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Uncovering the Energies and Reactivity of Aminoxyl Radicals by Mass Spectrometry David Marshall University of Wollongong

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Uncovering the Energies and Reactivity of Aminoxyl Radicals by Mass Spectrometry David Marshall University of Wollongong University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 2014 Uncovering the Energies and Reactivity of Aminoxyl Radicals by Mass Spectrometry David Marshall University of Wollongong Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Uncovering the Energetics and Reactivity of Aminoxyl Radicals by Mass Spectrometry A thesis submitted in (partial) fulfilment of the requirements for the award of the Degree Doctor of Philosophy from University of Wollongong by David Lachlan Marshall BNanotechAdv (Hons I) School of Chemistry April 2014 DECLARATION I, David L. Marshall, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the School of Chemistry, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. David L. Marshall April 2014 i ACKNOWLDGEMENTS This PhD would not have been possible without the support of many people, and I am strongly indebted to each and every one of them. First and foremost, a big thankyou to my supervisors Prof. Stephen Blanksby and Dr. Philip Barker. Over the past 7 years, through undergraduate research, Honours and into a PhD, you have been a constant source of ideas, encouragement, and support. Phil, your passion for science is contagious, and hopefully I’ve learned a few Ninja skills along the way. Leaving Wollongong won’t feel right without your face greeting me from the side of a truck every day. Steve, thankyou for your expertise and leadership and always being generous with your time. I look forward to our partnership developing further at QUT. To Dr. Anya Gryn’ova and Prof. Michelle Coote from ANU. It has been fantastic to work in collaboration with you, and admire the quality of your research. To Dr. Adam Trevitt and Dr. Berwyck Poad in the School of Chemistry at UoW, thankyou for your guidance, many helpful discussions, and willingness to let me loose with your instrumentation. To everyone in the School of Chemistry – students, administration staff, technical staff, academics – thankyou for making Building 18 a safe, fun and enjoyable place to study. A special thanks to all members of the mass spectrometry and laser chemistry groups past and present for your friendship, support, shared frustrations, and readiness to share ideas. Al, Bart, Ben, Celine, Chris, Jen, Jo, Kimberley, Mahendra, Marty, Monica, Nicole, Phil, Rachel, Shane, and Tom – I wish you all every success. To my family. My gratitude to you extends well beyond the time of this PhD. For the past 28 years, you’ve always been there for me, encouraged me to do my best, and supported me in every way. I love you. This is for you. ii ABSTRACT • 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 3 strengthened by adjacent heteroatom(s) (e.g., R = 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 iii 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 8 Å (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. iv TABLE OF CONTENTS Acknowldgements ....................................................................................... ii Abstract ....................................................................................................... iii Table of Contents ........................................................................................ v List of Figures .............................................................................................. x List of Schemes ......................................................................................... xiv List of Tables ............................................................................................ xvi List of Abbreviations............................................................................... xvii 1. Introduction ........................................................................................... 1 1.1 Free Radicals ................................................................................. 2 1.2 Free Radicals are Everywhere ...................................................... 4 1.3 Formation of Free Radicals........................................................... 5 1.4 Radical Ions .................................................................................. 7 1.5 Aminoxyl Radicals ....................................................................... 9 1.5.1 Applications .......................................................................................... 12 1.5.2 Polymer Stabilisers ............................................................................... 12 1.5.3 Nitroxide-Mediated Polymerisation ...................................................... 15 1.6 Characterisation of Free Radicals ............................................... 17 1.6.1 Solid Phase ............................................................................................ 17 1.6.2 Solution Phase ....................................................................................... 18 1.6.3 Gas Phase .............................................................................................. 21 1.7 Radical Ions and Mass Spectrometry ......................................... 23 1.7.1 Ion Sources ............................................................................................ 24 1.7.1.1 Electron Ionisation................................................................................... 24 1.7.1.2 Chemical Ionisation ................................................................................. 26 1.7.1.3 Photoionisation ........................................................................................ 28 1.7.1.4 Electrospray Ionisation ............................................................................ 30 1.7.2 Mass Analysers ..................................................................................... 32 1.7.2.1 Time-of-Flight ........................................................................................
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