HSQC and HMBC | hmbc
TheHMBC(HeteronuclearMultipleBondCorrelation)experimentgivescorrelationsbetweencarbonsandprotonsthatareseparatedbytwo,three,and,sometimesinconjugatedsystems,fourbonds.Directone-bondcorrelationsaresuppressed.Thisgivesconnectivityinformationmuchlikeaproton-protonCOSY.Theintensityofcrosspeaksdependsonthecouplingconstant,whichforthree-bondcouplingsfollowstheKarplusrelationship.Fordihedralanglesnear90degrees,thecouplingisnearzero.Thus,theabsenceofacrosspeakdoesntconfirmthatcarbon-protonpairsarema...
The HMBC (Heteronuclear Multiple Bond Correlation) experiment gives correlations between carbons and protons that are separated by two, three, and, sometimes in conjugated systems, four bonds. Direct one-bond correlations are suppressed.This gives connectivity information much like a proton-proton COSY. The intensity of cross peaks depends on the coupling constant, which for three-bond couplings follows the Karplus relationship. For dihedral angles near 90 degrees, the coupling is near zero. Thus, the absence of a cross peak doesnt confirm that carbon-proton pairs are many bonds apart.
Because of the wide range (0-14 Hz) of possible carbon-proton couplings, one often does two experiments. One optimized for 5 Hz couplings and the second optimized for 10 Hz. This gives the optimum signal-to-noise. Alternatively, a comprise value of 7-8 Hz can be used. There are also "accordion" versions that attempt to sample the full range of couplings.
The spectrum ...