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