October 2, 2009
NOESY, ROESY, COSY and TOCSY are all 2D NMR experiments that sound so similar but offer different pieces of information about the puzzle. When interpreting the NMR data, it is important to understand how the nuclei interact with each other. For example, the presence of a cross peak (a correlation off the diagonal) on a COSY dataset is a result of nuclei coupling through a bond(s) whereas a NOESY dataset measures NOE’s (Nuclear Overhauser Effect) through space regardless of the number of bonds separating the nuclei. An NOE is typically observed for nuclei that are separated no farther than 5 Å apart.
For the enantiomers example shown below, the NOESY and COSY experiments differ in the presence or absence of the cross peaks. A clear difference between the two experiments is the information provided on the diastereotopic protons of the CH2 group.
The NOESY spectrum, also outlined in Part 1, shows 2 correlations at (4.29,1.28) and (4.29,3.13) ppm. There are no NOE’s to the proton signal at 2.68 ppm. The DQF-COSY below shows two-bond and three-bond correlations at (4.29,3.13), (4.29,1.28) and (3.13,2.68) ppm. There are no four-bond correlations present as the 4J coupling constants are close to zero.