The Negative Ion Photoelectron Spectra of MoV and CrV Presentedby Beau Barker

Introduction Ground states: V 4s 2 3d 3 Cr 4s 1 3d 5 Mo 5s 1 4d 5 Early transition metal dimers are of interest because d-electrons are involved in bonding. The d-orbitals are large enough to participate in bonding. Dimers containing later transition metals, such as CrCu, Cu 2 or Ni 2, are dominated by s  bonding.

NPES Basics: Electron Kinetic Energy (eKE) Electron Binding Energy (eBE) Experiments involve the use of the photoelectric effect: The eKE is measured and the eBE found from the difference in energy between the eKE and the photon: eBE is equal to the difference in energy between the anion and neutral states:

NPES Basics : Selection Rules Electronic State Transitions: For example: If anion is a doublet the neutral could be either a singlet or triplet.  Vibrational Transitions: Intensities of vibrational transitions are governed by Franck-Condon overlap between the anion and neutral species.

NPES Basics : Information from Spectra Electron affinity: The energy difference between the ground states of the anion and neutral. Vibrational frequencies: Found for both anion and neutral species. Anharmonicities: If known accurately these can also yield bond dissociation energies (for diatomics). Geometry Changes: Only the changes in equilibrium geometry are found between anion and neutral. Orbitals: Bonding characteristics of the detached electron.

NPES Basics : Generic Diatomic

Anion Photoelectron Spectrometer

Experimental Setup Metal diatomics are produced by creating a plasma around a V metal rod, and then adding Cr(CO) 6 or Mo(CO) 6. Spectra were taken with flow tube at room temperature. 52 Cr 51 V  and 92 Mo 51 V  were mass selected. About 25 pA of CrV  and 0.5 pA of MoV  were produced under these conditions.

Likely Electronic States Neutral: V 4s 2 3d 3 Cr 4s 1 3d 5 Mo 5s 1 4d 5 Anions: V 4s 2 3d 4 Cr 4s 2 3d 5 Mo 5s 2 4d 5 Excited state: V 4s 1 3d cm -1 Anion 1    d  2 d  4 d  4 s  2 3  : d  2 d  4 d  3 s  2 s  * 1 Neutral 2  : d  2 d  4 d  3 s  2 4  : d  2 d  4 d  3 s  1 s  * 1 2    d  2 d  4 d  4 s  1 4    d  2 d  4 d  2 s  2 s  * 1

Ground State of Neutrals Known Resonant two-photon ionization studies by Morse and co-workers have established the ground states and bond lengths of CrV and MoV. 1,2 2  2.5 d  2 d  4 d  3 s  2 CASSCF/CASPT2 by Andersson on CrV electronic states were calculated. The ground state agrees with experiment. The states are multiconfigurational. The configuration above is weighted at 57%. 1) Sickafoose, S. M.; Langenberg, J. D.; Morse, M. D. J. Phys. Chem. A 2000, 104, ) Nagarajan, R.; Sickafoose, S. M.; Morse, M. D. J. Chem. Phys. 2007, 127, ) K. Andersson. Theo. Chem. Acct. 2003, 110, 218 correct

DFT: MoV and CrV Gaussian 03 CrV BPW91/6-311+G(df) 1 MoV BPW91/SDD (Stuttgart-Dresden ECP) Various electronic states were explored. In order to find the ground state. States relevant to the photoelectron spectra are presented here. 1 ) Gutsev,G.L.; Mochena, M.D.; Jena, P.; Bauschlicher, C.W.; Partridge, H. J. Chem. Phys. 2004, 121, 6785.

MoV: eV (488nm) Spectrum 2     3  3 2     1  + 2 12 1 High Binding Energy: 3 Transitions

MoV: Ground States 2    1   *=  2.5 *= 

MoV: Ground States Anion: 1   d  2 d  4 d  4 s  2 to Neutral: 2  d  2 d  4 d  3 s  2 Detachment from d  Experimental Calculated EA eV0.721 eV Δr e ± Å0.115 Å  e Anion 578 cm cm -1  e x e Anion  e Neutral 508 cm cm -1  e x e Neutral 2.8 cm SO  cm

MoV: First Excited Neutral x10 2 12 1

MoV: First Excited Neutral Anion: 1   d  2 d  4 d  4 s  2 to Neutral: 2   d  2 d  4 d  4 s  1 Detachment from s  Experimental Calculated T o eV1.307 eV Δr e ±0.053 Å Å  e Anion 572 cm cm -1  e x e Anion  e Neutral 651 cm cm -1  e x e Neutral 3.7 cm

MoV: Excited Anion State 2    3  3    0-0       0-1          1-0    0-0

MoV: Excited Anion State Anion: 3  d  2 d  4 d  3 s  2 s  * 1 to Neutral: 2  d  2 d  4 d  3 s  2 Detachment from s  * Experimental Calculated T eV0.130 eV Δr e ± Å Å  e Anion cm -1  e x e Anion SO  3-  2160 cm  e Neutral 508 cm cm -1  e x e Neutral 2.8 cm

CrV: 514.5nm (2.409 eV) Spectrum * 2     3  3 2 332 33 4      3  3 High Binding Energy: 3 Transitions Spectra reproduced from Ph.D. thesis by Simson Alex 1997

CrV: Ground State (2.409 eV) x    3  3

CrV: Ground State Spin-Orbit             2    3  3 0-0

CrV: Ground State Anion: 3  d  2 d  4 d  3 s  2 s  * 1 to Neutral: 2  d  2 d  4 d  3 s  2 Detachment from s  * Experimental Calculated * EA eV eV Δr e ± 0.03 Å Å  e Anion 409 cm cm -1  e x e Anion  e Neutral 520 cm cm -1  e x e Neutral 7.2 cm * DFT finds the 1   to be the anionic ground state; the 3  is 0.15 eV above it.

CrV: First Excited Neutral (2.409 eV) x5 2 332 3 * * * **** * ? ?

CrV: First Excited Neutral Anion: 3   d  2 d  4 d  3 s  2 s  * 1 to Neutral: 2   d  2 d  4 d  4 s  1 Two electron process Experimental Calculated T o eV eV Δr e ± 0.05 Å Å  e Neutral 715 cm cm -1  e x e Neutral 12.3 cm  e Anion 409 cm cm -1

Summary The ground electron state of the anions, MoV  and CrV , were found to be different. MoV - 1  + vs. CrV - 3   The force constants for the  states are large. 1    d  2 d  4 d  4 s  2 2    d  2 d  4 d  4 s  1 MoV - 1  Cr 2 1  g  MoV 2    8.19 Mo 2 1  g CrV 2   7.75Cu 2 1  g Units are mdyne/Å 1) Lombardi, J. R.; Davis, B. Chem. Rev. 2002, 102, 2431.

Thanks Doreen Leopold advisor Simson Alex for CrV experimental data Minnesota Supercomputing Institute Research Corporation

CrV: Ground State Spin-Orbit The uneven spin-orbit spacing in the anion 3  implies the existence of a low lying excited state; not seen in the spectrum. State Spacing Neutral  2.5-  cm -1 Anion  3-  270 cm -1 Anion  2-  1335 cm -1

Spin-Orbit States Can have spin-orbit transition when orbital angular momentum is not 0. Using a simple first order approach the spin- orbit states should be evenly spaced by A. E so =A  A is negative so highest  state is lowest. MoVCrV 3   427 cm cm -1  = 3,2,1 2   854 cm cm -1  = 2.5,1.5

MoV: High Binding Energy

CrV: Second Excited Neutral (2.409 eV)    3 

CrV: Second Excited Neutral (2.409 eV)  Anion: 3   d  2 d  4 d  3 s  2 s  * 1 to Neutral: 4   d  2 d  4 d  3 s  1 s  * 1  Detachment from a d  Experimental Calculated T o eV N/A Δr e 0.05 Å N/A  e Neutral 440 cm -1 N/A  e x e Neutral <4.0 cm -1 N/A  e Anion 409 cm cm -1

CrV High Binding Energy (2.601eV) 4    CrV  * 3343    3 