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Hydrated alizarin complexes: hydrogen bonding and proton transfer
Hyun Huh1, Sung Haeng Cho, Jiyoung Heo
1Department of Chemistry and WCU Department of Biophysics and Chemical Biology, Seoul National University, Seoul 151-747, Korea.
Physical Chemistry Chemical Physics : PCCP
|April 20, 2012
Summary
We studied alizarin (Az) hydration complexes using spectroscopy. For complexes with two or more water molecules, proton transfer occurs in the ground state, evidenced by broad spectral bands.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Molecular Interactions
Background:
- Alizarin (Az) is a molecule with two O-H groups capable of forming hydrogen bonds.
- Understanding hydration complexes is crucial for various chemical and biological processes.
Purpose of the Study:
- To investigate the hydrogen bonding structures and proton transfer in alizarin hydration complexes.
- To elucidate the role of water molecules in modifying alizarin's spectral properties and reactivity.
Main Methods:
- Supersonic jet expansion for generating stable molecular complexes.
- Fluorescence Excitation (FE) spectroscopy.
- Dispersed Laser Induced Fluorescence (LIF) spectroscopy.
- Visible-Visible Hole Burning (HB) spectroscopy.
- Fluorescence Detected Infrared (FDIR) spectroscopy.
- Theoretical calculations.
Main Results:
- Bare alizarin exhibits distinct O-H stretching vibrations due to intramolecular and weak intermolecular hydrogen bonds.
- The 1:1 alizarin-water complex (Az(H(2)O)(1)) shows three conformers, with the most stable featuring water acting as both a proton donor and acceptor.
- Hydration complexes with two or more water molecules (Az(H(2)O)(n), n≥2) display broad spectral features, unlike the sharp peaks of bare alizarin and the 1:1 complex.
- These broad bands suggest facile decay processes, potentially involving proton transfer in the excited state.
- FDIR spectroscopy indicates proton transfer by tunneling in the electronic ground state for Az(H(2)O)(n) (n≥2) upon O2-H2 vibration excitation.
Conclusions:
- The number of water molecules significantly influences the hydrogen bonding and proton transfer dynamics in alizarin complexes.
- Proton transfer, particularly via tunneling, becomes prominent in the ground state of larger alizarin hydration complexes.
- Spectroscopic techniques combined with theoretical calculations provide detailed insights into molecular interactions and reaction pathways.
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