Infrared spectra of ThH2, ThH4, and the hydride bridging ThH4(H2), (x = 1-4) complexes in solid neon and hydrogen

Article abstract of DOI:10.1021/jp710326k

Laser-ablated Th atoms react with molecular hydrogen to give thorium hydrides and their dihydrogen complexes during condensation in excess neon and hydrogen for characterization by matrix infrared spectroscopy. The ThH ₂, ThH₄, and ThH₄(H₂)_x (x = 1-4) product molecules have been identified through isotopic substitution (HD, D₂) and comparison to frequencies calculated by density functional theory and the coupled-cluster, singles, doubles (CCSD) method and those observed previously in solid argon. Theoretical calculations show that the Th-H bond in ThH₄ is the most polarized of group 4 and uranium metal tetrahydrides, and as a result, a strong attractive dihydrogen interaction was found between the oppositely charged hydride and H₂ ligands ThH₄(H₂)_x. This bridge-bonded dihydrogen complex structure is different from that recently computed for tungsten and uranium hydride super dihydrogen complexes but is similar to that recently called the dihydrogen bond (Crabtree, R. H. Science 1998, 282, 2000). Natural electron configurations show small charge flow from the Th center to the dihydrogen ligands.

Full text of DOI:10.1021/jp710326k

1754
J. Phys. Chem. A 2008, 112, 1754-1761
Infrared Spectra of ThH2, ThH4, and the Hydride Bridging ThH4(H2)x (x ) 1-4)
Complexes in Solid Neon and Hydrogen
Xuefeng Wang, Lester Andrews,* and Laura Gagliardi
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904-4319, and Department of
Physical Chemistry, UniVersity of GeneVa, 30 Quai Ernest Ansermet, CH-1211 GeneVa, Switzerland
ReceiVed: October 25, 2007
Laser-ablated Th atoms react with molecular hydrogen to give thorium hydrides and their dihydrogen complexes
during condensation in excess neon and hydrogen for characterization by matrix infrared spectroscopy. The
ThH2, ThH4, and ThH4(H2)x (x ) 1-4) product molecules have been identified through isotopic substitution
(HD, D2) and comparison to frequencies calculated by density functional theory and the coupled-cluster,
singles, doubles (CCSD) method and those observed previously in solid argon. Theoretical calculations show
that the Th-H bond in ThH4 is the most polarized of group 4 and uranium metal tetrahydrides, and as a
result, a strong attractive “dihydrogen” interaction was found between the oppositely charged hydride and H2
ligands ThH4(H2)x. This bridge-bonded dihydrogen complex structure is different from that recently computed
for tungsten and uranium hydride super dihydrogen complexes but is similar to that recently called the
“dihydrogen bond” (Crabtree, R. H. Science 1998, 282, 2000). Natural electron configurations show small
charge flow from the Th center to the dihydrogen ligands.
Introduction
hydrogen during condensation at 4 K. The ThH2 and ThH4
hydrides and the super dihydrogen thorium hydride complexes
ThH4(H2)x (x ) 1-4) are identified through infrared spectros-
copy and quantum chemical calculations. Owing to the unique
chemistry of thorium, the structure of this complex is computed
to be different from the uranium and tungsten analogues.3,6
The chemistry of early actinide atoms and reactions with other
elements have attracted great interest because 5f electrons are
involved in bonding while 4f electrons in lanthanide metals
remain in the core.1 This is dramatically demonstrated by the
involvement of 5f orbitals in the U-U multiple bonds in a
variety of compounds. The U-U multiple bond in U2 itself and
in compounds U2@C60, U2Ph2 (Ph ) phenyl), U2Cl6, U2Cl8
Experimental and Computational Methods
-
,
The experiment for investigating reactions of laser-ablated
thorium atoms with hydrogen has been described in detail
previously.7,12 The Nd:YAG laser fundamental (1064 nm, 10
Hz repetition rate with 10 ns pulse width) was focused onto a
rotating thorium target (Oak Ridge National Laboratory). Laser-
ablated metal atoms were codeposited with pure hydrogen or
hydrogen (0.3-4%) in excess neon onto a 4 K CsI cryogenic
window at 3 mmol/h for 30 min (for pure H2) and 60 min (for
neon). Deuterium gas and HD (Cambridge Isotopic Laborato-
ries) and selected mixtures were used in different experiments.
The laser energy was varied from 10 to 20 mJ/pulse. Reducing
radiation from the ablation plume as low as possible makes solid
H2 easier to form and minimizes its evaporation. Fourier
transform infrared (FTIR) spectra were recorded at 0.5 cm-1
resolution on Nicolet 750 with 0.1 cm-1 accuracy in the 4500-
400 region using an HgCdTe range B detector. Matrix samples
were annealed at different temperatures and were subjected to
broad-band irradiation using a medium-pressure mercury arc
lamp (Philips, 175W) with the outer globe removed.
Density functional theory calculations of thorium hydrides
and dihydrogen complexes are given for comparison. The
Gaussian 03 program13 was employed to calculate the structures
and frequencies of expected product molecules. All geometrical
parameters were fully optimized with the B3LYP functional.14
The 6-311++G(3df,3pd) basis set for hydrogen atom and SDD
pseudopotential for thorium atom were used.15,16 Analytical
vibrational frequencies were obtained at the optimized structures.
Single-point energies of optimized complex species were
U2(OCHO)4, and U2(OCHO)6 has been investigated theoreti-
cally, and the 5f, 6d, and 7s valence orbitals of U atom are
involved in bonding.2 With 5f orbital participation, a variety of
oxidation states are found for actinides, which leads to the
formation of unusual compounds and complexes with other
elements. For example, uranium atoms react with H2 to give
UH4, which in solid hydrogen ultimately forms the supercom-
plex UH4(H2)6.3 This molecule has potential interest as a metal
hydride with a large number of hydrogen atoms bound to
uranium. In contrast, Mo and W also form tetrahydride
molecules, but calculations suggest that these coordinate only
four dihydrogen ligands to give MH4(H2)4 complexes.4-6
The IR spectra of several thorium atom reaction products with
small molecules have been investigated in solid argon in this
laboratory, and several interesting reactions and products have
been observed.7 More recent investigations show that thorium
atoms react with CH4 and NH3 to form CH2dThH2 and HNd
ThH2 molecules, which reveal multiple bonding for thorium with
significant 5f orbital participation.8,9 Laser-ablated thorium atom
reactions with H2 have been studied in solid argon, and the major
ThH2 and ThH4 molecular species were identified.10 In the pure
solid state, such thorium hydride compounds as Th4H15, for
example, form and behave as weak coupling semiconductors.11
We report here the further investigation of thorium metal atom
reactions with molecular hydrogen in solid neon and pure
* To whom correspondence should be addressed. E-mail: lsa@virginia.edu.
10.1021/jp710326k CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/06/2008

Infrared Spectra of ThH2, ThH4, and the Hydride
J. Phys. Chem. A, Vol. 112, No. 8, 2008 1755
TABLE 1: Infrared Absorptions (cm-1) Observed from Reaction of Thorium and Isotopic Dihydrogen in Solid Neon and
Hydrogen
neon
HD
hydrogen
HD
identification
H2
D2
H2
D2
1511.0
1476.0
1078.3
ThH/ThD
ThH2
ThHD
ThD2
ThH4
ThH2D2
ThH2D2
ThD4
1475.5
1496.8, 1062sh
1055.0
1478.4
1054.5
1040.1
1053.9
1029.9
1455.7
1453.1
1496.2, 1454.9
1063.9, 1041.1
1393.1
1377.7
1389.3
1374.3
ThH4(H2)x
ThH4(H2)x
ThH2D2(HD)x
ThH2D2(HD)x
ThD4(D2)x
ThD4(D2)x
?
1372.3
991.3
1412, 1366
1008, 988
992.2
982.4
991.8
979.6
1345.8
1340.2, 959.2
?
?
959.2
calculated with coupled-cluster CCSD(T) methods.17a Natural
charges were calculated by means of natural bond orbital
(NBO)17b using built-in subroutines in the Gaussian 03 program,
and B3LYP and BPW91 functionals with larger (6-311++G-
(3df,3pd) and smaller (6-311++G(d,p) basis sets for hydrogen
atom were used.
Multiconfigurational complete active space (CASSCF)18
calculations followed by second-order perturbation theory
(CASPT2)19 were performed on ThH4(H2)4 and UH4(H2)6
complexes to understand the differences in the electronic
structure between these systems. The tetrahydride molecules
ThH4 and UH4 were also investigated at the same level of theory.
Single-point energy calculations were perfomed at the density
functional theory (DFT) optimized geometries.
Scalar relativistic effects were included using a Douglas-
Kroll-Hess Hamiltonian and relativistic ANO-RCC basis set
(2s1p for hydrogen and 9s8p6d5f2g1h for uranium and tho-
rium).20 In the CASSCF calculations, the orbitals formed by
linear combinations of 7s, 6d, and 5f orbitals of uranium/thorium
with 1s orbitals of the hydrogens were included in the active
space, resulting in active spaces formed of 12 electrons in 12
orbitals (12/12) up to 16 electrons in 16 orbitals (16/16). In the
following CASPT2 calculations, all orbitals up to and including
5d orbitals of uranium/thorium were kept frozen. This method
has proven successful for the study of actinide chemistry.21,22
on annealing. Two broad bands centered at 1393.1 and 1377.7
cm-1 appeared on deposition and increased markedly on later
annealing and irradiation while a 1435 cm-1 band decreased.
With D2/Ne, the upper bands shift to the Th-D stretching region
at 1078.3, 1054.5, 1040.1, 992.2, and 982.4 cm-1. The HD/Ne
experiments gave sharp bands at 1496.8, 1475.6, and 1454.9
cm-1 in the Th-H stretching region and at 1064.0, 1055.2, and
1041.1 cm-1 in the Th-D stretching region and gave broad
bands centered at 1411.8, 1371.0, 1006.1 and 988.4 cm-1
.
Spectra from a more dilute 0.3% sample show weak, sharp
1511.0, 1476.0, and 1455.7 cm-1 bands and weak, broad bands
at 1393 and 1378 cm-1. Annealing to 8 K slightly increased
the sharp bands with little effect elsewhere, but >320 nm and
then 240-380 nm irradiations decreased the 1476.0 and
increased the 1455.7 cm-1 band and increased broad features
at 1435, 1421, 1393, and 1377 cm-1. Subsequent annealings to
10 and 12 K increased both sharp bands and markedly increased
the upper broad bands and destroyed the lower broad bands.
Results
Infrared spectra are presented for Th atom reactions with H2
in excess neon and hydrogen, and the absorptions of reaction
products are listed in Table 1. Absorptions common to pure
hydrogen experiments, namely, H(H2)n, D(D2)n, H-(H2)n, D-(D2)n,
and D3+(D2)n, have been reported previously23,24 and are not
discussed again here. Trace impurity absorptions for CO2, CO,
H2O, and CH4 appeared in all spectra, but no reaction products
were observed with these molecules. Theoretical calculations
are used to support the product identifications.
In Solid Neon. Infrared spectra for the Th-H and Th-D
stretching regions are illustrated in Figures 1 and 2 for laser-
ablated thorium atoms codeposited with H2, HD, and D2 in neon
using 1-4% samples. In the Th-H stretching region, strong
absorptions at 1455.7 and 1476.0 cm-1 and a weaker band at
1511.0 cm-1 appeared on deposition. The 1455.7 cm-1 band
increased on >380 nm, >290 nm irradiation and annealing while
the 1476.0 cm-1 band decreased on irradiation but increased
Figure 1. Infrared spectra for the thorium atom and H2 reaction
products in neon at 4 K. (a) Th + H2 (1%) deposition for 60 min, (b)
after annealing to 7 K, (c) after 240-380 nm irradiation, (d) after
annealing to 8 K, (e) after annealing to 10 K, and (f) after annealing to
12 K. (g) Th + HD (4%) deposition for 60 min, (h) after annealing to
8 K, (i) after >320 nm irradiation, (j) after 240-380 nm irradiation,
(k) after >220 nm irradiation, and (l) after annealing to 11 K. Arrows
denote ThH4(H2)x complex absorptions.

1756 J. Phys. Chem. A, Vol. 112, No. 8, 2008
Wang et al.
Figure 2. Infrared spectra for the thorium atom and D2 reaction
products in neon at 4 K. (a) Th + D2 (2%) deposition for 60 min, (b)
after annealing to 7 K, (c) after 240-380 nm irradiation, (d) after
annealing to 8 K, (e) after annealing to 10 K, and (f) after annealing to
12 K. (g) Th + HD (4%) deposition for 60 min, (h) after annealing to
8 K, (i) after >320 nm irradiation, (j) after 240-380 nm irradiation,
(k) after >220 nm irradiation, and (l) after annealing to 11 K.
Figure 3. Infrared spectra for the thorium atom reaction with H2, HD,
and D2 in solid hydrogen, deuterium hydride, and deuterium at 4 K.
(a) Th + H2 deposition for 25 min, (b) after >380 nm irradiation, (c)
after >220 nm irradiation, and (d) after annealing to 6.4 K. (e) Th +
HD deposition for 30 min, (f) after >320 nm irradiation, and (g) after
annealing to 9 K. (h) Th + D2 deposition for 25 min, (i) after >380
nm irradiation, (j) after >220 nm irradiation, and (k) after annealing
to 8 K.
Final annealings to 13 and 14 K decreased the sharp bands and
increased the broad absorptions.
In Solid Hydrogen. Laser-ablated Th atom reactions with
H2 in solid hydrogen gave a very strong band at 1374.3 cm-1
with a shoulder at 1389.3 cm-1. Two very weak bands were
also observed at 1453.1 and 1478.4 cm-1. In solid D2, the
counterpart bands appeared at 979.6 and 991.8 cm-1 (broad)
with two weak bands at 1029.9 and 1053.9 cm-1, respectively.
Pure HD and Th gave a strong 1366.0 cm-1 band with a 1372.8
cm-1 shoulder and weaker bands at 1006.9, 989.9, and 984.8
cm-1. These spectra are compared in Figure 4.
In Solid Argon. The Th + H2 reactions in solid argon were
done again using 4 K temperature to trap reactive species more
efficiently, although spectra of Th + H2 in argon at 8 K have
been published. The initial ThH, ThH2, and ThH4 species are
the same as we have assigned in our earlier paper,10 but on
annealing, several broad bands centered at 1405, 1382, and 1367
cm-1 appeared. With deuterium, these bands shifted to 1006,
993, and 978 cm-1
.
Discussion
Infrared spectra of thorium hydrides and their dihydrogen
complexes will be assigned on the basis of isotopic shifts and
correlation with density functional frequency calculations.
ThH. The ThH diatomic molecule was observed at 1485.2
cm-1 in solid argon with ThD counterpart at 1060.2 cm-1 (H/D
ratio 1.4009).10 The weak neon matrix band at 1511.0 cm-1
increases slightly on near-UV irradiation and annealing and is
appropriate for assignment to ThH. The same absorption was
weak in the HD experiments. A band with analogous behavior
at 1078.3 cm-1 (H/D ratio 1.4013) in deuterium experiments is
assigned to ThD. The B3LYP harmonic frequency of 1493.9
cm-1 suggests a repulsive interaction and blue shift in the neon
matrix cage. The largest neon-to-argon shift observed here (25.8
cm-1) is for ThH, which has more of the Th center exposed for
matrix interaction than any of the higher hydrides. In this regard,
then, we are considering here a (Ng)xThH complex. We expect
Figure 4. Infrared spectra for the thorium atom reaction with H2 in
solid argon at 4 K. (a) Th + H2 (0.5%) deposition for 60 min, (b)
annealing to 30 K, (c) after annealing to 35 K, (d) after annealing to
40 K, and (e) after annealing to 45 K.
neon, the Th-H stretching mode of ThH2 was found at 1476.0
cm-1, which is 20 cm-1 higher than the same mode in solid
argon. The slightly higher frequency suggests a weaker binding
interaction of neon with Th than of argon. Such noble gas
actinide binding interactions have been discussed in earlier
papers.25 The Th-D stretching mode in solid neon was observed
at 1054.5, giving a 1.3998 H/D isotopic frequency ratio, which
is appropriate for the Th-H stretching mode. With HD in neon,
a new weak band at 1494.5 cm-1 (Th-H stretching region) and
a 1062 cm-1 shoulder (Th-D stretching region) were observed.
These bands are assigned to the appropriate stretching modes
of HThD, and their appearance is 18.5 and 8 cm-1 higher than
that ThH will have a gas-phase fundamental near 1500 cm-1
.
ThH2. Infrared spectra of ThH2 have been reported in our
earlier Th atom investigation with H2 in solid argon.10 In solid

Infrared Spectra of ThH2, ThH4, and the Hydride
J. Phys. Chem. A, Vol. 112, No. 8, 2008 1757
the antisymmetric stretching modes of ThH2 and ThD2, respec-
tively, meaning that the symmetric stretching modes are at the
same separation and higher still, namely, at 1518 and 1070 cm-1.
Our calculation (Table 2) predicts slightly larger (49 and 32
cm-1) mode separations and symmetric stretching modes only
15% as intense, which are not observed here. Although the ThH
and ThD bands are close to these positions, they do not track
with the stronger ThH2 and ThD2 bands on irradiation and
annealing. In solid hydrogen, a very weak absorption at 1478.4
cm-1 can be assigned to the ThH2 molecule adsorbed on the
matrix surface. However, any ThH2 in the bulk hydrogen matrix
will react with H2 to give ThH4 spontaneously. With solid
similar band contours and annealing behaviors. With D2, the
strong doublets shift to 1028.9 and 1024.9 cm-1, and two weak
doublets shift to 1020.8 and 1019.6 cm-1 and to 1017.9 and
1017.0 cm-1, revealing a very similar H/D ratio as for ThH4.
The counterpart bands in solid neon at 1435 cm-1 are weak,
and no counterparts are observed in solid hydrogen, suggesting
that this species is very reactive and can only be efficiently
trapped in the more polarizable argon matrix. A H2 complex
with the ThH4 core, ThH4(H2), is suggested for these bands.
The HD substituted spectra strongly support this ThH4(H2)
assignment. There are six new doublets found in both Th-H
and Th-D stretching regions, respectively, in Th atom reactions
with HD in argon. First of all, the above strong doublet bands
split into several new bands on symmetry lowering, which are
due to symmetric and antisymmetric ThH2 and ThD2 stretching
fundamentals perturbed by HD subunit in ThH2D2(HD). There
are totally three isotopomers for ThH2D2(HD), and all of the
isotopomers have the similar splitting pattern but different
wavenumber shifts. Three bands for ThH4(H2) at 1435.5, 1428.1,
and 1420.3 cm-1 in Th-H stretching region split to three pairs
at 1434.9 and 1470.3 cm-1, 1427.8 and 1452.7 cm-1, and 1421.6
and 1442.9 cm-1, and three bands for ThD4(D2) at 1025.9,
1020.8, and 1017.9 cm-1 in Th-D stretching region split to
another three pairs at 1026.4 and 1056.8 cm-1, 1020.9 and
1047.6 cm-1, and 1018.2 and 1036.8 cm-1, which are assigned
to three different ThH2D2(HD) isotopomers. Theoretical fre-
quency calculations match the observed values very well as
listed in Table 3.
deuterium, a weak ThD2 band was observed at 1053.9 cm-1
.
The appearance of ThH2 at 1475.5 cm-1 in the neon
experiment with HD, along with ThD2 at 1055.0 cm-1 and the
former appearing 0.5 cm-1 lower and the latter 0.5 cm-1 higher
than for the H2 and D2 reagents, respectively, suggests that some
weak reagent association occurs with the ThH2 product in the
neon matrix and, more importantly, that most of the ThH2
product is formed by combination of ThH and H atoms rather
than by insertion into the hydrogen reagent.
ThH4. In solid neon, a sharp band at 1455.7 cm-1 is
appropriate for the Th-H stretching mode of ThH4. The
deuterium counterpart band at 1040.1 cm-1 showed the same
annealing and irradiation behavior. The 1455.7 and 1040.1 cm-1
bands define a heavy metal hydride H/D ratio of 1.3996 and
can be assigned to antisymmetric Th-H and Th-D stretching
fundamentals of the tetrahydride on the basis of the following
evidence. With HD, the above bands split to 1496.2 and 1454.8
cm-1 and to 1063.9 and 1041.0 cm-1, respectively, which are
due to symmetric and antisymmetric ThH2 and ThD2 stretching
fundamentals in ThH2D2. This pattern of four Th-H(D)
stretching modes identifies a metal tetrahydride molecule as
The 1435.5 cm-1 band was assigned to ThH3 in our earlier
paper because early theoretical calculations did not give isotopic
frequencies, and the complicated HD isotopic shifts were
misleading. We do not have a definitive assignment for the
trihydride.
described for MH4 (M ) Sc, Y, La),26 ZrH4, and HfH4.27
-
ThH4(H2)2,3,4. A major broad band was observed at 1377.7
cm-1 with a shoulder at 1393.1 cm-1 in Th atom reactions with
H2 in solid neon, which appeared on deposition and which
increased on annealing. These bands are still due to the Th-H
stretching vibrations, but they are much lower than the same
modes for ThH2 and ThH4. The deuterium counterparts in neon
are at 992.2 and 982.4 cm-1. With HD in neon, two broad bands
were observed in Th-H stretching region at 1371.0 and 1411.8
Notice that the symmetric-antisymmetric mode separation for
ThH2D2 is less, namely, 41.4 and 22.9 cm-1, and that the
H-Th-H bond angle, which affects stretch-stretch interaction,
is also less than with ThH2. Our calculations predict this mode
separation to be 50.6 and 27.7 cm-1, which is in very good
agreement with the observed separations. In solid argon,10 the
absorptions of ThH4 and ThD4 were found at 1443.3 cm-1 and
1031.1 cm-1, respectively, and the mode separations for ThH2D2
are 43.4 and 23.6 cm-1, which are very close to the present
neon matrix values.
cm-1 and in Th-D stretching region at 1007.7 and 988.4 cm-1
.
This isotopic pattern suggests a metal tetrahydride structure for
this molecule, so a series of thorium tetrahydride super
dihydrogen complexes, ThH4(H2)x, is proposed. Arrows denote
ThH4(H2)x complex absorptions leading to the final, strongest
broad band.
In solid hydrogen, a weak band at 1453.1 cm-1 is appropriate
for ThH4. This band is located slightly lower than found for
ThH4 in neon but higher than in argon. This neon-hydrogen-
argon absorption shift is found in many metal hydrides.12 The
weak band survives on deposition but disappears on annealing,
suggesting that ThH4 is trapped on the surface, and reacts with
H2 to give higher order complexes with the hydrides. With solid
Theoretical calculations were performed at the CCSD(T) level
with one to six H2 molecules attached to ThH4, and the energy
profile is illustrated in Figure 6 and the Th-H stretching
vibrations are listed in Table 2. First, as shown in Figure 6,
this reaction is exothermic when one to four H2 molecules are
coordinated to ThH4, but it is endothermic when more H2
molecules are added, so the stable molecule in solid hydrogen
is most likely due to the ThH4 (H2)x complex with x ) 4.
Second, our frequency calculations show that the Th-H
stretching mode of ThH4(H2)4 supercomplex reaches maximum
red shift about 50 cm-1 from that of ThH4 itself, which is close
to the observed shift of about 70 cm-1. As five and six H2
molecules are added to ThH4, the Th-H vibrations return to
the Th-H stretching region of ThH4 itself as the ligands are
less strongly associated. Third, the calculated Th-H and Th-D
splittings in ThH2D2(HD)4 molecule match experimental values
very well. The calculated antisymmetric-symmetric Th-H and
D2, the absorption of ThD4 was observed at 1029.9 cm-1
.
The strong degenerate ν3 (t2) mode calculated using the
B3LYP method for ThH4 at 1448.7 cm-1 and ThD4 at 1028.1
cm-1 (H/D ) 1.4091) is in excellent agreement with experi-
mental values. The Th-H and Th-D symmetric and antisym-
metric stretching mode splittings calculated for ThH2D2 are only
17% larger than the observed separations.
ThH4(H2). In solid argon, strong doublet bands appeared at
1435.5 and 1434.1 cm-1 on deposition and increased greatly
on annealing following Th atom reactions with H2, which track
two weak doublets at 1428.1, 1426.4 cm-1 and 1420.3, 1418.8
cm-1 (Figure 4). These bands are located slightly lower than
the Th-H stretching vibration of ThH4, and they show very

1758 J. Phys. Chem. A, Vol. 112, No. 8, 2008
Wang et al.
TABLE 2: Calculated and Observed Th-H Stretching
Modes for Thorium Hydrides
species
calculated
observed
Ar
Ne
H2
ThH
1493.9 (276)
1059.0 (139)
1539.5 (84,ν1)
1511.0 1485.2
1078.3 1060.2
1480.1
ThD
ThH2 (1A1)
1490.8 (538, ν3) 1476.0 1455.6
1478.4
1053.9
ThD2
1090.2 (42)
1058.1 (271)
1515.9(295)
1073.6(173)
1542.5 (0)
1448.7 (8543)
1091.0 (0)
1028.1 (3433)
1499.0(369)
1448.4(841)
1056.3(277)
1928.6(451)
1442.2(846)
1428.6(731)
1420.5(767)
1055.6
1054.5 1040.3
1494.5 1467.7
ThHD
1062
1048.6
ThH4 (1A)
ThD4
1455.7 1435.4 (1434.1)a 1453.1
1040.1 1025.9 (1024.8)a 1029.9
1475.6 1487.3
1454.9 1443.9
1055.2 1056.6
1041.1 1033.0
ThH2D2
Figure 5. Infrared spectra for the thorium atom reaction with D2 in
solid argon at 4 K. (a) Th + D2 (0.5%) deposition for 60 min, (b)
annealing to 30 K, (c) after annealing to 35 K, (d) after annealing to
40 K, and (e) after annealing to 45 K.
ThH4(H2)
ThD4(D2)
1435.5 (1434.1)a
1428.1 (1426.4)a
1420.3 (1418.8)a
1075.3 (22)
1023.5 (430)
1013.6 (372)
1008.2 (390)
1420.2(7602)
1416.5(666)
1008.1(3862)
1004.8(340)
1415.7(767)
1407.1(669)
1403.5(634)
1005.1(391)
998.5(339)
1025.9 (1024.9)a
1020.8 (1019.6)a
1017.9 (1017.0)a
1405
ThH4(H2)2
ThD4(D2)2
ThH4(H2)3
1382
1367
1390
ThD4(D2)3
Figure 6. Total binding energy curves calculated for ThH4(H2)x
complexes using the CCSD(T) method at the B3LYP converged
minimum energy structures.
995.8(321)
ThH4(H2)4
ThD4(D2)4
1408.4(780)
1390.3(5822)
1000.3(395)
987.0(2972)
1378
982
1374
980
further change on annealing. This suggests that the final reaction
product is observed in solid hydrogen. In solid deuterium, these
bands shift to 979.6 and 991.8 cm-1 as shown in Figure 3. The
H/D ratio, 1.4029, is virtually the same as found for the broad
band in solid neon, 1.4024, and for the sharp ThH4 absorption
in solid neon, 1.3996. The two absorptions are due to antisym-
metric Th-H(D) stretching modes split by symmetry lowering
from isolated tetrahedral ThH4 to ThH4(H2)4 in C2V symmetry.
Our calculation predicts this splitting to be 16 cm-1, and we
ThH2D2(HD)4 1434.8(251)
1399.1(665)
1412
1366
1008
988
1372
992
1013.2(201)
995.8(374)
a Matrix site splitting for observed band.
TABLE 3: Calculated and Observed Th-H and Th-D
Stretching Modes for ThH4, ThD4, and Mixed ThH2D2
Isotopomers in the ThH4(HH) Complexesa
calcd
exptl
H2ThH2(H2)
1480.3(402) 1452.7 1488.6(416)
calcd
exptl
calcd
exptl
observe it to be 15 cm-1
.
HDThHD(H D)
H2ThD2(HD)
1470.3
Absorptions for ThH4(H2)2 and ThH4(H2)3 were observed at
1404 and 1388 cm-1, respectively, in the argon matrix, which
shift to 1006 and 994 cm-1 with deuterium. However, these
bands are weak in solid neon and hydrogen.
1520.0(41)
1442.2(846) 1435.5 1427.5(717) 1427.8 1442.0(837)
1428.6(731) 1428.1 1044.5(283) 1047.6 1033.6(208)
1420.5(767) 1420.3 1014.9(406) 1020.9 1008.9(404)
1434.9
1033.6
1018.2
The absorptions of ThH4(H2)4 were observed at 1368.1 and
1385.8 cm-1 in solid argon as shown in Figure 4. Because of
the more rigid argon matrix environment, these bands were not
observed on deposition but appeared on annealing and maxi-
mized on higher annealing. This gives a clear mechanism that
H2 molecules are added to ThH4 to form a super dihydrogen
complex.
Search for HThThH. In view of our observation of (UH)2
in analogous matrix isolation experiments,3,28 we have searched
diligently for evidence of the analogous interesting dithorium
counterpart molecule.29 If the ground-state molecule is linear,
a terminal Th-H stretching modes is expected in the 1350 cm-1
region. Unfortunately, the weak 1346 cm-1 band does not show
the computed HThThD isotopic shift for this identification. If
D2ThH2(HD)
1466.0(254) 1442.9 1075.3(22))
1420.1(752) 1420.6 1023.5(430) 1025.9
1050.1(296) 1056.8 1013.6(372) 1020.8
1023.8(440) 1026.4 1008.2(390) 1017.9
D2ThD2(D2)
a B3LYP/6-311++G(3df,3pd)/SDD.
Th-D mode separations are 35.7 and 17.4 cm-1, respectively,
and the observed band separations are 46 and 20 cm-1 in solid
hydrogen, which represents the maximum coordination of
dihydrogen possible.
The solid hydrogen spectra strongly support the ThH4(H2)4
assignment. After deposition, a very strong band at 1374.3 cm-1
with a shoulder at 1389.3 cm-1 was observed, which shows no

Infrared Spectra of ThH2, ThH4, and the Hydride
J. Phys. Chem. A, Vol. 112, No. 8, 2008 1759
TABLE 4: Natural Charges and Electron Configurations
Calculated for Metal Tetrahydrides and the ThH4(H2)4
Complexa
M-H
bond length
(Å)
natural
electron
configuration
M
H
c
ThH4
+2.631 -0.658
+2.458 -0.615
+1.822 -0.455
+1.647 -0.416
+1.297 -0.324
2.071
2.011
1.854
1.851
1.689
2.085
Th: [core] 7s0.49 5f0.16 6d0.74
H: 1s1.65
c
UH4
U: [core] 7s0.48 5f2.39 6d0.70
H: 1s1.60
HfH4
ZrH4
TiH4
Hf: [core] 6s0.73 5d1.44
H: 1s1.45
Zr: [core] 5s0.65 4d1.70
H: 1s1.40
Ti: [core] 4s0.67 3d2.0
H: 1s1.31
ThH4(H2)4 +2.678 -0.619
-0.025b
Th: [core] 7s0.38 5f0.24 6d0.71
H: 1s1.60
Hb: 1s1.02
a B3LYP/6-311++G(3df,3pd)/SDD. b H atom from hydrogen mol-
ecule. c With BPW91/6-311++G(3df,3pd)/SDD, natural charges of Th
and H for ThH4 are +1.759 and -0.440; natural charges of U and H
for UH4 are +1.640 and -0.410. With B3LYP/6-311++G(d,p)/SDD,
natural charges for Th and H are +1.873 and -0.468, and U and H
are +1.757 and -0.439.
the ground-state molecule is bridged, new bands are expected
in the 800-1100 cm-1 region where none are found. We believe
that the very small yield of ThH in these experiments, which
would dimerize to form HThThH, is the likely justification for
its absence from our spectra. In contrast, UH is produced in
higher yield, and its rhombic dimer is observed.3,28 CASPT2
calculations indicate that HThThH has a rhombic structure and
1
3
a A1g ground state, while DFT predicts a linear Ag ground
state.29
Bonding. The 5f orbital participation in the bonding for
thorium and uranium has been discussed extensively.1 Recently,
we reported the evidence for six hydrogen molecules bound to
the uranium tetrahydride center.3 The 5f orbitals must play a
very important role in bonding the dihydrogen ligands. A
different complex ThH4(H2)4 and structure are found for the
thorium case, in which H2 ligands are bridging the hydride
centers suggesting a different bonding mechanism within the
Th species. The U-H bond is computed to be 2.01 Å and the
Th-H bond is computed to be 2.07 Å in the tetrahydrides, which
is a difference typical of these metals. However, the U-H(H2)
distance of 2.38 Å is much shorter than the corresponding Th-
H(H2) distance of 2.53 Å. The average H2 submolecule binding
energies (2 kcal/mol for the six H2 in the UH4 complex3 and 4
kcal/mol for the four in the present ThH4 complex), however,
suggest a stronger interaction in the ThH4 complex.
To understand the bonding, we have calculated natural
charges and natural electron configurations using NBO17b for
the group 4 metal tetrahydrides, ThH4, and UH4, and the results
are listed in Table 4. We have found the Th-H bond in ThH4
to be the most polarized using both B3LYP and BPW91 density
functionals and different basis sets, which results in strong
intermolecular hydrogen bonding when a H2 molecule is
coordinated to ThH4. On the other hand, the hydride in ThH4 is
the most electronegative in these metal tetrahydrides, which also
interacts with H2 giving additional stabilization energy. Our
calculation for ThH4(H2) gave a 1.949 Å distance between H(Th)
and H(H2) (as shown in Figure 7), which is much shorter than
the normal H....H contact (2.4 Å),25 and this unusually short
distance suggests an attractive interaction between nonequivalent
hydrogen atoms in the ThH4(H2) complex. In fact, the coplanar
bridge-bonded structure of the dihydrogen molecule and of the
two hydride centers demonstrates an attractive interaction;
Figure 7. Structures for thorium hydrides calculated at the B3LYP/
SDD/6-311++G(3df,3pd) level of theory.
otherwise, the dihydrogen molecule would torque out of this
plane, as in other such complexes,3,6 which are dominated by
dihydrogen interaction with the metal centers. This attractive
interaction with short H...H distance between oppositely charged
hydrogen centers has been defined as a new type of hydrogen
bond,30 and several dihydrogen-bonded molecules with this type
of bonding interaction have been found.31
The nature of this interaction is revealed by comparison of
the natural charges for ThH4 and the ThH4(H2)4 complex (Table
4). The addition of the four dihydrogen ligands slightly elongates
the Th-H bonds and increases the positive charge at the thorium
center, and part of the negative hydride charge is shared with
the dihydrogen ligands. The 7s character in the configuration
decreases while the 5f character increases. The CASSCF
electron density plots in Figure 8 further show evolution of the
interaction. The smallest iso density (0.02) reveals a weak
interaction with the metal center, but as the iso density increases,
interaction with the H’s is also apparent.
The uranium tetrahydride has similar properties, but the U-H
bond polarization is less than the Th-H bond on the basis of
the natural charge calculation. As a result, the attractive
interaction in UH4(H2)6 depends on interaction with the uranium
metal center. The M-H bond polarization for d-type metal
tetrahydrides (TiH4, ZrH4, and HfH4) is much less, and weak
attractive interactions are expected in these molecules.
Inspection of the CASSCF wave function for both the U and
Th complexes shows that the ground state is a singlet in both
ThH4 and ThH4(H2)4, while it is a triplet in both UH4 and UH4-
(H2)6. There is not a change in the ground state in going from
the actinide-tetrahydride to the supersystem, unlike the more

1760 J. Phys. Chem. A, Vol. 112, No. 8, 2008
Wang et al.
Figure 8. Electron density plots for thorium tetrahydride and the tetradyhydrogen complex calculated at the CASSCF level of theory. Iso density
values given below.
Figure 9. Electron density plots for thorium tetrahydride tetradihydrogen (left) and uranium tetrahydride hexadihydrogen (right) complexes calculated
at the CASSCF level of theory. (The sixth dihydrogen is directly behind the U center.) Iso density 0.04 e/au3.
strongly bound WH4 case currently under investigation.6b In the
Th case, the bonding orbitals between Th and H(hydrides) are
mainly 6d on the Th, while in the U case there is a significant
5f U component. The interaction between Th/U and the H2
moieties seems slightly different for the two actinides: in the
Th case, all the H(H2) bear a partial charge of about +0.02,
while in the U case the H(H2) bear a partial charge that varies
from -0.03 to +0.04. Overall, they are more polarized. The
comparison of the total electronic densities at the same 0.04
isovalues shows that the electronic density delocalizes the bond
region more in the U case than in the Th case (Figure 9).
CHE03-52487, NCSA computing Grant CHE07-0004N, and
Swiss National Science Foundation Grant 200021-111645/1.
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