Low-power composite CPMG HSQMBC experiment for accurate measurement of long-range heteronuclear coupling constants.

A modified version of CPMG-HSQMBC pulse scheme is presented for the measurement of long-range heteronuclear coupling constants. The method implements adiabatic inversion and refocusing pulses on the heteronucleus. Low-power composite 180° XY-16 CPMG pulse train is applied on both proton and X nuclei during the evolution of long-range couplings to eliminate phase distortions due to co-evolution of homonuclear proton-proton couplings. The pulse sequence yields pure absorption antiphase multiplets allowing precise and direct measurement of the (n)J(XH) coupling constants regardless from the size of the proton-proton couplings. The applicability of the method is demonstrated using strychnine as a model compound. The selective 1D version of the method is also presented.