th  - 1987 - 51st ENC Conference

Thursday - AM
COHERENCE AUGMENTATION BY LIMITING
THE ENTROPY IN CATALYTIC HYDROGENATION
Daniel P. Weitekamp
Arthur Amos Noyes Laboratory of Chemical Physics
California Institute of Technology 127-72
Pasadena, CA 91125
The small equilibrium population differences between nuclear spin
energy levels at ambient temperatures restrict the size of NMR signals to less
than 10 -4 of their theoretical maxima. Such well-known methods of signal
enhancement as the Overhauser effect, ClDNP, and optical nuclear
polarization create large nonequilibrium nuclear magnetizations by taking
advantage of the coupling to unpaired electron spins. This talk will present
a fundamentally different approach to very large nonequilibrium
polarizations, which may be viewed as a coupling of nuclear spins to
rotational population differences through chemical reaction.
A consequence of the symmetrization postulate of quantum mechanics is
that in molecules with equivalent nuclei there is a strict correlation between
nuclear spin state and rotational state. In molecular hydrogen the even
rotational states have the singlet nuclear spin wavefunction (para-H2) and
may be prepared as a low-entropy room-temperature chemical reagent.
Symmetry-breaking molecular addition of para-H2 results in highly J-
ordered spin states in the product molecule, which upon rf irradiation give
rise to extremely large NMR signals.
The apparatus1 built to perform such reactions in the spectrometer will
be described. Experimental results at ambient temperatures show
enhancements of at least several hundred relative to equilibrium signals for
products and intermediates of homogeneous hydrogenation catalysis.
These results will be compared with the predictions of the density operator
theory of this phenomenon2 modified to include relaxation effects.
1. C.R. Bowers and D. P. Weitekamp, in preparation.
2. C. R. Bowers and D. P. Weitekamp, Phys. Rev. Lett. 57, 2645 (1986).