13C Carbon NMR Spectroscopy
Let’s start with the good news! Unlike the1H NMR, there isno integration and signal splitting in13C NMRspectroscopy. We are only looking at the number of signals that each non-equivalent carbon atom gives as a single peak!
The carbons being equivalent or nonequivalent is determined based on thesame principles we discussed for proton NMR. No need to dive deeper into figuring outhomotopic, enantiotopic, diastereotopic, or heterotopic. Simply find the carbons that are in the same environment based on symmetry, and if they are not related by symmetry, they are nonequivalent, and two signals will arise.
For example, below is the (stimulated)13C NMR spectrum of a symmetrical ether:
The symmetry plane indicates twoequivalent carbon atoms on each side and one in the middle, therefore, three signals are observed.
As expected, a similar molecule lacking symmetry gives more NMR signals:
Why There is NO Splitting Carbon13C NMR
Carbon nucleus resonates at a different frequency range than proton does, which makes it possible to have all the signals as singlets. However, you need to know thatsignal splitting in13C NMR by neighboringhydrogens does occur which leads to complicated splitting patterns. And that is why a technique calledbroadband decoupling is used. Most13C NMR spectra that you are going to see are decoupled.
Now, you may wonderwhy the neighboring carbons do not cause splitting since they resonate in the same frequency range. Carbon-carbon coupling is not observed because of thelow abundance of the13C isotope. Remember, the most abundant natural isotope of carbon is the12C which has an even number of protons and neutrons, so it is magnetically inactive and cannot be used in NMR. The13C isotope makes only 1%of the isotopes and that is the reason whycarbon NMR signals are weaker, and it takes a longer time to acquire a spectrum. In addition, there is what is calledgyromagnetic ratio which also affects the signal strength and it is four times lower than that of 1H.
13C NMR Chemical Shift
Let’s now mention thechemical shift values in carbon NMR. Just like the1H NMR, the reference point is the signal from TMS which again is set to 0 ppm. So, ignore this peak when analyzing a carbon NMR.
Most organic functional groups give signals from0-220 ppm. Here as well, the carbons connected toelectronegative elements resonate in thedownfield (higher energy) region. The signals in the200 ppm region are coming fromcarbonyl compounds.
Below is a representative13C spectrum and a table of themost important chemical shifts in13C NMR:
Among the carbonyls,aldehydes and ketones are in the most downfield region (past 200 ppm) since, unlike carboxylic acids, esters, amides, and others, they don’t have a heteroatom which is in resonance with the carbonyl group, thus reducing the partial positive charge of the C=O carbon. Remember, this is what we discussed in the reactivity of carbonyl cofounds in nucleophilic addition reactions such as theGrignard andreduction reactions.
Right next to the carbonyl region, you have theunsaturated region(100-160 ppm), and this includesalkenes, aromatic and other groups with π bonds. The onlyexception is alkynes which are not so much downfield because of their magnetic anisotropy which we discussed earlier in thechemical shift post.
In general, when you start analyzing a13C NMR,split the spectrum in twoparts bydrawing a line at 100 ppm; below this value you have thesaturated functional groups, and beyond that is theunstructured region.
Saturated carbon atoms connected toelectronegative heteroatoms give signals from 30-90 ppm. Themost upfield are thesp3 hybridized carbonatoms with different alkyl groups.
A few words about interesting features and exceptions in13C NMR
Iodine demonstrates what is called theHeavy-Atom-Effect. This goes counter to electronegativity as the large orbital of a bigger atom sometimes makes the carbon shielded, hence appearing at alower frequency. That’s why the scale ranges to negative ppm.
Like in the1H NMR,fluorine shows spin-spin splitting with13C atoms. The splitting by fluorine can be determined by then+1 rule since its spin is 1/2. One fluorine shits the chemical shift by 70-100 ppm.
Check Also
- NMR spectroscopy – An Easy Introduction
- NMR Chemical Shift
- NMR Chemical Shift Range and Value Table
- NMR Number of Signals and Equivalent Protons
- Homotopic Enantiotopic Diastereotopic and Heterotopic
- Homotopic Enantiotopic Diastereotopic Practice Problems
- Integration in NMR Spectroscopy
- Splitting and Multiplicity (N+1 rule) in NMR Spectroscopy
- NMR Signal Splitting N+1 Rule Multiplicity Practice Problems
- 13C NMR NMR
- DEPT NMR: Signals and Problem Solving
- NMR Spectroscopy-Carbon-Dept-IR Practice Problems
Practice
For each of the following compounds, predict the number of signals in a13C NMR spectrum:
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