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IR Spectroscopy of M+(Acetone)
Complexes (M=Mg, Al, Ca): CationCarbonyl Binding Interactions
E. D. Pillai, J. Velasquez, P.D. Carnegie, M. A. Duncan
Department of Chemistry, University of Georgia
Athens, GA 30602-2556
www.arches.uga.edu/~maduncan/
Why Study M+(Acetone)Complexes
• M+ binding in proteins often takes place at the carbonyl
groups of amino acids.
• Vibrational spectroscopy can provide insight into the
cation binding site and condition.
• In proteins, the carbonyl or “amide I” region or the IR
spectra are overlapped by absorptions from other
functional groups.
• IR spectroscopy of gas phase M+(Acetone) complexes
isolates the cation-carbonyl interaction providing valuable
insight into the bonding mechanism.
LaserVision OPO/OPA
2000-4500 cm-1
V
AgGaSe2 Crystal
800-1800 cm-1
0
Production of cold
metal ion complexes
with laser vaporization/
supersonic expansion.
Mass selection of cations
by time-of-flight.
Tunable infrared laser
photodissociation
spectroscopy.
Binding Energies of Complexes Relevant for This Study
Energy kcal/mol
Complex
Theory
Experimental
Mg+(Acetone)
41.3 (14455 cm-1)
45.6 (15960 cm-1)
41.4
Al+(Acetone)
41.5
42.7
Ca+(Acetone)
41.0
Mg+(Ar)
Al+(Ar)
Ca+(Ar)
3.70(1295 cm-1)
2.81(982.3 cm-1)
2.00(700 cm-1)
Rare Gas “Tagging”
Ar
Mg+-Ar
D0 = 1295 cm-1
IR(hn)
Mg+
~1700 cm-1
Mg+-CH3COCH3
D0 = 14445 cm-1
Mg+
Ar
1Duncan
et al. J.Chem. Phys. 1995, 103, 3293.
2Dunbar et al. J. Phys. Chem. 2005, 109, 1411.
+
Mg+(CH3COCH3)
2
1
0
8
400
800
m/z
1200
1600
1678
1731
The IRPD spectra are measured in
the Ar loss channel exhibit
resonances in the 1700 cm-1 region.
+
Mg (acetone)Ar
1653
+
Al (acetone)Ar
Mg+ and Ca+ complexes have two
bands in the region whereas Al+ has
only one.
1622
1674
+
Ca (acetone)Ar
All bands are red-shifted as
compared to free C=O stretch in
acetone
1643
Al+ complex band is shifted farthest
to the red.
1560 1580 1600 1620 1640 1660 1680 1700 1720
cm
-1
The C=O red shifts are consistent with M+
binding to the carbonyl of acetone
M
+
The bonding mechanism involves the M+
withdrawing electron density via a s- type
donation from the HOMO and HOMO –1
orbitals (b2 and b1) of acetone.
This weakens the C=O bond, thus lowering
its frequency.
For transition metals p-back bonding also
exists. However, Mg+, Al+, and Ca+ have no
d-electrons.
The Magnitude of the Red-Shifts
Ionic Radius
Al+
Mg+
Ca+
Al+ induces greatest red shift on
C=O stretch.
72 pm
82 pm
118 pm
The red shift is result of M+
polarizing electron density from
the carbonyl.
Such polarization of electron
density is optimized when the
charge density of the M+ is
greatest.
Al+ with its closed shell
configuration (3s2) most closely
resembles a point charge.
The Doublet Features in the Mg+ and Ca+
Complexes
Different isomeric structures for M+ binding are not
predicted by theory.
The possibility of enol- and keto- tautamers is intriguing
but DFT calculations show the enol species to lie at a
much higher energy.
0.0 kcal/mol
M+
M+
25 kcal/mol
Complex
Vibrational
Frequency (cm-1)
Mg+(Acetone)
sym C-C-C
stretch
839
C=O stretch
1680 (1663)
sym C-C-C
stretch
842
C=O stretch
1630 (1614)
sym C-C-C
stretch
831
C=O stretch
1686 (1669)
Al+(Acetone)
Ca+(Acetone)
* Results of B3LYP/6-311+G** calculations
The final possibility to
consider is a Fermi
resonance. A Fermi
resonance may occur when
there is an accidental neardegeneracy of any two or
more vibrational states
(fundamentals, overtones, or
combinations) having the
same frequency.
C-C-C stretch has same
symmetry as C=O (a1) and
its overtone is nearly
degenerate with C=O of Mg+
and Ca+.
For Al+, since C=O stretch is
shifted far, no neardegeneracy exists.
+
Mg -(acetone)-Ar
A Final Verification
13
C Substituted Acetone
The spectrum of Mg+ complex
with 13C isotopically substituted at
the carbonyl carbon is acquired.
1634
1667
The doublet appears but is redshifted further and the relative
intensities are changed.
1678
Natural Acetone
This behavior is consistent with a
Fermi resonance.
1653
1600
1620
1640
1660
cm
1680
-1
1700
1720
1740
Conclusions
IR spectroscopy of M+(Acetone) (M=Mg+, Al+, Ca+) in the C=O
stretch region reveals structures of these complexes.
The C=O stretch shifts to lower frequencies due to M+ binding
and can be explained electron density withdrawing mechanism
of the bonding.
The greatest shift is for the Al+ complex as the cation has the
largest charge density.
A Fermi resonance between the C=O stretch and the symmetric
C-C-C vibration occurs for Mg+ and Ca+ but not Al+.
Infrared Optical Parametric Oscillator/Amplifier (IR OPO/OPA)
OPO
532 nm
KTP
oscillator
OPA
idler
(1.4-2.1 µm)
signal (not
used)
2 crystals
angle+grating tuned
Tuning range
2000-4500 cm-1
KTA
diff. gen.
4 crystals
angle tuned
1064 nm
Power varies with wavelength
(15mJ-1mJ/pulse)
LaserVision
™
Linewidth ~0.3 cm-1