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