Efficient measurement of the sign and the magnitude of long‐range proton‐carbon coupling constants from a spin‐state‐selective HSQMBC‐COSY experiment

A spin state‐selective Heteronuclear Single‐Quantum Multiple‐Bond Connectivities (HSQMBC‐COSY) experiment is proposed to measure the sign and the magnitude of long‐range proton‐carbon coupling constants ( n J (CH); n  > 1) either for protonated or for non‐protonated carbons in small molecules. The simple substitution of the selective 180° 1 H pulse in the original selHSQMBC pulse scheme by a hard one allows the simultaneous evolution of both proton‐proton and proton‐carbon coupling constants during the refocusing period and enables a final COSY transfer between coupled protons. The successful implementation of the IPAP principle leads to separate mixed‐phase α/β cross‐peaks from which n J (CH) values can be easily measured by analyzing their relative frequency displacements in the detected dimension. Copyright © 2012 John Wiley & Sons, Ltd.