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Tuesday, June 3, 2008

COSY vs TOCSY

COSY (COrrelation SpectroscopY) and TOCSY (TOtal Correlation SpectroscopY) experiments are very common. Although both experiments provide a diagonally symmetric two dimensional contour plot with the one dimensional spectrum on the diagonal and correlations off of the diagonal, they have different information content. A COSY spectrum will have off-diagonal correlations between coupled spins whereas a TOCSY spectrum will have off-diagonal correlations between all spins in a spin system. For example, consider a spin system where A is coupled to B, B is coupled to C, and C is coupled to D. A COSY spectrum will have correlations between A and B, B and C, and C and D, whereas a TOCSY spectrum will have correlations between all of the spins. This is illustrated in the figure below for 3-heptanone. In this case, there are two spin systems (one on either side of the carbonyl group). The COSY spectrum shows correlations between adjacent spins whereas the TOCSY spectrum shows correlations between all of the spins in each of the two spin systems.

3 comments:

Ibrahim Shalayel said...

thnx alot, it is a very useful example... merci

Raja Venkatesh said...

I GOT IT 2D-TOSCY .BUT I CANT GET 2D-NOESY. MY COMPOUND MOLECULAR WEIGHT IS 1300-1400.I TRIED MIXING TIME FROM 50 TO 500 MS.BUT I AM NOT GETTING.PLEASE HELP ME

Glenn Facey said...

Raja,

Compounds with molecular weights of 1300 to 1400 typically have very small NOE's at typical field strengths. I suspect that is why you are unable to observe NOESY cross peaks. You should use a ROESY experiment which measures NOEs in the much smaller RF spin locking field. See this link: http://u-of-o-nmr-facility.blogspot.ca/2008/02/noesy-vs-roesy-for-large-molecules.html

Glenn