Two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy is an important analytical technique for determining the structure of organic compounds. Unlike one-dimensional NMR spectroscopy, in which signals are displayed along a single frequency axis, 2D NMR provides correlations between different nuclei or between different resonances. This additional information is particularly useful for assigning signals, establishing connectivity, identifying neighbouring atoms and distinguishing between closely related structures. Among the important 2D NMR experiments used in the spectroscopic identification of organic compounds are COSY and HETCOR. Although both experiments provide correlation information, they differ significantly in the type of nuclei involved, the mechanism of correlation and the structural information obtained. COSY stands for Correlation Spectroscopy. It is primarily a homonuclear 2D NMR experiment, most commonly performed using proton (^1H) NMR. In a conventional ^1H–^1H COSY spectrum, both dimensions represent proton chemical shifts. The main purpose of COSY is to identify protons that are spin-spin coupled to one another. Thus, it gives information about the connectivity or coupling relationship between neighbouring protons in a molecule. In a COSY spectrum, diagonal peaks correspond to the ordinary one-dimensional ^1H NMR signals. In addition to these diagonal peaks, cross-peaks appear at positions corresponding to pairs of protons that are coupled through scalar, or J, coupling. For example, if proton H_A is coupled with proton H_B, the COSY spectrum shows cross-peaks at the coordinates corresponding to H_A/H_B and H_B/H_A. These cross-peaks indicate that the two protons belong to the same spin-coupling system. This makes COSY especially useful for tracing proton–proton connectivity through a molecular framework. The most common COSY experiment detects through-bond coupling