BIRD-HMBC: improved detection of heteronuclear long-range couplings

A new 2D pulse sequence (BIRD–HMBC) designed for the detection of heteronuclear long-range connectivities is presented. In contrast to the basic HMBC experiment and its variants, unwanted 1J(C,H) signals are sampled together with nJ(C,H) signals but are cancelled in subsequent scans, taking advantage of a BIRD pulse sequence element. As a result, sensitivities close to the HMBC with only minor losses are measured. Excellent suppression degrees for unwanted 1J(C,H) residual peaks, superior to the state-of-the-art ACCORD–HMBC experiment, are achieved for the whole range of one-bond couplings. The influence of poorly calibrated pulse angles or off-resonance effects is marginal and options such as refocusing with additional 13C broadband decoupling or the ACCORDION element, applied for improved sampling of the whole nJ(C,H) coupling range, may easily be implemented. These attractive characteristics, together with the easy set-up, make this new experiment a valuable alternative to the HMBC experiment and its known variants for standard routine applications.