th  - 1988 - 51st ENC Conference

ITHU 10"55 I TIME DOMAIN ENDOR STUDIES OF DISORDERED SOLIDS: P. J. Tindall, M.
Bernardo, and H. Thomann, EXXON Corporate Research Laboratory, Route 22 East,
Annandale, N. J. 08801
Spectral simplification, resolution enhancement, and sensitivity enhancement are well
known advantages of multiple frequency techniques used in NMR. The ability to
coherently excite and coherently transfer longitudinal or transverse magnetization
among sub-levels of the spin system elgenstates is fundamental for the success of
most of these experiments and is only possible with time domain pulsed excitation. Ir
contrast to NMR, the most widely applied multiple resonance technique in ESR, the
ENDOR experiment, has traditionally been performed in the frequency domain. However,
recent advances in instrumentation have now made time domain ENDOR more feasible.
The time domain analog of the CW-ENDOR exper-lment is magnetization transfer (MT)
ENDOR using the Davies pulse sequence. MT-ENDOR has the advantage that the ENDOR
enhancement does not depend on the ratio of the electron and nuclear T 1 rates as it
does in CW-ENDOR. Furthermore, time domain excitation also makes possible more
complex double resonance experiments which depend on coherence transfer, such as
CT-ENDOR and splnor ENDOR recently demonstrated by Mehring et al. The general
applicability of these techniques to disordered solids will be governed by electron
T I and T m (phase memory) times which are typically shortened by disorder effects.
Fortunately, in many cases of interest, relaxation times for hydrocarbon radicals in
condensed hydrocarbons are sufficiently long for successful magnetization and
coherence transfer experiments even at room temperature. Experiments on transition
metal ion complexes and metal clusters are possible at liquid He temperatures. Some
recent time domain ENDOR results on isolated coal macerals, polyacetylene, and frozer
solutions of transition metal ion complexes will be presented.
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