Rotational-echo, nitrogen-14/carbon-13/proton triple resonance, solid-state, nuclear magnetic resonance: a probe of carbon-13-nitrogen-14 internuclear distances.
A novel triple resonance magic angle spinning (MAS) NMR expt. is reported that can be used to probe C-N interat. distances in polycryst. and amorphous solids, without the need for 15N isotopic labeling. Theory is presented for an S = 1 nucleus under conditions of MAS and a spin-locking radio frequency (r.f.) field. An off-resonance r.f. field is only effective in periods during the rotor cycle where it is of the same order of magnitude as the 1st order quadrupolar splitting Q. This occurs when Q changes sign (the zero crossing) and passages between the S = 1 Zeeman levels result. Numerical calcns. are used to det. the conditions for adiabatic passages. An adiabatic passage results in the greatest change in the d. matrix, inverting the Zeeman magnetization and creating quadrupolar order; faster passages, caused by faster MAS or a larger quadrupole coupling const. (e2qQ/h), result in the loss of some Zeeman magnetization to nonspin-locked coherences. Applying an off-resonance 14N spin-locking field, during the evolution period of a 13C spin-echo expt., alters the evolution of the dipolar coupled spin and leads to a loss of 13C intensity at the echo. Calcns. of the dephasing of 13C magnetization, caused by adiabatic 14N passages, are in good agreement with exptl. results obtained at slow spinning speeds for a sample of glycine, and an est. of the dipolar coupling between the N and directly bonded C can be made. Faster 14N passages result in less 13C dephasing. Despite the large value for e2qQ/h expected for the amide 14N nucleus in the polymer polyamide-6, significant dephasing is still obsd. for C atoms that are >3.7 .ANG. away from the N in the polymer chain. Methods for calcg. the 13C dephasing under conditions of fast 14N passages are considered.
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