A two-dimensional (2D) experiment has been used to show that N-14 irradiation and magic-angle spinning (MAS) results in population transfers between the N-14 Zeeman levels. This experiment was applied to a sample of N-acetyl-D,L-valine, a material where asymmetric doublets resulting from C-13-N-14 dipolar coupling are clearly resolved in the C-13 spectrum at a field of 7 T for carbon atoms directly bonded to the nitrogen atom. The C-13 transverse magnetization was allowed to evolve in the F1 and F2 dimensions, and the N-14 spins were irradiated during the mixing period. Cross-peaks were observed in the 2D C-13 spectrum between the two peaks of the CH asymmetric doublet. Since one peak of the doublet results primarily from coupling to the \0> state the other peak from coupling to the \ - 1> and \ + > states, population changes between the N-14 Zeeman levels have occurred during the mixing period. These population transfers are a consequence of the time dependence of the N-14 quadrupole splitting Q under MAS conditions and N-14 irradiation. Level anti-crossings of the N-14 Zeeman levels occur at the zero-crossings of Q, and a continuous and slow change in Q will result in the transfer of N-14 populations between the different Zeeman levels. If these passages are adiabatic, then the system returns to its original state after two zero-crossings. This is consistent with the experimental observation that the intensities of the cross-peaks for N-14 irradiation are greater for half a rotor period than a full rotor period.
