Z. Olender et al., C-13 CHEMICAL-SHIFT CORRELATION, SPIN-DIFFUSION AND SELF-DIFFUSION INISOTOPICALLY ENRICHED TROPOLONE, Journal of magnetic resonance. Series A, 120(1), 1996, pp. 31-45
Carbon-13 one-dimensional magnetization transfer and two-dimensional r
otor-synchronized exchange NMR experiments of magic-angle-spinning (MA
S) samples of solid tropolone are reported. Experiments were performed
on samples enriched to 25% C-13 in the carbonyl or the hydroxy carbon
s, as well as on a normal (unenriched) sample and a mixture of both. T
he result shows the presence of spin exchange between the two types of
carbons as well as between similar carbons in crystallographically in
equivalent, but symmetry-related sites. This effect is used to determi
ne the relative orientations of the various chemical-shift tensors and
, using the known crystal structure, also their orientation in the mol
ecular frame. The rates of the spin exchange are relatively fast in th
e enriched sample, the magnetization transfer being highly nonexponent
ial and almost independent of temperature. In the normal sample, the s
pin exchange is slow, the magnetization transfer is nearly exponential
, and the rates depend strongly on temperature. In the isotopically mi
xed sample, the spin-exchange rates are intermediate. These results in
dicate that, at high carbon-13 concentration, spin exchange is dominat
ed by spin diffusion, while in natural abundance it is dominated by th
ermally activated self diffusion. To simulate the spin-diffusion proce
ss, a method is introduced in which the dipolar interactions between a
ll the carbon-13 nuclei in the crystal are considered. The results rep
roduce well the experimental magnetization decay curves in the (25%) e
nriched sample and provide kinetic parameters for the spin-diffusion p
rocess, In normal tropolone, the spin-exchange experiments confirm the
occurrence of self diffusion in which the molecules undergo translati
onal jumps between different sites in the lattice. (C) 1996 Academic P
ress, Inc.