INFRARED PHOTODISSOCIATION SPECTROSCOPY OF Cr2+(H2O)Arn AND Cr+(H2O)Ar COMPLEXES P. D. CARNEGIE, B. BANDYOPADHYAY AND M. A. DUNCAN Department of Chemistry, University of Georgia, Athens, GA, 30602 www.arches.uga.edu/~maduncan/ U. S. Department of Energy

Motivation Understand solvation on a molecular scale Metal cation solvation is integral to several chemical and biological processes Most work has been performed on singly charged species Most metals exist in higher oxidation states in the bulk phase

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, Stace, Schwarz, etc.) Electronic Spectroscopy performed by Metz and coworkers Recently, IR spectroscopy by Williams and coworkers on Ca2+(H2O)n and Cu2+(H2O)n

LaserVision OPO/OPA 2000-4400 cm-1 Cold ions produced through laser vaporization/supersonic expansion Cations are mass selected Ion densities are too low for absorption Use IR-REPD

Metal Ion Water Complexes: M2+(H2O) Asymptotically Stable IP(Cr) 6.7 eV 2nd IP(Cr) 16.5 eV IP (H2O) 12.6 eV Asymptotically Unstable Difficulty in producing due to efficient charge transfer Other sources take advantage of bulk solution phase Stace and Schwarz have used techniques to ionize the complex In this source the singly charged complex is ionized

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) 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 OH Stretches

+ Combination Bands 3621 and 3687 are the OH stretching modes Combination band present in most M+(H2O)Ar2 Complex structure possibly from the hindered rotor vibration Combination Bands +

Internal Rotation

Hydroxide Formation Appearance of resonance to the red of fundamentals Bending overtone does not accurately describe the frequency Formation of the hydroxide reaction product 3271 cm-1 H-Cr3+(OH-) Cr2+(H2O)Ar4

Spectrum changes significantly at n = 6 Coordination of Cr2+ filled at n = 5 Ar begins to bind to hydrogens At n = 8 both hydrogens are bound to Ar

Conclusions IR photodissociation spectra obtained for mono- and dicationic Cr water complexes Larger red shifts in the OH stretches for the doubly charged complexes Rotationally resolved spectra for both analogues provide a direct comparison of both charged species Coordination of Cr2+ is six Formation of hydroxide reaction product