NMR Spectroscopy

In a nutshell

NMR spectroscopy utilises the changes in electron shielding of nuclei to measure their absorption at different frequencies. The nuclei must have an odd number of nucleons. The number of peaks represents the number of carbon environments and a table is provided for the chemical shifts.

NMR shows the structure of molecules

Nuclear magnetic resonance (NMR) spectroscopy is a technique which can be used to determine the structure of a molecule. There are two main types of NMR, $$^{13}C$$​ NMR which gives information about the carbon atoms and their environments and $$^{1}H$$ NMR which gives information about the hydrogen atoms and their environments. 

An atomic nucleus with an odd number of nucleons (protons and neutrons) is required. This is due to the nucleus having a nuclear spin which allows the nucleus to produce a weak magnetic field, like a bar magnet. NMR spectroscopy detects the strength of the magnetic field generated when a much larger external magnetic field is applied.

Hydrogen nuclei are single protons, so they have a spin. Carbon normally has $$6$$​ protons and $$6$$​ neutrons in its nucleus so this can't be detected. However, $$1 \%$$ of carbon atoms consist of the isotope $$^{13}C$$ which has an odd number of nucleons ($$6$$​ protons and $$7$$​ neutrons), which does have a spin. ​

Nuclei and magnetic fields

Nuclei are typically spinning in random directions. This means that there is no net magnetic field since it cancels out. When a strong external magnetic field is applied, it will either align or oppose the field, the nuclei which align with the external magnetic field have a slightly lower energy level than those which oppose it.   

Radio waves at the right frequency can flip the nuclei which are aligned with the external magnetic field, to oppose it and move towards a higher energy level. The nuclei can flip down to a lower energy level and emit radio waves. Initially, more nuclei are aligned with the external magnetic field, so there will be an absorption of energy, this is what NMR spectroscopy measures. 

Nuclei in different environments

A nucleus is shielded from the magnetic field by surrounding electrons. The atoms around it will affect the amount of electron shielding. If an electronegative atom, such as oxygen, is next to a carbon, the amount of electron shielding will decrease and cause a change in the chemical shift. 

This means that the nuclei of atoms in different environments will absorb a different amount of energy at different frequencies. The different energy absorptions at different frequencies are how an NMR spectrum allows the determination of a compound. 

TMS is used as a relative standard

Nuclei in different environments absorb energy at different frequencies. A standard substance is used $$(TMS)$$.

The standard substance is tetramethylsilane $$(TMS)$$, $$Si(CH_3)_4$$. This molecule has $$4$$ carbon atoms which are in identical environments, this means that only a single absorption peak is produced.  The peak which is produced by $$(TMS)$$ is far away from other frequencies. $$TMS$$ is inert, which means that it will not react with an unknown organic compound.

Chemical shift is measured in parts per million (ppm) relative to TMS. The peak produced by TMS is assigned a chemical shift value $$(\delta)$$​ of $$0$$. This is also used to calibrate the NMR spectrometer, so it is common to see a peak at $$\delta\ =\ 0$$. ​

​$$^{13}C$$ NMR and carbon environments

The number of carbon environments

The number of peaks on a $$^{13}C$$ NMR spectrum shows how many different carbon environments are present in a particular molecule. E.g. two peaks mean that there are two carbon environments. 

Aromatic rings can be slightly more complex but they still follow the rule of one peak for every carbon environment. Make sure to look out for lines of symmetry. 

Chemical shifts

To correspond to each peak in an NMR spectrum with a carbon environment a table with the chemical shifts and types of carbons will be provided. Chemical shifts can overlap, which can mean that matching the peaks to chemical shifts might not always be easy.  For example, a peak at $$\delta \ \approx\ 30$$ can be caused by either a $$C-C$$ or $$C-Cl$$ bond. 

Interpreting NMR spectra

​$$^{13}C$$ NMR spectra have few distinct peaks. Since some peaks may overlap with a few potential carbon environments, rule out as many possible environments, such as, if there is no hydroxyl peak it indicates a carboxylic acid is not present, therefore the peak may be an ester. 

Example

The diagram shows the carbon-13 NMR spectra for an species with the molecular formula of $$C_3H_6O_2$$​.

The peak which is  at $$\delta \ \approx\ 175$$ is likely to be an ester or a carboxylic acid.

The peak which is at $$\delta\ \approx\ 52$$ is either alcohol or ester. Since both oxygens are part of a carboxylic acid or ester, alcohol isn't possible. This peak, therefore, represents an ester.

The peak at $$\delta\ \approx\ 29$$ - a ppm value between $$20\ -\ 40$$ which is a higher ppm value, is likely to be a carbon atom adjacent to a carbonyl.