predicted theoretically: optimisation of the S 1 (ππ*) state, in which one H is
pointing toward the ring induces a strong puckering of the phenol chromophore.
The barrier to the proton transfer (PT) toward the phenol ring has been calculated to
be of the order 0.15 eV and is isomer independent. In this process the excited state
crosses the ground state, and the dynamics in this hot ground state leads to C α –C β
bond rupture after the proton transfer. This channel, dominant at low excitation
energies, is in competition with a second channel: the proton transfer from the NH 3
+
to the carbonyl, which is due to a crossing between S 1 and S 2 . Indeed the second
excited state (ππ* CO ) leads without barrier to a proton transfer to the carbonyl
group. Along this proton-transfer coordinate, S 2 crosses S 1 at around 0.7 eV above
the ππ* optimised energy and a second crossing with S 0 occurs below the ππ* state
(À1.37 eV). This triggers the dynamics on the ground-state surface leading to the
H 2 O+CO loss (m/z ¼ 136) and H 2 O+NH 3 loss (m/z ¼ 147). The initial ππ* state
decays through this channel by the S 1 /S 2 avoided crossing. This is the dominant
channel at higher energies (+1,000 cm
À1 see Fig. 9.6).
π* excitation
π* CO excitation
σ* excitation
Fig. 9.12 General scheme showing the connection between the electron localisation (i.e., excited
state) and the fragmentation channels in protonated tryptophan. Reprinted with permission from
[32]. Copyright [2007], American Institute of Physics
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