Weak nuclear spin singlet relaxation mechanisms revealed by experiment and computation

Nuclear spin singlet states are often found to allow long lived storage of nuclear magnetization, which can form the basis of novel applications in spectroscopy, imaging, and in studies of dynamic processes. Precisely how long such polarization remains intact, and which factors affect its lifetime is often difficult to determine and predict. We present a combined experimental/computational study to demonstrate that molecular dynamics simulations and ab initio calculations can be used to fully account for the experimentally observed singlet lifetimes in an organic molecule in solution. %Intermolecular interactions with Cl nuclei of the chloroform solvent are shown to contribute significantly to the relaxation. Paramagnetic relaxation due to dissolved oxygen is taken into account in a self-consistent manner. The correspondence between experiment and simulations is achieved without adjustable parameters. These studies highlight the importance of considering unusual and difficult-to-control mechanisms, such as dipolar couplings to low-gamma solvent nuclei, and to residual paramagnetic species, which often can represent lifetime limiting factors. These results also point to the power of molecular dynamics simulations to provide insights into little-known NMR relaxation mechanisms.

Pyridyl Phosphine Complexes in the Design of Hydration Catalysts

<p>Recent advances in homogeneous catalysis have identified the importance of ligands able to participate in the catalytic cycle. Particularly relevant to making chemistry “greener” are those ligands that solubilise the catalyst in aqueous solution, and those that are able to activate water molecules towards reaction with the metal complex or substrate. This thesis describes the synthesis and coordination chemistry of a novel ligand bearing 2-pyridylphosphine substituents attached to a 2,6-pyridyl backbone (²⁻pyrPNP, [(C₅H₄N)₂PCH₂]₂C₅H₃N). These components were selected for their abilities to interact with water through dearomatisation processes, hydrogen bonding, and the basic pyridyl nitrogen atoms. The synthesis of pure ²⁻pyrPNP described here represents a much improved method for the synthesis of pyridylphosphines compared to those published in the literature. This is demonstrated by comparison with the original synthetic route, which produced many intractable impurities, as well as by the ability of the new method to provide PhPNP from an economical and air-stable starting material. Reactions of ²⁻pyrPNP with rhodium precursors show complicated reactivity, including the potential formation of paramagnetic species. Investigation into the reactivity of ²⁻pyrPNP with analogous iridium precursors resulted in the synthesis of [(²⁻pyrPNP)Ir(cod)]Cl. This is the first crystallographically characterised complex containing a facially coordinated PNP ligand. The cod ligand can be removed with ethene and hydrogen to form bis(ethene) and chloroiridium(III) bis(hydride) complexes [(²⁻pyrPNP)Ir(C₂H₄)₂]Cl and [(²⁻pyrPNP)Ir(H)₂Cl], respectively. Both complexes contain meridionally-coordinated ²⁻pyrPNP. Preliminary investigations reveal that the iridium complexes are fairly successful nitrile hydration catalysts under aqueous conditions. In addition, the cod and bis(ethene) complexes bearing ²⁻pyrPNP are more active than the cod complex of the pyridyl-free PhPNP ligand.</p>

Pyridyl Phosphine Complexes in the Design of Hydration Catalysts

<p>Recent advances in homogeneous catalysis have identified the importance of ligands able to participate in the catalytic cycle. Particularly relevant to making chemistry “greener” are those ligands that solubilise the catalyst in aqueous solution, and those that are able to activate water molecules towards reaction with the metal complex or substrate. This thesis describes the synthesis and coordination chemistry of a novel ligand bearing 2-pyridylphosphine substituents attached to a 2,6-pyridyl backbone (²⁻pyrPNP, [(C₅H₄N)₂PCH₂]₂C₅H₃N). These components were selected for their abilities to interact with water through dearomatisation processes, hydrogen bonding, and the basic pyridyl nitrogen atoms. The synthesis of pure ²⁻pyrPNP described here represents a much improved method for the synthesis of pyridylphosphines compared to those published in the literature. This is demonstrated by comparison with the original synthetic route, which produced many intractable impurities, as well as by the ability of the new method to provide PhPNP from an economical and air-stable starting material. Reactions of ²⁻pyrPNP with rhodium precursors show complicated reactivity, including the potential formation of paramagnetic species. Investigation into the reactivity of ²⁻pyrPNP with analogous iridium precursors resulted in the synthesis of [(²⁻pyrPNP)Ir(cod)]Cl. This is the first crystallographically characterised complex containing a facially coordinated PNP ligand. The cod ligand can be removed with ethene and hydrogen to form bis(ethene) and chloroiridium(III) bis(hydride) complexes [(²⁻pyrPNP)Ir(C₂H₄)₂]Cl and [(²⁻pyrPNP)Ir(H)₂Cl], respectively. Both complexes contain meridionally-coordinated ²⁻pyrPNP. Preliminary investigations reveal that the iridium complexes are fairly successful nitrile hydration catalysts under aqueous conditions. In addition, the cod and bis(ethene) complexes bearing ²⁻pyrPNP are more active than the cod complex of the pyridyl-free PhPNP ligand.</p>

Rapid-scan electron paramagnetic resonance using an EPR-on-a-Chip sensor

Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, novel aspects of voltage-controlled oscillator (VCO)-based EPR-on-a-Chip (EPRoC) detectors are discussed, which have recently gained interest in the EPR community. More specifically, it is demonstrated that with a VCO-based EPRoC detector, the amplitude-sensitive mode of detection can be used to perform very fast rapid-scan EPR experiments with a comparatively simple experimental setup to improve sensitivity compared to the continuous-wave regime. In place of a MW resonator, VCO-based EPRoC detectors use an array of injection-locked VCOs, each incorporating a miniaturized planar coil as a combined microwave source and detector. A striking advantage of the VCO-based approach is the possibility of replacing the conventionally used magnetic field sweeps with frequency sweeps with very high agility and near-constant sensitivity. Here, proof-of-concept rapid-scan EPR (RS-EPRoC) experiments are performed by sweeping the frequency of the EPRoC VCO array with up to 400 THz s−1, corresponding to a field sweep rate of 14 kT s−1. The resulting time-domain RS-EPRoC signals of a micrometer-scale BDPA sample can be transformed into the corresponding absorption EPR signals with high precision. Considering currently available technology, the frequency sweep range may be extended to 320 MHz, indicating that RS-EPRoC shows great promise for future sensitivity enhancements in the rapid-scan regime.

Characterizing SPASM/twitch Domain-Containing Radical SAM Enzymes by EPR Spectroscopy

Owing to their importance, diversity and abundance of generated paramagnetic species, radical S-adenosylmethionine (rSAM) enzymes have become popular targets for electron paramagnetic resonance (EPR) spectroscopic studies. In contrast to prototypic single-domain and thus single-[4Fe–4S]-containing rSAM enzymes, there is a large subfamily of rSAM enzymes with multiple domains and one or two additional iron–sulfur cluster(s) called the SPASM/twitch domain-containing rSAM enzymes. EPR spectroscopy is a powerful tool that allows for the observation of the iron–sulfur clusters as well as potentially trappable paramagnetic reaction intermediates. Here, we review continuous-wave and pulse EPR spectroscopic studies of SPASM/twitch domain-containing rSAM enzymes. Among these enzymes, we will review in greater depth four well-studied enzymes, BtrN, MoaA, PqqE, and SuiB. Towards establishing a functional consensus of the additional architecture in these enzymes, we describe the commonalities between these enzymes as observed by EPR spectroscopy.

Antioxidative and Anti-Inflammatory Phytochemicals and Related Stable Paramagnetic Species in Different Parts of Dragon Fruit

In this study, we investigated the antioxidant and anti-inflammatory phytochemicals and paramagnetic species in dragon fruit using high-performance liquid chromatography (HPLC) and electron paramagnetic resonance (EPR). HPLC analysis demonstrated that dragon fruit is enriched with bioactive phytochemicals, with significant variations between each part of the fruit. Anthocyanins namely, cyanidin 3-glucoside, delphinidin 3-glucoside, and pelargonidin 3-glucoside were detected in the dragon fruit peel and fresh red pulp. Epicatechin gallate, epigallocatechin, caffeine, and gallic acid were found in the dragon fruit seed. Additionally, 25–100 mg × L−1 of dragon fruit pulp and peel extracts containing enrichment of cyanidin 3-glucoside were found to inhibit the production of reactive oxygen species (ROS), reactive nitrogen species (RNS), inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) in cell-based studies without exerted cytotoxicity. EPR primarily detected two paramagnetic species in the red samples. These two different radical species were assigned as stable radicals and Mn2+ (paramagnetic species) based on the g-values and hyperfine components. In addition, the broad EPR line width of the white peel can be correlated to a unique moiety in dragon fruit. Our EPR and HPLC results provide new insight regarding the phytochemicals and related stable intermediates found in various parts of dragon fruit. Thus, we suggest here that there is the potential to use dragon fruit peel, which contains anthocyanins, as a natural active pharmaceutical ingredient.

Rapid Scan Electron Paramagnetic Resonance using an EPR-on-a-Chip Sensor

Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, we present an improved design of a miniaturized EPR spectrometer implemented on a silicon microchip (EPR-on-a-chip, EPRoC). In place of a microwave resonator, EPRoC uses an array of injection-locked voltage-controlled oscillators (VCOs), each incorporating a 200 µm diameter coil, as a combined microwave source and detector. The individual miniaturized VCO elements provide an excellent spin sensitivity reported to be about 4 × 109 spins/√Hz, which is extended by the array over a larger area for improved concentration sensitivity. A striking advantage of this design is the possibility to sweep the MW frequency instead of the magnetic field, which allows the use of smaller, permanent magnets instead of the bulky and power-hungry electromagnets required for field-swept EPR. Here, we report rapid scan EPR (RS-EPRoC) experiments performed by sweeping the frequency of the EPRoC VCO array. RS-EPRoC spectra demonstrate an improved SNR by approximately two orders of magnitude for similar signal acquisition times compared to continuous wave (CW-EPRoC) methods, which may improve the absolute spin and concentration sensitivity of EPR-on-a-Chip at 14 GHz to about 6 × 107 spins/√Hz and 3.6 nM/√Hz, respectively.

Quantum-State Control and Manipulation of Paramagnetic Molecules with Magnetic Fields

Since external magnetic fields were first employed to deflect paramagnetic atoms in 1921, a range of magnetic field–based methods have been introduced to state-selectively manipulate paramagnetic species. These methods include magnetic guides, which selectively filter paramagnetic species from all other components of a beam, and magnetic traps, where paramagnetic species can be spatially confined for extended periods of time. However, many of these techniques were developed for atomic—rather than molecular—paramagnetic species. It has proven challenging to apply some of these experimental methods developed for atoms to paramagnetic molecules. Thanks to the emergence of new experimental approaches and new combinations of existing techniques, the past decade has seen significant progress toward the manipulation and control of paramagnetic molecules. This review identifies the key methods that have been implemented for the state-selective manipulation of paramagnetic molecules—discussing the motivation, state of the art, and future prospects of the field. Key applications include the ability to control chemical interactions, undertake precise spectroscopic measurements, and challenge our understanding of chemical reactivity at a fundamental level. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 72 is April 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.