In the Al-D region, new 1337.3 and 1293.3 cm-1 peaks
are due to AlD2, and a new 1317.6 band is appropriate for AlHD,
which is observed 18.3 cm-1 lower at 1299.3 cm-1 in solid
argon.25 A new 1326.3, 1325.1 cm-1 band appears to track with
the 1346.0, 1306.8 cm-1 pair.
The pure HD experiment (Figures 2e and 5) behaved
similarly. New bands at 1806.8 and 1315.5 cm-1 due to AlHD
in pure HD are 0.4 and 2.1 cm-1 different from AlHD in the
H2 + D2 sample. In addition, new absorptions are observed at
1844.5, 1840.1, 1835.2, 1831.5 cm-1 and at 1341.5, 1331.0 cm-1.
Lammertsma et al.51 performed a detailed series of calcula-
tions on Al2H4 structures, and concluded that ionic C3V and C2V
structures are slightly lower in energy than the D2d H2AlAlH2
form. Our B3LYP calculations agree with this conclusion and
provide frequencies and infrared intensities that support iden-
tification of the D2d form through the 1838.4, 1835.8, 1825.5
cm-1 band group in solid hydrogen. The lower energy ionic
C2V structure is predicted to exhibit a very strong a1 bridge
stretching mode at 1198 cm-1 (Table 2). No absorptions appear
in this region except for the very strong bridge stretching mode23
of Al2H2 at 1156.1 cm-1. The lowest energy HAl(H)3Al
structure is predicted to have its strongest IR absorption at 936
cm-1, which is antisymmetric stretching of the Al(µ-H)3Al
bonds along the Al-Al axis. A weak, photosensitive 924 cm-1
band is likely due to this species: the deuterium counterpart is
696 cm-1. The next strongest band is the degenerate H-Al
bending mode, computed at 821 cm-1, and the sharp 844.1 cm-1
band exhibits the same annealing and photolysis behavior, and
is likewise assigned to the ionic HAl(H)3Al isomer of Al2H4.
Although there is extensive synthetic chemistry for substituted
dialane (4) compounds,52 this is, we believe, the first experi-
mental evidence for dialane (4) itself, Al2H4.
Al2H6. The seven band DA set has straightforward chemistry.
The strongest three bands at 1932.3, 1408.1, and 1268.2 cm-1
are detected on laser-ablated Al deposition with normal hydro-
gen (absorbance 0.001). These bands double on annealing to
6.0 K and double again on λ > 290 nm photolysis while AlH
is reduced 90%, AlH3 increases 4-fold, and weaker DA bands
join at 1915.1, 835.6, 702.4, and 631.9 cm-1. Subsequent λ >
240 nm photolysis increases the DA bands 4-fold in concert
and the AlH3 bands 2-fold, while the next 6.5 K annealing
increases DA bands by 25% at the expense of AlH3, and a final
λ > 240 nm photolysis increases DA bands by 50% and AlH3
by 5% (Figure 1). Thus, AlH3 is produced on photoexcitation
of AlH with excess hydrogen,23-25 and DA is formed on
ultraviolet photolysis along with AlH3 and on annealing from
diffusion and reaction of AlH3. The seven DA bands then must
be considered for assignment to dialane, Al2H6. These bands
are produced in much greater yield in the softer hydrogen and
Two strong Al-H stretching modes are predicted at 1899
cm-1 (e, 544 km/mol) and 1880 cm-1 (b2, 409 km/mol) for
D2d symmetry Al2H4 (Table 2) where the former is an antisym-
(52) Uhl, W. Coord. Chem. ReV. 1997, 163, 1.
9
9224 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

Infrared Spectra of Aluminum Hydrides in Solid Hydrogen
A R T I C L E S
neon matrixes, which allow more diffusion, than the more rigid
argon matrix. Finally, the highest frequency five of these bands
are observed in para-H2 at 1933.5, 1918.6, 1406.1, 1264.3, and
835.2 cm-1 after ultraviolet photolysis.
for Al2H4, and 57, 51 cm-1 for Al2H6. Part of this discrepancy
is due to anharmonicity. The general agreement in these
calculated-observed differences underscores the predictive power
of quantum chemical calculations and further supports our
preparation and identification of Al2H6.
Similar experiments were done with pure normal-D2 and with
H2+D2 mixtures. The perdeuterio counterparts for AlD2, AlD3,
and all but the weakest DA band are listed in Table 1. The H/D
frequency ratios ranging from 1.379 to 1.364 (Table 3) are
characteristic of Al-H/Al-D vibrational modes. New informa-
tion obtained from H2 + D2 mixtures includes new absorptions
at 1925.5, 1411.7 cm-1 and at 1036.6 cm-1 in addition to
stronger AlH2D, AlHD2, and AlHD absorptions.23-26 The new
1925.5 cm-1 band between pure H2 product bands at 1932.3
and 1915.1 cm-1 (Figure 2d) and the new 1411.7 cm-1 band
between pure D2 product bands at 1414.9 and 1402.9 cm-1
indicate that the pure matrix bands are antisymmetric and
symmetric AlH2(AlD2) stretching modes, and their position
above AlH2 points to a terminal AlH2 subunit. Thus, the
intermediate bands are uncoupled Al-H and Al-D stretching
modes in a terminal AlHD group. In fact the H/D frequency
ratios for the antisymmetric Al-H stretching modes of AlH2
(b2), AlH3 (e), and Al2H6 (b1u) exhibit almost the same H/D
ratios (1.362-1.366). The 1036.6 cm-1 absorption is slightly
higher than 1028.5 cm-1 pure deuterium counterpart, and this
is consistent with an Al-(D)2-Al bridging subunit coupled to
Al-H motions. The pure HD experiment likewise produced
AlH2D, AlHD2 and AlHD plus sharp new absorptions at 1926.5,
1922.6, 1919.4, 1915.7 cm-1 (Figure 2f) and at 1416.8, 1415.0,
1412.9, 1406.4 cm-1. These bands are clearly due to terminal
Al-H and Al-D vibrations in mixed isotopic Al2HxDy (x + y
) 6) molecules. Unfortunately, the strongest Al(HD)Al bridge
mode is predicted to fall under the strong AlHD2 absorption.
Table 2 compares previous SCF and CCSD calculated
frequencies7,11 with the present B3LYP and BPW91 harmonic
predictions for Al2H6, which are expected to approach the
experimental anharmonic values.53 Notice that the BPW91-
computed Al-H stretching frequencies are closer to the
observed values, but the bending frequencies are below the
observed values. Liang et al.7 also performed similar SCF
calculations for B2H6 and Ga2H6 and showed that the calculated
frequencies for the seven strongest infrared active modes,
designated 8, 9, 13, 14, 16, 17, 18 in Table 2, are respectively
6.9, 16.4, 5.5, 9.1, 6.8, 11.6, and 8.7% higher than observed
values for B2H6 and 3.9, 14.7, 4.1, 6.5, 5.0, 10.5, and 8.6%
higher than observed for Ga2H6. Their calculated frequencies
for Al2H6 are likewise 6.7, 19.3, 6.5, 9.8, 7.1, 13.8, and 9.1%
higher than our hydrogen matrix DA absorptions. The correla-
tion in these % deviations between calculated and observed
M2H6 frequencies points to the identification of DA as dialane
Al2H6. The 1932 and 1915 cm-1 absorptions are antisymmetric
b1u and b3u terminal Al-H2 stretching modes. The 1408 and
1268 cm-1 bands are antisymmetric b2u and b3u stretching modes
of the bridged Al-(µ-H)2-Al subunit. The lower frequency 836,
632, and 702 cm-1 absorptions are due to AlH2 wag, rock, and
bending motions.7
Another means of comparison is the scale factor (observed/
calculated frequencies). For our B3LYP calculation, scale factors
are 0.971 and 0.974 for the terminal Al-H2 stretching modes,
0.949 and 0.981 for the bridge Al-(H)2-Al stretching motions,
and 0.965, 0.989, and 0.997 for the low-frequency vibrations.
Our scale factor for the e stretching mode of AlH3 is 0.967.
These typical scale factors for the B3LYP functional53 further
substantiate our identification of dialane from the matrix infrared
spectrum. Note that scale factors differ for different vibrational
modes: the potential functions will require anharmonic refine-
ment for better agreement. Confidence in this identification of
dialane is built on excellent agreement between observed and
calculated frequencies for seVen fundamental vibrations in three
different spectral regions.
Hydrogen-argon matrix shifts observed for the strongest DA
bands (Table 1) are slightly larger than those found for AlH2,
but the hydrogen-neon matrix shifts are smaller. Very weak
argon matrix DA bands were detected in our earlier argon matrix
investigations using high H2 concentrations.
Additional agreement is found in the calculated and observed
infrared intensities. The strongest band in the infrared spectrum,
by about a factor of 2, is the antisymmetric b3u Al-(µ-H)2-Al
bridge-bond stretching mode parallel to the Al-Al axis. This
1408 cm-1 band in our spectrum is broader than the 1268 cm-1
absorption, which is the antisymmetric b2u Al-(µ-H)2-Al
bridge-bond stretching mode perpendicular to the Al-Al axis.
However, the integrated experimental band intensities (Table
2) are in qualitative agreement with the predictions of theory.
The terminal Al-H2 b1u and b3u stretching modes are split by
interaction with the hydrogen matrix. The higher frequency b1u
mode is predicted (B3LYP) to be 3-fold stronger, and the
observed relative b1u and b3u mode intensities match nicely
(Figure 2c).
Weaker 1362.8 and 1227.8 cm-1 bands appear along with
DA on λ > 380, 290 nm photolysis but decrease when DA
increases markedly on λ > 240 nm irradiation (Figure 1). These
bands reappear on 6.5 K annealing while DA slightly increases.
The proximity to strong DA bands at 1408.1 and 1268.2 cm-1
suggests that a perturbed or aggregated DA species is also
formed in the hydrogen matrix. Finally, annealing produces a
satellite band 8 cm-1 lower than the strong e fundamental of
AlD3. Since we compute D3d and D3h structures to be unbound
(AlH3)2, this satellite is probably due to (AlD3)2.
Alkyl substituents stabilize aluminum hydrides,54 and the
stable compound (CH3)2Al(µ-H)2Al(CH3)2 has the same dibridged
Al(µ-H)2Al subunit as Al2H6.55 Recent B3LYP calculations of
the Al-H-Al bridge bond length find 1.76 Å, which is almost
the same as the present 1.74 Å value for Al2H6, and association
enthalpies computed for (CH3)2AlH are only 2 kcal/mol less
than for AlH3.56 Hence, these aluminum hydride compounds
have similar bridge bonding. Furthermore, calculated (B3LYP)
A comparison of calculated harmonic (B3LYP) and observed
anharmonic (hydrogen matrix) terminal Al-H stretching fre-
quencies (Tables 1-3) shows that calculated exceed observed
values by 40, 29 cm-1 for AlH2, 65 cm-1 for AlH3, 62, 54 cm-1
frequencies (1442, 1245 cm-1 56
and vapor-phase absorptions
)
(54) Schulz, S. Coord. Chem. ReV. 2001, 215, 1.
(55) Almenningen, A.; Anderson, G. A.; Forgaard, F. R.; Haaland, A. Acta Chem.
Scand. 1972, 26, 2315.
(53) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(56) Willis, B. G.; Jensen, K. F. J. Phys. Chem. A 1998, 102, 2613.
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9225

A R T I C L E S
(1368, 1215 cm-1 57
Wang et al.
)
observed for the Al(H)2Al bridge stretching
The photosensitive 1050.0, 1043.0 cm-1 band pair in solid
D2 is extremely close to our B3LYP prediction of 1058.4 (a1)
and 1056.9 cm-1 (b2) modes for AlD2-. Since our B3LYP
calculation found 1190.2 cm-1 for ν3 (t2) of AlD4-, only 7.0
modes are in excellent agreement for the (CH3)2AlH dimer,
(CH3)2Al(µ-H)2Al(CH3)2, and these diagnostic frequencies are
only slightly lower than our corresponding B3LYP (1483, 1292
cm-1) and observed (1408, 1268 cm-1) values for H2Al(µ-
H)2AlH2. A more recent investigation of gaseous [Me2AlH]2
reported a 1357 cm-1 infrared band and a more accurate 178
pm electron diffraction measurement for the Al-H bridge bond,
and corroborating MP2 calculations.58 These comparisons of
Al(H2)Al bridge stretching frequencies provide further evidence
for the present experimental identification of the binary hydride
dialane H2Al(µ-H)2AlH2 molecule.
A final comparison can be made with the infrared active
M-(µ-H)2-M bridge stretching modes for the three M2H6
molecules characterized to date, M ) B, Al, Ga. The ν13 (b1u)
modes are 1924, 1268, 1202 cm-1, respectively, and the ν17
(b3u) modes are 1615, 1408, 1273 cm-1, respectively.15,59 The
dialane frequencies are intermediate but much closer to digallane
than diborane values.
We believe that the infrared spectrum of Al2H6, containing
seven fundamental absorption bands in terminal Al-H2 stretch-
ing, bridge Al-(H)2-Al stretching, and Al-H2 bending regions,
which are nicely matched by quantum chemical frequency
calculations, makes a very strong case for the identification of
dialane and its characterization with the diborane bridge-bonding
model.60
Al2H2. The earlier argon matrix work assigned several bands
to Al2H2 species.23,50 One of these absorptions is observed at
1156.1 cm-1 in normal and at 1155.0 cm-1 in para-H2. The
former band shifts to 1102.6 cm-1 in pure HD and to 852.0
cm-1 in pure deuterium. Our B3LYP calculation predicts the
extremely intense b1u antisymmetric stretching mode of planar
dibridged Al2H2 at 1187 cm-1, but the corresponding b2u mode
is only 5% as strong, as found previously.50 The shift to 852.0
cm-1 and 1.357 H/D ratio are appropriate for a bridge stretching
mode. The sharp 844.1 cm-1 band does not track on λ > 530
nm photolysis, which destroys the 1156.1 cm-1 band in favor
of Al2H6. The 1156.1 cm-1 band is assigned to the most stable
Al2H2 isomer, dibridged Al-(µ-H)2-Al, which is the only form
observed in solid hydrogen.
cm-1 above the observed value in solid D2, the 1050.0 and
1043.0 cm-1 bands are assigned to ν1 (a1) and ν3 (b2) of AlD2
.
-
-
This observation means that AlD2 is stable in solid D2, but λ
-
> 290 nm radiation initiates reaction with D2 to form AlD4
.
-
However, the corresponding AlH2 species was not observed
in solid H2. This suggests that any AlH2 formed reacts
straightaway with H2 to form AlH4-, which is observed at
-
1638.1 cm-1 in solid H2.
-
Our B3LYP calculations show that AlH3 is a stable anion,
more so than BH3-, which has been characterized.30 The AlH3
-
anion is computed to be 10.1 kcal/mol more stable than AlH3,
to be pyramidal (112.7° H-Al-H angle), and to have a strong
ν3 (e) mode at 1698 cm-1. A sharp, very weak 1701.6 cm-1
absorption decreases on 6.2 K annealing and disappears on λ
> 380 nm photolysis: this band could be due to AlH3-, but we
cannot be certain without more evidence.
Solid (AlH3)n. When the hydrogen matrix samples are
annealed to 6.8 K, H2 evaporates, molecular aluminum hydrides
diffuse, aggregate and their absorptions decrease, and broad
absorptions appear at 1720 ( 20 and 720 ( 20 cm-1 (Figure
3a-c). These broad bands remain on the CsI window with
decreasing absorbance until room temperature is reached. The
deuterium matrix samples evaporate D2 at about 10 K and
aluminum deuterides produce a broad 1260 ( 20 cm-1
absorption (Figure 3d,e), which remains on the CsI window.
The frequency ratio 1720/1260 ) 1.365 demonstrates that these
bands are due to Al-H/Al-D vibrations. The aluminum/H2/
D2 samples produce similar broad bands after evaporation of
the matrix that are shifted slightly to 1740 ( 20, 1280 ( 20,
and 710 ( 20 cm-1 (Figure 3f).
The spectrum of pure solid (AlH3)n gives strong broad bands
at 1760 and 680 cm-1, solid (AlH3)n in Nujol reveals a very
strong, broad 1592 cm-1 band, and solid (AlD3)n gives a
corresponding 1163 cm-1 band.61,62 The present broad absorp-
tions at 1720 and 1260 cm-1 are therefore due to solid (AlH3)n
and (AlD3)n on the salt window. Since the Al-H bond is polar,16
the solid (AlH3)n network5 may align and bind electrostatically
to the ionic CsI lattice surface. The 100 cm-1 redshift in Nujol
is due to interaction with this host medium.
AlH4-, AlD4-, AlH3-, and AlD2-. The 1638.1 cm-1 absorp-
tion appears above a 1609.3 cm-1 photolysis product in the
argon/Al/H2 system, which has been conclusively identified as
AlH4- isolated in solid argon.48 The 1638.1 cm-1 band increases
on λ > 380, 290 nm photolysis but decreases on λ > 240 nm
irradiation (Figure 1). Furthermore, the 1638.1 cm-1 band
decreases in yield relative to the AlH3 band with increasing laser
energy. In pure deuterium the 1183.2 cm-1 counterpart is weak,
but a 1050.0, 1043.0 cm-1 doublet gives way on λ > 290 nm
photolysis and doubles the 1183.2 cm-1 absorption, but λ >
240 nm irradiation virtually destroys this feature. Pure HD gives
new counterpart absorptions at 1658.1 and 1193.9 cm-1 for
AlH2D2-, which supports assignment of the 1638.1 cm-1 band
to AlH4- in solid hydrogen. This is in agreement with the present
Our broad 720 cm-1 absorption corresponds to the 680 cm-1
solid band.61 These bands are higher than our 632 cm-1 Al2H6
absorption, which probably involves bending of the Al-H-Al
bridge bonds. A very weak 520 ( 20 cm-1 absorption on our
cold window (Figure 3e) may be the deuterium counterpart of
the 508 cm-1 Nujol band.
The crystal structure for solid (AlH3)n shows six-coordinate
aluminum with equivalent hydrogen atoms in bridge-bonding
arrangements between aluminum atoms.5 The average Al-H
distance is 1.72 Å. It is interesting to note that the solid Al-H
distance is intermediate between the 1.74 Å bridge and 1.58 Å
terminal Al-H distances calculated here for Al2H6 and that the
solid frequency of 1720 cm-1 is likewise intermediate between
and previous48 DFT calculations for the ν3 (t2) fundamental of
-
tetrahedral AlH4
.
(57) Grady, A. S.; Puntambekar, S. G.; Russell, D. K. Spectrochim. Acta 1991,
47A, 47.
(58) Downs, A. J. et al. Organometallics 2000, 19, 527.
(59) Duncan, J. L. J. Mol. Spectrosc. 1985, 113, 63.
(60) Price, W. C. J. Chem. Phys. 1948, 16, 894.
(61) Matzek, W. E.; Musinski, D. F. U.S. Patent 3,883,644, 1975; Chem. Abstr.
1975, 83, 45418.
(62) Roszinski, W.; Dantel, R.; Zeil, W. Z. Physik. Chem. (Frankfort) 1963,
36, 26.
9
9226 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003

Infrared Spectra of Aluminum Hydrides in Solid Hydrogen
A R T I C L E S
the terminal (1932, 1915 cm-1) and bridged (1408, 1268 cm-1
Al2H6 values observed here.
)
especially Figure 7a where Al2H4 dominates over the small
amount of AlH produced during deposition by the ultraviolet
spectroscopic diagnostic. Apparently the exothermic (-30.0(1.4
Evaporation of the H2, D2 matrix from mixed H, D-aluminum
hydride samples gave similar broad bands slightly blue shifted.
This shows a slight stretch-stretch interaction for Al-H-Al
bonds in the solid state and suggests that the symmetric
stretching modes in (AlH3)n are higher than the observed
antisymmetric stretching modes.
Finally, it is interesting to observe that these reactions take
Al and H2 to the molecular aluminum hydrides AlH3 and Al2H6,
which aggregate in the end to form solid (AlH3)n films. Thus,
we have explored aluminum-hydrogen chemistry from the
microscopic beginning to the macroscopic end using the matrix
isolation technique.
Reaction Mechanisms. The reaction of Al and H2 to form
AlH must be activated by photolysis24,25,63 or by excess energy
(kinetic or photon) from the ablation process.32,64 Even though
reaction 2 to form AlH2 is exothermic, it must also be activated
by ultraviolet photolysis.25,65 Furthermore, the observation of
AlH in solid H2 requires a barrier for reaction 4.
3
kcal/mol) dimerization of Al atoms to form Πu ground-state
Al266,67 is sufficient to activate one or two dihydrogen molecules
to form Al2H2 and Al2H4, reactions 7 and 8. This is consistent
with our previous nonobservation of B2 in the para-H2 matrixes
produced under conditions which yielded B2 in rare gas
matrixes.31 Hence, variable amounts of Al2H2 and Al2H4 are
observed from these “recombination” reactions on thermal Al
deposition with para-H2. Even though the Al2 reaction with 3H2
is even more exothermic (-87 kcal/mol, B3LYP), inserting (µ-
H)2 into the Al-Al bond in Al2H4 must require substantial
activation energy. However, dibridged Al-(µ-H)2-Al can be
activated by λ > 530 nm photolysis to form Al2H6 in solid
hydrogen. Furthermore, Al2 is similar to Ga2, which reacts
spontaneously with H2 in solid argon to form dibridged Ga(µ-
H)2-Ga.68 Finally, similar laser-ablated Ga experiments with
pure H2 give the analogous products including Ga2H2, Ga2H4,
Ga2H6, and GaH2-, which will be described in a later publica-
tion.69
Al + H2 f AlH + H (∆E ) +32 kcal/mol)45
(1)
Al2 (3Πu) + H2 f Al2H2 (1Ag) (∆E )
-33 kcal/mol, B3LYP) (7)
Al + H2 f AlH2 (∆E ) -14 kcal/mol, B3LYP) (2)
AlH + H f AlH2 (∆E ) -48 kcal/mol, B3LYP) (3)
AlH + H2 f AlH3 (∆E ) -26 kcal/mol, B3LYP) (4)
Al2 (3Πu) + 2H2 f Al2H4 (1A1) (∆E )
-55 kcal/mol, B3LYP) (8)
-
The stable AlH4 anion is formed by capture of electrons
The growth of AlH2 on annealing, Figure 1f, requires that H
atoms diffuse and react with AlH, reaction 3. The production
of H atoms in laser ablation/solid hydrogen experiments has
been documented through the observation of HO2 radical.49
Photoexcitiation of AlH at 3.0 eV (Figure 1c) in the presence
of hydrogen leads to the formation of AlH3, reaction 4. Then
Al2H6 is formed by the dimerization of AlH3, reaction 5.
Although a small growth of Al2H6 absorptions is found on
annealing the solid hydrogen samples, more growth is observed
on λ > 240 nm irradiation when the AlH3 monomer yield
increases markedly. A strong satellite absorption at 1370.5 cm-1
on the 1378.7 cm-1 AlD3 absorption is probably due to
unreacted (AlD3)2 dimers. The absence of a similar satellite
absorption for AlH3 suggests a small activation energy for
reaction 5 in the case of AlD3. The straightforward formation
of Al2H4 (D2d) involves the exothermic dimerization of AlH2,
reaction 6. We find evidence for one of the ionic Al2H4
isomers.51 With laser-ablated Al atoms, Al2H4 is formed on
sample deposition when the yield of AlH2 is large, and Al2H6
is favored after ultraviolet photolysis when the yield of AlH3 is
high.
-
from the ablation process. We note that no AlH4 counterpart
is observed with thermal Al atoms in para-H2. The electron
affinity of AlD2 is estimated as 25 kcal/mol (B3LYP). In solid
deuterium experiments, AlD2 is trapped and λ > 290 nm
photoexcitation leads directly to AlD4
-
-
.
AlH2 + H2 + e- f AlH4- (∆E ) -70 kcal/mol, B3LYP)
(9)
AlD2 + e- f AlD2- (∆E ) -25 kcal/mol, B3LYP) (10)
AlD2- + D2 f AlD4- (∆E ) -45 kcal/mol, B3LYP) (11)
The final reaction, the formation of solid aluminum hydride,
occurs on evaporation of the hydrogen matrix to allow associa-
tion of molecular aluminum hydrides, which can be monitored
by the loss of characteristic solid hydrogen absorptions.70 This
solid (AlH3)n is identified by agreement of broad infrared
absorption bands with spectra of the pure solid.61 Reaction 12
is estimated to be exothermic by 40 kcal per mole of AlH3, which
contributes to the stability of the solid.16 It appears, then, that
the failure to isolate pure Al2H6 is due to its polymerization
AlH3 + AlH3 f Al2H6 (∆E ) -35 kcal/mol)10,11,21 (5)
AlH2 + AlH2 f Al2H4 (∆E ) -56 kcal/mol, B3LYP) (6)
(66) Cai, M. F.; Dzugan, T. P.; Bondybey, V. E. Chem. Phys. Lett. 1989, 155,
A unique synthesis of Al2H4 is observed from the recombina-
tion of thermal Al atoms during deposition in para-H2; see
430.
(67) Fu, Z.; Lemire, G. W.; Bishea, G. A.; Morse, M. D. J. Chem. Phys. 1990,
93, 8420.
(68) Himmel, H.-J.; Manceron, L.; Downs, A. J.; Pullumbi, P. J. Am. Chem.
Soc. 2002, 124, 4448.
(69) Wang, X.; Andrews, L. Unpublished results (Ga in H2).
(70) Gush, H. P.; Hare, W. F. J.; Allin, E. F.; Welsh, H. L. Can. J. Phys. 1960,
38, 176.
(63) Parnis, J. M.; Ozin, G. A. J. Phys. Chem. 1989, 93, 1215.
(64) Kang, H.; Beauchamp, J. L. J. Phys. Chem. 1985, 89, 3364.
(65) Knight, L. B.; Woodward, J. R.; Kirk, T. J.; Arrington, C. A. J. Phys.
Chem. 1993, 97, 1304.
9
J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003 9227

A R T I C L E S
Wang et al.
into the more stable network solid, which is observed at the
end of these hydrogen matrix experiments.
Annealing these samples to remove the H2 matrix decreases
the sharp AlH3 and Al2H6 absorptions and produces broad
1720 ( 20 and 720 ( 20 cm-1 bands, which are due to solid
(AlH3)n on the CsI window. In fact laser-ablation of Al into
pure hydrogen appears to be an effective method for producing
an aluminum hydride film. This contrasts the case of B2H6,
which forms a molecular solid and gives broad bands near
isolated molecule values30 and the case of Ga2H6, which forms
an oligomer16 and gives similar film spectra15,69 after the
hydrogen matrix evaporates.
Small matrix shifts are observed between normal hydrogen
(3/4 ortho-H2, 1/4 para-H2)71 and para-H2. The formation of
Al2H4 during thermal Al atom deposition in para-H2 from the
exothermic combination of Al atoms assists in the matrix
identification of this novel molecule.
nAlH3 f (AlH3)n
(12)
The successful synthesis and characterization of Al2H6 is
made possible by special chemical and physical properties of
the pure hydrogen matrix. The “high” hydrogen concentration
ensures the formation of a large yield of AlH3 and the soft nature
of solid hydrogen allows diffusion and dimerization of AlH3 to
Al2H6. When the temperature of the solid hydrogen sample
approaches 7 K, H2 evaporates, the diffusion and reaction of
trapped aluminum hydride molecules is rapid and solid (AlH3)n
is formed.
Conclusions
Although many volatile binary boron hydride compounds are
known, binary aluminum hydride chemistry is limited to the
polymeric (AlH3)n solid. Our experimental characterization of
the dibridged Al2H6 molecule provides an important link
between the chemistries of boron and aluminum.
The reaction of laser-ablated Al atoms and normal-H2 during
co-deposition at 3.5 K forms AlH, AlH2, AlH3, and Al2H2 based
on infrared spectra, and the results of isotopic substitution (D2,
H2 + D2 mixtures, HD). Four new bands are assigned to Al2H4
with the D2d structure. Ultraviolet photolysis markedly increases
the yield of AlH3 and seven new absorptions for Al2H6 in the
infrared spectrum of the solid hydrogen sample. Complementary
results are found using thermal Al atoms and para-H2. These
seven vibrational frequencies are accurately predicted by
quantum chemical calculations for dibridged Al2H6, which is
isostructural with diborane.
Acknowledgment. We thank G. V. Chertihin and P. F. Souter
for work on earlier argon matrix experiments and NSF Grant
No. CHE00-78836 for financial support.
JA0353560
(71) Silvera, I. F. ReV. Mod. Phys. 1980, 52, 393.
9
9228 J. AM. CHEM. SOC. VOL. 125, NO. 30, 2003