title: 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.
authors.affiliation: Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4, 8010 Graz, Austria. grampp@ptc.tugraz.at
authors.forename: Günter
authors.initials: G
authors.lastname: Grampp
authors.forename: Birgit
authors.initials: B
authors.lastname: Grossmann
authors.forename: Jürgen
authors.initials: J
authors.lastname: Heinze
authors.forename: Stephan
authors.initials: S
authors.lastname: Landgraf
authors.forename: Kenneth
authors.initials: K
authors.lastname: Rasmussen