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ACD/Labs Blog

Arvin Moser
The explanation offered herein covers one possible scenario to a complicated situation. Please note that other explanations are possible. The LC/MS data shows an unknown at 1.8 minutes exhibiting an [M+H]+ at m/z 260 and an [M-H]- at m/z 256. The difference of 4 mass units lends itself into a scenario of conflicting data. One...

Arvin Moser
The purpose of this puzzle is to examine data collected from multiple detectors with the hopes of sorting out the conflicting data surrounding the unknown. In an attempt to identify the structure for an unknown, UPLC data was collected using a C18 column with a run time of 2.2 minutes. The UV, ESI- and ESI+...

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
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
There are two approaches to deciding which scenario best fits the NMR data in the puzzle. A structure elucidator can provide evidence that one scenario is more probable than an other and/or eliminate one scenario on the grounds of insufficient/contradictory data to support it. For the following 1H NMR spectrum, if the integral of the...

Arvin Moser
The goal of this puzzle is to distinguish between strong coupling and peak overlap. For the following 1H NMR spectrum, are the 1H signals at 7.58-7.59 ppm exhibiting strong coupling or are they two overlapping multiplets or both?

Arvin Moser
With intense solvent signals present on a spectrum, a smaller signal(s) can easily be missed. If 2D NMR data is available, then this extra information can assist in clarifying whether a small signal(s) is obscured by larger signals. On the 1H-13C HMBC below, the correlations for CDCl3/CHCl3 (due to 1J coupling responses and more) are...