Effect of Chemical Structure on the Electrochemical Cleavage of Alkoxyamines

A test set of 14 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-based alkoxyamines was studied via a combination of cyclic voltammetry and accurate quantum chemistry to assess the effect of substituents on electrochemical cleavage. The experimental oxidation potentials of alkoxyamines falling into the range of 1.1-1.6 V versus Ag/AgCl in acetonitrile, were well reproduced by theory (MAD 0.04 V), with values showing good correlation with the σR Hammett parameters of both the R-group and the OR-group in TEMPO-R. Importantly, most of the studied alkoxyamines underwent oxidative cleavage to form either TEMPO· and R+ or TEMPO+ and R·, with the former favored by electron-donating substituents on R (e.g., 2-oxolane, Ac, CH(CH3)Ph, i-Pr, t-Bu) and the latter by electron withdrawing substituents (Bn, allyl, CH(CH3)C(O)OCH3, C(CH3)2C(O)OCH3, CH(CH3)CN). Where R is not stabilized (e.g., R = CH2C(O)OCH3, Me, Et), fully or almost fully reversible oxidation - without cleavage - was observed, making these species promising candidates for battery applications. Finally, in the case of R = Ph, where N-O cleavage occurred, a phenoxy cation and an aminyl radical were generated. On the basis of these results, TEMPO-based alkoxyamines can provide a variety of electrochemically generated carbon-centered radicals and carbocations for use in synthesis, polymerization, and surface modification

Mechanism of Oxidative Alkoxyamine Cleavage: The Surprising Role of the Solvent and Supporting Electrolyte

In this work, we show that the nature of the supporting electrolyte and solvent can dramatically alter the outcome of the electrochemically mediated cleavage of alkoxyamines. A combination of cyclic voltammetry experiments and quantum chemistry is used to study the oxidation behavior of TEMPO-i-Pr under different conditions. In dichloromethane, using a noncoordinating electrolyte (TBAPF6), TEMPO-i-Pr undergoes reversible oxidation, which indicates that the intermediate radical cation is stable toward mesolytic fragmentation. In contrast, in tetrahydrofuran with the same electrolyte, oxidized TEMPO-i-Pr undergoes a rapid and irreversible fragmentation. In nitromethane and acetonitrile, partially irreversible oxidation is observed, indicating that fragmentation is much slower. Likewise, alkoxyamine oxidation in the presence of more strongly coordinating supporting electrolyte anions (BF4-, ClO4-, OTf-, HSO4-, NO3-) is also irreversible. These observations can be explained in terms of solvent- or electrolyte-mediated SN2 pathways and indicate that oxidative alkoxyamine cleavage can be "activated" by introducing coordinating solvents or electrolytes or be "inhibited" through the use of noncoordinating solvents and electrolytes

Identification of reduced host transcriptomic signatures for tuberculosis disease and digital PCR-based validation and quantification

Recently, host whole blood gene expression signatures have been identified for diagnosis of tuberculosis (TB). Absolute quantification of the concentrations of signature transcripts in blood have not been reported, but would facilitate diagnostic test development. To identify minimal transcript signatures, we applied a transcript selection procedure to microarray data from African adults comprising 536 patients with TB, other diseases (OD) and latent TB (LTBI), divided into training and test sets. Signatures were further investigated using reverse transcriptase (RT)-digital PCR (dPCR). A four-transcript signature (GBP6, TMCC1, PRDM1, and ARG1) measured using RT-dPCR distinguished TB patients from those with OD (area under the curve (AUC) 93.8% (CI95% 82.2-100%). A three-transcript signature (FCGR1A, ZNF296, and C1QB) differentiated TB from LTBI (AUC 97.3%, CI95%: 93.3-100%), regardless of HIV. These signatures have been validated across platforms and across samples offering strong, quantitative support for their use as diagnostic biomarkers for TB.Immunogenetics and cellular immunology of bacterial infectious disease

Identification of Reduced Host Transcriptomic Signatures for Tuberculosis Disease and Digital PCR-Based Validation and Quantification.

Recently, host whole blood gene expression signatures have been identified for diagnosis of tuberculosis (TB). Absolute quantification of the concentrations of signature transcripts in blood have not been reported, but would facilitate diagnostic test development. To identify minimal transcript signatures, we applied a transcript selection procedure to microarray data from African adults comprising 536 patients with TB, other diseases (OD) and latent TB (LTBI), divided into training and test sets. Signatures were further investigated using reverse transcriptase (RT)-digital PCR (dPCR). A four-transcript signature (GBP6, TMCC1, PRDM1, and ARG1) measured using RT-dPCR distinguished TB patients from those with OD (area under the curve (AUC) 93.8% (CI95% 82.2-100%). A three-transcript signature (FCGR1A, ZNF296, and C1QB) differentiated TB from LTBI (AUC 97.3%, CI95%: 93.3-100%), regardless of HIV. These signatures have been validated across platforms and across samples offering strong, quantitative support for their use as diagnostic biomarkers for TB

Computer-aided design of a destabilized RAFT adduct radical: Toward improved RAFT agents for styrene-block-vinyl acetate copolymers

High-level ab initio molecular orbital calculations indicate that a fluorine Z substituent significantly destabilizes the RAFT adduct radical, R'SO•(Z)SR, relative to known Z substituents. This destabilization of the RAFT adduct radical lowers the fragmentation enthalpy relative to normal dithioesters, but without stabilizing the C=S bond of the product RAFT agent, as in xanthate- or dithiocarbamate-mediated polymerization. On the basis of these calculations, it is predicted that, provided appropriate R groups are chosen, RAFT agents containing fluorine Z substituents (i.e., S=C(F)SR, fluorodithioformates, or "F-RAFT" agents) should provide a basis for improved control of monomers with reactive propagating radicals (such as vinyl acetate) and should have the advantage that their C=S bonds remain reactive enough for control of monomers with more stable propagating radicals (such as styrene) and hence the production of styrene-vinyl acetate copolymers

Effect of substituants on radical stability in reversible addition fragmentation chain transfer polymerization: An ab initio study

The effects of the R- and Z-substituents on radical stability in the reversible addition fragmentation chain transfer (RAFT) polymerization process have been studied via high level ab initio molecular orbital calculations. Radical stabilization energies (RSEs) of the RAFT-adduct radicals CH3SC•ZSR and corresponding leaving group radicals R• have been calculated for various combinations of Z = H, Cl, CCH, CHCH2, CN, CF3, NH2, CH3, Ph, Bz, naphthyl, OCH3, OCH2CH3, OCH(CH3)2, and OC(CH3)3 and R = CH2CN, C(CH3)2CN, Bz, CH(Ph)CH3, C(Ph)(CH3)2, CH2COOCH3, CH(COOCH3)CH3, CH2OCOCH3, and CH2CH3. The results were used in combination with the corresponding values of the enthalpies of the fragmentation reactions, CH3SC•(Z)SR → CH3SC(Z)S + •R and CH3SC•(Z)SR → •CH3 + SC(Z)SR, to examine the effects of the substituents on the stability of both the RAFT-adduct radicals and the corresponding thiocarbonyl compounds. The RAFT-adduct radicals are stabilized by electron donation from the two sulfur substituents, and this stability can be further enhanced by unsaturated π-accepting substituents (such as CN, phenyl, and naphthyl). In contrast, lone pair donor Z-substituents (such as Cl, NH2, and OCH3) have a much smaller effect on radical stability. The R-group, which can modify the donation ability of the SR-group, has a minimal effect on the stability of the RAFT-adduct as it is buffered by the second sulfur substituent. However, these orbital interactions do affect the strength of the breaking S−R bond, and this provides an important contribution to the trends in the fragmentation enthalpies. Steric effects on radical stability are also important, with bulky R- and Z-groups inducing conformational changes that interfere with these orbital interactions, sometimes with unexpected consequences. The substituent effects on the RAFT agents are qualitatively different; the agents are strongly stabilized by the lone pair donor Z-substituents and strongly destabilized by electron withdrawing groups (such as CN and CF3) in the R- and Z-positions. Moreover, steric effects are generally more significant, with bulky R- and Z-groups destabilizing the RAFT agent more than the corresponding RAFT-adduct radicals. As part of this work, the accuracy of the low-cost RMP2/6-311+G(3df,2p) method for studying addition−fragmentation processes in RAFT polymerization was evaluated

Electrostatics and Electrochemistry: Mechanism and Scope of Charge-Transfer Reactions on the Surface of Tribocharged Insulators

The phenomenon of surface electrification upon contact is a long-standing scientific puzzle, with for instance written accounts of charged samples of amber attracting feathers dating back to the 600 B.C. Electrostatic hazards associated with electrical insulators subject to mechanical friction are well documented, and the design of commercial products, such as copiers and laser printers, is based on the static charging of electrical insulators. Nonetheless, the physical-chemical origin of this phenomenon remains debated. This Perspective outlines recent advances in our understanding of the mechanism behind contact electrification, as well as the emerging research area of electrochemistry on insulators. Research is beginning to demonstrate how to exploit static charges present on insulating surfaces, with the goal of driving redox reactivity. These studies have helped to clarify the triboelectrification mechanism and have defined new platforms for electrochemiluminescence, metal nucleation, and mask-free lithography. This Perspective will help researchers working within electrochemistry, physics, green energy, sensing, and materials to gain an understanding of the implications of contact electrification to their respective fields. Special attention is given to the chemical, electronic, and mechanical factors influencing triboelectrochemical reactions, concluding with the perceived challenges facing further development of this field

Static Electrification of Plastics under Friction: The Position of Engineering-Grade Polyethylene Terephthalate in the Triboelectric Series

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim There is an emerging trend to replace moving metallic parts, such as bearings or bushes, with plastic components. The electrostatic hazard associated with plastic components subject to mechanical friction is well documented, but the magnitude as well as physical–chemical origin of this phenomenon remains debated. Using atomic force microscopy and Faraday pail measurements, the triboelectrification of Ertalyte®, a commonly used bearing-grade formulation of polyethylene terephthalate, when rubbed against other polymers and metals, is studied. The sign and magnitude of the net charge that Ertalyte® gains in relation to the chemical nature—electron affinity and ionization energy—of the contacting material are analyzed, concluding that this material should be located toward the negative end of the triboelectric series. It is also shown that large charge densities and fast charge decays result from contact of Ertalyte® with polymers of a small Derjaguin–Muller–Toporov (DMT) modulus and unstable ions, suggesting that ion transfer leads to the electrification of a dynamic insulator/insulator contact. These findings have immediate implications in the choice of the material used to manufacture plastic parts subject to friction and wear and to help address ongoing fundamental questions over the nature of the charge carriers that leads to static electricity

Comparison of the kinetics and thermodynamics for methyl radical addition to C=C, C=O and C=S double bonds

The barriers, enthalpies, and rate constants for the addition of methyl radical to the double bonds of a selection of alkene, carbonyl, and thiocarbonyl species (CH2=Z, CH3CH=Z, and (CH 3)2C=Z, where Z = CH2, O, or S) and for the reverse β-scission reactions have been investigated using high-level ab inito calculations. The results are rationalized with the aid of the curve-crossing model. The addition reactions proceed via early transition structures in all cases. The barriers for addition of methyl radical to C=C bonds are largely determined by the reaction exothermicities. Addition to the unsubstituted carbon center of C=C double bonds is favored over addition to the substituted carbon center, both kinetically (lower barriers) and thermodynamically (greater exothermicities). The barriers for addition to C=O bonds are influenced by both the reaction exothermicity and the singlet-triplet gap of the substrate. Addition to the carbon center is favored over addition to the oxygen, also both thermodynamically and kinetically. For the thiocarbonyl systems, addition to the carbon center is thermodynamically favored over addition to sulfur. However, in this case, the reaction is contrathermodynamic, addition to the sulfur center having a lower barrier due to spin density considerations. Entropic differences among corresponding addition and β-scission reactions are relatively minor, and the differences in reaction rates are thus dominated by differences in the respective reaction barriers