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ACD/Labs Blog

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...

The 1H-13C HMQC, HSQC, DEPT-HSQC, HSQC-TOCSY and HETCOR experiments offer the elucidator information on the proton-carbon connectivity. The interpretation process comes down to 3 basic assignments: the correlation belongs to a methyl, methylene or methine carbon. A methyl or methine carbon exhibits at most a single correlation between the 1H and 13C axes. A methylene...

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...

Among intense NMR resonances, weak resonances can be easily overlooked. This is commonly seen with 1H NMR spectra whereby 1H resonances with complicated couplings are spread out over a larger area in comparison to uncoupled 1H resonances that appear as sharp signals. The 1H NMR spectrum below shows 3 intense resonances at 2.39, 2.66 and...

As with many nitrogen-containing compounds, 15N chemical shift information can be critical in elucidating or confirming a candidate structure. Many elucidators extract 15N chemical shifts from such experiments as 15N NMR, 1H-15N HSQC or 1H-15N HMBC. Below are two tautomers, from a previous blog, that differ in the hybridization state of the nitrogen. The 15N...

Depending on the experimental conditions, signals from exchangeable protons such as NH and OH can be present on a 1H NMR spectrum. A key to interpreting a 1H NMR is distinguishing between CHn protons (where n = 1, 2 or 3) and exchangeable protons. Acquiring an 1H-13C 2D NMR experiment can assist in this process,...