Type D Site-Specified 3-Point PCET

Many ETs to an oxidant (or from a reductant) are accompanied by PT to a spedfic base (or from a specific add), usually via a preformed hydrogen bond. As discussed in Section 17.5, Type D PCET often describes the transfer of a net hydrogen atom in Nature. It is the specific coordinate for PT that distinguishes Type D PCET from its Type C counterpart of Section 17.4.1. 

Unlike proteins, which exert control over transfer distances by positioning amino acid residues according to the tertiary structure, most ternary PCET reactions studied in model systems to date are trimolecular reactions. This complicates kinetics measurements and analysis, and can mask the underlying physics. The PCET yield depends on both the association constant (K soc) lo form the PCET precursor complex, and the subsequent pseudo-bimolecular PCET rate constant (kpcET)- fi is imperative to decouple the measurement of K soc kpcET io 

System 26 uses an extrinsic base to tune the reducing power of a-hydroxy radicals [181]. The photochemically generated diphenylketyl radical serves as De/Dp-H, Ae is 1,2,4,5-tetracyanobenzene (TCB), and Ap is a substituted pyridine (py). In the absence of the pyridine to facilitate PT, the ET reaction from the ketyl radical to TCB is endothermic by 0.4 eV. Consequently, the diphenylketyl radical decays slowly to reconstitute the starting reactants with no net reaction taking place. When coupled to PT by pre-associating the 

The PCET reaction rate for 26 has been slowed by using a less basic proton acceptor, thus reducing the driving force for the concerted PCET reaction. When lutidine is replaced with 2-Cl-py, the driving force diminishes from 0.4 eV to 0.1 eV, and accordingly kp g-i- decreases by a factor of four. A KIE of 3.2 was measured when the deuterated diphenylketyl radical was used and Ap was 2-Cl-py. This magnitude certainly supports the mechanism of concerted PCET. 

Gust and their coworkers have developed 3-point PCET systems that incorporate all the necessary ET and PT components within molecular dyads and triads [184]. In 28, De is the singlet excited state of a free-base porphyrin, Ae/ 

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