th  - 1988 - 51st ENC Conference

~
(POSTER ABSTRACT) BARBARA LYONS/CORNELL UNIVERSITY
CHALLENGES TO THE CLASSICAL MODELS OF REACTIVITY
148 I The possibility of heavy atom tunneling, in this case that of
carbon atoms, has been investigated through calculational methods by several different
researchers in recent years. However, the molecules of interest used in the calculations
of these researchers have not been readily feasible as models for study in the
laboratory. To test the theory of heavy atom tunneling a suitable molecule needed to bE
found: one which would lend itself to available laboratory techniques. We turned to the
molecule semibullvalene as a possibility, due partly to its startlingly low energy of
activation(5.5+0.1kcal/mol at -140°C).
Semibul~alene undergoes a degenerate cope rearrangement at room temperature. The
classical'~C isotope effect for the rearrangement at -170°C is k../k.~=l.04. However
~ 1~
by performing a simplistic quantum mechanical calculation we preozcted that, including
tunneling contributions, the isotope effect for semibullvalen~3at -170°C should be
about kl2/k =2 8 The k value is determined from the fully C subst, molecule.
3 " " 13
To test t~zs hypothesis we turned to variable temperature n.m.r.line-shape analysis
using a 400 MHz spectrometer. Spectra were taken of the natural abundance molecule fro~
-160°C to -35°C in 3°C increments. Likewise, after the all C molecule had been synthesized,
spectra were taken in the same temperature range. A two-site exchange model
was used on a mainframe computer to calculate the relevant n.m.r, line-shapes for each
temperature. This gave us the rat~_constants for the natural abundance molecule.
However, upon comparing the all~3C spectra to the ~tural abundance spectra by overlap
plotting an unpleasant fact was noticed. The all--C line-shape width for each temperature
was the same as the natural abundance line-shape width for that same temperature.
Unfortunately only one conclusion was possible: that the fullyl3c molecule had
virtually the same rate constants at any given temperature. Therefore, in at least the
temperature range we were looking at, tunneling was not a major contributing factor
to the reaction process.
149
I 19F NMR STUDIES OF FLUORINE SUBSTITUTED Ba2YCu307_ x
C. E. Lee*, D. White, P.K. Davies, J. A. Stuart
University of Pennsylvania, Philadelphia, PA 19104
Gaseous phase exchange has been used to introduce substantial
concentrations of fluorine into orthorhombic perovskite powder
samples. The temperature dependence of the NMR lineshape and
signal amplitude in the normal and superconducting states are
presented for several fluorine concentrations. In general, it is found
that the presence of the oxy-fluoride phase does not significantly
affect the superconducting transition temperature of the bulk
sample, but does decrease the extent of field exclusion, i.e., the
Meissner effect of the bulk powder. Some preliminary spin lattice
relaxation data are presented in which the 19F NMR is used as a
probe of the electronic environment of the oxide in the normal and
superconcucting states.
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