Exchange repulsion excited state, singlet

Figure 1. Exchange repulsion between two closed-shell molecules and the dependence of the activation barrier on the double-triplet excited state coupled to the singlet.

In these equations J is the matrix element for electron repulsion due to electron exchange, K is the matrix element for coulombic interactions, and (ML ) is the metal complex in the excited state. Since both J and K are positive, the difference between E(S ) and (Ti) is 27, which results in E(S ) being greater than E(Ti). This higher energy of the singlet state over that of the triplet states is an important feature in understanding photochemical reactions. 

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