title: Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP.
abstract: The photoreactions of diazabicyclo[2,2,2]octane (DABCO) and triisopropylamine (TIPA) with the sensitizers anthraquinone (AQ) and xanthone (XA) or benzophenone (BP) were investigated by time-resolved photo-CIDNP (photochemically induced dynamic nuclear polarization) experiments. By varying the radical-pair concentration, it was ensured that these measurements respond only to self-exchange reactions of the free amine-derived radicals (radical cations DH (•) (+) or α-amino alkyl radicals D (•) ) with the parent amine DH; the acid-base equilibrium between DH (•) (+) and D (•) also plays no role. Although the sensitizer does not at all participate in the observed processes, it has a pronounced influence on the CIDNP kinetics because the reaction occurs through successive radical pairs. With AQ, the polarizations stem from the initially formed radical-ion pairs, and escaping DH (•) (+) then undergoes electron self-exchange with DH. In the reaction sensitized with XA (or BP), the polarizations arise in a secondary pair of neutral radicals that is rapidly produced by in-cage proton transfer, and the CIDNP kinetics are due to hydrogen self-exchange between escaping D (•) and DH. For TIPA, the activation parameters of both self-exchange reactions were determined. Outer-sphere reorganization energies obtained with the Marcus theory gave very good agreement between experimental and calculated values of ∆G (‡) 298.
authors.affiliation: Institut für Chemie, Martin-Luther-Universität Halle-Wittenberg, Kurt-Mothes-Str. 2, 06120 Halle/Saale, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Goez
authors.forename: Isabell
authors.initials: I
authors.lastname: Frisch
authors.forename: Ingo
authors.initials: I
authors.lastname: Sartorius
keywords: CIDNP; amines; electron transfer; free radicals; hydrogen transfer; ketones; kinetics; photochemistry; self-exchange