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

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.

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

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  1. 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/

  2. 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.

  3. LaserVision OPO/OPA 2000-4500 cm-1 AgGaSe2 Crystal 800-1800 cm-1 V 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.

  4. 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)

  5. Rare Gas “Tagging” Mg+-Ar 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.

  6. Mg+(CH3COCH3)

  7. 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.

  8. 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.

  9. Ionic Radius Al+ 72 pm Mg+ 82 pm Ca+ 118 pm The Magnitude of the Red-Shifts 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.

  10. 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

  11. 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 stretch C=O stretch 1630 (1614) Ca+(Acetone) sym C-C-C 831 stretch C=O stretch 1686 (1669) * Results of B3LYP/6-311+G** calculations

  12. 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.

  13. 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+.

  14. Infrared Optical Parametric Oscillator/Amplifier (IR OPO/OPA) 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™

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