Preparing NMR Samples on a Schlenk Line

Preparing an NMR sample to analyse a reaction mixture

Since thin layer chromatography is not compatible with many air and moisture sensitive compounds, it is common to analyse crude reaction mixtures by NMR spectroscopy prior to work-up or further manipulations.

Step 1: Cycle a J. Young’s NMR tube onto the Schlenk line with a minimum of three vacuum/inert gas cycles using an appropriate NMR tube adapter. A thermometer adapter or a rubber septum with a 5 mm hole at the bottom of the adapter provide a sufficiently air-tight seal that is ideal for securing the NMR tube in place.

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Cycling a J. Young’s NMR tube onto the Schlenk line using an adapter.

Step 2: Replace the glass stopper with a rubber septum under a flow of inert gas. Insert a bleed needle for 5-10 seconds to purge out any air that enters the system.

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Replacing the glass stopper with a rubber septum under inert gas.

Step 3: Add an aliquot of the reaction mixture (approximately 0.5 mL) to the NMR tube via cannula transfer (using a thin cannula) or with a syringe and needle.

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Adding an aliquot of crude reaction mixture to the NMR tube.

Step 4: Under a flow of inert gas, secure the Teflon valve onto the J. Young’s NMR tube. Close the stopcock or Teflon tap supplying the inert gas into the NMR tube adapter.

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Sealing the NMR tube under an inert atmosphere with a J. Young’s Teflon valve.

The protio-solvent in the NMR tube can now be removed under vacuum and the sample later redissolved in an appropriate deuterated solvent. Alternatively, the NMR sample can be run directly provided that the NMR spectrometer is setup for protio-solvents – this is particularly useful for heteronuclei such as ³¹P NMR spectroscopy.

Adding solvent or liquids to a sealed J. Young’s NMR tube on the Schlenk line

Step 1: Cycle the sealed J. Young’s NMR tube onto the Schlenk line using an appropriate adapter.

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Cycling a sealed J. Young’s NMR tube onto a Schlenk line.

Step 2: Under a flow of inert gas, remove the glass stopper, raise the NMR tube and remove the J. Young’s Teflon valve. Quickly lower the NMR tube and seal the adapter with a rubber septum. Insert a bleed needle through the septum for 10-15 seconds to purge out any air that is introduced into the system.

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Removing the Teflon valve under a flow of inert gas.

Step 3: Add the desired solvent or liquid reagent to the NMR tube via cannula transfer or needle and syringe.

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Adding solvent or liquid to the NMR tube.

Step 4: Under a flow of inert gas, secure the Teflon valve onto the J. Young’s NMR tube. Close the stopcock or Teflon tap supplying the inert gas into the NMR tube adapter.

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Sealing the NMR tube under an inert atmosphere with a J. Young’s Teflon valve.