Homonuclear correlation spectroscopy, or COSY, is a 2-dimensional NMR technique that provides information about coupled protons. Typically, the geminal and vicinal coupling are observed. For example, consider the COSY spectrum of ethyl acetate, where its 1D proton NMR spectrum is plotted along the vertical and horizontal axes with their corresponding chemical shift scale. Three spots on the diagonal corresponding to the three peaks in the 1D proton spectrum are called diagonal peaks. The COSY spectrum is symmetric about the diagonal, with signals above and below the diagonal called cross-peaks or off-diagonal peaks. An imaginary line is drawn from diagonal peaks parallel to the horizontal axis, leading to a cross peak. A vertical line is then drawn from the same cross peak towards the diagonal peak to correlate the peaks. The off-diagonal peaks correlating the multiplets at 1.25 and 4.2 ppm indicate that the methyl protons are coupled to the methylene protons. The signal at 2 ppm is not associated with cross-peaks because the protons are not involved in homonuclear coupling. A COSY spectrum is useful when overlapping multiplets complicate the corresponding 1D spectrum.
Homonuclear correlation spectroscopy or COSY is a 2D NMR technique that identifies coupled protons.
Consider the COSY spectrum of ethyl acetate. The corresponding 1D proton NMR spectrum is plotted along the horizontal and vertical axes.
The three spots on the diagonal correspond to the three peaks in the 1D spectrum, making COSY spectra symmetric about the diagonal.
The signals above and below the diagonal are called cross-peaks or off-diagonal peaks.
The off-diagonal peaks correlating the multiplets at 1.25 and 4.2 ppm indicate that the methyl protons are coupled to the methylene protons.
The signal at 2 ppm is not correlated to any cross-peaks because the protons are not involved in homonuclear coupling.
So, COSY spectra can identify coupling networks and therefore structural connectivity in molecules with complicated 1D spectra.
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