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ACD/Labs Blog

Arvin Moser
Like driving a car, the driver navigates the vehicle within the designated lanes on a roadway, thus using the lane markers as a reference. The same can be said when analyzing spectral data. Correctly referencing multiple NMR data is critical in a structure elucidation process. For the case where 1D and 2D NMR data is...

Arvin Moser
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...

Arvin Moser
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...

Arvin Moser
The best use of a set of tools for a structure elucidation are those that have a general target or goal in mind. Random experiments for the sake of collecting data are a sure sign of inexperience. With that said, Infrared (IR) spectroscopy is a great tool for identifying or confirming a functional group(s). It...

Arvin Moser
Like a COSY experiment, an HMBC dataset offers many combinations of atom connectivity. The goal for the elucidator is to assess each correlation and narrow down a set of fragments that support the data. For every correlation in a 1H-13C HMBC spectrum, an elucidator must decide whether a correlation corresponds to a 2J, 3J or...

Arvin Moser
Nine times out of ten, the consequence of an incorrect assignment of a 2D correlation to a 1D resonance is a structural dead end. That is, the fragments cannot be pieced together to present a clear-cut candidate(s). Dealing with ambiguity in an HMBC spectrum is a common area where the structure elucidation process can go...

Arvin Moser
The best approach when dealing with severe ambiguity in assigning a correlation(s) to a 1D resonance(s) in a 1H-13C HMBC spectrum is to take into account all possible assignments. The drawback to this approach is it adds to the complexity of the structure elucidation process rather than simplifying it. The HMBC spectrum below illustrates a...

Arvin Moser
Generally, a 1H-13C HMBC experiment offers a wealth of connectivity information about an unknown structure(s). However, an elucidator may be faced with the issue of ambiguity in assigning a correlation(s) to a 1D resonance(s). A correlation with an ambiguous assignment may be assessed in one of 5 ways: -reprocess the HMBC data using a different...

Arvin Moser
Failures, as are successes, are an integral part of a structure elucidator’s role. What differentiates a good elucidator from a bad one is the capability of an elucidator to learn from his/her failures. The most common obstacle that can hold a structure elucidation process from becoming a success is structural bias. (This is based on...

Arvin Moser
Attempting a challenging structure elucidation of an unknown and being unable to solve the problem can put a damper on a hectic workload and possibly on your skills as an elucidator. Emotionally, the excitement of working on a challenging elucidation problem leads into frustration — results are what count. Subsequently, the elucidator faces the following...