Infrared spectroscopy of Li(methylamine) n (NH 3 ) m clusters Nitika Bhalla, Luigi Varriale, Nicola Tonge and Andrew Ellis Department of Chemistry University of Leicester UK RI04

Gas phase clusters Solute, M =Solvent, S = MS MS 4 MS 8 MS 17 Evolution towards bulk solution properties

1.Motivation 2.Experimental 3.Vibrational photodepletion spectroscopy of Li(Ma) n (NH 3 ) m clusters where n + m = 4 4.Li(Ma)(NH 3 ) – non-resonant ionization-detected IR spectroscopy 5.Conclusion Content

Alkali metals dissolve in liquid ammonia to produce a blue coloured solution attributed to solvated electron formation Contribute to the study of alkali solvation by targeting finite-sized clusters as useful model systems. Our aim is to explore these issues by recording spectra of alkali-ammonia clusters Evolution of the unpaired electron from metal-bound to fully solvated Background M+M+ M+M+ e - (solvent) Dilute solution → strong blue colourConc. solution → strong bronze colour e-e-

Previously explored Li(NH 3 ) n clusters – the first solvation shell was shown to be full at n = 4 What happens for chemically similar but bulkier ligands e.g. CH 3 NH 2 (methylamine = Ma)? Explore the N-H stretching region of various Li(Ma) n (NH 3 ) m clusters to determine the impact of substituent on the cluster structure for n + m = 4 Motivation

Spectroscopic mechanism - depletion N-H = 0 N-H = 1 M-N dissociation limit Ground state population depletion by resonant IR absorption Predissociation N-H = 0 N-H  =  M(NH 3 ) n M + (NH 3 ) n  Assume rapid vibrational predissociation at energies above the metal- ammonia bond dissociation limit  Mass-selective detection of IR spectrum of M(NH 3 ) n through IR-induced depletion of M + (NH 3 ) n signal h UV

Experimental setup IR beam OPO/A Solvent gas UV beam photoionisation Metal ablation TOF-mass spectrometer

No depletion for n = 1-3; binding too strong isomer4 + 0 isomer Li(NH 3 ) 4 isomers

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

Li(Ma) n (NH 3 ) m mass spectrum TOF/μs 30

3+1 isomer (Ma in second shell)3+1 isomer (NH 3 in second shell) Structures of Li(Ma)(NH 3 ) isomer (0 eV) 0.30 eV0.33 eV

Vibrational spectrum of Li(Ma)(NH 3 ) 3

We do not seem to be able to account for the IR spectrum using the 4+0 isomer only With addition of the two types of 3+1 isomers we also struggle to account for the experimental spectrum. The best agreement with experiment comes when we add a contribution from the 3+1 isomer with Ma only in the 2 nd shell Why should there be almost no contribution from a 3+1 isomer with Ma in the inner solvation shell? Is this a steric effect which somehow favours Ma in the 2 nd shell in preference to NH 3 ? Vibrational spectrum of Li(Ma)(NH 3 ) 3

Vibrational spectrum of Li(Ma) 3 (NH 3 )

Preliminary investigation of mixed Li(Ma)(NH 3 ) n clusters – several others seen (not shown here) Full assignments not yet available – more ab initio calculations required, including (potentially) ab initio molecular dynamics Initial indication is that despite its additional bulk, four solvent molecules can fit into the first solvation shell even if NH 3 is replaced with a bulky Ma molecule Conclusions for Li(MA) n (NH 3 ) m (n + m = 4) clusters

For clusters n < 4 photodepletion is not feasible because Li-N binding energies exceed the energy of IR photon However to observe Li(Ma)(NH 3 ) → non-resonant ionisation detected spectroscopy In NID-IR the UV (λ) is below the ionisation threshold such that when an IR photon is added the system is taken to ionisation limit Enhancement of ion intensity is possible even when hν UV >AIE Detection of Li(Ma)(NH 3 ) using NID-IR

Vibrational spectrum of Li(Ma)(NH 3 ) IR + UV (NID-IR) N-H stretch in NH 3 N-H stretch in Ma

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