i-HMBC : Identifying two bond correlations

The HMBC experiment provides ¹H-¹³C correlations over multiple bonds and is essential for assigning small molecules. The main drawback to the HMBC is that it does not identify the length of the correlation and in some cases may be ambiguous. A recent publication describes a method, called i-HMBC, for distinguishing two bond HMBC correlations from longer ones.

Multiple bond correlations in HSQC spectra

¹H-¹³C HSQC spectra are designed to show correlations between carbons and their directly attached protons. Interpretation of all the correlations observed in a HSQC spectrum is an essential step in characterizing a new molecule. It is not uncommon, however, to observe HSQC correlations over more than one bond. At first these long range correlations can be quite confusing, but if one is aware of the possibility of long range correlations being present, they can actually help the analysis.

Measuring long range heteronuclear couplings

Three bond coupling constants are particularly useful for determining stereochemistry as the Karplus relationship relates them to torsion angles. Typically, however, only a qualitative analysis to define rotamers is used, as measuring the couplings accurately is not easy. Homonuclear ³J_HH couplings are normally obtained from ¹D ¹H spectra, and are fairly straightforward to measure, but heteronuclear ³J_CH couplings are more difficult to obtain. There are a variety of methods available for measuring long range heteronuclear couplings, each with their own advantages and disadvantages. Three of them, that have been implemented at the Skaggs NMR Facility, will be described below.