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