As≡UF3 molecule with a weak triple bond to uranium

Article abstract of DOI:10.1021/ic9005696

After reactions of uranium atoms with NF₃ and PF₃ to form the N≡UF₃ and P≡UF₃ molecules, the analogous reaction with AsF₃ produced the novel terminal arsenide As≡UF₃. This first molecule wi

Full text of DOI:10.1021/ic9005696

6594 Inorg. Chem. 2009, 48, 6594–6598
DOI: 10.1021/ic9005696
AstUF3 Molecule with a Weak Triple Bond to Uranium
Lester Andrews* and Xuefeng Wang
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319
::
Bjorn O. Roos
Department of Theoretical Chemistry, Chemical Center, University of Lund, POB 124, 2-221 00 Lund, Sweden
Received March 23, 2009
After reactions of uranium atoms with NF3 and PF3 to form the NtUF3 and PtUF3 molecules, the analogous reaction
with AsF3 produced the novel terminal arsenide AstUF3. This first molecule with a uranium-arsenic bond was
identified from matrix infrared spectra through comparison with spectra of the uranium nitride and phosphide species,
with spectra using other metals, and with frequencies computed by density functional and multiconfigurational wave
function methods. The latter calculation describes a weak triple bond to uranium in the AstUF3 molecule, which has
slightly less bonding and more antibonding character than the weak triple bond in PtUF3.
Introduction
FCtUF3 molecules.11-14 The well-known stability of ura-
nium fluorine bonds has made possible the straightforward
synthesis of the first UtC triple bond and simple terminal
UtN and UtP bond containing molecules. Through the
reaction of laser ablated U atoms with CHF3, NF3, and PF3,
the simple HCtUF3, NtUF3, and PtUF3 molecules have
been prepared, their matrix infrared spectra has been ob-
served, their vibrational frequencies and structure have been
calculated, and the bonding has been analyzed.14,15 The
UtN triple bond is the strongest as the more contracted 2p
orbitals of N bond more effectively with U 5f orbitals than
the more diffuse P 3p orbitals. This is also expected because
the electronegativity difference is larger and thus the ionic
contribution to the bond energy is greater in the case of N.
Thus, it should be possible to prepare the first U-As bonds
from the analogous reaction with AsF3, but these multiple
bonds will be weak. Parallel investigations show that Mo and
W atoms react with AsF3 upon excitation by laser ablation or
UV irradiation to form stable trigonal AstMF3 terminal
arsenides.16 Here follows a report on the formation, matrix
infrared spectrum, computed structure, and bonding in the
AstUF3 molecule.
Understanding uranium chemistry is important because of
the numerous far-reaching applications for his early actinide
element.1 The relatively large size and availability of 5f
orbitals for bonding interactions may facilitate unique cata-
lytic reactions.2 Interesting complexes with metal-ligand
multiple bonds including uranium nitrogen species have been
prepared.3-6 Such complexes with organoimido (UdNR)
and phosphinidene (UdPR) groups7,8 provide examples, but
no arsenic derivatives are known with any order of U-As
bond. In fact no compounds with U-As bonds are listed in
the Cambridge Structural Database.9 The heavier main
group elements are much less inclined to form multiple
bonds, and their chemistry reflects this difference.10
The first examples of uranium methylidene and methyli-
dyne molecules have been prepared in our laboratory using
laser-ablated uranium atoms as the reagent. These include
the CH2dUH2, CH2dUHF, CH2dUF2, HCtUF3, and
*To whom correspondence should be addressed. E-mail: lsa@virginia.
edu.
(1) Ephritikhine, M. Dalton Trans. 2006, 2501.
(2) Fox, A. R.; Bart, S. C.; Meyer, K.; Cummins, C. C. Nature 2008, 455,
34.
˚
(11) Lyon, J. T.; Andrews, L.; Malmqvist, P.-A.; Roos, B. O.; Yang, T.;
(3) Burns, C. J. Science 2005, 309, 1823.
Bursten, B. E. Inorg. Chem. 2007, 46, 4917.
(4) Brown, D. R.; Denning, R. G. Inorg. Chem. 1996, 35, 6158.
(5) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Science 2005, 309, 1835.
(6) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Batista, E. R.; Hay, P. J.
J. Am. Chem. Soc. 2006, 128, 10549.
(7) Brennan, J. G.; Andersen, R. A. J. Am. Chem. Soc. 1985, 107, 514.
(8) Arney, D. S. J.; Schnabel, R. C.; Scott, B. C.; Burns, C. J. J. Am. Chem.
Soc. 1996, 118, 6780.
(12) Lyon, J. T.; Andrews, L. Inorg. Chem. 2006, 45, 1847; (U+CH3X).
(13) Lyon, J. T.; Andrews, L.; Hu, H.-S.; Li, J. Inorg. Chem. 2008, 47,
1435.
(14) Lyon, J. T.; Hu, H.-S.; Andrews, L.; Li, J. Proc. Natl. Acad. Sci. 2007,
104, 18919, and unpublished results.
(15) Andrews, L.; Wang, X.; Lindh, R.; Roos, B. O.; Marsden, C. J.
Angew. Chem., Int. Ed. 2008, 47, 5366.
(9) www.ccdc.cam.ac.uk.
(10) Massey, A. G. Main Group Chemistry, 2nd ed.; John Wiley and Sons:
˚
(16) Wang, X.; Andrews, L.; Knitter, M.; Malmqvist, P.-A.; Roos, B. O.
New York, 2000.
J. Phys. Chem. A 2009, 113, 6064; (Gr. 6 + AsF3).
r
pubs.acs.org/IC
Published on Web 06/10/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6595
Results and Discussion
Uranium atoms, generated by laser-ablation from a solid
metal target (Oak Ridge National Laboratory, high purity,
depleted of 235U) were found to react with AsF3 (Ozark-
Mahoning, vacuum distilled from dry NaF) diluted in argon
during condensation on a 4 K cesium iodide window as
described previously.12,17 After reaction, infrared spectra
were recorded at 0.5 cm-1 resolution using a Nicolet 750
spectrometer with an Hg-Cd-Te B range detector. Samples
were annealed to allow reagent diffusion and further reaction
and later irradiated for 15 min periods by a mercury arc street
lamp (175 W) with the globe removed using a combination of
optical filters. Common absorptions in uranium experiments
using different reagents were limited to trace quantities of
UN2 and UO2.18,19 Common absorptions in AsF3 experi-
ments with different metals include very weak 808 and
785 cm-1 bands for AsF5 and a pair of strong bands at
699.7 and 668.4 cm-1 for the AsF2 free radical.20
Infrared spectra in the U-F stretching region are com-
pared in Figure 1 for U reaction products with NF3, PF3, and
AsF3. The only new bands unique to U and AsF3 are the
sharp features at 580.9, 578.5, 536.7, and 535.0 cm-1, which
increased about 30% on annealing to 20 K, increased slightly
on >220 nm ultraviolet irradiation, then decreased on
annealing to 30 K. These bands are very slightly shifted from
analogous 581.2, 579.0, 536.2, and 534.8 cm-1 bands pro-
ducedby the PF3 reaction product.15 Two weak blue satellites
at 542.7 and 541.1 cm-1 on the AsF3 reaction product are
very much weaker than the analogous 541.7 and 539.7 cm-1
bands with PF3, which supports the possibility that (PF3)
(PtUF3) complexes may contribute to that product spec-
trum. Although AsF3 is more reactive than PF3, we detect
only a trace of UF4 from the very weak 532.4 and 530.3 cm-1
bands that appear in the spectrum on annealing to 20 K.21
Recall that UF6 was observed in the much more reactive NF3
and U system,15 and weak bands increased on annealing for
UF4 as well.
Figure 1. Infrared spectra of U atom reaction products with NF3, PF3,
and AsF3 in the 630-520 cm-1 region. (a) Spectrumafterco-depositionof
laser-ablatedU and NF3 at0.3% in argonat 4 K for 60min, (b) after240-
380 nm irradiation for 40 min, (c) spectrum after co-deposition of laser-
ablated U and PF3 at 0.4% in argon at 4 K for 60 min, (d) after >220 nm
irradiation for 20 min, (e) after annealing to 20 K, (f) spectrum after co-
depositionoflaser-ablatedU and AsF3 at 0.5%in argonat 4 K for 60min,
(g) after annealing to 20 K, (h) after >220 nm irradiation for 20 min, and
(i) after annealing to 30 K. Bands labeled “c” are common to AsF3
experiments with other metals.
Calculations with density functional (DFT: B3LYP,
BPW91)23-27 and wave function (CASSCF/CASPT2) meth-
ods28,29 predict the strong U-F stretching frequencies for
AstUF3 to be near those for PtUF3, which are in satisfac-
tory agreement with experimental observations (Table 1).
The CASSCF predictions are higher than the B3LYP values,
which are 20 cm-1 higher than the observed values, and the
BPW91 frequencies, which are about 15 cm-1 higher than the
observed values, which is typical for computed harmonic
frequencies in comparison to observed anharmonic values.30
Both fundamentals are apparently split approximately 2 cm-1
by interaction with the argon matrix. The calculation of
vibrational frequencies is not an exact science, and the
correlation of frequencies computed by several different
methods is reassuring.
Our earlier investigations of Mo, W, and U reactions with
PF3 showed that insertion followed by R-F-transfer led to the
formation of terminal nitride and phosphide metal(VI) spe-
cies as represented by eqs 1 and 2 for uranium.15,22 The
analogous reaction for AsF3 is straightforward.
Calculations for the triplet AsFdUF2 intermediate con-
verged instead to a bridged As(F)dUF2 structure, which for
B3LYP is 11 kcal/mol lower in energy than AstUF3 with its
U þ NF3 f NF2;UF f NFdUF2 f NUF3 ð1Þ
(23) Frisch, M. J., et al. Gaussian 03, Revision D.01; Gaussian, Inc.:
Pittsburgh, PA, 2004.
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.;
U þ PF3 f PF2;UF f PFdUF2 f PUF3
ð2Þ
Parr, R. G. Phys. Rev. B 1988, 37, 785.
(25) (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.
(26) Frisch, M. J.; Pople, J. A.; Binkley, J. S J. Chem. Phys. 1984, 80,
U þ AsF3 f AsF2;UF f AsFdUF2 f AsUF3 ð3Þ
3265.
(17) (a) Souter, P. F.; Kushto, G. P.; Andrews, L.; Neurock, M. J. Am.
Chem. Soc. 1997, 119, 1682; (U + H2). (b) Andrews, L.; Cho, H.-G.
Organometallics 2006, 25, 4040; (review article).
::
(27) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100,
7535.
(28) Roos, B. O. The Complete Active Space Self-Consistent Field
Method and its applications in Electronic Structure Calculations. In Ad-
vances in Chemical Physics; Ab Initio Methods in Quantum Chemistry -II;
Lawley, K. P., Ed.; John Wiley & Sons Ltd.: New York, 1987; Chapter 69, p
399.
(18) Hunt, R. D.; Yustein, J. T.; Andrews, L. J. Chem. Phys. 1993, 98,
6070; (UN2).
(19) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1993, 98, 3690; and
references therein (OUO).
(20) (a) Khider Aljibury, A. J.; Redington, R. L. J. Chem. Phys. 1970, 52,
453. (b) Brum, J. L.; Hudgens, J. W. J. Chem. Phys. 1997, 106, 485.
(21) (a) Kunze, K. R.; Hauge, R. H.; Hamill, D.; Margrave, J. L. J. Chem.
Phys. 1977, 81, 1664. (b) Wang, X.; Andrews, L., spectra from unpublished
U and F2 experiments.
˚
(29) Andersson, K.; Malmqvist, P.-A.; Roos, B. O. J. Chem. Phys. 1992,
96, 1218.
(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. Orga-
nometallics 2006, 25, 118.
(22) Wang, X.; Andrews, L.; Lindh, R.; Veryazov, V.; Roos, B. O. J. Phys.
Chem. A 2008, 112, 8030.

6596 Inorganic Chemistry, Vol. 48, No. 14, 2009
Andrews et al.
Table 1. Observed and Calculated Frequencies for the PtUF3 and AstUF3 Molecules in the Singlet Ground Electronic States with C3v or Cs Structuresa
PtUF3
AstUF3
approximate description
obs
cal(C)
int
cal(L)
int
cal(B)
int
obs
cal(C)
int
cal(L)
int
cal(B)
int
U-F str, a1,a0
U-F str, e, a00
581.2,
579.0
536.2,
534.8
619
560
259
430
608
558
201
170
595
552
170
141
580.9,
578.5
536.7,
535.0
617
566
267
406
602
560
206
160
594
555
182
136
U-F str, a0
554
399
135
133
91
147
22
4
16
9
545
375
127
122
108
56
143
12
11
3
8
14
15
554
234
125
99
87
62
148
3
5
8
18
7
548
235
122
120
106
52
139
3
11
3
6
9
11
EtU str, a0
n. o.b
404
148
111
26
19
12
347
157
114
34
11
12
F-U-F bend, a1, a0
F-U-F bend, e, a00
F-U-F bend, a0
E-U-F bend, e, a00
E-U-F bend, a0
25
54
86
56
12
18
23
40
21
35
8
21
a Frequencies and intensities are in cm-1 and km/mol. Observed in an argon matrix. Frequencies and intensities computed with CASPT2 (C), B3LYP
(L), or BPW91(B) methods in the harmonic approximation. Symmetry notations are first for C3v from CASSCF and second for Cs from DFT. b Not
observed because of falling below instrument limit.
stronger U-F mode computed at 545 cm-1 and a slightly
weaker band at 528 cm-1 and for BPW91 this bridged
structure is isoenergetic with AstUF3 and has a stronger
(BPW91), and 2.495 A (B3LYP) bond lengths, slightly longer
than the triple bond in PtUF3. Although the DFT frequen-
cies are closer to the observed values, the structures are
distorted to Cs with two equal U-F bonds and one slightly
shorter U-F bond and two corresponding As-U-F angles
and one larger As-U-F angle, which average to near the
CASPT2 angle (Table 2). Similar distortions were found with
both DFT methods for NtUF3 and PtUF3 (Table 2), but
again the frequencies were predicted more closely to the
observed values.15 The DFT methods underestimate the
EtU bond lengths and overestimate the U-F bond lengths
compared to CASPT2.
˚
U-F mode at 575 cm-1 and slightly weaker band at 554 cm-1
.
Since DFT frequencies are usually above observed values,30
the B3LYP frequencies 34 and 8 cm-1 below the observed
are not compatible with the As(F)dUF2 assignment, and the
BPW91 frequencies do not fit the observed values as well
as the calculated set in Table 1. Similar calculations for
P(F)dUF2 find a 16 kcal/mol lower structure with frequen-
cies also too low using B3LYP and an 18 kcal/mol higher
energy structure with frequencies too low using BPW91.
However, our B3LYP calculation for the triplet NFdUF2
intermediate converged to a stable structure 24 kcal/mol
higher in energy than the NtUF3 molecule identified re-
cently.15 It appears that the hybrid density functional favors
the bridged structures for the heavier main group element,
which has been observed previously,31 but the pure density
functional favors the imine structures. Of most importance
here, it is clear that the calculated frequencies for the bridged
structures do not correlate with the observed values. The
matrix cage probably makes it more difficult for such a
bridged species to form, but without matrix relaxation of
reaction exothermicity, gas phase reaction (eq 3) would give
only the UF and AsF2 products. Thus, matrix relaxation of
the reaction energy released makes possible the stabilization
of the AstUF3 molecule in the matrix cage.
The active molecular orbitals that contribute to this triple
bond shown in Figure 3 reveal slightly lower bonding occu-
pancies and higher antibonding occupancies for AstUF3
than for PtUF3 (compare Figure 5 in ref 15). The natural
orbital occupation numbers for the σ- and π-bonding and
antibonding orbitals add to give a weak triple bond in
PtUF3 with an effective bond order of 2.39 and a weaker
triple bond in AstUF3 with an effective bond order of 2.21.
(The effective bond order bond order (EBO) is computed as
the difference between the occupation numbers of the bond-
ing and antibonding orbitals divided by two. A fully devel-
oped single bond has an EBO close to one, while the value
approaches zero as the bond dissociates.) We rationalized the
strong UtN bond as a more balanced covalent bond
between contracted U 5f orbitals in the UF3 subunit and
the smaller N 2p orbitals, and the diffuse P 3p orbitals do
not bond as effectively with U 5f orbitals.15 Arsenic extends
this view with the even more diffuse As 4p which bond even
less effectively with U 5f orbitals and form a still weaker
triple bond with the UF3 subunit. This is also expected
because the electronegativity difference is smaller, and thus
the ionic contribution to the bond energy is less in the case
of As.
The comparison with calculated and observed spectra for
PtUF3 and the present new results summarized in Table 1
substantiate the formation and identification of the AstUF3
molecule. This is, we believe, the first molecule to be formed
with a uranium-arsenic bond.
The C3v symmetry structures converged by CASSCF/
CASPT2 for the three EtUF3 molecules are illustrated in
Figure 2 and parameters are given in Table 2. Notice that as
the triple bond gets longer and weaker, the U-F bonds
become slightly shorter. The electronic structures in all these
compounds are very similar. They are characterized by ionic
uranium fluorine bonds and a covalent triple bond between
the metal and N, P, or As.
The triple bond energies can be estimated from the energy
of the main group atom, E, reaction with UF3 to form
EtUF3, where UF3 has a Cs structure.15 These CASSCF/
CASPT2 calculations are approximate without ZPE or
spin-orbit correction. The bond energies so calculated
decrease dramatically in this series from 110 kcal/mol for
UtN to 42 kcal/mol for UtP to 32 kcal/mol for the even
weaker UtAs bond as described above.
The AstUF3 molecule contains a weak triple bond to
˚
˚
uranium with calculated 2.544 A (CASPT2), 2.531 A
Although terminal pnictide complexes with uranium have
not been prepared, comparisons can be made to tungsten.
(31) Wang, X.; Andrews, L. Inorg. Chem. 2008, 47, 8159 (Gr 6 and
germane).

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6597
Figure 2. Structures of NtUF3, PtUF3, and AstUF3 in C3v symmetry calculated at the CASSCF/CASPT2 level of theory. Bond lengths in angstroms
and angles in degrees.
bond covalent radii,34 which provides further testament to
our description of the uranium-arsenic triple bond as weak.
Table 2. Structural Parameters Calculated for the NtUF3, PtUF3, and
AstUF3 Moleculesa
NtUF3
PtUF3
AstUF3
Conclusions
Like the uranium methylidyne systems,14 macroscopic
synthesis of terminal UtN and UtP triple bonds15 and
any U-As bonds has not yet been reported. This matrix
isolation work shows that with the assistance of strongly
electronegative fluorine ligands, the U(VI) oxidation state
can be stabilized, and terminal triple bonds can be formed
with N and reasonably well with the heavier analogues P
and As. Comparison of the nitride, phosphide, and arsenide
triple bonds to uranium demonstrates that U 5f orbitals
bond more effectively with the smaller nitrogen ligand
than with the softer phosphorus and even softer arsenic
species. This is also expected because the electronegativity
difference is larger, and the ionic contribution to the bond
energy is greater in the case of N. The UtAs bond so formed
is weak with a computed approximately 32 kcal/mol bond
energy.
parameter CAS B3L BPW CAS B3L BPW CAS B3L BPW
EtU
U-F
1.759 1.722 1.749 2.401 2.321 2.376 2.544 2.495 2.531
2.046 2.038, 2.056, 2.039 2.040, 2.049, 2.035 2.043, 2.047,
2.064 2.064
E-U-F 122.9 109.9, 117.2, 119.7 107.0, 111.0, 120.7 104.7, 109.2,
148.1 134.5 144.4 136.8 142.8 137.5
F-U-F 93.3 98.5, 94.9, 97.6 104.1, 99.3, 96.3 104.8, 100.9,
90.4 92.1 94.5 96.2 97.6 97.2
2.052 2.050
2.049 2.048
EBO
2.78
2.39
2.21
a Computed with CASPT2 (CAS), B3LYP(B3L), or BPW91(BPW)
methods. Bond lengths in angstroms and angles in degrees. EBO is
effective bond order as described in the text.
Theoretical Methods
After considerable success in correlating matrix spectra of
small metal containing molecules with frequencies cal-
culated by density functional theory (DFT),11-17 similar
calculations were carried out using the Gaussian 03
package,23 the B3LYP and BPW91 density functionals,24,25
the 6-311+G(3df) basis sets for F and As, and the small 60
electron SDD pseudopotential and basis set for uranium26,27
to provide a consistent set of vibrational frequencies for the
reaction products. Different spin states were computed to
locate the ground-state product molecules. Geometries were
fully relaxed during optimization, and the optimized geome-
try was confirmed by vibrational analysis. All of the vibra-
tional frequencies were calculated analytically with zero-
point energy included for the determination of reaction
energies.
To understand the bonding involving uranium, higher
level CASSCF28 and CASPT229calculations were performed
for the EtUF3 molecules. The basis set was of VTZP quality
with the primitives obtained from the relativistic ANO-RCC
basis sets: 6s5p3d1f for As, 5s4p2d1f for P, 4s3p2d1f for
F and N, and 9s8p6d4f2g1h for U.35,36 Scalar relativistic
effects are included in the calculations using the Douglas-
Kroll-Hess Hamiltonian as is standard in the MOLCAS
software. The active space was chosen to describe the UtE
Figure 3. Activemolecular orbitalsinAstUF3. Contourline0.05e/au3.
Natural orbital occupation numbers given below the orbital.
˚
A typical WtP bond length of 2.12 A in organometallic
complexes32 provides a reference to show that the triple bond
length computed here for uranium and phosphorus is 13%
longer than the tungsten analogues. The first arsenide com-
plex to be prepared, [(N3N)WtAs], revealed a WtAs bond
length of 2.29 A,33 and our computed UtAs bond length is
˚
11% longer. Finally, our CASPT2 UtAs bond length of
˚
2.54 A is 12% longer that the sum of recently tabulated triple
(32) Fox, A. R.; Clough, C. R.; Piro, N. A.; Cummins, C. C. Angew.
Chem., Int. Ed. 2007, 46, 973.
˚
(35) Roos, B. O.; Lindh, R.; Malmqvist, P.-A.; Veryazov, V.; Widmark,
P.-O. J. Phys. Chem. A 2004, 108, 2851.
::
::
(33) Scheer, M.; Muller, J.; Haser, M. Angew. Chem., Int. Ed. Engl. 1996,
35, 2492.
(34) Pyykko, P.; Riedel, S.; Patzschke, M. Chem.;Eur. J. 2005, 11, 3511.
˚
(36) Roos, B. O.; Lindh, R.; Malmqvist, P.-A.; Veryazov, V.; Widmark,
::
P.-O. Chem. Phys. Lett. 2005, 409, 295.

6598 Inorganic Chemistry, Vol. 48, No. 14, 2009
Andrews et al.
triple bonds: six active orbitals (three bonding and three
antibonding) with six active electrons. All valence electrons
plus the U 6s and 6p electrons were correlated in the CASPT2
calculations, which used the standard IPEA Hamiltonian
and an imaginary shift of 0.1 to remove some weak intruder
states. All calculations were performed with the MOLCAS-7
quantum chemistry software.37 The geometries were opti-
mized at the CASPT2 level and vibrational frequencies were
computed using numerical gradients and Hessians. All calcu-
lations were performed in Cs symmetry.
Acknowledgment. We gratefully acknowledge financial sup-
port from Subgrant No. 6855694 under Prime Contract No.
DE-AC02-05CH11231 to the Lawrence Berkeley National
Laboratory from the DOE, and NCSA computing Grant
CHE07-0004N to L.A. and from the Swedish Research Council
(VR) through the Linnaeus Center of Excellence on Organizing
Molecular Matter (OMM) to B.O.R. G. L. Schrobilgen and
M. J. Hughes kindly provided the AsF3 sample, and C. C.
Cummins shared helpful information.
::
˚
(37) Karlstrom, G.; Lindh, R.; Malmqvist, P.-A.; Roos, B. O.; Ryde, U.;
Veryazov, V.; Widmark, P.-O.; Cossi, M.; Schimmelpfennig, B.; Neogrady,
P.; Seijo, L. Comput. Mater. Sci. 2003, 28, 222.