Determination of heteronuclear long‐range H,X coupling constants from gradient‐selected HMBC spectra

A gradient‐selected J ‐HMBC experiment is presented. Coherence selection with magnetic field gradients results in HMBC spectra in which t 1 noise is very low or completely eliminated. This makes it possible to detect unequivocally very small (<2 Hz) long‐range H,X coupling constants. With a slight modification, the HMBC experiment can be used for the quantitative measurement of these couplings. The signal amplitude of spectra with increasing coupling evolution time τ shows the characteristic sin (πJ XH τ) dependence. By fitting a sine curve to the experimental data or by performing a 3D J ‐resolved HMBC experiment, accurate n J XH coupling constants are obtained. This method is especially useful for estimating coupling constants between protons and unprotonated heteronuclei, e.g. quaternary carbons, 15 N nuclei in labelled prolines or 31 P in organophosphates.