Nuclear Magnetic Resonance Spectroscopy (NMR spectroscopy)

What is Spectroscopy?

Spectroscopy is the study of the interaction of energy with matter.
When energy is applied to matter, it can be absorbed, emitted, cause a chemical change, or be transmitted.

 

In this article, we’ll see how detailed information about molecular structure can be obtained by interpreting results from the interaction of energy with molecules. In our study of nuclear magnetic resonance (NMR) spectroscopy we’ll focus our attention on energy absorption by molecules that have been placed in a strong magnetic field.
 
 

Nuclear Magnetic Resonance – NMR Spectroscopy

The nuclei of certain elements, including ¹H nuclei (protons) and ¹³C (carbon-13) nuclei, behave as though they were magnets spinning about an axis. When a compound containing protons or carbon-13 nuclei is placed in a very strong magnetic field and simultaneously irradiated with electromagnetic energy of the appropriate frequency, nuclei of the compound absorb energy through a process called magnetic resonance. The absorption of energy is quantized.

A graph that shows the characteristic energy absorption frequencies and intensities for a sample in a magnetic field is called a nuclear magnetic resonance (NMR) spectrum.

 

As a typical example, the proton (¹H) NMR spectrum of 1-bromoethane is shown in Figure below. We can use NMR spectra to provide valuable information about the structure of any molecule we might be studying. In the following sections we’ll explain how four features of a molecule’s proton NMR spectrum can help us arrive at its structure.

NMR spectrum of 1-bromoethane

1. The number of signals in the spectrum tells us how many different sets of protons there are in the molecule. In the spectrum for 1-bromoethane there are two signals arising from two different sets of protons. One signal (consisting of four peaks) is shown in blue and labeled (a). The other signal (consisting of three peaks) is in red and is labeled (b). These signals are shown twice in the spectrum, at a smaller scale on the baseline spectrum, and expanded and moved to the left above the base spectrum. [Don’t worry now about the signal at the far right of the spectrum (labeled TMS); it comes from a compound (tetramethylsilane) that was added to the 1-bromoethane so as to calibrate the positions of the other signals.]
2. The position of the signals in the spectrum along the x-axis tells us about the magnetic environment of each set of protons arising largely from the electron density in their environment.
3. The area under the signal tells us about how many protons there are in the set being measured.
4. The multiplicity (or splitting pattern) of each signal tells us about the number of protons on atoms adjacent to the one whose signal is being measured. In 1-bromoethane, signal (a) is split into a quartet of peaks by the three protons of set (b), and signal (b) is split into a triplet of peaks by the two protons of set (a).

 



 
 

Chemical Shift

• The position of a signal along the x-axis of an NMR spectrum is called its chemical shift.
• The chemical shift of each signal gives information about the structural environment of the nuclei producing that signal.
• Counting the number of signals in a ¹H NMR spectrum indicates, at a first approximation, the number of distinct proton environments in a molecule.

 

Tables and charts have been developed that allow us to correlate chemical shifts of NMR signals with likely structural environments for the nuclei producing the signals. ¹H NMR chemical shifts generally fall in the range of 13–0 ppm (d).

Approximate Proton Chemical Shifts

¹H NMR Approximate Chemical Shift Ranges

The chemical shift of a signal in an NMR spectrum depends on the local magnetic environment of the nucleus producing the signal. The local magnetic environment of a nucleus is influenced by electron density and other factors. The physical meaning of chemical shift values relates to the actual frequency of the NMR signals produced by the nuclei. The practical importance of chemical shift information is that it gives important clues about molecular structure. Each NMR signal indicates the presence of nuclei in a different magnetic environment.

Chemical shifts are measured along the spectrum axis using a delta (δ) scale, in units
of parts per million (ppm). When comparing one signal with another:

• A signal that occurs further to the left in the spectrum than another (i.e., at a higher δ or ppm value) is said to occur downfield.
• A signal to the right is said to occur upfield.

The terms upfield and downfield relate to the strength of the magnetic field (higher versus lower, respectively) that is required to bring the nuclei into resonance.

 The ¹H NMR spectrum of 1,4-dimethylbenzene (p-xylene), shown in Figure below, is a simple example that we can use to learn how to interpret chemical shifts. First, note that there is a signal at δ 0. The signal at δ 0 is not from 1,4-dimethylbenzene, but from tetramethylsilane (TMS), a compound that is sometimes added to samples as an internal standard to calibrate the chemical shift scale. If the signal from TMS appears at zero ppm, the chemical shift axis is calibrated correctly.

¹H NMR of 1,4-dimethylbenzene

Next we observe that there are only two other signals in the ¹H NMR spectrum of 1,4-dimethylbenzene, at approximately δ 7.0 and δ 2.3. The existence of just two signals implies that there are only two distinct proton environments in 1,4-dimethylbenzene, a fact we can easily verify for ourselves by examining its structure.

We say, then, that there are “two types” of hydrogen atoms in 1,4-dimethylbenzene, and these are the hydrogen atoms of the methyl groups and the hydrogen atoms of the benzene ring. The two methyl groups produce only one signal because they are equivalent by virtue of the plane of symmetry between them. Furthermore, the three hydrogen atoms of each methyl group are equivalent due to free rotation about the bond between the methyl carbon and the ring. The benzene ring hydrogen atoms also produce only one signal because they are equivalent to each other by symmetry.

Referring to Chemical Shifts Table given above, we can see that ¹H NMR signals for hydrogen atoms bonded to a benzene ring typically occur between δ 6 and 8.5, and that signals for hydrogen atoms on an sp3 carbon bonded to a benzene ring (benzylic hydrogens) typically occur between δ 2 and 3. Thus, chemical shifts for the signals from 1,4-dimethylbenzene occur where we would expect them to according to NMR spectral correlation charts.

In the case of this example, the structure of the compound under consideration was known from the outset. Had we not known its structure in advance, however, we would have used chemical shift correlation tables to infer likely structural environments for the hydrogen atoms. We would also have considered the relative area of the signals and signal multiplicity, and many other factors.

Integration of Signal Areas

The area under each signal in a ¹H NMR spectrum is proportional to the number of hydrogen atoms producing that signal.

In the ¹H NMR spectrum of 1,4-dimethylbenzene, you may have noticed curves that resemble steps over each signal. The height of each step (using any unit of measure) is proportional to the area of the NMR signal underneath it, and also to the number of hydrogen atoms giving rise to that signal. Taking the ratio of the step height associated with one signal to the step height associated with another provides the ratio of the areas for the signals, and therefore represents the number of hydrogen atoms producing one signal as compared to the other. Note that we are discussing the height of the integral steps, not the heights of the signals. It is signal area (integration), not signal height, that is important.

The area under each signal (shown with blue shading above) is what is measured (integrated) and taken as a ratio to compare the relative numbers of hydrogen atoms producing each signal in an NMR spectrum.

In Figure of NMR Spectrum of 1,4-dimethylbenzene we have indicated the relative step heights as 1.0 and 1.5 (in dimensionless units). Had these values not been given, we would have measured the step heights with a ruler and taken their ratio. Since the actual number of hydrogen atoms giving rise to the signals is not likely to be 1 and 1.5 (we cannot have a fraction of an atom), we can surmise that the true number of hydrogens producing the signals is probably 2 and 3, or 4 and 6, etc. For 1,4-dimethylbenzene the actual values are, of course, 4 and 6.

Whether NMR data are provided as in Figure of NMR Spectrum of 1,4-dimethylbenzene with an integral step over each signal, or simply with numbers that represent each signal’s relative area, the process of interpreting the data is the same because the area of each signal is proportional to the number of hydrogen atoms producing that signal. (It is important to note that in 13C NMR spectroscopy signal area is not relevant in routine analyses.)

Coupling (Signal Splitting)

Coupling, also referred to as signal splitting or signal multiplicity, is a third feature of ¹H NMR spectra that provides very useful information about the structure of a compound.

Coupling is caused by the magnetic effect of nonequivalent hydrogen atoms that are within 2 or 3 bonds of the hydrogens producing the signal.

The effect of the nearby hydrogens is to split (or couple with) the energy levels of the hydrogens whose signal is being observed, and the result is a signal with multiple peaks. (Notice that we have been careful to differentiate use of the words signal and peak. A group of equivalent atoms produces one signal that may be split into multiple peaks.) however, the importance of coupling is that it is predictable, and it gives us specific information about the constitution of the molecule under study.

The typical coupling we observe is from nonequivalent, vicinal hydrogens, that is, from hydrogens on adjacent carbons, separated by three bonds from the hydrogens producing the signal. Coupling can also occur between nonequivalent geminal hydrogens (hydrogens bonded to the same carbon) if the geminal hydrogens are in a chiral or conformationally restricted molecule.

A simple rule exists for predicting the number of peaks expected from vicinal coupling in ¹H NMR:

Number of peaks from vicinal coupling in a ¹H NMR signal = n + 1

Where n is the number of vicinal hydrogen atoms that are nonequivalent to those producing the signal.

This rule is applicable in general to achiral molecules without conformational barriers. The ¹H NMR spectrum of 1,4-dimethylbenzene is an example where n = 0 (in the above equation) regarding the hydrogen atoms producing the signals at δ 7.0 and at δ 2.3. There are no hydrogen atoms on the carbons adjacent to the methyl groups; hence n= 0 for the signal at δ 2.3, and the signal is a singlet (signals with only one peak are called singlets). And, since all of the hydrogen atoms on the ring are equivalent by symmetry and there are no adjacent nonequivalent hydrogen atoms, n=0 for the hydrogens on the ring producing the signal at δ 7.0, and hence this signal is a singlet as well.

The ¹H NMR spectrum of 1,1,2-trichloroethane, shown in Figure below, provides an example where n is not equal to zero, and coupling is therefore evident. In the spectrum of 1,1,2-trichloroethane we see two signals: one with three peaks and one with two peaks. These signals are called, respectively, a triplet and a doublet. The signal for the CHCl₂
hydrogen is a triplet because there are two hydrogen atoms on the adjacent carbon (n=2). The signal for the CH2Cl hydrogens is a doublet because there is one hydrogen on the adjacent carbon (n=1).

¹H NMR Spectrum of 1,1,2-trichloroethane

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Now that we have had an introduction to key aspects of ¹H NMR spectra (chemical shift, peak area, and signal splitting), we can start to apply ¹H NMR spectroscopy to elucidating the structure of unknown compounds. The following steps summarize the process:

1. Count the number of signals to determine how many distinct proton environments are in the molecule (neglecting, for the time being, the possibility of overlapping signals).
2. Use chemical shift tables or charts to correlate chemical shifts with possible structural environments.
3. Determine the relative area of each signal, as compared with the area of other signals, as an indication of the relative number of protons producing the signal.
4. Interpret the splitting pattern for each signal to determine how many hydrogen atoms are present on carbon atoms adjacent to those producing the signal and sketch possible molecular fragments.
5. Join the fragments to make a molecule in a fashion that is consistent with the data.

let’s interpret the ¹H NMR spectrum for a compound with the molecular formula C₃H₇Br.

1. First, we observe that there are three distinct signals, with chemical shifts of approximately δ 3.4, 1.8, and 1.1. One of these signals (δ 3.4) is noticeably downfield of the others, indicating hydrogen atoms that are likely to be near an electronegative group. This is not surprising given the presence of bromine in the formula. The presence of three distinct signals suggests that there are only three distinct proton environments in the molecule. For this example, this information alone makes it possible to reach a conclusion about the structure of the compound, since its molecular formula is as simple as C₃H₇Br. (Do you know what the compound is? Even if you do, you should still demonstrate that all of the information in the spectrum is consistent with the structure you propose.)
2. Next, we measure (or estimate) the step heights of the integral curves and reduce them to whole number ratios. Doing so, we find that the ratio is 2 : 2 : 3 (from the most downfield to the most upfield signal). Given a molecular formula that contains seven hydrogen atoms, we infer that these signals likely arise from two CH₂ groups and one CH₃ group, respectively. One of the CH₂ groups must bear the bromine. (Although you almost certainly know the structure of the compound at this point, let’s continue with the analysis.) At this point we can begin to sketch molecular fragments, if we wish.
3. Next we evaluate the multiplicity of the signals. The signal at δ 3.4 is a triplet, indicating that there are two hydrogen atoms on the adjacent carbon. Since this signal is downfield and has an integral value that suggests two hydrogens, we conclude that this signal is from the CH₂Br group, and that it is next to a CH₂ group. The signal at δ 1.8 is a sextet, indicating five hydrogen atoms on adjacent carbons. The presence of five neighboring hydrogen atoms (n=5, producing six peaks) is consistent with a CH₂ group on one side and a CH₃ group on the other. Lastly, the signal at δ 1.1 is a triplet, indicating two adjacent hydrogen atoms. Joining these molecular pieces together on paper or in our mind gives us BrCH₂CH₂CH₃ for the structural formula.

We have been careful in the above analysis to evaluate each aspect of the data (chemical shift, integration, and signal splitting). As you gain more skill at interpreting NMR data, you may find that just a portion of the data is sufficient to determine a compound’s identity. At other times, however, you will find that more data are necessary than solely a ¹H NMR spectrum. Combined analysis of ¹³C NMR, IR, and other information may be needed, for example. In the above case, knowing the molecular formula, conceiving of the possible isomers, and comparing these with the number of signals (i.e., distinct hydrogen environments) would have been enough by itself to come to the conclusion that the compound is 1-bromopropane. Nevertheless, when working a problem one should still check the final conclusion by verifying the consistency of all data with the proposed structure.

Solved Problem

What compound with molecular formula C₃H₆Cl₂ is consistent with the ¹H NMR spectrum shown in Figure below? Interpret the data by assigning each aspect of the spectrum to the structure you propose.

Strategy and Answer

 

The spectrum shown in Figure above shows only one signal (therefore its integral is irrelevant and not shown). This must mean that the six hydrogen atoms in the formula C₃H₆Cl₂ all exist in the same magnetic environment. The presence of two equivalent methyl groups is a likely scenario for six equivalent hydrogen atoms. The only way to have two identical methyl groups with the formula C₃H₆Cl₂ is for both chlorine atoms to be bonded at C2 resulting in the structure shown to the right.



 






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