School of Chemistry - Research Publications
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ItemEnvironmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals(AMER CHEMICAL SOC, 2017-07-20)A novel precursor to the distonic O- and C-centered radical cations Oxo+O• and Oxo+C• was designed and synthesized, which represents model systems for radicals produced during polyester degradation. The precursor is equipped with a nitrate functional group, which serves as a masked site for these alkoxyl and carbon radicals that are unleashed through collision-induced dissociation (CID). Oxo+O• and Oxo+C• feature a cyclic carboxonium ion as permanent charge tag to enable monitoring their ion-molecule reactions on the millisecond to second time scale in the ion trap of the mass spectrometer. The reactions of Oxo+O• and Oxo+C• with cyclohexene, cyclohexadiene, ethyl acetate, 1,1-dimethoxyethane, and 1,2-dimethoxyethane, which exhibit structural features present in both intact and defective polyesters, were explored through product and kinetic studies to identify "hot spots" for radical-induced damage in polyesters. All reactions with Oxo+O• were extremely fast and proceeded predominantly through HAT. Oxo+C• was about two orders of magnitude less reactive and did not noticeably damage aliphatic ester moieties through hydrogen abstraction on the time scale of our experiments. Radical addition to alkene π systems was identified as an important pathway for C-radicals, which needs to be included in polymer degradation mechanisms.
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ItemReaction of Distonic Aryl and Alkyl Radical Cations with Amines: The Role of Charge and Spin Revealed by Mass Spectrometry, Kinetic Studies, and DFT Calculations(Wiley, 2020-01-01)Gas‐phase reaction of the aromatic distonic radical cations 4‐Pyr+. and 3‐Pyr+. with amines led to formation of the corresponding amino pyridinium ions 4‐Pyr+NR2 and 3‐Pyr+NR2 through amine addition at the site of the radical, followed by homolytic β‐fragmentation. The reaction efficiencies range from 66–100 % for 4‐Pyr+. and 57–86 % for 3‐Pyr+., respectively, indicating practically collision‐controlled processes in some cases. Computational studies revealed that the combination of positive charge and spin makes nucleophilic attack by the amine at the site of the radical barrierless and strongly exothermic by about 175±15 kJ mol−1, thereby rendering ‘conventional’ radical pathways through hydrogen abstraction or addition onto π systems less important. Exemplary studies with 4‐Pyr+. showed that this reaction can be reproduced in solution. A similar addition/radical fragmentation sequence occurs also in the gas‐phase reaction of amines with the aliphatic distonic radical cation Oxo+C., showing that the observed charge‐spin synergism is not limited to aromatic systems.