Electron‐Self‐Exchange Kinetics of the Cyclooctatetraene/Cyclooctatetraene Radical‐Anion Couple

Abstract

Rate constants k(hom) and k(het) are reported for the homogeneous electron-self-exchange and the heterogeneous electrochemical electron-transfer reactions, respectively, of the cyclooctatetraene/cyclooctatetraene(-) (COT/COT(.-)) redox couple. In acetonitrile, the values k(hom) (298 K)=(5+/-3) x 10(5) M(-1) s(-1) and k(het) (295 K)=8 x 10(-3) cm s(-1) are found, whereas slightly faster rates are obtained in dimethylformamide, namely, k(hom) (298 K)=(1.6+/-0.6) x 10(6) M(-1) s(-1) and k(het) (295 K)=2 x 10(-2) cm s(-1). The k(hom) rates are obtained from electron spin resonance (ESR) line broadening whereas the k(het) rates are measured at a mercurized Pt electrode by using Nicolson's method. The slowness of both electron-transfer reactions is caused by the high inner-sphere reorganization energy that results from the inevitable conformational change that takes place upon going from the tub-like COT molecule to the planar COT(.-) anion. The rates are well-understood in terms of Marcus theory, including an additional medium inner-sphere mode which is responsible for the flattening of COT.

Experimental comparison of the Butler–Volmer and Marcus–Hush–Chidsey formalisms of electrode kinetics: The reduction of cyclooctatetraene at mercury hemispherical electrodes via cyclic and square wave voltammetries

“…This is known to be a quasireversible process with small value of the transfer coefficient, a, and purely diffusional behavior [1][2][3][4][5][6][7]. The reduction of COT at mercury electrodes has been previously studied in dimethylformamide (DMF) [3] and acetonitrile [6] solvents (see Table 1).…”

“…The electrochemical behavior of the reduction of cyclooctatetraene (COT) and its derivatives in aprotic solvents has been often studied [1][2][3][4][5][6][7] with the primary aim of explaining the unusual slowness of the first reduction of the neutral species to yield the corresponding radical anion:…”

Electron spin resonance

“…The rates are readily interpreted in terms of Marcus theory, including an additional medium inner-sphere mode which is responsible for the flattening of COT. 115 A spin-probe study has been made of whether the ESR changes that occur at the freezing-point of water reflect local or global phenomena. Essentially, at the freezing-point, the narrow triplet spectrum collapses into a single broad line.…”

Investigation of solvent dynamic effects on the electron self-exchange in two thianthrene couples with large inner reorganization energies

“…In contrast, the self-exchange of cyclooctatetraene (COT) and its radical anion, where the neutral molecule transforms from a tub-shaped to a planar conformation, has been studied. 10 ESR line broadening experiments show that the extraordinarily high inner-sphere reorganization energy of 158 kJ mol À1 at 298 K causes the slow rate constant of Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9/I, A-8010 Graz, Austria. E-mail: p.choto@student.tugraz.at k et = 5 AE 3 Â 10 5 M À1 s À1 in acetonitrile, which is three orders of magnitude lower than those of the systems mentioned above.…”