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.

1D TOCSY for Mixture Analysis

Many two dimensional techniques such as the 2D TOCSY, have similar 1D analogs where selective pulses are used to get effectively only one slice of the comparable 2D experiment. If you are only interested in a few correlations, then the 1D experiments can represent a tremendous time saving ( i.e. money saving). Below is a series of 1D TOCSY spectra run on a mixture of toluene, p-dichlorobenzene and pyridine. In this case, the technique can be used to separate out the components of the mixture. In the figure, the lower trace is the standard ¹H spectrum. In the other traces red arrows indicate the resonance selectively irradiated with the shaped pulse.

1H TOCSY

The TOCSY (TOtal Correlation SpectroscopY) sequence is a homonuclear experiment which produces a COSY-like plot. A COSY will give correlations between protons that are coupled to one another. A TOCSY, on the other hand, will give correlations between all protons in a given spin system. Below is the TOCSY spectrum of 3-heptanone. There are two ¹H spin systems; the protons on carbons 1 and 2 and the protons on carbons 4, 5, 6 and 7. The protons on carbons 1 and 2 have a correlation as they would in a COSY spectrum. The protons on carbons 4, 5, 6 and 7 are all correlated to one another whereas in a COSY spectrum, the main correlations would be between 4-5, 5-6 and 6-7 but not, for example, between the protons on carbons 4 and 7.