Q.4 The number of peaks in the 13 C-NMR spectrum of CDCl3 is ____
The 13C‑NMR spectrum of CDCl3 shows three peaks, appearing as a triplet near 77 ppm.
Introduction
In NMR spectroscopy, CDCl3 is one of the most commonly used deuterated solvents, and its own carbon signal is frequently visible near 77 ppm in 13C spectra. Knowing the number of peaks in the 13C NMR spectrum of CDCl3 is essential for correctly counting and assigning carbon signals in organic structure elucidation problems.
Why CDCl3 gives three peaks in 13C NMR
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CDCl3 (deuterated chloroform) has only one carbon atom, so without coupling effects it would show a single 13C signal.
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In reality, the carbon is directly bonded to deuterium (D, 2H), a nucleus with spin quantum number I=1, so the 13C nucleus is coupled to this spin‑1 nucleus.
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The multiplicity for coupling to a nucleus of spin I is given by 2I+1; for deuterium, 2×1+1=3, so the single carbon signal splits into a triplet with an intensity ratio of approximately 1:1:1.
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In typical broadband proton‑decoupled 13C experiments, 1H–13C couplings are removed, but 13C–D couplings are not, so the carbon of CDCl3 still appears as this characteristic triplet around 77 ppm.
Explanation in exam context (options logic)
Many multiple‑choice questions ask: “The number of peaks in the 13C‑NMR spectrum of CDCl3 is ____.”
Typical options might be:
| Option | Possible answer | Reasoning |
|---|---|---|
| A | 1 peak | Incorrect: counts only the number of carbons and ignores C–D coupling; CDCl3 carbon is split by deuterium. |
| B | 2 peaks | Incorrect: would match coupling to a spin‑1/2 nucleus (doublet), not to spin‑1 deuterium. |
| C | 3 peaks | Correct: carbon couples to one deuterium (I = 1), giving a triplet (2I+1=3). |
| D | 4 peaks | Incorrect: no combination of a single spin‑1 neighbor gives four lines; this does not match observed CDCl3 spectra. |
Thus, even though there is only one chemically unique carbon in CDCl3, the observed number of peaks in its 13C‑NMR spectrum is three, because of spin–spin coupling between 13C and the attached deuterium nucleus.
