Doctor of Philosophy
School of Chemistry
Marshall, David, Uncovering the Energies and Reactivity of Aminoxyl Radicals by Mass Spectrometry, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2014. https://ro.uow.edu.au/theses/4239
Aminoxyl radicals (R1R2NO•) are a well-known class of stable free radicals and play a major role in polymer chemistry as both reagents for controlled polymerisation processes and polymer stabilisers. Both of these processes involve cycling between aminoxyl radical and alkoxyamine (R1R2NO–R3) forms. Despite their widespread industrial use, there is surprisingly little known about the intrinsic energetics and reactivity of these species that could inform our understanding of their behaviours in complex polymer matrices or reaction mixtures. In this thesis mass spectrometry is used to uncover these fundamental properties of aminoxyl radicals in the gas phase.
Alkoxyamines were prepared with an ionisable carboxylic acid moiety, remote from the NOR3 functional group. These precursors were subjected to electrospray ionisation to generate the corresponding gas phase [M – H]– anions. The effect of different radical fragments (•R3) on the competitive homolysis of O–C and N–O bonds was examined by collision-induced dissociation (Chapter 2). These results demonstrate that cleavage of the O–C bond is dominant for most of the R3-substituents investigated but examples of preferential N–O homolysis were observed where the O–C bond was strengthened by adjacent heteroatom(s) (e.g., R3 = CH2F). These experimental findings are supported by theoretical calculations, which confirm trends in relative bond dissociation energetics. Importantly, calculations also predict that O–C bond dissociation energies are lowered by the presence of the remote carboxylate anion. The corollary of this finding is that gas phase acidities (GPAs) of the corresponding carboxylic acid moieties are greater in the presence of aminoxyl radicals than structurally related, but closed-shell, alkoxyamines.
To test computational predictions, relative and absolute GPAs were measured experimentally by applying the kinetic method to proton-bound dimers containing alkoxyamines and aminoxyl radicals bearing carboxylic acid groups. The results confirm the decreased basicity of anions in the presence of aminoxyl radicals, and by extension, the increased stability of aminoxyl radicals in the presence of an ostensibly remote anion (Chapter 3). Further experiments were undertaken to elucidate the relationship between the magnitude of stabilisation and the nature of the charge-tag (e.g., carboxylates, sulfates, alkoxides) and the spatial separation between charge and radical moieties. These studies demonstrate that stabilisation of the radical can be measured at intramolecular separations of almost 8A (Chapter 4). The consequences for this discovery in the use of distonic radical anions as models of neutral radicals are evaluated (Chapter 7).
Encouraged by the selective release of carbon-centred radicals upon collisional activation of alkoxyamines, this moiety was incorporated onto peptide N-termini with the aim of photodissociative radical-directed structure elucidation (i.e., peptide sequencing). Upon isolation of desired ions in a linear ion trap mass spectrometer, homolysis of the oxygen-carbon was studied as a function of laser wavelength, charge state and peptide structure (Chapter 5). Finally, combined electron spin resonance spectroscopy and mass spectrometry methods were employed to study the degradation of piperidine-based aminoxyl radicals in solution. In the presence of hydroxyl radicals generated by irradiation of photocatalytic TiO2 suspensions, multiple products are identified. The elucidation of these reaction mechanisms by experiment and theory provide a rationale for the well-documented time-dependent decrease in efficacy of piperidine stabilisers in polymer coatings.