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 (Δν_1/2 = 30 Hz) and a narrower one (Δν_1/2 = 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).