IR photodepletion and REMPI spectroscopy of LiNH2Men clusters

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IR photodepletion and REMPI spectroscopy of
Li(NH2Me)n clusters

• Tom Salter, Victor Mikhailov, Corey Evans and
  Andrew Ellis
• Department of Chemistry

International Symposium on Molecular Spectroscopy
22nd June 2006
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Content

• Background
• Experimental
• Results
• Li(MeNH2)n Experimental and theoretical
• Conclusions
• Future work

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Background

• Very little experimental or theoretical work
  undertaken on metal-methylamine clusters
• All solution phase ESR and neutron diffraction
• Will provide more information on the nature of
  the solvated electron
• Extrapolation back to bulk solution
• May expect to see stretches in C-H region along
  with N-H
• Increased steric bulk may affect onset of the
  closure of the first solvation shell
• Current results are only preliminary

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Spectroscopic mechanism

• Excitation of N-H stretching region with tuneable
  IR radiation
• Fixed wavelength UV laser, set just above IP,
  used to ionise clusters
• Resonance results in the loss of a solvent
  molecule leading to ion depletion
• Requires the solvent binding energy to be less
  than that of the IR photon and for
  predissociation to be fast enough (ns or less)

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Experimental
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Li(MeNH2)n

• Mass spectrum

• Up to 15-fold increase in ion production observed
  for n 1, 2 and 3
• Depletion seen for larger clusters

(a)

• (a) IR OFF
• (b) IR ON

(b)
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Li(MeNH2)n

• IR depletion spectra for n 4 and 5

n 2

• Only recorded so far using a fixed and short UV
  wavelength, 248 nm
• See depletion of large clusters and corresponding
  production of small clusters
• Under tighter IR focal conditions, production
  also seen in n 1 channel
• Possibility of fragmentation

n 3
n 4
n 5
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Calculations

• Calculations still underway
• B3LYP/
• 6-311G(d,p)
• More conformational isomers possible so
  systematic searching is more involved
• Lowest energy predicted spectra for n 1-4 show
  very similar trends making assignment problematic
• Difficult to be sure where ion production for
  small clusters is originating from

n 4 Experimental
n 1
n 2
n 3
n 4
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Calculations

• Possible that several low energy isomers are
  adopted resulting in spectral broadening
• 3 isomers predicted for n 2
• 12 isomers predicted for n 3
• gt20 isomer predicted for n 4
• Due to increased steric bulk from methyl group
• Calculations indicate closure of the first
  solvation shell with 3 methylamine molecules
• In contradiction to neutron diffraction data,
  which predicts closure of the first solvation
  shell with 4 molecules

• Dissociation energies
• n a) DFT
• 1 4596
• 2 4019
• 3 4160
• 4 1370
• Thus IR absorption for n ? 4 could induce loss of
  an ammonia molecule
• a) Lowest energy conformer in each case

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Improvements

• Contributions from larger clusters may present a
  serious problem for identification
• Possible solution would be excitation downstream
  from ionisation
• Small clusters will miss MCP
• Record spectra at wavelengths just above a
  cluster IP
• Aim to minimise fragmentation

Time-of-Flight extraction region
Excitation and ionisation in the same region.
Fragments detected
Excitation spatially separated from ionisation.
Fragments not detected
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Conclusions

• Preliminary spectroscopic data obtained
• Li(MeNH2)n clusters for n 4 and 5 show IR
  photodepletion spectra
• This depletion is mirrored by ion production for
  n 1, 2 and 3
• Calculations provide the first indication that
  the first solvation shell is closed with 3
  solvent molecules
• Plausible due to increased steric hindrance from
  methyl group
• Assignment problematic as spectra for n 1-4 are
  very similar
• Dissociation energies consistent with depletion
  from n 4
• Further work is required on these clusters, such
  as experimental determination of IP
• Confirm extrapolation to the bulk phase
• Identify trends confirming closure of the first
  solvation sphere

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Future Work

• Have a general methodology for recording
  mass-selected IR spectra of solute-solvent
  clusters
• Can explore other metal solutes, e.g.
• Other alkali metals
• Alkali metal clusters, e.g. Li2, Li3
• Alkaline earth and transition metals such as Cu
• Molecular solutes
• e.g. acids such as HCl or HNO3, salts, etc.
• Different solvents
• e.g. water, alcohols, methanol, acetonitrile, etc.

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Acknowledgements

• Funding principally EPSRC
• University of Leicester Centre for Mathematical
  Modelling
• Mechanical and electronic workshops

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Additional

• IR production spectrum for Li(NH3)1
• Peaks in C-H stretching region visible