N-14 POPULATION TRANSFERS IN 2-DIMENSIONAL C-13-N-14-H-1 TRIPLE-RESONANCE MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

A two-dimensional (2D) experiment has been used to show that N-14 irra diation and magic-angle spinning (MAS) results in population transfers between the N-14 Zeeman levels. This experiment was applied to a samp le of N-acetyl-D,L-valine, a material where asymmetric doublets result ing from C-13-N-14 dipolar coupling are clearly resolved in the C-13 s pectrum at a field of 7 T for carbon atoms directly bonded to the nitr ogen atom. The C-13 transverse magnetization was allowed to evolve in the F1 and F2 dimensions, and the N-14 spins were irradiated during th e mixing period. Cross-peaks were observed in the 2D C-13 spectrum bet ween the two peaks of the CH asymmetric doublet. Since one peak of the doublet results primarily from coupling to the \0> state the other pe ak from coupling to the \ - 1> and \ + > states, population changes be tween the N-14 Zeeman levels have occurred during the mixing period. T hese population transfers are a consequence of the time dependence of the N-14 quadrupole splitting Q under MAS conditions and N-14 irradiat ion. 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 t he transfer of N-14 populations between the different Zeeman levels. I f these passages are adiabatic, then the system returns to its origina l state after two zero-crossings. This is consistent with the experime ntal observation that the intensities of the cross-peaks for N-14 irra diation are greater for half a rotor period than a full rotor period.