ROTATIONAL ECHO N-14 C-13/H-1 TRIPLE-RESONANCE SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE - A PROBE OF C-13-N-14 INTERNUCLEAR DISTANCES/

A novel triple resonance magic angle spinning (MAS) NMR experiment is reported that can be used to probe carbon-nitrogen interatomic distanc es in polycrystalline and amorphous solids, without the need for N-15 isotopic labeling. Theory is presented for an S = 1 nucleus under cond itions 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 cyc le where it is of the same order of magnitude as the first order quadr upolar splitting Q. This occurs when Q changes sign (the zero crossing ) and passages between the S = 1 Zeeman levels result. Numerical calcu lations are used to determine the conditions for adiabatic passages. A n adiabatic passage results in the greatest change in the density matr ix, 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 N-14 spin-locki ng field, during the evolution period of a C-13 spin-echo experiment, alters the evolution of the dipolar coupled spin and leads to a loss o f C-13 intensity at the echo. Calculations of the dephasing of C-13 ma gnetization, caused by adiabatic N-14 passages, are in good agreement with experimental results obtained at slow spinning speeds for a sampl e of glycine, and an estimate of the dipolar coupling between the nitr ogen and directly bonded carbon can be made. Faster N-14 passages resu lt in less C-13 dephasing. Despite the large value for e2qQ/h expected for the amide N-14 nucleus in the polymer polyamide-6, significant de phasing is still observed for carbon atoms that are more than 3.7 angs trom away from the nitrogen in the polymer chain. Methods for calculat ing the C-13 dephasing under conditions of fast N-14 passages are cons idered.