High-Resolution Study of NMR Spin Echoes: “J Spectra”

The spin–spin relaxation of individual lines in a high‐resolution NMR spectrum has been studied by a technique which extracts an essentially “monochromatic” component from the Carr–Purcell spin‐echo response by means of a narrow‐band filter. The resulting selectivity significantly reduces the complexity of the modulation of the spin‐echo amplitude due to homonuclear spin–spin coupling, and further simplification may be achieved by double irradiation experiments. Echo modulation has been used as a precise measure of the 0.051‐Hz long‐range coupling in 3‐bromothiophene‐2‐aldehyde, normally hidden by magnet inhomogeneity. It is proposed that modulation and spin–spin relaxation effects are best analyzed in the general case by calculating the Fourier transformation of the envelope of the spin‐echo peaks. The general term suggested for this new mode of presentation is the “spin‐echo spectrum.” It consists of a set of resonance responses with widths determined by the relevant spin–spin relaxation times, at freueqncies determined by the various modulation components of the spin‐echo decay. The present study concentrates on operating conditions (low pulse repetition rate and “first‐order” spin coupling) where the resonance frequencies correspond to one‐half the sums and differences of the spin coupling constants. This special case of a spin‐echo spectrum is called a “$J$ spectrum.” When the spin‐echo response appropriate to a single group of chemically shifted nuclei is observed selectively, the transformation gives a “partial $J$ spectrum” which can be used as an aid to the assignment of the full spectrum. $J$ spectra are illustrated for a typical first‐order three‐spin system—the ring protons of methyl furoate.