1. Infrared spectroscopy of metal ion-water complexes
Biswajit Bandyopadhyay, Prosser D. Carnegie and Michael A. Duncan
Department of Chemistry, University of Georgia, Athens, GA, 30602
U. S. Department of Energy

2. Introduction
Interaction of water with metal ions is fundamental to understand the chemistry of solvation.
A molecular level understanding is obtained by studying these complexes in the gas phase.
Collision induced dissociation to measure the metal-water binding energies by Armentrout and coworkers.
Electronic spectroscopy of cation- water systems performed by the Brucat, Metz and the Duncan group.
ZEKE spectroscopy by the Blake group and the Duncan group.
Infrared Photodissociation Spectroscopy (IRPD) :
alkali metal cation-water complexes by Lisy and coworkers
alkali earth and main group by Inokuchi, Misaizu and coworkers
Transition metals and alkaline earth metal ions by Williams and coworkers
Transition metal ions by Duncan and coworkers.

3. Experimental

4. Argon “tagging” Ar elimination IR Photon
M+(H2O) bond energies are ~ kcal/mol ( cm-1)
Infrared photon energy ~ cm-1
For the M+(H2O)n clusters, water molecules in the second solvent shell have lower binding energies and can be eliminated by a single photon
M+-Ar bonds are weaker and argon falls off when the O-H stretches are excited.

5. Red shifts in O-H stretches
IR spectra of cation-water systems
Red shifts in O-H stretches
M+(H2O) B.E. vs. red shifts
Combination band1
The HOMO of water has partial bonding character.
Polarization of the electron due to metal cation removes the electron
density from the O-H bond –accounts for red shift
Red shifts depend on the extent of polarization
of water molecule by the metal cation. Closed shell
cations or metal ions with fewer d-electrons polarize
water the most – more red shift
1 P. D. Carnegie, A. B. McCoy, M. A. Duncan
J. Phys. Chem. A 113, 4849 (2009).

6. IR spectra of cation-water systems Intensity pattern switch
The intensity ratio of symmetric and
asymmetric stretch is 1: 18 for free water
In a metal ion –water complex this ratio is ~1:1
Asymmetric stretch-perpendicular
type vibration- less change in
dynamical dipole moment than the
symmetric stretch
Symmetric stretch-parallel type
vibration- Involves greater change in
dynamical dipole moment-gains
greater intensity

7. Partially resolved rotational structures
Sc+(H2O)Ar
Li+(H2O)Ar
A" = 13.4 cm-1
B", C" = 0.07, 0.07 cm-1
A' = 14.3 cm-1
B', C' = 0.07, 0.07 cm-1
B. O.sym = cm-1
B. O.asym = 3692 cm-1
TJ,K = 15, 40K
A'' = 13.7 cm-1
B'', C'' = cm-1
A' = 13.4 cm-1
B', C' = cm-1
B.O.sym = 3580 cm-1
B.O.asym = 3656 cm-1
T = 50 K C2
Most of the M+(H2O)Ar complexes have C2v symmetry
Ar binds to the M+ along the C2 axis. Only light H-atoms are off the axis and contributes to the moment-of-inertia along that axis
Rotational constants are close to cm-1
From the partially resolved sub-bands H-O-H bond angle can be calculated, assuming that
the O-H bond length does not change.

8. IR spectra of Mn+(H2O)Arn complexes
Different binding sites of argon atoms produce isomers

9. IR spectra of Zn+(H2O)nAr complexes
Appearance of 3425 cm-1 peak
shows that one of the O-H bonds
is interacting with the argon –
Coordination number 4.
Zn+(H2O)2Ar and Zn+(H2O)3Ar
Have similar looking spectra
Argon is off the C2 axis
s-orbital of the metal ion is
back polarized by water. Argon
does not want to attach opposite to water.

10. Slightly different spectral pattern due to reaction product?
IR spectrum of Ti+(H2O)Ar complex
Slightly different spectral pattern due to reaction product?
A″, A′=9.0, 11.8 cm-1
B.O =3661 cm-1
T J, K = 10, 20 K.
H-Ti2+-OH- ?
A″, A′=17.5, 15.0 cm-1
B.O =3664 cm-1

11. IR spectrum of V+(H2O)Ar complex
V+(H2O)Ne
Nb+(H2O)Ar
Nb+(H2O)Ne

12. IR spectra of U+(H2O) and Au +(H2O) complexes
U+(H2O)Ar2
Au+(H2O)Ar2

13. Conclusions Acknowledgements Red shifts in O-H stretching frequencies
Intensity pattern switch for O-H sym. and asym. stretches
Partially resolved rotational structures
Multiple argons produce isomers
Spectra with multiple waters provide information about coordination number
Insertion product complicates spectra for early transition metals
Argon tends to go to hydrogen of water molecule in case of Au+- and U+- water complexes
Acknowledgements
Prof. Mike Heaven (Emory University) for letting us borrow a uranium rod
U. S. Department of Energy for funding