Reactions of laser-ablated Nb and Ta atoms with N2: Experimental and theoretical study of M(NN)x (M = Nb, Ta; X = 1-4) in solid neon

Article abstract of DOI:10.1021/jp103067k

Reactions of laser-ablated niobium and tantalum atoms with dinitrogen in solid neon have been investigated using matrix-isolation infrared spectroscopy. The Nb(NN)_x and Ta(NN)_x (x = 1-4) molecules formed during sample deposition or o

Full text of DOI:10.1021/jp103067k

J. Phys. Chem. A 2010, 114, 6837–6842
6837
Reactions of Laser-Ablated Nb and Ta Atoms with N2: Experimental and Theoretical Study
of M(NN)x (M ) Nb, Ta; x ) 1-4) in Solid Neon
Zhang-Hui Lu,†,‡ Ling Jiang,† and Qiang Xu*,†,†
National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan, and
Graduate School of Engineering, Kobe UniVersity, Nada Ku, Kobe, Hyogo 657-8501, Japan
ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: May 17, 2010
Reactions of laser-ablated niobium and tantalum atoms with dinitrogen in solid neon have been investigated
using matrix-isolation infrared spectroscopy. The Nb(NN)x and Ta(NN)x (x ) 1-4) molecules formed during
sample deposition or on annealing are the major products. The products are characterized on the basis of
isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, and change of reagent concentration and
laser energy. Density functional theory calculations have been performed to understand the structures, ground
electronic states, and bonding characteristics of niobium and tantalum dinitrogen complexes. The overall
agreement between the experimental and calculated vibrational frequencies, relative absorption intensities,
and isotopic shifts supports the identification of these species from the matrix infrared spectra. The molecular
orbital analyses and plausible reaction pathways for the formation of the products are also discussed.
Introduction
element atoms with N2.5 Absorptions at 1965.2 and 1947.6 cm-1
were tentatively assigned to the stretching frequencies of
Nb(NN) and Ta(NN) in pure nitrogen.8a The Nb(NN)4 and
Ta(NN)4 molecules were characterized from isotope experiments
in pure nitrogen and excess argon without theoretical
calculation.8a However, much less is known about the M(NN)2-3
(M ) Nb, Ta) molecules. To gain insight into the whole series
of niobium and tantalum dinitrogen complexes, we investigated
the structures, ground electronic states, and bonding character-
istics of niobium and tantalum dinitrogen complexes by probing
the N-N vibration in reaction products with IR spectroscopy
and DFT calculations. Considering that solid neon may stabilize
some species that are difficult to observe in solid argon,14 we
have performed the reactions of laser-ablated niobium and
tantalum atoms with N2 molecules in excess neon, which
provides obvious evidence for the formation of the M(NN)x (M
) Nb, Ta; x ) 1-4) molecules.
Dinitrogen fixation and activation are one of the most
challenging and important subjects in chemistry. The interaction
of dinitrogen with metal atoms is of considerable interest from
an academic or an industrial viewpoint owing to its importance
in biological and catalytic systems.1-3 Since the discovery of
the first dinitrogen complex [Ru(NH3)5(N2)]2+ in 1965,4 the
synthesis, structural characterization, and reactivity of metal
dinitrogen complexes have been the subject of intensive studies,
and a series of metal dinitrogen complexes have been experi-
mentally and theoretically characterized.5 Taking the group 5
metal dinitrogen complexes as an example, neutral V(NN)n (n
) 1, 2, 4, 6),7 Nb(NN)n, and Ta(NN)n (n ) 1, 4, 6)8 complexes
have been reported in excess argon and pure nitrogen. Electron-
spin resonance spectroscopy has been applied to neutral V(NN)6
and Nb(NN)6 complexes in pure solid nitrogen.9 Recently, the
gas-phase metal ion complexes V+(N2)n (n ) 3-7)10 and
Nb+(N2)n (n ) 3-16)11 have been generated in a pulsed nozzle
laser vaporization source and investigated via infrared photo-
dissociation spectroscopy. Avenier and co-workers have studied
the activation of dinitrogen by an isolated surface Ta atom.12
In addition, various groups have studied the molecular adsorp-
tion of N2 onto group 5 metal clusters and surfaces.13 To
supplement the experimental work on these systems, ab initio
methods and density functional theory (DFT) have been
performed to understand the electronic structures and bonding
characteristics.7-13
Experimental and Theoretical Methods
The experiments for laser ablation and matrix isolation
infrared spectroscopy are similar to those previously reported.17
In short, the Nd:YAG laser fundamental (1064 nm, 10 Hz
repetition rate with 10 ns pulse width) was focused on the
rotating Nb and Ta targets. The laser-ablated Nb and Ta atoms
were co-deposited with N2 (99.95%, Suzuki Shokan Co., Ltd.)
in excess neon onto a CsI window cooled normally to 4 K by
means of a closed-cycle helium refrigerator. Typically, 5-25
mJ/pulse laser power was used. Isotopic 15N2 (99.8%, Shoko
Co., Ltd.) and 14N2 + 15N2 mixtures were used in different
experiments. In general, matrix samples were deposited for
30-60 min with a typical rate of 2-4 mmol per hour. After
sample deposition, IR spectra were recorded on a Bio-Rad FTS-
6000e spectrometer at 0.5 cm-1 resolution using a liquid-
nitrogen-cooled HgCdTe (MCT) detector for the spectral range
of 5000-400 cm-1. Samples were annealed at different tem-
peratures and subjected to broad-band irradiation (λ > 250 nm)
using a high-pressure mercury arc lamp (Ushio, 100 W).
DFT calculations were performed to predict the structures
and vibrational frequencies of the observed reaction products
Recent studies have shown that, with the aid of isotopic
substitution, matrix isolation infrared spectroscopy combined
with DFT calculation is very powerful in investigating the
spectrum, structure, and bonding of novel species and the related
reaction mechanisms.14-16 Various metal dinitrogen complexes
have been prepared in low-temperature matrix samples by
codeposition of laser-ablated transition-metal and main-group-
* To whom correspondence should be addressed. E-mail: q.xu@aist.go.jp.
† National Institute of Advanced Industrial Science and Technology
(AIST).
‡ Kobe University.
10.1021/jp103067k  2010 American Chemical Society
Published on Web 06/08/2010

6838 J. Phys. Chem. A, Vol. 114, No. 25, 2010
Lu et al.
Figure 3. Infrared spectra in the 2110-1850 cm-1 regions from
codeposition of laser-ablated Ta atoms with 0.08% N2 in Ne at 4 K.
(a) Spectrum obtained from initial deposited sample for 30 min, (b)
spectrum after annealing to 8 K, (c) spectrum after 10 min of broad-
band irradiation, and (e) spectrum after annealing to 10 K.
Figure 1. Infrared spectra in the 2120-1900 cm-1 regions from
codeposition of laser-ablated Nb atoms with 0.3% N2 in Ne at 4 K. (a)
Spectrum obtained from initial deposited sample for 30 min, (b)
spectrum after annealing to 8 K, (c) spectrum after 10 min of broad-
band irradiation, and (e) spectrum after annealing to 10 K.
Figure 4. Infrared spectra in the 2100-1850 cm-1 region from
codeposition of laser-ablated Ta atoms with 0.08% 14N2 + 0.08% 15N2
in Ne at 4 K. (a) Spectrum obtained from initial deposited sample for
30 min, (b) spectrum after annealing to 8 K, (c) spectrum after 10 min
of broad-band irradiation, and (e) spectrum after annealing to 10 K.
Figure 2. Infrared spectra in the 2100-1850 cm-1 region from
codeposition of laser-ablated Nb atoms with 0.2% 14N2 + 0.2% 15N2
in Ne at 4 K. (a) Spectrum obtained from initial deposited sample for
30 min, (b) spectrum after annealing to 8 K, (c) spectrum after 10 min
of broad-band irradiation, and (e) spectrum after annealing to 10 K.
involved. Doping with CCl4 has no effect on these bands, which
suggests that the reactions products are neutral. The stepwise
annealing and photolysis behavior of the product absorptions
is also shown in the figures and will be discussed below.
Density functional theory calculations have been carried out
for the possible isomers and electronic states of the potential
product molecules. Figure 5 shows the ground state structures
of the products. Table 2 reports a comparison of the observed
and calculated IR frequencies and isotopic frequency ratios for
the N-N stretching modes of the observed products. Energetic
analysis for possible reactions of Nb and Ta atoms with N2 is
given in Table 3. Due to the similarity of the bonding
mechanism between the two series of niobium and tantalum
dinitrogen complexes, molecular orbital depictions of the highest
occupied molecular orbitals (HOMOs) and HOMO-1s of the
representative Nb(NN)x (x ) 1-4) complexes are illustrated in
Figure 6.
using the Gaussian 03 program.18 The B3LYP density functional
method was utilized.19 The 6-311+G(d) basis set was used for
the N atoms,20 and the Los Alamos effective-core-potential plus
double-ꢀ (LANL2DZ) basis set was used for the Nb and Ta
atoms.21 All geometrical parameters were fully optimized, and
the harmonic vibrational frequencies were calculated with
analytical second derivatives. Trial calculations and recent
investigations have shown that such computational methods can
provide reliable information for metal complexes, such as
infrared frequencies, relative absorption intensities, and isotopic
shifts.15-17,22-24
Results and Discussion
Experiments have been done with N2 concentrations ranging
from 0.05% to 1.0% in excess neon. Typical infrared spectra
for the products in the selected regions are illustrated in Figures
1-4, and the absorption bands in different isotopic experiments
are listed in Table 1. These bands are observed in lower laser
power experiments and not favored with higher laser energy
and low N2 concentration, suggesting that one metal atom is
M(NN). In the Nb + N2 experiments, the absorption at 1939.5
cm-1 appears during sample deposition, increases upon sample
annealing, visibly decreases after broad-band irradiation, and
recovers after further annealing to higher temperature (Table 1

Study of M(NN)x in Solid Neon
J. Phys. Chem. A, Vol. 114, No. 25, 2010 6839
Figure 5. Optimized structures (bond lengths in angstroms, bond angles in degrees) of the niobium and tantalum (in parentheses) dinitrogen
complexes calculated at the B3LYP/6-311+G(d)-LANL2DZ level.
TABLE 1: IR Absorptions (in cm-1) Observed from Reaction of Laser-Ablated Nb and Ta Atoms with N2 in Excess Neon
14N2
15N2
14N2 + 15N2
14N2/15N2
assignment
Niobium
2149.4
2130.0
2100.1
2079.4
2049.4
2039.9
1998.8
1949.3
1939.5
2078.3
2059.5
2030.5
2010.8
1981.4
1972.4
1932.6
1884.6
1875.5
1.0342
1.0342
1.0343
1.0341
1.0343
1.0342
1.0343
1.0343
1.0341
Nb(NN)6 site
Nb(NN)6
Nb(NN)5
Nb(NN)4
(NN)xNb(NN)
Nb(NN)2
Nb(NN)3
Nb(NN)3
Nb(NN)
2079.4, 2051.0, 2036.9, 2023.2, 2009.4
2039.9, 1992.4, 1972.4
1998.8, 1983.3, 1958.6, 1932.5
1949.3, 1929.5, 1894.4, 1884.6
1939.5, 1875.3
Tantalum
2110.0
2092.0
2058.6
2035.9
1991.3
1978.7
1934.8
1921.8
2039.5
2022.5
1990.2
1968.3
1925.0
1913.5
1870.9
1859.4
1.0346
1.0344
1.0344
1.0344
1.0344
1.0341
1.0342
1.0336
Ta(NN)6
Ta(NN)5
Ta(NN)4
Ta(NN)3
Ta(NN)3
Ta(NN)2
Ta(NN)
2058.6, 2039.3, 2022.5, 2010.6, 1990.3
2035.4, 2014.3, 1988.5, 1969.0
1991.3, 1957.6, 1943.1, 1924.9
1978.7, 1934.8, 1913.5
1934.8, 1870.9
1921.7, 1859.0
Ta(NN) site
and Figure 1). The band shifts to 1875.5 cm-1 with 15N2, giving
a 1.0341 14N/15N isotopic frequency ratio, which is characteristic
of a N-N stretching vibration. The 14N2 + 15N2 experiment
reveals a clear 1939.5 and 1875.3 cm-1 mixed isotopic doublet
(Figure 2); hence, one NN subunit is involved in these
vibrations.26 The 1939.5 cm-1 band is therefore assigned to the
N-N stretching vibration of the Nb(NN) complex in solid neon,
which is in accord with the previous tentative assignments in
pure nitrogen (1965.2 cm-1)8a and excess argon (1926 and 1931
cm-1).8b In the Ta + N2 experiments, the absorption of the
analogous Ta(NN) molecule has been observed at 1934.8 cm-1
in solid neon (Table 1 and Figure 3-4), which is in agreement
with the previous 1947.6 cm-1 pure nitrogen tentative assign-
ment to the Ta(NN) species.8a
(1991.9 cm-1) and the 14N/15N isotopic frequency ratio (1.0350)
(Table 2) are consistent with the experimental values 1939.5
and 1.0341, respectively. Overall agreement between the
experimental and calculated results has also been obtained for
the Ta(NN) molecule (Table 2 and Figure 5).
M(NN)2. In the Nb + N2 experiments, the absorption at
2039.9 cm-1 appears during sample deposition, increases upon
sample annealing, almost disappears after broad-band irradiation,
and markedly recovers after further annealing to higher tem-
perature (Table 1 and Figure 1). The band shifts to 1972.4 cm-1
with 15N2, exhibiting isotopic frequency ratio (14N/15N, 1.0342)
characteristic of N-N stretching vibration. As can be seen in
Figure 2, a triplet isotopic pattern is observed in the mixed 14N2
+
15N2 isotopic spectra, which indicates that two N2 units are
DFT calculations have been performed for the Nb(NN) and
Ta(NN) molecules to support the above assignments. The
Nb(NN) and Ta(NN) molecules are predicted to have a linear
involved in the complex.26 Accordingly, this band is assigned
to the antisymmetric N-N stretching mode of the Nb(NN)2
molecule. This band of Nb(NN)2 appears to be a partially
resolved doublet (Figure 1 and 2), which may be due to the
N-N stretching vibration of this species in different matrix sites.
The Nb(NN)2 argon matrix counterpart was not identified in
6
4
geometry with a Σ+ and Σ+ ground state (Figure 5), respec-
tively, consistent with the previous theoretical calculations.8,11
For the Nb(NN) molecule, the scaled N-N stretching frequency

6840 J. Phys. Chem. A, Vol. 114, No. 25, 2010
TABLE 2: Comparison of Observed and Calculated IR Frequencies (in cm-1) and Isotopic Frequency Ratios for the Products
observed calculated
νN-N (intensity, mode)a
Lu et al.
species
Nb(NN)
νN-N
14N2/15N2
14N2/15N2
1939.5
2039.9
1.0341
1.0342
1991.9 (1032, σ)
2069.2 (2769, B2)
2128.3 (20, A1)
2040.0 (968, A1)
2077.6 (2280, B2)
2135.4 (16, A1)
2101.4 (0, B1g)
1.0350
1.0350
Nb(NN)2
Nb(NN)3
1949.3
1998.8
1.0343
1.0343
1.0350
1.0350
Nb(NN)4
2079.4
1.0341
2108.1 (2072 × 2, Eu)
1.0350
2182.9 (0, A1g)
Ta(NN)
Ta(NN)2
1934.8
1978.7
1.0343
1.0341
1875.8 (792, σ)
2018.1 (2628, B2)
2103.2 (0, A1)
1.0350
1.0350
Ta(NN)3
Ta(NN)4
1991.3
2035.9
1.0344
1.0344
1974.5 (1070, A1)
2025.0 (2373, B2)
2090.0 (3, A1)
1.0350
1.0350
2058.6
1.0344
2058.9 (2336 × 2, Eu)
1.0350
2060.2 (0, B1g)
2147.6 (0, A1g)
a Calculated frequencies are scaled by a factor of 0.9663;25 intensities (in parentheses) are given in kilometers per mole.
TABLE 3: Energetics for Possible Reactions of Nb and Ta
Atoms with N2Calculated at the B3LYP/
6-311+G(d)-LANL2DZ Level
reaction
energya
no.
reaction
(kcal/mol)
1
2
3
4
5
6
7
8
Nb(6D) + N2(1∑g+) f NbNN(6∑+)
-21.4
-18.0
-18.0
-21.1
-14.8
-16.1
-24.9
-25.2
NbNN(6∑+) + N2(1∑g+) f Nb(NN)2(6A1)
Nb(NN)2(6A1) + N2(1∑g+) f Nb(NN)3(4B2)
Nb(NN)3(4B2) + N2(1∑g+) f Nb(NN)4(4A1g)
Ta(4F) + N2(1∑g+) f TaNN(4∑+)
TaNN(6∑+) + N2(1∑g+) f Ta(NN)2(4B1)
Ta(NN)2(6A1) + N2(1∑g+) f Ta(NN)3(4B2)
Ta(NN)3(4B2) + N2(1∑g+) f Ta(NN)4(4A1g)
a A negative value of energy denotes that the reaction is
exothermic.
the previous experiments,8 but the Nb(CO)2 molecule was
observed at 1847.2 cm-1 in solid neon.22a
The analogous feature with Ta and N2 in neon appears during
sample deposition at 1978.7 cm-1, sharply increases upon
sample annealing, almost disappears after broad-band irradiation,
and recovers after further annealing to higher temperature (Table
1 and Figures 3 and 4). This band is assigned to the antisym-
metric N-N stretching modes of the Ta(NN)2 molecule on the
basis of isotopic shift and splitting patterns.
Figure 6. Molecular orbital depictions of the highest occupied
molecular orbitals (HOMOs) and HOMO-1s of the niobium dinitrogen
complexes.
absorption intensities, and isotopic shifts confirms the identifica-
tions of the Nb(NN)2 and Ta(NN)2 molecules from the matrix
IR spectra.
B3LYP calculations predict the Nb(NN)2 and Ta(NN)2
6
4
molecules to have C2V symmetry with a A1 and B1 ground
electronic state (Figure 5), respectively, which are slightly lower
in energy than the corresponding linear structures. Loss of one
electron in the Nb(NN)2 molecule results in a linear geometry.11
For the Nb(NN)2 molecule, the scaled antisymmetric N-N
stretching frequency is predicted at 2069.2 cm-1 (Table 2),
which is consistent with the experimental value (2039.9 cm-1).
The calculated band 2128.3 cm-1 with very low intensity (20
km/mol) is not readily observed, which is consistent with the
absence of the symmetric N-N stretching vibration of the
Nb(NN)2 molecule from the present experiments. As sum-
marized in Table 2, the calculated 14N/15N isotopic frequency
ratio of 1.0350 is also in accord with the experimental value
(1.0342). Similar results have been obtained for the Ta(NN)2
molecule (Table 2 and Figure 5). This agreement between the
experimental and calculated vibrational frequencies, relative
M(NN)3. In the reactions of laser-ablated Nb atoms with N2,
the 1998.8 and 1949.3 cm-1 bands weakly appear together
during sample deposition, visibly increase on sample annealing,
markedly decrease after broad-band irradiation, and recover after
further annealing to higher temperature (Table 1 and Figure 1).
The two bands shift to 1932.6 and 1884.6 cm-1, respectively,
in the 15N2 experiment with exactly the same 1.0343 14N/15N
isotopic frequency ratio. In the 14N2 + 15N2 experiment, two
sets of quartet bands, 1998.8/1983.3/1958.6/1932.5 and 1949.3/
1929.5/1894.4/1884.6 cm-1, have been observed (Table 1 and
Figure 2), which is reminiscent of the Rh(NN)3 and Fe(NN)3
complex in solid neon.27,28 Accordingly, these bands are suitable
for the N-N stretching vibration of the Nb(NN)3 molecule. The
corresponding N-N stretching frequencies of the analogous
Ta(NN)3 product have been observed at 2035.9 and 1991.3 cm-1

Study of M(NN)x in Solid Neon
J. Phys. Chem. A, Vol. 114, No. 25, 2010 6841
in solid neon (Table 1 and Figures 3 and 4). The matrix
counterparts for Nb(NN)3 and Ta(NN)3 were not identified in
the previous argon and pure nitrogen experiments.8
to the Nb(NN)6 molecule. The 2100.1 and 2049.4 cm-1 may
be due to the N-N stretching vibrations of Nb(NN)5 and
(NN)xNb(NN), respectively. Analogous potential Ta(NN)5 and
Ta(NN)6 molecules have been observed at 2092.0 and 2110.0
cm-1 in the present experiments (Table 1). The 2110.0 cm-1
band is in accord with previous 2106.4 cm-1 assignments to
Ta(NN)6 in an argon matrix.8a
The Nb(NN)3 and Ta(NN)3 assignments are strongly sup-
ported by the DFT frequency calculations (Table 2). The
Nb(NN)3 and Ta(NN)3 molecules are predicted to be T-shaped
4
with a B2 ground electronic state (Figure 5), similar to the
Rh(NN)3 molecule.27 Previously, B3LYP calculations predicted
Reaction Mechanism. At the present experimental condi-
tions, laser-ablated niobium and tantalum atoms react with N2
in the excess neon matrixes to produce the binary metal
dinitrogen complexes M(NN)x (M ) Nb, Ta; x ) 1-4). The
absorptions of the metal dinitrogen complexes increase on
annealing, suggesting the reactions to form these species are
spontaneous. The Nb(NN) and Ta(NN) molecules appear during
sample deposition and increase upon annealing (Figures 1 and
3 and Table 1), suggesting that these products may be formed
from the reactions of niobium and tantalum atoms with N2
(Table 3, reactions 1 and 5). Higher metal dinitrogen complexes
may be formed via the N2 addition (reactions 2-4 and 6-8).
As can be seen in Table 3 (reactions 1-8), all association
reactions are predicted to be exothermic (-14.8 to -25.2 kcal/
mol), in agreement with the experimental observations.
As illustrated in Figure 6, the highest occupied molecular
orbitals (HOMOs) in the Nb(NN) and Nb(NN)2 are of δ-type
with the contribution mainly from the Nb 4d atomic orbitals,
whereas the HOMO-1s are of σ-type and are largely Nb 5s and
4d in character. In addition, the HOMOs in Nb(NN)3 and
Nb(NN)4 are largely Nb 5s and 4d in character and are
nonbonding. The degenerate HOMO-1s in Nb(NN)3 and
Nb(NN)4 are the M-N π bonding orbitals, which are responsible
for the stability of these metal dinitrogen complexes. Such
bonding features hold true for the tantalum dinitrogen complexes.
5
the Nb+(N2)3 molecule to have a A1 ground state with a C2v
symmetry.11 In the Nb(NN)3 molecule, as an example, three
N-N stretching modes are calculated at 2135.4 cm-1 (16 km/
mol, A1), 2077.6 cm-1 (2280 km/mol, B2), and 2040.0 cm-1
(968 km/mol, A1) (Table 2). The band at 2135.4 cm-1 with very
low intensity (16 km/mol) is not readily observed, which is
consistent with the absence of this N-N stretching vibration
of the Nb(NN)3 molecule from the present experiments. The
calculated bands 2077.6 and 2040.0 cm-1 are consistent with
the experimental frequencies 1998.8 and 1949.3 cm-1, respec-
tively. As listed in Table 2, the calculated 14N2/15N2 isotopic
frequency ratio of 1.0350 is also in accord with the experimental
observation (1.0343). Similar results have also been obtained
for the Ta(NN)3 molecule (Table 2 and Figure 5).
M(NN)4. The 2079.4 cm-1 band with Nb and N2 in neon
weakly appears upon sample annealing, visibly increases after
broad-band irradiation, and slightly increases after further
annealing to higher temperature (Table 1 and Figure 1). This
band shifts to 2010.8 cm-1 with 15N2, exhibiting an isotopic
frequency ratio (14N/15N, 1.0341) characteristic of N-N stretch-
ing vibration. As can be seen in Figure 2, a quintet isotopic
pattern has been observed in the mixed 14N2 + 15N2 experiment,
which is the characteristic feature for the triply degenerate mode
of a tetrahedral species.26 This band is therefore assigned to
the N-N stretching vibration of the Nb(NN)4 molecule in solid
neon, which is 6.9 cm-1 above the argon matrix value.8a It is
noted that previous gas phase experiments observed the N-N
Conclusions
Niobium and tantalum dinitrogen complexes M(NN)x (M )
Nb, Ta; x ) 1-4) have been prepared by the reactions of laser-
ablated niobium and tantalum atoms with N2 in excess neon.
In the niobium experiments, the absorptions at 1939.5, 2039.9,
1998.8, 1949.3, and 2079.4 cm-1 are assigned to the N-N
stretching vibrations of the Nb(NN), Nb(NN)2, Nb(NN)3, and
Nb(NN)4 molecules, respectively, on the basis of the isotopic
shifts and mixed isotopic splitting patterns. Analogous Ta(NN)x
(x ) 1-4) products have been observed in the tantalum
experiments. DFT calculations have been performed, which lend
support to the experimental assignments of the matrix infrared
spectra.
stretching vibration of Nb+(N2)4 at 2264 cm-1 11
. Similar neon
matrix experiments of Ta and N2 give the absorption of the
analogous Ta(NN)4 molecule at 2058.6 cm-1 (Table 1 and
Figures 3 and 4). This band is in accord with previous 2040.9
cm-1 assignment to Ta(NN)4 in pure nitrogen.8a
Our DFT calculations predict the Nb(NN)4 and Ta(NN)4
molecules to have D4h symmetry with a 4A1g ground electronic
state (Figure 5), in accord with the previous report for a square-
planar D4h Nb(NN)4.8b The Nb+(N2)4 complex was also predicted
to have D4h symmetry with a 5B2g ground state in the previous
B3LYP calculations.11 For the Nb(NN)4 molecule, the scaled
antisymmetric N-N stretching frequency is predicted at 2108.1
cm-1 (Table 2), only 28.7 cm-1 lower than the present neon
matrix value. As summarized in Table 2, the calculated 14N2/
15N2 isotopic frequency ratio of 1.0350 is also in accord with
the experimental value (1.0341). Similar results have been
obtained for the Ta(NN)4 molecule (Table 2 and Figure 5). These
agreements of the Nb(NN)4 and Ta(NN)4 molecules are sup-
ported by the agreement between the experimental and the
calculated vibrational frequencies and isotopic shifts.
M(NN)5,6. In the Nb + N2 experiments, the 2149.4, 2130.0,
2100.1, and 2049.4 cm-1 bands appear after broad-band
irradiation, slightly increase on sample annealing, and show
N-N stretching frequency rations (Table 1). The mixed isotopic
structures for these bands were not well-resolved because of
band overlap and isotopic dilution. Compared with the previous
Nb + N2 system in pure nitrogen, for which the bands at 2140.2
and 2134.8 cm-1 were assigned to the Nb(NN)6 molecule,8a the
2149.4 and 2130.0 cm-1 bands in excess neon are probably due
Acknowledgment. This work was supported by AIST and a
Grant-in-Aid for Scientific Research (B) (Grant No. 17350012)
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan. Z.-H.L. acknowledges JASSO
and Kobe University for an Honors Scholarship.
References and Notes
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