Presentation on theme: "Infrared spectroscopy of metal ion-water complexes"— Presentation transcript:
1Infrared spectroscopy of metal ion-water complexes Biswajit Bandyopadhyay, Prosser D. Carnegie and Michael A. DuncanDepartment of Chemistry, University of Georgia, Athens, GA, 30602U. S. Department of Energy
2IntroductionInteraction 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 coworkersalkali earth and main group by Inokuchi, Misaizu and coworkersTransition metals and alkaline earth metal ions by Williams and coworkersTransition metal ions by Duncan and coworkers.
4Argon “tagging” Ar elimination IR Photon M+(H2O) bond energies are ~ kcal/mol ( cm-1)Infrared photon energy ~ cm-1For the M+(H2O)n clusters, water molecules in the second solvent shell have lower binding energies and can be eliminated by a single photonM+-Ar bonds are weaker and argon falls off when the O-H stretches are excited.
5Red shifts in O-H stretches IR spectra of cation-water systemsRed shifts in O-H stretchesM+(H2O) B.E. vs. red shiftsCombination band1The HOMO of water has partial bonding character.Polarization of the electron due to metal cation removes the electrondensity from the O-H bond –accounts for red shiftRed shifts depend on the extent of polarizationof water molecule by the metal cation. Closed shellcations or metal ions with fewer d-electrons polarizewater the most – more red shift1 P. D. Carnegie, A. B. McCoy, M. A. DuncanJ. Phys. Chem. A 113, 4849 (2009).
6IR spectra of cation-water systems Intensity pattern switch The intensity ratio of symmetric andasymmetric stretch is 1: 18 for free waterIn a metal ion –water complex this ratio is ~1:1Asymmetric stretch-perpendiculartype vibration- less change indynamical dipole moment than thesymmetric stretchSymmetric stretch-parallel typevibration- Involves greater change indynamical dipole moment-gainsgreater intensity
7Partially resolved rotational structures Sc+(H2O)ArLi+(H2O)ArA" = 13.4 cm-1B", C" = 0.07, 0.07 cm-1A' = 14.3 cm-1B', C' = 0.07, 0.07 cm-1B. O.sym = cm-1B. O.asym = 3692 cm-1TJ,K = 15, 40KA'' = 13.7 cm-1B'', C'' = cm-1A' = 13.4 cm-1B', C' = cm-1B.O.sym = 3580 cm-1B.O.asym = 3656 cm-1T = 50 KC2Most of the M+(H2O)Ar complexes have C2v symmetryAr 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 axisRotational constants are close to cm-1From the partially resolved sub-bands H-O-H bond angle can be calculated, assuming thatthe O-H bond length does not change.
8IR spectra of Mn+(H2O)Arn complexes Different binding sites of argon atoms produce isomers
9IR spectra of Zn+(H2O)nAr complexes Appearance of 3425 cm-1 peakshows that one of the O-H bondsis interacting with the argon –Coordination number 4.Zn+(H2O)2Ar and Zn+(H2O)3ArHave similar looking spectraArgon is off the C2 axiss-orbital of the metal ion isback polarized by water. Argondoes not want to attach opposite to water.
10Slightly different spectral pattern due to reaction product? IR spectrum of Ti+(H2O)Ar complexSlightly different spectral pattern due to reaction product?A″, A′=9.0, 11.8 cm-1B.O =3661 cm-1T J, K = 10, 20 K.H-Ti2+-OH-?A″, A′=17.5, 15.0 cm-1B.O =3664 cm-1
11IR spectrum of V+(H2O)Ar complex V+(H2O)NeNb+(H2O)ArNb+(H2O)Ne
12IR spectra of U+(H2O) and Au +(H2O) complexes U+(H2O)Ar2Au+(H2O)Ar2
13Conclusions Acknowledgements Red shifts in O-H stretching frequencies Intensity pattern switch for O-H sym. and asym. stretchesPartially resolved rotational structuresMultiple argons produce isomersSpectra with multiple waters provide information about coordination numberInsertion product complicates spectra for early transition metalsArgon tends to go to hydrogen of water molecule in case of Au+- and U+- water complexesAcknowledgementsProf. Mike Heaven (Emory University) for letting us borrow a uranium rodU. S. Department of Energy for funding