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COSY  for the Novice

 

COSY is a method to determine 1H-1H connectivities, thereby giving the spectroscopist valuable information for structural elucidation of an organic compound.   We chose sucrose as an example.

 

 

We start with the 1H spectrum in Deuterium oxide  (D2O).

 

 

 

The large peak at 4.80ppm is from HDO.  The only easy identification we have at this point is the position of H-1’, 5.4 ppm, the anomeric hydrogen on the pyranose ring.    We can use our COSY spectrum to find H-2’, H-3’ etc. 

 

COSY is an example of a 2D spectrum.  The data in a 2D spectrum are usually presented in the form of a contour plot, which resembles a topographical map used for land elevations.  A 2D spectrum is thus a 3-dimensional map.  The vertical and horizontal dimensions are frequency dimensions, labeled F2 and F1.  In the case of a COSY spectrum they both correspond to 1H chemical shifts and increase from top to bottom and from right to left.   The signal intensities are shown in the third dimension, above and below the plane of the plot, (both positive and negative).

Here is a double quantum filtered COSY from sucrose. 

 

 

There are two kinds of peaks in a COSY type spectra.  Peaks that are along the diagonal where F2 = F1 are called diagonal or auto peaks and don’t really give any information.  The real help is the position of the peaks off the diagonal, the cross peaks.   These provide the chemical shifts of two coupled H atoms.  There should be an equivalent cross peak on the top side of the diagonal for every one on the bottom side.

 

We start with the H-1’ at 5.40 ppm. Its auto peak is at F1=F2 = 5.40 ppm, and we see a nice cross peak at 3.53, 5.40, ergo we have found H-2’ at 3.53 ppm.  Now find the auto peak for H-2’ (close to the upper right corner) and look for other cross peaks.  We find another cross peak at 3.75, 3.53, this corresponds to H-3’ coupling with H-2’.  We continue this process to identify all the 1H signals from the pyranose ring. 

 

We run out of cross peaks past H-6’ ‘s.  Now for the furanose ring.  We need a starting point.  The easiest is the peak for H-3 at 4.20ppm.  There is no H at C-2.  Follow the cross peaks as before and we find H-4 and H-5. 

 

Unfortunately, H-1’s , H-6’s and H-6’ ‘s are all overlapped around 3.8 to 3.9ppm, and if we can find them at all it would take careful analysis of the fine structure.

 

2D spectra can be displayed in either an absolute value mode (all signals are positive) and a phase sensitive mode where the phase information is recovered.  Absolute value mode is adequate, if we only need to get the chemical shifts of coupled systems.  If we want coupling constant information, phase sensitive spectra COSY can be invaluable.  Below we expand the cross peak for the H-1’ – H-2’ connectivity.  The red areas correspond to negative intensities and the blue positive ones.   In DQF-COSY active couplings (i.e., the couplings generating the cross peak) are indicated by antiphase peaks (up-down).   Passive couplings (i.e., splittings from couplings to another spin  but not generating the cross peak)  by in phase peaks (up-up or down-down). So J1’2’ is the active coupling and is a small 2.5 Hz.  The passive coupling is J2’3’ and is a large 10 Hz. 

 

 

 

We have thus demonstrated just how powerful a technique COSY is.  There are other 2D techniques just as useful.