1. 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
U. S. Department of Energy

2. 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

3. 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

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

6. Metal Ion Water Complexes: M2+(H2O)
Asymptotically Stable
IP(Cr) eV
2nd IP(Cr) eV
IP (H2O) 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

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

8. Red Shift in OH Stretches

10. + 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 +

13. Internal Rotation

14. 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

15. 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

16. 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