Rotational echo 14N/13C/1H triple resonance solid-state nuclear magnetic resonance: A probe of 13C–14N internuclear distances

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Abstract
A novel triple resonance magic angle spinning (MAS) NMR experiment is reported that can be used to probe carbon–nitrogen interatomic distances in polycrystalline 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 first order quadrupolar splitting Q. This occurs when Q changes sign (the zero crossing) and passages between the S=1 Zeeman levels result. Numerical calculations are used to determine the conditions for adiabatic passages. An adiabatic passage results in the greatest change in the density matrix, inverting the Zeeman magnetization and creating quadrupolar order; faster passages, caused by faster MAS or a larger quadrupole coupling constant (e2qQ/h), result in the loss of some Zeeman magnetization to non-spin-locked coherences. Applying an off-resonance 14N spin-locking field, during the evolution period of a 13C spin-echo experiment, alters the evolution of the dipolar coupled spin and leads to a loss of 13C intensity at the echo. Calculations of the dephasing of 13C magnetization, caused by adiabatic 14N passages, are in good agreement with experimental results obtained at slow spinning speeds for a sample of glycine, and an estimate of the dipolar coupling between the nitrogen and directly bonded carbon 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 observed for carbon atoms that are more than 3.7 Å away from the nitrogen in the polymer chain. Methods for calculating the 13C dephasing under conditions of fast 14N passages are considered.