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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: Big-Daddy on July 01, 2013, 06:40:41 PM

Title: COSY spectra
Post by: Big-Daddy on July 01, 2013, 06:40:41 PM

What is the need for COSY NMR spectra? Can someone explain what they offer that normal one-dimensional ¹H spectra do not, and how they work in terms of the basic interpretation of the spectrum? I've tried searching the Internet but can't find any clear guides, particularly which explain the advantage of COSY over normal ¹H spectra.

Title: Re: COSY spectra
Post by: Illuminatus on July 01, 2013, 07:36:21 PM

Hi Big-Daddy,

I think the Harvey Mudd chemists explain it pretty well: http://www.hmc.edu/academicsclinicresearch/academicdepartments/chemistry/resources1/nmr1/2dnmrcosy.html

This answers both what the COSY result output offers as well as an example and explanation on how to interpret it.

Enjoy

Title: Re: COSY spectra
Post by: Big-Daddy on July 02, 2013, 05:02:03 PM

Thanks, here's the tricky bit:

Quote

Note that the diagonal within the box is also the spectrum for ethyl benzene as seen from "above." Off-diagonal peaks denote splitting between protons on adjacent carbons.

What diagonal, what box and what is meant by "off-diagonal peaks"?

Title: Re: COSY spectra
Post by: Corribus on July 02, 2013, 05:22:59 PM

Peaks along the diagonal refers to the diagonal line drawn from the bottom left corner to the top right corner.  You always get a diagonal  peak for every resonance in the spectrum.  There are four resonances in ethyl benzene spectrum (including TMS), and you get four diagonal peaks.  Off-diagonal peaks are any peaks that are not along the diagonal.  If you look at the spectrum on the link, you see that there is a set of four that form a sort of square near the top-right corner of the plot.  Two of these are your on-diagonal peaks for the benzene ethyl group.  The other two, off-diagonal, show that the two groups or protons are coupled together, as you'd expect.  There is no off-diagonal peak linking the aromatic resonances to those of the aliphatic chain (if there were, you would get two off-diagonal peaks, one in the top left corner of the plot and one the bottom right, which link these two).  This makes sense because the aliphatic and aromatic protons are not coupled.

If this isn't making sense, I will try to show it graphically for you.