On the Limited Stability of BDPA Radicals

Post-print (lokagerð höfundar)1,3‐Bis(diphenylene)‐2‐phenylallyl (BDPA)‐based radicals are of interest as polarizing agents for dynamic nuclear polarization (DNP). For this purpose, a BDPA‐nitroxide biradical, employing a phosphodiester linkage, was synthesized. Contrary to what is commonly assumed, BDPA‐derived radicals were observed to have limited stability. Hence, the effects of various factors on the stability of BDPA radicals were investigated. Solvent polarity was found to play a significant role on degradation; a polar BDPA radical was observed to degrade faster in a non‐polar solvent, whereas non‐polar radicals were more unstable in polar solvents. The rate of decomposition was found to increase non‐linearly with increasing radical concentration; a 2‐fold increase in concentration led to a 3‐fold increase in the rate of degradation. Collectively, these results indicate that the dimerization is a significant degradation pathway for BDPA radicals and indeed, a dimer of one BDPA radical was detected by mass spectrometry.This work is funded by Icelandic Research Fund (163393-052) from the Icelandic Centre for Research (Rannis). We thank Dr. S. Jonsdottir for assistance with collection of the NMR and HRMS (ESI) data; Dr. Frederic Mentink-Vigier for help with simulations and for valuable suggestions; and members of the Sigurdsson research group for helpful discussions.Peer reviewe

BDPA radicals with improved persistence for dynamic nuclear polarization

Dynamic nuclear polarization (DNP) has emerged as an important technique to enhance the signal intensity of nuclear magnetic resonance (NMR) spectroscopy by transferring spin polarization from unpaired electrons to nuclei of interest. Persistent organic radicals are a common source of unpaired electrons for DNP. Carbon-based radicals, such as trityl and 1,3-bisdiphenylene-2-phenylallyl (BDPA), are very promising polarizing agents at high magnetic fields. BDPA has the advantage that it is easier to synthesize than trityl, but its lack of solubility in aqueous media limits its application for structural studies of biomolecules by DNP NMR. This PhD thesis describes the design and synthesis of BDPA radicals that are soluble in aqueous media and have improved persistence. The first part revolves around the syntheses of phosphoester-derived BDPA radicals. Unexpectedly, these BDPA radicals, and BDPA radicals in general, were found to have limited persistence. Therefore, the effects of various factors on the persistence were investigated. Decomposition in the solid state was found to be due to oxidation while dimer formation was discovered to be a significant pathway of degradation in solution. In the second part, a new class of BDPA radicals is described, their synthesis and characterization. The radicals contain four alkyl/aryl-ammonium groups and are soluble in aqueous solution. More importantly, they are highly persistent in polar solvents due to reduced decomposition via dimerization. The third part describes the synthesis of three water-soluble BDPA-nitroxide biradicals. Different linkers were used to optimize the spin-spin interaction between the two radical centers. These biradicals are being investigated by our collaborators, as polarizing agents for DNP-NMR spectroscopy at high magnetic fieldsIcelandic Research Fund (163393-052), University of Iceland Research Fun

External Electric Field Induces a Mechanistic Crossover in the Reactivity of 1,3-Dienes with Sulfur Dioxide: Sulfolene <i>Versus</i> Sultine

1,3-Dienes (1) react with SO2 to produce sulfolenes (2) and sultines (3). Previous experiments and computational studies have shown that hetero-Diels–Alder (HDA) reaction producing the sultine is kinetically favorable compared to the chelotropic (CE) reaction producing the sulfolene. In this article, DFT calculations for a series of substituted 1,3-dienes show that under the influence of a moderate oriented external electric field (OEEF) of 2.0–4.0 V nm–1 along the reaction axis, the chelotropic reaction becomes the kinetically favorable reaction. In the absence of the OEEF, the destabilizing distortion involved in bringing the 1,3-diene and SO2 to the CE transition-state (TS) exceeds than for the HDA TS. However, under the influence of the OEEF, the strongly stabilizing electrostatic interactions in the CE TS effectively overcome the structural distortion energy. The enhanced dipole moment of the CE TS vis-à-vis the HDA TS under the OEEF accounts for the stabilization of the former

A stepper-piezo-based co-actuation paradigm for tool positioning in parallel spark micro-electro-discharge machining

We report a new method for tool positioning in micro-electro-discharge machining with multiple electrodes for generating parallel spark employing a combination of stepper motor and piezoactuator-based co-actuation method. The stepper motor was used for coarse positioning and the inequality arising due to difference in the tool size of multiple tools used in micro-electro-discharge machining was equated employing piezoactuation followed by electrical continuity test. Simultaneous sparks for two different electrodes could be observed employing this method. The voltage waveforms across the tool–workpiece interface also confirm the generation of simultaneous and parallel sparks across both the electrodes