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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
100 200 300
Cr2+(H2O)Ar
m
m = 0
6
m/z
Cr2+
Cr2+Arn
n = 1
5
Metal Ion Water Complexes: M2+(H2O)
IP(Cr) 6.7 eV
2nd IP(Cr) 16.5 eV
IP (H2O) 12.6 eV
•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
Asymptotically Stable
Asymptotically Unstable
Binding Energy (D0; kcal/mol) vs Photon Energy
Mn+-H2O Mn+-Ar
IR Photon
loss of argon
Ar
Ar
M+
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
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
3500 3600 3700 3800 3900
3762 38373735
3655
cm-1
3576
3764
36973623
Zn+(H2O)Ar
Cu+(H2O)Ar
2
O
H
H
O
H
H
3400 3500 3600 3700 3800 3900 4000
(4,3)
(3,2)(1,2)
(0,1)
(1,0)
Simulation
cm-1
A'' = 13.1 cm-1
B'',C'' = .056, .056 cm-1
A' = 13.0 cm-1
TJ,K
= 85, 130 K
B.O.sym
= 3620 cm-1
B.O.asym
= 3690 cm-1
(2,1)
(
(
Cr+(H2O)Ar
Experiment
111.1
(4,3)
(3,2)(1,2)
(0,1)
(1,0)
(2,1) Free OH Stretches
3500 3600 3700 3800 3900
3687
cm-1
Cr+(H2O)Ar
2
3621
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
50 100 150 200 250
- 20
75
95Cr2+(H2O)Ar
3
m/z
- 20
95
115Cr2+(H2O)Ar
4
- 20
115135
155Cr2+(H2O)Ar
6
135 155
Cr2+(H2O)Ar
8
195- 40 = 2 x 20
3000 3200 3400 3600 3800 4000
cm-1
3000 3200 3400 3600 3800 4000
3000 3200 3400 3600 3800 4000
3512Cr2+(H
2O)Ar
3
3292
3281
3531
Cr2+(H2O)Ar
4
3276
3546
Cr2+(H2O)Ar
5
free OH stretches
Internal Rotation
3000 3200 3400 3600 3800 4000
(,)
A" = 12.6 cm-1
B", C" = 0.05, 0.05 cm-1
A' = 12.8 cm-1
B', C' = 0.05, 0.05 cm-1
TJ,K
= 250 K
B. O.sym
= 3521 cm-1
B. O.asym
= 3586 cm-1
cm-1
simulation
experiment
(0,1
)
(1,0
)(2
,1)
(3,2
) (4,3
)
Cr2+(H2O)Ar
4
113.5
Hydroxide Formation
3000 3200 3400 3600 3800 4000
cm-1
113.5
H-Cr3+(OH-) Cr2+(H2O)Ar4
•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
3000 3200 3400 3600 3800 4000
3578
3380
Cr2+(H2O)Ar
6
cm-1
3265
3000 3200 3400 3600 3800 4000
3467
3260Cr2+(H
2O)Ar
8
•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
