In a COSY experiment, radio frequency pulses excite atomic nuclei inside a strong magnetic field. The instrument collects this data and creates a square plot on a computer screen. Both the horizontal and vertical axes display the normal one-dimensional proton NMR spectrum.
Peaks located along the main diagonal simply reproduce this standard one-dimensional spectrum. The peaks located away from the diagonal are known as cross-peaks. These cross-peaks are the most useful feature of the entire two-dimensional plot.
They mark specific pairs of protons that share a magnetic interaction called scalar coupling. Scalar coupling occurs when atomic nuclei interact through their shared chemical bonding electrons. By tracing these cross-peaks, chemists can map out the exact proton connectivity.
This mapping is vital when analyzing complex or entirely unknown chemical samples. In crowded spectra, overlapping signals make direct visual interpretation extremely difficult. COSY solves this issue by spreading the complex data across a second dimension.
This separates the overlapping peaks and clearly reveals the underlying molecular structure. COSY remains an essential tool for determining the exact shape of new organic molecules. It is also heavily used by biochemists for analyzing large complex biomolecules like proteins.
A common student misconception is that COSY shows all protons physically close in space. Instead, COSY only detects protons connected through a short path of chemical bonds.