Presentation on theme: "Infrared spectroscopy of metal ion-water complexes Biswajit Bandyopadhyay, Prosser D. Carnegie and Michael A. Duncan Department of Chemistry, University."— Presentation transcript:
Infrared spectroscopy of metal ion-water complexes Biswajit Bandyopadhyay, Prosser D. Carnegie and Michael A. Duncan Department of Chemistry, University of Georgia, Athens, GA, U. S. Department of Energy
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.
Argon tagging M + (H 2 O) bond energies are ~ kcal/mol ( cm -1 ) Infrared photon energy ~ cm -1 For the M + (H 2 O) 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. IR Photon Ar elimination
Red shifts in O-H stretches 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 Combination band 1 IR spectra of cation-water systems M + (H 2 O) B.E. vs. red shifts 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).
The intensity ratio of symmetric and asymmetric stretch is 1: 18 for free water 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 IR spectra of cation-water systems Intensity pattern switch In a metal ion – water complex this ratio is ~1:1
Partially resolved rotational structures From the partially resolved sub-bands H-O-H bond angle can be calculated, assuming that the O-H bond length does not change. C2C2 Most of the M + (H 2 O)Ar complexes have C 2v symmetry Ar binds to the M + along the C 2 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 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 = 3629 cm -1 B. O.asym = 3692 cm -1 T J,K = 15, 40K Li + (H 2 O)Ar 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 Sc + (H 2 O)Ar
IR spectra of Mn + (H 2 O)Ar n complexes Different binding sites of argon atoms produce isomers
IR spectra of Zn + (H 2 O) n Ar complexes Argon is off the C 2 axis -s-orbital of the metal ion is back polarized by water. Argon does not want to attach opposite to water. Appearance of 3425 cm -1 peak shows that one of the O-H bonds is interacting with the argon – Coordination number 4. Zn + (H 2 O) 2 Ar and Zn + (H 2 O) 3 Ar Have similar looking spectra
Slightly different spectral pattern due to reaction product? A, A=17.5, 15.0 cm -1 A, A=9.0, 11.8 cm -1 B.O =3664 cm -1 B.O =3661 cm -1 T J, K = 10, 20 K. H-Ti 2+ -OH - IR spectrum of Ti + (H 2 O)Ar complex ?
V + (H 2 O)Ar V + (H 2 O)Ne IR spectrum of V + (H 2 O)Ar complex Nb + (H 2 O)Ar Nb + (H 2 O)Ne
U + (H 2 O)Ar 2 Au + (H 2 O)Ar 2 IR spectra of U + (H 2 O) and Au + (H 2 O) complexes
Conclusions 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