Infrared spectra and density functional calculations of the SUO2 molecule

Article abstract of DOI:10.1021/ic900869f

Reactions of laser-ablated U atoms with SO₂ molecules gave the very stable U(VI) molecule, SUO₂, as the major product. Infrared absorptions for two new O=U=O stretching modes were observed In solid argon and neon. The band assignment

Full text of DOI:10.1021/ic900869f

6888 Inorg. Chem. 2009, 48, 6888–6895
DOI: 10.1021/ic900869f
Infrared Spectra and Density Functional Calculations of the SUO2 Molecule
Xuefeng Wang and Lester Andrews*
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319
Colin J. Marsden
Laboratoire de Chimie et Physique Quantiques, UMR 5626, IRSAMC, Universite Paul Sabatier, 118 Route de
Narbonne, 31062 Toulouse Cedex 9, France
Received May 5, 2009
Reactions of laser-ablated U atoms with SO2 molecules gave the very stable U(VI) molecule, SUO2, as the major
product. Infrared absorptions for two new OdUdO stretching modes were observed in solid argon and neon. The
band assignments were confirmed by appropriate 34SO2, S18O2, and S16,18O2 isotopic shifts. B3LYP and BPW91
density functional calculations were performed to determine molecular structure, vibrational frequencies, and isotopic
shifts. The C2v structure is analogous to those computed for UO3 and US3. Minor products were identified as SUO, the
SUO2+ cation, and the (SO2)(SUO2) adduct.
Introduction
of the uranium reaction with SO2 and the resulting analogous
SUO2 product.
The uranium(VI) oxide UO3 molecule has an interesting
almost T-shaped C2v structure. This unique structure
was first deduced from extensive oxygen isotopic substitution
in argon matrix infrared spectra with normal coordinate
analysis1,2 and later confirmed by SCF and B3LYP calcula-
tions.3-6 The UO3 molecule evaporated from oxygen
rich solid urania at 1550 °C, but the temperature had to be
increased to near 1900 °C to produce significant UO2
absorption in the matrix isolation experiments.1,2 Both
of these uranium oxide molecules were produced in the
reaction of laser ablated U atoms with oxygen.7 The sulfur
analogues US2 and US3 were formed in the reaction of laser
ablated U atoms with sulfur molecules, and calculations
found the analogous T-shaped C2v structure for US3.8 The
known stability of U(VI) species prompted our investigation
Laser ablated uranium atoms have provided numerous
new uranium bearing molecules with novel bonding, parti-
cularly the first examples of methylidene complexes such as
CH2dUH2, CH2dUHF, CH2dUF2, the methylidyne com-
plexes HCtUF3 and FCtUF3, and the terminal pnictides
NtUF3 and PtUF3.9-13 Such simple product molecules are
formed in straightforward reactions of U atoms with small
precursor molecules during condensation in excess argon.
Experimental and Computational Methods
Laser-ablated U atoms were reacted with SO2 in excess
argon or neon during condensation at 5 K using methods
described in our previous papers.14-16 The Nd:YAG laser
fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse
width) was focused onto a rotating uranium target (Oak
Ridge National Laboratory, high purity, depleted of 235U).
The uranium target was filed to remove oxide coating and
*To whom correspondence should be addressed. E-mail: lsa@virginia.
edu.
(1) Gabelnick, S. D.; Reedy, G. T.; Chasanov, M. G. J. Chem. Phys. 1973,
58, 4468 (UO2).
˚
(9) Lyon, J. T.; Andrews, L.; Malmqvist, P.-A.; Roos, B. O.; Yang, T.;
(2) Gabelnick, S. D.; Reedy, G. T.; Chasanov, M. G. J. Chem. Phys. 1973,
59, 6397 (UO3).
(3) Pyykko, P.; Li, J.; Runeberg, N. J. J. Phys. Chem. 1994, 98, 4809.
(4) Zhou, M.; Andrews, L.; Ismail, N.; Marsden, C. J. Phys. Chem A 2000,
104, 5495 (UO2 in solid neon).
Bursten, B. E. Inorg. Chem. 2007, 46, 4917.
(10) Lyon, J. T.; Andrews, L. Inorg. Chem. 2006, 45, 1847. (U + CH3X).
(11) Lyon, J. T.; Andrews, L.; Hu, H.-S.; Li, J. Inorg. Chem. 2008, 47,
1435.
(12) Lyon, J. T.; Hu, H.-S.; Andrews, L.; Li, J. Proc. Natl. Acad. Sci. U.S.
(5) Shamov, G. A.; Schreckenbach, G.; Vo, T. N. Chem.;Eur. J. 2007,
A. 2007, 104, 18919.
13, 4932.
(13) Andrews, L.; Wang, X.; Lindh, R.; Roos, B. O.; Marsden, C. J.
Angew. Chem., Int. Ed. 2008, 47, 5366.
(14) Andrews, L.; Citra, A. Chem. Rev. 2002, 102, 885 and references
ꢀ
(6) Iche--Tarrat, N.; Marsden, C. J. J. Phys. Chem A 2008, 112, 7632.
(7) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1993, 98, 3690 (UO2 and
UO3).
therein.
(8) Liang, B.; Andrews, L.; Ismail, N.; Marsden, C. Inorg. Chem. 2002, 41,
(15) Andrews, L. Chem. Soc. Rev. 2004, 33, 123 and references therein.
(16) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040 (review
article).
2811 (US2 and US3). Unpublished neon matrix spectrum of US2 reveals the
antisymmetric stretching mode at 442.3 cm-1
.
r
pubs.acs.org/IC
Published on Web 06/22/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6889
immediately placed in the vacuum chamber. Sulfur dioxide
(Matheson) was condensed and outgassed before sample
preparation with research grade argon and neon. A mixture
of S18O2 and S16,18O2 with a trace of S16O2 was prepared by
tesla coil discharge of 18O2 (YEDA) (about 1 Torr) in a 2 L
pyrex bulb containing a film of sulfur sublimed onto the walls
(about 5mg)byexternalheatingusingahotairgun. Asample
of 34SO2 was prepared from sulfur-34 (98% 34S, Cambridge
Isotope Laboratories) and oxygen. The laser energy was
varied about 10-20 mJ/pulse. FTIR spectra were recorded
at 0.5 cm-1 resolution on Nicolet 750 with 0.1 cm-1 accuracy
using an HgCdTe range B detector. Matrix samples were
annealedatdifferenttemperatures, and selected samples were
subjected to photolysis by a medium pressure mercury arc
lamp (Philips, 175W) with the globe removed.
Complementary density functional theory (DFT) calcula-
tions were performed using the Gaussian 03 program,17 the
hybrid B3LYP and pure BPW91 density functionals, and
6-311+G(3df) basis set for sulfur and oxygen atoms, and the
60electronSDDpseudopotential foruranium.18-21 All ofthe
geometrical parameters were fully optimized, and the harmo-
nic vibrational frequencies were obtained analytically at the
optimized structures.
Table 1. Infrared Absorptions from Products of Laser-Ablated
U Atom
(and U+) Reactions with Sulfur Dioxide in Excess Argon and Neon at 5 K
32S16O2 34S16O2 32S16,18O2 + 32S18O2 ratio(16/18)
Argon
identity
1210.9 1198.0
1167.4
969.4, 938.3
904.8
910.3, 880.9
875.5, 846.6
811.5
809.7
799
793.0, 774.0
736.7
705.0
1.03726
1.05244
1.05272
1.05267
1.05268
1.05607
1.05298
1.0600
(SO2)(SUO2)
SUO2+
UO2+
(SO2)(SUO2)
SUO2
SUO
987.5
952.5
927.3
891.2
857.0
852.6
847.0
821.7
775.7
745.6
987.5
952.5
927.3
891.1
857.0
852.6
847.0
821.5
UO3
(SO2)(SUO2)
SUO2
UO2
1.06163
1.05294
1.05759
UO3
Neon
1005.4 1005.4
986.8, 955.5
931.0
1.05222
1.05285
SUO+2
UO+2
(SO2)(SUO2)
UO2
SUO2
SUO
UO3
(SO2)(SUO2)
SUO2
UO3
980.2
936.0
915.2
904.5
871.5
865.5
855.0
835.5
760.4
980.2
936.0
915.2
904.4
871.5
869.7
888.7, 859.0
1.0532
1.05297
822.0
1.05292
855.5
835.3
760.4
805.4, 786.7
719.2
1.06203
1.05729
Results
Infrared spectra of laser ablated uranium atom reaction
products with SO2 in excess argon or neon during condensa-
tion at 5 K will be presented in turn. Experiments were also
done with S18O2 and 34SO2, and the isotopic frequencies are
listed in Table 1. Density functional calculations were per-
formed to support the identification of new reaction pro-
ducts. Weak absorptions for common species, such as SO,
SO3, and uranium oxides, have been identified in previous
papers.4,7,22,23
Infrared Spectra of U + SO2 Reaction Products. Infra-
red spectra of laser-ablated uranium atom reaction pro-
ducts with SO2 in excess argon are illustrated in Figure 1
a-e. Sample co-deposition reveals two major new sharp
bands at 891.2 and 821.7 cm-1, a very weak 857.0 cm-1
absorption, a weak 775.7 cm-1 band for UO2 in solid
argon,1,2,7 and weak broad bands at 1211, 927, 853, 848,
and 746 cm-1. Annealing the sample to 20 K increased all
of these features including sharp subpeaks at 852.6 and
745.6 cm-1, which are due to UO3 in solid argon.1,2,7
Ultraviolet irradiation (>220 nm) further increased all
bands except 857.0 cm-1 and UO2. Subsequent annealing
to 30 K again increased all absorptions, but a final
annealing to 35 K increased only the 1211, 927, and
848 cm-1 bands with a sharp subpeaks at 1210.9, 927.4,
and 847.0 cm-1 and slightly decreased the rest. Weak,
sharp absorptions at 987.5, 952.5, and 857.0 cm-1 are
shown clearly in Figure 2 using expanded absorbance
scale. The 952.5 cm-1 absorption has been assigned to
UO2+ in a previous report.4 The 987.5 cm-1 peak is
destroyed by ultraviolet irradiation but is reproduced on
further annealing, which also increased the UO2+ absorp-
tion. Similar behavior is found for the 857.0 cm-1 peak.
Spectra from an experiment using 34SO2 at lower
concentration gave virtually the same spectra (Figure 1
f,g,h,i) except the major sharp bands were shifted slightly
to 891.1 and 821.5 cm-1. The same annealing and photo-
lysis behavior was found: this time >320 nm irradiation
began the effect which >220 nm light completed. The
987.5 and 857.0 cm-1 peaks were not shifted, as illu-
strated in Figure 2.
The discharge of 18O2 with elemental sulfur in a pyrex
bulb resulted in the preparation of a convenient S18O2 and
S
16,18O2 mixture, which enabled characterization of the
new product absorptions. The major product bands
shifted to 846.6 and 774.0 cm-1 with higher frequency
mixed isotopic components at 875.5 and 793.0 cm-1 after
the intensity pattern of the precursor molecules. The
spectra shown in Figure 1 j, k, l, m reveal a slight growth
with ultraviolet irradiation and continued increase on 30
K annealing of these new bands in concert. The U18O2+
band shifted to 904.8 cm-1, the U18O3 bands to 809.7 and
705.0 cm-1, and the U18O2 to 736.7 cm-1, as reported
previously.1,2,4,5 These oxygen-18 substituted uranium
oxides show that most of the uranium oxide products
come from the precursor reaction as a very weak U16O2
peak is detected at 775.8 cm-1. The broadband that
increased on final annealing shifted to 881 cm-1 with a
subpeak at 880.9 cm-1 and exhibited a mixed isotopic
component at 910.3 cm-1. The sharp 987.5 cm-1 band
shifted to 969.4 cm-1 mixed and 938.3 cm-1 pure oxygen-
18 counterparts as shown in Figure 2 m.
(17) Frisch, M. J. et al. Gaussian 03, Revision D.01; Gaussian, Inc.:
Pittsburgh, PA, 2004.
(18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.;
Parr, R. G. Phys. Rev. B 1988, 37, 785.
(19) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Perdew, J. P.; Burke,
K.; Wang, Y. Phys. Rev. B 1996, 54, 16533 and references therein. (c) See
also: Becke, A. D. J. Chem. Phys. 1997, 107, 8554.
(20) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3265.
::
(21) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100,
Lower sample concentrations are required for neon
matrix experiments, and weaker product absorptions
result. Two experiments were done with different laser
energies. The major peaks appear at 904.5 and 835.5 cm-1
7535.
(22) Hopkins, A. G.; Brown, C. W. J. Chem. Phys. 1975, 62, 2511 (SO).
(23) Bondybey, V. E.; English, J. H. J. Mol. Spectrosc. 1985, 109, 221
(SO3).

6890 Inorganic Chemistry, Vol. 48, No. 14, 2009
Wang et al.
Figure 2. Infrared spectra in the 1000-830 cm-1 region for products
formed in the laser-ablated uranium atom reaction with sulfur dioxide
during condensation in excess argon at 5 K. Expanded absorbance scale
for the same spectra presented in Figure 1.
Figure 1. Infrared spectra in the 970-730 cm-1 region for products
formed in the laser-ablated uranium atom reaction with sulfur dioxide
during condensation in excess argon at 5 K. (a) Normal isotopic 32SO2
isotopic sample at 0.4% in argon co-deposited for 60 min, (b) after
annealing to 20 K, (c) after >220 nm irradiation, (d) after annealing to
30 K, (e) after annealing to 35 K, (f) 34SO2 isotopic sample at 0.2% in
argon co-deposited, (g) after annealing to 30 K, (h) after >220 nm
irradiation, (i) after annealing to 35 K, ( j) S18O2 and S16,18O2 isotopic
mixture at0.4%total co-deposited, (k) after >220nmirradiation, (l)after
annealing to 30 K, (m) after annealing to 35 K.
with broad red shoulders in solid neon. Annealing re-
duces the shoulder in favor of the sharp product bands,
Figure 3 a, b, c. Sharp weak bands are observed for UO2+
at 980.2 cm-1, for UO3 at 865.5 and 760.4 cm-1, and for
UO2 at 915.2 cm-1.4 Broad features increased on anneal-
ing at 936.0 and 855.0 cm-1. The photosensitive, sharp
988.6 cm-1 band is common to other metal experiments
with SO2 in solid neon and is near absorptions for the
isolated SO2- anion.24
Additional spectra are shown for the more productive
34SO2 experiment, Figure 3 f-j. Again annealing decreases
the broad red wing in favor of the sharp band. Next, 240-
380 nm irradiation slightly increases the two bands to-
gether whereas >220 nm decreases them together. Sub-
sequent annealing increases both sharp bands in concert
with 3:1 relative intensity. All peaks measure the same as
with natural isotopic precursor save the major sharp
Figure 3. Infrared spectra in the 1020-740 cm-1 region for products
formed in the laser-ablated uranium atom reaction with sulfur dioxide
during condensation in excess neon at 5 K. (a) Normal isotopic
32SO2 isotopic sample at 0.2% in argon co-deposited for 60 min, (b) after
annealing to 8 K, (c) after >220 nm irradiation, (d) after annealing to
10 K, (e) after annealing to 12 K, (f) 34SO2 isotopic sample at 0.2% in
argon co-deposited, (g) after annealing to 8 K, (h) after >220 nm
irradiation, (i) after annealing to 10 K, (j) after annealing to 12 K,
bands, which red-shifted 0.1 and 0.2 ( 0.05 cm-1
.
Reaction with the S18O2 and S16,18O2 mixture shifted
the major product bands to 859.0 and 786.7 cm-1 with
higher frequency mixed-isotopic components at 888.7
and 805.4 cm-1 after the intensity pattern of the precursor
molecules. The spectra shown in Figure 3 k, l, m, n reveal
a slight growth on annealing of these new bands in
concert. The U18O2+ band shifted to 931.0 cm-1, the
stronger U18O3 band to 822.0 cm-1, and the U18O2 band
to 869.7 cm-1, as reported previously.4 Another experi-
ment was done with a different oygen-18 enrichment to
confirm the above measurements.
(k)
S
18O2 and S16,18O2 isotopic mixture at 0.1% co-deposited,
(l) after annealing to 8 K, (m) after annealing to 10 K, (n) after annealing
to 12 K.
bands. Since the reaction of U and CO2 gave the insertion
product OUCO,25 the analogous species OUSO was
examined in detail. Two linear triplet electronic states
were characterized by computation; in the first, which is a
true minimum, the unpaired electrons are in sigma and
delta orbitals, whereas in the second, which is 8 kcal/mol
lower in energy but which has two degenerate imaginary
bending frequencies, they are in delta and phi orbitals. On
relaxation, the O-U-S angle decreases to around 135°;
the resulting species that contains U(IV) is a true mini-
mum, 95 kcal/mol higher in energy than the 1A1 uranium
Calculations. Density functional calculations were per-
formed for several types of products whose stoichiometry
is UO2S, with different topologies, spin states, and oxida-
tion states of uranium, to help assign the various IR
(24) Forney, D.; Kellogg, C. B.; Thompson, W. E.; Jacox, M. E. J. Chem.
Phys. 2000, 113, 86 and references therein (SO2-).
(25) Andrews, L.; Zhou, M.; Liang, B.; Li, J.; Bursten, B. E. J. Am. Chem.
Soc. 2000, 122, 11440 (OUCO).

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6891
Figure 4. Structures for the series of molecules UO3, SUO2, and US3 calculated at the (B3LYP/6-311+G(3df)/SDD) level of theory. Bond lengths in
angstrom and angles in degrees.
Table 2. Calculated Harmonic Vibrational Frequencies (cm-1) and Infrared Intensities (km/mol) for U Atom and U+ Cation Reaction Products with Sulfur Dioxidea
32SUO2
34SUO2
32SU16,18O2
32SU18O2
32SUO2
identity
B3LYP
B3LYP
918.86
B3LYP
903.3
B3LYP
872.5
BPW91
888.7(338)
825.0(83)
407.9(49)
162.8(19)
139.2(1)
918.86(403)
853.46(95)
426.8(65)
195.7(20)
153.5(1)
B2, OUO str
A1, OUO str
A1, US str
A1, OUS bend
B2, OUS bend
B1, OUO bend
853.40
415.8
195.5
151.9
145.2
821.5
426.7
191.0
150.5
141.8
804.7
426.6
185.7
148.2
138.3
145.3(33)
138.8(30)
32SUO2+
34SUO2+
32SU16,18O2+
32SU18O2+
identity
B3LYP
B3LYP
1027.5
939.9
331.9
210.9
179.3
127.1
B3LYP
1008.9
905.8
340.3
206.1
174.8
125.7
B3LYP
975.8
886.1
340.1
200.6
170.3
123.5
1027.5(282)
939.9(34)
340.4(20)
211.2(12)
179.3(21)
128.3(0)
B2, OUO str
A1, OUO str
A1, US str
A1, OUS bend
B1, OUO bend
B2, OUS bend
(SO2)(SUO2)
(
34SO2)-
(
18OSO)-
identity
B3LYPb
B3LYP
1233.0
952.5
882.8
757.5
520.3
472.2
B3LYP
1199.9
952.5
882.8
764.4
521.6
467.8
1245.7(259)
952.5(346)
882.8(108)
765.3(117)
522.6(128)
476.3(12)
-SdO str
antisym OUO str
sym OUO str
sym S-O str
SdU-O bend
SO2 bend
32SUO
34SUO
32SU18
O
identity
B3LYPc
B3LYP
904.6(290)
441.7(62)
80.5(21 ꢀ 2)
B3LYP
856.4(265)
453.4(64)
77.9(20 ꢀ 2)
B3LYPd
868(270)
426(79)
124(8)
904.6(290)
453.7(65)
80.9(21 ꢀ 2)
U-O str
U-S str
SUO bend
a B3LYP or BPW91 density functionals and 6-311+G(3df)/SDD. The major product SUO2 is a singlet state. b Singlet state. Only the six highest-
frequency vibrations are listed, those in the spectral range available. c This triplet state is linear, SU, 2.326 A and UO, 1.793 A. d This triplet state is bent
˚
˚
˚
˚
and 5 kcal/mol higher, SU, 2.407 A, UO, 1.815 A, SUO, 121°.
˚
the unique UdS bond in the trisulfide is 0.045 A shorter
(VI) sulfido-dioxide SUO2 molecule with a C2v almost
T-shaped structure. The lowest-energy quintet species
(U(II)) has Cs symmetry, with SO2 linked in a bidentate
mode through the two oxygen atoms; it is 104 kcal/mol
above SUO2. The least unstable triplet species identified is
planar, with an almost linear O-U-O grouping but with
substantially different U-O distances and a very acute
S-U-O angle of 43°; this system is 34 kcal/mol above
SUO2. It turns out that the observed IR spectra provide
no evidence for any of these higher-energy species, and
their computed spectra are sufficiently characteristic that
they would have been detected had they been formed in
significant quantity.
˚
than this bond in SUO2 where the UdO bonds are 0.022 A
shorter than the two equivalent UdO bonds in UO3.
Calculated vibrational frequencies for the SUO2 molecule
and related product molecules are listed in Table 2.
Natural charges and electron configurations are listed
in Table 3. As expected the charge on U decreases with S
substitution. In principle, a T-shaped isomer of SUO2 is
possible in which an oxygen atom is on the stem of the T,
making the two oxygen atoms non-equivalent; however,
geometry optimizations for this isomer with Cs symmetry
converged to the species shown in Figure 4, where the
unique S atom is on the stem of the T.
Structures for the series of molecules UO3, SUO2, and
US3 are compared in Figure 4. It is interesting to note that
Calculations were also done for the SUO triatomic
molecule, the SUO2+ cation, and the precursor adducts,

6892 Inorganic Chemistry, Vol. 48, No. 14, 2009
Wang et al.
Table 3. Structural Parameters and Physical Constants Computed for UO3,
SUO2, and US3 in Singlet States and C2v Symmetrya
parameter
UO3(B3)
SUO2(B3)
SUO2(BP)
US3(B3)
r(U-O,S top)
r(U-O,S side)
(O,S-U-O,S)
1.849
1.845
100.4
2.382
1.783
100.6
2.390
1.801
101.4
2.337
2.327
104.5
q(O,S top)b
q(U)b
-0.68
1.95
-0.63
-0.45
1.66
-0.61
-0.0.38
1.53
-0.58
-0.269
0.80
-0.265
q(O,S side)b
U(s)c
U(d)c
U(f)c
0.06
1.52
2.75
0.15
1.64
2.81
0.14
1.56
2.98
0.15
2.24
2.87
Figure 5. Structure calculated for the (SO2)(SUO2) adduct at the
(B3LYP/6-311+G(3df)/SDD) level of theory. Bond lengths in angstrom
and angles in degrees.
a Bond lengths and angles are in anstrom and degrees (see Figure 4).
All calculations performed at the B3LYP (listed as B3) or BPW91 (listed
as BP)//6-311+G(3df)//SDD level. b Natural atomic charges from NBO
analysis. c Natural electron configuration for U valence orbitals.
Discussion
The major product SUO2 molecule will be identified, and
bonding in the series of molecules UO3, SUO2, and US3
considered. The minor triatomic product SUO, the SUO2+
cation, and the precursor adduct, (O2S)(SUO2), will also be
assigned.
(O2S)(SUO2) and (SO2)(SUO2), in which SO2 binds to U
through S or O, respectively, using both B3LYP hybrid
and BPW91 pure density functionals to support the
identification of new product absorptions. The Cs (O2S)
(SUO2) complex was bound by 6 kcal/mol at the B3LYP
level (note that actinide thermochemistry is predicted
much more reliably by hybrid DFT than by GGA func-
SUO2 Identification. The strong new argon matrix
absorptions at 891.2 and 821.7 cm-1 shift to 846.6 and
774.0 cm-1 and define oxygen 16/18 isotopic frequency
ratios of 1.05268 and 1.06163. The former ratio is char-
acteristic of an antisymmetric OdUdO stretching mode,4
and the latter ratio is slightly larger than the harmonic
ratio for a totally symmetric OdUdO stretching mode
(1.06080). The appearance of one intermediate mixed
oxygen 16, 18 isotopic band for each mode, at 875.5
and at 793.3 cm-1 with relative intensities matching the
S18O2 and S16,18O2 precursor bands, demonstrates that
this new molecule contains two equivalent oxygen atoms.
These sharp bands can be measured to an accuracy better
than 0.1 cm-1, and the 34SO2 experiment reveals shifts to
891.1 and 821.5 cm-1. Although the former 0.1 cm-1 shift
may not be outside of experimental error, the latter
0.2 cm-1 difference indicates a measurable shift. Thus,
this molecule contains sulfur even though the UdS
stretching mode was not observed.
Our density functional frequency calculations at
the B3LYP level of theory find strong 918.86 cm-1
(403 km/mol) and 853.46 cm-1 (95 km/mol) frequencies
(infrared intensities). The former frequency does not shift
with sulfur-34, but the latter sustains a 0.06 cm-1 shift as
this symmetric OdUdO stretching mode couples very
slightly with the symmetric UdS stretching mode. Both
modes shift considerably with oxygen-18 substitution at
both atomic positions, and harmonic 16/18 ratios of
1.05318 and 1.06064 are defined. Single oxygen-18 sub-
stitution gives observed 1.01793 and 1.03619 oxygen 16/
18 ratios for comparison to computed 1.01727 and
1.03895 harmonic values. The asymmetry in the mixed
isotopic components within the oxygen isotopic triplets
(grouped in Figures 1 and 3), which is manifested in
the different 16,16/16,18 isotopic ratios 1.01793 and
1.03619 arises because of interaction between these two
modes in the mixed isotopic molecule of lower symmetry
and demonstrates further that the two major bands
are due to modes of different symmetry in the same
molecule. Very good agreement between calculated
and observed values for single and double oxygen-
18 substitution in both antisymmetric and symmetric
OdUdO stretching modes for the SUO2 molecule
tionals),26 the OUO modes blue-shifted by 18 and 13 cm-1
,
and the symmetric SO2 mode red-shifted by 18 cm-1. In
contrast the C1 (SO2)(SUO2) complex is substantially
more strongly bound, by 34 kcal/mol (B3LYP), the
OUO modes are blue-shifted by 25 and 17 cm-1, and a
very strong -SdO stretch is computed 66 cm-1 above the
symmetric SO2 mode. The C1 (SO2)(SUO2) complex no
doubt gains some stability from S-S bonding in the four-
membered ring structure (see Figure 5).
Our B3LYP calculation for the 2B2 ground state
SUO2+ cation is straightforward. It is interesting that
the B3LYP ionization energy for SUO2 (191 kcal/mol) is
almost as high as the value computed for UO3 (221 kcal/
mol). The latter is near the 10 eV appearance potential of
the gaseous UO3+ cation in Knudsen effusion mass
spectrometry experiments.27 Although the SUO2+ cation
has the same basic structure as the neutral molecule, the
bond lengths and angles computed are slightly different
˚
˚
(UO, 1.739 A; US, 2.502 A; OUS, 92.3°). The Mulliken
atomic spin densities show that ionization comes essen-
tially from the sulfur atom.
Several low-energy configurations for the SUO mole-
cule, which has a triplet ground state, are located in our
preliminary DFT calculations. Three orbitals are almost
degenerate for linear SUO: non-bonding f-φ, non-bond-
ing f-δ, and essentially non-bonding s-σ. At the B3LYP
level of theory, the δφ configuration for SUO lies lowest,
3
and we note that neutral UO2, which is linear, has a Φ
(φσ) ground state.4,28
ꢀ
ꢀ
(26) Clavaguera, C.; Marsden, C. J.; Begue, D.; Pouchan, C. Chem. Phys.
2004, 302, 1.
(27) Guido, M.; Balducci, G. J. Chem. Phys. 1991, 95, 5373 and references
therein.
(28) (a) Gagliardi, L.; Heaven, M. C.; Krogh, J. W.; Roos, B. O. J. Am.
Chem. Soc. 2005, 127, 86. (b) Fleig, T.; Jensen, H. J. Aa.; Olsen, J.; Visscher,
L. J. Chem. Phys. 2006, 124, 104106. (c) Infante, I.; Eliav, E.; Vilkas, M. J.;
Ishikawa, Y.; Kaldor, U.; Visscher, L. J. Chem. Phys. 2007, 127, 124308.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6893
verifies these assignments and the identification of this
new U(VI) molecule.
clear increase of the major SUO2 absorptions in solid
argon and neon on annealing indicates that reaction 1 is
spontaneous and requires no activation energy.
Similar information is found in the neon matrix spec-
trum. The major bands are 13.3 and 13.8 cm-1 higher in
the more weakly interacting neon matrix host, but the
isotopic characteristics are almost identical to the argon
matrix values. See for example the oxygen 16/18 iso-
topic frequency ratios and the 32S16,18O2 and 32S18O2
frequency patterns in Table 1. Nearly the same 12.9 and
14.8 cm-1 blue shifts from argon to neon are observed for
two modes of the 1A1 ground state UO3 molecule, which
are typical matrix shifts.29 These comparisons further
support our identification of the SUO2 molecule.
U þ SO2 f SUO2
ð1Þ
SUO2 Structure. The computed C2v structure for
SUO2, is analogous to that for UO3.1-6 Two bond length
changes in the UO3, SUO2, and US3 molecule series,
Figure 4, are noteworthy. The OdUdO bond lengths
˚
decrease 0.022 A on sulfur substitution in the third
˚
position, and this UdS bond is 0.045 A longer than
computed for US3. Our NBO analysis reveals an increase
in the U 5f configuration from 2.75 to 2.81 to 2.87 and U
6d configuration from 1.52 to 1.64 to 2.24 in the series
UO3, SUO2, and US3. The natural charges on U decrease
1.95f1.66f0.80 in the series, as less charge is withdrawn
by the less electronegative S, seems to sustain stronger
U-O bonds in UO3 compared to SUO2.
It is interesting the compare harmonic frequencies
calculated for the SUO2 molecule using the B3LYP
hybrid density functional and the BPW91 pure density
functional. As is generally found, the hybrid func-
tional predicts slightly higher frequencies than the pure
density functional.26,30 The OdUdO stretching fre-
quencies for the SUO2 molecule are predicted about 30
cm-1 higher with the hybrid functional. The gaseous
molecule is not known, but extrapolating from argon to
neon to gas phase predicts these two modes near 925 and
855 cm-1, which are very close to the B3LYP values. Note
that the lower BPW91 frequencies are within 5 cm-1 of the
argon matrix values. So here the BPW91 functional clearly
underestimates the OdUdO stretching frequencies, and
the B3LYP functional predicts frequencies very close to
extrapolated gas phase values for the SUO2 molecule.
It is also interesting to compare the calculated and
observed frequencies for the SUO2 molecule with those for
the analogous UO3 molecule. The calculated OdUdO
angle in the SUO2 molecule, 158.8°, is slightly smaller
than the corresponding 159.2° angle for the UO3 molecule
presumably owing to greater repulsions for the larger
sulfur electron cloud. This is attested by the still smaller
151.0° SdUdS angle in US3 calculated here with the same
method. On going from UO3 to SUO2 the antisymmetric
OdUdO stretching mode is blue-shifted 42.1 cm-1
(B3LYP) or 39.0 cm-1 (neon matrix), and the symmetric
mode is red-shifted 27.4 cm-1 (B3LYP), but this much
weaker mode is not observed for the UO3 molecule in
solid neon. Hence, the small decrease in the OdUdO
angle and interaction with S lone pairs triple the infrared
intensity of the symmetric OdUdO stretching mode in
the SUO2 molecule.
The C2v structure for UO3 is well established, and the
related SUO2 molecule follows suit with a very similar
structure. This is in marked contrast to the pyramidal,
C3v, structure of the similar WO3 molecule.32,33 One
is thus tempted to conclude that the special angular proper-
ties of the UO3, SUO2, and US3 molecular structures are
due to the involvement of U 5f orbitals, which are not
present with the symmetrical WO3 molecule. Dyall has
carefully discussed the consequences of f-orbital partici-
pation on molecular shapes.34 We note above the increase
in both U 5f and 6d configurations with increasing
electron density at the U center (decrease in positive
charge), but the increase in 6d is much more pronounced
that that for 5f. As a result the U-S bond lengths are
˚
more nearly equal in US3 (difference 0.01 A) than are the
˚
U-O bonds in UO3 (difference 0.042 A) where the latter
has the more dominant 5f involvement.
SUO2+ Identification. The sharp 987.5 cm-1 band
shows no sulfur-34 shift so this absorption is not due to
a precursor mode, and in this region it is therefore most
likely due to an antisymmetric OdUdO vibration. The
shift to 938.3 cm-1 and the oxygen 16/18 isotopic fre-
quency ratio 1.05244 are again characteristic of an anti-
symmetric OdUdO vibration, and the weak mixed 16, 18
component at 969.4 cm-1 with 16,18/16,16 ratio 1.01867
completes this characterization as found in similar iso-
topic frequency ratios for SUO2 above. In solid neon this
band blue shifts 18.0 cm-1 to 1005.5 cm-1. The dioxygen-
18 counterpart at 955.5 cm-1 again defines the antisym-
metric OdUdO vibration isotopic ratio 1.05233, and the
mixed oxygen isotopic counterpart at 986.8 cm-1 gives
almost the same single oxygen isotopic substitution ratio,
1.01895, as the argon matrix observations.
The SUO2 molecule is formed directly in the concerted
highly exothermic reaction 1, which is computed as -196
kcal/mol at the B3LYP level in the absence of spin orbit
coupling. The O atom dissociation from SO2 is suffi-
ciently high (121 kcal/mol)31 to make UO stripping
unlikely as attested by the detection of only a trace of
UO at 819.7 cm-1 beside the major product band in
Figure 1. Reaction 1 may proceed through insertion to
form OUSO followed by immediate rearrangement in the
matrix cage to the more stable U(VI) product SUO2. The
The position of this antisymmetric OdUdO vibration
above that for UO2+ itself (Table 1) suggested the possible
SUO2+ assignment, so calculations were performed for
the latter cation. Our B3LYP calculation predicted a very
strong antisymmetric OdUdO vibration for this 2B2
ground state cation at 1027.5 cm-1 with no sulfur-34 shift
(29) Jacox, M. E. Chem. Phys. 1994, 189, 149.
(30) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b)
Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 2937. (c) von
Frantzius, G.; Streubel, R.; Brandhorst, K.; Grunenberg, J. Organometallics
(32) Bare, W. D.; Souter, P. F.; Andrews, L. J. Phys. Chem. A 1998, 102,
8279. (Mo, W+O2).
ꢀ
2006, 25, 118. (d) Iche-Tarrat, N.; Marsden, C. J. J. Phys. Chem. A 2008, 112,
(33) Zhou, M.; Andrews, L. J. Chem. Phys. 1999, 111, 4230. (Cr, Mo,
7632.
W+O2 in neon).
(31) Okabe, H. J. Am. Chem. Soc. 1969, 73, 762. D[OS-O].
(34) Dyall, K. Mol. Phys. 1999, 96, 511.

6894 Inorganic Chemistry, Vol. 48, No. 14, 2009
Wang et al.
and 1.05298 harmonic 16,16/18,18 and 1.01844 harmonic
16,16/16,18 isotopic frequency ratios. The order-of-magni-
tude weaker symmetric counterpart predicted at 939.9 cm-1
was not observed. The above agreement with the frequency
calculation and isotopic effects for SUO2+ and the com-
parison with UO2+ all support our matrix identification of
SUO2+.
levels of computational theory than the preliminary
estimates offered here.
The SUO molecule is probably made in these experi-
ments by the U reaction with the SO byproduct of sample
irradiation during deposition. The SUO molecule
U þ SO f SUO
ð5Þ
The SUO2+ cation can be prepared three different ways
in these experiments. First, ionization of the neutral
product using hard ultraviolet radiation in the ablation
plume as shown in reaction 2 is the most direct process.
Annealing can foster the reaction of U+, also produced
by laser ablation and proposed to react with dioxygen
to form UO2+ on annealing in earlier experiments,4 with
SO2, as in highly exothermic reaction 3, which is com-
puted as -149 kcal/mol at the B3LYP level in the absence
of spin orbit coupling. Finally, the reaction of S with
UO2+ can also give the SUO2+ cation, reaction 4, which is
42 kcal/mol exothermic.
absorption increases on annealing while the SO band
decreases. Recall that the U and O2 reaction proceeds
on annealing.4 Reaction 5 is exothermic by 164 kcal/mol
(B3LYP, computed in the absence of spin orbit coupling).
Ultraviolet irradiation likely leads to S detachment,
which may preclude a significant yield on sample deposi-
tion under exposure to the intense ablation plume.
SUO þ uv f S þ UO þ anneal f SUO
ð6Þ
(SO2)(SUO2) identification. The 927.3 cm-1 absorp-
tion increases on annealing and reveals isotopic charac-
teristics similar to the major 891.2 cm-1 band for SUO2,
namely, the progressive shifts with one oxygen-18 to
910.3 cm-1 (ratio 1.01868) and two oxygen-18’s to
880.9 cm-1 (ratio 1.05267). No sulfur-34 shift was ob-
served. A weaker associated lower band was observed at
847.0 cm-1 (16/18 isotopic ratio 1.0600) again with no
sulfur-34 shift. Another weaker band at 1210.9 cm-1 with
the same profile as the above bands tracks with them on
annealing and reaches a final absorbance of one-fourth of
the 927.3 cm-1 band (not shown). This band is blue-
shifted 63.7 cm-1 from the symmetric SO2 stretching
mode in solid argon at 1147.2 cm-1, but its 12.9 and
43.5 cm-1 sulfur-34 and oxygen-18 shifts are close to
values for a simple S-O vibration. The above 16/18
isotopic frequency ratios again point to antisymmetric
and symmetric OdUdO stretching modes as found for
the major product molecule. The steady increase of these
new absorptions on annealing suggests a precursor ad-
duct, but the blue shifts (36.1 and 25.3 cm-1) in the
OdUdO stretching modes are unusual. This indicates
that any intermolecular interaction does not directly
involve the oxide centers. Only natural isotopic counter-
parts of the major bands were observed in solid neon at
936 and 855 cm-1, which represent 9 and 8 cm-1 blue
shifts from the argon matrix values.
SUO2 þ vac uv f SUO2þ
Uþ þ SO2 f SUO2þ
ð2Þ
ð3Þ
S þ UO2þ f SUO2þ
ð4Þ
SUO Identification. The very weak 857.0 cm-1 band
increases on annealing, decreases on ultraviolet irradia-
tion, but restores on subsequent annealing as illustrated
in Figures 1 and 2 (to absorbance 0.005). The band shows
no sulfur-34 shift but a large oxygen-18 shift to 811.5 cm-1
(ratio 1.05607). There is no evidence for a mixed oxygen
isotopic counterpart in the 820-840 cm-1 region. This
oxygen 16/18 isotopic frequency ratio is appropriate for a
single O vibrating against U as in UO itself, but UO
absorbs at 819.7 cm-1 in solid argon.1,2,4,7 A neon matrix
counterpart was observed at 871.5 cm-1 with both sulfur
isotopes, but no oxygen-18 counterpart could be detected.
This blue neon matrix shift is near those observed for
SUO2, and larger than that found for US2 (3.6 cm-1).8
These relatively small argon-neon matrix shifts suggest
that the electronic state is the same in both matrix hosts,
in marked contrast to the large change in UO2 on going
from solid argon to neon.4
Calculations were launched for this probable adduct
using both S and O coordination, and the C1 (SO2)
(SUO2) complex is predicted to be more strongly bound,
by 34 kcal/mol instead of only 6. The preference for SO2
to bind to U in SUO2, which is a fairly hard center,
through O rather than through S should not be a surprise:
SO2 protonates preferentially on O rather than on S, with
a difference of proton affinity at the B3LYP level of
theory of 53 kcal/mol. The OUO modes in the C1 (SO2)
(SUO2) complex are blue-shifted by 25 and 17 cm-1, and a
very strong -SdO stretch is computed 66 cm-1 above
the symmetric SO2 mode. This is significant because the
weak band observed 63.7 cm-1 above the symmetric
SO2 mode exhibits 1.01077 and 1.03726 isotopic 32/34
and 16/18 frequency ratios, which are in excellent agree-
ment with the 1.01030 and 1.03817 computed values.
Furthermore, no red-shifted symmetric SO2 mode coun-
terpart was found like that predicted for the S-bonded
adduct. The weaker, lower bond stretching modes
Our preliminary B3LYP calculations for the SUO
molecule, a relative of OUO and SUS,1,2,4,7,8 find a linear
triplet low energy state with strong computed U-O
stretching mode at 904.6 cm-1 and 16/18 ratio 1.0561
with no sulfur-34 displacement. The much weaker U-S
stretching mode computed at 453.7 cm-1 is too low for us
to observe. Another low energy triplet state is bent (121°)
and has a calculated U-O stretch at 867.6 cm-1 (16/18
ratio1.0566). Hence, the U-O stretching mode of SUO is
expected to fall in the 850-900 cm-1 range.
The weak 857.0 cm-1 argon matrix and 871.5 cm-1
neon matrix bands are probably due to the SUO molecule
although our case would be stronger with more intense
bands and the observation of both stretching modes. This
molecule is interesting because of its relationship to OUO
(linear) and SUS (bond angle near 120°), which have been
investigated in our laboratories.3,7,8 In addition SUO is
surely a multireference problem that will require higher

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6895
were not observed here. Overall the agreement between
the observed and computed spectra for the C1 (SO2)
(SUO2) complex is excellent and it substantiates this
assignment.
for antisymmetric and symmetric OdUdO stretching modes
and mixed isotopic components for the vibration of two
equivalent oxygen atoms. The UO3, SUO2, US3 molecules
have analogous C2v structures, based on density functional
calculations, but the analogous group 6 metal contain-
ing molecules have pyramidal structures.33,35,36 The SUO2
molecule acts as a fairly strong Lewis acid toward SO2 and
H2O, so a rich coordination chemistry of these molecules can
safely be anticipated. The SUO2+ molecular cation and the
SUO molecule have been detected through their strongest
absorptions. The SUO molecule promises to be a computa-
tional challenge like its UO2 and US2 counterparts.
SO2 þ SUO2 f ðSO2ÞðSUO2Þ
ð7Þ
Since the binding of SO2 to SUO2 turns out to be more
favorable than we had anticipated (34 kcal/mol), we
investigated the interaction between a simpler Lewis base
(H2O) and SUO2. The binding energy is still appreciable,
at 23 kcal/mol. It thus appears that there is a rich
coordination chemistry of SUO2 (and presumably also
for UO3) awaiting discovery.
Acknowledgment. We gratefully acknowledge financial
support from Subgrant 6855694 under Prime Contract
No. DE-AC02-05CH11231 to the Lawrence Berkeley
National Laboratory from the DOE and NCSA comput-
ing Grant CHE07-0004N to L.A.
Conclusions
Laser ablated U atoms react with SO2 on deposition and
on annealing in solid argon and neon to form the SUO2
molecule, which is identified from two strong OdUdO
stretching modes as predicted by DFT. These infrared
absorptions exhibit oxygen 16/18 isotopic frequency ratios
(35) Wang, X.; Andrews, L. J. Phys. Chem. A 2009, 113, in press. (Gr 6+
SO2).
(36) Liang, B.; Andrews, L. J. Phys. Chem. A 2002, 106, 6945. (Cr, Mo,
W+S2).