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ACD/Labs Blog

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...

Arvin Moser
Long range 2D NMR experiments do not necessarily provide information about all the connectivities. The following structure elucidation problem set is one such example. Based on the 1H-13C HMBC shown below, there is no evident correlation between the 1H at 5.31 ppm and the 13C at 21.1 ppm. Note the green box describes the region...

Arvin Moser
Typical for long range 2D NMR experiments, spectral data may exhibit more than one correlation for two coupled nuclei (e.g. A to B and B to A). The pairwise correlations offer an extra degree of confidence in the interpretation. For the following fragment, an 1H-13C HMBC correlation exists for the 1H 2.15 ppm to 13C...

Arvin Moser
Like any new process, it takes some practice to extract, understand and convert the information presented from a set of experimental NMR datasets into a fragment. The only fragment that can accommodate the set of restrictions from a 1H-13C HSQC and HMBC is 2,3-dimethylbutane-1,1-diyl. The green arrows illustrate the 2-3JCH coupling responses extracted from an...

Arvin Moser
The goal of this puzzle is to conceptualize a fragment(s) from the given information. In the following example, a set of protonated sp3 carbons were extracted from an HSQC experiment (not shown). The green arrows represent the 2-3JCH coupling responses extracted from an HMBC experiment. Based on these restrictions, what fragment(s) supports the data? Note...

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