A Quadrupole-Central-Transition 17 O NMR Study of Nicotinamide: Experimental Evidence of Cross-Correlation between Second-Order Quadrupolar Interaction and Magnetic Shielding Anisotropy.

We have examined the 17 O quadrupole-central-transition (QCT) NMR signal from [17 O]nicotinamide (vitamin B3) dissolved in glycerol. Measurements were performed at five magnetic fields ranging from 9.4 to 35.2 T between 243 and 363 K. We found that, in the ultraslow motion regime, cross-correlation between the second-order quadrupole interaction and magnetic shielding anisotropy is an important contributor to the transverse relaxation process for the 17 O QCT signal of [17 O]nicotinamide. While such a cross-correlation effect has generally been predicted by relaxation theory, we report here the first experimental evidence for this phenomenon in solution-state NMR for quadrupolar nuclei. We have discussed the various factors that determine the ultimate resolution limit in QCT NMR spectroscopy. The present study also highlights the advantages of performing QCT NMR experiments at very high magnetic fields (e.g., 35.2 T).