Unit 13: Nuclear Magnetic Resonance (NMR) Spectroscopy

Mapping the carbon-hydrogen framework of molecules to determine their exact structure.

13.7 Basic Principles (Nuclear Spin, Chemical Shift)

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique that provides detailed information about the structure of a molecule. It is based on the magnetic properties of certain atomic nuclei.

The principle relies on nuclear spin. Nuclei with an odd number of protons or neutrons (like ¹H and ¹³C) have a quantum mechanical property called spin, which causes them to behave like tiny magnets. When placed in a strong external magnetic field ($B_0$), these nuclear magnets can align either with the field (low-energy state) or against it (high-energy state).

By irradiating the sample with radio waves, we can provide the exact amount of energy needed to cause a nucleus to "flip" from the low-energy to the high-energy state. This absorption of energy is called resonance.

Chemical Shift

Crucially, the exact frequency of radio waves needed for resonance depends on the nucleus's local electronic environment. Electrons surrounding the nucleus create a small magnetic field that opposes the external field, "shielding" the nucleus.

  • Nuclei in electron-rich environments are highly shielded and require a lower frequency to resonate. They are said to be upfield.
  • Nuclei near electronegative atoms (like oxygen or halogens) have their electron density pulled away. They are deshielded and require a higher frequency to resonate. They are said to be downfield.

This difference in resonance frequency is called the chemical shift ($\delta$), measured in parts per million (ppm). An NMR spectrum plots the absorption of radio waves against the chemical shift, revealing the different electronic environments of the nuclei in the molecule.

Solved Examples:
  1. Which of the following nuclei are NMR-active: ¹H, ¹²C, ¹³C, ¹⁶O?
    Solution: ¹H and ¹³C are NMR-active because they have an odd number of nucleons. ¹²C and ¹⁶O have even numbers of both protons and neutrons, so they have no nuclear spin and are NMR-inactive.
  2. What does "resonance" mean in the context of NMR?
    Solution: Resonance is the absorption of energy from radio waves that causes a nucleus to flip its spin from a low-energy state to a high-energy state within an external magnetic field.
  3. What is meant by "shielding" in NMR?
    Solution: Shielding is the effect where the electron cloud around a nucleus creates a small magnetic field that opposes the external field, thus protecting or "shielding" the nucleus from the full strength of the external field.
  4. Would the protons in methane ($CH_4$) or chloromethane ($CH_3Cl$) be more shielded?
    Solution: The protons in methane ($CH_4$) would be more shielded. In chloromethane, the highly electronegative chlorine atom pulls electron density away from the protons, making them deshielded.
  5. What does a larger chemical shift value (further downfield) indicate?
    Solution: It indicates that the nucleus is in a more electron-poor environment (it is more deshielded).
  6. What is the standard reference compound used in NMR, and what is its chemical shift defined as?
    Solution: Tetramethylsilane (TMS), Si(CH₃)₄. Its chemical shift is defined as 0 ppm.
  7. Why are radio waves used in NMR instead of infrared radiation?
    Solution: The energy difference between nuclear spin states is very small and corresponds to the energy of photons in the radio wave region of the electromagnetic spectrum. Infrared radiation has too much energy and causes molecular vibrations instead.
  8. What are the two possible alignment states for a ¹H nucleus in an external magnetic field?
    Solution: Aligned with the field (lower energy) and aligned against the field (higher energy).
  9. What does "upfield" on an NMR spectrum mean?
    Solution: Upfield refers to the right-hand side of the spectrum, which corresponds to lower chemical shift values (closer to 0 ppm) and more shielded nuclei.
  10. Why is ¹³C NMR spectroscopy more difficult than ¹H NMR?
    Solution: The natural abundance of the NMR-active ¹³C isotope is only about 1.1%, whereas the ¹H isotope is nearly 100% abundant. This makes the ¹³C signals much weaker and harder to detect.

13.8 Proton NMR (¹H NMR): Chemical Shift, Integration, Splitting

A ¹H NMR spectrum provides three crucial pieces of information to determine a molecule's structure:

  1. Number of Signals: The number of distinct signals indicates the number of different electronic environments for protons in the molecule. Protons in identical environments are called chemically equivalent.
  2. Integration (Peak Area): The area under each signal is proportional to the number of protons in that environment. This is often shown as an integration ratio.
  3. Splitting Pattern (Spin-Spin Coupling): The signal for a set of protons is split into multiple peaks by the influence of non-equivalent protons on adjacent carbon atoms. The splitting pattern follows the n+1 rule, where 'n' is the number of protons on the adjacent carbon(s).
    • 0 adjacent protons → singlet (1 peak)
    • 1 adjacent proton → doublet (2 peaks)
    • 2 adjacent protons → triplet (3 peaks)
    • 3 adjacent protons → quartet (4 peaks)
Solved Examples:
  1. How many signals would you expect in the ¹H NMR spectrum of ethanol ($CH_3CH_2OH$)?
    Solution: There are three different proton environments: the -CH₃ group, the -CH₂ group, and the -OH group. Therefore, you would expect three signals.
  2. A ¹H NMR spectrum has two signals with an integration ratio of 2:3. What does this tell you?
    Solution: It tells you that for every two protons in one environment, there are three protons in the other environment.
  3. A signal in a ¹H NMR spectrum is a triplet. How many protons are on the adjacent carbon(s)?
    Solution: According to the n+1 rule, if the signal is a triplet (3 peaks), then n+1 = 3. Therefore, n = 2 protons on the adjacent carbon(s).
  4. Describe the expected ¹H NMR spectrum for chloroethane ($CH_3CH_2Cl$).
    Solution: There are two proton environments: -CH₃ and -CH₂. The integration ratio will be 3:2. The -CH₃ signal will be split into a triplet (n=2, n+1=3) by the adjacent -CH₂ group. The -CH₂ signal will be split into a quartet (n=3, n+1=4) by the adjacent -CH₃ group. The -CH₂ signal will be further downfield due to the deshielding effect of the chlorine atom.
  5. What splitting pattern would you expect for the circled protons in propane ($CH_3\underline{CH_2}CH_3$)?
    Solution: The central CH₂ group has 3 protons on the left and 3 on the right, for a total of n=6 adjacent protons. According to the n+1 rule, its signal would be split into a septet (7 peaks).
  6. How many signals would you expect in the ¹H NMR spectrum of benzene ($C_6H_6$)?
    Solution: Only one signal. Due to the symmetry of the ring, all six protons are in the exact same chemical environment.
  7. A signal is a singlet. What does this tell you about its neighbors?
    Solution: It has no non-equivalent protons on any adjacent carbons (n=0, n+1=1).
  8. A molecule has the formula C₂H₆O. Its ¹H NMR spectrum is a single sharp singlet. What is its structure?
    Solution: The formula fits ethanol ($CH_3CH_2OH$) or dimethyl ether ($CH_3OCH_3$). Ethanol would have three signals. Dimethyl ether has only one proton environment (all 6 protons are equivalent due to symmetry) and no adjacent protons, so it would give a single singlet. The structure is dimethyl ether.
  9. What is the "n+1 rule"?
    Solution: It is a rule that predicts the splitting pattern of a signal in ¹H NMR. The signal for a proton is split into 'n+1' peaks, where 'n' is the number of non-equivalent protons on adjacent carbons.
  10. A signal for a CH₃ group is a doublet. What does this tell you about its neighboring carbon?
    Solution: A doublet means n+1=2, so n=1. This tells you that the adjacent carbon atom has only one proton attached to it (it is a CH group).

13.9 Carbon-13 NMR (¹³C NMR): Chemical Shift, Number of Signals

¹³C NMR spectroscopy provides information about the carbon skeleton of a molecule. The principles are similar to ¹H NMR, but the interpretation is simpler.

In a standard broadband-decoupled ¹³C NMR spectrum, each signal appears as a single sharp line. We get two main pieces of information:

  • Number of Signals: The number of signals equals the number of non-equivalent carbon environments in the molecule. Carbons that are equivalent by symmetry will give a single signal.
  • Chemical Shift ($\delta$): The chemical shift of each signal (typically 0-220 ppm) indicates the type of carbon environment. For example, carbons in C=O bonds appear far downfield (>160 ppm), while carbons in alkanes appear far upfield (<50 ppm).

Unlike ¹H NMR, the peak heights or areas in a standard ¹³C NMR spectrum are not directly proportional to the number of carbons.

Solved Examples:
  1. How many signals would you expect in the ¹³C NMR spectrum of ethanol ($CH_3CH_2OH$)?
    Solution: There are two distinct carbon environments: the -CH₃ carbon and the -CH₂- carbon. You would expect two signals.
  2. How many signals would you expect in the ¹³C NMR spectrum of pentan-3-one ($CH_3CH_2COCH_2CH_3$)?
    Solution: Due to the molecule's symmetry, there are only three distinct carbon environments: the C=O carbon, the two equivalent -CH₂- carbons, and the two equivalent -CH₃ carbons. You would expect three signals.
  3. A compound has the formula C₄H₁₀. Its ¹³C NMR spectrum shows only one signal. What is its structure?
    Solution: The formula could be butane or 2-methylpropane. Butane has two carbon environments. 2-methylpropane has a central CH carbon and three equivalent CH₃ carbons, also giving two signals. Wait, there must be a mistake in the question or my reasoning. Let me re-evaluate. Oh, I see, the question implies a different isomer. Let me check again. Ah, butane has two signals (CH₃ and CH₂). 2-methylpropane has two signals (CH and CH₃). The question is flawed. Let me rephrase and answer a correct question. **Question: A compound with formula C₅H₁₂ shows a single peak in its ¹³C NMR spectrum. What is its structure?** **Solution:** A single peak means all carbons are chemically equivalent. This high degree of symmetry points to 2,2-dimethylpropane (neopentane), which has a central quaternary carbon and four equivalent methyl carbons. Wait, that's two signals. The only way to have one signal is if all carbons are identical. The only C₅H₁₂ isomer with one carbon environment is not possible. Let me try again with a better example. **Question: How many signals are in the ¹³C NMR spectrum of benzene ($C_6H_6$)?** **Solution:** Due to symmetry, all six carbon atoms are in the same environment. Therefore, there is only one signal.
  4. A ¹³C NMR spectrum shows a peak at 175 ppm. What type of carbon is likely present?
    Solution: A chemical shift greater than 160 ppm is characteristic of a carbon in a carbonyl (C=O) or carboxyl group.
  5. How many signals would be in the ¹³C NMR spectrum of propane ($CH_3CH_2CH_3$)?
    Solution: There are two distinct carbon environments due to symmetry: the two outer -CH₃ carbons are equivalent, and the central -CH₂ carbon is unique. There would be two signals.
  6. Why are there no splitting patterns in a standard ¹³C NMR spectrum?
    Solution: The spectra are typically run in "broadband-decoupled" mode, which uses a second radio frequency to constantly flip the spins of the attached protons. This removes the effect of coupling, so each carbon signal appears as a simple singlet.
  7. Which would have a larger chemical shift (be more downfield) in a ¹³C NMR spectrum: the carbon of a C-Cl bond or a C-Br bond?
    Solution: The carbon of the C-Cl bond. Chlorine is more electronegative than bromine, so it deshields the carbon atom more, causing it to resonate at a higher frequency (larger chemical shift).
  8. A molecule has 6 carbons, but its ¹³C NMR spectrum only shows 4 signals. What does this imply?
    Solution: This implies that the molecule has some symmetry, causing some of the carbon atoms to be chemically equivalent.
  9. How many signals would you expect in the ¹³C NMR spectrum of propan-1-ol ($CH_3CH_2CH_2OH$)?
    Solution: There is no symmetry in this molecule. All three carbon atoms are in different chemical environments. You would expect three signals.
  10. How many signals would you expect in the ¹³C NMR spectrum of propan-2-ol ($CH_3CH(OH)CH_3$)?
    Solution: This molecule has a plane of symmetry through the central carbon. The two outer -CH₃ carbons are equivalent, and the central -CH(OH)- carbon is unique. You would expect two signals.

13.10 Applications of NMR Spectroscopy

NMR is arguably the most powerful tool available to organic chemists for determining the structure of a molecule.

  • Structure Elucidation: By combining the information from ¹H NMR (number of proton environments, ratio of protons, and neighboring protons) and ¹³C NMR (number of carbon environments), chemists can piece together the exact connectivity of atoms in a molecule.
  • Medical Imaging (MRI): Magnetic Resonance Imaging (MRI) is a medical application of NMR principles. It uses the same principles of nuclear spin in a magnetic field, but focuses on the protons in water molecules within the body's tissues to create detailed, 3D images of soft tissues and organs without using harmful X-rays.
  • Purity Determination: NMR can be used to assess the purity of a sample. The presence of unexpected signals indicates that impurities are present.
Solved Examples:
  1. What is the primary application of NMR in organic chemistry?
    Solution: Structure elucidation - determining the detailed structure of molecules.
  2. What does the acronym MRI stand for?
    Solution: Magnetic Resonance Imaging.
  3. How is MRI different from a standard NMR experiment?
    Solution: While based on the same principles, NMR spectroscopy aims to get chemical information from a uniform sample, resulting in a spectrum. MRI uses gradients in the magnetic field to get spatial information, creating an image of a non-uniform object like the human body.
  4. A chemist runs a ¹H NMR spectrum on a product they believe is pure ethanol. They see the expected three signals but also a small, extra singlet. What does this suggest?
    Solution: The extra singlet suggests the presence of an impurity that has protons in an environment different from those in ethanol.
  5. Which technique, IR or NMR, gives more detailed information about the overall molecular skeleton?
    Solution: NMR spectroscopy gives far more detailed information about the carbon-hydrogen framework and connectivity. IR is primarily used to identify functional groups.
  6. Why is MRI particularly good for imaging soft tissues?
    Solution: Because soft tissues have a high water content, and MRI is extremely sensitive to detecting the signals from the hydrogen nuclei (protons) in those water molecules.
  7. Can NMR be used for quantitative analysis?
    Solution: Yes. The integration of signals in ¹H NMR is a powerful quantitative tool that gives the ratio of protons in different environments.
  8. What information from a ¹H NMR spectrum tells you about adjacent protons?
    Solution: The splitting pattern (e.g., singlet, doublet, triplet).
  9. What information from a ¹³C NMR spectrum tells you about molecular symmetry?
    Solution: The number of signals. If the number of signals is less than the number of carbons in the molecular formula, it indicates that the molecule has symmetry.
  10. Why is NMR a non-destructive technique?
    Solution: The energy from radio waves is very low and only flips nuclear spins; it does not break chemical bonds, so the sample can be recovered unchanged after analysis.

🧠 Quiz

Answer: Nuclear spin.

Answer: Radio waves.

Answer: The number of non-equivalent carbon environments.

Answer: 3 (from the n+1 rule).

Answer: The nucleus is in an electron-poor environment, often near an electronegative atom.

Answer: The relative number of protons in that environment.

Answer: Tetramethylsilane (TMS).

Answer: Two (the CH₃ protons are equivalent, and the CH₂ protons are different).

Answer: Magnetic Resonance Imaging (MRI).

Answer: A carbonyl carbon (C=O) from a ketone or aldehyde.

Answer: A singlet.

Answer: One.

Answer: The left-hand side, corresponding to larger chemical shift values and deshielded nuclei.

Answer: A triplet (split by the adjacent CH₂ group, n=2, n+1=3).

Answer: Its low abundance (1.1%) makes the signals very weak and requires more sensitive instruments or longer scan times.

Answer: Structure elucidation of molecules.

Answer: One.

Answer: No.

Answer: Chemically equivalent.

Answer: Protons (hydrogen nuclei) in water molecules.
⬅ Back to IR Spectroscopy Next: UV-Vis Spectroscopy ➡