Long-lived coherences for line-narrowing in high-field NMR

Remarkably narrow lines can be obtained in solution-state NMR spectra in high fields by exploiting the long life-times associated with superpositions of quantum states with different symmetry, known as long-lived coherences (LLCs). These can be compared with coherences that can be excited by extremely low frequency (ELF) irradiation in a vanishing static field. LLCs may be regarded as a special class of zero-quantum coherences (ZQCs). In small molecules in the extreme narrowing limit, the line-widths can be a factor narrower than in spectra of ordinary single-quantum coherences (SQCs), provided relaxation is driven only by the dipole-dipole interaction between the two spins that participate in the LLC. In the slow motion limit for large molecules, the improvement in line-widths can in principle be a factor in favorable cases, provided that extraneous relaxation mechanisms do not operate.