electronic spectroscopy of li(nh 3 ) 4 nitika bhalla, luigi varriale, nicola tonge and andrew ellis...
TRANSCRIPT
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Electronic spectroscopy of Li(NH3)4
Nitika Bhalla, Luigi Varriale, Nicola Tonge
and Andrew Ellis
Department of ChemistryUniversity of Leicester
UK
WI04
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1. Motivation
2. Experimental
3. Li(NH3)4 spectroscopic results
4. Link to solvated electron
5. Conclusions
Content
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Solute, M = Solvent, S =
MS4 MS8 MS17
Evolution towards bulk solution properties
Solvent-solute clusters in the gas phase
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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-
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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))
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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
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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
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Experimental setup
IR beamOPO/A
Solventgas
UV beamPhotoionisation
Metalablation
TOF-Mass spectrometer
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m/z
1
2 3
4
Photoionization mass spectra of Li(NH3)n
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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
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2T2 2A1 transition
Electronic spectrum of Li(NH3)4
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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)
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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)
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• 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
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• 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
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• 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
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Acknowledgments
Dr Corey Evans Funding/facilities
EPSRC
EPSRC National Computational Chemistry Service
UK resource centre for women in science