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Infrared spectroscopy of metal ion-water complexes

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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

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