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Infrared spectroscopy of Li(methylamine)n(NH3)m clusters
Nitika Bhalla, Luigi Varriale, Nicola Tonge
and Andrew Ellis
Department of ChemistryUniversity of Leicester
UK
RI04
Gas phase clusters
Solute, M = Solvent, S =
MS MS4 MS8 MS17
Evolution towards bulk solution properties
1. Motivation
2. Experimental
3. Vibrational photodepletion spectroscopy of Li(Ma)n(NH3)m clusters where n + m = 4
4. Li(Ma)(NH3) – non-resonant ionization-detected IR spectroscopy
5. Conclusion
Content
• Alkali metals dissolve in liquid ammonia to produce a blue coloured solution attributed to solvated electron formation
• Contribute to the study of alkali solvation by targeting finite-sized clusters as useful model systems.
• Our aim is to explore these issues by recording spectra of alkali-ammonia clusters
• 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-
• Previously explored Li(NH3)n clusters – the first solvation shell
was shown to be full at n = 4
• What happens for chemically similar but bulkier ligands e.g.
CH3NH2 (methylamine = Ma)?
• Explore the N-H stretching region of various Li(Ma)n(NH3)m
clusters to determine the impact of substituent on the cluster
structure for n + m = 4
Motivation
Spectroscopic mechanism - depletion
nN-H = 0
nN-H = 1
M-N dissociation
limit
Ground state population depletion by resonant IR absorption
Predissociation
nN-H = 0
nN-H = 1M(NH3)n
M+(NH3)n
Assume rapid vibrational predissociation at energies above the metal-
ammonia bond dissociation limit
Mass-selective detection of IR spectrum of M(NH3)n through IR-induced
depletion of M+(NH3)n signal
hUV
Experimental setup
IR beamOPO/A
Solventgas
UV beamphotoionisation
Metalablation
TOF-mass spectrometer
No depletion for n = 1-3; binding too strong
3 + 1 isomer 4 + 0 isomer
Li(NH3)4 isomers
Salter et al. J. Chem. Phys. 125, 034302 (2006))
Li(NH3)4 in mid IR 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
Li(Ma)4NH
3Li(Ma)
4
Li(Ma)3NH
3
Li(Ma)2(NH
3)2
Li(Ma)(NH3)3
Li(Ma)2
Li(NH3)3
LiMaNH3
LiMa
LiNH3
10 2015 25
Li(Ma)n(NH3)m mass spectrum
TOF/μs
30
3+1 isomer (Ma in second shell) 3+1 isomer (NH3 in second shell)
Structures of Li(Ma)(NH3)3
4+0 isomer (0 eV)
0.30 eV 0.33 eV
Vibrational spectrum of Li(Ma)(NH3)3
3100 3200 3300 3400
LiMa(NH3)3 (4+0)
LiMa(NH3)3 (3+1, NH
3 in 2nd shell)
Li(NH3)3Ma (3+1, Ma in 2nd shell)
Experimental
Wavenumber/cm-1
Vibrational spectrum of Li(Ma)(NH3)3
3100 3200 3300 3400
LiMa(NH3)3 (4+0)
Li(NH3)3Ma (3+1, Ma in 2nd shell)
Experimental
Wavenumber/cm-1
• We do not seem to be able to account for the IR spectrum
using the 4+0 isomer only
• With addition of the two types of 3+1 isomers we also struggle
to account for the experimental spectrum.
• The best agreement with experiment comes when we add a
contribution from the 3+1 isomer with Ma only in the 2nd shell
• Why should there be almost no contribution from a 3+1 isomer
with Ma in the inner solvation shell? Is this a steric effect which
somehow favours Ma in the 2nd shell in preference to NH3?
Vibrational spectrum of Li(Ma)(NH3)3
3000 3250 3500
Experimental
Li(Ma)3NH
3 (NH
3 in 2nd shell 3+1)
Li(Ma)3NH
3 (4+0))
Wavenumber/cm-1
Vibrational spectrum of Li(Ma)3(NH3)
• Preliminary investigation of mixed Li(Ma)(NH3)n clusters –
several others seen (not shown here)
• Full assignments not yet available – more ab initio calculations
required, including (potentially) ab initio molecular dynamics
• Initial indication is that despite its additional bulk, four solvent
molecules can fit into the first solvation shell even if NH3 is
replaced with a bulky Ma molecule
Conclusions for Li(MA)n(NH3)m (n + m = 4) clusters
• For clusters n < 4 photodepletion is not feasible because Li-N
binding energies exceed the energy of IR photon
• However to observe Li(Ma)(NH3) → non-resonant ionisation
detected spectroscopy
• In NID-IR the UV (λ) is below the ionisation threshold such that
when an IR photon is added the system is taken to ionisation
limit
• Enhancement of ion intensity is possible even when hνUV >AIE
Detection of Li(Ma)(NH3) using NID-IR
Vibrational spectrum of Li(Ma)(NH3)
IR + UV (NID-IR)
2800 3000 3200 3400 3600
Wavenumber/cm-1
N-H stretch in NH3
N-H stretch in Ma
Acknowledgments
Dr Corey Evans Funding/facilities
EPSRC
EPSRC National Computational Chemistry Service
UK resource centre for women in science