th  - 1987 - 51st ENC Conference

MK41
PARAHYDROGEN AND SYNTHESIS ALLOWS
DRAMATICALLY ENHANCED NUCLEAR ALIGNMENT
C. Russell Bowers* and Daniel P. Weitekamp
Arthur Amos Noyes Laboratory of Chemical Physics
California Institute of Technology 127-72
Pasadena, CA91125
Molecular hydrogen is known to come in two forms, para-H2 and
ortho-H2, which differ from one another in their nuclear spin states. A
consequence of the symmetrization postulate of quantum mechanics is that
these two forms are associated only with specific states of molecular rotation.
In particular, the lowest energy rotational state has the para nuclear spin
state, (1/2)t(]a~ >-ilia > ), so that low-temperature equilibration prepares the
protons predominantly in this non-magnetic state. This para-H2 may be
used even as a room temperature reagent, since conversion to the ortho form
is slow in the absence of a catal' y st.
This poster presents the ~irst experimental demonstration of the recent
prediction1 that the nuclear spin order ofpara-H2 can be converted by
chemical reaction into large v nonequilibrium NMR signals. The reaction
studied is the molecular addition of H2 to acrylonitrile to form propionitrile.
The reaction is performed in deuterochloroform solution in a standard NMR
tube at room temperature and pressure with the aid of Wilkinson's catalyst,
tris (triphenylphosphine) rhodium(I) chloride. The hydrogenation is initiated
by bubbling H2 gas through the solution containing acrylonitrile and catalyst.
If the gas has been enriched inpara-H2 by previous exposure to a
paramagnetic catalyst in liquidnitrogen, a n/4 pulse elicits an f.i.d, whose
Fourier transform shows antiphase multiplets at 'the expected propionitrile
frequencies as shown in)artn par~ ~a) of the figure. The amplitude of these lines is at
least two orders of magnitude greater than would result from the equilibrium
magnetization of the product formed. This is evidenced by the absence of
observable product signal in part b), obtained after a delay of several times T1,
even though the product is chemically stable.
(o)
(b)
f i ~ |
I I I
6 3 0
~ ) C.R. Bowers and D. P. Weitekamp, "rhe Transformation of
ppm
ymmetrization Order to Nuclear Spin Magnetization By Chemical
Reaction and NMR", Phys.Rev.Lett. 53, 2645 (1986).