The project comprised a number of complementary developments, which are presented here by method rather than chronologically.
One of the main objectives of the project was to increase the accuracy of spatially encoded diffusion-ordered NMR spectroscopy (SPEN DOSY), which provides a complete diffusion NMR data set in a single scan. We have first characterised the properties of the method in its initial form and produced a set of guidelines on how to use it. (Jacquemmoz et al., Magn. Reson. Chem. 60, 121 (2022)). We have then shown how to account for the non-uniformity of magnetic-field gradients, and this results in a significant improvement of the accuracy (Lorandel et al. J. Magn. Reson. 355, 107543). We have also adapted multivariate processing algorithms so that they can be used to “unmix” the spectra of compounds using SPEN DOSY data. These include direct exponential curve resolution algorithm, which required the design of an original frequency-swept pulse (Mishra et al., Chem. Commun. 57, 2384 (2021), Mishra et al. J. Magn. Reson. 334, 107114 (2021)), and speedy component resolution (Lorandel et al. Magn. Reson. Chem. 63, 49). These developments are now broadly accessible thanks to the distribution of pulse sequence code, and to the integration of the processing and analysis code in a widely used software library.
One of the initial objectives of the project was to develop single-scan diffusion NMR experiments that are applicable to continuously flowing samples. We have carried out theoretical and numerical investigation that illustrate the challenges of this approach, and ways to address them (Mishra et al., J. Chem. Phys. 158, 014204 (2023)). We have also shown that classical DOSY expeirments can be adapted such that they are applicable to continuously flowing samples, thanks to a combination of strategies that mitigate the effect of sample motion (Marchand et al. Chem. Eur. J. e202201175 (2022). The method was also used to monitor an organic chemical reaction.
A complementary set of development was carried out to obtain flow compatible ultrafast (UF) 2D NMR methods. We have shown that the use of adapted encoding schemes made it possible to obtain high-quality spectra with good repeatability, for flow rates that are relevant for online monitoring applications (Jacquemmoz et al., Analyst, 145, 478)). While that initial approach was limited in terms of the achievable spectral width, we have later shown how to obtain broadband spectra, with also improved suppression of the solvent signal (Lhoste et al. Analyst, 148, 5255). The method was used to monitor an organocatalysed reaction.
The flow compatible methods developed during the project were also used for the in-line monitoring of flow reactions. An original setup was assembled, using flow NMR at high-field to analyse the output of a flow reactor. We then used solvent-suppressed UF NMR to obtain 2D spectra for series of runs of a photochemical click reaction (Bazzoni et al., Chem. Eur. J. e202203240 (2023)). In this case the use of fast detection methods is motivated by the goal to run consecutive reactions in rapid sequence. Towards the integration of these methods within an intelligent flow reactor, we also showed how in-line detection by high-field NMR can guide the self-optimisation of reaction conditions by an autonomous flow reactor (El Sabbagh et al., React. Chem. Eng. 9, 2599 (2024)).
Sensitivity is often a limitation for the analysis of mixtures by NMR. Dissolution dynamic nuclear polarisation (D-DNP) is a method that provides high signal enhancement, but it is a single shot technique and required UF NMR to collect 2D spectra. We have shown that this was also possible for 2D experiments that are particularly well suited for mixtures analysis, namely total correlation spectroscopy and multiple-quantum spectroscopy (Singh et al., Chem. Commun. 57, 8035)). Other methods are being developed, with the aim of having a lower cost than D-DNP. HYPNOESYS is one such method, based on intermolecular polarisation transfer from highly polarised naphthalene molecules. We have shown that mixtures can be polarised with this approach, and that UF NMR can then be used to obtain 2D spectra from these mixtures (Parker et al., Angew. Chem. Int. Ed. e202312302 (2023)).
During the course of the project, a new class of single-scan ultraselective experiment was reported, which is particularly powerful for the rapid analysis of mixtures. Building on knowledge acquired while working on UF NMR methods, we have shown how to adapt these single-scan ultraselective experiments such that they become much more immune to the deleterious effects of translational molecular diffusion on sensitivity (Bazzoni et al., Angew. Chem. Ind. Ed. e202314598 (2023)).
