Study of the electronic structure of the actinide tetrabromides ThBr4 and UBr4 using ultraviolet photoelectron spectroscopy and density functional calculations

Article abstract of DOI:10.1063/1.1370945

Ultraviolet photoelectron spectroscopy and density functional theory was used to study the electronic structure of actinide tetrabromides. A high temperature photoelectron spectrometer was used to record photoelectron spectra for ThBr₄ and UBr₄. Vertical ionization energies were calculated for the tetrahalides and supporting matrix isolation experiments were carried out.

Full text of DOI:10.1063/1.1370945

Study of the electronic structure of the actinide tetrabromides ThBr 4 and UBr 4 using
ultraviolet photoelectron spectroscopy and density functional calculations
L. J. Beeching, J. M. Dyke, A. Morris, and J. S. Ogden
Citation: The Journal of Chemical Physics 114, 9832 (2001); doi: 10.1063/1.1370945
View online: http://dx.doi.org/10.1063/1.1370945
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/114/22?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Effect of spin-orbit coupling on the actinide dioxides AnO2 (An=Th, Pa, U, Np, Pu, and Am): A screened hybrid
density functional study
J. Chem. Phys. 137, 154707 (2012); 10.1063/1.4757615
Photoelectron spectroscopy and theoretical studies of UF5 − and UF6 −
J. Chem. Phys. 136, 194304 (2012); 10.1063/1.4716182
Publisher’s Note: “Probing the oxygen environment in UO 2 2 + by solid-state O 17 nuclear magnetic resonance
spectroscopy and relativistic density functional calculations” [J. Chem. Phys.132, 084501 (2010)]
J. Chem. Phys. 132, 109901 (2010); 10.1063/1.3363849
Probing the oxygen environment in UO 2 2 + by solid-state O 17 nuclear magnetic resonance spectroscopy and
relativistic density functional calculations
J. Chem. Phys. 132, 084501 (2010); 10.1063/1.3308499
Electronic spectrum of U O 2 2 + and [ U O 2 Cl 4 ] 2 − calculated with time-dependent density functional theory
J. Chem. Phys. 126, 194311 (2007); 10.1063/1.2735297
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

JOURNAL OF CHEMICAL PHYSICS
VOLUME 114, NUMBER 22
8 JUNE 2001
Study of the electronic structure of the actinide tetrabromides ThBr4
and UBr using ultraviolet photoelectron spectroscopy
4
and density functional calculations
L. J. Beeching, J. M. Dyke,a) A. Morris, and J. S. Ogden
Department of Chemistry, Southampton University, Southampton SO17 1BJ, United Kingdom
͑
Received 13 December 2000; accepted 20 March 2001͒
Ultraviolet photoelectron spectra of UBr and ThBr have been recorded in the gas-phase and
4
4
interpreted using relativistic density functional calculations. For ThBr , eight bands were observed
4
which are interpreted as ionization from the five Br 4p symmetry orbitals of a Br unit in T
4
d
Ϫ1
Ϫ1
Ϫ1
symmetry, with three of the bands ͓the (4t2) , (1t1) , and (3t2) ionizations͔ each being split
into two by spin–orbit interaction. The observed splittings are rationalized in terms of the Th 6p and
Br 4p contributions to the orbitals and by comparison with the known ultraviolet photoelectron
spectra of ThF and ThCl . The first vertical ionization energy ͑VIE͒ of ThBr was measured as
4
4
4
͑
͑
10.92Ϯ0.03͒ eV. UBr4 shows a very similar photoelectron spectrum with an extra band at
Ϫ1
9.65Ϯ0.02͒ eV VIE. This is associated with a (5t2)
(U 5f,6d) ionization. Supporting matrix
isolation infrared experiments were also carried out under very similar vaporization conditions to
those used in the photoelectron spectroscopy experiments to check the composition of the vapor
beams used. In these experiments, the T stretching modes of ThBr and UBr have been measured
2
4
4
Ϫ1
as 230Ϯ2 and 239Ϯ2 cm , respectively. Both the photoelectron and infrared matrix isolation
spectra are consistent with an effective tetrahedral geometry for UBr4 and ThBr4 . © 2001
American Institute of Physics. ͓DOI: 10.1063/1.1370945͔
I. INTRODUCTION
gas-phase electron diffraction studies indicated a structure of
C2v symmetry with equal metal-halogen bond lengths. Re-
8
The uranium and thorium tetrahalides play important
roles in nuclear fuel enrichment and reprocessing and they
cently a density functional study of the ground states of the
thorium tetrahalides has also provided estimates of equilib-
1
have been studied in some detail both in the solid state and
the gas phase. Uranium tetrafluoride is of particular impor-
tance in the nuclear industry as it acts as a precursor for both
rium bond lengths and harmonic vibrational frequencies of
9
ThX ͑XϭF, Cl, Br, and I͒. These tetrahalides were all
4
found to have tetrahedral minimum energy geometries in
their ground states. In summary, although reasonably de-
tailed experimental studies have been undertaken to probe
the electronic structure of the uranium and thorium tetrafluo-
rides and tetrachlorides, relatively little work has been un-
dertaken on the corresponding tetrabromides, UBr4 and
ThBr4 .
uranium metal and UF required for isotope enrichment pro-
6
cesses. UF is usually prepared from UO , extracted from
4
2
uranium ore, by reaction with HF at 850 K in a fluidized-bed
reactor. It is then directly fluorinated to produce UF for
6
isotope separation. Similarly thorium tetrafluoride is a key
intermediate for the reprocessing and recovery of thorium
1
metal.
Electron-impact high temperature mass spectrometric
1
0–12
The investigation of the electronic structure of the ura-
nium and thorium tetrahalides provides a good opportunity
to study the role of metal 5f electrons in chemical
studies of the uranium and thorium halides
have mea-
ϩ
sured the appearance energy of ThBr4 from ThBr as ͑10.4
Ϯ0.3͒ eV ͑Ref. 10͒ and UBr from UBr as ͑9.6Ϯ0.3͒ eV.
4
ϩ
11
4
4
2
–6
bonding.
The gas-phase He͑I͒ photoelectron spectra of
These represent the only available values for the first ioniza-
tion energies of these molecules.
UF4 , ThF , UCl4 , and ThCl4 have been recorded
4
2
previously. The spectra were first interpreted using non-
The aim of this present work was to record He͑I͒ photo-
electron spectra of ThBr and UBr in the vapor phase using
relativistic multiple-scattering SCF-X␣ calculations2 and
4
4
later reinterpreted using relativistic Hartree–Fock–Slater
a high temperature photoelectron spectrometer which uses
7
͑HFS͒ density functional calculations. For each tetrafluoride
radio frequency induction heating to vaporize a solid
sample.1
3–15
It was proposed to compare the photoelectron
or tetrachloride, the results are consistent with an effective
tetrahedral geometry with no evidence of any Jahn–Teller
distortion away from tetrahedral symmetry. In the uranium
tetrahalides, a distortion from tetrahedral symmetry is pos-
sible because of the open-shell degenerate ground state and
spectra obtained for ThBr and UBr with those previously
4
4
recorded for UF , ThF , UCl , and ThCl . Relativistic den-
4
4
4
4
sity functional calculations of vertical ionization energies
͑VIEs͒ were also performed for all six MX tetrahalides ͑M
4
ϭTh, U; XϭF, Cl, Br͒. Comparison with experimental VIEs
should allow ionic state assignment to be achieved for the
tetrabromides and trends in ionization energies and ionic
a͒Author to whom correspondence should be addressed; electronic mail:
jmdyke@soton.ac.uk
0021-9606/2001/114(22)/9832/8/$18.00
9832
© 2001 American Institute of Physics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Electronic structure of the actinide tetrabromides
9833
TABLE I. Comparison of the unit cell parameters used to reproduce the
white. Deposition times were typically 1–2 h and the argon
matrix was estimated to be at least 1000-fold in excess rela-
tive to the metal tetrabromide in the final matrix used for
spectroscopic study. Infrared spectra were recorded over the
range 4000–180 cm using a PE983 infrared spectrometer,
supported by standard data processing facilities.
x-ray diffraction pattern obtained from a UBr single crystal prepared in this
4
work, with those reported in the literature ͑Ref. 18͒.
Values from single
crystal diffraction
Reference
parameters
Ref. 18
Ϫ1
a
Crystal parameter
pattern
Space group
C2/m
11.01
8.77
C2/m
11.04͑2͒
8.76͑2͒
7.04͑1͒
90.00
In the photoelectron experiments, the actinide tetrabro-
mide molecules, UBr and ThBr , were produced in the gas
a
b
c
␣
␤
␥
4
4
phase by vaporizing a solid sample from an inductively
7.06
13,14
heated graphite furnace.
ThBr and UBr are known to
4 4
90.00
94.24
90.00
vaporize congruently.1
0–12
The temperature range over which
94.05͑8͒
90.00
vaporization occurred, and over which photoelectron spectra
were recorded with reasonable signal-to-noise ratios, was
aSingle crystal data were obtained using the Nonius COLLECT software, in-
corporating the DENZO Package ͑Refs. 38 and 39͒.
8
70–1000 K for ThBr and 820–970 K for UBr as esti-
4
4
mated from measurements made with a chromel–alumel
thermocouple attached to the furnace. For both tetrabro-
mides, it was found necessary to bake the sample on the
photoelectron spectrometer, initially at 550 K for 1 h to re-
state spin-orbit splittings in the metal tetrahalides deduced as
the halide ligand gets heavier. Supporting matrix isolation
experiments were also carried out under very similar vapor-
ization conditions in order to confirm the composition of the
move residual water and hydrogen bromide. Any MO Br2
2
͑MϭTh or U͒ that may have been present as a minor impu-
vapor and obtain infrared spectra of the MBr species.
rity in the MBr samples would have decomposed ͑probably
4
4
to less volatile MO and MBr ͒ at the temperatures at which
2
4
1
,17,20
II. EXPERIMENT
photoelectron spectra were obtained.
In a typical photoelectron experiment, a solid MBr4
sample was loaded into a graphite furnace in a glove bag
under dry nitrogen. This was then transferred as quickly as
possible to the photoelectron spectrometer, which was imme-
diately evacuated to minimize exposure to air. Both ThBr4
and UBr hydrolyze in air with UBr appearing to hydrolyze
ThBr was commercially available as an anhydrous solid
Pfaltz and Bauer, 99.5%͒. It was used as received, but
samples were kept in a desiccator prior to use and loaded
into a furnace under nitrogen prior to transfer to the photo-
electron spectrometer. This reduced, but did not eliminate,
hydration of the sample.
4
͑
4
4
more rapidly. The photoelectron spectrometer and induc-
UBr was prepared from UO ͑Strem Chemicals͒ using
4
2
1
6
tively heated furnace system used in this work have been
an existing preparative route. This involved passing bro-
described previously.1
3–15
The typical resolution of the spec-
mine in dry nitrogen over a heated UO /C mixture at 1300
2
trometer under the conditions at which He͑I͒ ͑21.22 eV͒ pho-
K. The product, which was collected downstream of the solid
toelectron spectra were recorded was 25–35 meV full width
UO /C mixture in a long reaction tube, was initially identi-
2
Ϫ1
1
7
at half maximum ͑FWHM͒, as measured for the (3p)
fied as UBr by its appearance as a tan colored powder
4
He͑I͒ ionization of argon. Spectra were calibrated using ar-
gon, methyl iodide, nitrogen, carbon monoxide, and hydro-
gen bromide. The transmission of the spectrometer, which
varies linearly with pass energy, was checked regularly using
which distinguished it from the main likely impurity,
UO Br , which is a brick-red color.
2
2
A small sample of the product was sealed in a Linde-
mann capillary and examined by x-ray diffraction using a
Nonius Kappa CCD diffractometer with Mo K␣ radiation
oxygen. Under conditions at which the MBr spectra were
4
recorded, it was found to be acceptable, as the fifth band of
oxygen could be observed with reasonable intensity relative
to the other four oxygen bands. Hence, the relative band
͑
␭ϭ0.710 69 Å͒. A good powder pattern was obtained from a
single one degree image, but this also showed single crystal
reflections. Following recording of a further ten one degree
images, it proved possible to index all observed reflections,
as one set, to give a cell analogous to that previously re-
intensities in the MBr spectra presented in this work are not
4
unduly affected by loss of intensity at lower kinetic energy.
ported for UBr ͑Ref. 18͒ ͑see Table I͒. The powder lines
4
were all compatible with this cell, indicating a homogeneous
sample.
III. COMPUTATIONAL DETAILS
The general features of the matrix isolation equipment
used for this work have been described previously.19 Matrix
isolation infrared studies on UBr and ThBr involved vapor-
Relativistic density functional theory ͑DFT͒ calculations
of vertical ionization energies were carried out for the ac-
tinide tetrahalides MX ͑MϭU, Th; XϭF, Cl, Br͒ to facili-
4
4
4
ization of the samples in the temperature range 720–800 K
and subsequent co-condensation with argon onto a cesium
iodide window cooled to ϳ12 K. The bromide samples were
contained in silica holders and extensively degassed at ϳ600
K in order to remove traces of HBr, arising from hydrolysis.
Final purification was by sublimation immediately prior to
co-condensation with argon. Pure samples of solid UBr4
were yellow-brown in color, while those of ThBr4 were
tate a complete valence ionic state assignment of their ultra-
violet photoelectron spectra.
Calculations for the actinide halides ThF , ThCl ,
4
4
ThBr , UF , UCl , and UBr were carried out using the
4
4
4
4
2
1
Amsterdam Density Functional ͑ADF͒ program on the Co-
lumbus machine at the Rutherford Appleton Laboratories.22
The ThBr and UBr results presented here assumed a tetra-
4
4
hedral geometry with metal–bromine bond lengths being
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

9834
J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Beeching et al.
8
taken from gas-phase electron diffraction studies ͑2.73 Å for
ThBr and 2.69 Å for UBr ͒.
4
4
For both uranium and thorium tetrabromide, a relativistic
frozen core was used. Calculations were carried out using the
metal core ͑a͒ frozen up to the 6p level, and ͑b͒ frozen up to
the 5d level. This allowed the effect of having the metal 6p
orbitals frozen or unfrozen on the computed VIEs to be in-
vestigated. For the bromine atoms, the 4s and 4p orbitals
were in the valence region in these calculations, while the
lower-lying orbitals were in the frozen core. Including the
Br 3d orbitals in the valence region was found to have neg-
ligible effect on computed VIEs. Therefore, with the metal
6
s and 6p orbitals not frozen, the valence space contained
the metal 6s, 6p, 7s, and 5f orbitals and the bromine 4s
and 4p orbitals. A triple zeta plus polarization ͑TZϩP͒
Slater-type basis set was used. Also, the gradient-corrected
BLYP functional, which includes the uniform electron gas
2
3
exchange, the Becke 88 correction to exchange and the
Lee–Yang–Parr correlation functional24 was utilized. Rela-
tivistic effects are taken into account using a relativistic fro-
zen core and a perturbation treatment in the valence
FIG. 1. Infrared spectra of the 200–300 cmϪ1 spectral region obtained for
UBr and ThBr isolated in argon matrices at 12 K.
4
4
33Ϯ3 cmϪ1, and assigned as the T stretch. This value is
34
2
5,26
2
region.
This method has been shown to give reliable
2
in very satisfactory agreement with the results obtained in
minimum energy geometries and ionization energies for
Ϫ1
compounds containing heavy elements.2
7–29
In order to cal-
this present work ͑239Ϯ2 cm , see Fig. 1͒, when one con-
siders that the gas phase band contour would be expected to
include an appreciable contribution from a hot band.
There appears to be no published infrared spectral data
culate VIEs, the transition state method was used. This in-
volved half an electron being removed from each orbital, in
turn, and the negative of the orbital energy of the resulting
partially filled orbital, obtained from a converged calcula-
tion, being equated to the VIE. These VIEs were then com-
pared with the experimentally measured VIEs. The transition
for molecular ThBr , but the experimental value of 230Ϯ2
4
Ϫ1
cm for the T2 stretching mode observed here compares
very favorably with the recommended value of 227 cm
Ϫ1
obtained from density functional calculations with a B3LYP
state method has been shown to give results which are
functional.9
equivalent to ⌬SCF calculations3
0,31
and this was confirmed
There are no other infrared active stretching modes pre-
by performing transition state and ⌬SCF calculations on
VIEs on ThBr4 .
dicted for Td UBr and ThBr and the infrared active T
4
4
2
Ϫ1
bends are anticipated to lie at ϳ45 cm ͑Refs. 8 and 9͒,
which is well below the lower limit of the infrared spectrom-
eter used. There is no evidence from these results of any
significant distortion from Td symmetry, and indeed, it can
be shown that if a static C3v distortion of more than ϳ5° in
bond angle were present in these tetrabromides, the effect
IV. RESULTS AND DISCUSSION
A. Matrix isolation studies
Figure 1 shows the argon matrix infrared spectra ob-
tained from samples of UBr and ThBr in the frequency
4
4
would be to split the observed ‘‘T ’’ stretching modes by ϳ8
Ϫ1
2
region 300–200 cm . One band is observed for each tetra-
Ϫ1
cm , which would be clearly visible in the matrix infrared
bromide, and this feature is assigned in each case to the T2
spectra.
stretching mode of the T monomer. The only other absorp-
d
These matrix isolation infrared measurements therefore
provide support for the purity of the tetrabromide effusive
beams used in these studies, and for the Td symmetry of
molecular UBr and ThBr . They also provide the first ex-
tions present in these spectra were bands arising from traces
of adventitious water at ϳ3700 and 1600 cm , and a num-
ber of sharp, weak features in the region 2300–2600 cm
Ϫ1
Ϫ1
4
4
which may be assigned to HBr and its oligomers. These lat-
ter features appeared at considerably lower vaporization tem-
peratures than the metal tetrabromide absorptions, and their
positions corresponded very closely to the absorptions previ-
ously reported for HBr in argon over a range of
perimental measurement of the T stretching mode in ThBr .
2
4
B. Photoelectron spectra
If the actinide tetrabromides, UBr and ThBr , have tet-
4
4
3
2
concentrations. It is particularly significant that no absorp-
rahedral geometries then the transformation properties of the
metal 7s, 6p, and 6d orbitals and the halogen 4s and 4p
orbitals can be readily determined. Also, metal 5f orbitals in
T symmetry transform as a , t , and t . One way of un-
tions were observed in the U–O ͑or Th–O͒ stretching region
͑
typically 700–1000 cmϪ1͒ indicating that under the vapor-
ization conditions used, no uranium ͑or thorium͒ oxybro-
mides were produced.
d
1
1
2
derstanding the valence energy level ordering as well as the
Previous studies on UBr have assumed T symmetry in
character of each valence level in these MBr molecules is to
4
d
4
8
,33,34
the gas-phase.
In an experimental gas-phase infrared
compute the valence energy levels of the Br group, at the
4
study, a single infrared-active band has been reported at
geometry it has in the MBr neutral molecule, and study the
4
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Electronic structure of the actinide tetrabromides
9835
FIG. 3. He͑I͒ photoelectron spectrum obtained for ͑a͒ ThBr and ͑b͒ UBr .
4
4
3
t Ͻ1eϽ1t Ͻ3a Ͻ4t .
2 1 1 2
These symmetry orbitals interact with the orbitals of the cen-
tral atom, i.e., 6p(t ), 5f(a ϩt ϩt ), 6d(eϩt ), and
2
1
1
2
2
7
s(a ) and the order changes to
1
3
a Ͻ3t Ͻ1eϽ1t Ͻ4t .
1 2 1 2
In ThBr , these are the outermost occupied five levels.
4
Mulliken analysis of the occupied levels in ThBr shows that
4
the next deepest levels, the 2t and 2a levels, are essen-
2
1
tially ligand 4s symmetry combinations with the 2t level
2
having some metal 6p(t ) character. The next lowest level,
2
the 1t level, is almost completely composed of Th 6p(t )
2
2
orbitals. UBr has two more electrons than ThBr and these
4
4
go into the 5t level which has metal 5f and 6d character.
2
The experimental He͑I͒ photoelectron spectra of UBr4
and ThBr are shown in Figs. 3͑a͒ and 3͑b͒. The 10.0–13.5
4
eV ionization energy regions of these spectra exhibit a re-
markable similarity and each spectrum shows eight repro-
ducible band maxima, labeled A–H in Fig. 3. UBr shows an
4
FIG. 2. Computed orbital energies for M, Br, Br , and MBr obtained from
non-relativistic density functional calculations for ͑a͒ MϭTh and ͑b͒ MϭU.
4
4
extra band which is not present in the ThBr spectra. This
4
has a vertical ionization energy ͑VIE͒ of ͑9.65Ϯ0.02͒ eV and
an adiabatic ionization energy ͑AIE͒ ͑band onset͒ of ͑9.35
Ϯ0.10͒ eV. These values compare with the only previously
The 1a1 molecular orbital in the MBr compounds is essentially a metal 6s
4
orbital. It is the lowest energy molecular orbital in the MBr compounds and
4
is too low in energy to be shown in ͑a͒ and ͑b͒.
determined value for the first AIE of UBr , from electron
4
1
1
impact mass spectrometry, of ͑9.6Ϯ0.3͒ eV. The corre-
effect of the introduction of the central metal M. This has
been done for ThBr and UBr . The eigenvalues obtained
from non-relativistic ADF density functional calculations
sponding values in ThBr , the band maximum and band on-
4
set of band A in Fig. 3͑a͒, are ͑10.92Ϯ0.02͒ eV and ͑10.28
Ϯ0.05͒ eV, which compare with the earlier value, from
electron impact mass spectrometry of ͑10.4Ϯ0.3͒ eV.10
Scans in the higher ionization energy region ͑13.5–21.0 eV͒
showed no bands that could be attributed to these tetrabro-
mides, although it was noted that the bands in the 10.0–13.5
4
4
have been plotted for M, Br , and MBr ͑for MϭTh and U͒
4
4
in Figs. 2͑a͒ and 2͑b͒.
The following order of the Br 4p symmetry orbitals was
obtained for the Br unit
4
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

9836
J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Beeching et al.
TABLE II. Comparison of experimental and computed VIEs ͑in eV͒ of ThBr , with band assignments.
4
Orbital
Th 5d core
Non
relativistic
Th 6p
core
Th 5d
core
Shifted
values
Relativistic
Experiment
Assignment
4
4
1
1
1
3
3
3
2
2
2
1
1
1
t2
t2
t1
t1
e
t2
t2
a1
t2
t2
a1
t2
t2
a1
6u3/2
4e5/2
5u3/2
4e1/2
4u3/2
3u3/2
3e5/2
3e1/2
2u3/2
2e5/2
2e1/2
1u3/2
1e5/2
1e1/2
10.83
10.86
11.02
11.19
11.60
12.01
12.23
12.43
23.35
23.37
23.59
10.82
11.08
11.00
11.16
11.56
11.93
12.23
12.40
22.84
23.39
23.56
26.33
32.67
49.87
10.92
11.18
11.10
11.26
11.65
12.02
12.32
12.50
22.94
23.49
23.66
26.43
32.77
49.97
10.92
11.23
11.12
11.33
11.85
12.11
12.40
12.67
a
A
C
B
D
E
F
G
H
¯
aBands outside the photon energy range used.
eV region were lower in intensity than the corresponding
bands observed for the tetrafluorides and tetrachlorides, and
hence any weak metal tetrabromide bands in the 13.5–21.0
eV region would be difficult to detect.
to all computed VIEs. This shift was 0.10 eV for ThBr and
4
0.08 eV for UBr . The spread in the computed VIEs, both
4
with and without the metal 6p orbitals in the valence region,
for bands A–H ͑1.60 and 1.58 eV for ThBr , 1.73 and 1.70
4
Tables II and III show the computed VIEs for ThBr and
4
eV for UBr ͒ compares very favorably with the experimental
4
UBr with the metal orbitals ͑a͒ frozen up to the 6p level
4
spread ͑1.75 eV for ThBr and 1.70 eV for UBr ͒ for both
4
4
͑the M 6p core in Tables II and III͒ and ͑b͒ frozen up to the
metal tetrabromides ͑see Tables II and III͒.
5
d level ͑the M 5d core in Tables II and III͒. As can be seen,
The assignment of the bands shown in Figs. 3͑a͒ and
͑b͒ relies on the computed VIEs of ThBr and UBr shown
the computed VIEs are all slightly too low. This is attributed
to ͑i͒ the use of a relatively small basis set, ͑ii͒ the use of a
frozen core, and ͑iii͒ the use of the transition-state method
which neglects some electron correlation energy change on
ionization, although the gradient corrected functional in-
cludes some electron correlation in each state involved in the
ionization process. In order to compare computed and ex-
perimental VIEs, for both tetrabromides the computed VIE
3
4
4
in Tables II and III and the assignment achieved previously
for the UV photoelectron spectra of the uranium and thorium
7
tetrafluorides and tetrachlorides, which is also based on
VIEs computed with relativistic density functional calcula-
tions.
It is well established that the spin-orbit splitting of the
outermost occupied t2 level in ThF4 and ThCl4 , the 4t2
level, is responsible for the first two bands in their UV pho-
Ϫ1
of band A ͓the first (4t2) VIE͔ was shifted to match the
experimental VIE of band A, and this shift was then applied
TABLE III. Comparison of experimental and computed VIEs ͑in eV͒ of UBr , with band assignments.
4
Orbital
U 5d core
Non
relativistic
U 6p
core
U 5d
core
Shifted
values
Relativistic
Experiment
Assignment
5
4
4
1
1
1
3
3
3
2
2
2
1
1
1
t2
t2
t2
t1
t1
e
t2
t2
a1
t2
t2
a1
t2
t2
a1
7u3/2
6u3/2
4e5/2
5u3/2
4e1/2
4u3/2
3u3/2
3e5/2
3e1/2
2u3/2
2e5/2
2e1/2
1u3/2
1e5/2
1e1/2
9.46
10.88
10.88
11.10
11.30
11.64
12.03
12.24
12.61
23.41
23.42
23.69
9.30
10.91
11.17
11.09
11.27
11.61
11.99
12.28
12.61
23.08
23.51
23.67
27.17
34.48
54.74
9.37
10.99
11.25
11.16
11.34
11.68
12.06
12.35
12.68
23.16
23.58
23.74
27.25
34.56
54.81
9.65
10.99
11.28
11.18
11.42
11.88
12.12
12.42
12.69
a
U(5f,6d)
A
C
B
D
E
F
G
H
aBands outside the photon energy range used.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Electronic structure of the actinide tetrabromides
9837
FIG. 4. Comparison of the ultraviolet photoelectron spectra of ThF4 ,
ThCl , and ThBr , and correlation of the ionic state assignment of the
FIG. 5. Comparison of the ultraviolet photoelectron spectra of UF , UCl ,
4
4
4
4
bands.
and UBr and correlation of the ionic state assignment of the bands.
4
toelectron spectra. This arises because the 4t level contains
2
a small amount of thorium 6p character and the Th 6p or-
bital shows a large spin-orbit splitting ͑the spin-orbit split-
ting in the 1t level of ThBr , which is mainly Th 6p in
bands is approximately 2:1. This splitting is not resolved in
the ThF and ThCl spectra ͑or the UF and UCl spectra͒.
4
4
4
4
The computed separations of the u3/2 and e5/2 components in
2
4
character, is 6.3 eV; similar values are also obtained for this
splitting in the other uranium and thorium tetrahalides͒. For
ThF , ThCl , and ThBr the experimental spin-orbit split-
ThF , ThCl , and ThBr are 0.08, 0.10, and 0.30 eV consis-
4
4
4
tent with a large halogen p and a small metal 6p contribution
to this level. The measured splitting of bands F and G in the
UBr and ThBr spectra are 0.30 and 0.29 eV, consistent
4
4
4
ting of the outermost t level stays approximately constant at
2
4
4
͑
0.35Ϯ0.06͒ eV with values of 0.41, 0.35, and 0.31 eV being
with the computed splittings of 0.29 and 0.30 eV obtained
from calculations with the metal 6p orbitals in the valence
region. These computed splittings reduce slightly, to 0.21
and 0.22 eV, on putting the metal 6p orbitals in the core.
measured. Table II confirms the origin of this splitting as it is
only reproduced when the metal 6p orbitals are not frozen
and included in the valence space. The 1t level is almost
1
completely halogen in character, and the computed spin-orbit
splittings in ThF , ThCl , and ThBr are 0.03, 0.05, and
Band H is assigned to ionization from the 3a orbital, which
is 3e1/2 relativistically.
1
4
4
4
0.16 eV. This splitting is only resolved in the photoelectron
A comparison of the assignment of the ThF , ThCl ,
4
4
spectra of ThBr and UBr and these components are as-
and ThBr spectra, and the UF , UCl , and UBr spectra are
4
4
4
4
4
4
signed to bands B and D. Although the 4t level is split into
presented in Figs. 4 and 5. Apart from the extra band seen in
the uranium tetrahalide spectra at low ionization energy,
which arises in each case from ionization from the outermost
2
u3/2 and e5/2 levels with the u3/2 level lying lower ͑expected
degeneracy ratio 2:1͒ and the 1t level is split into u and
1
3/2
e1/2 levels with the u3/2 level lying lower ͑expected degen-
eracy ratio 2:1͒, the bands A, B, C, and D in the experimen-
tal UBr and ThBr spectra are not sufficiently well resolved
5t (7u3/2) level, the assignments of the uranium and thorium
2
spectra are remarkably similar. The splitting of the ionic
Ϫ1
states that arise from the (4t2) ionization in ThF , ThCl ,
4
4
4
4
to estimate the A:C relative intensity ͑the 4t components͒
and ThBr ͑0.41, 0.35, and 0.31 eV͒ and in UF , UCl , and
2
4
4
4
and the B:D relative intensity ͑the 1t components͒. The 1e
UBr ͑0.30, 0.36, 0.29͒ are approximately constant, consis-
1
4
level is u3/2 relativistically and ionization from this level is
tent with it being associated with a spin-orbit splitting arising
Ϫ1
assigned to band E in ThBr ͑and UBr ͒. The (3t ) ion-
from a metal 6p contribution to the 4t level. In contrast, the
4
4
2
2
ization is expected to give two components u3/2 and e5/2
,
1t and 3t levels have spin-orbit splittings which are ex-
1
2
degeneracy ratio 2:1. Bands F and G in Figs. 3͑a͒ and 3͑b͒
are assigned to these ionizations and support for this assign-
ment is provided by the fact that the intensity ratio of these
pected, from the computed VIEs, to increase on going along
the series MF , MCl , MBr ͑with MϭTh or U͒ consistent
4
4
4
with these levels being halogen np in origin. This is consis-
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

9838
J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Beeching et al.
tent with the spectra, as these splittings are only resolved in
the tetrabromides.
ACKNOWLEDGMENTS
The authors thank EPSRC and the Leverhulme Trust for
financial support and L.J.B. is indebted to Leybold Ltd. and
BAT for a postgraduate studentship. Professor M. B. Hurst-
house and the National X-Ray Crystallographic Service
If the 1t level in each tetrahalide is composed solely of
1
halogen np valence orbitals, then neglecting off-diagonal
2
spin–orbit interactions, the T (u3/2Ϫe1/2) splitting is just
1
Ϫ3/4 ␨, where ␨ is the spin-orbit coupling constant for the
͑Southampton͒ are thanked for the x-ray diffraction data on
3
5,36
free halogen ion.
The negative sign implies that the split-
UBr single crystals, and the assistance of K. Holloway in
4
ting is inverted with the u3/2 component lying lower than the
the early stages of this work is also acknowledged. The au-
thors have also benefited from valuable discussions with Dr.
L. Gagliardi ͑Bologna͒ and Professor B. Roos ͑Lund͒.
5
e1/2 component, consistent with Hund’s third rule as the t1
shell in the tetrahalide ion is more than half full. ␨ values can
ϩ
ϩ
ϩ
be readily evaluated for F , Cl , and Br from atomic
tables as 0.04, 0.08, and 0.32 eV giving calculated 1t spin-
1
1
J. J. Katz, G. T. Seaborg, and T. R. Morss, The Chemistry of the Actinide
Elements, 2nd ed. ͑Chapman and Hall, London, 1986͒, Vol. 1.
J. M. Dyke, N. K. Fayad, A. Morris, and I. R. Trickle, J. Chem. Phys. 72,
3822 ͑1980͒.
E. Thibaut, J. P. Boutique, J. J. Verbist, J. C. Levet, and H. Noel, J. Am.
Chem. Soc. 104, 5266 ͑1982͒.
G. C. Allen, S. Hubert, and E. Simoni, J. Chem. Soc., Faraday Trans. 91,
2767 ͑1995͒.
orbit splittings in the uranium and thorium tetrafluorides, tet-
rachlorides and tetrabromides of 0.03, 0.06, and 0.24 eV.
The only observed 1t splittings occur for ThBr and UBr
2
1
4
4
3
with values of 0.21 and 0.24 eV being measured, in good
agreement with the value of 0.24 eV obtained from this
simple model. The 1t1 splittings in the tetrafluorides and
tetrachlorides are unresolved experimentally and are there-
4
5
G. C. Allen and J. W. Tyler, J. Chem. Soc., Faraday Trans. 1 83, 1355
͑
1987͒.
fore expected to be Ͻ0.10 eV. The 1t splittings computed
1
6
7
8
9
J. M. Dyke, G. D. Josland, A. Morris, P. M. Tucker, and J. W. Tyler, J.
Chem. Soc., Faraday Trans. 2 77, 1273 ͑1981͒.
P. M. Boerrigter, J. G. Snijders, and J. M. Dyke, J. Electron Spectrosc.
Relat. Phenom. 46, 43 ͑1988͒.
for ThF , ThCl , and ThBr by relativistic density func-
4
4
4
tional calculations are 0.03, 0.06, and 0.16 eV in good agree-
ment with the above values derived from atomic halogen ion
R. J. M. Konings and D. L. Hildenbrand, J. Alloys Compd. 271, 583
splittings. The 3t levels show splittings of 0.29 and 0.30 eV
2
͑
1998͒.
in ThBr and UBr which are slightly greater than the corre-
4
4
L. Gagliardi, C. K. Skylaris, A. Willetts, J. M. Dyke, and V. Barone, Phys.
Chem. Chem. Phys. 2, 3111 ͑2000͒.
D. L. Hildenbrand and K. H. Lau, J. Chem. Phys. 93, 5983 ͑1990͒.
K. H. Lau and D. L. Hildenbrand, J. Chem. Phys. 86, 2949 ͑1987͒.
D. L. Hildenbrand, K. H. Lau, and R. D. Brittain, J. Chem. Phys. 94, 8270
͑1991͒.
D. K. Bulgin, J. M. Dyke, F. Goodfellow, N. Jonathan, E. P. F. Lee, and
A. Morris, J. Electron Spectrosc. Relat. Phenom. 12, 67 ͑1977͒.
J. M. Dyke, A. Morris, G. D. Josland, M. P. Hastings, and P. D. Francis,
High. Temp. Sci. 22, 95 ͑1986͒.
J. M. Dyke, N. Jonathan, and A. Morris, Electron Spectroscopy: Theory,
Techniques and Applications ͑Academic, London, 1979͒, Vol. 3, p. 189.
G. Brauer, Handbook of Preparative Inorganic Chemistry, 2nd ed. ͑Aca-
demic, London, 1965͒, Vol. 2, p. 1440.
J. Katz and E. Rabinowitch, The Chemistry of Uranium, National Nuclear
Energy Series ͑McGraw-Hill, New York, 1951͒, Vol. 5.
J. C. Taylor and P. W. Wilson, Acta Crystallogr., Sect. B: Struct. Crys-
tallogr. Cryst. Chem. 30, 2664 ͑1974͒.
J. S. Ogden and R. S. Wyatt, J. Chem. Soc. Dalton Trans. , 859 ͑1987͒.
D. Brown, Halides of the Lanthanides ͑Wiley, New York, 1968͒.
Theoretical Chemistry Department of the Vrije Universiteit, Amsterdam,
http://chem.few.vu.nl/afdelingen/TC/index-en.html
sponding splittings in the 1t levels. The 3t splittings are
1
2
1
1
0
1
unresolved in the tetrafluorides and tetrachlorides.
The simple atomic spin-orbit model cannot be so readily
applied to the t2 ionic states, as the t2 molecular orbitals
contain a larger number of contributions, notably halogen
ns, halogen np␴, halogen np␲, metal 6d, metal 5f, and
12
1
1
1
1
3
4
5
6
3
5–37
metal 6p orbitals.
The density functional computed val-
ues of the 3t2 spin-orbit splitting for ThF , ThCl , and
4
4
ThBr are, however, 0.02, 0.10, and 0.30 eV. These values
4
are very similar to the splittings computed by relativistic
density functional calculations for the 1t levels, consistent
17
1
with the 3t levels also being composed of mainly halogen
2
18
np valence orbitals. However, the computed 3t values are
2
slightly larger and this is probably due to a small metal 6p
contribution to these levels. It is particularly notable that
when the metal 6p level is included in the core, the com-
19
2
2
0
1
puted 3t spin-orbit splitting reduces to the spin-orbit split-
2
22
Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11, OQZ, UK,
http://www.rl.ac.uk
A. D. Becke, Phys. Rev. A 38, 3098 ͑1988͒.
ting of the 1t level ͑from 0.29 to 0.22 eV in ThBr ͒.
1
4
2
2
2
3
4
5
In summary, the photoelectron and infrared matrix iso-
lation results obtained in this work are consistent with effec-
tive tetrahedral geometries for UBr and ThBr . The photo-
C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 ͑1988͒.
J. G. Snijders and E. J. Baerends, Mol. Phys. 36, 1789 ͑1978͒.
J. G. Snijders, E. J. Baerends, and P. Ros, Mol. Phys. 38, 1909 ͑1979͒.
T. Zeigler, J. G. Snijders, and E. J. Baerends, J. Chem. Phys. 74, 1271
͑1981͒.
E. J. Baerends, J. G. Snijders, C. A. de Lange, and G. Jonkers, in Local
Density Approximations in Quantum Chemistry and Solid State Physics,
edited by J. P. Dahl and J. Avery ͑Plenum, New York, 1984͒, p. 515.
J. G. Snijders and E. J. Baerends, in Electron Distributions and the
Chemical Bond, edited by M. B. Hall and P. Coppens ͑Plenum, New
York, 1982͒, p. 111.
4
4
26
27
electron spectrum of ThBr exhibits eight bands which arise
4
from ionization of the five outermost orbitals derived from
2
2
8
9
the Br 4p symmetry combinations in a Br tetrahedral unit,
4
with three of the bands split by spin-orbit interaction. The
UBr spectrum is very similar but it also exhibits a weak
4
band at lower ionization energy arising from ionization of a
5
t orbital, which is unfilled in ThBr and has two electrons
2 4
30
31
N. H. F. Beebe, Chem. Phys. Lett. 19, 290 ͑1973͒.
J. M. Dyke, A. Morris, A. M. A. Ridha, and J. G. Snijders, Chem. Phys.
67, 245 ͑1982͒.
in it in UBr . The origin of the observed spin-orbit splittings
4
can be understood on the basis of this combined experimen-
tal and theoretical study but improved calculations of ioniza-
tion energies of the uranium and thorium tetrahalides that
give improved agreement with experimental values would be
valuable.
32
͑
a͒ M. T. Bowers and W. H. Flygare, J. Chem. Phys. 44, 1389 ͑1966͒; ͑b͒
A. J. Barnes, H. E. Hallam, and G. F. Scrimshaw, Trans. Faraday Soc. 65,
150 ͑1969͒.
3
33
Y. S. Ezhov, V. I. Bazhanov, S. A. Komarov, V. G. Sevastyanov, and F.
Yuldashev, Vysokochist. Veshchestva 5, 197 ͑1989͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20

J. Chem. Phys., Vol. 114, No. 22, 8 June 2001
Electronic structure of the actinide tetrabromides
9839
3
3
4
5
37
38
V. M. Kovba and I. V. Chikh, Zh. Strukt. Khim. 24, 172 ͑1983͒.
J. C. Green, M. L. H. Green, P. J. Joachim, A. F. Orchard, and D. W.
Turner, Philos. Trans. R. Soc. London, Ser. A 268, 111 ͑1970͒.
B. D. Bird and P. Day, J. Chem. Phys. 49, 392 ͑1968͒.
R. N. Dixon and R. P. Tuckett, Chem. Phys. Lett. 140, 553 ͑1987͒.
R. Hooft, Nonius COLLECT Software ͑Nonius, Delft, The Netherlands,
1998͒.
3
6
39
Z. Otwinowski and W. Minor, Methods Enzymol. 276, 307 ͑1997͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.67.7
On: Wed, 03 Dec 2014 07:17:20