Infrared spectra and density functional calculations of triplet pnictinidene N÷ThF3, P÷ThF3 and As÷ThF3 molecules

Article abstract of DOI:10.1039/b909576d

Thorium atoms react with NF₃, PF₃, and AsF ₃ to produce the first actinide triplet state pnictinidene molecules, N÷ThF₃, P÷ThF₃, and As÷ThF ₃, which are trapped in solid argon and identifie

Full text of DOI:10.1039/b909576d

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www.rsc.org/dalton | Dalton Transactions
Infrared spectra and density functional calculations of triplet pnictinidene
NϬThF3, PϬThF3 and AsϬThF3 molecules
Xuefeng Wang and Lester Andrews*
Received 14th May 2009, Accepted 18th August 2009
First published as an Advance Article on the web 2nd September 2009
DOI: 10.1039/b909576d
Thorium atoms react with NF3, PF3, and AsF3 to produce the first actinide triplet state pnictinidene
molecules, NϬThF3, PϬThF3, and AsϬThF3, which are trapped in solid argon and identified from
infrared spectra and comparison to computed vibrational frequencies. Density functional theory
calculations for these lowest energy triplet state products converge essentially to C3v symmetry
structures. Spin density calculations show that the two unpaired electrons are mostly localized in
nitrogen 2p, phosphorus 3p, or arsenic 4p orbitals. Although thorium has a small spin density, the weak
degenerate pa molecular orbitals are populated entirely from the terminal N, P, or As based on DFT
natural bond orbital analysis. This is in contrast with HCϬThF3, which contains degenerate pa
molecular orbitals with 81% C and 19% Th character.
of optical filters, and then samples were annealed to allow reagent
Introduction
diffusion and further reaction.
The simple nitrenes CH3–N and CF3–N are reactive triplet
Following our work on reactions with the isoelectronic flu-
state intermediates.1,2 A characteristic reaction of alkyl nitrenes
is substituent rearrangement to give the lower energy imine.3
Methylene imine CH2 NH is the more stable isomer of triplet
CH3–N.4,5 In reactions of group 4 metal atoms with ammonia,
oroform molecule,14 theoretical computations were performed
using the Gaussian 03 program with the B3LYP hybrid and
BPW91 density functionals.15–17 The 6-311+G(3df) basis was
used to represent the electronic density of nitrogen, fluorine,
phosphorus and arsenic atoms and SDD pseudopotential and
basis for thorium.18,19 Frequencies were computed analytically,
and all energy values reported include zero-point vibrational
corrections. Natural bond orbital analysis was also performed
for the product molecules.15,20
=
=
the lower energy MH2 NH imine analogs were formed, but
reactions with NF3 gave instead the triplet state nitrene species
MF3–N, which is lower in energy owing to the formation of
stronger M–F bonds.6–8 Analogous reactions of Th atoms with
ammonia produced thorimine, ThH2 NH, and the higher energy
triplet thorium nitrene, ThH3–N, was not formed.9 The reaction
=
=
of uranium atoms with NH3 behaved similarly to give UH2 NH,
Results and discussion
but U reacted with NF3 and PF3 to produce the higher oxidation
10,11
≡
≡
state and lower energy terminal nitrides UF3 N and UF3 P.
Accordingly, we wish to investigate the Th reaction with NF3 in
an attempt to form the first terminal actinide nitrene derivative
ThF3–N, and we report here matrix infrared spectra and density
functional calculations on the new series of thorium based
pnictinidene products of thorium atom reactions with NF3, PF3
and AsF3.
Infrared spectra of products formed in the reactions of laser-
ablated thorium atoms with NF3, PF3, and AsF3 in excess argon
during condensation at 5 K will be presented in turn. Density func-
tional calculations were performed to support the identifications
of new reaction products. Bands common to experiments using
-
different laser ablated metals with NF3 [such as NF2 and NF2 ],
with PF3 [PF2, PF5, and PF2 ], and with AsF3 [AsF2 and AsF5]
-
have been identified previously and will not be mentioned again
here.21,22
Experimental and computational methods
Laser ablated Th atoms (Oak Ridge National Laboratory) were
reacted with NF3 (Matheson), PF3 (PCR Research), or AsF3
(Ozark-Mahoning, vacuum distilled from dry NaF) in excess
argon and neon during condensation at 5 K using the closed-
cycle refrigerator described elsewhere.12,13 Reagent gas mixtures
were typically 0.5% in argon. After the reaction with thorium,
infrared spectra were recorded at a resolution of 0.5 cm-1 using
a Nicolet 750 spectrometer with a Hg–Cd–Te range B detector.
Samples were later irradiated for 15 min periods by a mercury arc
street lamp (175 W) with the globe removed using a combination
Infrared spectra
Matrix infrared spectra for Th reactions with NF3 in excess argon
are compared in Fig. 1. The sharp bands at 525.6, 520.9, and
514.6 cm-1 in the spectra have been reported previously and
assigned to thorium tetrafluoride in solid argon.23 The latter
product absorptions were also observed after the laser-ablated
Th atom reaction in separate experiments with argon/fluorine
samples. The major new product absorptions for Th and NF3 are
at 575.5, 525.2, and 430.0 cm-1. These bands increase together on
UV irradiation doubling their absorbance (Fig. 1(a)–(d)) while a
minor set of bands closely positioned at 565.0, and 531.0 cm-1
is almost destroyed on the same irradiation sequence. Subsequent
Department of Chemistry, University of Virginia, P.O Box 400319,
Charlottesville, Virginia 22904-4319. E-mail: lsa@virginia.edu
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traces (a)–(d). Annealing to 30 K increases the second pair of
bands more, and further annealing to 35 K holds the second pair
and decreases the first pair about 20% (spectra (e),(f)).
The neon matrix spectrum for Th and PF3 (0.3%) is compared in
Fig. 2, traces (g)–(i). The major ThF4 bands are again at 534.1 and
529.3 cm-1, and a weak new absorption appears at 539.8 cm-1 with a
542.6 cm-1 shoulder along with an even weaker band at 567.9 cm-1
with a 566.5 cm-1 shoulder. Ultraviolet photolysis increases these
bands slightly, and annealing to 12 K favors the inside pair relative
to the outside pair, similar to the argon matrix observations.
Spectra for Th reactions with AsF3 are shown in Fig. 3. The
ThF4 bands are stronger than with PF3 but weaker than when
produced by the NF3 reaction. Stronger bands are observed at
532.0 and 528.9 cm-1 with weaker associated bands at 569.5 and
566.9 cm-1. Ultraviolet irradiation and annealing alter these four
bands in like fashion to that described above and associate the
inner and outer pairs accordingly.
Fig. 1 Infrared spectra for thorium atom reaction products with NF3 in
excess argon in the 580–420 cm-1 region. (a) Spectrum after co-deposition
of laser-ablated Th and NF3 at 0.4% in argon at 5 K for 60 min, (b) after
annealing to 20 K, (c) after 240–380 nm irradiation, (d) after >220 nm
irradiation for 20 min, and (e) after annealing to 30 K.
annealing to 35 K slightly decreased the absorptions. Reducing the
sample concentration from 0.4 to 0.2% decreased the relative yield
of ThF4 in the spectrum but had no effect on the photochemistry.
An experiment with 0.1% NF3 in neon and lower laser energy gave
weaker bands for ThF4 at 534.1 and 529.3 cm-1 and a weak new
536.1 cm-1, which increased on UV irradiation like the 525.3 cm-1
argon matrix band.
Infrared spectra for the corresponding Th atom reactions with
PF3 are illustrated in Fig. 2. The ThF4 bands were much weaker as
PF3 is a more stable compound than NF3. The major Th reaction
product at 528.3 cm-1 has a 531.3 cm-1 satellite and these are joined
by weaker bands at 569.1 and 566.3 cm-1. These bands sharpen
on annealing to 20 K and increase 10% on >290 nm irradiation,
but >220 nm irradiation increases the 528.3 and 569.1 cm-1 bands
20% and decreases the 531.3 and 566.3 cm-1 bands a like amount,
Fig. 3 Infrared spectra for thorium atom reaction products with AsF3 in
excess argon in the 580–500 cm-1 region. (a) Spectrum of argon–arsenic
trifluoride precursor, (b) spectrum after co-deposition of laser-ablated Th
and AsF3 at 0.4% in argon at 5 K for 60 min, (c) after annealing to 20 K,
(d) after >320 nm irradiation for 20 min, (e) after >220 nm irradiation for
20 min, and (f) after annealing to 30 K.
Calculations
Density functional theory calculations were done for potential
reaction product structures, energies, and vibrational frequencies.
Following previous investigations with these EF3 (E = N, P, As)
precursors,8,21 we expect reactions (1), (2), and (3) to occur.
3
=
Th( F) + NF3 → F2N–ThF → FN ThF2 → NϬThF3
(1)
(2)
3
=
Th( F) + PF3 → F2P–ThF → FP ThF2 → PϬThF3
3
=
Th( F) + AsF3 → F2As–ThF → FAs ThF2 → AsϬThF3 (3)
Fig. 2 Infrared spectra for thorium atom reaction products with PF3 in
excess argon and neon in the 580–500 cm-1 region. (a) Spectrum after
co-deposition of laser-ablated Th and PF3 at 0.4% in argon at 5 K for
60 min, (b) after annealing to 20 K, (c) after >290 nm irradiation for
20 min, (d) after >220 nm irradiation, (e) after annealing to 30 K, and (f)
after annealing to 35 K, (g) spectrum after co-deposition of laser-ablated
Th and PF3 at 0.3% in neon at 5 K for 60 min, (h) after >240–380 nm
irradiation for 20 min, and (i) after annealing to 12 K.
Our B3LYP calculations find these overall reactions to be
exothermic as the final triplet state products are 340, 169, and
215 kcal mol-1 lower in energy than the reagents, respectively. An
attempt to calculate the first step in reaction (1), singlet F2N–
ThF, converged instead to the second step, singlet FN ThF2,
which is 39 kcal mol-1 higher in energy than the final triplet
=
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Table 1 Observed and calculated fundamental frequencies for the
NϬThF3 nitrene complex in the 3A¢¢ ground electronic state with
approximate C3v symmetrya
Table 3 Observed and calculated fundamental frequencies for the
3
AsϬThF3 complex in the A¢¢ ground electronic state with approximate
C3v symmetrya
Approx. modeb NϬThF3
Approx. modeb AsϬThF3
Description
Obsc
Calc(B3) Int
Calc(BP) Int
Description
Obsc
Calc(B3) Int
Calc(BP) Int
Th–F str, a1
Th–F str, e
NϬTh str, a1
ThF3 def, e
ThF3 def, a1
N–Th–F def, e
575.5
525.2
430.0
573
533
454
117
111
109
34
212 ¥ 2 528
100 446
10 ¥ 2 115
28 104
22 ¥ 2 106
565
34
200 ¥ 2
84
10 ¥ 2
17
20 ¥ 2
Th–F str, a1
Th–F str, e
AsϬTh str, a1
ThF3 def, e
ThF3 def, a1
As–Th–F def, e
569.5
528.9
573
539
177
113
107
81
90
183 ¥ 2 533
23 174
562
82
173 ¥ 2
18
10 ¥ 2
9
5 ¥ 2
10 ¥ 2 104
10
6 ¥ 2
93
70
a Frequencies and intensities are in cm-1 and km mol-1. Frequencies
and intensities computed with B3LYP and BPW91/6-311+G(3df) in the
harmonic approximation using the SDD core potential and basis set
for metal atoms. b The mode symmetry notations are based on the C3v
structure. The degenerate modes were split 1 to 3 cm-1, and average
values are reported here. c Observed in an argon matrix. Argon matrix
site absorptions at 569.0 and 531.3 cm-1. Neon matrix counterpart for
strongest band at 536.1 cm-1.
a Frequencies and intensities are in cm-1 and km mol-1. Frequencies
and intensities computed with B3LYP and BPW91/6-311+G(3df) in the
harmonic approximation using the SDD core potential and basis set
for metal atoms. b The mode symmetry notations are based on the C3v
structure. Using B3LYP the degenerate modes were split 3 to 8 cm-1, and
average values are reported here, but BPW91 gave degenerate modes and
identical bond lengths. c Observed in an argon matrix. Argon matrix site
absorptions at 566.9 and 532.0 cm-1.
Table 2 Observed and calculated fundamental frequencies for the
3
PϬThF3 complex in the A¢¢ ground electronic S state with approximate
than those for BPW91, which is the expected relationship for these
calculations.24,25 Notice also that the structural parameters are also
nearly the same for the two functionals, which substantiates our
modeling of this triplet ground state molecule. It is interesting to
note that four of these calculations converged very close to C3v
symmetry and the other two converged to C3v symmetry (Table 4).
C3v symmetrya
Approx. modeb PϬThF3
Description
Obsc
Calc(B3) Int
Calc(BP) Int
Th–F str, a1
Th–F str, e
PϬTh str, a1
ThF3 def, e
ThF3 def, a1
P–Th–F def, e
569.1
528.3
573
539
256
114
108
86
81
190 ¥ 2 532
33 255
562
75
178 ¥ 2
29
9 ¥ 2
13
7 ¥ 2
Identification of the EϬThF3 molecules
11 ¥ 2 104
10
9 ¥ 2
97
78
Reactions of Th atoms with NF3 gave new 575.5, 525.2, and
430.0 cm-1 product absorptions, which are compared in Table 1
with frequencies calculated for the lowest energy product molecule.
The most intense band at 525.5 cm-1 is just above the strongest
band at 521.3 cm-1 from the CHF3 reaction with Th, which was
assigned to the antisymmetric degenerate Th–F stretching mode
of HCϬThF3 calculated at 526 cm-1 using the same methods.14
Likewise, the weaker 575.5 cm-1 band is also slightly higher
than the weaker 565.4 cm-1 band computed at 568 cm-1 for
the symmetric Th–F stretching mode of the carbon species.
This favorable comparison with observed and calculated Th–F
stretching frequencies for the isoelectronic HCϬThF3 molecule,
and agreement of the observed frequencies within 3–8 cm-1
with those computed for NϬThF3 (Table 1) substantiates this
identification and assignments. Only the strongest band was
observed, at 536.1 cm-1, in the neon matrix experiment, and the 11
blue shift is reasonable for this guest molecule.26 The weakest band
in solid argon at 430.0 cm-1 band is lower than the 502.1 cm-1 C–Th
stretching mode of HCϬThF3 and also lower than the 454 cm-1
computed value of the corresponding N–Th stretching mode,
but close enough to support this assignment. The observation
of this additional diagnostic frequency for the NϬThF3 molecule
supports its identification.
a Frequencies and intensities are in cm-1 and km mol-1. Frequencies
and intensities computed with B3LYP and BPW91/6-311+G(3df) in the
harmonic approximation using the SDD core potential and basis set
for metal atoms. b The mode symmetry notations are based on the C3v
structure. Using B3LYP the degenerate modes were split 3 to 6 cm-1, and
average values are reported here, but BPW91 gave degenerate modes and
identical bond lengths. c Observed in an argon matrix. Argon matrix site
absorptions at 566.3 and 531.3 cm-1. Neon matrix counterparts at 567.9
and 539.8 cm-1 with shoulders at 566.5 and 542.4 cm-1 for matrix sites.
state NϬThF3 product. Furthermore, the singlet state N–ThF3
is computed to be 41 kcal mol-1 higher energy than the triplet
state, which is comparable to the 44 kcal mol-1 triplet–singlet
separation computed for N–CF3.2 In like fashion, the second step
FP ThF2 molecule is 43 kcal mol-1 higher in energy than the final
=
=
PϬThF3 product of reaction (2), and the second step FAs ThF2 is
60 kcal mol-1 higher in energy than the final AsϬThF3 product
of reaction (3). In marked contrast with reaction (1), the reaction
=
with ammonia produced the singlet thorimine HN ThH2 final
product,9 and the triplet nitrene NϬThH3 is 86 kcal mol-1 higher in
energy owing to the relative strengths of the N–H and Th–H bonds.
The analogous relationship is found for the isoelectronic reactive
=
HN CH2 and NϬCH3 species, where the nitrene is computed to
be 50 kcal mol-1 higher energy than the imine using B3LYP.
The triplet state final product structures were calculated at the
B3LYP and BPW91 levels of theory, the frequencies are listed in
Tables 1–3, and the structural parameters are given in Table 4.
Notice that the two density functionals predict the frequencies
no more than 14 cm-1 apart, and the B3LYP values are higher
The weaker nearby 565.0 and 531.0 absorptions give way on
ultraviolet irradiation to the above bands assigned to the NϬThF3
lowest energy reaction product from the Th metal atom reaction
with NF3, and the latter bands are most likely due to a less stable
argon matrix trapping site. This proposal is supported by similar
observations in the analogous experiments with PF3 and AsF3.
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Table 4 Structural parameters and physical constants for triplet state pnictinidenes EϬThF3 (E = N, P, As) in Cs or C3v symmetrya
Parameter
NϬThF3(B3)
NϬThF3(BP)
PϬThF3(B3)
PϬThF3(BP)
AsϬThF3(B3)
AsϬThF3(BP)
r(EϬTh)
r(Th–F)
<(E–Th–F)
q(E)b
2.378
2.117/2.118
106.7/107.0
-0.71
2.65
-0.65
—
1.901
0.075
0.008
—
1.23
340
2.367
2.114/2.116
106.7/107.0
-0.66
2.45
-0.60
—
1.888
0.085
0.009
—
1.18
334
2.905
2.108/2.110
106.1/106.9
-0.50
2.43
-0.64
—
1.933
0.051
0.005
—
1.27
169
2.890
2.108
106.2
-0.51
2.27
-0.59
—
1.909
0.076
0.005
—
3.012
2.997
2.107
106.2
-0.27
2.04
2.107/2.109
106.2/107.2
-0.27
2.20
-0.64
q(Th)b
q(F)b
-0.59
s(E)c
s(M)c
s(F)c
1.976
0.003
0.007
1.983
-0.005
0.007
Md
1.31
193
1.15
215
1.21
222
DEe
a
Bond lengths and angles are in A and degrees. All calculations performed at the B3LYP or BPW91//6-311+G(3df)//SDD level. b Mulliken atomic
charges. c Mulliken atomic spin densities: value given for only one fluorine atom. d Molecular dipole moment in D. e Binding energy in kcal mol-1 relative
to Th + EF3.
˚
Although the PF3 precursor is less reactive as attested by
much weaker ThF4 absorptions, the major product absorption
at 528.3 cm-1 is stronger than the nitrogen counterpart. The
stronger 528.3 and weaker 569.1 cm-1 absorptions track together
and increase at the expense of the 566.3 and 531.3 cm-1 bands on
photolysis, which decreases but does not destroy the second pair
of bands. The assignment of these two pairs of bands to different
matrix sites is also supported by the neon matrix spectra, also
shown in Fig. 2, where the 567.9 and 539.8 cm-1 bands in neon
correspond to the 569.1 and 528.3 cm-1 bands in solid argon, and
the 524.6 and 566.5 cm-1 shoulders in neon correspond to the 565.0
and 531.0 cm-1 satellites in argon.
The symmetric and antisymmetric Th–F stretching modes for
PϬThF3 are predicted to be very close to those for NϬThF3,
and the observed bands correlate very well (Tables 1 and 2).
This correlation is even closer for the AsϬThF3 product where
the observed and calculated bands for both density functionals
(Table 3) are within 1 cm-1 of those for the PϬThF3 product.
Thus, the closely related observed and computed Th–F stretching
modes for the three thorium pnictinidene molecules substantiate
our assignments and identifications of these novel species.
Structure and bonding in the thorium pnictinidenes
Structural parameters computed for the triplet state pnictinidenes
EϬThF3 given in Table 4 reveal a progressive increase in the
E–Th bond length, but the Th–F bond lengths and E–Th–F
angles are the same within the error of calculation. The molecules
have essentially C3v symmetry structures, which are illustrated in
Fig. 4.
The Mulliken atomic spin densities add to 2.00, which char-
acterizes these triplet state EϬThF3 pnictinidene species. Notice
that the spin densities on N, P, and As increase from about 1.89 to
1.92 to 1.98, respectively, as progressively less spin is transferred
to the Th centers from the more diffuse np valence orbitals.
Fig. 4 Structures calculated for the thorium trifluoride pnictinidenes
Natural bond orbital analysis15,20 performed for this series reveal pa
using B3LYP/6-311+G(3df)/SDD methods.
molecular orbitals populated completely from the non-metal with
very little Th 6d overlap, which are shown in Fig. 5. The analogous
group 4 early transition metal pnictinidenes EϬMF3 exhibit more
metal nd valence orbital involvement in the weak p bonding
interaction.8 In addition the pa molecular orbitals for HCϬThF3
have a 12% contribution from C, and the pa molecular orbital plot
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electron density for bonding with N in the trihydride species, which
˚
has a much shorter 2.006 A computed N–Th bond length and Cs
symmetry.28 This is demonstrated by the Mulliken atomic spin
densities [1.28, 0.20, 0.22, 0.22, 0.08], which show considerable
delocalization of the two unpaired electrons. The non-degenerate
pa molecular orbitals are accordingly 88% N (100% p) and 12%
Th (5% p, 47% d, 48% f), and 80% N (21% s, 79% p) and 20% Th
(66% d, 30% f). In contrast there is no Th contribution to the pa
molecular orbitals in NϬThF3.
Conclusions
The group 15 trifluoride molecules NF3, PF3, or AsF3 were
reacted with laser-ablated Th atoms to produce the actinide triplet
state pnictinidene NϬThF3, PϬThF3, or AsϬThF3 molecules,
as identified by their matrix infrared spectra and comparison
to theoretically predicted vibrations. Density functional theory
calculations using B3LYP and BPW91 functionals converge
approximately to C3v symmetry structures with relatively long
terminal group 15 bonds to Th for these lowest energy triplet state
products. Spin density calculations show that the two unpaired
electrons are mostly localized in nitrogen 2p, phosphorus 3p, or
arsenic 4p orbitals and thorium has a small spin density. The weak
degenerate pa molecular orbitals are populated entirely from the
terminal N, P, or As atoms based on DFT natural bond orbital
analysis. In contrast, the related HCϬThF3 molecule28 exhibits
slightly more interaction through degenerate pa molecular orbitals
with 81% C and 19% Th character and the analogous U reaction
≡
with AsF3 produced the terminal arsenide As UF3 with a weak
triple bond.27
Acknowledgements
We gratefully acknowledge financial support from Subgrant No.
6855694 under Prime Contract No. DE-AC02-05CH11231 to
the Lawrence Berkeley National Laboratory from the DOE and
NCSA computing Grant No. CHE07-0004N to L. A.
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˚
≡
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≡
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