Explain why the 19F
NMR spectrum of
consists of a 1: 1: 1: 1 quartet. What would you expect to observe in the
NMR spectrum of
The Correct Answer and Explanation is:
To explain why the NMR spectrum consists of a 1:1:1:1 quartet, we need to understand how nuclear magnetic resonance (NMR) splitting patterns arise due to spin-spin coupling between neighboring nuclei.
Explanation of the 1:1:1:1 Quartet in NMR
A quartet in an NMR spectrum that shows equal intensities in a 1:1:1:1 ratio is characteristic of coupling with a nucleus that has a spin quantum number (I) of 3/2. One of the most common examples is carbon-13 (¹³C) NMR when a carbon is directly bonded to a fluorine atom (¹⁹F), or proton NMR of a hydrogen atom coupled to three equivalent nuclei that have spin I = 1/2, such as ¹⁹F or ³¹P.
However, the 1:1:1:1 quartet is most often observed in cases like:
- A proton (¹H) attached to a carbon that is bonded to a fluorine atom (since ¹⁹F has I = 1/2 and 100% natural abundance).
- A ¹³C nucleus bonded to a single ¹⁹F will also show a quartet due to the coupling between the carbon and fluorine nuclei.
In such a situation, the fluorine nucleus (¹⁹F) splits the NMR signal of the adjacent nucleus into 2nI + 1 peaks, where:
- n is the number of equivalent nuclei coupled (n = 1 for one fluorine),
- I is the nuclear spin (I = 1/2 for fluorine).
So the number of peaks = 2(1)(1/2) + 1 = 2 + 1 = 3, but this seems inconsistent with a quartet.
Actually, if a nucleus couples with a single nucleus of spin 3/2, such as ²⁷Al or ¹¹B, the expected number of lines = 2nI + 1 = 2(1)(3/2) + 1 = 4. This gives a 1:1:1:1 quartet.
What You Would Expect to Observe in Another NMR Spectrum
If the NMR spectrum is instead from a hydrogen nucleus attached to a carbon without coupling, or to a carbon coupled to nuclei with spin I = 0 (such as ¹²C or ¹⁶O), then you would observe a singlet.
In summary:
- A 1:1:1:1 quartet arises from coupling to a single spin-3/2 nucleus.
- Without such coupling, you would observe a singlet or different multiplet, depending on the number and type of neighboring magnetic nuclei.
