Environmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals

Abstract

A novel precursor to the distonic O- and C-centered radical cations OxoO and OxoC 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). OxoO and OxoC 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 OxoO and OxoC 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 OxoO were extremely fast and proceeded predominantly through HAT. OxoC 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.

Reaction of Distonic Aryl and Alkyl Radical Cations with Amines: The Role of Charge and Spin Revealed by Mass Spectrometry, Kinetic Studies, and DFT Calculations

“…We recently developed glycerol derivative 10 as precursor for Oxo + C . ( m / z 148), which is obtained through sequential CID via the cyclic tertiary carboxonium ion 11 , followed by homolytic scission of the O−NO 2 bond and loss of formaldehyde (Scheme c) . According to DFT calculations (see below for details) the spin density in these distonic radical cations is predominantly (ca.…”

“…We believe that the latter results from ring‐opening of Oxo + C . to 20 , followed by fragmentation into the acylium ion 21 , which is subsequently trapped by the amine (Scheme ). This assignment could be confirmed by CID on isolated 18 a in the ion trap, which revealed fragmentation into 21 .…”

“…+amine→products, were obtained under quasi‐thermal conditions by monitoring the decay of the Pyr +. signal at m / z 93 ([M] .+ ) as a function of reaction time under pseudo‐first order conditions . The rate coefficients for product formation, i. e ., Pyr + H, the adducts Pyr + −NR 2 as well as 16 , 17 and amine oxidation products were obtained from kinetic modelling of the experimental concentration‐time profiles using the program Dynafit 4 .…”

“…To better understand the interplay between charge and unpaired electron in these reactions, DFT calculations were performed at the M062X/6‐31+G* level of theory, which has been previously shown to provide sufficiently accurate energies to identify the most likely reaction pathways . The energies of the gas‐phase calculations are given as enthalpies, as they best describe chemical reactivity under low‐pressure bimolecular conditions, whereas for the reactions in solution (see Section 2.5) free energies are considered.…”