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ACD/Labs Blog

Tying together the information from a 1H NMR and COSY spectrum, an elucidator can piece together certain fragments. Below is the solution for Logic Puzzle #21. The 1H NMR spectrum shows four aromatic methine multiplets at 6.61, 7.15, 7.22 and 7.58 ppm with coupling constants less than 2.5 Hz. The small coupling constants points to...

One of the trickiest parts of interpreting a 1H-1H COSY experiment is deciding how to classify the correlations. The goal of this puzzle is to assess the COSY correlations and narrow down a set of fragments that support the data. An expansion of the aromatic region of a 1H NMR spectrum shows four methine multiplets...

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...

Reality #1: LC/MS has become the primary analytical check for the medicinal chemist. I Love NMR, but it’s the reality Reality #2: More often than not, NMR spectra are merely glanced at: Sometimes interpretation is based on the presence or absence of one peak (did my reaction go to completion?) How often are NMR spectra...

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...

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...

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...

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...

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...

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...