H2BC

The H2BC (Heteronuclear 2 Bond Correlation) experiment correlates carbons and protons that are two bonds away by using one-bond carbon-proton and proton-proton couplings. The H2BC is essentially an HSQC-COSY experiment or an HSQC-TOCSY with very short mixing time. The spectrum of sucrose at 500 MHz is shown below. The peaks outlined in green show two bond correlations between the 5' carbon and the 4' and 6' protons.To interpret the H2BC spectrum, one should start with the direct correlation HSQC peaks. For example, in the HSQC the 5' proton and carbon are at 3.9 ppm and 82 ppm, respectively. COSY then transfers proton signal to coupled 4' and 6' protons. These transferred signals appear at the 5' carbon frequency of 82 ppm but at the proton shifts of 4' and 6'.

 

A major disadvantage of H2BC is that quaternary carbons are not observed. Only carbons with attached protons are present. The H2BC is sometimes considered complimentary to the HMBC experiment since HMBC uses different types of couplings. That is, HMBC uses 2- and 3-bond carbon-proton while H2BC uses one-bond carbon-proton and proton-proton. Quaternary carbons are present in HMBC.