Transcript IR photodepletion and REMPI spectroscopy of Li(NH2Me) n clusters.ppt

IR photodepletion and REMPI spectroscopy
of Li(NH2Me)n clusters
Tom Salter, Victor Mikhailov, Corey Evans and Andrew Ellis
Department of Chemistry
International Symposium on Molecular Spectroscopy
22nd June 2006
Content
• Background
• Experimental
• Results
– Li(MeNH2)n Experimental and theoretical
• Conclusions
• Future work
Background
• Very little experimental or theoretical work
undertaken on metal-methylamine clusters
– All solution phase ESR and neutron diffraction
• Will provide more information on the nature of
the solvated electron
– Extrapolation back to bulk solution
• May expect to see stretches in C-H region along
with N-H
• Increased steric bulk may affect onset of the
closure of the first solvation shell
• Current results are only preliminary
Spectroscopic mechanism
• Excitation of N-H stretching region
with tuneable IR radiation
• Fixed wavelength UV laser, set just
above IP, used to ionise clusters
• 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)
Predissociation
nN-H=1
Li-N
Dissociation
limit
nN-H=0
Depletion
Experimental
Li(MeNH2)n
• Mass spectrum
0
1
2
4
3
5
6
7
8
n
•
Up to 15-fold
increase in ion
production
observed for n =
1, 2 and 3
•
Depletion seen
for larger
clusters
(a)
(b)
0
40
80
120
160
Mass/Daltons
200
240
280
• (a) IR OFF
•
(b) IR ON
Li(MeNH2)n
• IR depletion spectra for n = 4 and 5
Delpetion/Arb. Units
n=2
n=3
n=4
n=5
3000
3100
3200
Wavenumber/cm
3300
-1
3400
•Only recorded so far
using a fixed and
short UV wavelength,
248 nm
•See depletion of
large clusters and
corresponding
production of small
clusters
•Under tighter IR
focal conditions,
production also seen
in n = 1 channel
•Possibility of
fragmentation
Calculations
•
Calculations still
underway
•
B3LYP/
•
•
•
n = 4 Experimental
6-311++G(d,p)
n=1
More conformational
isomers possible so
systematic
searching is more
involved
n=2
Lowest energy
predicted spectra for
n = 1-4 show very
similar trends
making assignment
problematic
Difficult to be sure
where ion
production for small
clusters is
originating from
n=3
n=4
2800
2900
3000
3100
3200
Wavenumber/cm
3300
-1
3400
3500
Calculations
•
Possible that several low energy
isomers are adopted resulting in
spectral broadening
• Dissociation energies
– 3 isomers predicted for n = 2
– 12 isomers predicted for n = 3
– >20 isomer predicted for n = 4
n a)
DFT
1
2
3
4
4596
4019
4160
1370
– Due to increased steric bulk from
methyl group
•
Calculations indicate closure of the
first solvation shell with 3
methylamine molecules
– In contradiction to neutron diffraction
data, which predicts closure of the
first solvation shell with 4 molecules
•
Thus IR absorption for n  4
could induce loss of an ammonia
molecule
a) Lowest energy conformer in each case
Improvements
• Contributions from larger
clusters may present a serious
problem for identification
• Possible solution would be
excitation downstream from
ionisation
Time-of-Flight
extraction region
Excitation and ionisation in the same
region. Fragments detected
– Small clusters will miss MCP
• Record spectra at wavelengths
just above a cluster IP
– Aim to minimise fragmentation
Excitation spatially separated from
ionisation. Fragments not detected
Conclusions
•
Preliminary spectroscopic data obtained
•
Li(MeNH2)n clusters for n = 4 and 5 show IR photodepletion spectra
•
This depletion is mirrored by ion production for n = 1, 2 and 3
•
Calculations provide the first indication that the first solvation shell is
closed with 3 solvent molecules
– Plausible due to increased steric hindrance from methyl group
•
Assignment problematic as spectra for n = 1-4 are very similar
•
Dissociation energies consistent with depletion from n ≥ 4
•
Further work is required on these clusters, such as experimental
determination of IP
– Confirm extrapolation to the bulk phase
– Identify trends confirming closure of the first solvation sphere
Future Work
• Have a general methodology for recording mass-selected IR
spectra of solute-solvent clusters
• Can explore other metal solutes, e.g.
• Other alkali metals
• Alkali metal clusters, e.g. Li2, Li3
• Alkaline earth and transition metals such as Cu
• Molecular solutes
• e.g. acids such as HCl or HNO3, salts, etc.
• Different solvents
• e.g. water, alcohols, methanol, acetonitrile, etc.
Acknowledgements
• Funding – principally EPSRC
• University of Leicester Centre for Mathematical
Modelling
• Mechanical and electronic workshops
Additional
• IR production
spectrum for
Li(NH3)1
• Peaks in C-H
stretching
region
visible
2700
2800
2900
3000
3100
3200
-1
Wavenumber/cm
3300
3400
3500