Nuclear Magnetic Resonance (NMR) is one of the most important techniques for determining or confirming the structure of organic compounds. Knowing how to read the NMR spectrum is essential to decipher the intricate language of molecules on the screens of an NMR spectrometer.
In order to find out the structure of a molecule using NMR spectroscopy, we require a mixture of the sample itself, a reference molecule (TMS), and a deuterated organic solvent. But the most important thing after we get a spectrum is reading the NMR spectrum.
TMS: Exploring its role
Tetramethylsilane (TMS) is used as a chemical shift reference in 1H-NMR and 13C-NMR spectroscopy. By definition, TMS has a chemical shift of 0.00 ppm.
Why only TMS?
TMS is proposed as a reference material because it offers following advantages:
- The chemical shifts are largely temperature and concentration independent.
- It gives a single, sharp, and intense absorption peak because it has 12 hydrogen atoms with an equivalent hydrogen environment on all sides.
- Only a small quantity is needed.
- The TMS peak lies outside the ‘usual spectral region and is readily identifiable.
- It is largely inert and is not likely to react with most samples.
- It can be used for almost any solvent (except H2O, and D2O).
The concept of chemical shift in NMR spectroscopy is integral to understanding the resonant frequencies of spinning protons concerning a reference molecule. Electron density around a proton influences shielding or deshielding effects, causing various types of protons—be they aliphatic, aromatic, or aldehydic and situated near electronegative atoms—to exhibit distinct chemical shift ranges. The 1H chemical shift, in particular, plays a pivotal role in identifying functional groups within a molecule.
Peak integration, a fundamental aspect of NMR analysis, adheres to the following principles:
1. Number of Peaks
This reflects the quantity of resonating protons present in the sample.
2. Area under Proton Peak
The area under a proton signal’s peak is directly proportional to the number of equivalent hydrogen atoms contributing to a particular peak.
3. Position of Peaks
The positioning of peaks offers valuable insights into two key aspects:
- The nature of the proton (e.g., whether it is aliphatic, aromatic, or aldehydic).
- The identity of the atoms surrounding the proton, providing critical context for structural analysis.
Reading NMR Spectrums
1. Methyl Ethanoate (CH3(CO)OCH3)
The structure of methyl ethanoate shows 2 main peaks in the (H)NMR spectrum:
- A zero peak (for TMS).
- A peak at a chemical shift of 2.05 ppm for 3 protons of methyl (indicating they are near the carbonyl group).
- A peak at a chemical shift of 3.66 ppm for 3 protons of methyl (indicating they are near the electronegative element-O).
The peak of protons nearer to the oxygen atom shows more chemical shift because of the electron pull, causing it to “hog” the shared electrons in a molecule. This electron-hogging effect, in turn, deshields nearby protons, making them resonate at higher frequencies, resulting in a higher chemical shift in the NMR spectrum.
An integrated protons ratio of 1:1 shows that the number of protons in both places is the same.
2. Benzene (C6H6)
Benzene molecule gives rise to only one peak at 6 ppm which corresponds to 6 equivalent atoms.
The electrons around the benzene ring act as bodyguards, shielding the protons from the external magnetic field, causing them to resonate at lower frequencies. So, in the NMR spectrum, you’ll observe these protons at lower chemical shift values, often around 7-8 ppm, indicating their unique environment within the benzene structure.
The reason there is only one peak is that all protons here experience a similar effect of electron pull and push making them look alike to the eyes of the NMR spectrometer.
The splitting is caused by the hydrogens on the same (geminal hydrogens) or on the neighboring carbons (vicinal hydrogens). Only nonequivalent protons split the signal of the given proton(s). One adjacent proton splits an NMR signal into a doublet and two adjacent protons split the signal into a triplet.
Examples of Splitting Patterns
1. Doublet (d)
- Scenario: When a proton has one adjacent proton (nonequivalent) on an adjacent carbon atom.
- Example: In ethanol (CH3CH2OH), the proton (H) on the methyl group (CH3) is a doublet because it has one neighboring proton (H) on the adjacent carbon.
2. Triplet (t)
- Scenario: When a proton has two adjacent protons (nonequivalent) on an adjacent carbon atom.
- Example: In propanol (CH3CH2CH2OH), the proton (H) on the methylene group (CH2) is a triplet because it has two neighboring protons (H) on the adjacent carbon.
These splitting patterns occur due to the interactions between neighboring protons, and they provide valuable information about a molecule’s structure.
What is NMR spectroscopy?
NMR spectroscopy can be defined as an important technique that applies a magnetic field to an atomic nucleus (e.g., 1H, 13C, 15N) and radio frequency pulses to characterize the resonant frequency of that atomic nucleus according to its chemical or environmental surroundings.
How does nuclear magnetic spectroscopy (NMR) work?
When molecules are placed in a strong magnetic field, the nuclei of some atoms will begin to behave like small magnets. a broad spectrum of radio frequency waves will be applied to the sample the nuclei start to resonate at their own frequencies, The resonant frequencies of the nuclei are then measured and converted into an NMR spectrum that displays all of the right frequencies as peaks on a graph
According to the positions, intensities, and fine structure of resonance peaks, people can study the structures of molecules quantitatively.
What are the units used in NMR spectroscopy?
Megahertz (MHz) (106 Hz) and the value for nuclear magnetic resonance spectroscopy usually fall between 60–300 MHz.
What does NMR tell you about a compound?
NMR spectra provide us with important information:
- The number of different absorptions (signals, peaks) implies how many different types of protons are present.
- The amount of shielding shown by these absorptions implies the electronic structure of the molecule close to each type of proton.
- The intensities of the signals imply how many protons of each type are present.
- The splitting of the signals gives information about other nearby protons.
What is the use of an NMR spectrum in chemistry What information does it give us about unknown compounds?
Besides identification, NMR spectroscopy provides detailed information about the structure(s), reaction state(s), and chemical environment(s) of molecules.