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
Content
• Background
• Experimental
• Results
– Li(MeNH2)n Experimental and theoretical
• Conclusions
• Future work
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
Spectroscopic mechanism
N-H=0
N-H=1
Li-N Dissociation
limit
Depletion
Predissociation
• 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)
Experimental
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
0 40 80 120 160 200 240 280
876543210n
Mass/Daltons
• (a) IR OFF
• (b) IR ON
(a)
(b)
Li(MeNH2)n
• IR depletion spectra for n = 4 and 5
•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 = 2
n = 3
n = 4
3000 3100 3200 3300 3400
Del
petio
n/A
rb. U
nits
Wavenumber/cm-1
n = 5
Calculations• Calculations still
underway
• B3LYP/
6-311++G(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
2800 2900 3000 3100 3200 3300 3400 3500
Wavenumber/cm-1
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
– >20 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 45962 40193 41604 1370
• Thus IR absorption for n 4 could induce loss of an ammonia molecule
a) Lowest energy conformer in each case
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
Excitation spatially separated from ionisation. Fragments not detected
Excitation and ionisation in the same region. Fragments detected
Time-of-Flight extraction region
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
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.
Acknowledgements
• Funding – principally EPSRC
• University of Leicester Centre for Mathematical Modelling
• Mechanical and electronic workshops
• IR production spectrum for Li(NH3)1
• Peaks in C-H stretching region visible
Additional