Evidence Based on Free Energy Correlations

18) for the correlation between the proton transfer rate, kp and (the dependence on enters through the free energy of activation term AG see below) usually result in very good agreement between the observed reactivity of the photoacid and its Forster cycle value. 

AG/ is the solvent-dependent activation energy of the charge-exchange reaction when the total free energy change (AG° = RT log piCj ) in the proton transfer is equal to zero. Eqs. (12.19) and (12.20) are practically equivalent in the photoacidity range that has been studied so far which seems to display only normal reaction behavior where the proton transfer rate increases monotonically as a function of the increase in the relative strength of the base compared to the acid (see Fig. 

A very convincing support for the existence of solvent controlled proton dissociation reactions in aqueous solutions has risen from the theoretical studies of Ando and Hynes [105-108] who have studied the proton dissociation of simple mineral acids HCl and HF in aqueous solutions. The two acids seem to follow a solvent-controlled proton transfer mechanism with a Marcus-like dependence of the activation energy on the acid strength. Recently, a free energy relationship for proton transfer reactions in a polar environment in which the proton is treated quantum mechanically was found by Kiefer and Hynes [109, 110]. Despite the quite different conceptual basis of the treatment the findings bear similarity to those resulting from the Marcus equation Eq. (12.19) which has been used to correlate the proton transfer rates of photoacids with their piG [ 101,102 ] 

The case of 1-hydroxypyrene is illuminating in this respect. Being one of the first photoacids studied by Weller [9], its plG = -log (K ) was estimated by Weller using the Forster cycle, pK = 3.7, but the photoacid was not observed to dissociate in water [111]. Several explanations were offered for this apparent lack of con- 

5 kcal higher than that found for the sub-set of the proton-dissociation-to-water reactions (Eig. 12.11). 

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