Reaction Dynamics Probing with Vibrational Marker Modes

Reaction Dynamics Probing with Vibrational Marker Modes 

The bimolecular reaction dynamics of geminate recombination or acid-base neutralization have until recently been studied with time-resolved techniques probing electronic transitions. Time-resolved fluorescence using time-correlated single photon counting detection is limited to a time resolution of a few picoseconds. UV/vis pump-probe experiments, in principle, may have a time resolution of a few tens of femtoseconds, but may be hampered by overlapping contributions of 

At high base concentrations ( 1 M) practically identical rise times for photobase and acetic acid are observed, indicating a dominant direct deuteron transfer mechanism from the photoacid to the base. At these high concentrations the observed reaction dynamics are bimodal (Fig. 14.6). The two contributions to the signals can be ascribed to HPTS- OAc complexes with a pre-formed hydrogen bond along the reaction coordinate, and initially uncomplexed H PTS that first has to form an encounter pair with OAc before a reaction can proceed. The preformed complex shows deuteron transfer faster than 150 fs. In contrast, for the fraction of initially uncomplexed HPTS, where the reaction coordinate is estab- 

The present results on the reaction dynamics between HPTS and the family of acetate bases demonstrate that a base-induced sequential proton transfer mechanism at close acid-base proximities is at the heart of aqueous acid-base proton transfer reactions. The experimentally found, relatively long lived, intermediate ionic complex consisting of photobase, hydrated proton and carboxylic base, indicates the special property of carboxylic bases, a finding with important implica- 

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HA Reaction Dynamics Probing with Vibrational Marker Modes 449... 

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