Biradicaloid Minima

Except in the case of very exothermic reactions (cf. Example 6.7), molecules that have traveled from the reactant geometry to the biradicaloid minimum will have little chance of reaching the product geometry on the same surface, because the return to Sg from the pericyclic minimum or a nearby conical intersection will be very efficient. 

The structural dependence of biradicaloid minima discussed in Section 4.3.3 on an example of twisting of a double bond A=B can be extended to take solvent effects into account. Not only the nature of the atoms A and B but also polar solvents and counterions affect the stability of zwitterionic states and states of charged species. Then, depending on the solvent, a biradicaloid minimum can represent either an intermediate or a funnel for a direct reaction. 

Here, R(Sq) stands for a reactant molecule in the Sq minimum at its equilibrium ground state geometry, the arrow labeled hi/ indicates vertical excitation by photon absorption or by energy transfer to a spectroscopic minimum S in the Si or Ti state, and the other arrows indicate further fate of this initial vertically excited species. Travel on the Si or Ti surface may take the molecule to an excimer or exciplex minimum E and to a biradicaloid minimum B (an Sq - Si conical intersection is viewed as a limiting case of B). Vertical radiative or non-radiative return from S or E restores R in its ground state (a photophysical outcome). If the return to the Sq state occurs from B, it is still possible for R to be restored, but there also is some probability of forming a new product P (a photochemical outcome). 

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