Introduction to Nuclear Magnetic Resonance (NMR)

The structure and purity of any compound can be determined through the spectroscopic analytical technique, Nuclear Magnetic Resonance (NMR). The basic principle of the NMR is the interaction between nuclei of different atomic isotopes with a static magnetic field. In medical science, magnetic resonance imaging (MRI), the same as NMR, is used to diagnose various diseases. In the food industry, NMR determines the ratio of water and fat in foods. NMR is also used to measure the corrosive fluids in pipes.

Why do students struggle to learn carbon NMR?

1. Lack of basic knowledge

Basic knowledge is important to understand every topic. The first question about carbon NMR is; why do we call it carbon NMR? The answer is simple because we use isotopes of carbon in carbon NMR, mainly ¹³C. The other things that may confuse the students are the resonances of the nucleus and the magnet. So make the different resonances and their interaction clear to students.

2. Carbon NMR spectra 

It is difficult for the students to understand and analyze the carbon NMR spectra at the school and college levels. So, practice is a must to analyze it; take some simple spectra to the classroom and make them clear about the different types of peaks. Make them clear about the intensities of the peaks. Tell them how to differentiate the solvent peak from the sample peaks.

3. It’s conceptually challenging

Understanding the process of NMR and its spectra interpretation by theoretical means is very challenging. In NMR shapes, the signals tell everything; the students must know what different type of carbon peaks looks like. So to clear all these doubts through theoretical means is nearly impossible. Video graphics is the best option for understanding the spectra of NMR. Various NMR spectra analysis softwares are available online, like ‘Mnova’; you can use them to interpret any NMR spectra.

5 ways to make carbon NMR interesting for students

1. Review the basics of NMR

• Magnetic resonance: The emission/absorption of electromagnetic radiation when a magnetic field is applied to any atom or nuclei is called magnetic resonance.
• Isotopes: Atoms having similar atomic numbers but different atomic masses are called isotopes.
• NMR: Nuclear Magnetic Resonance (NMR) is an analytical technique through which the structure and purity of any compound can be determined.
• ¹H NMR: When we use an isotope of hydrogen in NMR, it is called ¹H NMR or proton NMR.
• ¹³C NMR: When we use an isotope of carbon in NMR, it is called ¹³C NMR or carbon NMR.
• Chemical shift: The resonance frequency of any atomic nucleus relative to a standard compound set at 0 ppm. Chemical shift is denoted by the symbol (δ) and is measured in ppm (parts per million).
• Nuclear spin: Spin is an intrinsic property of electrons, protons and neutrons describing the particles angular momentum. There are two different spin states: 1/2 and -1/2. In most atoms, the total spin is equal to 0 because the different spin states cancel each other out. Only isotopes that have an odd number of protons/neutrons have nonzero spins and are active in NMR experiments, such as 13C, 1H, and 15N.

2. Explain NMR Sample Preparation

There are different types of NMR: Solid state, liquid-state, and solution-state NMR. The way the sample is prepared will depend on the type of NMR. The most common is the solution state. For this, you will require a suitable solvent and the internal standard TMS - Tetramethyl silane.

Equipment: NMR tubes are used as they are designed specifically for the spectrometer. They are typically 5mm in diameter and have a small cap and an NMR adaptor which helps the tube stand upright. A glass pipette is used instead of a plastic pipette and micropipette because it is much thinner for a more precise transfer of the sample in the thin NMR tube.

How much solid to weigh: The mass of solid compounds for carbon NMR is typically between 20-50 mg. This is much more than the mass required for proton NMR - which is 1-2 mg, and is to do with the difference in technique sensitivity.

Solvents: The solvent used is typically deuterated. This means that the hydrogens in the solvent molecules are replaced with the isotope of hydrogen named Deuterium. It helps with the locking and shimming process inside the machine.

Reference Compound: TMS is the chosen reference compound used because the carbons and protons are in the exact same chemical environment and causes a resonance peak at 0 ppm. So it lets us know where the chemical shift scale begins. Adding TMS directly to your sample will be too concentrated, so you must dissolve a small drop in some solvent before you dissolve your solid.

In the case of a solid sample, these are the steps to follow:

• Weigh out 20-50 mg of solid
• Add one drop of TMS to some deuterated solvent
• Dissolve the solid in solvent mixed with TMS
• Pipette the sample solution in the NMR tube
• Place the tube in the NMR adaptor
• Wipe the tube with tissue paper

3. Explain C-13 NMR

Carbon-13 NMR spectroscopy is the application of nuclear magnetic resonance to the isotope of carbon-12. The isotope is used because the Carbon-13 nucleus behaves like a little magnet, which means it can either be aligned with an external magnetic field or opposed to it.

After the alignment of the nuclear spins by a constant magnetic field, a radiofrequency pulse is used to perturb it. The pulse is applied perpendicular to the constant field, and its frequency is adjusted in order to generate the desired NMR signal. Only isotopes that have an odd number of protons/neutrons have nonzero spins and are active in NMR experiments, such as 13C, 1H, and 15N.

Chemical Shifts in C-13 NMR

The chemical shift describes the resonant frequency of a nucleus compared to a standard in a magnetic field. The resonant frequency depends on the nucleus' local environment, such as binding partners and geometry. Figure 1 shows the range for carbon-13 NMR.

Figure 1: Functional Group chemical shift ranges for carbon-13 NMR (Image source Labster theory)

The range is 20 times the chemical shift range for proton NMR.

Carbon Environments

 We see resonances at different chemical shifts for functional groups because the carbons bonded in these groups are in different chemical environments. The chemical environment of a carbon atom is affected by the surrounding atoms. Electronegativity, pi electrons, inductive effects and large electron density are examples of the factors that contribute towards the varying chemical shifts.

The number of chemical environments can be identified from the number of peaks in the spectrum.

Figure 2: Carbon NMR spectrum of Ethanol (Image source Labster theory)

There are two peaks on the spectrum indicating two chemical environments. The aliphatic carbon and the alcohol group carbon. Remember that the number of peaks is not the same as identifying the number of carbon molecules. Instead, the peak height may give an indication if there is more than one carbon atom in that chemical environment. These carbons are referred to as Chemically equivalent.

Diamagnetic Anisotropy

Diamagnetic anisotropy is an effect caused by the pi electrons inducing their own magnetic field when placed under an external magnetic field. The pi electrons aren't held as tightly together and so behave differently to sigma electrons.

Figure 3: Diagram showing diamagnetic anisotropy in double bonds and aromatic systems (Image source Labster theory)

One region of the induced field opposes the applied field, equivalent to the shielding effect. The other aligns with the direction of the field which has an equivalent effect to deshielding. This is why in carbon-to-carbon double bonds we will see varying chemical shifts for both carbons.

4. Explain the difference between Proton and Carbon NMR

Proton and Carbon NMR are the spectra that differ in many ways.

Sensitivity

In terms of spectroscopic sensitivity carbon-13, NMR is a much less sensitive technique because of two factors, the gyromagnetic ratio and natural abundance. The gyromagnetic ratio is another parameter that determines this value. It is represented by the constant gamma and is a characteristic of each isotope. The sensitivity of a nucleus in NMR depends on gamma. The higher the value of gamma, the higher the sensitivity, and vice versa.

The value of gamma for the carbon-13 nucleus is 9.6, and for hydrogen-1, it is 27.4. The natural abundance of carbon-13 is 1.1%, whereas, for Hydrogen-1, it is 99.9%. The theory behind it is simple, there are more hydrogen atoms contained in a sample and so easier to detect.

Coupling

You don't see coupling in Carbon-13 NMR because of the low natural abundance. No carbons can come close enough to one another in proximity for coupling. This is why we don't see splitting in the spectrum. In the case of proton NMR, we see hydrogen coupling and hence splitting patterns in the peak. Figure 7 shows an example of peak splitting in a proton NMR spectrum.

Figure 4: Proton NMR spectrum of ethanol (Image source Labster theory).

5. Use virtual lab simulations

With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment.

In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.

In Labster’s interactive Carbon NMR: What is the mystery compound? Virtual Lab, students will learn about the principles of the analytical spectroscopic technique known as nuclear magnetic resonance. Understand the factors that cause the resonance peaks and train your chemistry mind to interpret the complete spec