Hydrogens in a magnet

Section 4 of 4

NMR, short for nuclear magnetic resonance, uses a strong magnet and radio waves to pick out the hydrogen atoms in a molecule.

A hydrogen atom's nucleus is a single proton, and a proton behaves like a tiny bar magnet. In a strong magnet, the tiny magnets line up along the field, a few more pointing with it than against it. A pulse of radio waves at just the right frequency flips them over, and as they settle back they give out a faint radio signal that the machine picks up.

Neighbours change the signal

The electrons around a hydrogen shield its nucleus a little from the big magnet. An oxygen atom nearby pulls electrons away, as you saw with the partial charges on O–⁠H and C–⁠O bonds. Hydrogens close to an oxygen feel more of the magnet and flip at a slightly higher frequency, so their peaks sit further left on an NMR spectrum. A peak's place on the scale is called its chemical shift.

Hydrogens in the same surroundings share one peak, and the size of the peak counts them. Ethanol, CH₃CH₂OH, has three kinds of hydrogen: three on the CH₃ carbon, two on the CH₂ carbon next to the oxygen, and one on the oxygen itself.

Ethanol's three kinds of hydrogen Ethanol's displayed formula, H-C-C-O-H with two more hydrogens above and below each carbon, with its hydrogens shaded in three groups. The CH3 group, the first carbon and its three hydrogens, is shaded teal: 3 hydrogens. The CH2 group, the second carbon and its two hydrogens, is shaded amber: 2 hydrogens. The OH group, the oxygen and its hydrogen, is shaded blue: 1 hydrogen. HCCOHHHHH CH₃ 3 H CH₂ 2 H OH 1 H
Ethanol's six hydrogens fall into three groups: 3 on the CH₃ carbon (teal), 2 on the CH₂ carbon (amber) and 1 on the oxygen (blue).
The NMR spectrum of ethanol An NMR spectrum on a scale from 6 on the left to 0 on the right, with three peaks. Near 5.3 on the left, a small blue peak for the OH hydrogen, 1 H. Near 3.7, an amber peak twice as tall for the CH2 hydrogens, 2 H. Near 1.2 on the right, a teal peak three times as tall for the CH3 hydrogens, 3 H. Hydrogens near the oxygen atom sit further left. CH₃ 3 H CH₂ 2 H OH 1 H 6 5 4 3 2 1 0 near oxygen away from oxygen
A simple NMR spectrum of pure ethanol. Each kind of hydrogen gives one peak, and the peak sizes, 1 : 2 : 3 from left to right, count the hydrogens of each kind.

In 1951, three scientists at Stanford University recorded one of the first spectra like this: ethanol's hydrogens gave three separate peaks, with sizes in the ratio 1 : 2 : 3, just like the figure above. A sharper machine splits each peak into a small cluster of lines, which tells a chemist how many hydrogens sit on the next carbon along.

MRI scanners

Hospitals use the same physics in MRI scanners, short for magnetic resonance imaging. About 60% of your body is water, and every water molecule carries two hydrogens. An MRI magnet is 30,000 to 60,000 times as strong as the Earth's magnetic field. Radio pulses flip the hydrogens in your body's water and fat, and the scanner times how quickly they settle back. That time depends on what surrounds the water, so muscle, fat, brain and fluid show up as different shades, and the scanner builds a picture of your body slice by slice, using only a magnet and radio waves.

Three ways to look at a molecule
MachineWhat it uses, and what it shows
infrared spectrometerinfrared light, to find which bonds the molecule has
mass spectrometeran electron beam and an electric push, to weigh the molecule and its pieces
NMR machinea strong magnet and radio waves, to count each kind of hydrogen

Quick check

Propanone, CH₃COCH₃, has six hydrogen atoms. How many peaks does its NMR spectrum show?

That's the end of lesson 23

In this lesson you found out:

  • Why each kind of bond absorbs its own infrared light
  • How to spot O–⁠H and C=O bonds in an infrared spectrum
  • How a mass spectrometer weighs a molecule and its pieces
  • What NMR shows about a molecule's hydrogens, and how MRI uses it
Start Lesson 24: Making molecules All lessons