IR Spectroscopy of M+(Acetone) Complexes (M=Mg, Al, Ca): Cation-

Carbonyl 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.

Production of coldmetal ion complexeswith laser vaporization/supersonic expansion.

Mass selection of cationsby time-of-flight.

Tunable infrared laserphotodissociationspectroscopy.

LaserVision OPO/OPA2000-4500 cm-1

AgGaSe2 Crystal800-1800 cm-1V

0

Binding Energies of Complexes Relevant for This Study

Complex Theory Experimental

Energy kcal/mol

Mg+(Acetone) 41.3 (14455 cm-1) 41.445.6 (15960 cm-1)

Al+(Acetone) 41.542.7

Ca+(Acetone) 41.0

Mg+(Ar) 3.70(1295 cm-1)Al+(Ar) 2.81(982.3 cm-1)Ca+(Ar) 2.00(700 cm-1)

Rare Gas “Tagging”

IR(hMg+-ArD0 = 1295 cm-1

Mg+-CH3COCH3

D0 = 14445 cm-1

+1Duncan et al. J.Chem. Phys. 1995, 103, 3293. 2Dunbar et al. J. Phys. Chem. 2005, 109, 1411.

~1700 cm-1

Mg+

Mg+

Ar

Ar

0 400 800 1200 1600

m/z

1

2

8

Mg+(CH3COCH3)

1560 1580 1600 1620 1640 1660 1680 1700 1720

cm-1

Mg+(acetone)Ar

Al+(acetone)Ar

Ca+(acetone)Ar

1653

1678

1622

1643

1674

1731

The IRPD spectra are measured in the Ar loss channel exhibit resonances in the 1700 cm-1 region.

Mg+ and Ca+ complexes have two bands in the region whereas Al+ has only one.

All bands are red-shifted as compared to free C=O stretch in acetone

Al+ complex band is shifted farthest to the red.

M+

The C=O red shifts are consistent with M+ binding to the carbonyl of acetone

The bonding mechanism involves the M+ withdrawing electron density via a - 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 -back bonding also exists. However, Mg+, Al+, and Ca+ have no d-electrons.

Ionic Radius

Al+ 72 pm

Mg+ 82 pm

Ca+ 118 pm

Al+ induces greatest red shift on C=O stretch.

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 Magnitude of the Red-Shifts

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.

M+M+0.0 kcal/mol 25 kcal/mol

The final possibility to consider is a Fermi resonance. A Fermi resonance may occur when there is an accidental near-degeneracy 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 near-degeneracy exists.

Complex VibrationalFrequency (cm-1)

Mg+(Acetone) sym C-C-C 839stretch

C=O stretch 1680 (1663)

Al+(Acetone) sym C-C-C 842stretch

C=O stretch 1630 (1614)

Ca+(Acetone) sym C-C-C 831stretch

C=O stretch 1686 (1669)

* Results of B3LYP/6-311+G** calculations

1600 1620 1640 1660 1680 1700 1720 1740

cm-1

13C Substituted Acetone

Mg+-(acetone)-Ar

1653

1678

Natural Acetone

1634

1667

A Final Verification

The spectrum of Mg+ complex with 13C isotopically substituted at the carbonyl carbon is acquired.

The doublet appears but is red-shifted further and the relative intensities are changed.

This behavior is consistent with a Fermi resonance.

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+.

LaserVision™

Power varies with wavelength (15mJ-1mJ/pulse)

Linewidth ~0.3 cm-1

Infrared Optical Parametric Oscillator/Amplifier (IR OPO/OPA)

OPO OPA

2 crystalsangle+grating tuned

4 crystalsangle tuned

signal (not used)

idler (1.4-2.1 µm)

532 nm

1064 nm

Tuning range2000-4500 cm-1

KTPoscillator

KTAdiff. gen.