2D NMR: Homonuclear Correlation Spectroscopy (COSY)

Diagonal peaks are the three spots in a COSY spectrum that correspond directly to the three peaks in the 1D proton NMR spectrum. They appear along the diagonal line of the 2D spectrum and represent protons at specific chemical shifts. These diagonal peaks serve as reference points for identifying cross-peaks and understanding the coupling relationships in the molecule.

Cross-peaks, or off-diagonal peaks, appear above and below the diagonal in COSY spectra and indicate which protons are coupled to each other. For example, in ethyl acetate, off-diagonal peaks correlating signals at 1.25 and 4.2 ppm show that methyl protons are coupled to methylene protons. These cross-peaks directly map the homonuclear coupling networks within the molecule, similar to concepts explored in the 2D NMR overview of homonuclear correlation techniques.

The signal at 2 ppm in ethyl acetate COSY shows no cross-peaks because those protons are not involved in homonuclear coupling with any other protons in the molecule. Only protons that are coupled through vicinal or geminal interactions produce off-diagonal peaks. This absence of cross-peaks itself provides structural information about molecular connectivity.

COSY identifies coupling networks and structural connectivity by mapping which protons are coupled to each other through cross-peaks. When overlapping multiplets complicate the 1D spectrum, COSY separates these signals into a 2D display, making it easier to resolve individual coupling relationships. This 2D approach reveals molecular structure that may be obscured in one-dimensional data.

A COSY spectrum is symmetric about the diagonal because the same coupling information appears in both the upper and lower halves of the 2D plot. If proton A is coupled to proton B, this relationship is recorded both above and below the diagonal, creating mirror-image cross-peaks. This symmetry reflects the reciprocal nature of homonuclear spin-spin coupling.

Geminal and vicinal couplings are observed in COSY through cross-peaks that connect coupled protons. Geminal coupling occurs between protons on the same carbon, while vicinal coupling occurs between protons on adjacent carbons. Both types of homonuclear coupling produce off-diagonal peaks that reveal the spatial and electronic relationships between protons in the molecule.

In a COSY spectrum, the 1D proton NMR spectrum is plotted along both the horizontal and vertical axes, each with its corresponding chemical shift scale. This arrangement allows direct correlation between 1D peaks and their 2D positions. The diagonal peaks align with the 1D spectrum peaks, while cross-peaks extend perpendicular to show coupling relationships between different chemical shifts.