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