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Published year: 20134
Origin and Scope of Long-Range Stabilizing Interactions and Associated SOMO–HOMO Conversion in Distonic Radical Anions

Abstract (Expand)

High-level quantum-chemical methods have been used to study the scope and physical origin of the significant long-range stabilizing interactions between nonmutually conjugated anion and radical moieties … in SOMO–HOMO converted distonic radical anions. In such species, deprotonation of the acid fragment can stabilize the remote radical by tens of kilojoules, or, analogously, formation of a stable radical (by abstraction or homolytic cleavage reactions) increases the acidity of a remote acid by several pKa units. This stabilization can be broadly classified as a new type of polar effect that originates in Coloumbic interactions but, in contrast to standard polar effects, persists in radicals with no charge-separated (i.e., dipole) resonance contributors, is nondirectional, and hence of extremely broad scope. The stabilization upon deprotonation is largest when a highly delocalized radical is combined with an initially less stable anion (i.e., the conjugate base of a weaker acid), and is negligible for highly localized radicals and/or stable anions. The effect is largest in the gas phase and low-polarity solvents but is quenched in water, where the anion is sufficiently stabilized. These simple rules can be employed to design various switchable compounds able to reversibly release radicals in response to pH for use in, for example, organic synthesis or nitroxide-mediated polymerization. Moreover, given its wide chemical scope, this effect is likely to influence the protonation state of many biological substrates under radical attack and may contribute to enzyme catalysis.

Authors: Ganna Gryn’ova, Michelle L. Coote

Date Published: 3rd Oct 2013

Publication Type: Journal

Switching Radical Stability by pH-Induced Orbital Conversion

Abstract (Expand)

In most radicals the singly occupied molecular orbital (SOMO) is the highest-energy occupied molecular orbital (HOMO); however, in a small number of reported compounds this is not the case. In the … present work we expand significantly the scope of this phenomenon, known as SOMO–HOMO energy-level conversion, by showing that it occurs in virtually any distonic radical anion that contains a sufficiently stabilized radical (aminoxyl, peroxyl, aminyl) non-π-conjugated with a negative charge (carboxylate, phosphate, sulfate). Moreover, regular orbital order is restored on protonation of the anionic fragment, and hence the orbital configuration can be switched by pH. Most importantly, our theoretical and experimental results reveal a dramatically higher radical stability and proton acidity of such distonic radical anions. Changing radical stability by 3–4 orders of magnitude using pH-induced orbital conversion opens a variety of attractive industrial applications, including pH-switchable nitroxide-mediated polymerization, and it might be exploited in nature.

Authors: Ganna Gryn'ova, David L. Marshall, Stephen J. Blanksby, Michelle L. Coote

Date Published: 1st Jun 2013

Publication Type: Journal

Which Side-Reactions Compromise Nitroxide Mediated Polymerization?

Abstract (Expand)

The mechanism of the nitroxide mediated polymerization (NMP) is well understood, however less is known about the side-reactions that interfere and in certain cases severely compromise it. Experimental … studies inevitably involve model fitting leading to at times contradictory conclusions as to which elementary side-reactions are behind the failure of a given NMP system. In the present work we use high-level quantum-chemical calculations to obtain the rate coefficients of the various side-reactions, both suggested previously and considered here for the first time, and first principles PREDICI kinetic simulations to identify the most deleterious side-reactions involved in the TEMPO, SG1 and DPAIO mediated polymerization of styrene, acrylate and methacrylate monomers. We show that the core mechanism for the thermal decomposition of alkoxyamines differs between the uni- and polymeric species, which often makes such experiments not suitable for modelling the NMP conditions. We also find that the main side-reaction responsible for the failure of TEMPO and SG1 in methacrylate homopolymerization is an intramolecular alkoxyamine decomposition (often referred to as ‘disproportionation’) via a Cope-type elimination, however in the case of SG1 the polymerization outcome is additionally affected by the equilibrium constant of alkoxyamine bond homolysis. On the basis of these findings, complemented by a thorough analysis of available experimental data, we define guidelines for minimising occurrence of the side-reactions and thus improving NMP. Finally, the accurate first principles rate parameters reported in this study should prove useful for subsequent kinetic modelling oriented at optimising different polymerization conditions.

Authors: Ganna Gryn'ova, Ching Yeh Lin, Michelle L. Coote

Date Published: 2013

Publication Type: Journal

Computational Evaluation of the Sulfonyl Radical as a Universal Leaving Group for RAFT Polymerisation

Abstract (Expand)

The present study investigates the performance of the sulfonyl radical, i.e. •SO2Ph, as a universal leaving group in reversible addition–fragmentation chain-transfer (RAFT) polymerisation. The sulfonyl … radical is widely used as a radical initiator and has already been proved successful as a leaving group in an atom-transfer radical polymerisation. Our results, obtained using high-level ab initio computational methodology under relevant experimental conditions, indicate superior performance of the sulfonyl compared with a reference cyanoisopropyl group in controlling RAFT of a wide range of monomers. Importantly, the presence of sulfonyl chain ends in the polymers so formed opens attractive possibilities for further functionalisation. Potential synthetic routes to the R-sulfonyl RAFT agents are discussed.

Authors: Ganna Gryn'ova, Tamaz Guliashvili, Krzysztof Matyjaszewski, Michelle L. Coote

Date Published: 2013

Publication Type: Journal

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