by
Prof.Huib J. Bakker(FOM-Institute of Atomic and Molecular Physics, Amsterdam, The Netherlands.)
→
Europe/Stockholm
FA31 (AlbaNova)
FA31
AlbaNova
Description
Proton transport in water is of fundamental importance in chemistry and biology.
The mobility of the proton is much higher than would be expected based on the size of its solvated structure, indicating that the transport involves a mechanism other than ionic diffusion.
The proton in water is believed to occur mainly in two hydration structures, the so-called H9O4+ (Eigen) and H5O2+ (Zundel) structures.
Recent ab initio molecular dynamics simulations have provided evidence that the transport of protons in water involves a rapid interchange between the Eigen and Zundel forms.
This interchange takes place by a rearrangement of covalent and hydrogen bonds only, leading to a displacement of the protonic charge without moving the protonic mass (Grotthuss mechanism).
We studied the mechanism of proton-transfer reactions in liquid water using femtosecond vibrational pump-probe spectroscopy. By resonantly exciting the O-H+ stretching mode of the H9O4+ (Eigen) and by probing the subsequent absorption change over a broad frequency range, the dynamics of the proton is observed in real time. The lifetime of the protonic stretching mode is found to be approximately 120 fs, shorter than for any other vibration in liquid water. We observe the interconversion between the H9O4+ (Eigen) and H5O2+ (Zundel) structures hydration structures of the proton and find that this interconversion occurs on an extremely fast ( < 100 fs) time scale.
Similar proton conduction mechanisms turn out to be active in the proton transfer between photo-acids and bases dissolved in water. We studied the mechanism of proton transfer in aqueous environments between the photoacid 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) and acetate. By directly probing the vibrational absorption of the proton with femtosecond mid-infrared pulses, we can follow the trajectory of the proton from the HPTS donor to its eventual position on the acetate acceptor. We find that the proton transfer proceeds via Grotthuss conduction through hydrogen-bonded water wires of variable lengths that connect the acid with the base.
