![]() It includes nuclear quantum effects in a tractable manner using assumptions about the proton’s chemical environment and the motions which modulate the proton transfer distance. #Cplot pro transfer free#The modeling is robust, in the sense that it includes only a few free model parameters while using an extensive set of experimentally determined parameters. #Cplot pro transfer series#We apply this to model experimental temperature-dependent rates and kinetic isotope effects for a series of synthetic complexes. In this work, we will use a model based on work by Kuznetsov and Ulstrup, which is a special case of the more general semiclassical vibronic treatment, and which will be described in the next section. One approach is computational modeling of reaction rates using a semiclassical vibronic treatment. (25-34) Modeling these reactions is challenging in part due to the extreme sensitivity to dynamic fluctuations in geometry on both the reactant and product electronic surfaces. Such motions are implicated in enzymatic catalysis. This strong distance dependence also means that molecular vibrations and motions accessible at room temperature can have significant effects on tunneling rates by modulating the proton transfer distance. The increased mass of the proton relative to that of an electron makes proton tunneling much more strongly dependent on the distance. (10-24) One parameter in particular that has received attention is the proton transfer distance. A number of groups have investigated phenol oxidations in model systems to probe the key parameters which define CEPT reactivity. Phenols have been of particular interest in that regard because of the prevalence of tyrosine residues in electron transfer pathways in radical enzymes. Model systems have been useful for understanding of the key parameters defining CEPT reactivity. The results also suggest that a correlation of rate vs proton tunneling distance for the series of compounds is complicated by a concomitant variation of other relevant parameters. These results question the general applicability of this model. This discrepancy is likely because the assumption in the model of Morse-shaped proton potential energy surfaces is inappropriate for (strongly) hydrogen-bonded systems. Applying this model to the experimental data yields the conclusion that donor–acceptor compression is more facile in the compounds with shorter PT distance however, this is contrary to independent calculations for the same compounds. We use this kinetic data to evaluate a commonly employed theoretical model for proton tunneling which includes a harmonic distribution of proton donor–acceptor distances due to vibrational motions of the molecule. An Arrhenius analysis of temperature dependent data shows that the difference in reactivity arises primarily from differences in activation energies. The differences in rates, as well as the magnitude of the kinetic isotope effect (KIE = k H/ k D), both generally correlate with DFT calculated proton donor–acceptor distances. After correcting for differences in driving force, it is found that the intrinsic PCET rate constant still varies by 2 orders of magnitude. Photoinitiated proton-coupled electron transfer (PCET) kinetics has been investigated in a series of four modified tyrosines linked to a ruthenium photosensitizer in acetonitrile, with each tyrosine bearing an internal hydrogen bond to a covalently linked pyridine or benzimidazole base. ![]()
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