Some articles on hmbc:

Two-dimensional Nuclear Magnetic Resonance Spectroscopy - Heteronuclear Through-bond Correlation Methods - Heteronuclear Multiple-bond Correlation Spectroscopy (HMBC)
... HMBC detects heteronuclear correlations over longer ranges of about 2–4 bonds ... In HMBC, this difficulty is overcome by omitting one of these delays from an HMQC sequence ... There is a modification of the HMBC method which suppresses one-bond signals, leaving only the multiple-bond signals ...