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?

Blog on Hiatus from July 26 to August 6, 2010

Due to some business travel, the blog P2C2E will be on hiatus for 2 weeks from July 26 to August 6. Posts will resume the following week.

Stay tuned for more logic puzzles.

Logic Puzzle #5: Coupling + Tilting + Integration = Benzene … Solution

The goal of this puzzle is to determine the respective benzene ring systems that would exhibit the following 1H NMR spectrum.

The number of substituted benzene ring systems that would exhibit the following 1H NMR spectrum is 3 (assuming no repeating units). The keys to solving this puzzle lie with the determination of which multiplets are coupled in combination with the integral information.

 

Starting with the easy set of multiplets, M08 and M07 are coupled to each other. The second order effects (evident by tilting towards each other) and the similar coupling constants (roughly +/- 0.2 Hz) support this claim. Along side the integral ratio of 2:2, this pattern is intrinsic for a para-substituted benzene ring. Next, the multiplets M01, M03 and M05 are coupled. The 1:2:2 integral ratio and identical coupling constants support a mono-substituted benzene ring. Finally, M02, M04 and M06 are coupled. The 1:1:1 integral ratio and meta-coupling pattern (post#2 link) indicate the presence of a 1,2,4-trisubstituted benzene ring system.

Logic Puzzle #5: Coupling + Tilting + Integration = Benzene

The goal of this puzzle is to determine the respective benzene ring systems that would exhibit the following 1H NMR spectrum.

For the following aromatic region of the 1H NMR spectrum, how many benzene ring systems are present?

Logic Puzzle #5: Coupling + Tilting + Integration = Benzene

The goal of this puzzle is to determine the respective benzene ring systems that would exhibit the following 1H NMR spectrum.

For the following aromatic region of the 1H NMR spectrum, how many benzene ring systems are present?