Ganna Gryn'ova

Computational Design of Cyclic Nitroxides as Efficient Redox Mediators for Dye‐Sensitized Solar Cells

Cyclic nitroxide radicals represent promising alternatives to the iodine-based redox mediator commonly used in dye-sensitized solar cells (DSSCs). To date DSSCs with nitroxide-based redox mediators have achieved energy conversion efficiencies of just over 5 % but efficiencies of over 15 % might be achievable, given an appropriate mediator. The efficacy of the mediator depends upon two main factors: it must reversibly undergo one-electron oxidation and it must possess an oxidation potential in a range of 0.600-0.850 V (vs. a standard hydrogen electrode (SHE) in acetonitrile at 25 °C). Herein, we have examined the effect that structural modifications have on the value of the oxidation potential of cyclic nitroxides as well as the reversibility of the oxidation process. These included alterations to the N-containing skeleton (pyrrolidine, piperidine, isoindoline, azaphenalene, etc.), as well as the introduction of different substituents (alkyl-, methoxy-, amino-, carboxy-, etc.) to the ring. Standard oxidation potentials were calculated using high-level ab initio methodology that was demonstrated to be very accurate (with a mean absolute deviation from experimental values of only 16 mV). An optimal value of 1.45 for the electrostatic scaling factor for UAKS radii in acetonitrile solution was obtained. Established trends in the values of oxidation potentials were used to guide molecular design of stable nitroxides with desired E(ox)°, and a number of compounds were suggested for potential use as enhanced redox mediators in DSSCs.

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

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.

Theory And Practice Of Uncommon Molecular Electronic Configurations

The electronic configuration of the molecule is the foundation of its structure and reactivity. The spin state is one of the key characteristics arising from the ordering of electrons within the molecules set of orbitals. Organic molecules that have open‐shell ground states and interesting physicochemical properties, particularly those influencing their spin alignment, are of immense interest within the up‐and‐coming field of molecular electronics. In this advanced review, we scrutinize various qualitative rules of orbital occupation and spin alignment, viz., the aufbau principle, Hunds multiplicity rule, and dynamic spin polarization concept, through the prism of quantum mechanics. While such rules hold in selected simple cases, in general the spin state of a system depends on a combination of electronic factors that include Coulomb and Pauli repulsion, nuclear attraction, kinetic energy, orbital relaxation, and static correlation. A number of fascinating chemical systems with spin states that fluctuate between triplet and open‐shell singlet, and are responsive to irradiation, pH, and other external stimuli, are highlighted. In addition, we outline a range of organic molecules with intriguing non‐aufbau orbital configurations. In such quasi‐closed‐shell systems, the singly occupied molecular orbital (SOMO) is energetically lower than one or more doubly occupied orbitals. As a result, the SOMO is not affected by electron attachment to or removal from the molecule, and the products of such redox processes are polyradicals. These peculiar species possess attractive conductive and magnetic properties, and a number of them that have already been developed into molecular electronics applications are highlighted in this review. WIREs Comput Mol Sci 2015, 5:440–459. doi: 10.1002/wcms.1233

Which side-reactions compromise nitroxide mediated polymerization?

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.

Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions

Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds--based on 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)--with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONE via an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered.

Entropy-Driven Selectivity for Chain Scission: Where Macromolecules Cleave

We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high-temperature (HT) NMR spectroscopy of self-assembled supramolecular diblock systems as well as temperature-dependent size-exclusion chromatography (TD SEC) of covalently bonded Diels-Alder step-growth polymers.

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

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.

Mechanisms and energetics of potassium channel block by local anesthetics and antifungal agents.

Many drug molecules inhibit the conduction of several families of cation channels by binding to a small cavity just below the selectivity filter of the channel protein. The exact mechanisms governing drug-channel binding and the subsequent inhibition of conduction are not well understood. Here the inhibition of two K(+) channel isoforms, Kv1.2 and KCa3.1, by two drug molecules, lidocaine and TRAM-34, is examined in atomic detail using molecular dynamics simulations. A conserved valine-alanine-valine motif in the inner cavity is found to be crucial for drug binding in both channels, consistent with previous studies of similar systems. Potential of mean force calculations show that lidocaine in its charged form creates an energy barrier of ∼6 kT for a permeating K(+) ion when the ion is crossing over the drug, while the neutral form of lidocaine has no significant effect on the energetics of ion permeation. On the other hand, TRAM-34 in the neutral form is able to create a large energy barrier of ∼10 kT by causing the permeating ion to dehydrate. Our results suggest that TRAM-34 analogues that remain neutral and permeable to membranes under acidic conditions common to inflammation may act as possible drug scaffolds for combating local anesthetic failure in inflammation.

Chemical shift and coupling constant analysis of dibenzyloxy disulfides

Dialkoxy disulfides have found applications in the realm of organic synthesis as an S2 or alkoxy donor, under thermal and photolytic decompositions conditions, respectively. Spectrally, dibenzyloxy disulfides possess an ABq in the (1)H NMR, which can shift by over 1.1ppm depending on the substituents present on the aromatic ring, as well as the solvent employed. The effect of the said substituents and solvent were analyzed and compared to the center of the ABq, geminal coupling, and the differences in chemical shifts of the individual doublets. Additionally, quantum-chemical calculations demonstrated the intramolecular H-bonding arrangement, found within the dibenzyloxy disulfides.

Use of spin trap technique for kinetic investigation of elementary steps of RAFT-Polymerizaton

© 2012 Filippov et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Use of Spin Trap Technique for Kinetic Investigation of Elementary Steps of RAFT-Polymerization