MULTIPLET

About: The Science of Multiplet

If you can solve this…

A Multiplet puzzle whose shapes correspond to the ethyl-acetate molecule.
A Multiplet puzzle.

…you could solve this.

Proton NMR spectrum of ethyl acetate, with three peak groups labelled by their hydrogen positions.
1H NMR measurement of ethyl acetate, a common chemical used in nail polish remover and in the decaffeination of coffee beans. Image by T. van Schaik via Wikimedia Commons, CC BY-SA 3.0.

What you're looking at

The second picture is a real NMR (nuclear magnetic resonance) measurement of ethyl acetate. NMR is the technique chemists use to figure out what molecules look like by reading the signals they give off in a strong magnetic field.

A large NMR spectrometer: a silver domed magnet on a blue cryostat, surrounded by access stairs in a lab.
If you've ever had an MRI, you have essentially been inside of an NMR machine — the underlying physics are the same. Photo by Adville via Wikimedia Commons, CC BY-SA 4.0.

The result is a number line dotted with peaks, and the chemist's job is to work backward from those peaks to the structure that produced them: which pieces connect to which, where the neighbors are, what the whole molecule must look like. That's the same loop Multiplet asks of you — read the peaks, deduce the connections, build the structure.

Reading the peaks

Both pictures answer the same questions.

Where does each peak sit on the line?

Multiplet

That's a circle's position, which depends on the identity of surrounding shapes.

NMR

It depends on what surrounds each atom: what it's attached to, and what those atoms are attached to.

The number line position is called chemical shift in NMR.

How spiky is each peak?

Multiplet

The letters above each group tell you how many neighbors a circle has and what they are.

NMR

Those same neighbors split the peak into its characteristic spiky shape, telling the chemist where the neighboring atoms sit in the molecule.

Four NMR multiplet patterns shown as peak shapes on a baseline: a doublet, a doublet of doublets, a triplet of doublets, and a quartet of triplets.
Examples of peaks split into multiplets in NMR.

What are the shapes?

Multiplet

Each shape has a fixed number of connections.

NMR

Real atoms behave the same way; carbon makes four bonds like a square, oxygen two like a lens, nitrogen three like a triangle.

The puzzle's shapes stand in for atoms.

Real-world measurements

Skeletal chemical structure of dexamethasone, a multi-ring steroid with hydroxyl, ketone, and fluorine substituents.
Dexamethasone, a steroid drug used to treat inflammation.
Dense proton NMR spectrum of dexamethasone, with many overlapping peaks across the chemical-shift axis.
NMR measurement of dexamethasone. Image by Natasha M. A. Speakman via Wikimedia Commons, CC BY 4.0.

Real measurements are dense. Dozens of peaks, many of them overlapping, each carrying a clue about a molecule that may have hundreds of atoms. A chemist might spend an afternoon teasing one structure out of a single measurement, and a career learning to do it well. Every drug in your medicine cabinet, every new material, every suspect compound in a forensics lab — at some point, someone sat down with a plot like this one and worked backward to the structure. Multiplet may look like a friendlier version, with its colorful arrays of shapes, but underneath it runs on the same logic and deductive reasoning that scientists around the world use every day to map the molecular world.