Solvent Effects for Nonadiabatic Proton Transfer in the Benzophenone/N,N-Dimethylaniline Contact Radical Ion Pair

Abstract

Picosecond absorption spectroscopy is employed in determining the dynamics of proton transfer within a variety of substituted benzophenones/N,N-dimethylaniline in a series of alkanenitrile solvents. A correlation of the rate constants for proton transfer with driving force reveals a normal region and an inverted region for proton transfer. The kinetics are analyzed within two theoretical frameworks for nonadiabatic proton transfer. The Borgis−Hynes model, which is based upon a low-frequency promoting mode that modulates the tunneling frequency, is found to be only in qualitative accord with the experiment. The Lee−Hynes model, which not only includes a low-frequency promoting mode but also allows for contributions from vibrational excitation of the proton reaction coordinate in the reactant and product states, is found to give an excellent fit to the experimental data.

Theoretical aspects of tunneling proton transfer reactions in a polar environment

“…In analogy to ET reactions,97–98 inverted regime behavior would be the eventual decrease of the rate constant with increasingly negative reaction free energy Δ G RXN . Such behavior has been reported experimentally;114, 117–120, 125–127 indeed very good agreement has been achieved114, 117–119, 125–127 between the experimental findings and certain of the tunneling rate constant equations presented in the third section, limited to the 0–0 tunneling transition for both the transfer of a proton and a deuteron. This is quite encouraging in a certain sense, but is disturbing in another sense emphasized by Peters 121.…”

“…The situation is not so straightforward. It is likely that special characteristics of the experimental systems studied in Refs 114, 125–127 and 117–119 are involved in allowing inverted regime behavior to be observed. Indeed, Fig.…”

“…Dotted lines use the analytical form for the KIE versus Δ G RXN behavior in Eqn (29), using Eqn (26) to evaluate $\bar {\alpha }'_{oL} $ . (b) Experimental k H/ k D values for tunneling PT systems versus reaction asymmetry Δ G RXN for substituted benzophenone – N , N dimethylanaline contact radical ion pairs in butanenitrile (□) and pentanenitrile (◊) from Peters et al .,118 as well as substituted benzophenone – N‐ methylacridan contact radical ion pairs in benzene (○) Peters et al 119. The dotted line simply emphasizes the quantitative trend for the three sets of data …”

“…As we noted above, the first feature predicts that there is no ‘inverted region’ behavior for tunneling PT. (Further comments on an ‘inverted regime’ are given in the concluding remarks in reference to experimental data 117–121…”

Single Molecule Acid-Base Kinetics and Thermodynamics

“…This number is somewhat lower than the 7:85 kcal=mol value previously measured on a similar thin film polymer obtained using temperature dependent IR spectroscopy [10]. Proton exchange rates and activation enthalpies for nonaqueous, nonpolymer solutions have been reported as 10 6 -10 12 s ÿ1 and 0:1-10 kcal=mol, respectively [16]. The slower rates we measure here, relative to those in solution, should not be too surprising as proton exchange, and exchange in general, involves a reorganization of the local matrix.…”