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ACD/Labs Blog

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...

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...

Arvin Moser
A 1H-13C HSQC is frequently used to assist in assigning a carbon as CH, CH2 or CH3 (additional blog here). The purpose of this puzzle is to examine a non-standard assignment. The 1H-13C HSQC below shows 2 correlations for each carbon at 121 and 134 ppm. Given also the information on the 1H integrals on the F2...

Arvin Moser
Couplings can be affected by the torsion angle. The couplings can be expressed mathematically with a cos θ relationship. At certain values of θ, the couplings are expected to be relatively weaker to non-existent. Based on the torsion angle, fragment B is expected to exhibit a prominent coupling between the red and gold nuclei. Fragment A,...

Arvin Moser
For a simple case, the torsion angle (or dihedral angle) is described as the angle between 4 contiguous atoms or 3 successive bonds. In NMR, the magnitude of the coupling is directly related to the torsion angle between the vicinal nuclei (see the Karplus equation for more details). Below are two animations, A and B, for...