Missing Signals in HMBC Data

The HMBC sequence is very useful at establishing long range correlations between protons and carbon. It is particularly useful at finding carbonyl or quaternary carbons when only small amounts of sample are available. The HMBC spectrum has the same appearance as an HMQC or HSQC spectrum except that the responses are between protons and carbon separated by 2, 3 or possibly even 4 bonds. People often ask why are some signals missing from the data? Like the DEPT experiment discussed yesterday, the HMBC sequence is optimized for a particular coupling constant (usually about 10 Hz). If the long range proton-carbon coupling is close to zero then the signal will be absent from the spectrum. Such absences can give you valuable structural information as 3 bond proton-carbon coupling follows a Karplus type relationship.