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Infrared Photodissociation Spectroscopy of
TM+(N2)n (TM=V,Nb) Clusters
E. D. Pillai, T. D. Jaeger, M. A. Duncan
Department of Chemistry, University of Georgia
Athens, GA 30602-2556
www.arches.uga.edu/~maduncan/
U.S. Department of Energy
Why Study TM-Nitrogen?
• Biological systems require N2 as components of proteins, nucleic
acids, etc. But N2 is highly inert (IP = 15.08 eV, BE = 225 kcal/mol).
Nitrogenases catalyze N2 reduction and carry metal centers such as
Fe, Mo, V.
• Large scale ammonia synthesis uses Fe as catalyst.
N2 + H2
Fe catalyst
350 - 1000 atm
2NH3
300 - 500 oC
• N2 is isoelectronic to CO, C2H2 which are prevalent throughout
inorganic and organometallic chemistry
• N2 activation gauged by change in N-N bond distance or N-N
vibrational frequency
Previous Work
• Electronic spectroscopy of M+(N2) (M = Mg, Ca) by
Duncan and coworkers.
• CID studies by Armentrout and coworkers for Fe and Ni
with N2
• FT-ICR studies by H.Schwarz and coworkers, and
electronic spectroscopy by Brucat and coworkers on
Co+(N2)
• Theoretical studies on TM-N2 carried out by Bauschlicher
• ESR spectra for V(N2)6 and Nb(N2)6 done by Weltner.
• IR studies using matrix isolation on M(N2) (M = V, Cr,
Mn, Nb, Ta, Re) done by Andrews and coworkers
Experimental Bond Energies*
Direct absorption in our experiments
is not possible due low ion densities.
Solution is photodissociation.
Ni+(N2)n
IR photon
2359 cm-1 ~ 7 kcal/mol
Small clusters may fragment via
multiphoton process.
Large clusters will be easier to fragment
Fe+(N2)n
n= 1
2
3
4
5
Bond Energy (kcal/mol)
13
19
10
13
15
n= 1
2
3
4
V+(CO)n
n= 1
2
3
4
5
6
* Armentrout and coworkers
Bond Energy (kcal/mol)
27
27
14
2
Bond Energy (kcal/mol)
27
22
17
21
22
24
LaserVision OPO/OPA
2000-4500 cm-1
Production of cold
metal ion complexes
with laser vaporization/
supersonic expansion.
Mass selection of cations
by time-of-flight.
Tunable infrared laser
photodissociation
spectroscopy.
Nb+(N2)n
Nb+
6
4
2
10
5
n= 1
16
200
Mass
400
600
Fragmentation of Nb+(N2)n
6
7
9
6
8
6
5
Fragmentation ends at
n = 6 suggesting that
this cluster is more stable.
7
n=6
100
200
300
mass
400
500
Infrared Photodissociation Spectra for Nb+(N2)n
Free N2 mode
2359 cm-1
n=4
2265
Fragmentation is inefficient for the n =
1-3 clusters.
n=3
The n=4 cluster shows fragmentation 95
cm-1 red of the free N2 stretch
n=2
2100
2200
2300
cm
-1
2400
Dewar-Chatt-Duncanson Model of p-bonding
s
N
N
TM
s-donation from occupied
1pu or 3sg N2 orbital into
empty d-orbitals of the metal
s
N
TM
p
p
N
N
TM
N
p- type back donation from
filled dxy, dyz, dxz orbitals to
pg* orbitals of N2
N
TM
Both factors weaken the N-N bonding in nitrogen.
The N-N stretching frequencies shift to the red.
N
2212
n=7
Spectra show a red shift of
95 cm-1 for n=4 as compared to
free N2 stretch
2214
n=6
An additional red shift of 60 cm-1
is observed for n>4 cluster sizes
2204
n=5
n=4
The spectra of n=6 has a lower
S/N ratio suggesting the complex
is harder to dissociate owing to
unusual stability
2265
n=3
2100
2200
2300
cm
-1
2400
B3LYP/ DGDZVP Nb+
6-311+G* N
De= 33.8 kcal/mol
Freq = 2291 cm-1
Osc. Strength = 55 km/mol
De= 18.6 kcal/mol
Freq = 2160 cm-1
Osc. Strength = 169 km/mol
De= 8.3 kcal/mol
Freq = 2209 cm-1
Osc. Strength = 376 km/mol
De= 19.7 kcal/mol
Freq = 2262 cm-1
Osc. Strength = 354 km/mol
1. DFT calculations favor linear over T-shaped structures ( De ~ 15 –
20 kcal/mol
2. T-shaped complexes red-shift N-N stretch by 150-200 cm-1 whereas
linear complexes red shift by 50-100 cm-1.
Nb+
+
Grnd state: 4d4 5D
Nb (N2)3
1st state: 4d35s 5F
6.7 kcal/mol
2nd state: 4d4 3P
15.9 kcal/mol
5
A1
3
B2
2100
2200
2300
2400
Spectrum has two modes
because there are only two
equivalent N2
+
Nb (N2)4
5
2265
Single peak spectrum points
to a high symmetry structure.
DFT (B3LYP) calculations for
the n = 4 complex for the 5D spin
state show good correspondence
to the IR spectra.
B2g
3
A1g
2100
2200
2300
cm
-1
2400
What is causing the additional red
shifts for the n>4 clusters ?
Nb+(N2)5
1.
3
A2
5
In addition all spectra are single peak
signifying that no isomers are present.
B1
2.
2100
2200
Other structures such as T-shaped or
inserted complexes? DFT studies
consistently predict linear structures over
T-shaped structures. Energy differences ~
15 kcal/mol and 20 kcal/mol.
2300
2400
A change in spin state? DFT (B3LYP)
calculations for the n = 5 for triplet spin
state shows better correspondence to IR
spectrum than the quintet state.
Also triplet state is found to be lower
in energy by ~ 15 kcal/mol
Comparison of Nb+(N2)n and V+(N2)n
Nb+(N
Greater red-shifts for Nb+(N2)n than V+(N2)n
2)n
V+(N2)n
2271
2212
2258
n=7
n=7
2214
n=6
n=6
2271
2204
2258
n=5
n=5
2288
n=4
2265
n=4
n=3
2100
n=3
2200
2300
2400
2500 2100
2200
2300
2400
2500
s
N
N
TM
p
N
N
TM
1.
N2 and CO are p-accepting ligands and so dp-back donation
is expected to dominate the bonding interaction.
2.
d orbitals more diffuse for second row TM leading to better
s-d hybridization.
3.
Frequency shifts for V+(N2)n and Nb+(N2)n seems to justify
this reasoning.
Conclusions
IR spectroscopy coupled with DFT calculations of Nb+(N2)n
reveals the structures of these clusters.
The spectra show that N2 binds in an “end on” configuration to
Nb+.
The results also reveal possible evidence for a change in
multiplicity in the metal cation due to solvation effects.
The N-N stretch in Nb+(N2)n red shifts further than in V+(N2)n
consistent with the previous conclusions based on various TM(CO)n systems that p-back donation is the more significant
interaction in these TM-ligand systems.
+
Nb (N2)n
n=10
n=9
n=8
2212
n=7
2214
n=6
2100
2200
2300
2400