th  - 1988 - 51st ENC Conference

is6 I
ROTATING FRAME COHERENCE TRANSFER DUE TO TUNNELLING
Eric R. Johnston Haverford College Haverford, PA 19041
Slow tunnelling of protons and heavier atoms in chemical reactions is a
subject of considerable interest but the unambiguous demonstration of it is
difficult. Unlike thermally activated chemlcal exchange (which randomly averages
chemical shift differences) tunnelling coherently averages such differences and
its effects in NMR are expected to be quite different from those due to exchange.
In particular in the Hennig-Limbach rotating frame exchange experiment (i) the
tunnelling frequency should appear not in the decay rate of the spln-locked
magnetization but as an oscillation imposed upon the decay (2). Moreover,
tunnelling is expected to be invisible in selective inversion transfer exper-
iments in contrast to chemical exchange.
The theory describing these effects is briefly d¢scussed and experiments
illustrating them are presented. We have employed coherent spin decoupllng as
a (mathematically equivalent) model for a tunnelling process in experiments on
an AX spin system which involve spin locking of the X doublet components in the
presence of weak resonant decoupling of the A spin. The question of proton
tunnelling in tetraphenyl porphine (TPP) ~s cr~t~cally addressed ~n light of o6r
results and the possibility of employing a spin locking experiment to detect
slow tunnelling is assessed.
i. J. Hennlg and H. H. Limbach, J. Ma~n. Reson., 49, 322 (1982).
2. E. Johnston, J. Magn. Reson., in press (July, 1988).
157
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Satish Arora § and Mohan CharP
* Department of Chemistry, Rice University, Houston, Texas 77254;
§ College of Pharmacy, DDI, University of Texas, Austin, Texas 75712;
+~ Baylor MRI Center, Woodlands, Texas 77381
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