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

Abstract

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.

Real‐Time Band‐Selective Homonuclear Proton Decoupling for Improving Sensitivity and Resolution in Phase‐Sensitive J‐Resolved Spectroscopy

“…Consequently,c hemical-shift evolution is refocused, and the spin echo at the end of the t 1 period is modulated by only homonuclear scalar couplings. [1] The spin echo removes static-field inhomogeneity,w hich allows the precise measurement of J couplings along the F 1 dimension, provides natural line width (approximately 1/pT 2 ), [9] and a4 5 8-tilted spectrum produces one singlet peak per chemical site along the F 2 dimension , [10] thus allowing unambiguous assignment in crowded spectralregions.One major drawbackofconventional J-resolved spectroscopy is the poor resolution, which is attributed to the fact that only the absolute value mode of the experiment is feasible; the presence of broad dispersive antiphase peaks forbids pure-phase recording of the spectrum.…”

High‐precision measurement of internuclear distances using solid‐state NMR

“…Its central part is the spin-echo double-resonance (SEDOR) (35) pulse sequence. In a 2D form, this sequence has been used in heteronuclear J spectroscopy (36,37) and to measure 13 CÀ À 15 N dipolar couplings in a single crystal (38). Our scheme starts with ACP to boost polarization of the 13 C nuclei (29).…”

1H Nuclear‐Magnetic‐Resonance Investigation of Oxidized Fe4S4 Ferredoxin from Thermotoga maritima

“…T, and T, measurements were performed at 312.8 K in D,O solution using the inversion-recovery method (Vold et al, 1968) and the Can-Purcell-Freeman-Hill (Carr and Purcell, 1954 ;Freeman and Hill, 1971) sequence, respectively. These data were processed with the Bruker UXNMR software.…”