Infrared Spectroscopy of Doubly-Charged Metal-Water Complexes Biswajit Bandyopadhyay, Prosser D. Carnegie and Michael A. Duncan Department of Chemistry, University of Georgia, Athens, GA, 30602 www.arches.uga.edu/~maduncan/ U. S. Department of Energy

Motivation Understanding solvation at the molecular level Metal-cation solvation plays key role in many chemical and biological processes Most work has been performed on singly charged metal-water complexes J. Phys. Chem. 2008, 112, 6237

Previous Work First studies focused on bulk phase measurements of multiply charged metal complexes Difficulty in producing stable species in the gas phase Generated ions by electrospray and analyzed with mass spectrometric methods (Kebarle, Posey, Williams, Metz, Schwarz, etc.) Electronic Spectroscopy performed by Metz and coworkers, Stace and coworkers Infrared spectroscopy by Williams and coworkers, Duncan and coworkers

Experimental

Mass spectrum of the ions produced in the laser vaporization cluster source Density of doubly charged ions is much less than singly charged ions. This is also seen with other sources.

Metal Ion-Water Complexes: M2+(H2O) M IP(eV) 2nd IP(eV) Cr 6.70 16.50 Mn 7.17 15.10 Sc 6.30 12.35 V 6.50 14.13 IP (H2O) 12.6 eV Asymptotically Stable Asymptotically Unstable Asymptotically unstable doubly charged metal-water complexes are produced by carefully adjusting the vaporization laser power, laser timing with respect to the gas pulse, alignment of the laser to the metal rod and gas pulse width N. R. Walker, G. A. Grieves, J. B. Jaeger, R. S. Walters and M. A. Duncan. Int. J. Mass. Spec. 2003, 228, 285

Binding Energy (D0; kcal/mol) vs Photon Energy Mn+-H2O Mn+-Ar Cr+ 30.9 (10,807 cm-1)a 6.7 (2338 cm-1) c Cr2+ 84.3 (29,510 cm-1)b 37.3 (13,050 cm-1) d H2O O-H sym 3657 cm-1 O-H asym 3756 cm-1 vibrations H-O-H bend 1595 cm-1 IR Photon loss of argon Ar M+ a. Armentrout and coworkers, JACS 1994, 116, 3525 c. Brucat and coworkers, Chem. Phys. Lett. 1991, 177, 380. b. Bock and coworkers, Inorg. Chem. 1998, 372, 4425

Red Shift 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 –Red shift Intensity Pattern The intensity ratio of symmetric and asymmetric stretches 1: 18 for water molecule. Combination band Asymmetric Stretch 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 Symmetric Stretch C2

Comparison of red shifts and intensity pattern of O-H stretches of the M+(H2O) and M2+(H2O) complexes Red shift is larger for metal dication- water complex as polarization is more Symmetric Stretch Asymmetric Stretch C2 Intensity ratio of sym. and asym. stretches is ~2:1 for the metal dication-water complex 9

Partially Resolved Rotational structure- gives geometry of the complex and temperature.

11

Hydroxide formation Sc3+(OH-)H(Ar)6 12

Infrared Spectra of Doubly Charged Vanadium-Water-Argon complexes V3+(OH-)H(Ar)6

Conclusions Acknowledgement IRPD spectra for singly and doubly charged metal water complexes obtained OH stretching frequencies are red shifted Symmetric stretch gains IR intensity relative to asymmetric stretch Sifts in band positions and IR intensities greater for dications Acknowledgement Michael A. Duncan DOE for funding