COSY
Correlation Spectroscopy · Also known as: COSY NMR, 2D COSY, 1H-1H COSY
Correlation Spectroscopy (COSY) is a two-dimensional NMR technique that correlates proton chemical shifts through scalar coupling (J-coupling), revealing which protons are magnetically coupled and hence bonded through multiple bonds. Developed by Aue, Bartholdi, and Ernst in 1976, COSY became one of the most important tools in structural elucidation, enabling chemists to map out proton connectivity patterns and deduce molecular topology without isotopic labeling.
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When to use it
Apply COSY to any organic structure elucidation problem to determine which protons are magnetically coupled. It is most powerful for small to medium-sized molecules (< 1000 Da) with well-resolved proton spectra. COSY assumes scalar coupling dominates over dipolar effects and that the sample is stable during the relatively long acquisition time. For larger biomolecules, related techniques like HSQC or HMQC provide better resolution through heteronuclear correlation.
Strengths & limitations
- Directly visualizes J-coupling connectivity, making molecular topology transparent
- Unambiguous identification of spin systems and coupled proton sets
- Works with any organic compound containing protons without need for isotopic enrichment
- Robust and relatively quick; standard COSY takes minutes on modern spectrometers
- Limited to homonuclear coupling (1H-1H); cannot directly reveal carbon or nitrogen connectivity without additional experiments
- Spectral crowding in complex molecules leads to peak overlap and ambiguous cross-peak assignments
- Magnetic field inhomogeneity and long acquisition times can cause baseline distortion in 2D spectra
- Coupling constant determination requires careful lineshape analysis, not direct from COSY
Frequently asked
What is the difference between COSY cross-peaks and diagonal peaks?
Diagonal peaks appear at positions where F1 and F2 frequencies are identical, marking each proton's chemical shift. Cross-peaks appear off the diagonal where two different protons are coupled; their position indicates the chemical shift of both coupled partners.
How many bonds away can J-coupling be observed in COSY?
COSY primarily observes two- and three-bond couplings (2J and 3J), which are strongest. Four-bond (4J) and longer-range couplings are much weaker but can appear in well-resolved spectra, potentially creating ambiguity.
Why does COSY take much longer to acquire than a simple 1D proton NMR?
COSY must repeat the pulse sequence many times with different t1 evolution delays to build the full 2D dataset. A typical COSY acquires 256-512 t1 increments, each with multiple scans, whereas a simple 1D spectrum may require only 16-32 scans.
Sources
- Aue, W. P., Bartholdi, E., & Ernst, R. R. (1976). Two-dimensional spectroscopy. Application to nuclear magnetic resonance. The Journal of Chemical Physics, 64(5), 2229-2246. DOI: 10.1063/1.432450 ↗
- Bax, A., & Davis, D. G. (1985). MLEV-17-based two-dimensional homonuclear magnetization transfer spectroscopy. Journal of Magnetic Resonance, 65(2), 355-360. DOI: 10.1016/0022-2364(85)90018-6 ↗
How to cite this page
ScholarGate. (2026, June 3). Correlation Spectroscopy. ScholarGate. https://scholargate.app/en/spectroscopy/cosy
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
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