IR photodepletion and REMPI spectroscopy of Li(NH 2 Me) n clusters Tom Salter, Victor Mikhailov, Corey Evans and Andrew Ellis Department of Chemistry International Symposium on Molecular Spectroscopy 22 nd June 2006

Content Background Experimental Results –Li(MeNH 2 ) 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(MeNH 2 ) n Mass spectrum Up to 15-fold increase in ion production observed for n = 1, 2 and 3 Depletion seen for larger clusters (a) IR OFF (b) IR ON (a) (b)

Li(MeNH 2 ) 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 n = 5

Calculations Calculations still underway B3LYP/ 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

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 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(MeNH 2 ) 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. Li 2, Li 3 Alkaline earth and transition metals such as Cu Molecular solutes e.g. acids such as HCl or HNO 3, 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(NH 3 ) 1 Peaks in C-H stretching region visible Additional