Quantitative high-resolution on-line NMR spectroscopy in reaction and process monitoring

The R2D3 approach towards fast quantitative NMR: maintaining accuracy and reducing the experimental time

Quantitative NMR experiments, especially on low-abundance nuclei such as 13C, can be extremely time-consuming due to the various constraints to ensure the quality of the results: a high number of scans and a long recovery delay. We propose the combination of the DEFT pulse sequence and the R2D2 method, hereafter referred to as R2D3. The addition of DEFT to the R2D2 method reduces the quantitative limitations imposed by partial saturation. The parameters influencing the accuracy were evaluated with simulations and the quantitative performance of R2D3 was assessed by observing the trueness and precision for three different samples. The effects of different processing steps - the number of added rows and apodization - are also discussed. A precision of 1% or less was obtained in almost all the cases, showing that the R2D3 approach can drastically decrease the experimental time while retaining the key aspects of a quantitative experiment. A high time gain factor can be achieved, close to that of INEPT and without its drawbacks, when trueness is less critical than precision. The R2D3 method will particularly benefit qNMR applications based on the observation of heteronuclei and the analysis of a large sample series.

Evaluation of pure shift NMR methods for the analysis of complex metabolite mixtures with a benchtop NMR spectrometer

Compact, low-cost and cryogen-free benchtop nuclear magnetic resonance (NMR) spectrometers have become an attractive option for analysing complex biological mixtures. The implementation of spectral simplification methods such as pure shift spectroscopy is particularly important to circumvent the ubiquitous peak overlaps that occur due to the limited resolution of compact instruments. Pure shift (PS) strategies consist of eliminating the signal multiplicity induced by the homonuclear J-couplings observed in 1D 1H NMR spectra, thereby simplifying the spectral information. This paper provides an analytical evaluation of optimised pure shift spectroscopy pulse sequences for the analysis of metabolite mixtures on a recent benchtop NMR spectrometer. Six PS pulse sequences were carefully evaluated, based on three families of PS techniques: 1D projections of 2D J-resolved spectra, Zangger-Sterk (ZS) and PSYCHE. The methods were evaluated in terms of resolution, sensitivity, spectral purity and repeatability. Among the strategies we explored, 1D projections of 2D J-resolved double-echo (J-RES DE) spectra, combined with an improved processing strategy, appeared to offer the best compromise based on these analytical criteria. The potential of this method was illustrated on a complex sample from the food industry and enabled key metabolites to be detected with improved resolution and sensitivity, showing that PS NMR could be used for a rapid (22 minutes) profiling of complex metabolite mixtures on a benchtop NMR spectrometer.

Overhauser Dynamic Nuclear Polarization Enables Single Scan Benchtop 13C NMR Spectroscopy in Continuous-Flow

Benchtop 13C NMR spectroscopy is highly attractive for reaction and process monitoring. However, insufficient premagnetization and low signal intensities largely prevent its application to flowing liquids. We show that hyperpolarization by Overhauser dynamic nuclear polarization (ODNP) can be used to overcome these problems, as ODNP operates on short time scales and results in strong 13C signal enhancements. Benchtop 13C NMR spectra with ODNP enhancement acquired in continuous-flow are reported here for the first time. We have investigated two ODNP approaches: direct ODNP, which transfers the polarization of unpaired electrons to 13C nuclei via direct hyperfine coupling, and indirect ODNP, in which the electron polarization is first transferred to 1H nuclei before a polarization transfer pulse sequence finally transfers the polarization to the 13C nuclei. Experiments were carried out for three pure solvents and a mixture for different flow rates. The results show significant 13C signal enhancements for both approaches. However, their performance varies for different substances, depending on the strength and type of the hyperfine interaction as well as on the relaxation properties, but by selecting a suitable approach, good single-scan 13C NMR spectra can be obtained with benchtop NMR, even at high flow rates.

Real-time monitoring of fermentation processes in wine production with benchtop 1H NMR spectroscopy

The flexibility, affordability and ease of use of benchtop 1H NMR spectroscopy makes it potentially very interesting for assessing the quality of wine types and monitoring the fermentation process. However, the low spectral resolution of benchtop 1H NMR spectroscopy and the complexity of the mixtures hinder the direct quantification of important wine parameters and, thus, prevent its widespread use as an analytical tool in wineries. We show here that these problems can be solved using model-based data processing. In a first step, the accuracy of the new approach was evaluated by analyzing gravimetrically prepared test mixtures representing different fermentation stages. Good agreement was found, demonstrating the reliability of the new method. In a second step, benchtop 1H NMR spectroscopy combined with model-based data processing was used for the real-time monitoring of real fermentation media. Wine fermentation processes with different feed strategies (batch and fed-batch) were investigated and compared and the evolution of important wine constituents as well as effects caused by the different feeding strategies were monitored, demonstrating the applicability of the new approach also in demanding applications. This opens the way to using benchtop NMR spectroscopy for optimization and decision making in wine production.

Pure Shift NMR in Continuous Flow

Flow NMR is an expanding analytical approach with applications that include in-line analysis for process control and optimisation, and real-time reaction monitoring. The samples monitored by flow NMR are typically mixtures that yield complex 1D 1H spectra. "Pure shift" NMR is a powerful approach to simplifying 1H NMR spectra, but its standard implementation is not compatible with continuous flow because of interference between sample motion and the position-dependent spin manipulations that are required in pure shift NMR. Here we show that pure shift NMR spectra can be successfully collected for continuously flowing samples, thanks to an adapted acquisition scheme, robust solvent suppression, and a velocity-compensation strategy. The resulting method is used to collect ultrahigh resolution reaction monitoring data. Pure shift NMR spectra are expected to benefit many applications of flow NMR.

Using machine learning to improve the hard modeling of NMR time series

Modeling time series of NMR spectra is a useful method to accurately extract information such as temporal concentration profiles from complex processes, e.g. reactions. Modeling these time series by using nonlinear optimization often suffers from high runtimes. On the other hand, using deep learning solves the modeling problem quickly, especially for single spectra with separated peaks. However, the accuracy decreases significantly when peaks overlap or cross. We propose a hybrid approach combining the strengths of both methods while mitigating their drawbacks. This hybrid methods improves on a previous work (Meinhardt et al., 2022) and employs neural networks to predict initial parameters for the optimization algorithm, which only needs to fine-tune the parameters afterwards. We present results for both constructed and experimental data sets and achieve improvements in both runtime and accuracy.

Polarization transfer methods for quantitative analysis of flowing mixtures with benchtop 13C NMR spectroscopy

Benchtop NMR spectroscopy is attractive for process monitoring; however, there are still drawbacks that often hamper its use, namely, the comparatively low spectral resolution in 1H NMR, as well as the low signal intensities and problems with the premagnetization of flowing samples in 13C NMR. We show here that all these problems can be overcome by using 1H-13C polarization transfer methods. Two ternary test mixtures (one with overlapping peaks in the 1H NMR spectrum and one with well-separated peaks, which was used as a reference) were studied with a 1 T benchtop NMR spectrometer using the polarization transfer sequence PENDANT (polarization enhancement that is nurtured during attached nucleus testing). The mixtures were analyzed quantitatively in stationary as well as in flow experiments by PENDANT enhanced 13C NMR experiments, and the results were compared with those from the gravimetric sample preparation and from standard 1H and 13C NMR spectroscopy. Furthermore, as a proxy for a process monitoring application, continuous dilution experiments were carried out, and the composition of the mixture was monitored in a flow setup by 13C NMR benchtop spectroscopy with PENDANT. The results demonstrate the high potential of polarization transfer methods for applications in quantitative process analysis with benchtop NMR instruments, in particular with flowing samples.

Advances in continuous polymer analysis in flow with application towards biopolymers

Biopolymers, polymers derived from renewable biomass sources, have gained increasing attention in recent years due to their potential to replace traditional petroleum-based polymers in a range of applications. Among the many advantages of biopolymers can be included their biocompatibility, excellent mechanical properties, and availability from renewable feedstock. However, the development of biopolymers has been limited by a lack of understanding of their properties and processing behaviours. Continuous analysis techniques have the potential to hasten progress in this area by providing real-time insights into the properties and processing of biopolymers. Significant research in polymer chemistry has focused on petroleum-derived polymers and has thus provided a wealth of synthetic and analytical methodologies which may be applied to the biopolymer field. Of particular note is the application of flow technology in polymer science and its implications for accelerating progress towards more sustainable and environmentally friendly alternatives to traditional petroleum-based polymers. In this mini review we have outlined several of the most prominent use cases for biopolymers along with the current state-of-the art in continuous analysis of polymers in flow, including defining and differentiating atline, inline, online and offline analysis. We have found several examples for continuous flow analysis which have direct application to the biopolymer field, and we demonstrate an atline continuous polymer analysis method using size exclusion chromatography.

Graphical abstract

Mechanistic analysis by NMR spectroscopy: A users guide

A 'principles and practice' tutorial-style review of the application of solution-phase NMR in the analysis of the mechanisms of homogeneous organic and organometallic reactions and processes. This review of 345 references summarises why solution-phase NMR spectroscopy is uniquely effective in such studies, allowing non-destructive, quantitative analysis of a wide range of nuclei common to organic and organometallic reactions, providing exquisite structural detail, and using instrumentation that is routinely available in most chemistry research facilities. The review is in two parts. The first comprises an introduction to general techniques and equipment, and guidelines for their selection and application. Topics include practical aspects of the reaction itself, reaction monitoring techniques, NMR data acquisition and processing, analysis of temporal concentration data, NMR titrations, DOSY, and the use of isotopes. The second part comprises a series of 15 Case Studies, each selected to illustrate specific techniques and approaches discussed in the first part, including in situ NMR (^1/2H, ^10/11B, ¹³C, ¹⁵N, ¹⁹F, ²⁹Si, ³¹P), kinetic and equilibrium isotope effects, isotope entrainment, isotope shifts, isotopes at natural abundance, scalar coupling, kinetic analysis (VTNA, RPKA, simulation, steady-state), stopped-flow NMR, flow NMR, rapid injection NMR, pure shift NMR, dynamic nuclear polarisation, ¹H/¹⁹F DOSY NMR, and in situ illumination NMR.

Quantitative and convenient real-time reaction monitoring using stopped-flow benchtop NMR

We present a stopped-flow benchtop NMR system (composed of commercially available hardware components) that allows for quantitative reaction monitoring to be completed with relative ease, even with experimentally complex reaction systems.

High Flow-Rate Benchtop NMR Spectroscopy Enabled by Continuous Overhauser DNP

Analysis of a fast-flowing liquid with NMR spectroscopy is challenging because short residence times in the magnetic field of the spectrometer result in inefficient polarization buildup and thus poor signal intensity. This is particularly problematic for benchtop NMR spectrometers because of their compact design. Therefore, in the present work, different methods to counteract this prepolarization problem in benchtop NMR spectroscopy were studied experimentally. The tests were carried out with an equimolar acetonitrile + water mixture flowing through a capillary with a 0.25 mm inner diameter at flow rates up to 2.00 mL min^-1, corresponding to mean velocities of up to 0.7 m s^-1. Established approaches gave only poor results at high flow rates, namely, using a prepolarization magnet, using a loopy flow cell, and using a T₁ relaxation agent. To overcome this, signal enhancement by Overhauser dynamic nuclear polarization (ODNP) was used, which is based on polarization transfer from unpaired electron spins to nuclear spins and happens on very short time scales, resulting in high signal enhancements, also in fast-flowing liquids. A corresponding setup was developed and used for the studies: the line leading to the 1 T benchtop NMR spectrometer first passes through a fixed bed with a radical matrix placed in a Halbach magnet equipped with a microwave cavity to facilitate the spin transfer. With this ODNP setup, excellent results were obtained even for the highest studied flow rates. This shows that ODNP is an enabler for fast-flow benchtop NMR spectroscopy.

Microfluidic platform for serial mixing experiments with in operando nuclear magnetic resonance spectroscopy

We present a microfluidic platform that allows in operando nuclear magnetic resonance (NMR) observation of serial mixing experiments. Gradually adding one reagent to another is a fundamental experimental modality, widely used to quantify equilibrium constants, for titrations, and in chemical kinetics studies. NMR provides a non-invasive means to quantify concentrations and to follow structural changes at the molecular level as a function of exchanged volume. Using active pneumatic valving on the microfluidic device directly inside an NMR spectrometer equipped with a transmission-line NMR microprobe, the system allows injection of aliquots and in situ mixing in a sample volume of less than 10 μL.

Perspectives in process analytics using low field NMR

Low field NMR is a powerful analytical tool which creates an enormous added value in process analytics. Based on specific applications in process analytics and perspectives for low field NMR in form of spectroscopy, relaxation, diffusion, and imaging in quality control, diverse applications and technical realizations like spectrometers, time domain NMR, mobile NMR sensors and MRI will be discussed.

Engineering aspects of FlowNMR spectroscopy setups for online analysis of solution-phase processes

In this article we review some fundamental engineering concepts and evaluate components and materials required to assemble and operate safe and effective FlowNMR setups that reliably generate meaningful results.

Exploration of continuous-flow benchtop NMR acquisition parameters and considerations for reaction monitoring

This study focused on fundamental data acquisition parameter selection for a benchtop nuclear magnetic resonance (NMR) system with continuous flow, applicable for reaction monitoring. The effect of flow rate on the mixing behaviors within a flow cell was observed, along with an exponential decay relationship between flow rate and the apparent spin-lattice relaxation time (T1*) of benzaldehyde. We also monitored sensitivity (as determined by signal-to-noise ratios; SNRs) under various flow rates, analyte concentrations, and temperatures of the analyte flask. Results suggest that a maximum SNR can be achieved with low to medium flow rates and higher analyte concentrations. This was consistent with data collected with parameters that promote either slow or fast data acquisition. We further consider the effect of these conditions on the analyte's residence time, T1*, and magnetic field inhomogeneity that is a product of continuous flow. Altogether, our results demonstrate how fundamental acquisition parameters can be manipulated to achieve optimal data acquisition in continuous-flow NMR systems.

Taking compact NMR to monitoring real reactions in large-scale chemical industries-General considerations and learnings from a lab-scale test case

The considerations for use of compact nuclear magnetic resonance in a large-scale industrial environment clearly differ from those in academic and educational settings and even from those in smaller companies. In the first part of this article, these differences will be discussed along with the additional requirements that need to be fulfilled for successful applicability in different use cases. In the second part of the article, outcomes from different research activities aiming to fulfill these requirements will be presented with a focus on an online reaction-monitoring study on a lab-scale nucleophilic chlorination reaction.

NMR spectroscopy goes mobile: Using NMR as process analytical technology at the fume hood

Nuclear magnetic resonance (NMR) is potentially a very powerful process analytical technology (PAT) tool as it gives an atomic resolution picture of the reaction mixture without the need for chromatography. NMR is well suited for interrogating transient intermediates, providing kinetic information via NMR active nuclei, and most importantly provides universally quantitative information for all species in solution. This contrasts with commonly used PAT instruments, such as Raman or Flow-infrared (IR), which requires a separate calibration curve for every component of the reaction mixture. To date, the large footprint of high-field (≥400 MHz) NMR spectrometers and the immobility of superconducting magnets, coupled with strict requirements for the architecture for the room it is to be installed, have been a major obstacle to using this technology right next to fume hoods where chemists perform synthetic work. Here, we describe the use of a small, lightweight 60 MHz Benchtop NMR system (Nanalysis Pro-60) located on a mobile platform, that was used to monitor both small and intermediate scale Grignard formation and coupling reactions. We also show how low field NMR can provide a deceptively simple yes/no answer (for a system that would otherwise require laborious off-line testing) in the enrichment of one component versus another in a kilogram scale distillation. Benchtop NMR was also used to derive molecule specific information from Flow-IR, a technology found in most manufacturing sites, and compare the ease at which the concentrations of the reaction mixtures can be derived by NMR versus IR.

Artificial neural networks for quantitative online NMR spectroscopy

Industry 4.0 is all about interconnectivity, sensor-enhanced process control, and data-driven systems. Process analytical technology (PAT) such as online nuclear magnetic resonance (NMR) spectroscopy is gaining in importance, as it increasingly contributes to automation and digitalization in production. In many cases up to now, however, a classical evaluation of process data and their transformation into knowledge is not possible or not economical due to the insufficiently large datasets available. When developing an automated method applicable in process control, sometimes only the basic data of a limited number of batch tests from typical product and process development campaigns are available. However, these datasets are not large enough for training machine-supported procedures. In this work, to overcome this limitation, a new procedure was developed, which allows physically motivated multiplication of the available reference data in order to obtain a sufficiently large dataset for training machine learning algorithms. The underlying example chemical synthesis was measured and analyzed with both application-relevant low-field NMR and high-field NMR spectroscopy as reference method. Artificial neural networks (ANNs) have the potential to infer valuable process information already from relatively limited input data. However, in order to predict the concentration at complex conditions (many reactants and wide concentration ranges), larger ANNs and, therefore, a larger training dataset are required. We demonstrate that a moderately complex problem with four reactants can be addressed using ANNs in combination with the presented PAT method (low-field NMR) and with the proposed approach to generate meaningful training data. Graphical abstract.

Application of a new method for simultaneous phase and baseline correction of NMR signals (SINC)

Recently, we presented a new approach for simultaneous phase and baseline correction of nuclear magnetic resonance (NMR) signals (SINC) that is based on multiobjective optimization. The algorithm can automatically correct large sets of NMR spectra, which are commonly acquired when reactions and processes are monitored with NMR spectroscopy. The aim of the algorithm is to provide spectra that can be evaluated quantitatively, for example, to calculate the composition of a mixture or the extent of reaction. In this work, the SINC algorithm is tested in three different studies. In an in-house comparison study, spectra of different mixtures were corrected both with the SINC method and manually by different experienced users. The study shows that the results of the different users vary significantly and that their average uncertainty of the composition measurement is larger than the uncertainty obtained when the spectra are corrected with the SINC method. By means of a dilution study, we demonstrate that the SINC method is also applicable for the correction of spectra with low signal-to-noise ratio. Furthermore, a large set of NMR spectra that was acquired to follow a reaction was corrected with the SINC method. Even in this system, where the areas of the peaks and their chemical shifts changed during the course of reaction, the SINC method corrected the spectra robustly. The results show that this method is especially suited to correct large sets of NMR spectra and it is thus an important contribution for the automation of the evaluation of NMR spectra.

Prediction of flow effects in quantitative NMR measurements

A method for the prediction of the magnetization in flow NMR experiments is presented, which can be applied to mixtures. It enables a quantitative evaluation of NMR spectra of flowing liquid samples even in cases in which the magnetization is limited by the flow. A transport model of the nuclei's magnetization, which is based on the Bloch-equations, is introduced into a computational fluid dynamics (CFD) code. This code predicts the velocity field and relative magnetization of different nuclei for any chosen flow cell geometry, fluid and flow rate. The prediction of relative magnetization is used to correct the observed reduction of signal intensity caused by incomplete premagnetization in fast flowing liquids. By means of the model, quantitative NMR measurements at high flow rates are possible. The method is predictive and enables calculating correction factors for any flow cell design and operating condition based on simple static T₁ time measurements. This makes time-consuming calibration measurements for assessing the influence of flow effects obsolete, which otherwise would have to be carried out for each studied condition. The new method is especially interesting for flow measurements with compact medium field NMR spectrometers, which have small premagnetization volumes. In the present work, experiments with three different flow cells in a medium field NMR spectrometer were carried out. Acetonitrile, water, and mixtures of these components were used as model fluids. The experimental results for the magnetization were compared to the predictions from the CFD model and good agreement was observed.

Benchtop NMR for Online Reaction Monitoring of the Biocatalytic Synthesis of Aromatic Amino Alcohols

Online analytics provides insights into the progress of an ongoing reaction without the need for extensive sampling and offline analysis. In this study, we investigated benchtop NMR as an online reaction monitoring tool for complex enzyme cascade reactions. Online NMR was used to monitor a two-step cascade beginning with an aromatic aldehyde and leading to an aromatic amino alcohol as the final product, applying two different enzymes and a variety of co-substrates and intermediates. Benchtop NMR enabled the concentration of the reaction components to be detected in buffered systems in the single-digit mM range without using deuterated solvent. The concentrations determined via NMR were correlated with offline samples analyzed via uHPLC and displayed a good correlation between the two methods. In summary, benchtop NMR proved to be a sensitive, selective and reliable method for online reaction monitoring in (multi-step) biosynthesis. In future, online analytic systems such as the benchtop NMR devices described might not only enable direct monitoring of the reaction, but may also form the basis for self-regulation in biocatalytic reactions.

Fast time-resolved NMR with non-uniform sampling

NMR spectroscopy is a versatile tool for studying time-dependent processes: chemical reactions, phase transitions or macromolecular structure changes. However, time-resolved NMR is usually based on the simplest among available techniques - one-dimensional spectra serving as "snapshots" of the studied process. One of the reasons is that multidimensional experiments are very time-expensive due to costly sampling of evolution time space. In this review we summarize efforts to alleviate the problem of limited applicability of multidimensional NMR in time-resolved studies. We focus on techniques based on sparse or non-uniform sampling (NUS), which lead to experimental time reduction by omitting a significant part of the data during measurement and reconstructing it mathematically, adopting certain assumptions about the spectrum. NUS spectra are faster to acquire than conventional ones and thus better suited to the role of "snapshots", but still suffer from non-stationarity of the signal i.e. amplitude and frequency variations within a dataset. We discuss in detail how these instabilities affect the spectra, and what are the optimal ways of sampling the non-stationary FID signal. Finally, we discuss related areas of NMR where serial experiments are exploited and how they can benefit from the same NUS-based approaches.

Online reaction monitoring by single-scan 2D NMR under flow conditions

Single-scan 2D NMR based on spatial encoding can be used to monitor chemical reactions with a flow unit in realistic reaction conditions.

Enabling technologies in polymer synthesis: accessing a new design space for advanced polymer materials

This review discusses how developments in laboratory technologies can push the boundaries of what is achievable using existing polymer synthesis techniques.

Monitoring Heterogeneously Catalyzed Hydrogenation Reactions at Elevated Pressures Using In-Line Flow NMR

We present a novel setup that can be used for the in-line monitoring of solid-catalyzed gas-liquid reactions. The method combines the high sensitivity and resolution of a stripline NMR detector with a microfluidic network that can withstand elevated pressures. In our setup we dissolve hydrogen gas in the solvent, then flow it with the added substrate through a catalyst cartridge, and finally flow the reaction mixture directly through the stripline NMR detector. The method is quantitative and can be used to determine the solubility of hydrogen gas in liquids; it allows in-line monitoring of hydrogenation reactions and can be used to determine the reaction kinetics of these reactions. In this work, as proof of concept we demonstrate the optimization of the Pd-catalyzed hydrogenation reactions of styrene, phenylacetylene, cyclohexene, and hex-5-en-2-one in a microfluidic context.

Nonstationary Two-Dimensional Nuclear Magnetic Resonance: A Method for Studying Reaction Mechanisms in Situ

Nuclear magnetic resonance spectroscopy (NMR) is a versatile tool of chemical analysis allowing one to determine structures of molecules with atomic resolution. Particularly informative are two-dimensional (2D) experiments that directly identify atoms coupled by chemical bonds or a through-space interaction. Thus, NMR could potentially be powerful tool to study reactions in situ and explain their mechanisms. Unfortunately, 2D NMR is very time-consuming and thus often cannot serve as a "snapshot" technique for in situ reaction monitoring. Particularly difficult is the case of spectra, in which resonance frequencies vary in the course of reaction. This leads to resolution and sensitivity loss, often hindering the detection of transient products. In this paper we introduce a novel approach to correct such nonstationary 2D NMR signals and raise the detection limits over 10 times. We demonstrate success of its application for studying the mechanism of the reaction of AgSO₄-induced synthesis of diphenylmethane-type compounds. Several reactions occur in the studied mixture of benzene and toluene, all with rather low yield and leading to compounds with similar chemical shifts. Nevertheless, with the use of a proposed 2D NMR approach we were able to describe complex mechanisms of diphenylmethane formation involving AgSO₄-induced toluene deprotonation and formation of benzyl carbocation, followed by nucleophilic attacks.

Synthesis of α-fluoro-α,β-unsaturated esters monitored by 1D and 2D benchtop NMR spectroscopy

For optimization and control of pharmaceutically and industrially important reactions, chemical information is required in real time. Instrument size, handling, and operation costs are important criteria to be considered when choosing a suitable analytical method apart from sensitivity and resolution. This present study explores the use of a robust and compact nuclear magnetic resonance (NMR) spectrometer to monitor the stereo-selective formation of α-fluoro-α,β-unsaturated esters from α-fluoro-β-keto esters via deprotonation and deacylation in real time. These compounds are precursors of various pharmaceutically active substances. The real-time study revealed the deprotonation and deacylation steps of the reaction. The reaction was studied at temperatures ranging from 293 to 333 K by interleaved one-dimensional ¹ H and ¹⁹ F and two-dimensional ¹ H-¹ H COSY experiments. The kinetic rate constants were evaluated using a pseudo first-order kinetic model. The activation energies for the deprotonation and deacylation steps were determined to 28 ± 2 and 63.5 ± 8 kJ/mol, respectively. This showed that the deprotonation step is fast compared with the deacylation step and that the deacylation step determines the rate of the overall reaction. The reaction was repeated three times at 293 K to monitor the repeatability and stability of the system. The compact NMR spectrometer provided detailed information on the mechanism and kinetics of the reaction, which is essential for optimizing the synthetic routes for stepwise syntheses of pharmaceutically active substances.

Modular transmission line probes for microfluidic nuclear magnetic resonance spectroscopy and imaging

Microfluidic NMR spectroscopy can probe chemical and bio-chemical processes non-invasively in a tightly controlled environment. We present a dual-channel modular probe assembly for high efficiency microfluidic NMR spectroscopy and imaging. It is compatible with a wide range of microfluidic devices, without constraining the fluidic design. It collects NMR signals from a designated sample volume on the device with high sensitivity and resolution. Modular design allows adapting the detector geometry to different experimental conditions with minimal cost, by using the same probe base. The complete probe can be built from easily available parts. The probe body mainly consists of prefabricated aluminium profiles, while the probe circuit and detector are made from printed circuit boards. We demonstrate a double resonance HX probe with a limit of detection of 1.4 nmol s^-1/2 for protons at 600 MHz, resolution of 3.35 Hz, and excellent B₁ homogeneity. We have successfully acquired ¹H-¹³C and ¹H-¹⁵N heteronuclear correlation spectra (HSQC), including a ¹H-¹⁵N HSQC spectrum of 1 mM ¹⁵N labeled ubiquitin in 2.5 μl of sample volume.

Scalability of Visible-Light-Induced Nickel Negishi Reactions: A Combination of Flow Photochemistry, Use of Solid Reagents, and In-Line NMR Monitoring

The scale up of light-induced nickel-catalyzed Negishi reactions is reported herein, with output rates reaching multigram quantities per hour. This level of throughput is suitable to support preclinical medicinal chemistry programs in late lead optimization, where tens of grams to hundreds of grams of final product is needed. Adjusting reaction times and concentrations was critical in achieving this robust output. This example demonstrates how visible photochemistry and use of solid metal reagent can be used and how the progress of the reaction can be followed by in-line NMR monitoring.

TReNDS-Software for reaction monitoring with time-resolved non-uniform sampling

NMR spectroscopy, used routinely for structure elucidation, has also become a widely applied tool for process and reaction monitoring. However, the most informative of NMR methods-correlation experiments-are often useless in this kind of applications. The traditional sampling of a multidimensional FID is usually time-consuming, and thus, the reaction-monitoring toolbox was practically limited to 1D experiments (with rare exceptions, e.g., single-scan or fast-sampling experiments). Recently, the technique of time-resolved non-uniform sampling (TR-NUS) has been proposed, which allows to use standard multidimensional pulse sequences preserving the temporal resolution close to that achievable in 1D experiments. However, the method existed only as a prototype and did not allow on-the-fly processing during the reaction. In this paper, we introduce TReNDS: free, user-friendly software kit for acquisition and processing of TR-NUS data. The program works on Bruker, Agilent, and Magritek spectrometers, allowing to carry out up to four experiments with interleaved TR-NUS. The performance of the program is demonstrated on the example of enzymatic hydrolysis of sucrose.

Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors

A “Benchtop” NMR spectrometer is used for detailed monitoring of controlled and free radical polymerisations performed in batch and continuous reactors both offline and in real-time.

Online monitoring of the kinetic isotope effect in chemical reactions with 1H and 19F low-field NMR spectroscopy

The kinetic isotope effect (KIE) describes the change in the rate of a chemical reaction by substituting one of the atoms in the reactants with one of its isotopes. Investigating the KIE and its temperature dependency in reactions renders information for reconstructing chemical processes and confirming the rate-determining step. However, conventional methods to study the KIE, e.g. by calorimetry, conductivity, titration, Raman spectroscopy etc., require calibration and sophisticated handling of the reaction calorimeter, and the data are obtained at irregular and sparse intervals. This current study employs a compact NMR system as an alternative means to determine the temperature dependency of the reaction rate and, thus, the KIE, as well as the activation energy, enthalpy, and entropy of each reaction. Here the neutral hydrolysis of acetic anhydride and ethyl trifluoroacetate was studied in H₂O, D₂O and H₂O-D₂O mixtures with ¹H and ¹⁹F NMR spectroscopy. The activation energies for the hydrolysis of acetic anhydride with D₂O and H₂O were found to be 45 ± 2 kJ mol^-1 and 40 ± 2 kJ mol^-1, respectively. The activation energies of ethyl trifluoroacetate hydrolysis via¹⁹F NMR spectroscopy were determined to 46.7 ± 1 kJ mol^-1 and 54.9 ± 1 kJ mol^-1 for the reaction with H₂O and D₂O, respectively, and via¹H NMR spectroscopy to 48 ± 3 kJ mol^-1 and 55.8 ± 1 kJ mol^-1. The differences in rate constants and activation energies for both reactions in H₂O and D₂O are due to the kinetic isotope effect, involving the breakage and formation of O-H and O-D bonds during the rate-determining step. The proton inventory studies were performed for both the reactions for determining the isotopic fractionation factors for the given transition states of the reactions which help to predict the reaction mechanisms of other similar reactions. The compact NMR system is a relevant and practical tool to unmask precise reaction pathways, by tracing the KIE in real time with densely sampled data, which are essential for obtaining accurate rate constants.

Flow reactors integrated with in-line monitoring using benchtop NMR spectroscopy

The state-of-the-art flow reactors integrated with in-line benchtop NMR are thoroughly discussed with highlights on the strengths and weaknesses of this emerging technology.

Desktop NMR and Its Applications From Materials Science To Organic Chemistry

NMR spectroscopy is an indispensable method of analysis in chemistry, which until recently suffered from high demands for space, high costs for acquisition and maintenance, and operational complexity. This has changed with the introduction of compact NMR spectrometers suitable for small-molecule analysis on the chemical workbench. These spectrometers contain permanent magnets giving rise to proton NMR frequencies between 40 and 80 MHz. The enabling technology is to make small permanent magnets with homogeneous fields. Tabletop instruments with inhomogeneous fields have been in use for over 40 years for characterizing food and hydrogen-containing materials by relaxation and diffusion measurements. Related NMR instruments measure these parameters in the stray field outside the magnet. They are used to inspect the borehole walls of oil wells and to test objects nondestructively. The state-of-the-art of NMR spectroscopy, imaging and relaxometry with compact instruments is reviewed.

Desktop NMR spectroscopy for real-time monitoring of an acetalization reaction in comparison with gas chromatography and NMR at 9.4 T

Monitoring of chemical reactions in real-time is in demand for process control. Different methods such as gas chromatography (GC), mass spectroscopy, infrared spectroscopy, and nuclear magnetic resonance (NMR) are used for that purpose. The current state-of-the-art compact NMR systems provide a useful method to employ with various reaction conditions for studying chemical reactions inside the fume hood at the chemical workplace. In the present study, an acetalization reaction was investigated with compact NMR spectroscopy in real-time. Acetalization is used for multistep synthesis of the variety of organic compounds to protect particular chemical groups. A compact 1 T NMR spectrometer with a permanent magnet was employed to monitor the acid catalyzed acetalization of the p-nitrobenzaldehyde with ethylene glycol. The concentrations of both reactant and product were followed by peak integrals in single-scan ¹H NMR spectra as a function of time. The reaction conditions were varied in terms of temperature, agitation speed, catalyst loading, and feed concentrations in order to determine the activation energy with the help of a pseudo-homogeneous kinetic model. For low molar ratios of aldehyde and glycol, the equilibrium conversions were lower than for the stoichiometric ratio. Increasing catalyst concentration leads to faster conversion. The data obtained with low-field NMR spectroscopy were compared with data from GC and NMR spectroscopy at 9.4 T acquired in batch mode by extracting samples at regular time intervals. The reaction kinetics followed by either method agreed well. The activation energies for forward and backward reactions were determined by real-time monitoring with compact NMR at 1 T were 48 ± 5 and 60 ± 4 kJ/mol, respectively. The activation energies obtained with gas chromatography for forward and backward reactions were 48 ± 4 and 51 ± 4 kJ/mol. The equilibrium constant decreases with increasing temperature as expected for an exothermic reaction. The impact of dense sampling with online NMR and sparse sampling with GC was observed on the kinetic outcome using the same kinetic model. Graphical abstract Acetalization reaction kinetics were monitored with real-time desktop NMR spectroscopy at 1 T. Each data point was obtained at regular intervals with a single shot in 15 s. The kinetics was compared with sparsely sampled data obtained with GC and NMR at 9.4 T.

"Click" analytics for "click" chemistry - A simple method for calibration-free evaluation of online NMR spectra

Driven mostly by the search for chemical syntheses under biocompatible conditions, so called "click" chemistry rapidly became a growing field of research. The resulting simple one-pot reactions are so far only scarcely accompanied by an adequate optimization via comparably straightforward and robust analysis techniques possessing short set-up times. Here, we report on a fast and reliable calibration-free online NMR monitoring approach for technical mixtures. It combines a versatile fluidic system, continuous-flow measurement of ¹H spectra with a time interval of 20s per spectrum, and a robust, fully automated algorithm to interpret the obtained data. As a proof-of-concept, the thiol-ene coupling between N-boc cysteine methyl ester and allyl alcohol was conducted in a variety of non-deuterated solvents while its time-resolved behaviour was characterized with step tracer experiments. Overlapping signals in online spectra during thiol-ene coupling could be deconvoluted with a spectral model using indirect hard modeling and were subsequently converted to either molar ratios (using a calibration-free approach) or absolute concentrations (using 1-point calibration). For various solvents the kinetic constant k for pseudo-first order reaction was estimated to be 3.9h^-1 at 25°C. The obtained results were compared with direct integration of non-overlapping signals and showed good agreement with the implemented mass balance.

Coupling and optimisation of online nuclear magnetic resonance spectroscopy and mass spectrometry for process monitoring to cover the broad range of process concentration

Real time online monitoring of chemical processes can be carried out by a number of analytical techniques, including optical and vibrational spectroscopies, nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). As each technique has unique advantages and challenges, combinations are an attractive option. The combination of a 500-MHz ¹ H NMR and a small footprint mass spectrometer to monitor a batch reaction at process concentration was investigated. The mass spectrometer was coupled into the flow path of an online reaction monitoring NMR. Reaction mixture was pumped from a 100-ml vessel to an NMR flow tube before returning to the vessel. Small aliquots were diverted into a sampling make-up flow using an active flow splitter and passed to the mass spectrometer. Advantages of the combination were observed. ¹ H NMR was ideal for quantitation of high level components, whereas MS showed a greater capability for detecting those at low level. In preliminary experiments MS produced a limited linear relationship with concentration (0.02% to 2% relative concentration, 0.01 mg/ml-1.25 mg/ml), because of signal saturation at the higher concentrations. NMR was unable to detect components below 0.1% relative to concentration maximum. Optimisation of sample transfer to the MS extended the linearity to 10% relative to the concentration maximum. Therefore, the combination of online NMR and MS allows both qualitative and quantitative analysis of reaction components over the full process range. The application of the combination was demonstrated by monitoring a batch chemical reaction and this is described. Copyright © 2016 John Wiley & Sons, Ltd.

Continuous-flow synthesis of fluorine-containing fine chemicals with integrated benchtop NMR analysis

Fluorine fine chemical synthesis takes a step ahead using integrated NMR online-analysis with a benchtop NMR spectrometer.