The fundamental component for all of these macromolecules is carbon. The macromolecules are a subset of organic molecules (any carbon-containing liquid, solid, or gas) that are especially important for life. Describe the role of functional groups in biological moleculesĬells are made of many complex molecules called macromolecules, such as proteins, nucleic acids (RNA and DNA), carbohydrates, and lipids.Explain why carbon is important for life.One fluorine shits the chemical shift by 70-100 ppm.By the end of this section, you will be able to: The splitting by fluorine can be determined by the n+1 rule since its spin is 1/2. Like in the 1H NMR, fluorine shows spin-spin splitting with 13C atoms. That’s why the scale ranges to negative ppm. This goes counter to electronegativity as the large orbital of a bigger atom sometimes makes the carbon shielded, hence appear at lower frequency. Iodine demonstrates what is called the Heavy-Atom-Effect. The most upfield are the sp 3 hybridized carbon atoms with different alkyl groups.Ī few words about interesting features and exceptions in 13C NMR Saturated carbon atoms connected to electronegative heteroatoms give signal from 30-90 ppm. 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. The only exception are the alkynes which are not so much downfield because of their magnetic anisotropy which we discussed earlier in the chemical shift post. 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. Remember, this is what we discussed in the reactivity of carbonyl cofounds in nucleophilic addition reactions such as the Grignard and reduction reactions. The signals in 200 ppm region are coming from carbonyl compounds.īelow is a representative 13C spectrum and a table of most important chemical shifts in 13C NMR:Īmong 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. Here as well, the carbons connected to electronegative elements resonate in the downfield (higher energy) region. Most organic functional groups give signal from 0-220 ppm. So, ignore this peak when analyzing a carbon NMR. Just like the 1H NMR, the reference point is the signal from TMS which again is set to 0 ppm. Let’s now mention the chemical shift values in carbon NMR. In addition, there is what is called gyromagnetic ratio which also affects the signal strength and it is four times lower than that of 1H. 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. 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. Carbon-carbon coupling is not observed because of the low abundance of the 13C isotope. Now, you may wonder why the neighboring carbons do not cause splitting since they resonate in the same frequency range. Most 13C NMR spectra that you are going to see are decoupled. And that is why a technique called broadband decoupling is used. However, you need to know that signal splitting in 13C NMR by neighboring hydrogens does occur which leads to complicated splitting patterns. The symmetry plane indicates two equivalent carbon atoms on each side and one in the middle, therefore, three signals are observed.Īs expected, a similar molecule lacking symmetry gives more NMR signals:Ĭarbon nucleus resonates at a different frequency range than proton does, which makes it possible to have all the signals as singlets. Simply, find the carbons that are in the same environment based on symmetry and if they are not, they are nonequivalent, and two signals will arise.įor example, below is the (stimulated) 13C NMR spectrum of a symmetrical ether: No need for diving deeper in figuring out homotopic, enantiotopic, diastereotopic or heterotopic. The carbons being equivalent or nonequivalent is determined based on the same principles we discussed for proton NMR. We are only looking at the number of signals that each non-equivalent carbon atom gives as a single peak! Let’s start with the good news! Unlike the 1H NMR, there is no integration and signal splitting in 13C NMR spectroscopy.
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