Logic Puzzle #7: Almost Missed It … Solution 2

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 more intense in comparison to the weak correlation at approximately 6.9 and 77.3 ppm. In this case, the weak correlation is attributed to a quaternary carbon obscured by a set of intense solvent signals.

 

Logic Puzzle #7: Almost Missed It … Solution

‘How many signals are present?’ is such a simple question, and yet, it is a fundamental question to an elucidator. Mistaken a signal or overlook one and the elucidator can run the risk of wasting time and effort.

The 13C [1H] NMR spectrum below shows 3 discernible signals that are attributed to the solvent CDCl3. In addition, one can begin to speculate on weaker signals; there might a fourth signal at ~77.2 ppm (most likely due to residual CHCl3), possibly a fifth at ~77.5 ppm, perhaps a sixth one at ~77.3 ppm and maybe more.

The next step is to examine additional data and verify whether the ‘weak’ signals are real or not. This can include:

1. comparing the weak signals to other structural signals,

2. applying deconvolution/peak fitting to this region,

3. checking 2D NMR data,

4. acquiring data in a different solvent,

5. modifying the acquisition parameters to exclude the solvent or increase S/N, etc.

Logic Puzzle #7: Almost Missed It

When dealing with small sample concentrations of an unknown compound, the NMR region where a large solvent signal appears rarely gets a second look. However, a large NMR signal can easily obscure a small signal. From one elucidator to another, check the solvent signal for any structural signals.

In the 13C [1H] NMR spectrum below, how many signals are present?

If the spectrum is zoomed in, how many signals can be seen now?

Will the Real Ring System Please Stand Up?

Based on experience, fused ring systems on an unknown structure tend to conjure up variations of odd connections. The possibilities can seem endless.

The diagram, shown below, samples a subset of fused ring systems for a given dataset. It is important to note that although certain systems may be more appealing to the eye, it is imperative for an elucidator to evaluate each variation prior to accepting or rejecting the proposed candidate.

Logic Puzzle #6: Dealing with an Extra 13C Peak … Solution

Socrates is known for saying ‘Know thyself’, along the same line, chemists should ‘Know thy instrument’. The example below is one such case.

The 13C NMR [1H] spectrum below exhibits 6 signals. If an unknown compound comprises of 7 carbon atoms, then the following scenarios, or combinations thereof, are possible to account for missing or extra carbon peaks on the spectrum:

1. the 13C peaks are too weak to be clearly evident,

2. the 13C peaks are overlapping (equivalent or coincidental), and/or

3. the 13C peaks may pertain to instrument artefacts, mixtures or impurities.   

The carbon signals are 111 and 118 ppm may account for 2 carbons atoms each. This is based on the relative intensities of the peaks and this is a characteristic prevalent for aromatic carbons. The total count is now at 8 atoms. The higher atom count indicates the presence of an artefact or impurity. The signal at the exact centre of the spectrum, 100 ppm, can be attributed to a quadrature spike – an artefact produced from the instrument. If so, the total count matches the known carbon count.

Although this analysis is not conclusive, it is worth noting that there are several other possible interpretations and only additional data will help substantiate one possibility over the other.

Thank you Adolfo, Maxa and Serge for your comments.

In Memory of an Uncle

My uncle, Aldo Moser, passed away due to complications from a yearlong battle with pancreatic cancer. I would like to dedicate this post to him.

I would like to recount a nice story about working side-by-side with my uncle, who worked as a licensed electrician.

About 6 years ago, I remember being awakened on an early Saturday morning by some hammering. It turned out that uncle Aldo and my dad were in the process of installing new electrical outlets and needed to chisel out a portion of the house foundation to fit the outlet. Curious to learn how to do the work, I had asked if I could be of any assistance. Uncle Aldo handed me the chisel and hammer and took the time to explain what needed to be done. With the patience of a gentle teacher, uncle Aldo offered tips on chiselling out the concrete, setting the outlet in place and hooking up the wires for the outlet.

The picture below is of my uncle in January 2010 enjoying the challenge of putting together a jigsaw puzzle.

“The best and most beautiful things in the world cannot be seen or even touched. They must be felt within the heart.” Helen Keller

Logic Puzzle #6: Dealing with an Extra 13C Peak

Structure elucidation by NMR involves a deep understanding of various technical aspects behind data acquisition. Being aware of how the instrument works can facilitate the process and reduce the aggravation.

For today’s puzzle, an unknown compound is known to comprise of 7 carbon atoms and exhibit the following 13C NMR spectrum with 1H decoupling. How can the 7 atoms be accounted for in the spectrum below?