A DEPT-135 spectrum shows three positive peaks and one negative peak. The broadband-decoupled ¹³C spectrum shows those four peaks plus one additional peak. What can you conclude about the extra peak?
AIt is a CH₂ carbon, which points down in DEPT-135 and was missed
BIt is a quaternary carbon with no attached hydrogens
CIt is an artifact caused by incomplete decoupling
DIt is a CH carbon that was folded over in the DEPT experiment
Quaternary carbons — those with no attached hydrogens — appear in the broadband-decoupled ¹³C spectrum but produce no signal in DEPT-135 because DEPT relies on polarization transfer from attached protons. A peak present in the full ¹³C spectrum but absent in DEPT-135 is diagnostic of a quaternary carbon (C, carbonyl carbon, quaternary alkyl carbon, etc.). This is precisely the situation where comparing the two spectra is essential.
Question 2 Multiple Choice
Why is ¹³C NMR roughly 6,000 times less sensitive than ¹H NMR?
ABecause carbon nuclei are larger and harder to excite with radiofrequency pulses
BBecause the ¹³C isotope has only ~1.1% natural abundance and a gyromagnetic ratio about one-quarter that of ¹H
CBecause carbon-carbon bonds prevent the magnetization from relaxing properly
DBecause ¹³C nuclei have too many neutrons to respond to the applied magnetic field
Two independent factors multiply to create the sensitivity gap. First, only ~1.1% of all carbon atoms are the NMR-active ¹³C isotope (the rest are ¹²C, which is NMR-silent). Second, the gyromagnetic ratio of ¹³C is about one-quarter that of ¹H, which affects both the resonance frequency and the size of the detectable signal. Together, these factors reduce intrinsic sensitivity by roughly 6,000-fold, requiring longer acquisition times, more scans, or more concentrated samples.
Question 3 True / False
In a DEPT-135 experiment, most types of carbon atoms produce peaks — they differ mainly in whether the peak points up or down.
TTrue
FFalse
Answer: False
This is the most important practical point about DEPT-135: quaternary carbons (those with no attached hydrogens) produce NO peak at all. They disappear entirely from the spectrum because DEPT relies on polarization transfer from attached ¹H nuclei to ¹³C, and quaternary carbons have no such protons. This is why DEPT-135 must always be compared with a broadband-decoupled ¹³C spectrum to identify the quaternary carbons that DEPT misses.
Question 4 True / False
The wider chemical shift range of ¹³C NMR compared to ¹H NMR means that ¹³C peaks from structurally distinct carbons are less likely to overlap.
TTrue
FFalse
Answer: True
¹³C NMR spans roughly 0–220 ppm while ¹H NMR spans only 0–12 ppm. This 18-fold wider range means that even carbons in similar environments are more likely to be resolved as distinct peaks. For complex molecules, this separation is a practical advantage: counting the peaks in a ¹³C spectrum gives the number of chemically distinct carbon environments in the molecule with much less ambiguity than counting ¹H peaks.
Question 5 Short Answer
Why must a broadband-decoupled ¹³C spectrum and a DEPT experiment typically be run together rather than relying on DEPT alone for structure determination?
Think about your answer, then reveal below.
Model answer: DEPT-135 does not detect quaternary carbons at all, so DEPT alone provides an incomplete carbon count. Running broadband-decoupled ¹³C gives the total number of distinct carbon environments, while DEPT-135 classifies each as CH₃/CH (positive), CH₂ (negative), or quaternary (absent in DEPT). Any peak in the full ¹³C spectrum that has no DEPT counterpart must be a quaternary carbon. Without both experiments, you would not know how many quaternary carbons the molecule contains.
The two experiments are complementary. Broadband decoupling maximizes sensitivity and gives a complete carbon count but destroys multiplicity information. DEPT-135 recovers that multiplicity information (hydrogen count per carbon) but misses quaternary carbons. Together they give both the count and the type of every carbon, which is essential for structural assignment — especially for molecules containing carbonyl groups, quaternary stereocenters, or aromatic carbons bearing substituents rather than hydrogens.