Doctor of Philosophy
School of Chemistry
Kirk, Benjamin B., Investigating the reactions of gas-phase radicals using distonic ions, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3374
Free radicals are involved as reactive intermediates in a diverse range of processes including hydrocarbon combustion, the formation of photochemical smog, lipid perox- idation, cell apoptosis, polymerisation and synthetic cyclisation reactions. The intrinsic reactivity of free radicals and the inherent difficulty involved in their controlled gener- ation and isolation has led the experimentalist to develop ever more complex methods of determining their ultimate fate in these reactions. A systematic mass spectrometric and computational study of archetypal charge-tagged phenyl and cyclohexyl radicals was undertaken to elucidate the reactivity of these radicals towards O2. Central to this study was the modification of a commercial linear ion-trap mass spectrometer which facilitated (i) injection of neutral gases and liquid reagents into the buffer gas of the instrument and (ii) introduction of a laser pulse through the ions isolated within the ion-trap. These modifications were essential to allowing measurement of ion-molecule reactions which were used to probe the structure and reactivity of radical ions isolated within the ion-trap. In addition, photolysis experiments may now be undertaken using this instrument to probe isolated species or generate free radicals within the ion-trap by photodissociation of photoactive precursors.
Distonic phenyl and cyclohexyl radical ions with carboxylato anion and N,N,N- trimethylammonium cation motifs were synthesised using ion-trap mass spectrom- etry. The reactions of N,N,N-trimethylammoniumphenyl radical cation and 4- carboxylatophenyl radical anion with O2 were found to proceed through formation of a peroxyl radical intermediate. In the cationic case, the phenylperoxyl radical was gen- erated in sufficient quantity for subsequent isolation. The major degradation product in both cases was the phenoxyl radical, which differs significantly from long-standing mechanistic and computational predictions which suggest loss of CO2 to generate cyclopentadienyl radical as the major reaction product of this reaction. Collision in- duced dissociation of the isolated phenylperoxyl radical demonstrated that both O and CHO· may be lost directly from the phenylperoxyl radical, with a computational study suggesting that loss ofCHO· may be competitivewith CO2 loss at ambient temperatures.
A combined computational and experimental study suggests that the 4- carboxylatocyclohexylperoxyl radical anion degrades by loss of HO·2 and HO· to form alkenes and isomeric epoxides mirroring previous investigations of the neutral archetype. Alternative pathways leading to formation of cyclohexanone or ring-opened isomers were not observed. Subsequent isolation of the nascent cyclohexylperoxyl radical in the presence of O2 resulted in no reaction, suggesting that the radical does not isomerise to a hydroperoxycyclohexyl radical as has previously been suggested.
The reaction of the isolated 4-carboxylatocyclohexylperoxyl radical anion and N,N,N-trimethylammoniumphenylperoxyl radicals with NO· and NO·2 was also inves- tigated. Allowing the distonic cyclohexylperoxyl radical to react with NO· resulted in no reaction, while its reaction with NO·2 was not possible as the dicarboxylate precursor was found to be depleted by electron transfer to form NO–2. In contrast, reaction of the isolated phenylperoxyl radical with NO· and NO·2 resulted in products consistent with ejection of NO·2 and NO·3, respectively, as previously reported in the literature.
While these results highlight the strengths of using a distonic radical ion approach to investigating radical reactions and their ability to model the reactivity of their neutral counterparts, limitations were also identified which may restrict their utility. This may include (i) the complicated nature of multiple neutral reagents in the ion-trap which leads to competing reactions with the isolated radicals. This may limit the investigation of step-wise reactions which require different neutral reagents. (ii) The fixed pressure of the ion-trap which may limit the collisional cooling of reactive or fragile product ions leading to their absence or low ion-count in a given spectrum. (iii) Destabilisation or degradation of reactive species such as radicals due to the isolation process which was found to impart significant energy on the isolated species, and (iv) the limited structural information provided when a product is in low abundance or collision induced dissociation fails due to the inherent structural stability of the ion resulting in ejection of the ion prior to fragmentation.