The compound quadricyclo(2.2.1.0 2,6 .0 3,5 )heptane, more commonly referred to as quadricyclane (QC), has enjoyed a rich history of chemical investigation. From its initial discovery in 1936, 1 it has been considered for use as a solar energy storage medium, 2 an optical computer memory system, 3 and most recently, and perhaps most realistically, as a high energy aviation fuel. 4 Although several studies have been conducted concerning the nonaqueous chemistry of QC, including its valence isomerization to norbornadiene, [5][6][7][8][9] little is known about the chemistry that might be expected should QC be released into the (aqueous) environment. 10 The study of QC and its isomers by electrochemical means allows for careful assessment of the relevant redox chemistry in a timely fashion, as these processes occur very slowly under typical environmental conditions. The issue of release into the environment is of major concern given the possibility of its use as an aviation fuel additive. Toward this end, we report here a study of the aqueous redox chemistry of QC and nortricyclanol, a hydrolysis product that is readily formed when QC is present in aqueous solutions.QC is a seven-membered carbon quadricycle containing two cyclopropyl, one cyclobutyl, and two cyclopentyl rings (Scheme 1). The compound is a stable liquid at room temperature with a fairly high vapor pressure under normal conditions (bp 108ЊC at 740 mm Hg). The strain energy has been reported as approximately 340 kJ mol Ϫ1 . 11 The unusual reactivity of the system is due to this ring strain and generally results in cleavage of one or both of the C 2 -C 6 and C 3 -C 5 cyclopropyl bonds. QC readily undergoes photochemically, 12 electrochemically, 13 and chemically 14 initiated conversion to its lower energy valence isomer, norbornadiene (NB).There is a relatively high thermal barrier for the conversion of QC to NB (385 kJ mol Ϫ1 ), 15 which can be overcome with irradiation in the presence of a sensitizer. The conversion of QC to NB is achieved easily through a free-radical chain reaction. The initiation of this chain reaction is a one-electron oxidation to form the QC ϩ• radical cation, which quickly rearranges to the NB ϩ• radical cation. This species forms NB by oxidation of an additional QC molecule, in a step that generates another QC ϩ• radical cation, thus continuing the propagation. 15 Until recently, the structures and number of intermediates in this conversion were unknown. Through the use of chemically induced dynamic nuclear polarization (CIDNP) experiments, 16 Roth and Schilling were able to verify the existence of both the elusive QC ϩ• and the longer-lived NB ϩ• cation radicals. Very rapid isomerization of QC ϩ• to NB ϩ• precluded the direct spectroscopic observation of QC ϩ• until time-resolved electron spin resonance spectroscopy 17 was enlisted in the search. Turro et al. established the lifetime of QC ϩ• to be ϳ10 Ϫ6 s (with a thermal barrier of 38 kJ mol Ϫ1 for conversion to NB ϩ• ) 15 and also used laser flash photolysis and time-resol...
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