Proton NMR spectroscopy is the most widely used nuclear magnetic resonance spectroscopic technique. 1H represents a hydrogen atom that consists of 1 proton, 1 electron, and 0 neutrons. It is due to the presence of a single proton in its nucleus that the 1H nucleus is also simply known as a proton. So, NMR spectroscopy performed with respect to proton nuclei within the molecules of a substance is called proton NMR spectroscopy.
This article explores everything about proton NMR, including its working principle, the chemistry behind it, and how to interpret an NMR spectrum for molecular structure determination.
What is proton NMR spectroscopy
Proton NMR spectroscopy is a type of NMR spectroscopy. It is based on studying hydrogen-1 (or 1H) nuclei present in different electronic and chemical environments within a molecule. A 1H nucleus possesses a specific angular moment (I) and is always spinning about its axis. It has a total of two magnetic spin states (m) i.e., +1/2 and -1/2. When placed in an external magnetic field (B0), most of the 1H nuclei present in the sample occupy a low energy spin state by aligning with B0 while the others align against B0 and occupy a high energy spin state. The nuclei spinning at the lower energy then flip their spin orientation by absorbing radiowave frequency (in MHz) equal to the energy gap (∆ E) between the two spin states. The absorbed radiowave frequency also called resonance frequency ultimately helps in structural determination.
What is the working principle of proton NMR spectroscopy
- The NMR spectrometer consists of a permanent magnet.
- A small amount of sample is dissolved in an NMR-compatible solvent, and a small amount of tetramethyl silane (TMS) is added as the standard reference. The sample solution so prepared is filled into a glass cuvette.
- The glass cuvette is held between the opposite poles of the magnet.
- 1H-nuclei present in the sample molecules experience the external magnetic field and occupy two different spin orientations.
- The radio frequency oscillator then supplies energy to these spinning 1H nuclei. The nuclei spinning at a lower energy flip to the higher energy orientation by absorbing radio frequency equal to ∆ E.
- The absorbed energy is consequently emitted as the spinning nuclei return to a stable low energy level.
- The emitted radiations reach the detector which then sends a signal to the recorder and plots the proton NMR spectrum as a graph of signal intensity versus absorbance frequency. The frequency is measured in units of chemical shift.
- All the 12 protons in TMS are equivalent so they give a strong signal at a high frequency. This signal is considered the reference signal. All the sample signals are measured with reference to this signal, moving from right to left on the NMR spectrum.
What does the proton NMR spectrum tell you
The chemical shift (symbol δ) of a proton is the difference between its resonance frequency and the reference peak on the NMR spectrum. The chemical and electronic environments within a molecule strongly affect a proton’s NMR signal. If a concerned nucleus is surrounded by a dense electronic cloud, it gets strongly shielded from the externally applied magnetic field.
A greater shielding effect leads to a lower resonance frequency and thus a lower chemical shift value. An NMR peak closer to the reference signal is thus obtained. This is called the upfield NMR shift.
However, in the case of a hydrogen (1H) nucleus, there is only one electron present around it, so it is only weakly shielded from B0. Thus, the 1H nuclei generally give strong NMR peaks with significant δ values. Any electronegative atom present in a molecule near the proton strongly attracts its electron density. The proton gets deshielded and experiences a stronger external magnetic field effect. ∆ E increases so the proton’s resonance energy also increases. Its NMR signal shifts farthest away from the reference peak.
This deshielding effect is even more pronounced if the proton is attached to a benzene ring. A greater deshielding effect leads to a higher chemical shift value. An NMR peak farthest away from the reference signal is thus obtained. It is known as a downfield NMR shift.
How to read an NMR spectrum
A low-resolution proton NMR spectrum shows a single peak for each 1H nucleus from a particular chemical environment. Let’s understand this concept further with the help of an example.
Low-resolution 1H-NMR spectrum of ethanol (CH3CH2OH)
- Three different types of protons are present in the ethanol molecule. Three protons from methyl (CH3), two protons from ethyl (CH2), and one proton from the hydroxyl (OH) functional group.
- The low-resolution 1H-NMR spectrum of ethanol displays three peaks at different chemical shift values.
- The most deshielded OH proton gives a peak farthest away from the reference peak at δ=5.5 ppm.
- CH2 protons give a peak at δ=3.7 ppm while the CH3 protons give a peak at δ=1.2 ppm. The CH3 protons lie farthest away from the electronegative O atom so these are most shielded and thus possess the lowest resonance frequency so the smallest chemical shift value.
- The intensity of the CH3 peak is highest because it represents three atomic nuclei followed by the intensity of the CH2 peak followed by the OH peak intensity.
The singlet CH2 and CH3 peaks obtained on the low-resolution 1H-NMR spectrum of ethanol split further in its high-resolution 1H-NMR spectrum to give detailed structural information. This peak splitting introduces us to other important concepts in proton NMR i.e., scalar coupling and spin multiplicity.
What is scalar coupling in proton NMR
The scalar coupling phenomenon originates in organic molecules in which the spin of two nuclei is connected via chemical bonds. Thus, scalar coupling is also called spin-spin coupling. The magnetic moment experienced by the resonating proton gets influenced by the spin orientation of the nearby protons.
For instance, in the ethanol molecule, the carbon carrying the CH3 protons is bonded to another C atom which contains two individually spinning protons. The two H nuclei from CH2 can either align with the external field, against the field, and/or one with and the other against the field (in two different possibilities).
Aligning with the applied magnetic field, the CH2 protons enhance the effect of B0 on CH3 protons. Contrarily, aligning against the applied field, the CH2 protons can lead to a reduced B0 influence on CH3 protons. In this way, the singlet peak obtained for CH3 protons in the low-resolution 1H-NMR spectrum of ethanol splits into three peaks in the ratio 1:2:1 in its high-resolution spectrum.
Similarly, the CH3 protons split the CH2 peak into a quartet in the ratio of 1:3:3:1 in the high-resolution 1H-NMR spectrum of ethanol (CH3CH2OH). The characteristic shape of a split NMR peak is called spin multiplicity. It can be calculated using the n+1 rule where n denotes the protons attached to a carbon adjacent to the carbon atom carrying targeted 1H nuclei. n=3 for CH2 bonded to CH3 so the CH2 peak splits into 3+1=4 i.e., a quartet.
The splitting ratio/ peak intensity in spin-spin coupling can be determined from Pascal’s triangle given below.
We should also note that the protons in an identical chemical environment do not split their characteristic 1H-NMR peak. For example, there is no influence of the two CH2 protons on each other’s resonance frequency.
The magnitude of peak splitting is measured as a J coupling constant. J coupling constant depends on the interaction between a pair of protons. It is defined by the nuclei type and the distance between them (in chemical bonds). Thus, the split distance in the CH3 quartet is equal to a specific J coupling constant. A typical J coupling constant for aliphatic protons is 7 Hz.
High-resolution 1H-NMR spectrum of ethanol (CH3CH2OH)
- The high-resolution spectrum of ethanol shows the same three peaks i.e., those obtained for CH3, CH2, and OH protons. But here the CH3 and CH2 peaks are split into a triplet and a quartet respectively.
- The singlet peak due to OH proton remains as such as this proton is directly bonded to an electronegative O atom and not to a C atom. Additionally, the OH NMR peak does not split because, in a sample containing a large number of ethanol molecules, the OH protons undergo rapid exchange with each other and with protons present in water. Thus, δ also depends on sample concentration and the solvent, also called the solvent effect.
- The OH peak in the high-resolution NMR spectrum of ultradry ethanol appears as a triplet.
- In this way, we can identify an unknown organic molecule by looking at its 1H-NMR spectrum and recognizing the different functional groups present in it as well as their bonding sequence.
Some of the typical 1H-NMR chemical shift values in the table given below can help you in interpreting any given proton NMR spectrum.
| Type of proton | Chemical environment of proton | Examples | Chemical shift (δ) range |
| alkane | -CH3, -CH2-, -CH- |
0.9-1.7 | |
| alkyl next to C=O | CH3-C=O | 2.2-3.0 | |
| C-H | alkyl next to the aromatic ring | CH3-Ar, -CH2-Ar |
2.3-3.0 |
| alkyl next to an electronegative atom | CH3-O, -CH2-Cl |
3.2-4.0 | |
| attached directly to a benzene ring | Ar-H | 6.0-9.0 | |
| aldehyde | R-CHO | 9.3-10.5 | |
| alcohol | R-OH | 0.5-6.0 | |
| O-H | phenol | Ar-OH | 4.5-7.0 |
| carboxylic acid | R-COOH | 9.0-13.0 | |
| alkyl amine | R-NH- | 1.0-5.0 | |
| N-H | aryl amine (aniline) | Ar-NH2 | 3.0-6.0 |
| amide | R-CONH- | 5.0-12.0 |
Let’s practice another example using these values.
The 1H-NMR spectrum of acetic acid (H3C-COOH)
- Four protons are present in an acetic acid molecule i.e., three protons in the methyl (CH3) group while one proton in the carboxylic acid (COOH) functional group.
- The three CH3 protons are magnetically equivalent because they have an identical chemical environment. Each of the three H is bonded to the same carbon (C) atom. These three protons have an identical chemical shift value (δ = 2.0 ppm) and produce a single peak on the 1H-NMR spectrum.
- The H bonded to an oxygen atom in COOH gives another single peak on the 1H-NMR spectrum at δ= 11.5 ppm. The presence of O in the vicinity of the targeted proton leads to a strong deshielding effect so a higher chemical shift is witnessed.
- Therefore, the proton NMR spectrum of acetic acid displays two peaks. The intensity of the CH3 peak is three times stronger than that of the COOH peak. This is because the area under the peak is directly proportional to the number of atomic nuclei producing that peak.
Overview
Information that can be obtained from a proton NMR spectrum
Here is a video tutorial for you to revise all the basics learned about proton NMR through this article.
Consult our main article on NMR spectroscopy to learn some fascinating applications and uses of this extremely powerful spectroscopic technique.
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References
1. J.W.Akitt and B.E.Mann (2017). NMR and Chemistry: An introduction to modern NMR spectrum London, Taylor and Francis.
2.Pretsch, E., P. Buhlmann and M. Badertscher (2020). 1H NMR Spectroscopy. Structure Determination of Organic Compounds: Tables of Spectral Data. Berlin, Heidelberg, Springer Berlin Heidelberg: 167-254.