Getting COSY with the TOCSY Experiment
2D NMR experiments can provide a wealth of information to aid in the structural elucidation of chemical compounds. Of the many 2D NMR experiments available, the homonuclear correlation spectroscopy (COSY) sequence is one of the most popular and is used to identify which spin systems are directly coupled to each other. As an example, the 1H-1H COSY spectrum of 1-propanol recorded on the NMReady-60 is shown below in Figure 1.
Figure 1. 1H-1H COSY spectrum of 1-propanol in D2O.
As you can clearly see, an off-diagonal cross peak is observed between the triplet at 3.6 ppm (the CH2 group highlighted in red) and the multiplet at 1.6 ppm (the CH2 group highlighted in yellow). As well, a separate cross peak is between the multiplet at 1.6 ppm and the triplet at 1.0 ppm (the CH3 group highlighted in green), as expected based on the structure of 1-propanol. A more complicated example of a COSY spectrum has been presented in a previous blog post.
A closely related 2D experiment is the total correlation spectroscopy (TOCSY) experiment. Like the COSY experiment, the TOCSY experiment will reveal cross peaks between spin systems that are directly coupled to each other. However, cross peaks are also observed between all spin systems which are connected by a chain of couplings. This feature makes the TOCSY experiment very useful for identifying larger interconnected network of spin couplings in larger molecules such as peptides, proteins, oligosaccharides, and polysaccharides. To illustrate the difference, the TOCSY spectrum of 1-propanol recorded on the NMReady-60 is displayed in Figure 2.
Figure 2. 1H-1H TOCSY spectrum of 1-propanol in D2O.
As expected, the same cross peaks seen in the COSY spectrum are present in the TOCSY spectrum. The difference is the extra cross peak (purple circle) between the triplet at 3.6 ppm (red highlight) and the signal at 1.0 ppm (green highlight).
Please don’t hesitate to contact us if you have any questions about the COSY and TOCSY experiment or if you want to see how our instrument can be incorporated into your workflow!