Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

The reactivity of carbon-centered distonic radical ions has been of interest for decades. The existence of distonic radical ions was first postulated by Gross and McLafferty in the early 1970’s.1,2 In 1978, Bouma, MacLeod and Radom reported experimental results that supported theoretical predictions of the existence of a stable ring-opened ethylene oxide distonic radical cation.3 Ions with spatially separated charge and radical sites were coined as “distonic ions” by Yates, Bouma, and Radom4 in 1984. They later refined this definition5 to correspond to radical ions generated by ionization of a zwitterion, ylide, or diradical. Eberlin and co-workers later introduced the term “distonoid”, meaning distonic like, to encompass any radical ion that displays distonic “character” (i.e., ions with a high degree of discrete (non-mandatory) charge-spin separation) and is over-looked as a result of the strict distonic ion definition.6 However, the “distonoid” classification is not commonly used by the scientific community. Currently, the term “distonic” is widely accepted and used to denote ions with formally separated charge and radical sites even if they do not fall into the formal definition.7 According to the conventional valence bond description, the charge and radical sites are on adjacent atoms in α-distonic ions while they are separated by one and two atoms in β- and γ-distonic ions, respectively. A vast amount of experimental and theoretical studies were dedicated to distonic ions from the 1980’s to 1990’s, which have been previously reviewed separately by Hammerum and Kenttamaa.8,9,10 The focus of this review is on gaseous ions with one or more aryl radical sites, a subgroup of distonic radical cations. The interest in these distonic ions was initially sparked by the limited knowledge on the reactivity of neutral phenyl radicals and their diradical counterparts in spite of the vast amount of research dedicated to these reactive intermediates.11–70 Many such mono- and diradicals have been investigated as they are thought to play a vital role in numerous fields, including combustion,11–13 polymerization,14–16 atmospheric chemistry,17–19 interstellar chemistry,20 organic synthesis8 and the biological activity of certain drugs.21–33 In the 1990’s, the formation of such aromatic diradicals in naturally occurring anti-tumor antibiotics was associated with their DNA-cleaving ability.21–33 The two radical sites are thought to abstract a hydrogen atom from each strand of double stranded DNA, thus causing irreversible DNA cleavage. Since then, theoretical and experimental research on aryl mono- and diradicals has boomed. An area of special interest has been the mechanistic understanding of hydrogen atom abstraction by these radicals from small organic and biological molecules both in solution34–45 and in the gas phase.46–70 However, the ability to predict the rates of such seemingly simple reactions has proven challenging due to a poor understanding of the nature of the transition states for these reactions. Further, the examination of the chemical properties of neutral radicals is a challenge due to the difficulty to cleanly generate them both in solution and in the gas phase. In order to address the above difficulties, studies were carried out in the early 1990’s on distonic radical cations’ ion-molecule reactions inside mass spectrometers as ions can be easily manipulated in this environment.46–70 Distonic radical cations that have a phenyl radical site spatially separated from a chemically inert charge site were found to almost exclusively undergo radical reactions at the radical site(s) in the gas phase46–70 and lately also in solution.45 Hence, examination of these distonic ions will provide information on the properties of phenyl mono- and diradicals. A special benefit of using mass spectrometry to study above species is that the desired charged radical can be isolated before examining its reactivity. Hence, the precursors to any products formed in these gas-phase experiments are known, which is not always true for solution experiments wherein highly reactive molecules cannot be isolated. The chemical properties of many aryl mono-, di- and triradicals have been successfully examined in mass spectrometers by using this ‘distonic ion approach’.49,50,52 The results obtained in these studies provide valuable information on the relative reactivities of mono- and polyradicals, which would otherwise not be available. This paper reviews the current knowledge of the properties and reactivity of distonic radical ions with aryl radical sites and the mechanisms of these reactions. Distonic phenyl radical ions generated within peptides are not included due to space limitations and also since these radicals are usually generated as precursors to less reactive nonaromatic peptide radicals that are the true interest of the researchers. However, this is an important and exciting new field of distonic ion research that should be reviewed separately.