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.

with laser vaporization/ supersonic expansion. LaserVision OPO/OPA 2000-4500 cm-1 AgGaSe2 Crystal 800-1800 cm-1 V 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) 41.4 45.6 (15960 cm-1) Al+(Acetone) 41.5 42.7 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(hn) Mg+-Ar ~1700 cm-1 Mg+-CH3COCH3 D0 = 1295 cm-1 IR(hn) ~1700 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.

Mg+(CH3COCH3)

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.

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 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+ 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 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. 25 kcal/mol 0.0 kcal/mol

For Al+, since C=O stretch is shifted far, no near-degeneracy exists. 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 Vibrational Frequency (cm-1) Mg+(Acetone) sym C-C-C 839 stretch C=O stretch 1680 (1663) Al+(Acetone) sym C-C-C 842 C=O stretch 1630 (1614) Ca+(Acetone) sym C-C-C 831 C=O stretch 1686 (1669) * Results of B3LYP/6-311+G** calculations

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+.

Infrared Optical Parametric Oscillator/Amplifier (IR OPO/OPA) Tuning range 2000-4500 cm-1 idler (1.4-2.1 µm) KTP oscillator KTA diff. gen. 532 nm signal (not used) 4 crystals angle tuned 2 crystals angle+grating tuned 1064 nm Power varies with wavelength (15mJ-1mJ/pulse) Linewidth ~0.3 cm-1 LaserVision™