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ACD/Labs Blog

Arvin Moser
Diastereotopic protons that are also anisochronous exhibit different chemical shifts. As a general rule, the proximity of a chiral centre will influence the likelihood of diastereotopic proton signals being anisochronous. For the structure below, the CH2 labeled A is most likely to be anisochronous due to the adjacent chiral carbon. IUPAC: methyl 7-hydroxy-3,5-dioxo-7-phenylheptanoate InChi Key:...

Arvin Moser
Diastereotopic protons that are also anisochronous exhibit different chemical shifts. The purpose of this puzzle is to understand a general trend for CH2 groups. If the following structure, shown below, were dissolved in CDCl3 and a 1H NMR spectrum were acquired on a 500 MHz instrument, which of the following three CH2’s would most likely...

Arvin Moser
In the previous post, the solution noted that the multiplets at 1.2 and 3.5 ppm were related to a single compound (labeled B on the 1H NMR spectrum below). This post will attempt to clarify this reasoning. The relative integrals for the multiplets at 1.2 and 3.5 ppm are 1.20 and 0.81, respectively. The ratio...

Arvin Moser
The following publication by Gottlieb et al. (linked here) offers an excellent collection of 1H and 13C chemical shifts to commonly-used NMR solvents. From the 1H NMR spectrum below, the first step is to differentiate the signals pertaining to the unknown from those of the residual solvent. The key here lies with the integral values....

Arvin Moser
Residual solvents are typically organic volatile chemicals that are not completely removed from the sample. Many residual solvents exhibit a characteristic pattern when examined via NMR. The goal of this puzzle is to identify a residual solvent by NMR. The aliphatic region of the 1H NMR spectrum below is for an unknown acquired in CDCl3....

Arvin Moser
When examining NMR data, it is important to understand that coupling constants and patterns can be misinterpreted. The following example is one that can be avoided. For an unknown compound, the 1H NMR spectra below shows the same coupling pattern with different interpretations for the coupling constants. Spectra A relies on chemical shifts listed at...

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...