Efficient spin diffusion of transverse13C magnetization

Abstract

Relatively efficient spin diffusion among unprotonated carbons with large chemical-shift anisotropies can be achieved by a 13 C nuclear magnetic resonance multiple-pulse sequence with a lowduty cycle of -5% on the "C 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 "flipflop" 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% 13 COO-labeled poly(vinyl acetate), with 13 C in ester groups accounting for 7% of all heavy atoms, magnetization equilibrates within 20 ms over a volume of (0.9 nm)', corresponding to a molecular mass of 500 Da, while the T 2 relaxation time of the total transverse magnetization is -40 ms. The spin diffusion coefficient is estimated as D = 3 ± 1.5 nm 2/s.

A tilted rotatory frame method for the measurement of nuclear spin diffusion coefficients in solids doped with paramagnetic centers: Mn-Doped CaF2

“…or 6~ 1.2(T,q v6 nm (21) with time constants in microseconds for S = 5/2. One expects therefore 6 close to 1-2 nm.…”

“…The validity of this law is however dubious when protons have different chemical shifts and a large distribution of distances, as it is the case when considering backbone and side group protons and intra-and interchain pairs of protons or when intrachain and interchain contributions to the second moment are partially averaged by motions. It is likely that in these cases specific pathways exist for spin diffusion which break the above scaling [21]. For these reasons, a method of measurement of D O in homopolymers or single phases of compatible blends would be of much value.…”

Signal loss in 1D magic-angle spinning exchange NMR (CODEX): radio-frequency limitations and intermediate motions

“…In our previous paper, 10 we suggested a reference experiment with nominally no recoupling by grouping together pairs of p pulses, which was subject to limitations for the reason that the finite pulse lengths do not permit a pulse timing that leads to truly zero recoupling-this would only be possible if all p pulses were delta pulses and could be placed at multiples of T R . In this vein, ''minimal recoupling'' reference experiments have recently been proposed by Schmidt-Rohr and coworkers 22 in order to better normalize for the action of adverse T 1 relaxation effects during recoupling experiments involving dipolar couplings to quadrupolar spins (RIDER), 23 which in fact pose similar problems to the one at hand, i.e., loss of phase coherence on the recoupling time scale.…”