COSY
Definition and meaning of COSY in chemistry.
COSY (COrrelation SpectroscopY) is a two-dimensional nuclear magnetic resonance (NMR) technique that reveals which protons in a molecule are coupled to each other through chemical bonds, typically two to three bonds apart.
In more detail
In a COSY spectrum, both axes display the same one-dimensional proton NMR spectrum, producing a square plot. Peaks along the diagonal simply reproduce the normal 1D spectrum, while off-diagonal "cross-peaks" mark pairs of protons that share scalar (J) coupling. By tracing cross-peaks, chemists can map out proton-proton connectivity even in crowded spectra where overlapping signals make direct interpretation difficult, making COSY essential for structure elucidation of organic molecules and biomolecules.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Full name | COrrelation SpectroscopY |
| Technique type | 2D homonuclear NMR (usually 1H-1H) |
| Detects | Scalar (J) coupling, typically 2-3 bonds apart |
In the COSY spectrum of ethanol (CH3CH2OH), a cross-peak connects the CH3 signal (~1.2 ppm) with the CH2 signal (~3.7 ppm), confirming these two proton groups are vicinally coupled (3J coupling across the C-C bond).
Frequently asked questions
How does COSY differ from NOESY?
COSY maps through-bond scalar (J) coupling between nearby protons, while NOESY maps through-space dipolar coupling (the nuclear Overhauser effect), which reveals protons close together in 3D space regardless of how many bonds separate them.
Why is a COSY spectrum symmetric about its diagonal?
Because J-coupling between two protons is mutual, a cross-peak linking proton A to proton B appears at both (A,B) and (B,A), so the pattern is mirrored across the diagonal.