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 tau shows the characteristic sin (pi J(XH) tau) 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, N-15 nuclei in labelled prolines or P-31 in organophosphates.