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ACD/Labs Blog

Arvin Moser
Given a set of data points, the elucidator aims to conjure up a variety of scenarios that are consistent with the data at hand. This puzzle aims to draw out an unlikely, but worth considering, scenario that needs to be verified. The expanded region for the 1H-13C HSQC-DEPT spectrum presents two scenarios that are consistent...

Arvin Moser
For this puzzle, the objective is to extract structural information from the NMR data. However, the underlying message is to check all possible atomic combinations so that no plausible fragment gets overlooked. For an unknown compound, an expanded region of the 1H-13C HSQC-DEPT spectrum below shows two 1H multiplets and two negatively-phased correlations (blue). The...

Arvin Moser
With each piece understood, a fragment(s) will emerge. From the 1H-13C HSQC spectrum, there are 3 CHn groups where n represents 1, 2 or 3 protons. The proton integrals indicate 2 CHs and 1 CH3 group. The CH3 group is observed as a singlet with 1H and 13C chemical shifts at 3.35 and 56.93 ppm;...

Arvin Moser
Piece by piece, fragment by fragment, an unknown compound is constructed until it is solved. The purpose of this puzzle is to focus on solving a fragment. The 1H-13C HSQC spectrum, shown below, is for an organic three-carbon fragment. The fragment is known to have 2 open sites that connect back to the structure. What...

Arvin Moser
If an elucidator is too successful differentiate an impurity signal from a signal belonging to a compound-of-interest, then the following may be useful: foreknowledge of the prep work, experience with similar compounds and impurities, variations within the spectral data, trial-and-error, etc. From the 1H NMR spectrum, the integral of the 1H signal at 2.52 ppm...

Arvin Moser
Even with ‘good’ data, every elucidation of an unknown offers the potential for impurities to hinder the process. The task develops into differentiating signals pertaining to an unknown from the impurities. The 1H NMR and 1H-13C HSQC spectra below are for an unknown (solvent signal not shown). Are all the signals from a single unknown...

Arvin Moser
In addition to spin-spin splitting or J-coupling, the chemical shift difference of two nuclei can offer a clue into the type of coupling present. For unsaturated sp2 carbons, diastereotopic geminal protons tend to exhibit a small chemical shift difference, less than 45 Hz, in comparison to that seen with vicinal protons. Please note that this...

Arvin Moser
An interpretation that is biased is not necessarily a bad thing; it can help speed up the work involved in an elucidation of an unknown. On the flipside, an incorrect interpretation based on a bias can really hamper an elucidation. From the 1H -13C HMQC spectrum, the two correlations aligned with the carbon signal at...

Arvin Moser
Data misinterpretation based on personal bias happens more often than an elucidator likes to admit. The purpose of this puzzle is to examine a potential pitfall while interpreting data from a 2D NMR spectrum. For an unknown compound, the 1H -13C HMQC spectrum below shows two correlations both of which are assigned to the same...

Arvin Moser
The intensity of a correlation in relation to other correlations on a 2D NMR spectrum can be just as important in a structure elucidation as assessing the position and phase of a correlation. The example below presents one such case. From the 1H-13C HSQC in the previous post, the correlations differ significantly in intensity. The weaker...