Anion Photoelectron Spectroscopic Studies of NbC 4 H 4 ‾, NbC 6 H 6 ‾ and NbC 6 H 4 ‾ Products of Flow Tube Reactions of Niobium with Butadiene Melissa A. Baudhuin, † Praveenkumar Boopalachandran, † Stephen R. Miller, ‡ D. Alex Schnepper, † Doreen G. Leopold † † University of Minnesota – Chemistry Department ‡ Gustavus Adolphus College – Chemistry Department June 17 th,

Nb reactions with small unsaturated hydrocarbons Gas phase systems are not complicated by solvent effects Provide benchmark values for computational studies Serve as models for understanding processes occurring in real catalytic reactions ▫ C-H bond activation ▫ C-C bond formation and/or activation ▫ Dehydrogenation ▫ Intracluster cyclization → May contribute to the discovery of improved industrial catalysts 2 ORGANOMETALLIC COMPLEXES

Flowing Afterglow Ion-Molecule Reactor Magnetic Sector Mass Analyzer Photoelectron Spectroscopy Chamber Slits Argon Ion Laser Electron Kinetic Energy Analyzer Chamber 1 Chamber 2 Chamber 3 Chamber 4 Chamber 5 hν (488 or 514 nm) 3 Electron Detector ANION PHOTOELECTRON SPECTROMETER

4 INTERPRETING DATA (eKE) Electron Kinetic Energy (eBE) Electron Binding Energy eBE = hν - eKE X+ e ‾ ←X‾X‾ +hνhν X*+ e ‾ ←X‾X‾ +hνhν X‾X‾ X X* 1 eV ≈ 8065 cm -1

5 FLOW TUBE REACTION PRODUCTS OF Nb + C 4 H 6 Peak #Molecule 1NbC 2 H 2 O ‾ 2NbC 4 H 4 O ‾ 3NbC 6 H 8 ‾ 4NbC 8 H 8 ‾ 5NbC 8 H 10 O ‾ 6NbC 10 H 14 ‾ 7Nb 2 C 4 H 4 ‾ 8Nb 2 C 6 H 2 ‾ 9Nb 2 C 6 H 4 ‾ 3 Nb 2 C 8 H 4 ‾

6 PHOTOELECTRON SPECTRUM OF NbC 4 H 4 − AT 488 nm Nb + Butadiene 1 eV ≈ 8065 cm -1

7 PHOTOELECTRON SPECTRUM OF NbC 4 H 4 − AT 488 nm Calculated: B3LYP/gen [Nb → SDD, C and H → G(df,pd)] Nb + Butadiene

8 PHOTOELECTRON SPECTRUM OF NbC 4 H 4 − AT 488 nm Neutral moleculeNb + C 4 H 6 exp. Electron Affinity (eV)0.995 Mode 2ν M-L (cm -1 )375 Mode 1ν op (cm -1 )665 AnionNb + C 4 H 6 exp. Mode 2ν M-L (cm -1 )335 C s symmetry Nb + Butadiene X̃ 2 A' ← X̃ 3 A '

9 Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, PHOTOELECTRON SPECTRUM OF NbC 4 H 4 − AT 488 nm Neutral moleculeNb + C 4 H 6 exp.Nb + C 2 H 4 exp. Electron Affinity (eV) ± Mode 2ν M-L (cm -1 ) ± 15 Mode 1ν op (cm -1 ) ± 20 AnionNb + C 4 H 6 exp.Nb + C 2 H 4 exp. Mode 2ν M-L (cm -1 )335- C s symmetry Nb + Butadiene X̃ 2 A' ← X̃ 3 A '

10 Miller, S. R. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Reagent: Ethylene PHOTOELECTRON SPECTRUM OF NbC 4 H 4 − AT 488 nm Nb + Butadiene X̃ 2 A' ← X̃ 3 A '

11 PHOTOELECTRON SPECTRUM OF NbC 6 H 6 − AT 488 nm Inset: Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – Nb + Butadiene C 6v symmetry X̃ 2 A 1 ← X̃ 3 A 1 2 A 1 ← X̃ 3 A 1

12 PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm A B A NbC 6 H 4 ‾ NbC 6 H 4 NbC 6 H 4 * B Nb + Butadiene

cm cm -1 Neutral moleculeTransition A Electron Affinity (eV)1.110 Mode 1ν M-L (cm -1 )360 AnionTransition A Mode 1ν M-L (cm -1 )315 PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm Nb + Butadiene MoleculeNbNbC 4 H 4 NbC 6 H 6 NbC 6 H 4 EA (eV)0.918 ± 0.005* *Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Nbs1d4s1d4 Nb ‾ s2d4s2d4

cm cm -1 Neutral moleculeTransition ATransition B eBE (eV) Mode 1ν M-L (cm -1 )360 AnionTransition ATransition B Mode 1ν M-L (cm -1 ) PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm Nb + Butadiene

15 PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm A B A NbC 6 H 4 ‾ NbC 6 H 4 NbC 6 H 4 * ‾ B Perhaps Transition A is due to an excited state of the anion and accesses the same state of the neutral as Transition B: Nb + Butadiene

16 Relative probability that Transition A would occur: Thermal Equilibrium? 2 x eV 3690 cm -1 PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm A B A NbC 6 H 4 ‾ NbC 6 H 4 NbC 6 H 4 * ‾ B ∆E

17 Maybe: Transition A and B access different neutral states Transition A and B are from different isomers PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm A B Nb + Butadiene

NbC 6 H 4 ‾ DFT ANALYSIS 18 Identify the isomer: DFT (B3LYP/gen) ▫ Small # of active modes in spectrum → expect a higher symmetry structure C 2v -C 4 H 4 -Nb-C 2 C s -C 4 H 4 -Nb-C 2 C 2v -Nb-benzyne (planar) C s -Nb-benzyne (nonplanar) gen: Nb → SDD & C and H → G(df,pd)

19 Calculated: B3LYP/gen [Nb → SDD, C and H → G(df,pd)] Note: * TDB3LYP (assumed to be a vertical transition) CONCLUSIONS: NbC 6 H 4 − PHOTOELECTRON SPECTRUM

20 Current Group Members Dr. Doreen Leopold (Adviser) Alex Schnepper Former Group Members Dr. Praveen Boopalachandran Srijay Rajan Dr. Steve Miller Dr. Evan Millam Dr. Tim Marcy Funding NSF ACKNOWLEDGEMENTS

QUESTIONS 21

22

Nb-benzyne vs benzyne Å 1.39 Å 1.40 Å 1.09 Å 1.39 Å 1.40 Å No evidence of a C ≡ C stretch Calculated: B3LYP/gen [Nb → SDD, C and H → G(df,pd)] 1.34 Å 1.40 Å 1.09 Å 1.40 Å 1.09 Å

24 StateIsomer abbrev.E rel (eV)Stable* 5B25B2 C 2v -Nb-benzyne0.0000Yes 5 A"C s -Nb-benzyne0.0003Yes 3B23B2 C 2v -Nb-benzyne0.0587Yes 3 A"C s -Nb-benzyne0.0590Yes 1A11A1 C 2v -Nb-benzyne0.5570Yes 1 A'C s -Nb-benzyne0.5570Yes 1 A'C s -C 4 H 4 -Nb-C Yes 3 A'C s -C 4 H 4 -Nb-C Yes 3B13B1 C 2v -C 4 H 4 -Nb-C No 1A11A1 C 2v -C 4 H 4 -Nb-C No 5 A'C s -C 4 H 4 -Nb-C No 5B25B2 C 2v -C 4 H 4 -Nb-C No NbC 6 H 4 ‾ DFT ANALYSIS C 2v -Nb-benzyne C s -Nb-benzyne C s -C 4 H 4 -Nb-C 2 Note (*) Stable means no imaginary frequencies in the calculation Anion State: Singlet, triplet, and quintet states of each isomer were calculated

Flow Tube Configuration C4H6C4H6 25

Previous Work: Nb + C 2 H 4 Miller, S. R. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Photoelectron Spectrum of NbC 4 H 4 ‾ NbC 4 H 4 + e‾ ← NbC 4 H 4 ‾ + hν A DFT (B3LYP/gen) Isomer Neutral molecule Transition A Exp. Transition A Calc. Electron Affinity (eV)0.997 ± Mode 2ν M-L (cm -1 )385 ± Mode 1ν op (cm -1 )660 ± C s symmetry Transition A X̃ 2 A' ← X̃ 3 A' Calculated: B3LYP/gen gen: Nb → SDD & C and H → G(df,pd) 26

Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – Previous Work: Nb + C 2 H 4 Transition A X̃ 2 A 1 ← X̃ 3 A 1 Transition B 2 A 1 ← X̃ 3 A 1 Calculated: B3LYP/gen gen: Nb → SDD & C and H → G(df,pd) Neutral molecule Transition A Exp. Transition A Calc. Transition B Exp. Transition B Calc. eBE (eV)0.893 ± Mode 4ν M-L (cm -1 )382 ± Mode 3ν op (cm -1 )740 ± ± 20- Mode 2v CH (cm -1 )--955 ± 20- Mode 1v CH (cm -1 )3060 ± ± 30- Anion Transition A Exp. Transition A Calc. Transition B Exp. Transition B Calc. Mode 4ν M-L (cm -1 )375 ± Mode 3ν op (cm -1 )730 ± Summary of experimental and calculated results of NbC 6 H 6 ‾ and NbC 6 H 6 ‾ C 6v symmetry 27

28 C s symmetry Neutral moleculeTransition A Nb + C 2 H 4 exp. Transition A Nb + C 4 H 6 exp. Electron Affinity (eV)0.997 ± Mode 2ν M-L (cm -1 )385 ± Mode 1ν op (cm -1 )660 ± Anion Transition A Nb + C 2 H 4 exp. Transition A Nb + C 4 H 6 exp. Mode 2ν M-L (cm -1 )-335 Note (*) indicates that these transition are simulated Franck-Condon fits PHOTOELECTRON SPECTRUM OF NbC 4 H 4 − AT 488 nm

29 C 6v symmetry Note (*) indicates that these transition are simulated Franck-Condon fits Neutral moleculeTransition A Nb + C 2 H 4 exp. Transition A Nb + C 4 H 6 exp. Electron Affinity (eV)0.893 ± Mode 4ν M-L (cm -1 )382 ± Mode 3v op (cm -1 )740 ± Anion Transition A Nb + C 2 H 4 exp. Transition A Nb + C 4 H 6 exp. Mode 4ν M-L (cm -1 )375 ± PHOTOELECTRON SPECTRUM OF NbC 6 H 6 − AT 488 nm

30 PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm

31 PHOTOELECTRON SPECTRUM OF NbC 6 H 4 − AT 488 nm

NbC 6 H 4 − Experimental Results at Room Temperature and 488 nm 32 Neutral moleculeTransition FTransition Z eBE (eV) Mode 1ν M-L (cm -1 ) AnionTransition FTransition Z Mode 1ν M-L (cm -1 )315370

33 Photoelectron Spectrum of NbC 6 H 4 − : Theory versus Experiment C 2v symmetry C s symmetry Calculated: B3LYP/gen [Nb → SDD, C and H → G(df,pd)] Note: * TDB3LYP (assumed to be a vertical transition)

PREVIOUS WORK Anion photoelectron spectroscopic studies of anion products produced by flow tube reactions of Nb with ethylene Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – ← With Ethylene ← Without Ethylene 34

PREVIOUS WORK: Nb + C 2 H 4 Miller, S. R. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Photoelectron Spectrum of NbC 4 H 4 ‾ NbC 4 H 4 + e‾ ← NbC 4 H 4 ‾ + hν 35

Miller, S. R. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Photoelectron Spectrum of NbC 4 H 4 ‾ NbC 4 H 4 + e‾ ← NbC 4 H 4 ‾ + hν Broad Spectral Features Unresolved transitions No information on origin position or vibrational frequencies Suspected to come from at least one additional isomer 36 PREVIOUS WORK: Nb + C 2 H 4

Miller, S. R. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Marcy, T. P. Ph.D. Thesis, University of Minnesota: Minneapolis, MN, Photoelectron Spectrum of NbC 4 H 4 ‾ NbC 4 H 4 + e‾ ← NbC 4 H 4 ‾ + hν A DFT (B3LYP/gen) Isomer C s symmetry gen: Nb → SDD & C and H → G(df,pd) Transition A X̃ 2 A' ← X̃ 3 A' 37

Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – Photoelectron Spectrum of NbC 6 H 6 ‾ NbC 6 H 6 + e‾ ← NbC 6 H 6 ‾ + hν 38 PREVIOUS WORK: Nb + C 2 H 4

Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – Photoelectron Spectrum of NbC 6 H 6 ‾ NbC 6 H 6 + e‾ ← NbC 6 H 6 ‾ + hν B A DFT (B3LYP/gen) Isomer C 6v symmetry Transition A X̃ 2 A 1 ← X̃ 3 A 1 Transition B 2 A 1 ← X̃ 3 A 1 gen: Nb → SDD & C and H → G(df,pd) 39 PREVIOUS WORK: Nb + C 2 H 4

40 PHOTOELECTRON SPECTRUM OF NbC 6 H 6 − AT 488 nm Inset: Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – C 6v symmetry A Neutral moleculeNb + C 4 H 6 exp. Electron Affinity (eV)0.891 Mode 2ν M-L (cm -1 )377 Mode 1v op (cm -1 )740 Anion Nb + C 4 H 6 exp. Mode 2ν M-L (cm -1 )372 A Nb + Butadiene

41 PHOTOELECTRON SPECTRUM OF NbC 6 H 6 − AT 488 nm Inset: Miller, S.R.; Marcy, T.P.; Millam, E.L.; Leopold, D.G. J. Am. Chem. Soc. 2007, 129, 3482 – C 6v symmetry A Neutral moleculeNb + C 4 H 6 exp.Nb + C 2 H 4 exp. Electron Affinity (eV) ± Mode 2ν M-L (cm -1 ) ± 10 Mode 1v op (cm -1 ) ± 15 Anion Nb + C 4 H 6 exp.Nb + C 2 H 4 exp. Mode 2ν M-L (cm -1 ) ± 15 A Nb + Butadiene

42 PHOTOELECTRON SPECTRUM OF NbC 6 H 6 − AT 488 nm Calculated: B3LYP/gen [Nb → SDD, C and H → G(df,pd)] Nb + Butadiene