IR photodepletion spectroscopy of Li(NH3)n
clusters for n = 4-7
Tom Salter, Victor Mikhailov, Corey Evans and Andrew Ellis
Department of Chemistry
International Symposium on Molecular Spectroscopy
22nd June 2006
Content
• Background
• Experimental
• Results
– Experimental Li(NH3)n (n = 4 - 7)
– Theoretical Li(NH3)n (n = 4 and 5)
• Conclusions
Background
• Alkali metals dissolved in ammonia produce a blue coloured
solution attributed to solvated electron formation
• In small alkali-ammonia clusters the outer valence electron of the
alkali atom may not fully detach? How is this process affected by
the size of the cluster? Is the inner solvation shell in gas phase
clusters full after four ammonia molecules have been added as in
the liquid phase?
• Our aim is to explore these issues by recording spectra of alkaliammonia clusters
Ion versus neutral clusters
• Much previous and exciting spectroscopic work on
charged solute-solvent complexes, e.g. work of Farrar,
Duncan, Lisy, Johnson, and many others
• Our effort is focused on uncharged clusters, and in
particular an attempt to record size-selective midinfrared spectra of metal-solvent clusters.
Previous gas-phase work
• Prior experimental work on alkaliammonia clusters has focused
mainly on photoionization threshold
measurements
Ref. 1
– Studies of M(NH3)n show a rapid
decrease in IP with n for n  4
– Suggests closing of 1st solvation shell at
n=4
– Independent of identity of alkali metal
1
K. Fuke et al., J. Phys. Chem. A 1997, 101, 3078-3087
=Li, =Na, =Cs
Experimental
Spectroscopic mechanism
• Excitation of N-H stretching region with
tuneable IR radiation
• Fixed wavelength UV laser, set just
above IP, used to ionise clusters
Predissociation
nN-H=1
• Resonance results in the loss of a
solvent molecule leading to ion
depletion
• Requires the solvent binding energy to
be less than that of the IR photon and
for predissociation to be fast enough
(ns or less)
Li-N
Dissociation
limit
nN-H=0
Depletion
Li(NH3)n
• Mass spectrum
0
0
UV wavelength = 409. 5 nm
4
1
20
40
60
5
80
m/z
6
100
7
120
x
140
(a)
•
(a) IR OFF
(b)
• (b) IR ON
Li(NH3)n
• IR depletion spectra for n = 4 -7
n=4
Depletion/Arb. Units
n=5
n=6
n=7
3050
3100
3150
3200
3250
Wavenumber/cm
-1
3300
3350
•N-H stretch, red
shifted from free
ammonia by
~200 cm-1
•Interaction with
Li weakens the
N-H bonds
Ab initio calculations on Li(NH3)4
•
Relative energies of conformers DFT/MP2, 6-311++G(d,p)
4+0
0
0
eV
3+1
0.28
0.34
eV
2+2
0.99
1.13
eV
1+3
1.18
2.00
eV
Ab initio calculations on Li(NH3)4
• Experimental and DFT spectra
Experimental
4+0
3+1
3050
3100
3150
3200
Wavenumber/cm
3250
-1
3300
3350
Accounting for the Li(NH3)4 spectrum
• Vibrational structure is NOT consistent with one or more ammonia
molecules in a second solvation shell.
• Our current ab initio predictions suggest a single-shell tetrahedral
structure for the ground state of Li(NH3)4 – this too is inconsistent
with the spectra.
• Explanation may be
– (i) That several low energy conformers based broadly on a singleshell (4+0) structure may contribute and/or
– (ii) The ground state is not fully tetrahedral, leading to loss of
vibrational degeneracies. Interestingly, in recent neutron diffraction
studies of concentrated Li:NH3 solutions, a distorted tetrahedral
arrangement of four ammonia molecules in the inner solvation shell is
required to fit the data.
Ab initio calculations on Li(NH3)5
•
Relative energies of conformers DFT/MP2, 6-311++G(d,p)
4+1
0
0
eV
3+2
0.25
0.30
eV
3+2
0.28
3+2
0.29
eV
033
eV
Ab initio calculations on Li(NH3)5
•Additional bands
for n = 5, 6 and 7
at higher
frequencies, i.e.
smaller red-shift
from free
ammonia
Experimental
4+1
•Consistent with
ammonia
molecules in a
second solvation
shell
3+2
3100
3200
Wavenumber/cm
3300
-1
Calculated dissociation energies
n a)
DFT
MP2
1
2
3
4
5
4888
4657
4858
3347
1029
3973
3748
4314
3380
837
• Thus IR absorption for n  4 could induce loss of an ammonia
molecule
a)
Lowest energy conformer in each case
Conclusions
• See depletion for clusters with n  4 - determined by the
dissociation energy for loss of one ammonia
• No depletion seen for smaller clusters – dissociation energy too
large
• IR depletion spectrum and ab initio predictions for Li(NH3)4
support a single solvation shell structure (probably distorted
tetrahedral), in agreement with condensed phase data
• Additional blue-shifted bands in spectra for n  5 are explained by
one or more hydrogen bonded ammonia molecules held in second
solvation shell.
– Demonstrates that the inner solvation shell is full at n = 4 in the gas
phase.
Acknowledgements
• Funding – principally EPSRC
• EPSRC National Computational Chemistry Service
• Mechanical and electronic workshops
Additional
• Smaller Li(NH3)n clusters
n=1
n=2
n=3
n=4
3050
3100
3150
3200
Wavenumber/cm
3250
-1
3300
3350