13C Carbon NMR Spectroscopy
Let’s start with the good news! Unlike the 1H NMR, there is no integration and signal splitting in 13C NMR spectroscopy. 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 the same principles we discussed for proton NMR. No need to dive deeper into figuring out hom*otopic, 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 two equivalent 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:
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 that signal splitting in 13C NMR by neighboring hydrogens does occur which leads to complicated splitting patterns. And that is why a technique called broadband decoupling is used. Most 13C NMR spectra that you are going to see are decoupled.
Now, you may wonder why the neighboring carbons do not cause splitting since they resonate in the same frequency range. Carbon-carbon coupling is not observed because of the low abundance of the 13C isotope. Remember, the most abundant natural isotope of carbon is the 12C which has an even number of protons and neutrons, so it is magnetically inactive and cannot be used in NMR. The 13C isotope makes only 1% of the isotopes and that is the reason why carbon NMR signals are weaker, and it takes a longer time to acquire a spectrum. In addition, there is what is called gyromagnetic ratio which also affects the signal strength and it isfour times lower than that of1H.
Let’s now mention the chemical shift values in carbon NMR. Just like the 1H 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 from 0-220 ppm. Here as well, the carbons connected to electronegative elements resonate in the downfield (higher energy) region. The signals in the 200 ppm region are coming from carbonyl compounds.
Below is a representative 13C spectrum and a table of the most important chemical shifts in 13C 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 the Grignard and reduction reactions.
Right next to the carbonyl region, you have the unsaturated region (100-160 ppm), and this includes alkenes, aromatic and other groups with π bonds. The only exception is alkynes which are not so much downfield because of their magnetic anisotropy which we discussed earlier in the chemical shift post.
In general, when you start analyzing a 13C NMR, split the spectrum in two parts by drawing a line at 100 ppm; below this value you have the saturated functional groups, and beyond that is the unstructured region.
Saturated carbon atoms connected to electronegative heteroatoms give signals from 30-90 ppm. The most upfield are the sp3 hybridized carbon atoms with different alkyl groups.
Iodine demonstrates what is called the Heavy-Atom-Effect. This goes counter to electronegativity as the large orbital of a bigger atom sometimes makes the carbon shielded, hence appearing at a lower frequency. That’s why the scale ranges to negative ppm.
Like in the 1H NMR, fluorine shows spin-spin splitting with 13C atoms. The splitting by fluorine can be determined by the n+1 rule since its spin is 1/2. One fluorine sh*ts 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
- hom*otopic Enantiotopic Diastereotopic and Heterotopic
- hom*otopic 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
1.
For each of the following compounds, predict the number of signals in a 13C NMR spectrum:
a)
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