HMBC ( Heteronuclear Multiple Bond Correlation )

2D HMBC (Heteronuclear Multiple Bond Correlation) experiment correlates chemical shifts of two types of nuclei separated from each other with two or more chemical bonds. This is an improvement of the old COLOC experiment. For example ¹H,¹³C-HMBC correlates chemical shifts of protons with carbons separated with two or three bonds, in some cases even with the more distant ones. The zero and double quantum coherences with lowpass filtration for elimination of the single quantum coherences are usually applied in this experiment.

¹H,¹³C-HMBC is frequently used for assigning of quaternary and carbonyl carbons.

Chemical shift of one nucleus - usually ¹H is detected in the directly measured dimension, while chemical shift of second nucleus (heteronucleus or X-nucleus) is recorded in the indirect dimension.

Several betterments was proposed to increase sensivity, reduce artifacts (like ¹J_CH coupling), measure coupling constants, and others.





Ref.
NMR wiki
http://www.sciencesoft.net/AssembleIt.html