* C.J. Jameson. Chem. Rev. 91, 1375-95 (1991).
Monday, May 11, 2020
NMR and the Liquid-Gas Interface
Most NMR spectra are recorded for liquid or solid phase samples. Many chemists have not even considered measuring NMR spectra of gas phase samples. Such spectra are indeed possible to record and the information available from such spectra has been studied and reviewed in detail.* In our first high school science classes we learn that molecules in the gas phase diffuse much more quickly than those in the liquid phase and that there is an equilibrium between the liquid and gas phases. These two elementary concepts can be demonstrated nicely with 1H NMR spectroscopy.
A suitable sample was prepared by putting 1-2 µL of acetone in a standard 5 mm NMR tube. A greased rubber plug was then forced into the tube such that it resided about 6 cm above the bottom of the NMR tube. This was done to limit the volume over which the vapour could diffuse to that of the active volume of the probe coil. The tube was then sealed with a torch to prevent the loss of sample. The sample contained a small amount of liquid in the bottom of the NMR tube and a mixture of acetone vapour and air above the liquid. A sketch of the sample is shown in the figure below.
The 600 MHz 1H data were collected in a cryoprobe at 298 K without a 2H lock. The magnet was shimmed using the 1H FID. The 1H spectrum has two resonances, a broad one at ~2.2 ppm (Δν1/2 = 30 Hz) and a narrower one (Δν1/2 = 4 Hz) at ~3.8 ppm due to liquid and gaseous acetone, respectively. The large 30 Hz line width for the liquid resonance is due to the magnetic susceptibility discontinuity boundary between the droplet of liquid with the glass and vapour interfaces. There may also be broadening as the droplet resides near the edge of the homogeneous region of the magnetic field. A DOSY spectrum, acquired with δ = 0.5 msec and Δ = 4.9 msec, illustrates the vastly different molecular diffusion rates between the liquid and gaseous phases of acetone. An EXSY spectrum, acquired with a 2 second mixing time, clearly shows exchange peaks between the liquid and the gas phases, illustrating the liquid-gas equilibrium.
* C.J. Jameson. Chem. Rev. 91, 1375-95 (1991).
* C.J. Jameson. Chem. Rev. 91, 1375-95 (1991).
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8 comments:
Thanks for the interesting post. I have seen papers that measured the diffusion coefficient of liquid acetone as ~4 x 10^-9 m2/s at 25ºC (e.g. Krüger, G. J.; Weiss, R. Z. Naturforsch Tl. A 1970, 25, 777). Do you know why your value for the liquid is faster here (~10^-7)?
Hi Glenn, thanks a lot for this beautiful example of detection of NMR signals from both the liquid and the gas phases in an NMR sample. I just wonder up to what volume the droplet could be increased, as 1-2 uL look rather small. Do you have any practical experience related to that?
Thanks in advance.
Jesús
Unknown,
The faster than expected diffusion for the liquid measured here may be because the quantity of liquid is quite small and it is in exchange with the faster diffusing gas. The measured diffusion constant be skewed towards that of the gas.
Glenn
Jesus,
I chose to use a very small quantity of liquid at the bottom of the tube so that the intensity of the liquid and gas resonances would be comparable. I suspect that if one used much more than a couple of micro litres, the liquid line would grossly overwhelm the gas line.
Glenn
Hi Glenn,
First, thanks for the many interesting posts on this blog, including this one. Do you have any estimation on the number of gas molecules you are looking at here?
Best wishes,
Sander
A pratical question: If I just have a droplet of acetone at the bottom of the tube, the liquid acetone will not be in the active volume, it will be outside the coil. How can I then detect signal from it? Or do you position the sample in a manner, that the liquid acetone is just inside the coil?
Sander,
Based on the integral ratio and (very) approximate volume of liquid, I would estimate the equivalent of 0.5 mg of acetone in the vapour phase.
Glenn
Eva,
I did not lift the NMR tube to center the droplet in the coil. I set the depth as usual. The droplet is just outside of the coil. The 90 degree pulse was the same within experimental error for both the liquid and gas lines. Since the liquid line was so broad, I assumed that it came from the tiny droplet on the bottom of the tube however, I suppose the liquid line could be from a very thin film of liquid on the NMR tube walls inside the active region of the coil.
Glenn
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