C-13 CHEMICAL-SHIFT CORRELATION, SPIN-DIFFUSION AND SELF-DIFFUSION INISOTOPICALLY ENRICHED TROPOLONE

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.