Quantum mechanical tunneling intramolecular reactions

Comparing these observations to the requirements of TST, we can immediately see a number of problems. Even apart from the fact that the mass of the proton requires it to be treated as a quantum mechanical particle, so that even if there were a well-defined barrier, we would still need to take the possibility of tunneling into account. Transfer of the proton is directly coupled, or may even be driven by a redistribution of electron density in the molecule. In excited-state intramolecular proton transfer (ESIPT) reactions, the redistributed charge almost certainly provides the driving force. The generic picture for such is reaction is due to WeUer [7, 8], who was the first to realize that the enormous Stokes shift of about 10 000 cm he observed in the fluorescence of salicylic acid (X = OH) could be a consequence of a rapid proton transfer in the excited state. 

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