Mass Spectrometry & Spectroscopy 15


the thermodynamic barrier for rotation of the carbon-nitrogen bond can be calculated. With a benchtop instrument, NMR experiments can now follow a similar pattern with prediction, experiment, and result happening in the same session.


Reaction Monitoring


The ability to easily integrate flow cells into benchtop NMR also extends the benefits for both research and teaching beyond structural elucidation. With benchtop NMR, analysis can be run in minutes to determine the success of a reaction – no more running down to the user facility or sending samples to external companies for routine analysis.


One simple example is the flow rate and temperature dependent esterification of ethanol and ethanoic acid catalysed by sulphuric acid (reaction shown in Figure 5). The product synthesised, ethyl ethanoate, a solvent that is used for varnishes, lacquers, dry cleaning, stains, fats, and nitrocellulose.


Figure 5: Flow rate and temperature dependent esterification of ethanol and ethanoic acid catalysed by sulphuric acid.


To investigate the rate constant dependence on temperature, a flow cell set-up (Figure 6) was used with X-Pulse and the reaction performed at varying temperatures.


Table 2: Temperature dependent variation in rate constant.


Figure 6: Schematic (above) and physical set-up (below) in a fume hood showing how flow chemistry is incorporate.


Table 2 shows the temperature dependent variation in rate constant. NMR additionally enables determination of enthalpy (ΔH), entropy (ΔS) (associated with reaching the transition state from the reactants, and the Gibbs Free Energy (ΔG). For more information on how this was calculated please see our app note here.


Conclusion


The availability of cryogen free, benchtop NMR has enabled analytical scientists, university teaching courses, and industrial


research facilities to bring the NMR technique right into the heart of their labs. Benchtop NMR has removed the high upfront and maintenance cost of traditional high-field NMR, and with the arrival of the true broadband X-Pulse NMR incorporating comprehensive flow and temperature control, a single instrument now addresses needs ranging from student teaching, right through to high end R&D and industrial QA/QC.


References 1. E. O. Stejskal & J. E. Tanner, J. Chem. Phys., 1965, 42, 288-292.


2. M. Defernez; E. Wren; A. D. Watson; Y. Gunning; I. J. Colquhoun; G. L. Gall; D. Williamson & E. K. Kemsley, Food Chem., 2017, 216, 106-113


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