1. Infrared Photodissociation Spectroscopy of TM + (N 2 ) 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

2. Biological systems require N 2 as components of proteins, nucleic acids, etc. But N 2 is highly inert (IP = 15.08 eV, BE = 225 kcal/mol). Nitrogenases catalyze N 2 reduction and carry metal centers such as Fe, Mo, V. Large scale ammonia synthesis uses Fe as catalyst. N 2 is isoelectronic to CO, C 2 H 2 which are prevalent throughout inorganic and organometallic chemistry N 2 activation gauged by change in N-N bond distance or N-N vibrational frequency Why Study TM-Nitrogen?

3. Previous Work Electronic spectroscopy of M + (N 2 ) (M = Mg, Ca) by Duncan and coworkers. CID studies by Armentrout and coworkers for Fe and Ni with N 2 FT-ICR studies by H.Schwarz and coworkers, and electronic spectroscopy by Brucat and coworkers on Co + (N 2 ) Theoretical studies on TM-N 2 carried out by Bauschlicher ESR spectra for V(N 2 ) 6 and Nb(N 2 ) 6 done by Weltner. IR studies using matrix isolation on M(N 2 ) (M = V, Cr, Mn, Nb, Ta, Re) done by Andrews and coworkers

4. Experimental Bond Energies * Ni + (N 2 ) n Bond Energy (kcal/mol) n = 1 27 2 27 3 14 4 2 V + (CO) n Bond Energy (kcal/mol) n = 1 27 2 22 3 17 4 21 5 22 6 24 * Armentrout and coworkers Fe + (N 2 ) n Bond Energy (kcal/mol) n = 1 13 2 19 3 10 4 13 5 15 Direct absorption in our experiments is not possible due low ion densities. Solution is photodissociation. IR photon 2359 cm -1 ~ 7 kcal/mol Small clusters may fragment via multiphoton process. Large clusters will be easier to fragment

5. Production of cold metal ion complexes with laser vaporization/ supersonic expansion. Mass selection of cations by time-of-flight. Tunable infrared laser photodissociation spectroscopy. LaserVision OPO/OPA 2000-4500 cm -1

6. Nb + (N 2 ) n Nb + 2 4 5 6 n= 1 10 16 Mass

7. Fragmentation ends at n = 6 suggesting that this cluster is more stable. Fragmentation of Nb + (N 2 ) n n = 6 7 8 9 5 6 6 6 7

8. Free N 2 mode 2359 cm -1 Infrared Photodissociation Spectra for Nb + (N 2 ) n Fragmentation is inefficient for the n = 1-3 clusters. The n=4 cluster shows fragmentation 95 cm -1 red of the free N 2 stretch n = 2 n = 3 n = 4 2265

9. Dewar-Chatt-Duncanson Model of  -bonding Both factors weaken the N-N bonding in nitrogen. The N-N stretching frequencies shift to the red.  -donation from occupied 1  u or 3  g N 2 orbital into empty d-orbitals of the metal    - type back donation from filled d xy, d yz, d xz orbitals to  g * orbitals of N 2

10. Spectra show a red shift of 95 cm -1 for n=4 as compared to free N 2 stretch An additional red shift of 60 cm -1 is observed for n>4 cluster sizes The spectra of n=6 has a lower S/N ratio suggesting the complex is harder to dissociate owing to unusual stability

11. B3LYP/ DGDZVP  Nb + 6-311+G*  N D e = 33.8 kcal/mol Freq = 2291 cm -1 Osc. Strength = 55 km/mol D e = 18.6 kcal/mol Freq = 2160 cm -1 Osc. Strength = 169 km/mol D e = 19.7 kcal/mol Freq = 2262 cm -1 Osc. Strength = 354 km/mol D e = 8.3 kcal/mol Freq = 2209 cm -1 Osc. Strength = 376 km/mol 1.DFT calculations favor linear over T-shaped structures ( D e ~ 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.

12. Nb + Grnd state: 4d 4 5 D 1 st state: 4d 3 5s 5 F 6.7 kcal/mol 2 nd state: 4d 4 3 P 15.9 kcal/mol Spectrum has two modes because there are only two equivalent N 2

13. 2265 DFT (B3LYP) calculations for the n = 4 complex for the 5 D spin state show good correspondence to the IR spectra. Single peak spectrum points to a high symmetry structure.

15. What is causing the additional red shifts for the n>4 clusters ? 1.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. In addition all spectra are single peak signifying that no isomers are present. 2.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 Nb + (N 2 ) 5

16. Comparison of Nb + (N 2 ) n and V + (N 2 ) n Greater red-shifts for Nb + (N 2 ) n than V + (N 2 ) n Nb + (N 2 ) n V + (N 2 ) n

17. 1.N 2 and CO are  -accepting ligands and so d  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 + (N 2 ) n and Nb + (N 2 ) n seems to justify this reasoning.

18. Conclusions IR spectroscopy coupled with DFT calculations of Nb + (N 2 ) n reveals the structures of these clusters. The spectra show that N 2 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 + (N 2 ) n red shifts further than in V + (N 2 ) n consistent with the previous conclusions based on various TM- (CO) n systems that  -back donation is the more significant interaction in these TM-ligand systems.