Philosophy to Chemistry to Elucidation

Logic Puzzle #30: Unknown Coupling Pattern … Solution

Coupling patterns that overlap can be deceptively simple.

There are two possible coupling patterns for the CH2 signal at 3.39 ppm (top spectrum): a singlet or two 'doublets' (AB spin system) tilted towards each other. The bottom spectrum shows an example of two 'doublets' where the outer signals at 3.34 and 3.43 ppm are barely visible.

From the original problem set (top spectrum), the outer signals are not visible, as such, the coupling pattern is not clear. More information can be inferred from the structure. For example, if the CH2 protons are anisochronous, then they will exhibit different chemical shifts and thus rule out the singlet option.

Thank you TLP for you comment.

Logic Puzzle #30: Unknown Coupling Pattern

Coupling patterns can be deceptively simple.

For an unknown compound, the expanded region of a 1H NMR spectrum below shows a signal at 3.39 ppm for a CH2 group. Is the coupling pattern for this two spin system a 'true' first-order singlet?

Logic Puzzle #29: Adjusting the Threshold

Spectral data that exhibit weak signals (low S/N) can easily be misinterpreted. A good approach is to check the data against other data to confirm whether the signal is real or an artefact.

For an unknown compound, the 1H -13C HSQC spectrum below is shown at two different intensity thresholds. The aromatic protons at 6.97 and 7.00 ppm equate to 4 non-exchangeable hydrogen atoms. Is the correlation between the 1H signal at 6.97 ppm and the carbon at 122 ppm real or an artefact?

Logic Puzzle #27: Recognizing the Protonated Carbon

For this puzzle, the objective is to extract structural information from the NMR data. However, the underlying message is to check all possible atomic combinations so that no plausible fragment gets overlooked.

For an unknown compound, an expanded region of the 1H-13C HSQC-DEPT spectrum below shows two 1H multiplets and two negatively-phased correlations (blue). The 1H integrals show the correlations to be 1 Hydrogen atom each. The 1H multiplets are both aligned with the 13C shift at 107.9 ppm. Is the 13C shift at 107.9 ppm due to a CH, CH2 or CH3 group? Is there another fragment that should be considered?

Logic Puzzle #25: Zero Filling at its Best … Solution

For each increment of zero filling, the number of data points defining a line shape is increased. The trade-off is the size of the FID increases proportionately. An FID with 1K = 1024 points will produce a file of ~ 50 kb (including data parameters). The goal is to balance the file size with the amount of gain from applying zero filling.

Spectra A and B appeal aesthetically due to the higher level of smoothness across the signals. Whereas the original data, spectrum E, is jagged and presents less than ideal line shapes.

Analysts typically apply 1 to 2 zero fillings; this equates to adding respectively 1 to 2 increments of zeroes equal to the number of data points.

Logic Puzzle #25: Zero Filling at its Best

Zero filling is the process of adding a string of zeroes to the end of an FID. The following puzzle aims at understanding how zero filling impacts the line shape of a doublet.

The following 1H NMR spectrum was collected with a total of 4K points (selective region is shown as spectrum E). Applying different amounts of zero filling will modify the line shape of the doublet. Which spectrum(a) offers visually the best line shape for the doublet?

Logic Puzzle #23: That’s a Fragment?

Piece by piece, fragment by fragment, an unknown compound is constructed until it is solved. The purpose of this puzzle is to focus on solving a fragment.

The 1H-13C HSQC spectrum, shown below, is for an organic three-carbon fragment. The fragment is known to have 2 open sites that connect back to the structure. What fragment(s) can be determined with the available information?

Logic Puzzle #22: The Art of Uncovering the Molecular Formula

It is fairly route for an elucidator investigating an unknown compound to relate data from multiple spectroscopies. This puzzle aims at establishing a molecular formula from a set of spectroscopic data.

For an unknown organic, a ESI(neg.)-ToF mass spectrum for a singly-charged species and a 13C NMR spectrum are supplied below. In addition, the 1H NMR and 1H-13C HSQC spectra (not shown) indicate a ratio of 5 CH: 1 CH2: 1 NH. What is the molecular formula for the unknown compound?

Logic Puzzle #21: COSY 3J or 4J or 5J? … Solution

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 an aromatic system exhibiting meta coupling or geminal coupling for an unsaturated system. Note the 2JHH coupling response was not part of the question.

The 1H-1H COSY spectrum shows that proton signals at 6.61 and 7.15 ppm are coupled to each other and likewise for the signals at 7.22 and 7.58 ppm. Since the intensity of the off-diagonal correlations are significantly weaker in relation to the diagonal, meta coupling is most likely the case here.

As such, the off-diagonal correlations on the COSY experiment relate to 4JHH coupling responses.

Thank you Djalma for your comment.

Logic Puzzle #21: COSY 3J or 4J or 5J?

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 at 6.61, 7.15, 7.22 and 7.58 ppm with coupling constants less than 2.5 Hz. The 1H-1H COSY spectrum below points to two off-diagonal correlations: between 6.61 and 7.15 ppm and another at 7.22 and 7.58 ppm. Are the correlations on the COSY experiment relating to 3JHH or 4JHH or 5JHH coupling responses?