Accurate measurement of small heteronuclear coupling constants from pure-phase α/β HSQMBC cross-peaks

► A selective HSQMBC experiment to generate pure-phase multiplets without undesired proton-proton coupling modulation. ► Long-range proton-carbon coupling constants directly measured from relative cross-peak displacement of α/β multiplets. ► Accurate measurement of proton-carbon coupling constants over a wide range of experimental variables. ► Reliable validation of proton-carbon coupling constants measurements by three different analysis.