Infrared Spectroscopy of Metal Ions and Clusters:
Inorganic Chemistry in the Gas Phase
Michael A. Duncan
Department of Chemistry, University of Georgia, Athens, GA 30602
maduncan@uga.edu
http://www.arches.uga.edu/~maduncan
Department of Energy
Air Force Office of Scientific Research
Allen Ricks, Tim Cheng, Biswajit Bandyopadyay, Zach Reed
Infrared spectroscopy of ions and clusters:
Metal complexes, protonated species and carbocations
M+(H2O)n, M2+(H2O)n
M+(CO)n,, M+(CO2)n
M+(acetone)n
M+(benzene)n
H+(H2O)n
H+(CO2)n
H+(H2)n
H+(N2)n
H+(C2H2)n
H+(benzene)n
C2H3+, C3H3+, C3H5+, C4H9+, …
IR spectroscopy probes fundamental
bonding interactions and structures.








Ligand vibrational shifts
Structures, isomers
Coordination spheres
Metal spin states
Solvation
Intracluster reactions
Proton transfer intermediates
Astrophysical ions
full mass spectrum
activate mass gate;
select one cluster mass
excite at turning point
Production of cold cations and complexes with laser
vaporization in a supersonic expansion.
photofragments
Mass selection of cations by time-of-flight.
Photodissociation spectroscopy with an infrared OPO laser.
Measure fragment intensity versus wavelength to record
IR spectrum.
parent ion depletion
Tunable IR Spectroscopy
LaserVision Tunable Infrared Laser System
designed by Dean Guyer
Tuning range: 600-4300 cm-1
Linewidth: ~1.0 cm-1
2000-4300 cm-1
Tunable mid-IR
2.3-5.0 m
OPA
OPO
532 nm
KTP
oscillator
1 crystal
angle tuned
idler
signal
(not used)
KTA
diff. gen.
+ amp of idler beam
4 crystals
angle tuned
600-2200 cm-1
Tunable
4.5-16.7 m
AgGaSe2
diff. gen.
1 crystal
angle tuned
1064 nm
Pumped by pulsed, seeded Nd:YAG laser,
e.g., Spectra Physics PRO-230.
IR Photodissociation Spectroscopy and Rare Gas Tagging
The density of mass-selected ions is too low for absorption spectroscopy.
Therefore, we use photodissociation, with sensitive TOF mass spectrometer detection.
However, the bonds in our ions are often too strong (D0>20 kcal/mol)
to be broken with infrared light (e.g., C-H stretch of 3100 cm-1 = 9 kcal/mol). Our laser
power is too low for multiphoton absorption.
In larger complexes, external ligands (not in contact with ion) can be eliminated.
For others, we attach a weakly bound (1-2 kcal) “tag” atom to enhance fragmentation.
The tag elimination (only when light is absorbed) provides indirect evidence of absorption.
Use computations on ion with & without tag to reveal the degree of perturbation.
Ar
+
Mass selected ion
+ h (tunable IR) 
+
Fragment ion after argon elimination
detected vs IR frequency
Metal Carbonyls: The Importance of the 18 Electron Rule
Do Cations Behave the Same way as Neutrals?
How Far Can We Stretch the Rule?
+
0
10
5
15
Co(CO)n
Neutral vs Ion Complexes:
Fe(CO)5 d6s2 + 5x2
Co+(CO)5 d8 + 5x2
+
Cr(CO)6 d5s1 + 6x2
5
10
15
Co(CO)nH2O
500
600
Mn+(CO)6 d5s1 + 6x2
0
100
200
300
400
m/z
Co+(CO)n ion-molecule complexes: Some ligands are
coordinated directly to ion; others are bound weakly
like “solvent” due to low temperature conditions in
supersonic expansion.
Co+(CO)n Fragmentation
5
All complexes larger than n=5
fragment by sequential ligand
elimination terminating at the n=5
complex.
7
6
8
9
8
Photofragmentation “breakdown”
mass spectrum.
Weakly bound ligands are eliminated
more easily by IR than those strongly
bound to the ion.
Coordination number is five!
7
Bond Energies:
6
4
n=1 (1.80 eV); n=2 (1.58); n=3 (0.85); n=4 (0.78).
5
150
200
250
m/z
300
Armentrout and coworkers,
J. Am. Chem. Soc. 1995, 117, 6994.
IR excitation:
CO stretch near 2150 cm-1 = 0.27 eV
IR Photodissociation Spectroscopy
Small clusters: Elimination of Ar
Large clusters: Elimination of CO
2135
+
Co (CO)nArm
2148
Co+(CO)n
J. Phys. Chem. A 113, 4701 (2009)
2140
2165
2150
9
2137
2149
(5,1)
2166
8
2138
2149
(4,1)
2166
2141
2165
2154
7
2156
(3,1)
2150
2165
2168
2137
6
(2,1)
2156
5
(1,2)
2050
2100
2150
-1
cm
2200
2250
2050
2100
2150
cm
-1
2200
2250
Spin States:
In isolated metal cation, electrons spread out and have same spin to avoid each
other. As more ligands are added, ligand-electron repulsion causes spin
pairing. For Co+, spin changes from triplet to singlet upon addition of 5th CO.
+
Co (CO)nArm
n,m = 1,2
Co(CO)nArm+
(6,0)
singlet, five-coord.
triplet, six-coord.
2,1
2,0
(5,1)
1
A1
3
A1
3,0
3,1
(4,1)
1
A2
3
A2
4,0
5,0
2050
2100
2150
cm
-1
2200
2250
6
Mn(CO)+n
n=0
7
8
6
7
5
n=6
3
150
9
200
14
0
200
250
m/z
400
m/z
600
300
Mn+ is isoelectronic to Cr.
Expect and find coordination
of six.
Mn+(CO)n
2119
n=9
2172
Mn+ is isoelectronic to Cr.
Expect and find coordination
of six.
2121
n=8
2172
2122
n=7
2172
n=6
2000
2114
2100
2200
cm
2300
-1
J. Am. Soc. Mass Spectrom. 21, 739 (2010).
Metal Carbonyl Bonding Interactions
Both sigma donation and  backbonding
Weaken the C-O bond and thus lower the
C-O stretching frequency.
M
M
sigma donation
 back-bonding
Neutral vs Ion Complexes: C-O stretch
Fe(CO)5 d6s2
2013, 2034 cm-1
Co+(CO)5 d8
2140, 2150
Cr(CO)6 d5s1
2003
Mn+(CO)6 d5s1
2122
Cation species have more sigma donation and less backbonding,
but smaller red shifts. Backbonding is more important for red shift!
+
Au -(CO)n
“Non-classical carbonyls”
d10 systems –have blueshifted CO stretch
n=6
Free
CO
n=5
n=4
theory by Mark Gordon,
Iowa State
n=3
2140
2160
2180
2200
2220
-1
cm
2240
2260
2280
JPCA 112, 1907 (2008).
+
Cu(CO)nAr
+
Cu(CO)nNe
2202
2198
n=4
n=4
All copper carbonyls blue shifted.
2210
2191
n=3
n=3
2225
2195
n=2
n=2
2100
2200
-1
cm
Neon vs argon tagging different
at small sizes.
2300 2100
2200
-1
cm
2300
How far can we push the 18 electron rule?
Can seven-ligand species be stable?
+
V(CO)7
For V+, the seven-CO complex
has 18 electrons (d4 + 7x2 =18).
V+(CO)7 should be stable???
However, the V+(CO)7 species
has the same spectrum as
V+(CO)6, with an extra weak
band corresponding to “free” CO.
+
V(CO)6 Ar
The extra CO acts as the “tag”,
much the same as Ar.
Therefore, the 18-electron,
seven-coordinate species
does not form for vanadium!
2000
2050
2100
2150
cm
-1
2200
2250
+
Nb(CO)8
Nb+ and Ta+ are isoelectronic
To V+, so again the seven-coordinate
species would have 18 electrons.
Loss of 1 CO
The Nb+(CO)8 species has different
spectra in different fragment channels.
The loss-of-two CO’s channel has a
spectrum like that of Nb+(CO)6. The
loss-of-one CO channel has a spectrum
matching that calculated for the
seven-coordinate species.
Loss of 2 CO
Therefore, there are two isomers
present corresponding to BOTH the
six- and seven-coordinate species!
2000
2050
2100
2150
cm
2200
-1
2250
2300
2350
+
Ta(CO8) , loss of CO
Bigger ion:
Ta+ forms ONLY the
seven-coordinate species!
J. Am. Chem. Soc. 131, 9176 (2009).
+
Ta(CO)7 -Ar, loss of Ar
DFT
seven coord.
Ta+(CO)7
DFT
6+1 coord.
2000
2050
2100
cm
2150
-1
2200
Capped octahedral structure;
Not pentagonal bipyramid.
U(CO)8
+
Single bands consistent
with high symmetry structures.
Free CO (2143)
18 electron rule for transition metals
changes to 22 electrons for actinides
s2p6f14
2080
Structures and shifts reproduced
by theory.
UO2(CO)5
Red shift for U+ because f electrons
are available for back-bonding.
+
2194
Less backbonding for higher charge
state core ion. Electrostatic
interaction leads to blue shift.
(same as seen for Au+ and Cu+)
2000
2100
2200
cm
-1
2300
2400
V+(CO2)n
Charge-quadrupole binding
1
0
0
50
2
1
100
4
3
2
3
150
200
5
4
250
7
6
5
7
6
300
m/z
V(CO2)n
350
+
VO(CO2)n
400
450
500
550
600
+
Coordination of four
J. Chem. Phys. 120, 10037 (2004)
V+(CO
2)n
2361
2353
n=11
2377-2380 band assigned
to CO2 attached to metal
2377
2352-2353 band assigned
to external CO2
2402
2361 band assigned to
“squeezed” second layer
2376
2353
2402-2402 band assigned
to CO2 attached to core
ion changed by intracluster
reaction
2401
n=10
2353
n=9
2378
2402
Intracluster reaction occurs
but only when second sphere
ligands are present
2380
2352
n=8
Oxide-carbonyl?
Oxalate?
2250
2300
2350
2400
Energy / cm-1
J. Chem. Phys. 120, 10037 (2004)
2450
2500
Carbonyly-carboxylate?
Oxide-carbonyl
unreacted
carbonyl-carboxylate
oxalate
Expanded tuning range to investigate reaction products:
V(CO2)7
2348
3715 3737
3614
2376
asym st. (2349)
1800
1500
2000
V(CO2)6
2500
cm
-1
3000
combination bands
bend (667)
1000
+
3780
sym st. (1333)
637/664
1237/1267
1363
+
3500
4000
1379
1017
1270
1367
1375
1273
1243
1366
1239
VO(CO2)4
V(CO2)10
1786
+
1800 band appears for all clusters
larger than n=6.
+
There is no oxide stretch
in these clusters, unless we
intentionally make/select the oxide.
1791
1374
V(CO2)8
1272
+
+
1363
1371
1236
1268
V(CO2)7
1800
No carbonyl stretch either.
1361
1370
1236 1267
V(CO2)6
+
Reaction does not involve oxide or
carbonyl.
1360
V(CO2)4
1250
1000
1200
1400
cm
-1
1600
+
1800
Oxalate moiety has
two vibrational bands in
about the right places to
explain the reaction.
1363
2376
2347
638 665
Why does it only happen
when second-sphere ligands
are present?
Associated with spin change
quintet  triplet?
1268
1236
1800
unscaled theory
500
1000
1500
2000
cm
-1
2500
3000
Conclusions
Coordination numbers, spin states and ligand shifts for metal carbonyls.
Intracluster reactions.