Infrared Spectra of the HAnX and H2AnX2 Molecules (An=Th and U, X=Cl and Br) in Argon Matrices Supported by Electronic Structure Calculations

Article abstract of DOI:10.1002/chem.201805372

Uranium and thorium hydrides are known as functional groups for ligand stabilized complexes and as isolated molecules under matrix isolation conditions. Here, the new molecular products of the reactions of laser-ablated U and Th atoms with HCl and with HBr, namely HUCl, HUBr and HThCl, HThBr, based on their mid and far infrared spectra in solid argon, are reported. The assignment of these species is based on the close agreement between observed and calculated vibrational frequencies. The H?U and U?³⁵Cl stretching modes of HUCl were observed at 1404.6 and 323.8 cm^?1, respectively. Using DCl instead to form DUCl gives absorption bands at 1003.1 and 314.7 cm^?1. The corresponding bands of HThCl are 1483.8 (H?Th) and 1058.0 (D?Th), as well as 340.3 and 335.8 cm^?1 (Th?³⁵Cl), respectively. HUBr is observed at 1410.6 cm^?1 and the BP86 computed shift from HUCl is 6.2 cm^?1 in excellent agreement. The U?H stretching frequency increases from 1383.1 (HUF), 1404.6 (HUCl), 1410.6 (HUBr) to 1423.6 cm^?1 (UH) as less electronic charge is removed from the U?H bond by the less electronegative substituent. These U?H stretching frequencies follow the Mayer bond orders calculated for the three HUX molecules. A similar trend is found for the Th counterparts. Additional absorptions are assigned to the H₂AnX₂ molecules (An=U, Th, X=Cl, Br) formed by the exothermic reaction of a second HX molecule with the above primary products.

Full text of DOI:10.1002/chem.201805372

DOI: 10.1002/chem.201805372
Full Paper
&
Actinides
Infrared Spectra of the HAnX and H2AnX2 Molecules (An=Th and
U, X=Cl and Br) in Argon Matrices Supported by Electronic
Structure Calculations
Lin Li+,[a] Tony Stꢀker+,[a] Lester Andrews,*[a, b] Helmut Beckers,*[a] and Sebastian Riedel[a]
35
Abstract: Uranium and thorium hydrides are known as func-
tional groups for ligand stabilized complexes and as isolated
molecules under matrix isolation conditions. Here, the new
molecular products of the reactions of laser-ablated U and
Th atoms with HCl and with HBr, namely HUCl, HUBr and
HThCl, HThBr, based on their mid and far infrared spectra in
solid argon, are reported. The assignment of these species is
based on the close agreement between observed and calcu-
340.3 and 335.8 cmꢀ1 (Thꢀ Cl), respectively. HUBr is ob-
served at 1410.6 cmꢀ1 and the BP86 computed shift from
HUCl is 6.2 cmꢀ1 in excellent agreement. The UꢀH stretching
frequency increases from 1383.1 (HUF), 1404.6 (HUCl), 1410.6
(HUBr) to 1423.6 cmꢀ1 (UH) as less electronic charge is re-
moved from the UꢀH bond by the less electronegative sub-
stituent. These UꢀH stretching frequencies follow the Mayer
bond orders calculated for the three HUX molecules. A simi-
lar trend is found for the Th counterparts. Additional absorp-
tions are assigned to the H2AnX2 molecules (An=U, Th, X=
Cl, Br) formed by the exothermic reaction of a second HX
molecule with the above primary products.
35
lated vibrational frequencies. The HꢀU and Uꢀ Cl stretching
modes of HUCl were observed at 1404.6 and 323.8 cmꢀ1, re-
spectively. Using DCl instead to form DUCl gives absorption
bands at 1003.1 and 314.7 cmꢀ1. The corresponding bands
of HThCl are 1483.8 (HꢀTh) and 1058.0 (DꢀTh), as well as
Introduction
of the 5f and 6d orbitals, which makes the former hybridiza-
tion unfavorable on bending, and thus UO2 is a linear mole-
3
Early actinide metal atoms and their reactions with small mole-
cules have attracted great interest in part because 5f electrons
are involved in their bonding whereas 4f electrons remain in
the core for lanthanide metals.[1] It is also of interest to com-
pare the relative participation of 5f electrons in the bonding
for analogous molecules bearing Th and U atoms. The dioxides
are a good case in point. A linear configuration for ThO2 would
have strong Th 6d-O 2p bonds. However, ThO2 is bent because
the 5f is higher in energy than the 6d so that 5f–6d hybridiza-
tion provides for lowering the energy on bending.[2] In UO2,
there is increasing difference between the sizes and energies
cule.[2] Uranium dioxide UO2 has a F2,u ground state made up
of the (7s)1(5f(f))1 configuration.[3,4]
Uranium reacts with hydrogen to form the UH3 solid, which
is a useful reagent for the synthesis of uranium compounds.[5,6]
Thorium is quite electropositive and it oxidizes readily in air.[5]
Uranium and thorium hydrides are known as functional groups
for ligated complexes[5] and as isolated molecules under matrix
isolation conditions.[7–9] Simple binary uranium and thorium hy-
dride molecules were first prepared by reaction of the laser-ab-
lated metal atoms with H2 diluted in argon with condensation
at 7 K. Matrix infrared spectra were obtained for isolated UHx,
and ThHx (x=1–4) molecules, and a hydrogen-bridge-bonded
molecule identified as U2H2 was reported in 1997.[7–9] Later
work using hydrogen as the matrix and reagent also produced
uranium and thorium hydride complexes involving side-bound
dihydrogen molecules.[9,10]
[a] L. Li,+ T. Stꢀker,+ Prof. em. Dr. L. Andrews, Dr. habil. H. Beckers,
Prof. Dr. S. Riedel
Institut fꢀr Chemie und Biochemie
Freie Universitꢁt Berlin
Fabeckstr. 34–36
Berlin (Germany)
E-mail: h.beckers@fu-berlin.de
The first monomeric monohydride complexes for thorium
and uranium, HTh[N(SiMe3)2]3 and HU[N(SiMe3)2]3 metal(IV)
state compounds, were communicated in 1979 by Turner,
Simpson and Andersen.[11] These workers next discovered a
novel H–D exchange process for the uranium derivative by stir-
ring a pentane solution under D2 gas which gave complete ex-
change for D and was reversible with H2 gas over pressure.[12]
The thorium hydride compound was also found to undergo
complete and reversible exchange under similar conditions.[12]
In addition to their importance for the synthesis of novel Th
and U complexes, these monomeric monohydrides reveal
[b] Prof. em. Dr. L. Andrews
Chemistry Department
University of Virginia
Charlottesville VA, 22904-4319 (USA)
E-mail: lsa@virginia.edu
[+] L. Li and S. Stꢀker contributed equally to this work.
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201805372. It contains additional tables of
frequencies and figures of spectra and Cartesian coordinates.
Chem. Eur. J. 2019, 25, 1 – 12
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good vibrational spectra for comparison to the HThꢀCl, HUꢀCl,
HThꢀBr and HUꢀBr, which are subjects of this paper. The
HTh[N(SiMe3)2]3 compound revealed a ThꢀH stretching mode
at 1480 cmꢀ1 and the DTh[N(SiCD3)2]3 isotope ThꢀD stretching
mode shifted to 1060 cmꢀ1. Their ratio 1480:1060=1.396 con-
firmed these absorptions as heavy metal hydride and deuter-
ide stretching modes. Likewise for the HU[N(SiMe3)2]3 derivative
the UꢀH stretching mode was assigned as 1430 cmꢀ1 and the
deuterated material gave UꢀD modes at 1020 and 1027 cmꢀ1
in the two communications.[11,12] Taking the average to calcu-
late the UꢀH/UꢀD frequency ratio, 1430:1023=1398, which is
again characteristic of a heavy metal hydride vibrational mode,
and a verification of their vibrational assignments. Our first ad-
venture into hydrogen halide reactions with these early acti-
nide atoms gave HꢀUF and DꢀUF absorption bands at 1383.1
and 988.3 cmꢀ1 and a 1.399 frequency ratio.[13,14] Likewise for
HꢀThF and DꢀThF with frequencies 1464.8 and 1046.0 cmꢀ1
the same frequency ratio (1.400) for these heavy metal hy-
drides was obtained.[15]
uct yield and better signal-to-noise spectra. The 1064 nm fun-
damental of a Nd:YAG laser (10 Hz repetition rate with 10 ns
pulse width, and a pulse energy up to 55 mJ) was focused
onto the rotating metal targets. Infrared (IR) spectra were re-
corded at a resolution of 0.5 cmꢀ1 on a FTIR vacuum spectrom-
eter (Bruker Vertex 80v) equipped with a transfer optic, Mid-IR
MCT detector (4000–450 cmꢀ1) or liquid helium cooled FIR bol-
ometer (680–180 cmꢀ1). Matrix samples were annealed at dif-
ferent temperatures, and selected samples were irradiated by
455 or 365 nm LED light. DCl for these experiments was syn-
thesized by reacting D2O with SiCl4: D2O (0.2 mL) was slowly
added to SiCl4 (3 mL) at ꢀ788C using a dry ice/ethanol bath.
The bath was removed and the system slowly warmed back to
room temperature. The DCl gas was passed through a trap
cooled to ꢀ1208C and collected by two ꢀ1968C liquid nitro-
gen traps.[23]
Preliminary calculations at the density functional theory
(DFT)[24] level using the Turbomole V7.1 program[25] were per-
formed with the GGA BP86[26–28] exchange-correlation or the
B3LYP[29] hybrid exchange-correlation functionals. Since both
methods produced structures and vibrational frequencies in
good agreement with our CCSD(T) values of HThX, the faster
and simpler BP86 GGA functional combined with the triple-z
basis set def-TZVP[30,31] for all elements was finally used for all
calculations. Scalar relativistic effects were included by employ-
ing effective core potentials (def-ECP)[31] with 60 electrons at U
and Th. Spin-orbit coupling effects were not incorporated. For
the HAnX (An=U, Th and X=Cl and Br) species calculations
were supplemented by single point calculations at the CCSD(T)
level with the augmented triple-z basis set aug-cc-pVTZ[32] on
H, Cl and Br, and the cc-pVTZ-PP[33] basis set on U and Th em-
ploying the small-core pseudo potential ECP60MDF.[34] All
CCSD(T) calculations for HThX species were performed using
the CFOUR V1 program,[35] whereas Molpro 2015.1[36] was used
for all HUX species. The 2, 10, 18, 8 and 8 inner electrons on F,
Cl, Br, Th and U were excluded from the correlation treatment
in CFOUR, while the default settings were used in Molpro. All
obtained electronic ground state configurations obeyed the
“Aufbau principle” with positive HOMO–LUMO gaps. The vibra-
tional analyses were carried out in the harmonic approximation
for all molecules and isotopologues. In order to estimate the
amount of the anharmonic correction to the vibrational fre-
quencies of the 1,2HAnX molecules, additional anharmonic cor-
rections were calculated at the CCSD(T) level for 1,2HThCl using
vibrational second-order perturbation theory[37,38] (VPT2). Calcu-
lations of AIM charges,[39] electron densities at the bond critical
point[39] (1b) and Mayer Bond Orders[40] were carried out using
Multifwn 3.5[41] on the basis of the BP86 wave functions.
A few more examples of Th and U hydride complexes will
be summarized here. The synthesis and molecular structure of
a novel uranium(III) bisphosphine hydride have been reported.
Of particular interest here is the very strong infrared absorp-
tion (1219 cmꢀ1) for the hydride, which shifted (870 cmꢀ1) in
the analogue prepared with D2.[16] Although these absorptions
are substantially lower (211 and 150 cmꢀ1) than found for the
above metal(IV) complexes, the H/D frequency ratio (1.40) veri-
fies that they are mechanically due to a relatively pure U and
H vibration. Thus, there must be some interaction within this
complex that reduces the UꢀH bond stretching force constant
(and likely also the bond energy) relative to those for the
above metal(IV) complexes.
Another terminal monomeric UꢀH complex has been pre-
pared by reacting [U(COTTIPS2)Cp*R] (COTTIPS2 =C8H6(SiiPr3-1,4)2)
with H2. This uranium(IV) hydrido complex, [HU(COTTIPS2)Cp*], is
special for its instability with respect to hydrogen loss.[17] In
order to find out if tris(pentamethyl cyclopentadienyl) chemis-
try would be possible with thorium, the (C5Me5)3ThH complex
was prepared.[18] Again Th and U analogues have been synthe-
sized this time for [(C5Me5)2MH2]2).[19] In this case the trivalent
monohydride exists as an equilibrium mixture with the tetrava-
lent dihydride. A thorium(III) monohydride complex has even
been prepared in a bimetallic system.[20] Only recently isolable
crystalline molecular complexes of UII have been reported.[21]
The scarce nature of ligated UII complexes makes our matrix
isolated HAnX molecules (An=Th, U; X=F, Cl, Br) an even
more important contribution to early actinide metal chemistry.
Experimental and Theoretical Methods
Computational Results
Laser-ablated U and Th atoms were reacted with the HCl and
HBr gases (Linde AG) in argon host gas during their deposition
at 12 K using a closed-cycle helium refrigerator (Sumitomo
Heavy Industries, RDK-205D) inside of a self-made vacuum
chamber, which has been described in more detail in our pre-
vious works.[14,15,22] Early experiments with U and HCl were also
done with 5 and 15 K substrates, but 12 K gave a higher prod-
All possible spin states have been considered for the HAnX
and H2AnX2 (An=U, Th and X=Cl and Br) species. Table 1 lists
fully optimized structures obtained at the DFT BP86 level for
all HAnX species and zero-point-energy (ZPE) corrected DFT
relative energies for their low-lying electronic states including
the configuration of the lowest four MOs, as well as CCSD(T)
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electronic energies obtained by the DFT calculations predict
several low-lying excited electronic states for both, the HUX
and H2UX2 species due to the presence of partially filled near-
degenerate metal centered d- and f-type orbitals. Uranium-
centered spin densities for HUF, HUCl and HUBr are depicted
in Figure S1. Although due to their multi-reference nature the
electronic configuration of the HUX molecules cannot unam-
biguously be evaluated at the DFT level, the valence molecular
orbital Scheme of HUX shown in Figure 1 reveals that the
singly occupied valence molecular orbitals are energetically
well separated from the binding HꢀU and XꢀU MOs, suggest-
ing that the strong static correlated electrons have little influ-
ence on the molecular structure (see Table 1). As shown in
Table 3 for two selected low-lying electronic states of the HUX
molecules the different HUX states have very similar computed
vibrational frequencies. While our computed frequencies can
be safely assigned to a given matrix-isolated HUX species (see
below), neither our experimental frequencies nor the DFT re-
Table 1. Calculated structures and ZPE corrected relative energies of low-
lying electronic states of HAnX molecules (X=Cl, Br; An=Th, U) at the
BP86 (normal font) and CCSD(T) (italic font) level.
Bond length [ꢂ] Angle [8] Erel [kJmolꢀ1
HꢀAn AnꢀX HAnX
CCSD(T)[a] DFT
]
Electronic state
HThCl
X1A’ (16a’2 6a’’2 17a’2 18a’2)
2.021
2.035
2.060
2.071
2.509 126.7
2.536 129.5
2.549 115.5
2.559 125.2
0
0
5
A3A’’ (6a’’2 17a’2 18a’1 7a’’1)
B3A’ (6a’’2 17a’2 18a’1 19a’1)
HThBr
33
39
11
X1A’ (22a’2 9a’’2 23a’2 24a’2)
2.021
2.673 129.4
0
0
2.034
2.696
131.3
A3A’’ (9a’’2 23a’2 24a’1 10a’’1) 2.058
2.714 116.7
2.723 126.4
33
35
3
8
B3A’ (9a’’2 23a’2 24a’1 25a’1)
2.068
2.037
HUCl
X5A’ (18a’1 7a’’1 8a’’1 19a’1)
2.527 104.1
2.522 101.3
2.523 101.7
2.505 106.2
0
0
2
–
0
5
0
30
5
A A’ (18a’1 19a’1 20a’1 21a’1) 2.036
B5A’’ (18a’1 19a’1 7a’’1 20a’1)
3A’ (17a’2 18a’2 7a’’1 8a’’1)
2.034
2.025
HUBr
5
X A’ (10a’’1 24a’1 11a’’1 25a’1) 2.032
2.695 105.7
2.695 105.7
2.690 102.9
2.671 106.4
0
0
3
–
2
4
0
31
A5A’ (24a’1 25a’1 26a’1 27a’1) 2.032
5
B A’’ (24a’1 25a’1 10a’’1 26a’1) 2.032
3A’ (23a’2 24a’2 10a’’1 11a’’1)
2.025
[a] CCSD(T) relative energies (Erel) based on CCSD(T) single point energies
at BP86 minimum structures. A plot of selected frontier orbitals is shown
in Figure S2.
single point relative energies. For the HUX species the elec-
tronic ground state energies were obtained at the CCSD(T)
level by single point calculations at fully DFT optimized struc-
tures, while for the HThX species CCSD(T) optimized electronic
ground state structures are given. Since structure relaxations
only accounted for 0.6 and 0.4 kJmolꢀ1 energy difference for
HThCl and HThBr, it can be assumed the error of the ZPE cor-
rected relative CCSD(T) energies is well below 1 kJmolꢀ1 due
to error cancelation. Table 2 contains electronic configurations
and structures of the H2AnX2 species, as well as relative ener-
gies of low-lying electronic states of the H2UX2 species. The
Figure 1. Valence molecular orbital diagram of HUF and HUBr calculated at
the RO-BP86 level of theory. Atomic orbital energies of s(H) and p(F/Br) are
adjusted to meaningful positions. Atomic orbital participations have been
indicated with full lines (>10%), dashed lines (3–10%) and dotted lines (1–
3%).
Table 2. Calculated structures and ZPE corrected relative energies of lowest and low-lying electronic states of H2ThX2 and H2UX2 molecules (X=Cl, Br) at
the BP86 level.
Bond length [ꢂ]
MꢀAn
Angle [8]
HAnH
Erel
[kJmolꢀ1
]
Electronic state (configuration) [Point group]
HꢀAn
HAnX
XAnX
H2ThCl2
1A1 (C2v)
H2ThBr2
1A1 (C2v)
2.067
2.062
2.567
2.721
107.8
107.8
107.2
107.2
118.2
117.9
–
–
H2UCl2
X3B (17a2 16b2 18a1 17b1) [C2]
A3A’ (19a’2 20a’2 21a’1 22a’1) [Cs]
1.998
1.994, 1.997
2.005
2.514
2.512
2.515
2.512
101.7, 107.9
105.9, 107.9
110.6
109.8
108.1
104.0
114.7
127.5
120.7
110.2
136.0
0
1
3
7
2
2
1
B3B2 (14a1 6b1 11b21 15a1 ) [C2v]
2
2
1
1
C3A2 (14a1 6b1 11b2 7b1 ) [C2v]
2.006
101.7
H2UBr2
X3A’’ (28a’2 29a’2 23a’’1 30a’1) [Cs]
A3A’ (28a’2 29a’2 30a’1 31a’1) [Cs]
B3A’’ (36a’2 15a’’2 37a’1 16a’’1) [Cs]
1.986, 1.990
1.995, 1.990
1.992
2.678
2.672
2.678, 2.666
2.678
105.7, 100.5
108.6, 105.3
104.5, 108.3
102.4
105.3
107.3
107.5
106.6
135.8
121.0
122.9
138.1
0
0
0
2
2
1
1
C3A2 (20a1 9b1 7a2 21a1 ) [C2v]
1.993
10
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Table 3. HAnX molecules (X=F, Cl, Br; An=Th, U): AIM charges, Mayer bond orders (B.O.) and electron densities at the bond critical points (1b) obtained
at the BP86/def-TZVP level.
ꢀ1
~
n(H-An) [cm
]
AIM charge
M
Mayer B.O.
1b [au]
State
Obs.(Ar)
calcd
H
X
HꢀM
XꢀM
HꢀM
XꢀM
HUF
X5A’
B5A’’
X5A’
B5A’’
X5A’
B5A’’
1407.8
1403.9
1425.7
1428.5
1432.0
1427.8
ꢀ0.553
ꢀ0.555
ꢀ0.549
ꢀ0.548
ꢀ0.551
ꢀ0.549
1.240
1.241
1.190
1.184
1.167
1.162
ꢀ0.687
ꢀ0.685
ꢀ0.640
ꢀ0.637
ꢀ0.618
ꢀ0.613
0.926
0.928
0.936
0.937
0.939
0.939
1.095
1.100
1.233
1.238
1.213
1.216
0.0879
0.0879
0.0904
0.0907
0.0906
0.0912
0.1346
0.1360
0.0836
0.0834
0.0694
0.0704
1383.1
1404.6
1410.6
HUCl
HUBr
HU
1423.6
1464.8
1483.8
1485.4
1485.2
HThF
HThCl
HThBr
HTh
X1A’
X1A’
X1A’
1507.2
1515.5
1518.2
ꢀ0.544
ꢀ0.543
ꢀ0.545
1.237
1.156
1.112
ꢀ0.693
ꢀ0.613
ꢀ0.575
0.934
0.940
0.940
1.259
1.416
1.415
0.1047
0.1060
0.1062
0.1421
0.0944
0.0807
sults allow an unambiguous conclusion on the electronic
ground states of the observed species. Our preliminary DFT cal-
culation yield singlet and quintet ground state spin multiplici-
ties for the HThX and HUX species, respectively, with their
lowest triplet states between 30 and 39 kJmolꢀ1 above the
ground state (Table 1).
Table 4. Percentage of f-orbital participations in the HꢀAn and XꢀAn
bonds obtain by orbital composition analysis with Mulliken partition.
Bond
HUF
HUCl
HUBr
HThF
HThCl
HThBr
HꢀAn
XꢀAn
2.88
3.13
2.95
2.11
3.12
1.97
5.51
4.34
7.11
3.29
7.97
2.85
Table S1 contains calculated vibrational frequencies and in-
tensities employing a harmonic approximation at both, the
DFT BP86/def-TZVP (all HAnX species, X=Cl, Br) and CCSD(T)
(only HThX) levels for all H/D, 35/37Cl and 79/81Br isotopologues.
Additional anharmonic vibrational analysis were performed for
1,2HThCl at the CCSD(T) level using vibrational second-order
perturbation theory[37,38] (VPT2). The anharmonic corrections to
the HꢀTh and DꢀTh stretching modes amounts to ꢀ39 and
ꢀ20 cmꢀ1, respectively, and it can be assumed that these an-
harmonic corrections can approximately be applied to other
computed harmonic 1,2HꢀAn vibrational stretching frequencies
as well. We note that our calculated HꢀU stretching frequen-
cies of the HUX molecules at the BP86/def-TZVP level increases
from 1407.8 (HUF), 1425.7 (HUCl) to 1427.8 cmꢀ1 (HUBr), which
suggests an increasing strength of the corresponding HꢀU
bond. This conclusion is supported by the Mayer Bond
Order[40] which increases from 0.926 (HUF), 0.936 (HUCl) to
0.939 (HUBr), as well as by an increasing electron density at
the bond critical point (1b)[39] between U and H in the different
species (Table 3). It rises from 0.0879 (HUF), 0.0904 (HUCl) to
0.0906 (HUBr) as less electronic charge is removed from the
UꢀH bond by the less electronegative substituent. The same
effect is observed for the HThX species, with a less pro-
nounced impact on the Mayer Bond Order. Based on a Mullik-
en partition analysis we also found an increasing f-orbital par-
ticipation in the HꢀAn bonds of the HAnX molecules, with X=
F<Cl<Br, while the opposite is true for the AnꢀX bond,
which decreases going from F to Br (Table 4). The f-orbital par-
ticipation is found to be larger for the Th species.
topologues are listed in Table S3. The H3UX3 species converged
1
to singlet A1 ground states in the C2v point group, which is fa-
vored by 17 and 16 kJmolꢀ1 for H3UCl3 and H3UBr3 over the C3v
isomer (Table S4). No other spin states of the H3UX3 species
were considered. Calculated ground state vibrational frequen-
cies and intensities of the H3UX3 species are listed in Table S5.
The optimized ground state structures for the HAnX, H2AnX2
und H3AnH3 species are depicted in Figure 2 and listed in
Table S6.
Experimental Results
Figure 3 illustrates IR spectra for the major products from the
reactions of laser ablated U atoms with HCl and with HBr in
ꢀ
excess argon. The strongest bands are the HCl2 features at
ꢀ
697.0 and 956.2 cmꢀ1 and the HBr2 bands at 728.3, 892.6 and
1054.7 cmꢀ1, which were also observed with Th.[42–45] The laser
ablation process generates vacuum-UV, UV and visible light,
which irradiates the condensing sample, and metal cations and
electrons in addition to the major flux of metal atoms that em-
anate from the laser focal point. The first strong bands are due
ꢀ
to the HCl2 anion made by reaction of Clꢀ with HCl where
chloride anion arises from the Cl atom photolysis product after
ꢀ
electron capture.[42] The HBr2 anion is formed likewise.[43,44]
ꢀ
The DCl2 counterparts are observed in Figure 4(c) at 463.9
and 729.1 cmꢀ1. Other bands at 775.5 and 1051.0 cmꢀ1 are due
to UO2[46]and UN2[47] from the U atom reaction with trace air im-
purity in our vacuum system. The weak 819.5 band is due to
UO.[46] The sought reaction product at 1404.6, labeled HUCl in
Figure 3, exhibits shoulder satellite absorptions at 1422.4 and
1388.8 cmꢀ1, and the HUBr counterpart at 1410.6 cmꢀ1 shows
side bands at 1425.6, 1389.7 and also 1388.8 cmꢀ1. The blue-
The lowest electronic states for the H2AnX2 species have
been found to be singlet and triplet states for thorium and
uranium, respectively (Table 2, Table S2). No convergence was
reached for singlet states of H2UX2 species. All vibrational fre-
quencies of the H2An2X species and their H/D substituted iso-
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Figure 3. Infrared spectra of the reaction products of laser-ablated U with
HCl (top) and HBr (bottom) (0.5%) each seeded in excess Ar and co-deposit-
ed for 150 min at 12 K. The 1023.1 band is due to an impurity in the HBr
sample.
Figure 4. Infrared spectra of the reaction products of laser-ablated U with
HCl and with a DCl/ HCl mixture seeded in excess Ar and co-deposited for
150 min at 12 K: (a) U+HCl (0.5%) in Ar, (b) difference spectrum after an-
nealing to 25 K, (c) U+DCl/HCl approximately 1:1 (0.5%) mixture at 12 K in
Ar, (d) difference spectrum after annealing to 35 K.
Figure 2. Structures calculated for HAnX, H2AnX2 and H3UX3 molecules
(An=Th, U; X=Cl, Br) using BP86/def-TZVP.
to 20 K, and they decrease upon exposure to 455 nm light for
15 min, but they increase a little on annealing to 25 K and a
lot more on final annealing to 35 K. The new band at 1410.6
(HUBr) has satellites at 1425.6, 1389.7 and 1388.8 cmꢀ1, and
thus it appears much like the HUCl band at 1404.6 in the top
spectrum of Figure 3.
shifted shoulder band for HUCl appears to be a matrix site as
it is removed by annealing to 25 K, while the sharp 1404.6
band increases slightly (Figure 4(b)). The latter sharp 1388.8
+
band for HO2 and the sharp 952.1 band for UO2 are resolved
in the HBr experiment.[46,48] Other features at 1569.8 and
1531.4 cmꢀ1 for HCl and 1553.1 and 1532.9 for HBr will be dis-
cussed later. From Figure 4(b), the annealing difference spec-
The lower two scans in Figure 4 were taken from the highest
DCl enrichment used with U. The two product bands labeled
HUCl at 1404.6 and DUCl at 1003.1 cmꢀ1 have almost equal ex-
perimental intensities (5/4) but the infrared intensity of the
HUCl mode is calculated to be double that of DUCl, so this
means that the DCl enrichment is 8/5 over HCl. Annealing dif-
ference spectra show that both HUCl and DUCl increase slight-
ly on annealing, but there is no effect from 455 nm LED pho-
tolysis. The very sharp shoulder at 952.1 cmꢀ1, resolved from
ꢀ
trum also shows that the sharp bands for HCl2 increase, but
the broad blue wing on the 697.0 band decreases. The UO2 ab-
sorption also increases slightly as this U atom reaction with O2
is spontaneous.[3,46]
Figure S3 shows annealing spectra from the U and HBr ex-
periment. The very strong 728.3, 892.6 and 1054.7 cmꢀ1 bands
ꢀ
ꢀ
+ [3,46]
are due to the HBr2 anion[43] where a second combination
the 956.2 cmꢀ1 band of the HCl2 anion, is due to UO2
,
band is also observed: these absorptions increase on annealing
which is shown more clearly in the Figure 3 bottom spectrum.
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Figure 6. Infrared spectra of the reaction products of laser-ablated Th with
HCl (top spectrum) and HBr (bottom spectrum) (0.5%) each seeded in argon
at 12 K and co-deposited for 150 min.
Figure 5. IR spectra of the reaction products of laser-ablated U with HCl/DCl
seeded in excess of Ar co-deposited for 150 min at 12 K: (a) U+HCl (0.5%)
co-deposited in Ar, (b) difference spectrum after annealing to 25 K.
(c) U+DCl/HCl (0.5%) co-deposited in Ar, (d) difference spectrum after an-
nealing to 25 K. Noise spikes of the bolometer are marked by an asterisk.
Spectra were recorded in the far IR region using the bolome-
ter detector and plotted for U and HCl in the 260–360 cmꢀ1
region, Figure 5. It is clearly more difficult to obtain good spec-
tra of matrix isolated species in this region. The deposited
sample revealed absorptions in the 320 cmꢀ1 region, which in-
creased and sharpened after annealing to 25 K. The band at
323.8 cmꢀ1 exhibited the same behavior on annealing as the
1404.6 cmꢀ1 band labeled HUCl in Figure 3. The band at
313.3 cmꢀ1 is assigned to UCl2 based on agreement with a
312 cmꢀ1 band assigned to UCl2 from the U+Cl2 reaction, and
the 296 cmꢀ1 nitrogen matrix band assigned to UCl2 using two
different methods of production.[49] The 15 cmꢀ1 red matrix
shift for PbCl2 in this region allows a prediction of 311 cmꢀ1 for
UCl2 in solid argon with the reasonable assumption of the
same N2 to Ar matrix shift for PbCl2 and UCl2. Blue photolysis
at 455 nm (Figure S4) appeared to increase both bands. The
UCl4 band at 345 cmꢀ1 is not observed.[50] Similar experiments
were done with HCl/DCl samples and the spectra are shown in
Figure 5(c),(d). Unfortunately, our BP86 calculation predicts a
9 cmꢀ1 red shift for DUCl which puts this band near UCl2. The
25 K annealing difference spectrum shows increases in the
323.8, 314.7 and UCl2 bands.
Figure 7. IR spectra of the reaction products of laser-ablated Th with DCl/
HCl at 3:1 in excess Ar. (a) Th+DCl/ HCl (0.5%) in Ar co-deposited for
180 min at 12 K, (b) difference spectrum after annealing to 30 K.
Figure 7 illustrates the spectra from laser ablated Th reac-
tions with a DCl/HCl 3:1 mixture and with DCl. The same
ꢀ
ꢀ
strong HCl2 and DCl2 anion bands were observed (not
shown). The primary new product feature labeled HThCl at
1483.8 cmꢀ1 exhibited a series of satellites beginning at 1463.0
and the DThCl counterpart at 1058.0 with a sharp well-resolved
satellite at 1044.4 and a 1067.4 cmꢀ1 shoulder. The HThCl/
DThCl frequency ratio, 1.4024, is appropriate for a heavy metal
hydride stretching mode. Unlike the uranium counterparts,
there are no significant bands to higher wavenumbers of the
main bands. Annealing to 20 K increased the primary feature
slightly (not shown), but annealing to 30 K decreased the
1483.8 band and increased the 1454.5 absorption in the ThꢀH
stretching region and decreased the 1058.0 band and in-
creased the 1044.4 band in the ThꢀD region. Figure S5 shows
annealing spectra from the Th and HBr experiment.
Figure 6 compares products from the laser-ablated Th atom
reaction with HCl and HBr. Again, the hydrogen dihalide
anions were observed like in the U atom reaction spectra as
they were made from the common reagents with photons and
electrons added in the ablation process. These bands increased
on sample annealing. Sharp bands at 735.0 and 787.5 cmꢀ1 are
due to ThO2 and the band at 876.7 cmꢀ1 arises from ThO.[8,51]
The ThO2 bands increase slightly on annealing as the Th reac-
tion with O2 is a spontaneous process. The important new
band using HCl appears at 1483.8 with clear satellites lower at
1436.8, 1446.3, and 1463.0 cmꢀ1: the latter bands increase very
slightly on annealing to 20 K. The new band is at 1485.6 with
satellite at 1464.3 cmꢀ1 using HBr. Both spectra contained the
1388.8 spike for HO2.[48]
Far IR spectra were recorded in several experiments with
thorium in different samples. The top two spectra in Figure 8
are for the Th+Cl2 reaction with product absorptions at 340.5
and 330.5 cmꢀ1. The higher band is in good agreement with
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likely HThCl also absorbs at 340.3 cmꢀ1. Annealing to 25 K does
increase the 335.8 band and decrease ThCl2 as before, but
judgment on the 340.3 band is not clear.
Discussion
The new metal bearing product absorptions will be assigned
and compared with computed frequencies and energies for
their formation. Tables 5 and 6 list the observed and computed
frequencies for the major product HAnX molecules (An=Th, U;
X=Cl, Br). Bonding trends in these chemical series will be con-
sidered.
HUCl: The major new product absorption with uranium and
hydrogen chloride appears at 1404.6 cmꢀ1 upon sample co-
deposition. Annealing to 25 K resulted in a 20% increase in
Figure 8. IR spectra of the reaction products of laser-ablated Th with Cl2 or
HCl seeded in excess of Ar co-deposited at 12 K: (a) Th+Cl2 (0.5%) in Ar,
(b) difference spectrum after annealing to 30 K, (c) Th+HCl (0.5%) in Ar,
(d) difference spectrum after annealing to 20 K. Noise spikes of the bolome-
ter are marked by an asterisk.
Table 5. Comparison of observed and computed harmonic vibrational
frequencies (in cmꢀ1) for the HUCl, DUCl and HUBr.[a]
Exptl (argon)
Calcd
BP86/def-TZVP
the Beattie observation of bands at 340.4 and 342.7 cmꢀ1 for
ThCl4 in solid argon.[50] We are not aware of any spectra for
ThCl2 so our 330.5 cmꢀ1 absorption is a good bet for ThCl2 in
solid argon. Our BP86 calculations predicted the strongest
band for ThCl2 to be 20 cmꢀ1 lower than that for ThCl4 and our
bands are 10 cmꢀ1 apart. Annealing to 30 K decreased the
lower band with little effect on the higher absorption. The
next two spectra come from an experiment with Th and HCl
and the higher band is stronger than the lower band (Fig-
ure 8(c),(d)), the reverse of the top scan using Cl2. This opens
the possibility that something else like HThCl could be on top
of a weak ThCl4 band. Annealing to 20 K decreased both
bands, but ThCl4 did not decrease in scan (b) so HThCl might
be sharing the absorption at 340.3 cmꢀ1. Two spectra from an
experiment with Th and HCl/DCl are shown in Figure 9. The
first thing to notice is a new band at 335.8 cmꢀ1 and the 330.5
band is as strong as the ThCl2 band. Thus, assignment of the
new band at 335.8 cmꢀ1 to DThCl is straightforward and quite
HU35Cl
HU37Cl
DU35Cl
DU37Cl
HU79Br
HU81Br
1404.6
323.8
1404.65
–
1003.1
314.7
1003.1
–
1410.6
–
1410.6
–
1425.79 (326)
328.66 (89)
1425.79 (326)
322.70 (90)
1010.79 (164)
319.62 (54)
1010.79 (164)
312.32 (53)
1432.02 (340)
282.78 (87)
1432.02 (340)
282.57 (86)
[a] A complete list of the computed frequencies is given in Table S1. Com-
puted intensities are given in parentheses (kmmolꢀ1).
Table 6. Comparison of observed and computed harmonic vibrational
frequencies (in cmꢀ1) for the HThCl, DThCl and HThBr molecules.[a]
Exptl (argon)
Calcd
CCSD(T)/aVTZ(-PP)
BP86/def-TZVP
1515.61 (363)
HTh35Cl
1483.8
340.3
1541.19 (384)
1502.24 (379)
352.53 (55)
346.15 (57)
349.92 (66)
HTh37Cl
DTh35Cl
1483.8
–
1058.0
1515.60 (363)
342.27 (65)
1074.39 (183)
1541.19 (384)
346.51 (50)
1092.54 (193)
1072.83 (191)
342.71 (49)
338.7 (49)
335.8
346.35 (51)
DTh37Cl
HTh79Br
HTh81Br
1058.0
1074.38 (183)
338.29 (49)
1518.33 (305)
270.08 (27)
1518.32 (305)
269.71 (27)
1092.53 (53)
334.85 (47)
1543.80 (416)
309.00 (16)
1543.80(416)
308.85(15)
–
1485.4
–
1485.4
–
Figure 9. IR spectra of the reaction products of laser-ablated Th with HCl/
DCl at 2/1 in excess Ar co-deposited at 12 K: (a) Th+DCl/HCl (0.5%) in Ar,
(b) Th+DCl/HCl (0.5%) difference spectrum after annealing to 20 K. Noise
spikes of the bolometer are marked by an asterisk.
[a] A complete list of the computed harmonic frequencies is given in
Table S1. Computed intensities are given in parentheses (kmmolꢀ1). An-
harmonic frequencies are indicated in italics.
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this band, which is illustrated in Figure 4. The DUCl counterpart
shows the same band profile and growth on annealing at
1003.1 cmꢀ1. The high HUCl/DUCl frequency ratio 1406.6/
1003.1=1.4002 is characteristic of a very heavy metal hydride
vibration. (The H/D frequency ratio for the vibration of a hydro-
gen atom against a baseball would be 1.4142). The DFT calcu-
lated values using the BP86[26–28] exchange-correlation function-
al for the UꢀH/D stretching mode in the harmonic approxima-
tion given in Table 5 are 1425.8 and 1010.8 cmꢀ1, respectively,
which are 1.5 and 0.8% higher than the observed values, and
in very good agreement for this level of theory especially for a
uranium hydride vibration. The 1134.0 cmꢀ1 absorption exhibits
the same band contour as the above strong HUCl band, but
we cannot match it with any of the other vibrational modes
for HUCl, nor can we find a deuterium or HBr counterpart. This
band also appears with Zr ablation and HCl, so it is most likely
due to an impurity in our HCl gas sample.
form HUBr is almost as exothermic as the chlorine counterpart
[reaction (2)]:
U þ HBr ! HUBr DH ¼ ꢀ301 kJ molꢀ1
ð2Þ
H2UCl2 and H2UBr2
The strong bands for HUCl and for HUBr have no major lower
frequency satellites, but weaker bands appear at 1569.8 and
1531.4 cmꢀ1 above HUCl and at 1553.1 and 1532.9 cmꢀ1 above
HUBr (Figure 3). Our DCl experiment revealed a 1094 counter-
part for the strongest 1531.4 band with a 1.400 H/D frequency
ratio for this UꢀH stretching mode. Our BP86 calculations pre-
dict the strongest two UꢀH stretching modes for H2UCl2
(Table 7) to be 67 and 93 cmꢀ1 higher than computed for the
UꢀH mode discussed above for HUCl, and likewise for H2UBr2
Far IR spectra in Figure 5 reveal the most obvious new band
at 323.8 cmꢀ1 which increases on annealing like the stronger
counterpart at 1404.6 cmꢀ1. Our DFT BP86 calculations find the
two lowest modes to be mixed UꢀCl stretch and HꢀUꢀCl
bend, and the stronger of these two bands to be at
328.7 cmꢀ1 which is just above the new band we observed so
it is the best assignment we have for this mode of HUCl. The
analogous mode for DUCl shifts down into the region for UCl2,
but fortunately a shoulder absorption at 314.7 increases on an-
nealing and is appropriate for the DUCl counterpart. Obviously,
our confidence in these far-IR assignments is not up to the
higher level we have for the mid-IR absorptions of HUCl and
DUCl. The increase for the above 1404.6 and 1003.1 cmꢀ1
bands on annealing the solid matrix to 25 or 35 K shows that
their formation reactions are spontaneous. Reaction (1) is
322 kJmolꢀ1 exothermic at the (ZPE corrected) BP86 level of
theory.
Table 7. Comparison of observed and computed harmonic vibrational
frequencies (in cmꢀ1) for the H2AnCl2 and H2AnBr2 molecules.
Exptl (argon)
Calcd[a]
BP86/def-TZVP
H2UCl2
D2UCl2
H2UBr2
H2ThCl2
D2ThCl2
H2ThBr2
1569.8
1531.4
1094
1518.77 (347)
1492.42 (565)
1076.00 (176)
1059.01 (288)
1531.12 (331)
1504.01 (488)
1506.22 (401)
1468.91 (575)
1067.12 (203)
1042.32 (294)
1511.63 (439)
1476.80 (545)
1553.1
1532.9
1463.2
1044.4
1464.3
[a] A complete list of the computed frequencies is given in Table S3. Com-
puted intensities are given in parentheses (kmmolꢀ1).
U þ HCl ! HUCl DH ¼ ꢀ322 kJ molꢀ1
ð1Þ
to be 72 and 99 cmꢀ1 higher than the calculated UꢀH mode
for HUBr (Table 5). Now we turn to the two observed bands
listed above for the product of reaction (3) to be 127 and
165 cmꢀ1 higher than HUCl and those above for the product of
reaction (4) at 122 and 143 cmꢀ1 higher than HUBr. However,
the computed UꢀH modes for H2UCl2 are 39 and 51 cmꢀ1
lower (about 3%) than the observed value and 22 and 29 cmꢀ1
lower (less than 2% of the observed value) than the observed
bands for the dibromide. It is also possible for matrix site split-
tings to be involved with these absorptions. We assign the
above pairs of new absorptions to the dihalides H2UCl2 and
H2UBr2 whose energies of formation are also exothermic.
HUBr
The HBr experiments were performed for a chemical compari-
son with HCl. First, the strong HBr product band at
1410.6 cmꢀ1 in Figure 3 has the same profile as the HUCl coun-
terpart, and the new band increases about 30% on annealing
to 20 K (see Figure S3), but irradiation at 455 nm results in a
slight decrease for this new absorption. Annealing to 25 and
30 K continues substantial increases, but final annealing to
35 K decreases the 1410.6 cmꢀ1 band. The HUBr band at
ꢀ
1410.6 has almost the same annealing behavior as HBr2 since
HUCl þ HCl ! H2UCl2 DH ¼ ꢀ255 kJ molꢀ1
HUBr þ HBr ! H2UBr2 DH ¼ ꢀ251 kJ molꢀ1
ð3Þ
ð4Þ
both require the HBr reagent. First annealing to 15 or 20 K also
decreases the HCl and HBr Q branch bands. The 6.0 cmꢀ1 blue
shift for the major 1410.6 band from HUCl at 1404.6 is
matched by a 6.2 cmꢀ1 blue shift in the BP86 frequency calcu-
lation for HUBr. This agreement shows that the approximations
in each calculation are nearly the same. The calculated har-
monic UꢀH frequency for HUBr at 1432.0 cmꢀ1 is 1.5% higher
than the observed value, which is appropriate. The reaction to
H3UCl3 and H3UBr3
We computed these molecules (Table S4 and S5) with hopes of
finding them in our spectra, but there were no absorptions in
the region about 40 cmꢀ1 above H2UCl2 and H2UBr2 where the
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strongest absorptions for H3UCl3 and H3UBr3 are predicted to
Spectra for the Th and HCl reaction products in the far-IR
region are complicated by the presence of ThCl4 and ThCl2 in
the same 10 cmꢀ1 space. The top two spectra in Figure 8 ex-
amine the Th+Cl2 reaction and give product absorptions at
340.5 and 330.5 cmꢀ1 for ThCl4 and ThCl2.[50] Annealing to 30 K
decreased the lower band with little effect on the higher ab-
sorption. The next two spectra come from an experiment with
Th and HCl and the higher band is stronger than the lower
band, the reverse of the top scan using Cl2. This suggests that
HThCl could be on top of a weak ThCl4 band. Annealing to
20 K decreased both bands. Two scans from an experiment
with Th and HCl/DCl are shown in Figure 9. The new band at
335.8 cmꢀ1 has got to come from DCl, and the 340.3 band is in
the right place for the HCl counterpart. Thus, assignment of
the new band at 335.8 cmꢀ1 to DThCl follows and probably
HThCl also absorbs near 340.3 cmꢀ1 based on the DFT frequen-
cy calculations in Table 6.
1
occur. These molecules converged to singlet A1 ground states
(Table S4), with the mer-isomers (C2v) 17 and 16 kJmolꢀ1 lower
in energy for X=Cl and Br, respectively, compared to the fac-
isomer (C3v). The calculated structures are shown in Figure 2.
Their formations are exothermic [reactions (5) and (6)], but
much less so than reactions (3) and (4).
H2UCl2 þ HCl ! H3UCl3 DH ¼ ꢀ54 kJ molꢀ1
H2UBr2 þ HCl ! H3UBr3 DH ¼ ꢀ59 kJ molꢀ1
ð5Þ
ð6Þ
HThCl and HThBr
The strong product band in the thorium atom reaction with
HCl appeared at 1483.8 cmꢀ1, which is 79 cmꢀ1 above the ura-
nium counterpart. This band increased slightly on annealing to
20 K along with the 1463.2 cmꢀ1 satellite band (see Figure S6).
The DThCl counterpart at 1058.0 had a sharp well resolved sat-
ellite at 1044.4 and a 1067.4 cmꢀ1 shoulder. Annealing to 30 K
in this HCl/DCl experiment decreased the 1483.8 cmꢀ1 band
and increased the satellite band at 1454.5 while the
1058.0 cmꢀ1 DThCl counterpart also decreased and the 1044.4
Finally, we note that the computed increase in the HꢀAn
stretching frequencies in the series HAnF<HAnCl<HAnBr<
HAn (An=U, Th, X=F, Cl, Br) is nicely confirmed by the experi-
mental spectra (see Table 8).
ꢀ
ꢀ
satellite increased (Figure 7). The same strong HCl2 and DCl2
H2ThCl2 and H2ThBr2
anion bands were observed as in Figure 4. The HThCl/DThCl
frequency ratio, 1483.8/1058.0=1.4024, is also indicative of a
heavy metal hydride vibration. Unlike the uranium counter-
parts, no significant absorption bands were observed above
the main bands. Our anharmonic vibrational analysis at the
CCSD(T) level (Table 6) predicted this HThCl frequency
18.5 cmꢀ1 or 1.2% higher than the observed value. Also, the
harmonic BP86 wavenumber, which is 31.8 cmꢀ1 higher than
the observed value, is in very good agreement considering an
anharmonic correction of ꢀ39 cmꢀ1. We must remember here
that a red matrix shift on the order of ten wavenumbers is also
expected for the observed frequencies, but since no AnH mol-
ecules have been observed in the gas phase this is an esti-
mate. The corresponding frequency observed for HThBr
1485.4 cmꢀ1 is 1.6 cmꢀ1 higher than the frequency observed
for HThCl and our BP86 calculation predicted a 2.7 cmꢀ1 differ-
ence, again showing that the errors in each calculation are
almost the same. The BP86 frequency for HThBr is 32.9 cmꢀ1 or
2.2% higher than observed, which is certainly very good agree-
ment for this harmonic frequency calculation. Finally, the ThH
diatomic molecule absorption at 1485.2 cmꢀ1 in solid argon is
within experimental error of the HThBr band at 1485.4 cmꢀ1
but not with HThCl at 1483.8 cmꢀ1. However the band profiles
are very different: ThH exhibits a sharp symmetrical band.[8]
The broader bands with site splittings assigned here to HThCl
and HThBr are not due to ThH.
DFT calculations find the strong thorium dichloride and dibro-
mide ThꢀH frequencies just below the strong HThCl and
HThBr bands, in contrast to their uranium counterparts. Fig-
ures 6 and 7 show the spectra. Take the first band at
1463.2 cmꢀ1 below the strong 1483.8 band for HThCl and its
deuterium counterpart at 1044.4 and their ratio is 1.4010,
which is perfect for a ThꢀH stretching mode (Figure S7 and
S9). The sharp bands that increase on 30 K annealing at
1446.3, 1454.5 and at 1034.2, 1039.8 are matrix sites for
H2ThCl2 and D2ThCl2. Their H/D frequency ratios are 1.3985 and
1.3988 (Figure 7). The HBr counterpart at 1464.3 is 21.1 cmꢀ1
below the strong HThBr band and the first satellite band on
the HThCl band is 20.6 cmꢀ1 lower than the strong band. The
near agreement of these frequency differences and the only
1.6 cmꢀ1 difference between the main HThCl and HThBr bands
tells us that we are dealing with halogen counterparts of the
same product molecules. Again, their exothermicities are
nearly the same. The annealing spectra (Figure 7) reveal de-
creases in the HThCl and DThCl bands and increases in the
H2ThCl2 and D2ThCl2 absorptions on annealing to 30 K, which
supports the participation of reactions (9) and (10). The differ-
ences between the H2ThCl2 band at 1463.2 and the H2ThBr2
band at 1464.3 cmꢀ1 is just 1.1 cmꢀ1 (see Table 7), a small dif-
ference near that has also been indicated above between the
main HThCl and HThBr bands.
The thorium reactions with HCl and HBr are even more exo-
thermic than their uranium counterparts. The slight growth of
HThCl and HThBr on annealing to 20 K shows that reactions (7)
and (8) are also spontaneous.
HThCl þ HCl ! H2ThCl2 DH ¼ ꢀ404 kJ molꢀ1
HThBr þ HBr ! H2ThBr2 DH ¼ ꢀ401 kJ molꢀ1
ð9Þ
ð10Þ
Conclusions
Th þ HCl ! HThCl DH ¼ ꢀ419 kJ molꢀ1
Th þ HBr ! HThBr DH ¼ ꢀ410 kJ molꢀ1
ð7Þ
ð8Þ
Laser-ablated U and Th atoms react by insertion with HCl and
with HBr to form the metal (II) state HUCl, HUBr and HThCl,
Chem. Eur. J. 2019, 25, 1 – 12
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HThBr molecules with considerable exothermicity, which are
identified by their mid and far infrared spectra in solid argon.
Our assignment of these reaction products is based on the
close agreement between observed and calculated vibrational
frequencies and with frequency differences between these
molecules. The most interesting trend observed here is the in-
crease in UꢀH stretching frequencies from 1383.1 (HUF),
1404.6 (HUCl), 1410.6 (HUBr) to 1423.6 cmꢀ1 (UH) as less elec-
tronic charge is removed from the UꢀH bond by the less elec-
tronegative substituent (Table 8). An analogous trend is ob-
served for the ThꢀH stretching modes of the thorium counter-
parts 1464.8 (HThF), 1483.8 (HThCl), 1485.4 (HThBr) to
1485.2 cmꢀ1 (ThH).[8] It is important to notice that the comput-
ed increase in the HꢀAn stretching frequencies in the HAnF<
HAnCl<HAnBr<HAn (An=U, Th, X=F, Cl, Br) series are in
very good agreement with these trends in the experimental
spectra (see Table 8).
Keywords: density functional calculations · infrared · matrix
isolation · thorium · uranium
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Acknowledegements
We gratefully acknowledge the Zentraleinrichtung fꢀr Daten-
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Manuscript received: October 26, 2018
Revised manuscript received: November 28, 2018
Version of record online: && &&, 0000
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FULL PAPER
Uranium and thorium hydrides are
known as functional groups for ligand
stabilized complexes and as isolated
molecules under matrix isolation condi-
tions. Here, the new molecular products
of the reactions of laser-ablated U and
Th atoms with HCl and with HBr,
&
Actinides
L. Li, T. Stꢀker, L. Andrews,* H. Beckers,*
S. Riedel
&& – &&
Infrared Spectra of the HAnX and
H2AnX2 Molecules (An=Th and U,
X=Cl and Br) in Argon Matrices
Supported by Electronic Structure
Calculations
namely HUCl, HUBr and HThCl, HThBr,
based on their mid and far infrared
spectra in solid argon, is reported.
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