Ultra High-Resolution NMR: Sustained Induction Decays of Long-Lived Coherences
Long-lived coherences (LLCs) in homonuclear pairs of chemically inequivalent spins can be excited and sustained during protracted radio frequency irradiation periods that alternate with brief windows for signal observation. Fourier transformation of the sustained induction decays recorded in a single scan yields NMR spectra with line-widths in the range 10 < Delta v < 100 mHz, even in moderately inhomogeneous magnetic fields. The resulting doublets, which are reminiscent of J-spectra, allow one to determine the sum of scalar and residual dipolar interactions in partly oriented media. The signal intensity can be boosted by several orders of magnitude by "dissolution" dynamic nuclear polarization (DNP).