A 506-PP NMR tube (Wilmad) is blown onto a 7 cm long piece of Pyrex tubing (thin wall; O.D. 5 mm) and a female 14/20 ground glass joint. The NMR tube, gas inlet adapter, vacuum transfer apparatus, a modified spinner (Figure B.1), chemical precursor, Krytox™ grease, and rubber bands are brought inside a glovebox. The NMR tube is loaded with the precursor (typically 7 mg or 10 μmol) and the apparatus is assembled. Krytox™ grease is used on all ground glass joints and the joints are held together with rubber bands. The stopcocks of the apparatus are closed, the apparatus is removed from the glovebox, and then the apparatus is attached to a Schlenk line through the hose connectors on both the vacuum transfer apparatus and the gas inlet adapter. A Schlenk flask containing the NMR solvent (in our case, CDCl2F) is also attached to the vacuum
transfer apparatus (Figure B.2). Stopcocks A and B are opened and the stopcock to the Schlenk line is cycled between vacuum and N2 to remove the air between stopcock A and the solvent flask.
Stopcock B is closed and the CDCl2F is cooled with an ice/water bath to reduce the vapor pressure
of the solvent (b.p., 9 °C). Stopcocks C and D are opened to evacuate the NMR tube and then stopcock D is closed to place the apparatus under static vacuum. A Dewar flask is filled with liquid nitrogen and placed underneath the NMR tube so that the solid sample inside the NMR tube is just below the level of the liquid nitrogen. The Schlenk flask containing the solvent is opened briefly to condense just enough of the solvent on top of the sample to cover it. The liquid nitrogen bath is removed and the solvent is allowed to warm to dissolve the solid sample. If the sample does not completely dissolve in the solvent, then more solvent is condensed into the tube. Once the solid has been completely dissolved, the sample is submerged in liquid nitrogen just enough to
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freeze the solution. The solvent flask is opened and the Dewar flask containing the liquid nitrogen slowly raised to condense and freeze fresh solvent on top of the solution.
Figure B.1: Pictures of the modified spinner and the spinner extension (length: 40”, OD: 7/8”, ID: 1/2”). The extension has four small holes (1/8” diameter) to allow pressure equalization between its interior and exterior.
Once a sufficient volume of solvent has been transferred to the NMR tube, the solvent flask is closed and stopcock D is opened. The apparatus is back-filled with N2. The vacuum transfer
apparatus and solvent flask are removed and the gas inlet adapter is plugged with a rubber septum. A portion of the pure CDCl2F is thawed, and triflic acid is added by means of a gas-tight syringe.
The rubber septum is replaced with a glass stopper. The entire sample is frozen and evacuated, and then the NMR tube is flame sealed under dynamic vacuum. A Dewar flask is filled with liquid nitrogen and capped with a polyethylene cap that has a 5-mm hole. The NMR sample is inserted
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through the hole to keep the sample frozen while the NMR instrument probe is cooled. The cap thermally insulates the spinner to minimize deposition of ice.
Figure B.2: A diagram of the apparatus used to add solvent to the NMR sample. The vacuum transfer apparatus and solvent flask are removed before addition of triflic acid.
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The acquisition of NMR spectra at -130 °C presents several technical challenges. The sample tube is brittle at these temperatures and is prone to break when inserted into the probe. When the probe is kept at cryogenic temperatures for extended periods of time, water can condense at the top of the probe stack and drip down into the instrument. At -130 °C, the flow of gas to the instrument must be increased in order to eject and insert samples. If, upon ejection, the sample is arrested in the probe due to buildup of ice, the pressure inside the probe can increase. The violent decompression that results when the sample breaks free can damage the probe. To minimize the occurrence of complications, we made several modifications to the procedure for using the NMR instrument when conducting experiments below -80 °C.
The first modification is to insert and eject samples manually. To do this, the sample tube is equipped with a modified spinner. A threaded, hollow stick made from Nylon 6/6, hereby referred to as the spinner extension (Figure B.1), is screwed onto the spinner and the sample tube is lowered into the probe by hand. The extension is long enough to protrude from the top of the probe stack when the sample is inserted (Figure B.3). An O-ring (high-purity silicone, OD: 1- 1/8”, ID: 7/8”) at the top of the extension ensures that the extension and the sample remain centered in the instrument. The use of the extension prevents spinning of the sample. However, our NMR instrument does not spin below -90 °C even without usage of the stick. When spectra have been acquired and the experiment is over, the extension and sample are lifted out of the probe.
Another modification we made is to insulate the top of the probe stack with Tygon® tubing (length: 12”, OD: 1-5/8”, ID: 1-1/4”) (Figure B.3). The tubing is fitted over the probe stack and acts as a sleeve to prevent atmospheric water from condensing onto the probe stack. Thermally insulating the probe stack reduces the water that condenses and drips into the instrument, which occurs when the instrument is kept cold continuously for multiple hours. Additionally, the
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insulating tubing is flushed continuously with N2 by means of a black vinyl line placed inside the
tubing. A slow flow of N2 discourages the diffusion of atmospheric water into the probe. The
tubing is removed before insertion or ejection of the NMR sample, and then replaced afterward.
Figure B.3: Diagram of the top of the NMR instrument. The sample is seated in the probe at the bottom of the spinner extension, which is screwed into the spinner. The extension protrudes out from the probe stack for ease of handling. The O-ring keeps the extension and sample centered in the instrument. The tube and N2 gas line discourage atmospheric water from entering the probe