December 8, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
When peak picking a 2D NMR experiment, past weblogs have advocated zooming in on correlations especially in cases dealing with ambiguity. Depending on the data collection parameters, a 1H-13C HMBC experiment can contain paired 1J coupling responses. Without careful scrutiny of the data, these extra responses can be misinterpreted as long-range correlations (2J or longer).
The 1H -13C HMBC spectrum below indicates two peaks picked, thus, correlating proton 1.24 ppm to carbon 20.47 ppm and proton 1.35 ppm to carbon 23.07 ppm. This is an easy interpretation that cannot possibly be wrong, or is it?
Before accepting the peak picking, it is best to take a step back—actually zoom out a little further to get the bigger picture. The correlations from the 1H -13C HMBC spectrum are 1J coupling responses and thus not long-range correlations. The blue lines indicate the paired 1J coupling responses. When assigning long-range correlations, 1J coupling responses are best not to be picked.
NOTE: Although it is possible for a long-range correlation to overlap with a 1J coupling response, it may be wise to examine the volumes of the responses for any significant differences.