th  - 1988 - 51st ENC Conference

196 1
TR FLUOROETHOXY DERIVATIVES: SELECTIVE DEACTIVATION OF
OXYGEN CONTAINING FUNCTIONAL GROUPS IN LANTHANIDE INDUCED SHIFTS
AND/OR RELAXATION NMR STUDIES. C. Wild*, C. Tsiao*, T. E. Glass,
J. Roy, H. C. Dorn, Chem. Dept. VPI&SU, Blacksburg, VA 24061.
During the last twenty years, a considerable number of
lanthanide shift reagents (LSR) have been used for structural
studies in organic chemistry. These shift reagents generally
function as weak Lewis acids which can form weak complexes with
nucleophilic functional groups present in the substrate of
interest. For the case of polyfunctional molecules, most
structural studies have been hampered because of the posssiblity
of complexation at the various nucleophilic sites in a given
molecule.
To overcome this problem, we have made use of trifluoro-
ethoxy group to selectively deactivate oxygen containing func-
tional groups towards complexation with lanthanide shift
reagents. Our initial studies illustrate the utility of these
reagents by comparing the lanthanide induced shifts (LIS) of
several trifluoroethyl ketals with their corresponding ethyl
analogs. The practical aspects of these reagents are explored
in a study which involved the selective deactivation of
specific sites in several polyfunctional molecules. In this
manner, structural information (e.g. cis/trans isomer assign-
ments) can be obtained from the LIS and spin-lattice relaxation
(TI) data.
197 I TIME DOMAIN ENDOR STUDIES OF DISORDERED SOLIDS: P. J. Tindall, H.
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. In
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-ENDORand splnor ENDOR recently demonstrated by Mehring et al. The general
applicability of these techniques to disordered solids will be governed by electron
T 1 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 frozen
solutions of transition metal ion complexes will be presented.
197