The Sobolewski, Domcke, Dedonder, Jouvet Model
The combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as a nucleic bases and aromatic amino acids. The key role in this picture is played by excited singlet states of ps* character, which have repulsive potential-energy functions with respect tothe stretching of OH or NH bonds. The 1ps* potential-energy functions intersect not only the bound potential-energy functions of the 1pp* excited states, but also that of the electronic ground state. Via predissociation of the 1pp* states and a conical intersection with the ground state, the 1ps* states trigger an ultrafast internal-conversion process, which is essential for the photostability of biomolecules.
1) the H atom can tunnel through the barrier which lead to a strong H/D effect on the excited state lifetime. example: phenol lifetime 1ns, deuterated phenol lifetime 10ns
2) the second crossing lead to internal conversion.
3) If the OH bond is linked to a good Hydrogen acceptor, this lead to an H atom transfer. The (NH3)n clusters are the good example
Generalization
Depending of the relative position of the pp* relative to the ps* one get different behavior
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High gap: phenol, indole lifetime depend on the vibrational energy, strong H/D effect |
no gap: DNA bases short lifetime, very limited excitation spectra
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inverse gap : Pyrrole short lifetime and fast ejection of the H atom
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One consequence : the H transfer in phenol-NH3