Master essential MCAT spectroscopy concepts, including IR and NMR, with clear explanations, high-yield strategies, and practice questions with answers.
(Note: This guide is part of our MCAT Organic Chemistry series.)
Table of Contents
Part 1: Introduction to IR and NMR spectroscopy
Part 2: IR spectroscopy
a) Experimental procedure
b) Key functional groups
c) Example
d) Related forms of spectroscopy
Part 3: NMR Spectroscopy
a) Experimental procedure
b) Reading a spectrum
c) Splitting and coupling
d) Example
Part 4: High-Yield Terms
Part 5: Passage-Based Questions and Answers
Part 6: Standalone Questions and Answers
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Part 1: Introduction to IR and NMR spectroscopy
Since molecules are so small, how do chemists determine the structure of an unknown molecule? If researchers cannot observe single molecules with the naked eye, how do we verify the identities of chemical products and reactants?
The answer lies in spectroscopy and other experimental techniques. In this guide, we'll cover everything you need to know about two of the most important forms of spectroscopy: infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. From the fundamental experimental principles to how to interpret the resulting data, by the end of this guide, you'll be equipped for anything the MCAT has in store for you come test day.
Note that while you may have learned about IR and NMR spectroscopy in your undergraduate courses, the level of depth required to do well on these topics for the MCAT is significantly lower.
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Part 2: IR spectroscopy
a) Experimental procedure
Infrared spectroscopy, or IR spectroscopy, involves the casting of infrared light through a molecular sample. The energy from the light then causes covalent bonds within the molecule to vibrate. These bonds can vibrate by stretching and bending.
As the light passes through the sample, a spectrometer measures the percentage of light that passes through the other end. This percentage is expressed as the percent transmittance of light over a range of frequencies. The frequencies at which the light is absorbed depend on the types of functional groups present in the sample. Thus, IR spectroscopy can identify specific functional groups and types of bonds in a compound.
One way to visualize the vibrations of the covalent bonds is by using the spring model. Imagine two balls connected by a spring. When the tension of the spring is increased, the frequency of its vibration will increase. Additionally, when the strength of the bond is increased, the frequency at which it vibrates will increase. Lastly, if the mass of the two balls is increased, the frequency will decrease.
These are the same principles behind tuning a guitar: the tighter a string is tuned, the higher its pitch will be. When the strength of a guitar string is increased, a thick string is used instead of a thin string, the frequency also changes. Thicker strings, like those you’ll find on a bass guitar, produce lower frequencies than the thinner strings you’ll find on a regular guitar.
Figure 1 Stretching and bending of covalent bonds between atoms, as visualized by the spring model.
These intramolecular vibrations and rotations are elicited and detected with IR spectroscopy.
b) Key functional groups
The plot of an IR spectrum has percent transmittance on the y-axis and the wavenumber on the x-axis.
The fingerprint region is a special range of wavenumbers from around 1500 to 500 cm-1 that is unique to a certain compound. For the MCAT, you'll be working within a different range, from around 4000 to 400 cm-1. The table below lists the key functional groups for which you should memorize their characteristic wavenumbers. These, in particular, are important because they are characteristic of biological compounds. Table 1 Key functional groups and their wavenumber
| Functional Group | Wavenumber |
|---|---|
For more information on the way they behave in reactions, be sure to refer to our guide on important functional groups.
c) Example
Let’s take what has been discussed so far and apply it using an example problem. Try to identify the functional groups present in the IR spectrum below.
Figure 2 An unlabelled IR spectrum.
First, note that there is a broad peak at around 3,300 to 3,000 cm-1. This corresponds with the presence of an alcohol functional group (O-H). Next, there is another peak, present around the 1800 to 1600 cm-1 range. This indicates that the compound has a carbonyl group (C=O). The peaks we see towards the very right are within the fingerprint region and are out of scope for the MCAT.
Figure 3 An IR spectrum of a molecule that contains -OH and C=O functional groups.
On the MCAT, you may be asked to identify the molecular compound that has generated a corresponding IR spectrum. Let’s try an example, referring to the IR spectrum we’ve been working with.
Which of the following structures is most likely the compound that generated the IR spectrum?
A)
B)
C)
D)
Further, note that choice D is unique because it is the only one lacking a carbonyl group. Since the IR spectrum has confirmed the presence of a C=O bond in the molecule (a peak at around ~1,750 cm-1), this cannot be the right structure.
Finally, choices A and B differ in the presence of an alcohol group. Since the IR spectrum has confirmed the presence of an -OH bond in the molecule (a broad peak at ~3,000 cm-1), choice B cannot be the correct structure. Therefore, choice A, which contains a carboxylic acid, is the only molecule presented in the answer options that could have generated the IR spectrum.
d) Related forms of spectroscopy
While spectroscopy using light from the infrared region is widely used, other frequencies of light may also be used for spectroscopy.
Spectroscopy using visible light can be useful to detect the change in concentration of a colored indicator. Indicator molecules can be used to measure the progress of a reaction. For instance, the dye malachite green binds to free inorganic phosphate to turn a vibrant green. The dye will absorb light at green wavelengths while transmitting light at a red wavelength, since red and green are complementary colors.
Spectroscopy using ultraviolet light can be useful for detecting conjugated electron systems within aromatic molecules. These ultraviolet wavelengths interact with pi-electron and nonbonding electron transitions to produce different frequencies of transmitted light.
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Part 3: NMR spectroscopy
a) Experimental procedure
Nuclear magnetic resonance spectroscopy, or NMR spectroscopy, is a type of spectroscopy that allows scientists to determine the structure of a molecule, including the presence of key functional groups. This technique takes advantage of the fact that the nucleus of an atom can be in one of many different magnetic states when placed in an external magnetic field. One magnetic state corresponds with alignment with the magnetic field and low energy, while the other corresponds with unalignment with the magnetic field and higher energy.
Molecular samples are placed in a magnetic field and exposed to different frequencies of light, the absorption of which can cause shifts between magnetic states. Depending on atoms present in the molecule and its magnetic environment, absorption will occur at different frequencies. Measuring this absorption at different frequencies thereby allows the determination of any elements present, along with the structure of the molecule.
b) Reading a spectrum
NMR spectroscopy plots list frequency on their y-axis and chemical shift (δ), measured in parts per million, on the x-axis. Similar to IR plots, this value decreases the further right you go on an NMR spectrum.
Moving toward the left of an NMR spectroscopy plot (toward higher chemical shifts) is referred to as going downfield. Moving toward the right of an NMR spectroscopy plot (toward lower chemical shifts) is referred to as going upfield. This terminology is important when discussing deshielding.
Deshielding is a phenomenon that occurs when electron-withdrawing groups pull electrons away from the core of an atom, making it more susceptible to the magnetic field. Electron-withdrawing groups tend to contain elements of very high electronegativity. (For more information on this, be sure to refer to our guide on periodic trends.) This “pulling away” of electron density makes the nucleus more susceptible to magnetic shifts, thus moving a peak further downfield. Conversely, shielding occurs when electron-donating groups increase the electron density near the core of an atom, reducing its susceptibility to the magnetic field. This moves a peak further upfield.
Figure 4 Electron-withdrawing groups vs electron-donating groups
On test day, you will most likely encounter problems involving H-NMR spectroscopy. For this type of spectroscopy, certain types of bonded protons (in the form of hydrogen atoms) cause a specific chemical shift. The table below summarizes the chemical shifts you should memorize and know.
Table 2 NMR-Spectroscopy values of differently bonded carbons
| Type of proton | Chemical shift / δ (ppm) |
|---|---|
c) Splitting and coupling
Spin-spin splitting, also known as coupling, occurs when hydrogen atoms attached to adjacent carbons interfere with one another's magnetic environment. These protons, which are often located in the same chemical environment and bonded to the same heteroatoms, are considered to be equivalent protons. The coupling constant (J) is used to define the strength of these magnetic interferences. It is measured in Hertz. Ultimately, the presence of splitting of protons in a magnetic field is useful because it results in distinguishing features between isomers.
Depending on the number of adjacent carbons, any peak on an NMR spectrum may be split up into subpeaks. We can predict the number of subpeaks that will arise using the formula, n + 1, where n is equal to the number of hydrogen atoms that are three bonds away from the hydrogen atom of interest. In other words, the carbon atoms that the hydrogen atoms are bonded to are neighboring.
If two subpeaks result on the spectrum, the trace is referred to as a doublet. If three subpeaks result, they are referred to as a triplet. A set of any greater number of subpeaks is referred to as a multiplet.
d) Example
Let's take what we've discussed and apply it using an example.
Given the compound below and its 1H-NMR spectrum, determine which sets of hydrogen atoms correspond with each peak.
Figure 5 An NMR spectrum of a known molecule.
Let's start with functional group A. There are no hydrogen atoms on adjacent carbons that are within three bonds. Using the n + 1 formula, we would expect 0 + 1 = 1 subpeaks that are grouped together. Therefore, the singlet at around 2 ppm corresponds with group A.
Next, let's look at group B. There are three hydrogen atoms on the carbon directly adjacent to this group. Using the n + 1 formula, we would expect 3 + 1 = 4 subpeaks that are grouped together on the spectrum. Thus, the multiplet at around 4 ppm corresponds with this group.
Finally, group C has two adjacent hydrogen atoms in group B. After plugging in the numbers to the n + 1 formula, we would expect 2 + 1 = 3 subpeaks grouped together. Therefore, the triplet at around 1 ppm corresponds with group C.
Figure 6 Mapped NMR spectroscopy with each functional group paired to a chemical shift
Recognizing Common Characteristic Group Absorptions and the Fingerprint Region
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Part 4: High-yield terms
Infrared (IR) spectroscopy: allows one to identify key functional groups using infrared light; detects specific frequencies at which known covalent bonds vibrate
Percent transmittance: amount of infrared light that passes through a molecular sample; listed on the y-axis of an IR spectrum
Fingerprint region: region within 1500 to 500 cm-1 of an IR spectrum that contains peaks unique to a certain compound
Nuclear magnetic resonance (NMR) spectroscopy: provides identification of the structure of a molecule using an applied magnetic field that impacts the alignment of atomic nuclei
Tetramethylsilane (TMS): standard used in NMR spectroscopy
Downfield: towards the left on an NMR spectrum; greater chemical shift (δ)
Upfield: towards the right on an NMR spectrum; lesser chemical shift (δ)
Deshielding: occurs when electron-withdrawing groups pull electrons away from the core of an atom, making the atom more susceptible to an applied magnetic field
Shielding: occurs when electron-donating groups increase the electron density near the core of an atom, reducing its susceptibility to an applied magnetic field
Splitting: occurs when hydrogens attached to adjacent carbon atoms interfere with one another's magnetic environment
Doublet: in NMR spectra, a peak with two subpeaks
Triplet: in NMR spectra, a peak with three subpeaks
Multiplet: in NMR spectra, a peak with four or more subpeaks
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Part 5: Passage-based questions and answers
The World Health Organization's List of Essential Medicines is a directory of medications that have been deemed by the scientific community to be vital for any health system. Among these medications are household medications such as acetaminophen. Acetaminophen is commonly used to treat fevers and pain.
Several students are interested in determining the structure of acetaminophen using spectroscopy. Spectroscopy is a laboratory technique that uses electromagnetic radiation and its interactions with matter to determine key characteristics of compounds. This includes functional groups and the types of bonds present in a molecule. Two common spectroscopy techniques include infrared spectroscopy and nuclear magnetic resonance spectroscopy.
First, the students took a sample of acetaminophen and irradiated it with infrared light. A spectrometer was used to detect the frequencies at which absorption occurred. The resulting data is shown in Figure 1.
Figure 1 Resulting absorbance spectrum for acetaminophen
Next, the students took a sample of the drug and subjected it to NMR spectroscopy. The data from this procedure is shown in Figure 2.
Figure 2 NMR spectroscopy for acetaminophen
Passage created for practice purposes only
Question 1: Which of the following is the range for the fingerprint region of an IR spectrum?
A) 4000 – 400 cm-1
B) 2000 – 0 cm-1
C) 1500 – 500 cm-1
D) 4000 – 500 cm-1
A) Carbonyl
B) Carboxylic acid
C) Alcohol
D) Amino
Question 3: Which of the following statements most accurately characterizes NMR spectroscopy?
A) It uses a magnetic field to measure the mass of each individual atom in a compound
B) It uses a magnetic field to manipulate the alignment of atomic nuclei
C) It allows researchers to determine key functional groups but not the overall structure of a molecule
D) It separates molecules based on their charge
Question 4: Which of the following functional groups is most likely to generate the furthest downfield chemical shift in Figure 2?
A) TMS
B) sp3 hybridized carbon
C) Carboxylic acid
D) Aldehyde
Question 5: Based on the information in the passage, which of the following is most likely not found in acetaminophen?
A) Aromatic carbons
B) sp2 hybridized carbon
C) Alcohol
D) Carboxylic acid
Answer key for passage-based questions
- Answer choice C is correct. The fingerprint region is the special range that is unique to a certain compound. It spans the region of wavenumbers from 1500 to 500 cm-1 (choice C is correct).
- Answer choice C is correct. A broad peak at around ~3,000 cm-1 corresponds with the presence of an -OH bond, and thus an alcohol group (choice C is correct). A sharp peak at 1,750 cm-1 corresponds with the presence of a carbonyl group (choice A is incorrect). Broad peaks at 3,000 cm-1 and 1,750 cm-1 indicate the presence of both of these functional groups and thus suggest the presence of a carboxylic acid (choice B is incorrect). A sharp peak at around ~3,300 cm-1 corresponds with the presence of an amino group (choice D is incorrect).
- Answer choice B is correct. Nuclear magnetic resonance spectroscopy, or NMR spectroscopy, allows researchers to determine the overall structure of a molecule (choice C is incorrect). This is accomplished by applying a magnetic field that manipulates the alignment of atomic nuclei (choices A and D are incorrect).
- Answer choice D is correct. The most downfield chemical shift is the one most to the left. In Figure 1, this is the peak just below 10 ppm, which has been generated by an aldehyde functional group (choice D is correct). By definition, TMS has a chemical shift of 0 ppm (choice A is incorrect). An sp3 carbon generates a chemical shift around δ=0 – 2 ppm (choice B is incorrect). Carboxylic acid carbons generate a chemical shift around δ=10 - 13 ppm (choice C is incorrect).
- Answer choice D is correct. The resulting NMR spectrum contains peaks between 6 – 9 ppm, which indicates the presence of aromatic carbons. These carbons are sp2 hybridized (choices A and B are incorrect). The broad peak at around ~3,000 cm-1 in the IR spectrum indicates the presence of an alcohol group (choice C is incorrect). Thus, by process of elimination, a carboxylic acid is most likely to be absent in the structure of acetaminophen (choice D is correct).
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Part 6: Standalone questions and answers
Question 1: Which of the following most accurately characterizes IR spectroscopy?
A) It uses ultraviolet light to detect the presence of specific functional groups
B) It uses infrared light to detect specific frequencies at which covalent bonds vibrate
C) It uses an applied magnetic field to measure the alignment of atomic nuclei
D) It uses infrared light to measure the mass of the different atoms present in a molecule
Question 2: What is tetramethylsilane?
A) A standard used to calibrate NMR spectra
B) A denaturing agent necessary for NMR spectroscopy
C) A fluorescent tag that allows one to detect the presence of a compound of interest
D) None of the above
Question 3: Which of the following splitting patterns would be observed in an NMR spectrum of ethanol?
A) One doublet and one triplet
B) Two doublets and one singlet
C) Two doublets and one triplet
D) One multiplet and one triplet
Question 4: What is the expected chemical shift of the functional group shown below?
A) 0 – 2
B) 6 – 9
C) 9 – 10
D) 10 – 13
Question 5: Which of the following best describes the occurrence of deshielding?
A) When electron-donating groups make the nucleus of an atom more susceptible to the applied magnetic field
B) When electron-withdrawing groups make the nucleus of an atom more susceptible to the applied magnetic field
C) When electron-donating groups make the nucleus of an atom less susceptible to the applied magnetic field
D) When electron-withdrawing groups make the nucleus of an atom less susceptible to the applied magnetic field
Answer key for standalone questions
Answer choice B is correct. Infrared spectroscopy uses infrared light sources at various frequencies (choices A and C are incorrect). This allows a researcher to determine the frequencies that cause particular covalent bonds to vibrate and stretch (choice D is incorrect).
Answer choice A is correct. Tetramethylsilane, or TMS, is a chemical standard used to calibrate NMR spectra. It has a chemical shift of 0 ppm (choices B, C, and D are incorrect).
Answer choice D is correct. The methyl group (-CH₃) at the end of the ethanol molecule is adjacent to two hydrogens. Using our n+1 formula, we would expect 2 + 1 = 3 subpeaks together. Thus, a triplet should be present (choice B is incorrect). The hydrogens found on the other carbon are adjacent to the three hydrogens on the methyl group. This gives us 3 + 1 = 4 subpeaks grouped together. Therefore, a multiplet should also be present (choices A and C are incorrect).
Answer choice D is correct. The functional group shown is a carboxylic acid group, which has a chemical shift of around δ=10 – 13 ppm (choice D is correct). An sp³ carbon would result in a chemical shift of δ=0 – 2 ppm (choice A is incorrect). An aromatic carbon would result in a chemical shift of δ=6 – 9 ppm (choice B is incorrect). Finally, an aldehyde group would result in a chemical shift of δ=9 – 10 ppm (choice C is incorrect).
Answer choice B is correct. Deshielding occurs when electron-withdrawing groups pull electrons away from an atom's nucleus (choices A and C are incorrect). This makes the atom's protons more susceptible to the applied magnetic field (choice D is incorrect).
