th  - 1988 - 51st ENC Conference

198 I NUMERICAL STUDIES OF STIMULATED ESEEM WAVEFORMS: H. Jin and H.
omann, Corporate Research Laboratory, EXXON Research and Engineering Company,
Route 22 East, Annandale, N. J. 08801
Electron spin echo envelope modulation (ESEEM) spectroscopy has proven to be a
powerful method for studying hyperfine and quadrupolar interactions of nuclei
coordinated to paramagnetic electron centers. Important areas of application
include the study of nitrogen coordination in metalloproteins; surface adsorbate
interactions of supported transition metals; and the electron density distribution
on organic paramagnetic radicals important in photosynthesis and in photoexcited
triplet states. Analysis of the ESEEM patterns for S-I/2, I -I spin systems in
randomly oriented solids is usually performed using frequency spectrum analysis.
ESEEM spectra are simulated by calculating the superposition of the two powder
pattern quadrupolar spectra obtained when the isotropic hyperfine coupling adds or
subtracts to the Zeeman field. Such simulations will accurately predict ESEEM
frequencies but will usually not even give qualitatively correct modulation depths
in the ESEEM waveform or reproduce the correct linewidths in the ESEEM spectrum.
These depend on the anisotropic hyperfine interactions and are therefore important
spectroscopic parameters which can reveal additional chemical information about the
coordination complex. This provides the impetus for numerical studies of the ESEEM
spectrum and time domain waveforms in which the psuedodipolar as well as the
isotropic hyperfine and quadrupolar interactions are retained. Some preliminary
numerical results of this study will be presented in this poster. In particular, we
explore the importance of the relative magnitudes of the Zeeman, isotropic and
psuedodipolar hyperfine, and quadrupolar interactions in determining the modulation
depth of the ESEEM waveform. We also explore the ESEEM waveform and frequency
spectrum in the presence of anisotropic hyperfine interactions with only a partial
cancellation of the Zeeman field by the isotropic hyperfine coupling.
-- 1 99 l HIGH PRESSURE 13C CROSS-POLARIZATION AND SPIN RELAXATION STUDY OF
ADAMANTANE: K.O. Prins and D. van der Putten, Van der Waals Laboratory, University
of Amsterdam, Postbus 20216, 1000 HE Amsterdam, The Netherlands.
The poster presents a description of a double resonance probe suitable for IH-I~C
cross-polarization experiments at hydrostatic pressure up to 10 kbar. The probe is
placed in a liquid nitrogen cryostat, constructed inside the 13 cm bore of a 4.2 T
superconducting magnet.
Cross-polarization has been used in a study of the effect of high pressure on
molecular reorientation in the orientationally disordered solid phase I and in the
ordered phase II of adamantane. It is shown that knowledge of the iH and 13C
relaxation times T and T allows distinction between isotropic rotational diffusion
i 10
and discrete reorientations in the two solid phases. In phase I adamantane spends
a non-negligible time between its equilibrium orientations. In phase II the experimen-
tal results are well described by a discrete reorientational model. A broadening of
the 13C resonance observed while spin-locking the protons occurs at increasing
pressure.
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