14: COSY

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COSY Spectroscopy*

Imagine if you could do the homonuclear spin decoupled experiment for all hydrogens in the molecule in one spectrum.

COSY is a 2D NMR experiment (homonuclear COrrelation SpectroscopY) that runs decoupling pulses along the entire frequency range and records all coupling.

This technique is used to identify spins which are coupled to each other (usually up to four bonds).

A cross-peak always shows up along the diagonal, where a peak on one axis corresponds with the identical peak on the other (“self-correlations”, in black). These are simply the ¹H spectra.

Draw a line through the diagonal.

Correlations that represent coupling show up both above and below the diagonal. In this spectrum, there are two correlations in red and blue.

Use a ruler to connect these correlations to the original peaks (black on the diagonal) and the chemical shifts.

These correlations are usually represented with double headed arrows on the structure and placed in a new column on our data tables.

* All spectra are either from SDBS (Japan National Institute of Advanced Industrial Science and Technology) or simulated.

2-Butanone: An Introduction to COSY

The ¹H NMR Spectrum of 2-butanone (C₄H₈O) is shown below.

COSY Spectrum for 2-butanone

Diagonal peaks correspond to the peaks in a 1D-NMR experiment.

Draw a line through the diagonal.

The cross peaks (correlations)indicate couplings between pairs of nuclei.

Draw the lines between the correlations and the shift.
Draw the correlations on the structure above with doubleheaded arrows.

When there is no coupling, no correlation is expected to appear.

Identify the peak that has no correlations. Which set of hydrogens is this?

Draw Your Own COSY

The ¹H spectrum of iodobutane is shown below:

Predict the COSY spectrum for this structure.

Distinguish Isomers using COSY

The ¹H NMR Spectrum of Compound A (C₁₀H₁₀O₂) is below.

The singlet at 3.8 ppm is ____________.
The position of the aldehydic H at _______ ppm can easily allow for the assignment of the peaks in the H atoms on the double bond in the straight chain.
There also appears to be an alkene at (_________ ppm) and an aromatic ring at (_________ ppm )

But the connectivity is a little bit trickier as you could imagine two possible structures:

The COSY on the next page will allow us to see correlations.

Distinguish Isomers using COSY (cont.)

Sometimes, COSY or 2D correlation spectroscopy shows more correlations than are visible in 1D spectrum. Thus, you will occasionally see correlations to an apparent singlet.

COSY Spectrum of C₁₀H₁₀O₂.