NMR spectroscopy in plain words
Nuclear magnetic resonance (NMR) spectroscopy puts a sample in a strong magnet and listens for radio signals from certain atomic nuclei, usually hydrogen-1 and carbon-13. Tiny differences in those signals reveal how the atoms are connected, which makes NMR the chemist's main tool for working out molecular structure.
The short version
- Nuclei with an odd number of protons or neutrons, such as ¹H and ¹³C, have spin and act like tiny magnets. ¹²C and ¹⁶O do not.
- In a magnetic field the spins line up with (lower energy) or against (higher energy) the field. A radio-frequency pulse at just the right frequency flips them: that is resonance.
- Electrons around a nucleus shield it slightly, so nuclei in different chemical surroundings resonate at slightly different frequencies: the chemical shift, measured in parts per million (ppm, δ).
- In ¹H NMR, a signal from protons with n equivalent neighbors splits into n + 1 peaks, and the area under each signal is proportional to the number of protons.
Spin, field and resonance
Many nuclei have a property called spin, which gives them a magnetic moment, like a tiny bar magnet. Outside a magnet these little magnets point in random directions. Inside a strong field (called B₀), a spin-½ nucleus such as ¹H can take one of two states: aligned with the field, which is slightly lower in energy, or against it, slightly higher. There are only a few more nuclei in the lower state than the upper one, and the energy gap is very small, so NMR is a relatively insensitive technique that needs strong magnets.
The energy gap corresponds to a radio frequency. Hit the sample with radio waves of exactly that frequency and nuclei absorb energy and flip; as they return, they give off a signal the instrument detects. The resonance frequency is proportional to the field strength. This is often called the Larmor frequency. For ¹H it is about 42.6 MHz per tesla:
| Magnet | ¹H frequency | ¹³C frequency |
|---|---|---|
| 1.5 T (common hospital MRI) | ≈ 64 MHz | ≈ 16 MHz |
| 4.7 T | 200 MHz | 50 MHz |
| 11.7 T | ≈ 500 MHz | ≈ 125 MHz |
| 23.5 T (the strongest commercial magnets) | ≈ 1,000 MHz | ≈ 250 MHz |
Chemists often name an instrument by its ¹H frequency: a "500 MHz NMR" has an 11.7-tesla magnet.
Chemical shift and shielding
If every hydrogen nucleus resonated at exactly the same frequency, NMR would be useless. They do not, because the electrons around each nucleus create a small opposing field. A nucleus surrounded by more electron density is shielded: it feels slightly less of the external field and resonates at a slightly lower frequency, toward the right of a spectrum ("upfield"). A nucleus next to an electronegative atom such as oxygen, or near a C=O or aromatic ring, is deshielded and appears further left ("downfield").
These differences are tiny, a few hundred hertz out of hundreds of millions, so they are expressed relative to a standard, tetramethylsilane (TMS), set at 0:
δ (ppm) = (distance of the peak from TMS in Hz) ÷ (spectrometer frequency in MHz)
Example: a peak 400 Hz from TMS on a 200 MHz instrument has δ = 400 ÷ 200 = 2.0 ppm. On a 500 MHz instrument the same proton appears 1,000 Hz from TMS, and 1,000 ÷ 500 = 2.0 ppm again. Using ppm means spectra from any magnet can be compared directly.
| Type of hydrogen | Typical δ (ppm) |
|---|---|
| C–H in an alkane chain | about 0.9–1.7 |
| C–H next to C=O | about 2–2.7 |
| C–H on a carbon bonded to O or a halogen | about 3–4.5 |
| H on an aromatic ring | about 6.5–8 |
| Aldehyde H (CHO) | about 9.5–10 |
Almost all ¹H signals fall between 0 and 10 ppm (carboxylic acid O–H, at about 10–12, is a well-known exception). ¹³C signals spread over a much wider range, roughly 0–220 ppm.
Splitting and integration
The n + 1 rule
Protons on neighboring carbons feel each other's small magnetic fields, which splits a signal into several lines. In simple cases, a set of protons with n equivalent neighboring protons gives n + 1 peaks. Chemically equivalent protons do not split each other.
Bromoethane, CH₃CH₂Br, is the classic example:
- The CH₂ protons (near bromine, deshielded) have 3 neighbors on CH₃, so they appear as a quartet (3 + 1 = 4 peaks) at about 3.4 ppm.
- The CH₃ protons have 2 neighbors on CH₂, so they appear as a triplet (2 + 1 = 3 peaks) at about 1.7 ppm.
The peak heights within a multiplet follow Pascal's triangle: a triplet is about 1 : 2 : 1, a quartet 1 : 3 : 3 : 1. The spacing between the lines is the coupling constant J, measured in hertz (about 7 Hz in bromoethane). Groups that split each other share the same J, which helps you match partners.
Integration
The area under a ¹H signal is proportional to the number of hydrogens producing it. In bromoethane the quartet and triplet areas are in the ratio 2 : 3. The instrument reports these areas as an integral, so you can count hydrogens in each environment.
¹³C NMR
Only about 1.1% of natural carbon is ¹³C, so carbon spectra are weaker and take longer. They are usually recorded with the hydrogen coupling removed ("decoupled"), so each different carbon appears as a single line. Counting lines tells you how many kinds of carbon the molecule has; peak areas in routine ¹³C spectra are not reliably proportional to the number of carbons.
What NMR is used for
Chemists use NMR daily to confirm that a reaction made the intended product, to check purity, and to identify unknown compounds. Larger versions determine the shapes of proteins in solution and measure metabolites in body fluids. The same physics, applied to the hydrogen in water and fat in the body, gives medical imaging: see MRI basics: T1, T2 and T2*.
Memory aid
Read a ¹H spectrum by asking four questions, "How many, where, how big, how split?"
- How many signals? = how many kinds of hydrogen.
- Where (shift)? = what they are next to; left means deshielded.
- How big (integration)? = how many H in each kind.
- How split? = number of neighbors + 1.
And for shielding: "Electrons are an umbrella" — more electrons, more shade from the field, signal further right.
A verse to remember it
Check yourself
Why does ¹²C give no NMR signal but ¹³C does?
¹²C has 6 protons and 6 neutrons (both even), so it has no nuclear spin. ¹³C has 7 neutrons, an odd number, so it has spin and is NMR-active.
A peak is 600 Hz from TMS on a 300 MHz spectrometer. What is its chemical shift?
600 ÷ 300 = 2.0 ppm.
In ethanol's ¹H spectrum (ignoring the OH), what splitting do you expect for the CH₃ and CH₂ groups?
The CH₃ has 2 neighbors on CH₂, so a triplet. The CH₂ has 3 neighbors on CH₃, so a quartet (the OH proton usually exchanges quickly and does not split it in ordinary samples).
What does the integral of a ¹H signal tell you?
The relative number of hydrogens that produce that signal.