th  - 1988 - 51st ENC Conference

THU 11:35 I
DYNAMIC NUCLEAR POLARIZATION IN THE NUCLEAR ROTATING FRAME, R~A.
Wind, H. Lock, L. Li and G.E. Maciel, Department of Chemistry,
Colorado State University, Ft. Collins, CO 80523.
Traditionally Dynamic Nuclear Polarization (DNP) has been used to enhance the
nuclear polarization directed along the external magnetic field. Several DNP mecha-
nisms can be responsible for this polarization enhancement, depending on the time-
dependence and character of the electron-nuclear interactions: the Overhauser
effect, the Solid-state effect and the direct and indirect thermal mixing effects.
In solids containing organic radicals all mechanisms often determine the observed
enhancement. It will be shown that it is also possible to enhance the nuclear
polarization in its rotating frame. This is obtained by irradiating with a strong
r.f. field at the nuclear Larmor frequency during the microwave irradiation. Com-
pared with the conventional method of DNP this technique has several advantages:
(i) the build-up time of the nuclear polarization is determined by the rotating-
frame relaxation time, which is often much shorter than the Zeeman relaxation time;
(ii) The 'forbidden' transition rate, which determines the enhancement due to the
indirect thermal mixing effect, is several orders ofmagnitude larger in the rotating
frame than in the laboratory frame. As a result the enhancement due to this effect
is one to two orders of magnitude larger in the rotating frame than in the labora-
tory frame; (iii) If the Zeeman relaxation time is short, the DNP enhancement in the
laboratory frame can be reduced, whereas the rotating-frame enhancement remains
large; (iv) rotating-frame DNP of abundant spins opens the possibility for multiple-
contact cross-polarization (CP) experiments and for CP experiments with long contact
times, independent of the value of the rotating-frame relaxation time.
Applications will be shown of rotating-frame 1H DNP and 13C-1H CP experiments
in doped styrene and coal.
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