Electronic spectroscopy of Li(NH 3 ) 4 Nitika Bhalla, Luigi Varriale, Nicola Tonge and Andrew Ellis Department of Chemistry University of Leicester UK WI04

1.Motivation 2.Experimental 3.Li(NH 3 ) 4 spectroscopic results 4.Link to solvated electron 5.Conclusions Content

Solute, M =Solvent, S = MS 4 MS 8 MS 17 Evolution towards bulk solution properties Solvent-solute clusters in the gas phase

Alkali metals dissolve in liquid NH 3 to produce a coloured solution – attributed to solvated electron formation Contribute to the study of alkali solvation by targeting finite-sized clusters as useful model systems Explore these issues by recording spectra of alkali-ammonia clusters as a function of size To follow the evolution of the unpaired electron from metal-bound to fully solvated Background M+M+ M+M+ e - (solvent) Dilute solution → strong blue colour Conc. solution → strong bronze colour e-e-

Likely constituents of a Li/NH 3 solution (guided by DFT calculations) (NH 3 ) n Li(NH 3 ) 4 + Li(NH 3 ) 4 e 3 ) n Li(NH 3 ) 4 + e (NH 3 ) n [Li(NH 3 ) 4 + e 3 ) n ] r Li(NH 3 ) 4 e (NH 3 ) n 2e 3 ) n [Li(NH 3 ) 4 ] r Li(NH 3 ) Li concentration (mol %) (Adapted from E. Zurek, P. P. Edwards, R. Hoffmann, Angew. Chemie 48, 8198 (2009))

R. Hoffmann et al., Angew. Chemie 48, 8198 (2009) ‘Li’ 2p ← 2s transition The DFT prediction is that the absorption maximum of Li(NH 3 ) 4 cluster will nearly coincide with that of the solvated electron in liquid ammonia TD- DFT prediction of the electronic spectrum of Li(NH 3 ) 4

Ground state Excited state M-N dissociation limit Ground state population depletion by resonant laser absorption Predissociation M(NH 3 ) n M + (NH 3 ) n  Assume rapid predissociation at energies above the metal-ammonia bond dissociation limit  Mass-selective detection of IR spectrum of M(NH 3 ) n through laser-induced depletion of M + (NH 3 ) n signal h UV Spectroscopic mechanism

Experimental setup IR beam OPO/A Solvent gas UV beam Photoionisation Metal ablation TOF-Mass spectrometer

m/z Photoionization mass spectra of Li(NH 3 ) n

Salter et al. J. Chem. Phys. 125, (2006)) Li(NH 3 ) 4 in mid-infrared excitation 2 4 Antisymm stretch Single solvation shell n = 4 Li(NH 3 ) 4

2 T 2  2 A 1 transition Electronic spectrum of Li(NH 3 ) 4

 3, LiN 4 deformation (e)  5, Li-N stretch (a 1 ) (Jahn-Teller active) Expanded view of electronic spectrum of Li(NH 3 ) 4 L. Varriale, N. M. Tonge, N. Bhalla, A. M. Ellis, J. Chem. Phys. 132, (2010)

Predicted and measured vibrational wavenumbers of Li(NH 3 ) 4 Li(NH 3 ) 4 Li(ND 3 ) 4 Mode Symmetry Theory a) Expt b) Theory a) Expt b) 1 a2a t1t e t2t a1a (149) c) 6 t1t t2t e t2t t2t a1a c)

First electronic spectra recorded for Li(NH 3 ) 4 The broad Li(NH 3 ) 4 spectrum overlaps with that of the solvated electron in the near-IR Vibrational structure is observed and can be resolved, with clear evidence for major Jahn-Teller distortion in the first excited electronic state Conclusions

Ab initio calculations on the excited electronic state are required to fully understand the spectrum Even with only four NH3 molecules added to Li, we already have spectroscopic behaviour with strong similarities to the fully solvated electron in liquid ammonia Conclusions

Investigation of other Li(NH 3 ) n clusters, i.e. both larger and smaller (as seen in talk TG08 – Electronic spectra of LiNH 3 ) Explore other metals, including other alkalis, alkaline earths and rare earths, along with other solvents (see talk RI04 – Infrared spectroscopy of Li(methylamine) n (NH 3 ) m clusters) Future work

Acknowledgments Dr Corey Evans Funding/facilities EPSRC EPSRC National Computational Chemistry Service UK resource centre for women in science