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ACD/Labs Blog

Arvin Moser
The purpose of this post is to demonstrate how a small difference can result in a big misinterpretation. For an unknown compound, the 1H NMR spectra below shows the same coupling pattern with different interpretations for the coupling constants. Which of the two spectra A or B shows the better interpretation? Why?

Arvin Moser
If a chemist can(not) recognize the pattern, then they can(not) understand the structure. Remember pattern recognition comes with practice.   From the 1H NMR spectrum, the coupling pattern at 3.10 ppm exhibits four signals in a 1:2:2:1 ratio. The coupling pattern is most consistent with a doublet of doublets (dd) and an AB spin system...

Arvin Moser
Spin-spin splitting or J-coupling is used to elucidate and verify a candidate structure. The observed coupling pattern (also referred to as splitting or multiplet pattern) serves as a guide in determining the neighbouring groups. Hence, the wrong coupling pattern can send an elucidator on a wild-goose chase. The following 1H NMR spectrum below is for...

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
An effective elucidator is capable of connecting information from multiple sources with the goal of eliminating the inconsistent candidates. In this puzzle, the candidates are molecular formulae (MFs). The previous blog listed 4 potential MFs differing in mass difference and RDBE. The mass difference is not a good filter as the mass differences for the...

Arvin Moser
When the spectral data is sparse, the list of potential candidate structures can seem endless. The task then becomes sifting through the spectral data as a means to filter the candidates. The purpose of this puzzle is to perform such a task. The following unknown compound with a mass of 380.1 +/- 0.5 Da comprises...

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...

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