Potential energy surface path avoidance

In photochemical applications, where minima in the second lowest singlet potential energy surface (S ) are sought, this is an unproductive type of perturbation of a perfect biradical, and tends to be avoided by photochemical paths. If there is an Sq - Si degeneracy, it occurs at the geometry of the perfect biradical, and only if the biradical is of the axial type. 

Fig. 20. Schematic representation of the reaction coordinate for tryptophan fluorescence quenching induced by hydrogen transfer and aborted decarboxylation. The electronic nature of the Si surface changes character along the Si path due to two avoided crossings between jSi and S2 The first one occurs between the covalent state and the ionic La state along the reaction coordinate that interconverts the i9i-Min and. Si-Exc minima. The second one occurs between the ionic La state and the biradical Bi, state along the tautomerization coordinate that leads to the excited-state tautomerized form S -Taut. This point does not corresponds to a minimum on the potential-energy surface and it is found that evolution along a decarboxylation coordinate leads to a -Si /-So conical intersection, where efficient radiationless decay to the ground state takes place. The values of the relevant structural parameters are given in A. Data from Ref. 102.

The course of the radiationless decay of any bound excited electronic state of a polyatomic molecule is determined by the relative disposition of its potential energy surfaces and the interactions between than. Strong interactions between zero order electronic states of the same synunetry result in excited state potential hypersurfaces that can contain several local minima in a large molecule. On the other hand conical intersections between more weakly interacting states of different synunetry (avoided aossings) are characterized by holes in an excited state surface and can provide a path for radiationless relaxation ova or through a finite potential energy barrier. 

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