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要旨:
Relatively efficient spin diffusion among unprotonated carbons with large chemical-shift anisotropies can be achieved by a13C nuclear magnetic resonance multiple-pulse sequence with a lowduty cycle of ∼5% on the13C channel, which minimizes sample heating and reduces cumulative effects of pulse imperfections. The spin diffusion occurs among transverse-magnetization isochromats, while the total transverse magnetization is a conserved quantity under the average Hamiltonian. The “flip-flop” term of the dipolar-coupling average Hamiltonian is the same as in the full dipolar coupling, i.e., its scaling factor is unity. For a sample of 40%13COO-labeled poly(vinyl acetate), with13C in ester groups accounting for 7% of all heavy atoms, magnetization equilibrates within 20 ms over a volume of (0.9 nm)3, corresponding to a molecular mass of 500 Da, while the T2 relaxation time of the total transverse magnetization is ∼40 ms. The spin diffusion coefficient is estimated asD = 3 ± 1.5 nm2/s
