Development of an in Situ NMR Photoreactor To Study Environmental Photochemistry

Steady-State Free Precession (SSFP) NMR Spectroscopy for Sensitivity Enhancement in Complex Environmental and Biological Samples Using Both High-Field and Low-Field NMR

Nuclear magnetic resonance (NMR) spectroscopy is a valuable and complementary tool in environmental research, but it is underutilized due to the cost, size, and maintenance requirements of standard "high-field" NMR spectrometers. "Low-field" NMR spectrometers are a financially and physically accessible alternative, but their lower sensitivity and increased spectral overlap limit the analysis of heterogeneous environmental/biological media, especially with fast-relaxing samples that produce broad, low-intensity spectra. This study therefore investigates the potential of the steady-state free precession (SSFP) experiment to enhance signal-to-noise ratios (SNRs) of fast-relaxing, complex samples at both high- and low-field. SSFP works by obtaining steady-state transverse signal using a train of equally spaced radiofrequency pulses with the same flip angle and a time between pulses less than the transverse relaxation time, allowing for thousands of scans to be summed in a short time period. Here, 13C-SSFP is applied to samples of varying complexity (egg white, dissolved organic matter, and crude oil) at low-field and at high-field for testing and comparison. The potential of in vivo SSFP NMR is additionally investigated by applying 31P-SSFP to live Eisenia fetida at high-field. In some samples, SSFP increased 13C SNR by over 2000% at both high-field and low-field compared to standard 13C NMR and enabled detection of peaks that were not observable by standard 13C NMR. Ultimately, SSFP holds great potential for improving analysis of fast-relaxing, complex samples, which could in turn make low-field NMR spectroscopy a more effective tool not only in environmental/biological research but also in numerous other disciplines.

Monitoring off-resonance signals with SHARPER NMR - the MR-SHARPER experiment

We demonstrate an extension to the SHARPER (Sensitive Homogenous and Refocussed Peaks in Real Time) NMR experiment which allows more than one signal to be monitored simultaneously, while still giving ultra-sharp, homo- and hetero-decoupled NMR signals. This is especially valuable in situations where magnetic field inhomogeneity would normally make NMR a problematic tool, for example when gas evolution is occurring during reaction monitoring. The originally reported SHARPER experiment only works for a single, on-resonance NMR signal, but here we demonstrate the Multiple Resonance SHARPER approach can be developed, which in principle can acquire multiple on-/off-resonance signals simultaneously while retaining the desirable properties of the parent sequence. In practice, the case of two resonances, e.g. those of a reactant and a product, will most of the time be considered for MR-SHARPER, as illustrated here.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

NMR spectroscopy of wastewater: A review, case study, and future potential

NMR spectroscopy is arguably the most powerful tool for the study of molecular structures and interactions, and is increasingly being applied to environmental research, such as the study of wastewater. With over 97% of the planet's water being saltwater, and two thirds of freshwater being frozen in the ice caps and glaciers, there is a significant need to maintain and reuse the remaining 1%, which is a precious resource, critical to the sustainability of most life on Earth. Sanitation and reutilization of wastewater is an important method of water conservation, especially in arid regions, making the understanding of wastewater itself, and of its treatment processes, a highly relevant area of environmental research. Here, the benefits, challenges and subtleties of using NMR spectroscopy for the analysis of wastewater are considered. First, the techniques available to overcome the specific challenges arising from the nature of wastewater (which is a complex and dilute matrix), including an examination of sample preparation and NMR techniques (such as solvent suppression), in both the solid and solution states, are discussed. Then, the arsenal of available NMR techniques for both structure elucidation (e.g., heteronuclear, multidimensional NMR, homonuclear scalar coupling-based experiments) and the study of intermolecular interactions (e.g., diffusion, nuclear Overhauser and saturation transfer-based techniques) in wastewater are examined. Examples of wastewater NMR studies from the literature are reviewed and potential areas for future research are identified. Organized by nucleus, this review includes the common heteronuclei (¹³C, ¹⁵N, ¹⁹F, ³¹P, ²⁹Si) as well as other environmentally relevant nuclei and metals such as ²⁷Al, ⁵¹V, ²⁰⁷Pb and ¹¹³Cd, among others. Further, the potential of additional NMR methods such as comprehensive multiphase NMR, NMR microscopy and hyphenated techniques (for example, LC-SPE-NMR-MS) for advancing the current understanding of wastewater are discussed. In addition, a case study that combines natural abundance (i.e. non-concentrated), targeted and non-targeted NMR to characterize wastewater, along with in vivo based NMR to understand its toxicity, is included. The study demonstrates that, when applied comprehensively, NMR can provide unique insights into not just the structure, but also potential impacts, of wastewater and wastewater treatment processes. Finally, low-field NMR, which holds considerable future potential for on-site wastewater monitoring, is briefly discussed. In summary, NMR spectroscopy is one of the most versatile tools in modern science, with abilities to study all phases (gases, liquids, gels and solids), chemical structures, interactions, interfaces, toxicity and much more. The authors hope this review will inspire more scientists to embrace NMR, given its huge potential for both wastewater analysis in particular and environmental research in general.

LED-NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition

We report that an NMR spectrometer equipped with a high-power LED light source can be used to study a fast enantioselective photocatalytic [2+2] cycloaddition. While traditional ex situ applications of NMR provide considerable information on reaction mechanisms, they are often ineffective for observing fast reactions. Recently, motivated by renewed interest in organic photochemistry, several approaches have been reported for in situ monitoring of photochemical reactions. These previously disclosed methods, however, have rarely been applied to rapid (<5 min) photochemical reactions. Furthermore, these approaches have not previously been used to interrogate the mechanisms of photocatalytic energy-transfer reactions. In the present work, we describe our experimental setup and demonstrate its utility by determining a phenomenological rate law for a model photocatalytic energy-transfer cycloaddition reaction.

Flow-based in vivo NMR spectroscopy of small aquatic organisms

NMR applied to living organisms is arguably the ultimate tool for understanding environmental stress responses and can provide desperately needed information on toxic mechanisms, synergistic effects, sublethal impacts, recovery, and biotransformation of xenobiotics. To perform in vivo NMR spectroscopy, a flow cell system is required to deliver oxygen and food to the organisms while maintaining optimal line shape for NMR spectroscopy. In this tutorial, two such flow cell systems and their constructions are discussed: (a) a single pump high-volume flow cell design is simple to build and ideal for organisms that do not require feeding (i.e., eggs) and (b) a more advanced low-volume double pump flow cell design that permits feeding, maintains optimal water height for water suppression, improves locking and shimming, and uses only a small recirculating volume, thus reducing the amount of xenobiotic required for testing. In addition, key experimental aspects including isotopic enrichment, water suppression, and 2D experiments for both ¹³ C enriched and natural abundance organisms are discussed.

Process analytical technology (PAT) as a versatile tool for real-time monitoring and kinetic evaluation of photocatalytic reactions

Combining in situ Raman spectroscopy with multivariate data analysis enables the real-time monitoring and kinetic evaluation of photocatalytic reactions. The applicability is demonstrated on the photooxidation of 4-methoxythiophenol.

Combination of illumination and high resolution NMR spectroscopy: Key features and practical aspects, photochemical applications, and new concepts

In the last decade, photochemical and photocatalytic applications have developed into one of the dominant research fields in chemistry. However, mechanistic investigations to sustain this enormous progress are still relatively sparse and in high demand by the photochemistry community. UV/Vis spectroscopy and EPR spectroscopy have been the main spectroscopic tools to study the mechanisms of photoreactions due to their higher time resolution and sensitivity. On the other hand, application of NMR in photosystems has been mainly restricted to photo-CIDNP, since the initial photoexcitation was thought to be the single key to understand photoinduced reactions. In 2015 the Gschwind group showcased the possibility that different reaction pathways could occur from the same photoexcited state depending on the reaction conditions by using in situ LED illumination NMR. This was the starting point to push the active participation of NMR in photosystems to its full potential, including reaction profiling, structure determination of intermediates, downstream mechanistic studies, dark pathways, intermediate sequencing with CEST etc. Following this, multiple studies using in situ illumination NMR have been reported focusing on mechanistic investigations in photocatalysis, photoswitches, and polymerizations. The recent increased popularity of this technique can be attributed to the simplicity of the experimental setup and the availability of low cost, high power LEDs. Here, we review the development of experimental design, applications and new concepts of illuminated NMR. In the first part, we describe the development of different designs of NMR illumination apparatus, illuminating from the bottom/side/top/inside, and discuss their pros and cons for specific applications. Furthermore, we address LASERs and LEDs as different light sources as well as special cases such as UVNMR(-illumination), FlowNMR, NMR on a Chip etc. To complete the discussion on experimental apparatus, the advantages and disadvantages of in situ LED illumination NMR versus ex situ illumination NMR are described. The second part of this review discusses different facets of applications of inside illumination experiments. It highlights newly revealed mechanistic and structural information and ideas in the fields of photocatalyis, photoswitches and photopolymerization. Finally, we present new concepts and methods based on the combination of NMR and illumination such as sensitivity enhancement, chemical pump probes, experimental access to transition state combinations and NMR actinometry. Overall this review presents NMR spectroscopy as a complementary tool to UV/Vis spectroscopy in mechanistic and structural investigations of photochemical processes. The review is presented in a way that is intended to assist the photochemistry and photocatalysis community in adopting and understanding this astonishingly powerful in situ LED illumination NMR method for their investigations on a daily basis.

Digital microfluidics and nuclear magnetic resonance spectroscopy for in situ diffusion measurements and reaction monitoring

In recent years microcoils and related structures have been developed to increase the mass sensitivity of nuclear magnetic resonance spectroscopy, allowing this extremely powerful analytical technique to be extended to small sample volumes (<5 μl). In general, microchannels have been used to deliver the samples of interest to these microcoils; however, these systems tend to have large dead volumes and require more complex fluidic connections. Here, we introduce a two-plate digital microfluidic (DMF) strategy to interface small-volume samples with NMR microcoils. In this system, a planar microcoil is surrounded by a copper plane that serves as the counter-electrode for the digital microfluidic device, allowing for precise control of droplet position and shape. This feature allows for the user-determination of the orientation of droplets relative to the main axes of the shim stack, permitting improved shimming and a more homogeneous magnetic field inside the droplet below the microcoil, which leads to improved spectral lineshape. This, along with high-fidelity droplet actuation, allows for rapid shimming strategies (developed over decades for vertically oriented NMR tubes) to be employed, permitting the determination of reaction-product diffusion coefficients as well as quantitative monitoring of reactive intermediates. We propose that this system paves the way for new and exciting applications for in situ analysis of small samples by NMR spectroscopy.

Analysis of Polycyclic Aromatic Hydrocarbon (PAH) Mixtures Using Diffusion-Ordered NMR Spectroscopy and Adsorption by Powdered Activated Carbon and Biochar

Analysis of polycyclic aromatic hydrocarbons (PAHs) in air and water sources is a key part of environmental chemistry research, since most PAHs are well known to be associated with negative health impacts on humans. This study explores an approach for analyzing PAH mixtures with advanced nuclear magnetic resonance (NMR) spectroscopic techniques including high-resolution one-dimensional (1D) NMR spectroscopy and diffusion-ordered NMR spectroscopy (DOSY NMR). With this method, different kinds of PAHs can be detected and differentiated from a mixture with high resolution. The adsorption process of PAH mixtures by PAC and biochar was studied to understand the mechanism and assess the method.

Online monitoring of a photocatalytic reaction by real-time high resolution FlowNMR spectroscopy

We demonstrate how FlowNMR spectroscopy can readily be applied to investigate photochemical reactions that require sustained input of light and air to yield mechanistic insight under realistic conditions. The Eosin Y mediated photo-oxidation of N-allylbenzylamine is shown to produce imines as primary reaction products from which undesired aldehydes form after longer reaction times. Facile variation of reaction conditions during the reaction in flow allows for probe experiments that give information about the mode of action of the photocatalyst.

Systemic Homeostasis in Metabolome, Ionome, and Microbiome of Wild Yellowfin Goby in Estuarine Ecosystem

Data-driven approaches were applied to investigate the temporal and spatial changes of 1,022 individuals of wild yellowfin goby and its potential interaction with the estuarine environment in Japan. Nuclear magnetic resonance (NMR)-based metabolomics revealed that growth stage is a primary factor affecting muscle metabolism. Then, the metabolic, elemental and microbial profiles of the pooled samples generated according to either the same habitat or sampling season as well as the river water and sediment samples from their habitats were measured using NMR spectra, inductively coupled plasma optical emission spectrometry and next-generation 16 S rRNA gene sequencing. Hidden interactions in the integrated datasets such as the potential role of intestinal bacteria in the control of spawning migration, essential amino acids and fatty acids synthesis in wild yellowfin goby were further extracted using correlation clustering and market basket analysis-generated networks. Importantly, our systematic analysis of both the seasonal and latitudinal variations in metabolome, ionome and microbiome of wild yellowfin goby pointed out that the environmental factors such as the temperature play important roles in regulating the body homeostasis of wild fish.

Environmental Nuclear Magnetic Resonance Spectroscopy: An Overview and a Primer

NMR spectroscopy is a versatile tool for the study of structure and interactions in environmental media such as air, soil, and water as well as monitoring the metabolic responses of living organisms to an ever changing environment. Part review, part perspective, and part tutorial, this Feature is aimed at nonspecialists who are interested in learning more about the potential and impact of NMR spectroscopy in environmental research.

Natural products and morphogenic activity of γ-Proteobacteria associated with the marine hydroid polyp Hydractinia echinata

Illumina 16S rRNA gene sequencing was used to profile the associated bacterial community of the marine hydroid Hydractinia echinata, a long-standing model system in developmental biology. 56 associated bacteria were isolated and evaluated for their antimicrobial activity. Three strains were selected for further in-depth chemical analysis leading to the identification of 17 natural products. Several γ-Proteobacteria were found to induce settlement of the motile larvae, but only six isolates induced the metamorphosis to the primary polyp stage within 24h. Our study paves the way to better understand how bacterial partners contribute to protection, homeostasis and propagation of the hydroid polyp.

Analysis of DOM phototransformation using a looped NMR system integrated with a sunlight simulator

Photochemical transformation plays an important role in functionalizing and degrading dissolved organic matter (DOM), producing one of the most complex mixtures known. In this study, using a flow-based design, nuclear magnetic resonance (NMR) spectroscopy is directly interfaced with a sunlight simulator enabling the study of DOM photodegradation in situ with high temporal resolution over 5 days. Samples from Suwannee River (Florida), Nordic Reservoir (Norway), and Pony Lake (Antarctic) are studied. Phototransformation of DOM is dominated by the degradation of aromatics and unsaturated structures (many arising from lignin) into carboxylated and hydroxylated products. To assess longer term changes, the samples were continuously irradiated for 17.5 days, followed by the identification a wide range of compounds and assessment of their fate using off-line 2D-NMR. This study demonstrates the applicability of the looped system to follow degradation in a non-targeted fashion (the mixture as a whole) and target analysis (tracing specific metabolites), which holds great potential to study the fate and transformation of contaminants and nutrients in the presence of DOM. It also demonstrates that components that remain unresolved in 1D NMR can be identified using 2D methods.

Chemical properties of dissolved organic matter derived from sugarcane rind and the impacts on copper adsorption onto red soil

Dissolved organic matter (DOM), as the most active organic carbon in the soil, has a coherent affinity with heavy metals from inherent and exogenous sources. Although the important roles of DOM in the adsorption of heavy metals in soil have previously been demonstrated, the heterogeneity and variability of the chemical constitution of DOM impede the investigation of its effects on heavy metal adsorption onto soil under natural conditions. Fresh DOM (FDOM) and degraded DOM (DDOM) from sugarcane rind were prepared, and their chemical properties were measured by Fourier-transform infrared spectrometry (FTIR), excitation-emission matrix (EEM) fluorescence spectroscopes, nuclear magnetic resonance (NMR), and molecular weight distribution (MWD). They were also used in batch experiments to evaluate their effects on the adsorption of Cu(II) onto farmland red soil. Based on our results, the chemical structure and composition of DDOM greatly varied; compared with FDOM, the C/O ratio (from 24.0 to 9.6%) and fluorescence index (FI) (from 1.4 to 1.0) decreased, and high molecular weight (>10 kDa) compounds increased from 23.18 to 70.51%, while low molecular weight (<3 kDa) compounds decreased from 56.13 to 12.13%; aromaticity and humification degree were markedly enhanced. The discrepancy of FDOM and DDOM in terms of chemical properties greatly influenced Cu(II) adsorption onto red soil by affecting DOM-Cu(II) complex capacity. The FDOM inhibited the adsorption of Cu(II), while DDOM promoted adsorption, which was significantly influenced by soil pH. Maximum adsorption capacity (Q _m) was 0.92 and 5.76 mg g^-1 in the presence of FDOM and DDOM, respectively. The adsorption process with DDOM could be better described by the Langmuir model, while that with FDOM was better described by the Freundlich model. The impacts caused by the dynamic changes of the chemical properties of DOM under natural conditions should therefore be considered in the risk assessment and remediation of soils contaminated with heavy metals.

Toward Matching Optically and NMR Active Volumes for Optimizing the Observation of Photo-Induced Reactions by NMR

In the recent years photo-induced reactions are becoming increasingly popular in many fields of chemistry comprising biological conversions, material/environmental science and synthesis. NMR monitoring of such reactions has been shown being advantageous and several strategies of providing an efficient irradiation of the NMR sample have been developed and reported. Here we show that adjusting the optical properties of the investigated solution to the active volume detected by the NMR experiment is valuable. This is shown with the help of three examples comprising photo-isomerization, photo-induced polymerization and CIDNP-detected bond cleavage. Adjusting the photo-active volume to the NMR-detectable portion of the sample provides a substantially more realistic kinetic information, background suppression and reduction of thermal and diffusional effects.