August 29, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
When working with multiple NMR datasets, it is key to set a reference point(s) in order to align all the datasets. The elucidation purpose — as opposed to standard NMR purposes of referencing to a standard such as TMS — behind referencing the spectral data to a common point(s) is to facilitate the interpretation and thus minimize any incorrect grouping of data. For heteronuclear 2D NMR experiments, such as 1H-13C HMQC and HMBC experiments, two reference points are needed. Typically, an elucidator selects a peak from a 1H and a 13C NMR spectrum as the reference points and the 2D NMR data is adjusted accordingly. If a 13C NMR spectrum is not available, as is the case sometimes, then a correlation on an HSQC or HMQC experiment can serve as the stand-in reference point.
From the 1H NMR spectrum below, the resonance at 5.90 ppm is selected as the reference point for the 1H dimension. The resonance at 5.90 ppm is a good choice for a reference point, as it is isolated from other signals and thus is clearly visible. In the case where there are multiple choices for a reference point, I typically lean towards an intense singlet as the 2D counterpart tend to be easy to locate on a 2D NMR spectrum.
Checking the COSY data, the diagonal correlation at 5.9, 5.9 ppm is properly aligned to the 1D resonance at 5.9 ppm. Therefore, no further data manipulation is needed. The HMQC spectrum also shows good alignment with the 1H resonance. Since there is no 1D 13C NMR spectrum available, the correlation at 122.2 ppm in the HMQC is selected as the 13C reference point for the HMBC. The F1 dimension of the HMBC spectrum needs to be referenced as the carbon at 123.3 ppm is off by 1.1 ppm.
