Electronic spectroscopy of Li(NH3)4

Nitika Bhalla, Luigi Varriale, Nicola Tonge

and Andrew Ellis

                                  

Department of ChemistryUniversity of Leicester

UK

WI04

1. Motivation

2. Experimental

3. Li(NH3)4 spectroscopic results

4. Link to solvated electron

5. Conclusions

Content

Solute, M = Solvent, S =

MS4 MS8 MS17

Evolution towards bulk solution properties

Solvent-solute clusters in the gas phase

Alkali metals dissolve in liquid NH3 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+e- (solvent)

Dilute solution → strong blue colour Conc. solution → strong bronze colour

e-

Likely constituents of a Li/NH3 solution (guided by DFT calculations)

(NH3)n Li(NH3)4

+ Li(NH3)4

e-@(NH3)n

Li(NH3)4+

•e-@ (NH3)n [Li(NH3)4

+• e-@(NH3)n ]r

Li(NH3)4•e-@ (NH3)n

2e-@(NH3)n

[Li(NH3)4 ]r

Li(NH3)4-

0 4 8 12 16 20

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(NH3)4 cluster will nearly

coincide with that of the solvated electron in liquid ammonia

TD- DFT prediction of the electronic spectrum of Li(NH3)4

Ground state

Excited state

M-N dissociation

limit

Ground state population depletion by resonant laser absorption

Predissociation

M(NH3)n

M+(NH3)n

Assume rapid predissociation at energies above the metal-ammonia bond

dissociation limit

Mass-selective detection of IR spectrum of M(NH3)n through laser-induced

depletion of M+(NH3)n signal

hUV

Spectroscopic mechanism

Experimental setup

IR beamOPO/A

Solventgas

UV beamPhotoionisation

Metalablation

TOF-Mass spectrometer

m/z

1

2 3

4

Photoionization mass spectra of Li(NH3)n

Salter et al. J. Chem. Phys. 125, 034302 (2006))

Li(NH3)4 in mid-infrared excitation

Experimental

3050 3100 3150 3200 3250 3300 3350

3+1

Wavenumber/cm -1

4+0

24 Antisymm stretch

Single solvation shell

n = 4

Li(NH3)4

2T2 2A1 transition

Electronic spectrum of Li(NH3)4

3, LiN4 deformation (e)

5, Li-N stretch (a1)

6000 6200 6400

335132513151

5251

31 32 33 34 35 36

00

Inten

sity /

Arbit

ary U

nits

Wavenumber / cm-1

(Jahn-Teller active)

Expanded view of electronic spectrum of Li(NH3)4

L. Varriale, N. M. Tonge, N. Bhalla, A. M. Ellis, J. Chem. Phys. 132, 161101 (2010)

Predicted and measured vibrational wavenumbers of Li(NH3)4

Li(NH3)4 Li(ND3)4

Mode Symmetry Theory a) Expt b) Theory a) Expt b)

1 a2 51 35

2 t1 66 47

3 e 68 74 59 65

4 t2 76 65

5 a1 231 186 212 (149) c)

6 t1 311 231

7 t2 322 260

8 e 402 304

9 t2 494 472

10 t2 1165 886

11 a1 1172 1242 890 1026 c)

• First electronic spectra recorded for Li(NH3)4

• The broad Li(NH3)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(NH3)n clusters, i.e. both

larger and smaller (as seen in talk TG08 – Electronic

spectra of LiNH3)

• Explore other metals, including other alkalis, alkaline

earths and rare earths, along with other solvents

(see talk RI04 – Infrared spectroscopy of

Li(methylamine)n(NH3)m clusters)

Future work

Acknowledgments

Dr Corey Evans Funding/facilities

EPSRC

EPSRC National Computational Chemistry Service

UK resource centre for women in science