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.