Infrared spectra and quantum chemical calculations of the uranium carbide molecules UC and CUC with triple bonds

Article abstract of DOI:10.1021/ja102475t

Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm^-1. A new band at 827 cm ^-1 becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm^-1 gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the C≡U≡C ground state as linear ³∑u⁺ with 1.840 A bond length and molecular orbital occupancies for an effective bond order of 2.83. Similar calculations with spin-orbit coupling show that the U≡C diatomic molecule has a quintet (λ = 5, δ = 3) ground state, a similar 1.855 A bond length, and a fully developed triple bond of 2.82 effective bond order.

Full text of DOI:10.1021/ja102475t

Published on Web 05/26/2010
Infrared Spectra and Quantum Chemical Calculations of the
Uranium Carbide Molecules UC and CUC with Triple Bonds
†
,†
‡
‡
Xuefeng Wang, Lester Andrews,* Per-Åke Malmqvist, Bj o¨ rn O. Roos,
§
§
§
|
Ant o´ nio P. Gon c¸ alves, Cl a´ udia C. L. Pereira, Joaquim Mar c¸ alo, Claude Godart,
|
and Benjamin Villeroy
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia, 22904-4319,
Department of Theoretical Chemistry, Chemical Center, UniVersity of Lund, POB 124,
2
-221 00 Lund, Sweden, Unidade de Ci eˆ ncias Qu ´ı micas e Radiofarmac eˆ uticas, Instituto
Tecnol o´ gico e Nuclear/CFMCUL, 2686-953 SacaV e´ m, Portugal, and Chimie M e´ tallurgique des
Terres Rares, ICMPE, CNRS, 2/8 rue Henri Dunant, 94320 Thiais, France
Received March 31, 2010; E-mail: lsa@virginia.edu
Abstract: Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid
-
1
argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm . A new
-1
band at 827 cm becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic
frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm-1
gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic
of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the CtUtC ground state
3
+
as linear Σ
.83. Similar calculations with spin-orbit coupling show that the UtC diatomic molecule has a quintet
Λ ) 5, Ω ) 3) ground state, a similar 1.855 Å bond length, and a fully developed triple bond of 2.82
effective bond order.
u
with 1.840 Å bond length and molecular orbital occupancies for an effective bond order of
2
(
Introduction
over solid UC at 2400-2700 K and for UC from the pressure-
2
independent equilibrium UC (g) + U(g) ) 2UC(g) over a
2
Uranium carbides are hard, refractory ceramic materials which
exist in several stoichiometries including uranium monocarbide,
UC, diuranium tricarbide, U C , and uranium dicarbide, UC .
2 3 2
6
uranium-boron alloy plus carbon at 2500-2800 K. Subsequent
mass spectroscopic measurements of atomization energies
suggest that polyatomic uranium carbides will contribute to the
1,2
Uranium dicarbide is used as a fuel for nuclear reactors, usually
in the form of solid pellets either as the carbide or mixed with
the dioxide. Such fuel is proposed in designs for nuclear-
powered rockets owing to better power density. Thus, uranium
carbides are appropriate as fuels for new generations of nuclear
7
vapor phase of the uranium-carbon system over 3000 K.
The reaction of laser-ablated U atoms and CO molecules
produced the stable CUO molecule, which is isoelectronic with
2+
8-10
the ubiquitous UO
approach cannot be applied to binary uranium carbides such as
CUC since clean sources of molecular C as a reagent,
2
uranyl dication.
Unfortunately, this
3
-5
reactors.
2
Solid uranium carbides are very stable, and small molecules
containing uranium and carbon will also be stable and probably
contain multiple bonds if methods can be devised to prepare
and investigate these molecules. Mass spectroscopic evidence
particularly with the carbon-13 isotopic enrichment necessary
for vibrational analysis, are not readily available. Hence, we
must find a new method to produce U and C atoms for reaction
under conditions where novel molecular products can be
isolated, and laser evaporation of arc melted uranium/carbon
alloy targets, which can be prepared with carbon-13 using a
newly developed technique, appeared to be promising. Accord-
ingly, U/C alloy targets were laser evaporated at temperatures
in excess of 3300 K, and the resulting atoms were reacted in a
solid argon matrix environment and trapped at 8 K.
2
has been obtained for UC as a trace component in the vapor
†
University of Virginia.
University of Lund.
Instituto Tecnol o´ gico e Nuclear/CFMCU.
ICMPE.
‡
§
|
(
(
1) (a) Austin, A. E. Acta Crystallogr. 1959, 12, 159–161. (b) Bowman,
A. L.; Arnold, G. P.; Witteman, W. G.; Wallace, T. C.; Nereson, N. G.
Acta Crystallogr. 1966, 21, 670–671.
2) (a) Atoji, M. J. Chem. Phys. 1967, 47, 1188–1189. (b) Wells, A. F.
Structural Inorganic Chemistry, 5th ed.; Oxford University Press:
Oxford, UK, 1984.
(6) (a) Norman, J. H.; Winchell, P. J. Phys. Chem. 1964, 68, 3802–3805,
and references therein. (b) Gingerich, K. A J. Chem. Phys. 1969, 50,
2255–2256.
(
(
(
3) Hunt, R. D.; Lindemer, T. B.; Hu, M. Z.; Del Cul, G. D.; Collins,
(7) Gupta, S. K.; Gingerich, K. A. J. Chem. Phys. 1979, 71, 3072–3080.
(8) Zhou, M. F.; Andrews, L.; Li, J.; Bursten, B. E. J. Am. Chem. Soc.
1999, 121, 9712–9721.
J. L. Radiochim. Acta 2007, 94, 225–232.
4) Utton, C. A.; De Bruycker, F.; Boboridis, K.; Jardin, R.; Noel, H.;
Gueneau, C.; Manara, D. J. Nucl. Mater. 2009, 385, 443–448.
5) (a) Vasudevamurthy, G.; Knight, T. W. Nuclear Tech. 2008, 163, 321–
(9) Li, J.; Bursten, B. E.; Liang, B.; Andrews, L. Science 2002, 295, 2242–
2245.
3
4
27. (b) Petti, D.; Crawford, D.; Chauvin, N. MRS Bull. 2009, 34,
(10) Andrews, L.; Liang, B.; Li, J.; Bursten, B. E. J. Am. Chem. Soc. 2003,
125, 3126–3139.
0–45.
8
484 9 J. AM. CHEM. SOC. 2010, 132, 8484–8488
10.1021/ja102475t  2010 American Chemical Society

Uranium Carbide Molecules UC and CUC
A R T I C L E S
Multiple bonding between uranium and main group elements
1
1-17
has become of considerable interest.
numerous UdO bonds, but fewer UdNX and UdCX
are known. The imido (AndNX), phosphinidene (AndPX), and
Uranium forms
2
bonds
1
3-17
N-U-N molecular linkages have been prepared,
simple NHdUH , CH dUH , CH dUHX, CH
CHtUX molecules have been produced and identified in
matrix isolation experiments and characterized by density
and the
2
2
2
2
2
dUX , and
2
3
1
8-22
functional theory calculations.
Ligand-supported uranium
carbene complexes have been prepared, and the uranium-carbon
bond has been measured in uranium phosphoylide and nucleo-
philic carbene complexes, but the bond lengths are much longer
23,24
than computed for the above simple methylidene complexes.
Hence, the preparation and characterization of isolated binary
uranium carbon bearing molecules containing multiple bonds
is of practical and fundamental importance.
Figure 1. Infrared spectra of uranium and carbon atom reaction products
-
1
in the 900-780 cm region: (a) U and C from laser evaporation of alloy
1
2
13
Experimental Details
target [U/C ) 1/4, C/ C ) 1/1] codeposited in excess argon at 8 K for
0 min, (b) after annealing to 20 K, (c) after annealing to 30 K, (d) after
290 nm irradiation for 20 min, (e) after annealing to 30 K, and (f) after
6
>
The material evaporated from arc-melted, depleted uranium/
carbon alloy targets using a pulsed YAG laser, as described
annealing to 35 K.
2
5
previously for pure metals, was collected in a condensing argon
stream at 6-8 K. A standard arc furnace melting method was
employed for reacting cleaned uranium turnings and graphite
Although spark plasma sintering is commonly used to produce
dense metals, intermetallics, and ceramics, there are no reports on
the application of this technique to produce dense solids from
amorphous starting powders. This new treatment of solid carbon-
2
6
pieces, and alloy targets with U/C mole ratios of 1/2, 1/4, and
/7 were used to produce new infrared absorptions. Powder X-ray
patterns showed that these targets are mixtures of UC and graphite.
1
2
1
3 will find other applications that require stable isotopic substitu-
Unfortunately, this method did not work for amorphous carbon-13
powder, which is the only available form of pure solid carbon-13
tion in spectroscopy, intermetallics, and ceramics.
27
Results and Discussion
isotopic material. However, the spark plasma sintering technique
2
8
employed for ceramics was adapted to make a dense pellet of
solid carbon-13 from amorphous carbon-13 powder. This carbon-
Infrared spectra of the reaction products formed from ablated
material were recorded, and the freshly deposited sample reveals
very weak bands that increase on annealing to 20 and 30 K.
1
3 pellet was then arc melted with normal isotopic graphite and
cleaned uranium turnings to form the U/C-12/C-13 and U/C-13
pellets employed here (U/C mole ratios 1/4).
2
9-31
Weak C
1
3
, C
2
O, and UO bands
are observed at 2039.2,
-1
968.9, and 820.0 cm , and a moderate CO absorption appears
at 2138.5 cm in these experiments. Weak bands at 852.6 and
804.4 cm are in agreement with absorptions reported for the
-
1
(
(
11) Burns, C. J. Science 2005, 309, 1823–1824.
-
1
12) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Palmer, P. D.; Batista,
E. R.; Hay, P. J. Science 2005, 310, 1941–1943.
triplet state CUO molecule isolated in and interacting with
(
(
13) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Science 2005, 309, 1835–
9,10
argon.
8
Significant new reaction product bands appeared at
1
838.
-
1
91.4 and 827.4 cm , and the C
3
band also increased on
14) Gagliardi, L.; Pyykk o¨ , P. Angew. Chem., Int. Ed. 2004, 43, 1573–
1
576.
annealing. Ablation of a U/C-13 pellet produced these new
(
(
15) Ephritikhine, M. Dalton Trans. 2006, 2501–2516.
-1
13
-1
3
bands shifted to 859.9 and 797.0 cm with C at 1960.9 cm ,
16) Brennan, J. G.; Andersen, R. A. J. Am. Chem. Soc. 1985, 107, 514–
and 13
-1
2
C O at 1918.4 cm .
5
16.
(17) Arney, D. S.; Burns, C. J.; Schnabel, R. C. J. Am. Chem. Soc. 1996,
18, 6780–6781.
(18) Wang, X.; Andrews, L.; Marsden, C. J. Chem.sEur. J. 2005, 14, 9192–
(
The most important experiments were performed using the
target containing uranium with half carbon-12 and half carbon-
1
1
3, which is necessary to determine the carbon stoichiometry
9
201 (NH)UH ).
2
of the new product molecules from vibrational spectroscopy.
These spectra, shown in Figure 1 after sample deposition,
annealing, and >290 nm irradiation, contained carbon isotopic
19) (a) Lyon, J. T.; Andrews, L.; Malmqvist, P.-Å.; Roos, B. O.; Yang,
T.; Bursten, B. E Inorg. Chem. 2007, 46, 4917–4925. (b) Roos, B. O.;
Lindh, R.; Cho, H.-G.; Andrews, L. J. Phys. Chem A 2007, 111, 6420–
6
424 (CH
20) Lyon, J. T.; Andrews, L. Inorg. Chem. 2006, 45, 1847–1852
CH dUHX).
21) Lyon, J. T.; Andrews, L.; Hu, H.-S.; Li, J. Inorg. Chem. 2008, 47,
435–1442 (CH dUF ).
22) Lyon, J. T.; Hu, H.-S.; Andrews, L.; Li, J. Proc. Natl. Acad. Sci. U.S.A.
007, 104, 18919–18924 (CHtUX ). CHtUF has a 2.56 effective
UC bond order.
2 2
dUH ).
-
1
doublets for the first bands at 852.6, 837.0 cm and at 804.3,
(
(
(
-
1
(
2
789.1 cm for the CUO molecule containing a single carbon
9
,10
atom.
An isotopic doublet was observed at 827.4 and 797.0
1
2
2
-1
cm for the lower new product and a triplet at 891.4, 880.6,
and 859.9 cm for the higher new band. An isotopic doublet
was also observed at 2138.5 and 2091.4 cm for CO and
CO, respectively, with absorbances of 0.024 and 0.018. These
-
1
2
3
3
-
1
12
(
23) (a) Cramer, R. E.; Maynard, R. B.; Paw, J. C.; Gilje, J. W. J. Am.
Chem. Soc. 1981, 103, 3589. (b) Stevens, R. C.; Bau, R.; Cramer,
R. E.; Afzal, D.; Gilje, J. W.; Koetzle, T. F. Organometallics 1990,
13
CUO products are probably made here first on annealing by
reaction of UO evaporated from the alloy target surface with C
atoms in the matrix and on >290 nm photolysis as observed
9
, 694.
(
(
(
(
(
24) Cantat, T.; Arliguie, T.; Noel, A.; Thuery, P.; Ephritikhine, M.; Le
Floch, P.; Mezailles, N. J. Am. Chem. Soc. 2009, 131, 963–972.
25) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040–4053
(
review article).
(29) Weltner, W., Jr; McLeod, D., Jr. J. Chem. Phys. 1964, 40, 1305.
(30) (a) Jacox, M.; Milligan, D. E.; Moll, N. G.; Thompson, W. E. J. Chem.
Phys. 1965, 43, 3734. (b) DeKock, R. L.; Weltner, W., Jr; McLeod,
D., Jr. J. Am. Chem. Soc. 1971, 93, 7106.
26) Kruger, O. L.; Armstrong, J. L. ReV. Sci. Instrum. 1964, 35, 156 (arc
furnace).
27) Cambridge Isotope Laboratories, Cambridge, MA, www.isotope.
com.
(31) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1993, 98, 3690 (UO, UO
2
28) Omori, M.; Hirai, T. New Ceram. 1994, 7, 23 (spark plasma sintering).
3
and UO ).
J. AM. CHEM. SOC. 9 VOL. 132, NO. 24, 2010 8485

A R T I C L E S
Wang et al.
Table 1. Calculated Structural Parameters and Harmonic
matrix value is higher and nearer to the computed values, which
is the typical matrix host relationship where argon interacts more
a
Frequencies for UC and CUC
4
0
-1
molecule
CASPT2
B3LYP
BPW91
obsd in Ar
strongly with the guest molecule. Since the 44 cm argon to
neon matrix shift is larger than normal, it is possible that a
different electronic state of UC is trapped in solid argon than
in solid neon, which will be investigated through further
calculations. It is difficult to predict the gas phase UC ground
state fundamental frequency from these observations, but 900
b
UC
X
1.855
938
1.859
934 (116)
1.834/180
1.864
908 (100)
1.846/180
952 (σ
891 (σ
5
827.4
891.4
b
CUC
1.840/180
933 (σ
89 (σ
3
+
c
Σ
u
u
)
g
976 (σ
u
, 252)
, 0)
u
, 228)
, 0)
, 89 × 2)
8
, 0) 918 (σ
g
g
1
10 (π
u
, 88 × 2) 54 (π
u
-1
(
10 cm is reasonable. Finally these calculations substantiate
a
Calculations performed at the CASPT2//ANO-RCC-VTZP, B3LYP,
this preparation and identification of the UC diatomic molecule.
Next, the new isotopic triplet with 1/2/1 relative intensity
components is clearly due to a new molecule with two
equivalent carbon atoms as double-statistical weight applies to
the two equivalent 12-13 and 13-12 orientations. Furthermore,
a linear CUC molecule would have an antisymmetric U-C
stretching mode with a harmonic carbon-12/carbon-13 isotopic
frequency ratio of 1.0370, which is slightly higher than the
observed 1.0366 value owing to anharmonicity in the observed
frequencies. Thus, the mixed carbon isotopic triplet and the
b
or BPW91//6-311+G(3df)//SDD level. Bond lengths/angles are in Å
and deg. Frequencies in cm with mode symmetry, intensities, km/
mol, in parentheses. CASPT2 values are corrected for anharmonicity.
c
-1
8
-10
before from the U + CO reaction.
3
The C absorption
doubled on the annealing sequence, which also attests to the
diffusion and reaction of C atoms in these samples. The
observation of a known single U atom bearing species, CUO,
suggests strongly that these new product molecules also most
likely contain single U atoms, a fact substantiated by the carbon
-
1
magnitude of the carbon-13 shift for the 891.4 cm absorption
clearly identify the linear CUC molecule.
isotopic vibrational analysis below. Finally, the C
3
absorption
produced a mixed carbon isotopic sextet, which confirms
randomization of the carbon atoms in these samples.
2
9
Preliminary density functional calculations using the BPW91
3
+
functional predict a linear CUC molecule in the Σ
u
ground
First, the new carbon isotopic doublet at 827.4 and 797.0
cm- is due to another single carbon atom bearing species, and
it exhibits the carbon-12/carbon-13 isotopic frequency ratio
state with intense antisymmetric harmonic stretching frequency
1
-
1
12
13
at 952.0 cm (228 km/mol intensity), CU C component at
-
1
9
39.5 cm (208 km/mol) with symmetric counterpart at 868.8
1
.0381. This is near and slightly lower than the 1.0388 value
-1
13
13
-1
cm (12 km/mol), and CU C counterpart at 918.0 cm (212
km/mol). The observed isotopic ratios 891.4/880.6 ) 1.0123
and 891.4/859.9 ) 1.0366 are slightly lower than the calculated
ratios (1.0133 and 1.0370) for the reason discussed above.
Furthermore, the asymmetry in the carbon isotopic triplet is due
to the unobserved symmetric stretching mode falling at lower
frequency (calculated for CU C at 890.9 cm ) and interacting
with the antisymmetric mode in the mixed 12CU C isotopic
molecule of lower symmetry. As a result, the computed intensity
of the central component is reduced by 12 km/mol, which
appears in the symmetric mode counterpart given above. The
observed carbon isotopic multiplet absorbances also reveal this
calculated for a harmonic U-C diatomic oscillator, which is
appropriate for the UC diatomic molecule with a small anhar-
monic contribution to the vibrational potential function. Quan-
tum chemical calculations using the hybrid B3LYP and pure
25,32-34
BPW91 density functionals as performed previously
find
a quintet ground state for UC with 1.859 and 1.864 Å bond
lengths and 934 and 908 cm- harmonic vibrational frequencies,
respectively (Table 1). The wave function based CASPT2
12
12
-1
1
13
3
5-37
method with spin orbit coupling
results in the quintet
ground state (Λ ) 5, Ω ) 3) for UC with the 1.855 Å bond
length and a 945 cm- harmonic or 938 cm anharmonic
frequency. The calculation of vibrational frequencies is not an
exact science, but the close agreement of two different density
functional and state of the art wave function frequency calcula-
1
-1
-
1
behavior: 891.4 cm (0.0044 absorbance, ( 0.0001), 880.6
-
1
-1
cm (0.0083 a), and 859.9 cm (0.0045 a). Although there is
a small isotopic effect on intensity, the carbon-12 and -13 in
these samples are approximately equally abundant, and the
central mixed isotopic component should in the absence of any
interaction be stronger, about 0.0089 a. It is observed to be
weaker owing to interaction and intensity sharing with the
nearby symmetric stretching mode of the mixed isotopic
-
1
tions points to a higher frequency for UC than the 827 cm
argon matrix observation. Subsequent neon matrix experiments
find this carbon isotopic doublet blue-shifted to 871.6 and 839.6
-1
cm , which is more in line with the results of calculations that
38,39
still slightly overestimate the observed frequency.
The neon
-
1
molecule. A very weak band at 822.6 cm (0.0007 a) tracks
-
1
(
32) Frisch, M. J., et al. Gaussian 09, ReVision A.02, Gaussian, Inc.:
Wallingford, CT, 2009.
with the 880.8 cm band, and this position and intensity are
appropriate for such an assignment. The B3LYP density
functional typically finds a slightly higher frequency, and the
(
33) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
Y.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Becke, A. D. Phys.
ReV. A 1988, 38, 3098. (d) Perdew, J. P.; Burke, K.; Wang, Y. Phys.
ReV. B 1996, 54, 16533, and references therein.
-
1
38,39
976 cm value for CUC is in accord.
The CASPT2 calculations with dynamical correlation find
(
(
34) (a) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,
3
+
the ground state as linear Σ
u
with 1.840 Å bond length and
3
265. (b) K u¨ chle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys.
994, 100, 7535.
-
1
1
predict a 933 cm antisymmetric C-U-C stretching funda-
mental, which is corrected for anharmonicity. The Σ state is
g
35) Roos, B. O., The Complete Active Space Self-Consistent Field Method
and its Applications in Electronic Structure Calculations. In AdVances
in Chemical Physics; Ab Initio Methods in Quantum Chemistry - II;
Lawley, K. P., Ed.; John Wiley & Sons Ltd.: New York, 1987; Chapter
1 +
higher in energy and unstable toward bending but stays
comfortably higher than the triplet minimum (the bent minimum
is about 3 kcal/mol higher than the linear triplet minimum).
The agreement of three different computed values just above
6
9, p 399.
(
(
36) Andersson, K.; Malmqvist, P.-Å.; Roos, B. O. J. Chem. Phys. 1992,
9
6, 1218.
-
1
37) Roos, B. O.; Lindh, R.; Malmqvist, P.-Å.; Veryazov, V.; Widmark,
P.-O. J. Phys. Chem. A 2005, 109, 6575 (ANO-RCC-VTZP) basis.
38) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
39) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 2937–
the 891.4 cm argon matrix value confirms our identification
of the linear, symmetrical CUC molecule.
(
(
2
941.
(40) Jacox, M. E. Chem. Phys. 1994, 189, 149.
8
486 J. AM. CHEM. SOC. 9 VOL. 132, NO. 24, 2010

Uranium Carbide Molecules UC and CUC
A R T I C L E S
3
+
Figure 3. CASPT2 molecular orbitals for the Σ
u
CUC ground state plotted
-
3
using an isodensity of 0.04 e au . Occupation numbers are given in
parentheses.
1
+
)2
For
electrons, a Σ
g
ground state, and a triple bond of type (σ
g
2
4
4
43-45
(
σ
u
) (π
g
u
) (π ) with recently computed EBO of 2.87.
NUN, the N 2s orbital is considered as “semicore”, and the σ
molecular orbitals are built from nitrogen 2p σ orbitals.
However, for CUC the σ
instead C 2s and not 2p. The carbon 2p orbitals appear in the
half-filled, paramagnetic σ and σ molecular orbitals coupled
g
and σ
u
molecular orbitals involve
Figure 2. CASPT2 molecular orbitals for the quintet (Λ ) 5, Ω ) 3) UC
-3
ground state plotted using an isodensity of 0.04 e au . Occupation numbers
are given in parentheses.
g
u
with the uranium 6d σ and 5f σ, respectively. Qualitatively,
this can be understood since carbon is lighter, and it therefore
neither wants nor has so many 2p electrons.
These experiments also provide important information on
the reactivity of atomic uranium. Atomic U in the ground
The UC and CUC bond lengths computed by three quantum
chemical methods are given in Table 1. The sum of triple bond
covalent radii given by Pyykk o¨ et al., 178 pm, is slightly
shorter than our three computed values.
4
1
state does not react with CO or N
CUO or NUN are formed.
2
, but on photoexcitation,
The oxygen molecule is,
however, more reactive, and the U insertion with dioxygen
CASPT2 molecular orbitals for quintet (Λ ) 5, Ω ) 3) UC are
plotted in Figure 2. The doubly occupied σ and σ* and degenerate
pairs of π and π* MOs are shown in the top left corner, and the
effective bond order (EBO) determined from bonding minus
antibonding occupancies divided by two is 2.82, which character-
izes a fully developed triple bond. The singly occupied nonbonding
and atomic-like orbitals contain 3.92 electrons. Mulliken atomic
spin densities computed on U and C in the UC molecule are 3.52
and 0.48: U has donated 0.48 e to C, and the resulting dipole
moment is 3.94 D. Molecules with large dipole moments typically
have large dipole derivatives, and they tend to have high infrared
intensities, which is the case for UC.
1
0,44
4
5
gives OUO on annealing in a 18-24 K argon matrix. Here
we show that U and C atoms also react in a cold argon matrix.
Laser ablation of the U/C alloy samples produces predomi-
nantly atomic species as the initial spectra show virtually no
product absorptions. The atom reaction 1 clearly proceeds
to form UC, and CUC is produced by the second reaction 2
with another C atom. Dicarbon, C
sample deposition, which is supported by the detection of
O, and we expect the insertion reaction of U with C to
2
, is probably formed on
C
2
2
be spontaneous as it is with dioxygen. The enthalpy change
Similar CASPT2 molecular orbitals for CUC are plotted in
Figure 3. Notice two pairs of σ and σ* and four pairs of π and
π* occupied MO’s, which determine the net bonding, and the
singly occupied σ and σ* MO’s, which result in a triplet ground
state. The EBO is 2.83, which is comparable to that in UC.
Although CUC and CUO are both U(VI) molecules, the UtC
bond in CUO is slightly shorter (1.756 Å, BPW91) and the U-C
stretching frequency higher (1047.5 cm- for the singlet state
in solid neon) than for CUC. This may be due in part to the
slightly higher positive charge on uranium in CUO than in
for reaction 3 from mass spectroscopic thermochemical data
7
is -158 ( 3 kcal/mol. Finally, the growth of C
3
on annealing
attests to sequential carbon atom reactions in these samples.
U + C f UC
(1)
(2)
(3)
UC + C f CUC
1
U + C f CUC
2
4
2
CUC, which results in a larger ionic component and stronger
bonding in CUO than in CUC. Finally, the UtC bonds
characterized here are shorter and stronger than those in the
C + C f C
(4)
2
3
2
2
3
simple CHtUX methylidyne complexes.
It is interesting to compare the triple bonding in the similar
molecules CUC and NUN where the latter has two more
(43) (a) Gagliardi, L.; La Manna, G.; Roos, B. O. Faraday Discuss. 2003,
124, 63. (b) Vlaisavljevich, B.; Gagliardi, L.; Wang, X.; Liang, B.;
Andrews, L.; Infante, I. To be published, 2010.
(
44) Hunt, R. D.; Yustein, J. T.; Andrews, L. J. Chem. Phys. 1993, 98,
6070 (UN ).
(45) Zhou, M.; Andrews, L.; Ismail, N.; Marsden, C. J. Phys. Chem A
2000, 104, 5495 (UO in solid neon and argon).
(
(
41) Pyykk o¨ , P.; Riedel, S; Patzschke, M. Chem.sEur. J. 2005, 12, 3511.
42) Mulliken atomic charges, BPW91. CUO: C,-0.74; U, 1.59; O,-0.85.
CUC: U, 1.42; C,-0.71.
2
2
J. AM. CHEM. SOC. 9 VOL. 132, NO. 24, 2010 8487

A R T I C L E S
Wang et al.
Conclusions
Acknowledgment. We gratefully acknowledge financial support
from DOE Grant No. DE-SC0001034, NCSA computing Grant No.
CHE07-0004N, the Swedish Research Council (VR) for Grant No.
We have prepared binary uranium carbon bearing molecules
in optical spectroscopic quantities from U and C atom reactions
and identified them from carbon-13 substitution in the matrix
infrared spectrum and comparison with frequencies calculated
by three quantum chemical methods. Laser-evaporated U and
C atoms collected in solid argon at 8 K react directly without
activation energy on warming the matrix sample to 20-30 K
and form the novel UtC and CtUtC molecules with
uranium-carbon triple bonds as described by CASPT2 molec-
ular orbital calculations.
70526201, and the Funda c¸ a˜ o para a Ci eˆ ncia e a Tecnologia through
the Ci eˆ ncia 2007 Program (Portugal).
Supporting Information Available: Complete ref 32. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA102475T
8
488 J. AM. CHEM. SOC. 9 VOL. 132, NO. 24, 2010