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Friday, July 18, 2008

Second Order 1H NMR Spectra of Isopropyl Groups

One of the first things a chemistry student learns about NMR is how to interpret the coupling patterns in first order NMR spectra. With the high magnetic fields available for NMR today, this really goes a long way in interpreting spectra. Chemistry students also learn that when the chemical shift difference between two spins is comparable to their coupling constant that second order NMR spectra are observed and furthermore, that these second order spectra are "very complicated". Many do not bother to understand the line shapes. Sadly, many students carry around laptops with software packages capable of simulating these spectra and do not even know it. Several years ago, a student came to me with a proton NMR spectrum of an isopropylsilyl compound and asked why he could not see the typical septet - doublet isopropyl pattern in the spectrum. He was very concerned that he did not have the right compound. I told him he had a second order spectrum and that it was just as "beautiful" as any first order spectrum.

The first figure below shows simulations (carried out in TOPSPIN) for an isopropyl group as a function of the C-H chemical shift. One can see the typical septet -doublet pattern when the chemical shift difference between the methyl and CH protons is much greater than the coupling constant. When the shift difference is comparable to the coupling constant, complicated second order spectra are obtained. When the shift difference is zero one obtains a singlet.

The left panel of the second figure shows the isopropyl region of the experimental NMR spectrum of (triisopropylsilyl)acetylene. The complicated second order spectrum is simulated in the right hand panel.

Thank you to Mattieu Leclere for providing the sample used in the figure above.

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