Tunnel Spectroscopy by Dipolar-Driven NMR
At low static fields nmr transitions are driven at least in part by the dipole-dipole interactions being modulated by the applied rf field. The result is that the Δm = 2 transition is comparable in intensity with the main Δm = 1 transition. More importantly, transitions which are symmetry forbidden at high field may become allowed at low field. This applies to the tunnelling sidebands of CH3 and NH4 groups observed at low temperatures, where the transitions involve a change in proton permutation symmetry. By carrying out nmr in fields of the order of 0-0.02 T tunnel sidebands are easily observed. This has opened a new window of tunnel spectroscopy for tunnel frequencies from 40 kHz - 1 MHz. Several examples will be shown. To maintain reasonable sensitivity, a field cycling procedure is used in which the initial polarisation and final measurement are carried out at high fields. Apart from measuring tunnel frequencies, a number of interesting and unexpected features have been observed. The small tunnel frequencies accessible to this technique are characteristic of CH3 groups terminating aliphatic chains. It therefore provides a sensitive probe of the environment of CH3 groups in a very wide range of materials.
KeywordsLithium Ketone Dimethyl Alkane Suffix
Unable to display preview. Download preview PDF.