infrared spectroscopy of li(methylamine) n (nh 3 ) m clusters nitika bhalla, luigi varriale, nicola...
TRANSCRIPT
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